Abstract

Current challenges to antiretroviral therapy have opened new vistas in the search for novel drugs from natural products. This review focusses on plants as sources of inhibitors for human immunodeficiency virus type 1 (HIV-1) reverse transcriptase. Based on a systematic search of the literature, anti-HIV-1 reverse transcriptase activity was recorded for 132 plant species in 100 genera and 51 families. Seven families comprise 52.6% of plant species with anti-reverse transcriptase activity: Lamiaceae (13.7%), Fabaceae (10.7%), Euphorbiaceae (9.9%), Clusiaceae (6.1%), Asteraceae (4.6%), Combretaceae (4.6%), and Moraceae (3.0%). The repertoire of anti-reverse transcriptase active compounds includes (−)-catechin, 1,8-cineole, 3,4-di-O-caffeoylquinic acid, 5,7-dimethoxy-6-methylflavone, apigenin, baicalein, betulinic acid, caffeic acid, cis-3-hexene-1-ol, eugenol, euscaphic acid, gallic acid, hoslunddiol, limonene, naringenin, oleanolic acid, p-cymene, pomolic acid, quinic acid, rosmarinic acid, stigmasterol, thymol, ursolic acid, α-bergamotene, α-pinene, and γ-terpinene. Among the IC₅₀ values are 0.10 µg/ml (Uvaria angolensis), 3 µg/ml (Hemidesmus indicus), 2.3 µg/ml (Adansonia digitata), 6.24 µg/ml (Caesalpinia coriaria), 7.2 µg/ml (Terminalia sericea), 17.4 µg/ml (Hypoxis hemerocallidea), and 79 µg/ml (Moringa oleifera). The chemical diversity and activity profiles of HIV-1 reverse transcriptase inhibitors from plants reveal two recurring motifs: the structure of several active anti-reverse transcriptase compounds mimics nucleoside analogues, and numerous anti-reverse transcriptase phytochemicals have pleiotropic effects and heterogenous pharmacological benefits during infection and disease. To accelerate drug discovery and development, this review recommends the urgent need to tap into the rich vein of indigenous knowledge of putative anti-HIV/AIDS medicinal plants (reverse pharmacology), determine pan-assay interference compounds, analyze structure–activity relationships, and conduct more clinical trials.

Keywords:
HIV-1; Reverse transcriptase; Inhibitors; Phytochemicals; Inhibitory profiles

Introduction

Globally, about 36.9 million people were living with human immunodeficiency virus (HIV) in 2017 (UNAIDS, 2018UNAIDS, 2018. Global HIV & AIDS Statistics – 2018 Fact Sheet. http://www.unaids.org/en/resources/fact-sheet.
http://www.unaids.org/en/resources/fact-... ). HIV infection compromises the body's immune system leading to acquired immunodeficiency syndrome (AIDS). HIV type 1 (HIV-1) reverse transcriptase (RT) possesses both RNA-dependent DNA polymerase (RDDP) and ribonuclease H (RNase H) activities that work in tandem to convert viral genomic single-stranded RNA to double-stranded DNA that is then integrated into the DNA of the infected host cell (Figiel et al., 2018Figiel, M., Krepl, M., Park, S., Poznański, J., Skowronek, K., Gołąb, A., Ha, T., Šponer, J., Nowotny, M., 2018. Mechanism of polypurine tract primer generation by HIV-1 reverse transcriptase. J. Biol. Chem. 293, 191-202.). RT-associated RDDP and RNase H functions are essential for HIV-1 genome replication. Among the HIV-1 RT inhibitors, non-nucleoside reverse transcriptase inhibitors (NNRTI) constitute a prominent class of drugs which for almost 20 years has served as the cornerstone of combination antiretroviral therapy (cART) (Poongavanam et al., 2018Poongavanam, V., Namasivayam, V., Vanangamudi, M., Al Shamaileh, H., Veedu, R.N., Kihlberg, J., Murugan, N.A., 2018. Integrative approaches in HIV-1 non-nucleoside reverse transcriptase inhibitor design. Wires Comput. Mol. Sci. 8, http://dx.doi.org/10.1002/wcms.1328.
http://dx.doi.org/10.1002/wcms.1328... ; Sluis-Cremer, 2018Sluis-Cremer, N., 2018. Future of nonnucleoside reverse transcriptase inhibitors. Proc. Natl. Acad. Sci. USA , .
https://doi.org/10.1073/pnas.1720975115... ). NNRTI are small molecules that bind to HIV-1 RT at a site distinct from the DNA polymerase active site of the enzyme and block HIV-1 reverse transcription via an allosteric mechanism of action (Sluis-Cremer, 2018Sluis-Cremer, N., 2018. Future of nonnucleoside reverse transcriptase inhibitors. Proc. Natl. Acad. Sci. USA , .
https://doi.org/10.1073/pnas.1720975115... ). The conformation of RT for RNA hydrolysis is distinctly different from that for DNA synthesis and reveals a structural cavity which serves as a target for RT inhibition (Tian et al., 2018Tian, L., Kim, M.S., Li, H., Wang, J., Yang, W., 2018. Structure of HIV-1 reverse transcriptase cleaving RNA in an RNA/DNA hybrid. Proc. Natl. Acad. Sci. USA 115, 507-512.).

Nevirapine was the first NNRTI approved in 1996 by the United States of America (USA) Food and Drug Administration (FDA) for the treatment of HIV-1 infection, followed by delavirdine in 1997, efavirenz in 1998, etravirine in 2008, and rilpivirine in 2011 (Sluis-Cremer, 2018Sluis-Cremer, N., 2018. Future of nonnucleoside reverse transcriptase inhibitors. Proc. Natl. Acad. Sci. USA , .
https://doi.org/10.1073/pnas.1720975115... ). About 21.7 million people were accessing cART in 2017 (UNAIDS, 2018UNAIDS, 2018. Global HIV & AIDS Statistics – 2018 Fact Sheet. http://www.unaids.org/en/resources/fact-sheet.
http://www.unaids.org/en/resources/fact-... ). However, the efficiency of NNRTI is undermined by adverse events, poor drug–drug interactions, and drug-resistant variants of HIV-1 RT (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.; Peltenburg et al., 2018Peltenburg, N.C., Bierau, J., Bakker, J.A., Schippers, J.A., Lowe, S.H., Paulussen, A.D., van den Bosch, B.J.C., Leers, M.P.G., Hansen, B.E., Verbon, A., 2018. Erythrocyte Inosine triphosphatase activity: a potential biomarker for adverse events during combination antiretroviral therapy for HIV. PLoS ONE 13, e0191069.). Even in naïve patients who are not yet on cART, HIV-1 mutant variants with residues that confer resistance to RT inhibitors form small pockets of the viral population (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.).

In African countries such as Kenya, Nigeria, South Africa, Uganda, Zambia, and Zimbabwe, the overall prevalence of HIV resistance is 5.6%; this includes 3.3% associated with NNRTI (Hamers et al., 2011Hamers, R.L., Wallis, C.L., Kityo, C., Siwale, M., Mandaliya, K., Conradie, F., Botes, M.E., Wellington, M., Osibogun, A., Sigaloff, K.C., Nankya, I., Schuurman, R., Wit, F.W., Stevens, W.S., van Vugt, M., de Wit, T.F., 2011. PharmAccess African Studies to Evaluate Resistance (PASER). HIV-1 drug resistance in antiretroviral-naive individuals in sub-Saharan Africa after rollout of antiretroviral therapy: a multicentre observational study. Lancet Infect. Dis. 11, 750-759.). People living with HIV who develop drug-resistant HIV have few other treatment options except regimens based on ritonavir-boosted protease inhibitors (Sörstedt et al., 2018Sörstedt, E., Carlander, C., Flamholc, L., Hejdeman, B., Svedhem, V., Sönnerborg, A., Gisslén, M., Yilmaz, A., 2018. Effect of dolutegravir in combination with nucleoside reverse transcriptase inhibitors on people living with HIV who have pre-existing nucleoside reverse transcriptase inhibitor mutations. Int. J. Antimicrob. Agents 51, 733-738.). Still, protease and integrase inhibitors result in inferior virological outcomes, more HIV resistance, and are less likely recommended by current treatment protocols (Orkin et al., 2018Orkin, C., Llibre, J.M., Gallien, S., Antinori, A., Behrens, G.M.N., Carr, A., 2018. Nucleoside reverse transcriptase inhibitor-reducing strategies in HIV treatment: assessing the evidence. HIV Med. 19, 18-32.). Severe liver toxicity affects 8–23% of HIV-infected patients receiving indinavir and tenofovir (Lemoine and Ingiliz, 2012Lemoine, M., Ingiliz, P., 2012. Liver injury in HIV monoinfected patients: should we turn a blind eye to it? Clin. Res. Hepatol. Gas. 36, 441-447.), and cART is associated with nephrotoxicity (Hamzah and Post, 2009Hamzah, L., Post, F.A., 2009. HIV and kidney disease. Medicine 37, 365-367.). Development of HIV resistance and toxicity subtract from the efficacy of and adherence to cART.

Against this backdrop, it is important to discover alternative HIV-1 RT inhibitors from plants. Therefore, the current review details the chemical diversity and biological activity profiles of HIV-1 RT inhibitors from plants. Detailed knowledge of plant chemical compounds that inhibit RDDP and RNase H functions is significant in the search for novel antiretroviral drugs. In the face of current challenges to cART, this review may inspire a new future where plants are the frontier for more efficacious HIV-1 RT inhibitors, in addition to creating a strong bioprospecting pipeline for innovative bioentrepreneurs to advance the invention of new HIV-1 RT drugs from plants.

Methodology

To prevent author bias, this review was carried out using a comprehensive and systematic data mining approach. To obtain pertinent literature, the key words “HIV reverse transcriptase inhibitors” and “plants” were concomitantly searched in Google Scholar, Elsevier's pay-to-view service ScienceDirect, Scopus, Scielo, and PubMed Central, the United States of America National Library of Medicine's digital archive of biomedical and life sciences journal literature. Literature sources included peer reviewed journal articles, conference/seminar proceedings, PhD theses, refereed books and abstracts. Further, only plants with known anti-RT activities were included in the study. Mushrooms, algae, plants with unknown anti-RT activities, and plants with anti-HIV-1 integrase or anti-HIV-1 protease activities were excluded.

Families and species of plants, phytochemical compounds and anti-RT profiles mainly half maximal inhibitory concentration (IC₅₀) were recorded. Where IC₅₀ values were absent, the following anti-RT parameters were documeted: effective concentration (EC₅₀), inhibitory dose (ID₅₀), median effective dose (ED₅₀), and percentage inhibition of RT. Many of the RT assays used a non-radioactive HIV-RT colorimetric ELISA kit from Roche, Germany. International Plant Names Index (http://www.ipni.org) and The Plant List (http://www.theplantlist.org) were used to verify names of plant families and species. PubChem Structure Search was used to corroborate chemical structures of phytochemical compounds. The results of the review are presented and discussed in the following sections.

Plants, phytochemical compounds and anti-HIV RT activities

Box 1 shows the plant species, genera and families with their phytochemical diversity and HIV-1 RT inhibitory profiles. Overall, HIV-1 RT inhibitory activity was found in 132 plant species distributed across 100 genera and 51 families. Most of the plant species were principally distributed among the following families (Fig. 1): Lamiaceae (13.7%), Fabaceae (10.7%), Euphorbiaceae (9.9%), Clusiaceae (6.1%), Asteraceae (4.6%), Combretaceae (4.6%), and Moraceae (3.0%). These seven families consist of 52.6% of plant species with anti-HIV-1 RT activity. Plant species in the family Lamiaceae potently inhibit HIV-1 replication (Bedoya et al., 2001Bedoya, L.M., Sanchez-Palomino, S., Abad, M.J., Bermejo, P., Alcami, J., 2001. Anti-HIV activity of medicinal plant extracts. J. Ethnopharmacol. 77, 113-116.; Reichling et al., 2008Reichling, J., Nolkemper, S., Stintzing, F.C., Schnitzler, P., 2008. Impact of ethanolic Lamiaceae extracts on herpesvirus infectivity in cell culture. Complement. Med. Res. 15, 313-320.; Geuenich et al., 2008Geuenich, S., Goffinet, C., Venzke, S., Nolkemper, S., Baumann, I., Plinkert, P., Reichling, J., Keppler, O.T., 2008. Aqueous extracts from peppermint, sage and lemon balm leaves display potent anti-HIV-1 activity by increasing the virion density. Retrovirology 5, .
https://doi.org/10.1186/1742-4690-5-27... ). Other studies have found that plant species in the family Fabaceae and Euphorbiaceae inhibit HIV-1 RT (Chang et al., 1995Chang, C.W., Lin, M.T., Lee, S.S., Liu, K.C.C., Hsu, F.L., Lin, J.Y., 1995. Differential inhibition of reverse transcriptase and cellular DNA polymerase-α activities by lignans isolated from Chinese herbs, Phyllanthus myrtifolius Moon, and tannins from Lonicera japonica Thunb and Castanopsis hystrix. Antivir. Res. 27, 367-374.; Ng et al., 1997Ng, T.B., Huang, B., Fong, W.P., Yeung, H.W., 1997. Anti-human immunodeficiency virus (anti-HIV) natural products with special emphasis on HIV reverse transcriptase inhibitors. Life Sci. 61, 933-949.; Matsuse et al., 1998Matsuse, I.T., Lim, Y.A., Hattori, M., Correa, M., Gupta, M.P., 1998. A search for anti-viral properties in Panamanian medicinal plants: the effects on HIV and its essential enzymes. J. Ethnopharmacol. 64, 15-22.; Matthée et al., 1999Matthée, G., Wright, A.D., König, G.M., 1999. HIV reverse transcriptase inhibitors of natural origin. Planta Med. 65, 493-506.). Chinsembu and Hedimbi (2010)Chinsembu, K.C., Hedimbi, M., 2010. Ethnomedicinal plants and other natural products with anti-HIV compounds and their putative modes of action. Int. J. Biotech. and Mol. Biol. Res. 6, 74-91. found that 65 plant species mostly from the Fabaceae family had anti-HIV activity and Prinsloo et al. (2018)Prinsloo, G., Marokane, C.K., Street, R.A., 2018. Anti-HIV activity of southern African plants: current developments, phytochemistry and future research. J. Ethnopharmacol. 210, 133-155. documented 56 anti-HIV plant species from Southern Africa. Other studies have also documented anti-HIV-1 RT from plants (Li et al., 2018aLi, C., Zhang, C., Zhou, H., Feng, Y., Tang, F., Hoi, P.M.M., He, C., Ma, D., Zhao, C., Lee, S.M.Y., 2018. Inhibitory effects of betulinic acid on LPS-induced neuroinflammation involve M2 microglial polarization via CaMKKβ-dependent AMPK activation. Front. Mol. Neurosci. 11, .
https://doi.org/10.3389/fnmol.2018.00098... ,bLi, J., Zhao, Y.L., Huang, H.Y., Wang, Y.Z., 2017. Phytochemistry and pharmacological activities of the genus Swertia (Gentianaceae): a review. Am. J. Chin. Med. 45, 667-736.,cLi, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ; Ornano et al., 2018Ornano, L., Feroci, M., Guarcini, L., Venditti, A., Bianco, A., 2018. Anti-HIV agents from nature: natural compounds from Hypericum hircinum and carbocyclic nucleosides from iridoids. Stud. Nat. Prod. Chem. 56, 173-228.; Palacios, 2018Palacios, P.J., 2018. The Determination and Comparison of the in vitro Lectin, Antimicrobial and Anti-HIV Reverse Transcriptase Activities of Three South Texas Fabaceae Leaf Extracts, Doctoral Dissertation. Texas A&M International University, http://hdl.handle.net/2152.4/130.
http://hdl.handle.net/2152.4/130... ).

In the leading plant families, the following phytochemicals were predominant: (−)-catechin, 1,8-cineole, 3,4-di-O-caffeoylquinic acid, 5,7-dimethoxy-6-methylflavone, apigenin, apigenin-7-O-glucoronide, apigenin-7-O-glucoside, apigenin-7-O-rutinoside, baicalein (1), betulinic acid, caffeic acid, cis-3-hexene-1-ol, eugenol, euscaphic acid, gallic acid, hoslunddiol, limonene, naringenin, oleanolic acid, p-cymene, pomolic acid, quinic acid, rosmarinic acid, stigmasterol, thymol, thymoquinone, ursolic acid, α-bergamotene (2), α-pinene, β-pinene and γ-terpinene. Other notable phytochemical compounds from plant extracts with anti-RT activities were: 1,3,6,7-tetrahydroxyxanthone or mangiferin (3), 1,5-dicaffeoylquinic acid (4), 1,8-cineole, 2-(3,3-dimethylallyl)-1,3,7-trihydroxyxanthone, 2-methylene-5-(1-methylethyl), 3-O-acetyl-aleuritolic acid, 3-O-acetyl-erythrodiol, 6-hydroxyl kaempferol 3-O-arabinoglucoside, 6β-angeloyloxy-3β,8α-dihydroxyeremophil-7(11)-en-12,8β-olide, 6β-angeloyloxy-3β,8β-dihydroxyeremophil-7(11)-en-12,8α-olide, agathisflavone (5), amentoflavone (6), angoletin (7), anisic acid, apetalic acid, artemisinin (8), bergenin, calanolides B and C, caloinophyllin A, camphene, camphor, centratherin and its derivative isocentratherin, chlorogenic acid, corydine, dicaffeoyl acids, digitoxigenin-3-O-glucoside, ellagic acid, ferulic acid, friedelane-3-one-28-al, friedelin, fumaric acid, garcinuntabiphenyls A–C, garcinuntins A–C, garciosones A–D, garcisaterpenes A and C, hinokiflavone, hinokiflavone, inophyllum B, isobavachalcone, lupenediol, lupenoic acid, luteolin (9), michellamine B, morelloflavone, nobiletin, norisoboldine, pentamethylquercetin, p-hydroxybenzoic acid, proteins such as acaconin and acafusin, punicalin (10), protostanes, quercelagetin 3,7-dimethylether, quercetin, repandusinic acid, rhusflavanone, robustaflavone, robustaflavone, shikimic acid, stigmasta-4,22-dien-3-one, vanillic acid, vismiaphenone D, volkensiflavone, α-thujone, and xanthohumol (11).

In terms of anti-RT profiles, the IC₅₀ values included 0.10 µg/ml for Uvaria angolensis, 3 µg/ml for Hemidesmus indicus, 2.3 µg/ml for Adansonia digitata, 6.24 µg/ml for Caesalpinia coriaria, 7.2 µg/ml for Terminalia sericea, 17.4 µg/ml for Hypoxis hemerocallidea, and 79 µg/ml for Moringa oleifera. EC₅₀ values included 4.2 µg/ml for extracts of Andrographis paniculata, 1 µM for michellamine B isolated from a fraction of Ancistrocladus korupensis, 20.9 µg/ml for artemisinin (8) isolated from Artemisia annua, 1–2 µg/ml for water extracts of Petasites japonicus, 0.50 µg/ml for xanthohumol (11) isolated from Humulus lupulus, and 11 µg/ml for vismiaphenone D isolated from Vismia cayennensis. ED₅₀ values included 7.6 µg/ml for a p-cymene and γ-terpinene-rich fraction of Thymus serpyllum, 1.6 µg/ml for rosmarinic acid isolated from Melissa officinalis, 7.1 µg/ml for polyphenolics isolated from Perilla frutescens, and 7.6 µg/ml for essential oils from Lavandula dentata.

Knowledge of plant species, genera and families in addition to their phytochemical diversity and HIV-1 RT inhibitory profiles may help demystify the use of plant remedies for managing HIV infection. This is important because in at least fourteen African countries, 80% or more of people who were estimated to be eligible for cART under the 2013 WHO guidelines were not on cART as of December 2012 (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Nine out of every ten people have an unmet need for cART in sub-Saharan African countries such as Angola, Cameroon, Central African Republic, Chad, Cote d’Ivoire, Democratic Republic of the Congo, Ethiopia, Ghana, Kenya, Lesotho, Malawi, Mozambique, Nigeria, South Africa, South Sudan, Togo, Uganda, Tanzania, Zambia and Zimbabwe. In 2007, only less than 10% of all HIV-infected children in need of cART in sub-Saharan Africa were actually receiving therapy (Eley and Nuttall, 2007Eley, B., Nuttall, J., 2007. Antiretroviral therapy for children: challenges and opportunities. Ann. Trop. Paediatr. 27, 1-10.). In these countries (especially for children), plant products with known anti-RT efficacy can become the new window of dispensary in the management of HIV infection and AIDS.

A remarkable diversity of phytochemical structures including proteins, terpenoids, coumarins, xanthones, alkaloids, flavonoids, polyphenols, and polysaccharides are capable of rendering HIV-1 RT less active (Ng et al., 1997Ng, T.B., Huang, B., Fong, W.P., Yeung, H.W., 1997. Anti-human immunodeficiency virus (anti-HIV) natural products with special emphasis on HIV reverse transcriptase inhibitors. Life Sci. 61, 933-949.). Plants may serve as sources of new active leads that may be developed further into anti-HIV drug candidates. In a 24 week study, a Chinese preparation of fourteen plants increased plasma CD4 counts and reduced HIV viral loads (Deng et al., 2014Deng, X., Jiang, M., Zhao, X., Liang, J., 2014. Efficacy and safety of traditional Chinese medicine for the treatment of acquired immunodeficiency syndrome: a systematic review. J. Trad. Chin. Med. 34, 1-9.). In Zambia, the Sondashi Formula (SF2000), tested in human HIV patients, reduced viral loads and increased CD4 counts (Chinsembu, 2009; 2015Chinsembu, K.C., 2009. Model and experiences of initiating collaboration with traditional healers in validation of ethnomedicines for HIV/AIDS in Namibia. J. Ethnobiol. Ethnomed., http://dx.doi.org/10.1186/1746-4269-5-30.
http://dx.doi.org/10.1186/1746-4269-5-30... ). Phytochemicals such as calanolides (coumarins), ursolic and betulinic acids (triterpenes), and baicalin (flavonoid) are promising candidates for anti-HIV RT agents (Salehi et al., 2018Salehi, B., Kumar, N.V.A., Şener, B., Sharifi-Rad, M., Kılıç, M., Mahady, G.B., Vlaisavljevic, S., Iriti, M., Kobarfard, F., Setzer, W.N., Ayatollahi, S.A., Ata, A., Sharifi-Rad, J., 2018. Medicinal plants used in the treatment of human immunodeficiency virus. Int. J. Mol. Sci. 19, .
https://doi.org/10.3390/ijms19051459... ).

By 2006, more than fifty chemical compounds with varying levels of anti-HIV activity had been isolated from plants (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Although the use of plants is a strong pipeline for the discovery of HIV-1 RT inhibitors, the anti-HIV-1 RT evidence presented in this paper provides a fresh quest to reconsider the mainstreaming of plant medications in the treatment of HIV/AIDS. In resource-poor settings, anti-RT plants can be used as herbal drugs to manage HIV/AIDS. Medicinal plants are not a replacement for cART (Chinsembu, 2009Chinsembu, K.C., 2009. Model and experiences of initiating collaboration with traditional healers in validation of ethnomedicines for HIV/AIDS in Namibia. J. Ethnobiol. Ethnomed., http://dx.doi.org/10.1186/1746-4269-5-30.
http://dx.doi.org/10.1186/1746-4269-5-30... ), but they do increase the options available to AIDS patients, especially those faced by side-effects due to antiretroviral drugs, HIV resistance and treatment failure (WHO, 1989WHO, 1989. In vitro screening of traditional medicines for anti-HIV activity: memorandum from a WHO meeting. Bull. World Health Organ. 87, 613-618.; Homsy et al., 2004Homsy, J., King, R., Tenywa, J., Kyeyune, P., Opio, A., Balaba, D., 2004. Defining minimum standards of practice for incorporating African traditional medicine into HIV/AIDS prevention, care, and support: a regional initiative in eastern and southern Africa. J. Altern. Complement. Med. 10, 905-910.). Anti-HIV-1 RT compounds from plants may provide leads to novel and more efficacious drugs to lessen the global burden of HIV/AIDS. Conservation of anti-HIV plants is important to ensure sustainability and avoid medicinal plant extinction. As research into new anti-HIV agents focuses on novel structures and (or) new action mechanisms (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.), the chemical diversity of plant compounds in this review can be utilized as herbal antiretroviral drugs or used in the search for novel compounds with anti-RT activities.

Recurring motifs in chemical diversity and activity profiles of HIV-1 RT inhibitors from plants

Structure dictates function

The first recurring motif illustrated in this review relates to the structure of the plant compounds with anti-HIV-1 RT inhibitory activity. Much like the first ART drug azidothymidine (AZT) which was approved for AIDS treatment in 1987, several active anti-RT plant compounds mimic nucleoside analogues in one structural shape or form. They appear to be close homologues that share structural and functional similarities with NNRTI. This is true for chemical compounds such as (−)-catechin, 1,5-dicaffeoylquinic acid (4), agathisflavone (5), amentoflavone (6), angoletin (7), apigenin, artemisinin (8), baicalein (1), betulinic acid, ellagic acid, gallic acid, lupeol, luteolin (9), mangiferin (3), oleanolic acid, p-cymene, punicalin (10), rosmarinic acid, shikimic acid, stigmasterol, ursolic acid, xanthohumol (11), α-bergamotene (2), and γ-terpinene. Corollary to the principle that structure dictates function, it is unsurprising that these active compounds putatively inhibit or reduce the activity of RT, thus making HIV-infected cells produce fewer virions.

Pleiotropic effects

The second recurring motif is that numerous anti-HIV-1 RT phytochemicals enumerated in this review affect various targets and confer heterogenous pharmacological actions and health benefits. For example, andrographolide, a labdane diterpenoid produced by A. paniculata (Rao et al., 2004Rao, Y.K., Vimalamma, G., Rao, C.V., Tzeng, Y.M., 2004. Flavonoids and andrographolides from Andrographis paniculata. Phytochemistry 65, 2317-2321.), has a broad range of therapeutic applications including anti-inflammatory, anti-platelet aggregation activities and potential antineoplastic properties (Burgos et al., 2005Burgos, R.A., Seguel, K., Perez, M., Meneses, A., Ortega, M., Guarda, M.I., Loaiza, A., Hancke, J.L., 2005. Andrographolide inhibits IFN-γ and IL-2 cytokine production and protects against cell apoptosis. Planta Med. 71, 429-434.). Discovered in the leaves of the A. korupensis in Cameroon, michellamines are atropisomeric alkaloids with strong anti-HIV RT inhibitory activities (Manfredi et al., 1991Manfredi, K.P., Blunt, J.W., Cardellina, J.H., McMahon, J.B., Pannell, L.L., Cragg, G.M., Boyd, M.R., 1991. Novel alkaloids from the tropical plant Ancistrocladus abbreviatus inhibit cell killing by HIV-1 and HIV-2. J. Med. Chem. 34, 3402-3405.; Vlietinck et al., 1998Vlietinck, A.J., De Bruyne, T., Apers, S., Pieters, L.A., 1998. Plant-derived leading compounds for chemotherapy of human immunodeficiency virus (HIV) infection. Planta Med. 64, 97-109.). They block HIV-induced cellular fusion and inhibit mutant HIV-1 RT. Michellamines are especially useful against RT from HIV-2 particularly found in and around Cameroon (Supko and Malspeis, 1994Supko, J.G., Malspeis, L., 1994. Determination of michellamine B in biological fluids by high-performance liquid chromatography with fluorescence detection. Anal. Biochem. 216, 52-60.). In addition to their anti-HIV activities, michellamines have anti-parasitic and anti-leukaemic properties (Tshitenge et al., 2018Tshitenge, D.T., Feineis, D., Mudogo, V., Kaiser, M., Brun, R., Seo, E.J., Efferth, T., Bringmann, G., 2018. Mbandakamine-type naphthylisoquinoline dimers and related alkaloids from the Central African liana Ancistrocladus ealaensis with antiparasitic and antileukemic activities. J. Nat. Prod. 81, 918-933.). β-Amyrin, an oleanane-type pentacyclic triterpenoid, known to have many antiviral activities (Xiao et al., 2018Xiao, S., Tian, Z., Wang, Y., Si, L., Zhang, L., Zhou, D., 2018. Recent progress in the antiviral activity and mechanism study of pentacyclic triterpenoids and their derivatives. Med. Res. Rev. 38, 951-976.), is also present in the genus Moringa (Rani et al., 2018Rani, A., Zahirah, N., Husain, K.B., Kumolosasi, E., 2018. Moringa genus: a review of phytochemistry and pharmacology. Front. Pharmacol. 9, http://dx.doi.org/10.3389/fphar.2018.00108.
http://dx.doi.org/10.3389/fphar.2018.001... ).

Lupeol is a pharmacologically active triterpenoid endowed with several medicinal properties not least in colorectal cancer where it downregulates cell viability and activates cell apoptosis (Chen et al., 2018aChen, M.C., Hsu, H.H., Chu, Y.Y., Cheng, S.F., Shen, C.Y., Lin, Y.J., Chen, R.J., Viswanadha, V.P., Lin, Y.M., Huang, C.Y., 2018. Lupeol alters ER stress-signaling pathway by downregulating ABCG2 expression to induce oxaliplatin-resistant LoVo colorectal cancer cell apoptosis. Environ. Toxicol. 33, 587-593.,bChen, Y., Du, K., Li, J., Bai, Y., An, M., Tan, Z., Chang, Y.X., 2018. A green and efficient method for the preconcentration and determination of gallic acid, bergenin, quercitrin, and embelin from Ardisia japonica using nononic surfactant genapol X-080 as the extraction solvent. Int. J. Anal. Chem., http://dx.doi.org/10.1155/2018/1707853.
http://dx.doi.org/10.1155/2018/1707853... ). Parvez et al. (2018)Parvez, M.K., Alam, P., Arbab, A.H., Al-Dosari, M.S., Alhowiriny, T.A., Alqasoumi, S.I., 2018. Analysis of antioxidative and antiviral biomarkers β-amyrin, β-sitosterol, lupeol, ursolic acid in Guiera senegalensis leaves extract by validated HPTLC methods. Saudi Pharm. J. 26, 685-693. reported that lupeol decreases the level of reactive oxygen species and restores antioxidant enzyme activities in the liver, and induces growth inhibition and apoptosis in a hepatocellular carcinoma cell line. Lupeol is a lead compound for the generation of more effective drugs against Influenza A and herpes simplex virus (Parvez et al., 2018Parvez, M.K., Alam, P., Arbab, A.H., Al-Dosari, M.S., Alhowiriny, T.A., Alqasoumi, S.I., 2018. Analysis of antioxidative and antiviral biomarkers β-amyrin, β-sitosterol, lupeol, ursolic acid in Guiera senegalensis leaves extract by validated HPTLC methods. Saudi Pharm. J. 26, 685-693.). Isolated from Guiera senegalensis, a broad-spectrum African folk medicinal plant, lupeol (6.72 µg/mg) exerts anti-hepatitis B virus activity (Parvez et al., 2018Parvez, M.K., Alam, P., Arbab, A.H., Al-Dosari, M.S., Alhowiriny, T.A., Alqasoumi, S.I., 2018. Analysis of antioxidative and antiviral biomarkers β-amyrin, β-sitosterol, lupeol, ursolic acid in Guiera senegalensis leaves extract by validated HPTLC methods. Saudi Pharm. J. 26, 685-693.).

HIV is associated with greater malaria mortality (Chintu et al., 1995Chintu, C., Luo, C., Bhat, G., DuPont, H.L., Mwansa-Salamu, P., Kabika, M., Zumla, A., 1995. Impact of the human immunodeficiency virus type-1 on common pediatric illnesses in Zambia. J. Trop. Pediatr. 41, 348-353.) and acute malaria induces temporal increases in the HIV viral load (Vamvaka et al., 2014Vamvaka, E., Twyman, R.M., Christou, P., Capell, T., 2014. Can plant biotechnology help break the HIV – malaria link?. Biotechnol. Adv. 32, 575-582.). HIV also triggers a wide range of opportunistic infections often misdiagnosed as malaria and HIV/malaria co-morbidity reduces the effectiveness of both antimalarial and antiretroviral drugs (Vamvaka et al., 2014Vamvaka, E., Twyman, R.M., Christou, P., Capell, T., 2014. Can plant biotechnology help break the HIV – malaria link?. Biotechnol. Adv. 32, 575-582.). Given that co-administration of non-artemisinin antimalarial and antiretroviral drugs increases the risk of drug-related toxicity (Brentlinger et al., 2006Brentlinger, P.E., Behrens, C.B., Micek, M.A., 2006. Challenges in the concurrent management of malaria and HIV in pregnancy in sub-Saharan Africa. Lancet Infec. Dis. 6, 100-111.), the action of artemisinin (8) on HIV-1 RT is noteworthy because its semi-synthetic derivatives can concurrently be used against HIV and Plasmodium falciparum malaria.

Corilagin is an ellagitannin first isolated in 1951 from C. coriaria. Apart from inhibiting HIV-1 RT, it also blocks hepatitis C virus (Reddy et al., 2018Reddy, B.U., Mullick, R., Kumar, A., Sharma, G., Bag, P., Roy, C.L., Sudha, G., Tandon, H., Dave, P., Shukla, A., Srinivasan, P., Nandhitha, M., Srinivasan, P., Das, S., 2018. A natural small molecule inhibitor corilagin blocks HCV replication and modulates oxidative stress to reduce liver damage. Antivir. Res. 150, 47-59.), attenuates allergy (Zhou et al., 2018aZhou, J., Ci’an Zhang, Y.S., Wang, L., Zhang, J., Li, F., Mao, W., 2018. Corilagin attenuates allergy and anaphylactic reaction by inhibiting degranulation of mast cells. Med. Sci. Monit. 24, 891-896.,bZhou, X., Shang, J., Wang, J., Jiang, B., Wang, Q., 2018. Antioxidant activity of extracts from the aril of Torreya fargesii Franch. and its protection on the oxidation of DHA algal oil. CyTA-J. Food 16, 381-389.), and alleviates cholestasis (Yang et al., 2018aYang, F., Wang, Y., Li, G., Xue, J., Chen, Z.L., Jin, F., Luo, L., Zhou, X., Ma, Q., Cai, X., Li, H.R., Zhao, L., 2018. Effects of corilagin on alleviating cholestasis via farnesoid X receptor-associated pathways in vitro and in vivo. Br. J. Pharmacol. 175, 810-829.,bYang, S., Xu, M., Lee, E.M., Gorshkov, K., Shiryaev, S.A., He, S., Sun, W., Cheng, Y.-S., Hu, X., Tharappel, A.M., Lu, B., Pinto, A., Farhy, C., Huang, C.-T., Zhang, Z., Zhu, W., Wu, Y., Zhou, Y., Song, G., Zhu, H., Shamim, K., Martínez-Romero, C., García-Sastre, A., Preston, R.A., Jayaweera, D.T., Huang, R., Huang, W., Xia, M., Simeonov, A., Ming, G., Qiu, X., Terskikh, A.V., Tang, H., Song, H., Zheng, W., 2018. Emetine inhibits Zika and Ebola virus infections through two molecular mechanisms: inhibiting viral replication and decreasing viral entry. Cell Discov. 4, .
https://doi.org/10.1038/s41421-018-0034-... ). Xanthohumol (11) is the main prenylated chalcone in the female inflorescences of Humulus lupulus, commonly known as hops. This hop-derived prenylated flavonoid has anticancer effects and significantly decreases age-related oxidative stress, inflammation and apoptosis as it regulates several pathways linked to proliferation and apoptosis (Fernández-García et al., 2018Fernández-García, C., Rancan, L., Paredes, S.D., Montero, C., de la Fuente, M., Vara, E., Tresguerres, J.A., 2018. Xanthohumol exerts protective effects in liver alterations associated with aging. Eur. J. Nutr., http://dx.doi.org/10.1007/s00394-018-1657-6.
http://dx.doi.org/10.1007/s00394-018-165... ).

An ansamycin antibiotic originally isolated from the Ethiopian shrub Maytenus serrata, maytansine induces microtubule disassembly and disrupts mitosis (Kasilo et al., 2017Kasilo, O.M., Kofi-Tsekpo, M., Gachathi, F., 2017. Medicinal and aromatic plants of the World-Africa. Medicinal and Aromatic Plants of the World-Africa, vol. 3. Springer, Dordrecht, pp. 77–90.). Since maytansine exhibits cytotoxicity against many tumour cell lines and inhibits tumour growth in vivo, it is an important antineoplastic and antimicrobial compound (Kusari et al., 2017Kusari, S., Eckelmann, D., Spiteller, M., 2017. In situ MALDI-HRMS-imaging of Maytenus senegalensis during the germination process reveals the ecological role of the important anticancer compound maytansine. Planta Med. Int. Open , .
https://doi.org/10.1055/s-0037-1608245... ). Hinokiflavone and other related biflavonoids including amentoflavone (6), robustaflavone, agathisflavone (5), rhusflavone and rhusflavanone are the cytotoxic principle of Rhus succedanea (Lin et al., 1989Lin, Y.M., Chen, F.C., Lee, K.H., 1989. Hinokiflavone, a cytotoxic principle from Rhus succedanea and the cytotoxicity of the related biflavonoids. Planta Med. 55, 166-168.). These compounds are leading candidates in anti-HIV drug discovery (Parveen et al., 2018Parveen, M., Malla, A.M., Ahmad, F., Azaz, S., 2018. Anti-HIV drug discovery struggle: from natural products to drug prototypes. In: Atta-ur-Rahman, Anjum, S., El-Seedi, H. (Eds.), Natural Products in Clinical Trials, vol. 1, pp. 165–232.). In addition, hinokiflavone also has antioxidant and hepatoprotective effects (Zhou et al., 2018aZhou, J., Ci’an Zhang, Y.S., Wang, L., Zhang, J., Li, F., Mao, W., 2018. Corilagin attenuates allergy and anaphylactic reaction by inhibiting degranulation of mast cells. Med. Sci. Monit. 24, 891-896.,bZhou, X., Shang, J., Wang, J., Jiang, B., Wang, Q., 2018. Antioxidant activity of extracts from the aril of Torreya fargesii Franch. and its protection on the oxidation of DHA algal oil. CyTA-J. Food 16, 381-389.; Abdel-Kader et al., 2018Abdel-Kader, M.S., Abulhamd, A.T., Hamad, A.M., Alanazi, A.H., Ali, R., Alqasoumi, S.I., 2018. Evaluation of the hepatoprotective effect of combination between hinokiflavone and glycyrrhizin against CCl4 induced toxicity in rats. Saudi Pharm. J. 26, 496-503.).

Putranjivain A possesses anti-herpes virus activity (Cheng et al., 2004Cheng, H.Y., Lin, T.C., Yang, C.M., Wang, K.C., Lin, L.T., Lin, C.C., 2004. Putranjivain A from Euphorbia jolkini inhibits both virus entry and late stage replication of herpes simplex virus type 2 in vitro. J. Antimicrob. Chemother. 53, 577-583.). It also inhibits HIV attachment and penetration, and blocks late stage HIV replication. Mallotojaponin, a major constituent of pericarps of Mallotus japonicas, confers anticancer and anti-HIV effects (Satomi et al., 1994Satomi, Y., Arisawa, M., Nishino, H., Iwashima, A., 1994. Antitumor-promoting activity of mallotojaponin, a major constituent of pericarps of Mallotus japonicus. Oncology 51, 215-219.; Chauthe et al., 2012Chauthe, S.K., Bharate, S.B., Periyasamy, G., Khanna, A., Bhutani, K.K., Mishra, P.D., Singh, I.P., 2012. One pot synthesis and anticancer activity of dimeric phloroglucinols. Bioorg. Med. Chem. Lett. 22, 2251-2256.). β-Sitosterol is one of several phytosterols with anti-HIV activity. Since it significantly inhibits the growth of cancer cells without harming normal human cells (treatment with β-sitosterol triggers apoptosis as evidenced by caspase-3 and -9 activation), it is an important compound in the prevention and therapy of human cancers (Rajavel et al., 2018Rajavel, T., Packiyaraj, P., Suryanarayanan, V., Singh, S.K., Ruckmani, K., Devi, K.P., 2018. β-Sitosterol targets Trx/Trx1 reductase to induce apoptosis in A549 cells via ROS mediated mitochondrial dysregulation and p53 activation. Sci. Rep.-UK 8, http://dx.doi.org/10.1038/s41598-018-20311-6.
http://dx.doi.org/10.1038/s41598-018-203... ). The first flavone-xanthone C-glucoside, swertifrancheside, inhibits HIV-1 RT (Khan, 2017Khan, A.S., 2017. Woody plants with possible anti-HIV activity. In: Medicinally Important Trees. Springer, Cham, pp. 109–131.). This flavonoid is also an important lead in the discovery of anti-infective agents (Gautam et al., 2017Gautam, M., Acharya, D., Ali Bhat, Z., Kumar, D., 2017. Future leads: natural products as anti-infective agent. Nat. Prod. J. 7, 84-96.).

Baicalein (1) is a flavone originally isolated from the roots of Scutellaria baicalensis and Scutellaria lateriflora. Besides its neuroprotective effects (Zhao et al., 2018aZhao, C., Rakesh, K.P., Mumtaz, S., Moku, B., Asiri, A.M., Marwani, H.M., Manukumar, H.M., Qin, H.L., 2018. Arylnaphthalene lactone analogues: synthesis and development as excellent biological candidates for future drug discovery. RSC Adv. 8, 9487-9502.,bZhao, Q., Chen, X.Y., Martin, C., 2016. Scutellaria baicalensis, the golden herb from the garden of Chinese medicinal plants. Sci. Bull. 61, 1391-1398.), baicalein suppresses the proliferation of human cervical cancer cells and induces apoptosis of liver cancer cells (He et al., 2018aHe, K., Yu, X., Wang, X., Tang, L., Cao, Y., Xia, J., Cheng, J., 2018. Baicalein and Ly294002 induces liver cancer cells apoptosis via regulating phosphatidyl inositol 3-kinase/Akt signaling pathway. J. Cancer Res. Ther. 14, S519-S525.
https://doi.org/10.4103/0973-1482.235356... ,bHe, S., Simpson, B.K., Sun, H., Ngadi, M.O., Ma, Y., Huang, T., 2018. Phaseolus vulgaris lectins: a systematic review of characteristics and health implications. Crit. Rev. Food Sci. Nutr. 58, 70-83.; Lian et al., 2018Lian, H., Hui, Y., Xiaoping, T., Wei, T., Jiyi, X., Xiaolan, Y., 2018. Baicalein suppresses the proliferation of human cervical cancer cells via Notch 1/Hes signaling pathway. J. Cancer Res. Ther., http://dx.doi.org/10.4103/0973-1482.204899.
http://dx.doi.org/10.4103/0973-1482.2048... ). Betulinic acid is a naturally occurring pentacyclic triterpenoid which inhibits topoisomerase. It has antiretroviral, antimalarial, anti-inflammatory and anticancer properties (Li et al., 2018aLi, C., Zhang, C., Zhou, H., Feng, Y., Tang, F., Hoi, P.M.M., He, C., Ma, D., Zhao, C., Lee, S.M.Y., 2018. Inhibitory effects of betulinic acid on LPS-induced neuroinflammation involve M2 microglial polarization via CaMKKβ-dependent AMPK activation. Front. Mol. Neurosci. 11, .
https://doi.org/10.3389/fnmol.2018.00098... ,bLi, J., Zhao, Y.L., Huang, H.Y., Wang, Y.Z., 2017. Phytochemistry and pharmacological activities of the genus Swertia (Gentianaceae): a review. Am. J. Chin. Med. 45, 667-736.,cLi, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ; Kumar et al., 2018Kumar, P., Singh, A.K., Raj, V., Rai, A., Keshari, A.K., Kumar, D., Maity, B., Prakash, A., Maiti, S., Saha, S., 2018. Poly(lactic-co-glycolic acid)-loaded nanoparticles of betulinic acid for improved treatment of hepatic cancer: characterization, in vitro and in vivo evaluations. Int. J. Nanomed. 13, 975-990.; Zhang et al., 2018Zhang, H., Ma, Z.F., 2018. Phytochemical and pharmacological properties of Capparis spinosa as a medicinal plant. Nutrients 10, http://dx.doi.org/10.3390/nu10020116.
http://dx.doi.org/10.3390/nu10020116... ). Cheng et al. (2018)Cheng, Y.B., Liu, F.J., Wang, C.H., Hwang, T.L., Tsai, Y.F., Yen, C.H., Wang, H.C., Tseng, Y.H., Chien, C.T., Chen, Y.A., Chang, F.R., Wu, Y.C., 2018. Bioactive triterpenoids from the leaves and twigs of Lithocarpus litseifolius and L. corneus. Planta Med. 84, 49-58. found that 3-epi-betulinic acid isolated from two Lithocarpus species exerts strong anti-HIV activity comparable to abacavir, a drug used for treating HIV/AIDS. Gallic acid, also known as 3,4,5-trihydroxybenzoic acid, is a type of phenolic acid found in plants. Gallic acid, bergenin and quercitrin are used to treat pneumonia, cancer and HIV infection (Chen et al., 2018aChen, M.C., Hsu, H.H., Chu, Y.Y., Cheng, S.F., Shen, C.Y., Lin, Y.J., Chen, R.J., Viswanadha, V.P., Lin, Y.M., Huang, C.Y., 2018. Lupeol alters ER stress-signaling pathway by downregulating ABCG2 expression to induce oxaliplatin-resistant LoVo colorectal cancer cell apoptosis. Environ. Toxicol. 33, 587-593.,bChen, Y., Du, K., Li, J., Bai, Y., An, M., Tan, Z., Chang, Y.X., 2018. A green and efficient method for the preconcentration and determination of gallic acid, bergenin, quercitrin, and embelin from Ardisia japonica using nononic surfactant genapol X-080 as the extraction solvent. Int. J. Anal. Chem., http://dx.doi.org/10.1155/2018/1707853.
http://dx.doi.org/10.1155/2018/1707853... ; Prinsloo et al., 2018Prinsloo, G., Marokane, C.K., Street, R.A., 2018. Anti-HIV activity of southern African plants: current developments, phytochemistry and future research. J. Ethnopharmacol. 210, 133-155.). An active green tea compound, epigallocatechin gallate (a type of catechin), also known as epigallocatechin-3-gallate, is the ester of epigallocatechin and gallic acid. In addition to inhibiting HIV-1- RT, epigallocatechin gallate inhibits Zika virus entry into host cells (Sharma et al., 2017aSharma, M., Hotpet, V., Sindhura, B.R., Kamalanathan, A.S., Swamy, B.M., Inamdar, S.R., 2017. Purification, characterization and biological significance of mannose binding lectin from Dioscorea bulbifera bulbils. Int. J. Biol. Macromol. 102, 1146-1155.,bSharma, N., Murali, A., Singh, S.K., Giri, R., 2017. Epigallocatechin gallate, an active green tea compound inhibits the Zika virus entry into host cells via binding the envelope protein. Int. J. Biol. Macromol. 104, 1046-1054.).

Ellagic acid is a polyphenolic antioxidant found in numerous plant foods including raspberries, strawberries, cranberries, walnuts, pecans and pomegranates. Since it inhibits HIV-1 infection in vitro, ellagic acid is a potential novel microbicide (Promsong et al., 2018Promsong, A., Chuenchitra, T., Saipin, K., Tewtrakul, S., Panichayupakaranant, P., Satthakarn, S., Nittayananta, W., 2018. Ellagic acid inhibits HIV-1 infection in vitro: potential role as a novel microbicide. Oral Dis. 24, 249-252.). It has anti-proliferative (González-Sarrías et al., 2017González-Sarrías, A., Núñez-Sánchez, M.Á., García-Villalba, R., Tomás-Barberán, F.A., Espín, J.C., 2017. Antiproliferative activity of the ellagic acid-derived gut microbiota isourolithin A and comparison with its urolithin A isomer: the role of cell metabolism. Eur. J. Nutr. 56, 831-841.) and antioxidant properties (Dalvi et al., 2017Dalvi, L.T., Moreira, D.C., Andrade, R., Ginani, J., Alonso, A., Hermes-Lima, M., 2017. Ellagic acid inhibits iron-mediated free radical formation. Spectrochim. Acta A Mol. Biomol. Spectrosc. 173, 910-917.), prevents the destruction of the p53 gene by cancer cells (Ahire et al., 2017Ahire, V., Kumar, A., Mishra, K.P., Kulkarni, G., 2017. Ellagic acid enhances apoptotic sensitivity of breast cancer cells to γ-radiation. Nutr. Cancer 69, 904-910.), and exerts anti-depressant-like actions (Bedel et al., 2017Bedel, H.A., Kencebay Manas, C., Özbey, G., Usta, C., 2017. The antidepressant-like activity of ellagic acid and its effect on hippocampal brain derived neurotrophic factor levels in mouse depression models. Nat. Prod. Res., http://dx.doi.org/10.1080/14786419.2017.1385021.
http://dx.doi.org/10.1080/14786419.2017.... ). Morosetti et al. (2017)Morosetti, G., Criscuolo, A.A., Santi, F., Perno, C.F., Piccione, E., Ciotti, M., 2017. Ellagic acid and Annona muricata in the chemoprevention of HPV-related pre-neoplastic lesions of the cervix. Oncol. Lett. 13, 1880-1884. report the use of ellagic acid in the chemoprevention of human papilloma virus-related pre-neoplastic lesions of the cervix. Ellagic acid also inhibits mutagenesis and carcinogenesis by forming adducts with DNA, thereby masking binding sites to be occupied by the mutagen or carcinogen. Nigranoic acid, a triterpenoid from Schisandra sphaerandra, inhibits HIV-1 RT (Khan, 2017Khan, A.S., 2017. Woody plants with possible anti-HIV activity. In: Medicinally Important Trees. Springer, Cham, pp. 109–131.) and its esters act as novel human neutrophil elastase inhibitors (Huang et al., 2015Huang, G., Feng, L., Liu, B., He, Y., Li, Y., Chen, Y., 2015. Synthesis and biological evaluation of nigranoic acid esters as novel human neutrophil elastase inhibitors. Nat. Prod. Res. 29, 1650-1656.). Nigranoic acid protects against cerebral ischaemia–reperfusion injury (Feng et al., 2015Feng, T., Liu, Y., Li, C., Li, Z., 2015. Protective effects of nigranoic acid on cerebral ischemia–reperfusion injury and its mechanism involving apoptotic signaling pathway. Cell Biochem. Biophys. 71, 345-351.).

Ursolic acid (ursane-type pentacyclic triterpenoid), also known as urson, prunol, micromerol or malol is one of the most promising therapeutic plant compounds with hepatoprotective activity and potent antiviral activity especially against herpes simplex virus and human hepatitis C virus (Parvez et al., 2018Parvez, M.K., Alam, P., Arbab, A.H., Al-Dosari, M.S., Alhowiriny, T.A., Alqasoumi, S.I., 2018. Analysis of antioxidative and antiviral biomarkers β-amyrin, β-sitosterol, lupeol, ursolic acid in Guiera senegalensis leaves extract by validated HPTLC methods. Saudi Pharm. J. 26, 685-693.). Shikimic acid, commonly known by its anionic form shikimate, is an important biochemical intermediate in plants and microorganisms. It is a cyclohexene, a cyclitol and a cyclohexanecarboxylic acid; its name comes from the Japanese flower shikimi. Nabavi et al. (2018)Nabavi, S.M., Nabavi, S.F., Sureda, A., Caprioli, G., Iannarelli, R., Sokeng, A.J.T., Braidy, N., Khanjani, S., Moghaddam, A.H., Atanasov, A.G., Daglia, M., Maggi, F., 2018. The water extract of tutsan (Hypericum androsaemum L.) red berries exerts antidepressive-like effects and in vivo antioxidant activity in a mouse model of post-stroke depression. Biomed. Pharmacother. 99, 290-298. reported that a shikimic acid rich extract of Hypericum androsaemum presents protective effects during post-stroke depression, most likely due to antioxidant activity. Rosmarinic acid is an antioxidant and anti-inflammatory polyphenol found in a variety of plants. It improves learning and memory (Farr et al., 2016Farr, S.A., Niehoff, M.L., Ceddia, M.A., Herrlinger, K.A., Lewis, B.J., Feng, S., Welleford, A., Butterfield, D.A., Morley, J.E., 2016. Effect of botanical extracts containing carnosic acid or rosmarinic acid on learning and memory in SAMP8 mice. Physiol. Behav. 165, 328-338.) but inhibits tumour-associated carbonic anhydrase isoenzymes and enzymes (Gülçin et al., 2016Gülçin, İ., Scozzafava, A., Supuran, C.T., Koksal, Z., Turkan, F., Çetinkaya, S., Bingöl, Z., Huyut, Z., Alwasel, S.H., 2016. Rosmarinic acid inhibits some metabolic enzymes including glutathione S-transferase, lactoperoxidase, acetylcholinesterase, butyrylcholinesterase and carbonic anhydrase isoenzymes. J. Enzyme Inhib. Med. Chem. 31, 1698-1702.).

The terpinenes are a group of isomeric hydrocarbons that are classified as monoterpenes. A precursor of p-cymene, γ-terpinene is one of the three isomeric hydrocarbons with anti-HIV-1 RT actions. It is a natural antioxidant and anti-inflammatory agent isolated from a variety of plant sources including essential oils of citrus fruits (Ramalho et al., 2015Ramalho, T.R., Oliveira, M.T., de Araujo Lima, A.L., Bezerra-Santos, C.R., Piuvezam, M.R., 2015. Gamma-terpinene modulates acute inflammatory response in mice. Planta Med. 81, 1248-1254.). Myricetin is a member of the flavonoid class of polyphenolic compounds, with antioxidant properties and anti-HIV-1 RT activity (Pasetto et al., 2014Pasetto, S., Pardi, V., Murata, R.M., 2014. Anti-HIV-1 activity of flavonoid myricetin on HIV-1 infection in a dual-chamber in vitro model. PLOS ONE 9, e115323.). It is commonly derived from vegetables, fruits, nuts, berries, tea, and is also found in red wine (Đorđević et al., 2018Đorđević, N., Novaković, M., Pejin, B., Živković, M., Savić, A., Mutić, J., Tešević, V., 2018. An insight into chemical composition and biological activity of Montenegrin Vranac red wine. Sci. Hortic. – Amsterdam 230, 142-148.). Naringenin, a bitter and colourless flavanone predominant in grapefruit, attenuates cART-induced sperm DNA fragmentations and testicular toxicity (Adana et al., 2018Adana, M.Y., Akang, E.N., Peter, A.I., Jegede, A.I., Naidu, E.C.S., Tiloke, C., Chuturgoon, A.A., Azu, O.O., 2018. Naringenin attenuates highly active antiretroviral therapy-induced sperm DNA fragmentations and testicular toxicity in Sprague-Dawley rats. Andrology 6, 166-175.). Lim et al. (2018)Lim, Y.J., Kim, J.H., Pan, J.H., Kim, J.K., Park, T.S., Kim, Y.J., Lee, J.H., Kim, J.H., 2018. Naringin protects pancreatic β-cells against oxidative stress-induced apoptosis by inhibiting both intrinsic and extrinsic pathways in insulin-deficient diabetic mice. Mol. Nutr. Food Res., http://dx.doi.org/10.1002/mnfr.201700810.
http://dx.doi.org/10.1002/mnfr.201700810... found that naringenin protects pancreatic β-cells against oxidative stress-induced apoptosis.

Apart from inhibiting HIV-1 RT, the following active compounds also have other multiple effects. N-docosanol, a long chained alcohol, has anti-herpes simplex virus activity. It was approved by the FDA as a topical treatment for herpes simplex labialis (Hung et al., 2015Hung, P.Y., Ho, B.C., Lee, S.Y., Chang, S.Y., Kao, C.L., Lee, S.S., Lee, C.N., 2015. Houttuynia cordata targets the beginning stage of herpes simplex virus infection. PLOS ONE 10, e0115475.). Flavonoids such as quercetin, isoquercitrin, rutin, hyperin, and quercitrin exhibit antimutagenic and free radical scavenging capacity (Hung et al., 2015Hung, P.Y., Ho, B.C., Lee, S.Y., Chang, S.Y., Kao, C.L., Lee, S.S., Lee, C.N., 2015. Houttuynia cordata targets the beginning stage of herpes simplex virus infection. PLOS ONE 10, e0115475.). The phenolic compound chlorogenic acid has antipyretic and antibiotic activities (Hung et al., 2015Hung, P.Y., Ho, B.C., Lee, S.Y., Chang, S.Y., Kao, C.L., Lee, S.S., Lee, C.N., 2015. Houttuynia cordata targets the beginning stage of herpes simplex virus infection. PLOS ONE 10, e0115475.). Tryptamine and ferulic acid significantly inhibit HIV-1 integrase (Sanna et al., 2018aSanna, C., Rigano, D., Corona, A., Piano, D., Formisano, C., Farci, D., Ballero, M., Chianese, G., Tramontano, E., Taglialatela-Scafati, O., Esposito, F., 2018. Dual HIV-1 reverse transcriptase and integrase inhibitors from Limonium morisianum Arrigoni, an endemic species of Sardinia (Italy). Nat. Prod. Res., http://dx.doi.org/10.1080/14786419.2018.1434649.
http://dx.doi.org/10.1080/14786419.2018.... ,bSanna, C., Rigano, D., Cortis, P., Corona, A., Ballero, M., Parolin, C., Del Vecchio, C., Chianese, G., Saccon, E., Formisano, C., Tramontano, E., Esposito, F., 2018. Onopordum illyricum L., a Mediterranean plant, as a source of anti HIV-1 compounds. Plant Biosyst., http://dx.doi.org/10.1080/11263504.2018.1439118, An Interna-tional Journal Dealing with all Aspects of Plant Biology.
http://dx.doi.org/10.1080/11263504.2018.... ). Smilarly, secocycloartanes are a class of triterpenoids with novel structures showing anti-HIV and antitumour activities (Li et al., 2017aLi, J., Zhao, Y.L., Huang, H.Y., Wang, Y.Z., 2017. Phytochemistry and pharmacological activities of the genus Swertia (Gentianaceae): a review. Am. J. Chin. Med. 45, 667-736.,bLi, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ). According to Rahim et al. (2018)Rahim, A., Saito, Y., Miyake, K., Goto, M., Chen, C.H., Alam, G., Morris-Natschke, S., Lee, K.H., Nakagawa-Goto, K., 2018. Kleinhospitine E and cycloartane triterpenoids from Kleinhovia hospita. J. Nat. Prod. 81, 1619-1627., cycloartane triterpenoids exhibit potent anti-proliferative activities against multidrug-resistant cancers and sub-micromolar anti-HIV activity.

Lectins are a ubiquitous group of carbohydrate-binding proteins. Some of the microbial and plant lectins are griffithsin, actinohivin, concanavalin-A, cyanovirin-N, microvirin, and banana lectin (BanLec). During HIV infection, opportunistic pathogens including enveloped viruses, bacteria, fungi, and protozoa are neutralized by lectins because they display sugar-coated macromolecules on their surfaces, making them suitable targets for lectins (Mazalovska and Kouokam, 2018Mazalovska, M., Kouokam, J.C., 2018. Lectins as promising therapeutics for the prevention and treatment of HIV and other potential coinfections. BioMed. Res. Int. , .
https://doi.org/10.1155/2018/3750646... ). Emetine is an FDA approved drug for amoebiasis and several viruses. Cephaeline, a structural desmethyl emetine analogue, inhibits Zika virus polymerase activity and Ebola virus entry (Yang et al., 2018aYang, F., Wang, Y., Li, G., Xue, J., Chen, Z.L., Jin, F., Luo, L., Zhou, X., Ma, Q., Cai, X., Li, H.R., Zhao, L., 2018. Effects of corilagin on alleviating cholestasis via farnesoid X receptor-associated pathways in vitro and in vivo. Br. J. Pharmacol. 175, 810-829.,bYang, S., Xu, M., Lee, E.M., Gorshkov, K., Shiryaev, S.A., He, S., Sun, W., Cheng, Y.-S., Hu, X., Tharappel, A.M., Lu, B., Pinto, A., Farhy, C., Huang, C.-T., Zhang, Z., Zhu, W., Wu, Y., Zhou, Y., Song, G., Zhu, H., Shamim, K., Martínez-Romero, C., García-Sastre, A., Preston, R.A., Jayaweera, D.T., Huang, R., Huang, W., Xia, M., Simeonov, A., Ming, G., Qiu, X., Terskikh, A.V., Tang, H., Song, H., Zheng, W., 2018. Emetine inhibits Zika and Ebola virus infections through two molecular mechanisms: inhibiting viral replication and decreasing viral entry. Cell Discov. 4, .
https://doi.org/10.1038/s41421-018-0034-... ).

Nitidine is a bioactive plant benzophenanthridine alkaloid with marked anticancer, neuroprotective, antimalarial, anti-HIV, analgesic, anti-inflammatory, and antifungal activities (Khan et al., 2018Khan, H., Hadda, T.B., Touzani, R., 2018. Diverse therapeutic potential of nitidine, a comprehensive review. Curr. Drug Metab., http://dx.doi.org/10.2174/1389200219666180628165643.
http://dx.doi.org/10.2174/13892002196661... ). Despite its outstanding therapeutic potential, it has not yet been subjected to clinical trials (Khan et al., 2018Khan, H., Hadda, T.B., Touzani, R., 2018. Diverse therapeutic potential of nitidine, a comprehensive review. Curr. Drug Metab., http://dx.doi.org/10.2174/1389200219666180628165643.
http://dx.doi.org/10.2174/13892002196661... ). Schisandrin B and deoxyschisandrin and other lignans with a dibenzo-cyclooctadiene skeleton have antihepatotoxic, antiasthmatic, and anti-tumour effects (Su et al., 2018Su, L.L., Cheng, X., Ding, X.Y., Mao, C.Q., Lu, T.L., Hao, M., Ping, L., Qin, S., 2018. Simultaneous quantification of five lignans from Schisandra chinensis in various tissues of rats. Acta Chromatogr. , .
https://doi.org/10.1556/1326.2018.00387... ). Wikstroelides (daphnane diterpenes) inhibit hepatitis B virus (Li et al., 2018aLi, C., Zhang, C., Zhou, H., Feng, Y., Tang, F., Hoi, P.M.M., He, C., Ma, D., Zhao, C., Lee, S.M.Y., 2018. Inhibitory effects of betulinic acid on LPS-induced neuroinflammation involve M2 microglial polarization via CaMKKβ-dependent AMPK activation. Front. Mol. Neurosci. 11, .
https://doi.org/10.3389/fnmol.2018.00098... ,bLi, J., Zhao, Y.L., Huang, H.Y., Wang, Y.Z., 2017. Phytochemistry and pharmacological activities of the genus Swertia (Gentianaceae): a review. Am. J. Chin. Med. 45, 667-736.,cLi, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ). Curcuminoids have anti-inflammatory, antioxidant, anticancer, and anti-HIV-1 integrase activities (Cunha Neto et al., 2018Cunha Neto, F., Marton, L.T., de Marqui, S.V., Lima, T.A., Barbalho, S.M., 2018. Curcuminoids from Curcuma longa: new adjuvants for the treatment of crohn's disease and ulcerative colitis?. Crit. Rev. Food Sci. Nutr., http://dx.doi.org/10.1080/10408398.2018.
http://dx.doi.org/10.1080/10408398.2018... ). As noted in many of the cases, anti-HIV-1 RT active compounds have a multiplicity of pleiotropic pharmacological effects and benefits during infection and disease.

Towards the development of plant-derived anti-HIV-1 RT drugs

Fig. 2 shows pillars for the discovery and development of anti-HIV-1 RT inhibitors from plants. On one hand, the pillars act as drivers, prerequisite technical knowledge and conditions that can initiate, support and accelerate the discovery and development of plant-derived anti-HIV-1 RT inhibitors from plants. On the other, the pillars also act as bottlenecks, namely critical resources whose inefficiencies create delays, limit the throughput or impede the research and development process from progressing. To accelerate drug discovery and development, this review considers four important pillars: tapping into the rich vein of indigenous knowledge of putative anti-HIV/AIDS medicinal plants (reverse pharmacology), determining pan-assay interference compounds, analysis of structure–activity relationships, and conducting more clinical trials.

Reverse pharmacology

About 21 drugs sold in 2007 to treat HIV-1 infection were all obtained by chemical synthesis, and none from natural products (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Yet, indigenous knowledge, coupled with a history of safe use and ethnopharmacological efficacy, present a faster approach to discover new anti-HIV-1 agents from plants. This new approach, now called reverse pharmacology (Chinsembu, 2009Chinsembu, K.C., 2009. Model and experiences of initiating collaboration with traditional healers in validation of ethnomedicines for HIV/AIDS in Namibia. J. Ethnobiol. Ethnomed., http://dx.doi.org/10.1186/1746-4269-5-30.
http://dx.doi.org/10.1186/1746-4269-5-30... ), is a trans-disciplinary approach focused on indigenous knowledge of medicinal plants, experimental observations and clinical experiences (Simoes-Pires et al., 2014Simoes-Pires, C., Hostettmann, K., Haouala, A., Cuendet, M., Falquet, J., Graz, B., Christen, P., 2014. Reverse pharmacology for developing an antimalarial phytomedicine. The example of Argemone mexicana. Int. J. Parasitol. – Drug 4, 338-346.). Reverse pharmacology promises to shorten the classical drug discovery process (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Tapping into the rich vein of ethnobotanical knowledge can unveil putative plant medicines that could provide crucial and quick leads to the development of new drugs against HIV-1 RT.

Plants continue to be a very important resource for HIV/AIDS remedies and beneficial compounds. In fact, given rampant drug shortages in poor countries, even qualified western-trained medical doctors sometimes prescribe herbal remedies for the management of HIV/AIDS (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). In many resource-poor settings, various plants used to treat HIV-1 infection have known anti-RT activity. In Ethiopia, acetone extracts from the plant Combretum molle inhibit HIV-1 RT (Asres and Bucar, 2005Asres, K., Bucar, F., 2005. Anti-HIV activity against immunodeficiency virus type 1 (HIV-I) and type II (HIV-II) of compounds isolated from the stem bark of Combretum molle. Ethiop. Med. J. 43, 15-20.). Phytochemical investigations of this fraction showed that the HIV-1 RT inhibitory properties are mediated by two tannins and two oleanane-type pentacyclic triterpene glycosides. A natural agent, gallotannin, isolated from C. molle, inhibits RDDP activity of HIV-1 RT. Similar results were obtained in South Africa where the methanol extract of the roots of C. molle had the highest inhibitory effect on the RNase H activity of HIV-1 RT (Bessong et al., 2005Bessong, P.O., Obi, C.L., Andréola, M.L., Rojas, L.B., Pouységu, L., Igumbor, E., Meyer, J.J., Quideau, S., Litvak, S., 2005. Evaluation of selected South African medicinal plants for inhibitory properties against human immunodeficiency virus type 1 reverse transcriptase and integrase. J. Ethnopharmacol. 99, 83-91.).

L-Canavanine from Sutherlandia frutescens has activity against HIV but synergistically interacts with the efflux of nevirapine (Prinsloo et al., 2018Prinsloo, G., Marokane, C.K., Street, R.A., 2018. Anti-HIV activity of southern African plants: current developments, phytochemistry and future research. J. Ethnopharmacol. 210, 133-155.). D-Pinitol, also from S. frutescens, has been suggested as a treatment for wasting in cancer and AIDS patients. Recent evidence shows that D-pinitol, chemically known as 3-O-methyl-D-chiro-inositol, is an active ingredient of Diospyros mespliformis leaves, whose crude extracts inhibit HIV (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). D-Pinitol is also known to possess anti-diabetic activity. Catechin (a flavonoid), bergenin (a C-galloyl-glycoside) and betulinic acid have been isolated from Peltophorum africanum, an important southern African ethnomedicinal plant for HIV/AIDS (Chinsembu, 2015Chinsembu, K.C., 2015. Bioprospecting for ‘green diamonds’: medicinal plants used in the management of HIV/AIDS-related conditions. In: Chinsembu, K.C., Cheikyoussef, A., Mubengwegwi, D., Kandawa-Schulz, M., Kasanda, C.D.,Kazembe, L. (Eds.), Indigenous Knowledge of Namibia. University of NamibiaPress, Windhoek, pp. 9–40.; 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.).

In South Africa, Bessong et al. (2005)Bessong, P.O., Obi, C.L., Andréola, M.L., Rojas, L.B., Pouységu, L., Igumbor, E., Meyer, J.J., Quideau, S., Litvak, S., 2005. Evaluation of selected South African medicinal plants for inhibitory properties against human immunodeficiency virus type 1 reverse transcriptase and integrase. J. Ethnopharmacol. 99, 83-91. screened seventeen aqueous and methanol extracts of nine South African medicinal plants. The plants were ethnobotanically selected and evaluated for inhibitory properties against HIV-1 RT. Inhibitory action was seen with the stem-bark of P. africanum (IC₅₀ 3.5 µg/ml) and the roots of C. molle (IC₅₀ 9.7 µg/ml). Extracts of the leaves of Sudanese plant Combretum hartmannianum totally inhibits HIV-1 RT at a concentration of 66 µg/ml (Ali et al., 2002Ali, H., König, G.M., Khalid, S.A., Wright, A.D., Kaminsky, R., 2002. Evaluation of selected Sudanese medicinal plants for their in vitro activity against hemoflagellates, selected bacteria, HIV-1-RT and tyrosine kinase inhibitory, and for cytotoxicity. J. Ethnopharmacol. 83, 219-228.). Other experiments by Chukwujekwu et al. (2014)Chukwujekwu, J.C., Ndhlala, A.R., De Kock, C.A., Smith, P.J., Van Staden, J., 2014. Antiplasmodial, HIV-1 reverse transcriptase inhibitory and cytotoxicity properties of Centratherum punctatum Cass. and its fractions. S. Afr. J. Bot. 90, 17-19. attest that Centratherum punctatum leaves have HIV-1 RT inhibitory activity (IC₅₀ = 52.4 µg/ml). The plant C. punctatum is commonly known as the Brazilian bachelor button, a refreshing pineapple-scented bushy perennial plant. In Brazilian traditional medicine, it is used as a treatment for heart ailments. Its leaves have antimicrobial, antioxidant and antiproliferative properties (Chukwujekwu et al., 2014Chukwujekwu, J.C., Ndhlala, A.R., De Kock, C.A., Smith, P.J., Van Staden, J., 2014. Antiplasmodial, HIV-1 reverse transcriptase inhibitory and cytotoxicity properties of Centratherum punctatum Cass. and its fractions. S. Afr. J. Bot. 90, 17-19.).

Woradulayapinij et al. (2005)Woradulayapinij, W., Soonthornchareonnon, N., Wiwat, C., 2005. In vitro HIV type 1 reverse transcriptase inhibitory activities of Thai medicinal plants and Canna indica L. rhizomes. J. Ethnopharmacol. 101, 84-89. studied the in vitro HIV-1 RT inhibitory activities of Thai medicinal plants. They tested water and 80% ethanol extracts of twenty Thai medicinal plants used to treat AIDS for their HIV-1 RT inhibitory activity. The water extracts of Ipomoea carnea subsp. fistulosa (aerial parts), Vitex glabrata (branch), Vitex trifolia (aerial part), Vitex negundo (aerial part), Canna indica (rhizome), and Justicia gendarussa (aerial part) showed HIV-1 RT inhibition ratio higher than 90% at a 200 µg/ml concentration. Although T. sericea wields very strong activity against the HIV-1 RDDP and RNase H with 98% and 99% inhibition, respectively (Bessong et al., 2004Bessong, P.O., Obi, C.L., Igumbor, E., Andreola, M.L., Litvak, S., 2004. In vitro activity of three selected South African medicinal plants against human immunodeficiency virus type 1 reverse transcriptase. Afr. J. Biotechnol. 3, 555-559.), the plant may contain high levels of toxic tannins (Ndhlala et al., 2013Ndhlala, A.R., Ncube, B., Okem, A., Mulaudzi, R.B., Van Staden, J., 2013. Toxicology of some important medicinal plants in southern Africa. Food Chem. Toxicol. 62, 609-621.; Wink and Van Wyk, 2008Wink, M., Van Wyk, B.E., 2008. Mind-Altering and Poisonous Plants of the World. Timber Press, Portland, pp. 464.). In experiments performed in Tanzania, Moshi and Mbwambo (2005)Moshi, M.J., Mbwambo, Z.H., 2005. Some pharmacological properties of extracts of Terminalia sericea roots. J. Ethnopharmacol. 97, 43-47. observed that, with the exception of the dichloromethane and petroleum ether extracts, all the intermediate and polar extracts of T. sericea were toxic with values ranging from 5.4 to 17.4 mg/ml. This result suggests that plants that show anti-HIV-1 RT activity should also be evaluated for cytotoxicity.

Although Africa has the highest burden of HIV/AIDS, few scientific studies that screen plants for anti-RT activity are conducted on the continent (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Primary data on chemical diversity and activity profiles of HIV-1 RT inhibitors from plants are mostly available from studies done outside Africa. Whereas the Chinese represent about 41% of all research efforts on anti-HIV natural products, the African share of research on anti-HIV natural products is a paltry 16% (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.). Most of the studies done in Africa are ethnobotanical and are inconclusive. While Sub-Saharan Africa accounts for about 70% of all HIV infections, it is ironic that the search for novel anti-HIV-1 RT treatments and the hotbeds of research and development for anti-HIV-1 RT plants are in China where HIV prevalence rate is less than 0.1% among adult (Chinsembu, 2016Chinsembu, K.C., 2016. Green Medicines: Pharmacy of Natural Products for HIV and Five AIDS-Related Infections. Toronto, Canada, Africa in Canada Press.).

Africa is not investing enough of her resources in the search for new anti-HIV-1 RT inhibitors from plants. This means that Africa will continue to look to the outside world for novel drugs to manage HIV/AIDS. Africa has failed to sow the seed of self-reliance in terms of searching for new plant remedies to combat HIV/AIDS. This is disheartening considering that Africa is endowed with abundant medicinal plant resources. More concerted research efforts on the search for drugs from putative anti-HIV-1 RT plants are urgently required in Africa. Indigenous knowledge of plants used as putative herbal drugs for HIV-1 infection is a good starting point in the development of new plant-based HIV-1 RT drugs.

Filter out pan-assay interference compounds

Although plant products represent an enormous source of pharmacologically useful compounds and are often used as the starting point in modern drug discovery, many biologically interesting plant compounds are not being pursued as potential drug candidates because they lack well-defined modes of action (Li et al., 2015Li, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ). For example, it is unlikely that pheophytin-a (Kapewangolo et al., 2017Kapewangolo, P., Kandawa-Schulz, M., Meyer, D., 2017. Anti-HIV activity of Ocimum labiatum extract and isolated pheophytin-a. Molecules 22, 1763.), a degradation product of chlorophyll which represents chlorophyll that has lost the central Mg²⁺ ion, would be developed into an anti-HIV-1 RT drug. Similarly, the anti-RT therapeutic application of andrographolide is limited because this bioactive bicyclic diterpenoid is a highly promiscuous compound that interacts with numerous and unknown cellular targets; hence it is not seriously considered as a potential drug candidate (Li et al., 2015Li, L., Wijaya, H., Samanta, S., Lam, Y., Yao, S.Q., 2015. In situ imaging and proteome profiling indicate andrographolide is a highly promiscuous compound. Sci. Rep-UK 5, .
https://doi.org/10.1038/srep11522... ). In general, the development of novel anti-RT drugs from plants is an uphill task because the classical drug discovery route is long (takes about 12–15 years) and the expenditure is high; around US$ 1 billion (Farooq et al., 2016Farooq, T., Hameed, A., Rehman, K., Ibrahim, M., Qadir, M.I., Akash, M.S.H., 2016. Antiretroviral agents: looking for the best possible chemotherapeutic options to conquer HIV. Crit. Rev. Eukaryot. Gene Expr. 26, 363-381.). In particular, in vivo testing and ultimately human clinical trials need to be carried out on key lead plant compounds. Continuous evaluation of anti-HIV-1 RT compounds should also be rigorously pursued because pharmaceutical companies do not progress anti-HIV-1 RT screening hits that are photoreactive, for example 2-aminothiazoles (Baell and Holloway, 2010Baell, J.B., Holloway, G.A., 2010. New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays. J. Med. Chem. 53, 2719-2740.).

Sink et al. (2010)Sink, R., Gobec, S., Pecar, S., Zega, A., 2010. False positives in the early stages of drug discovery. Curr. Med. Chem. 17, 4231-4255. caution that although high-throughput screening (HTS) is one of the most powerful approaches available for identifying new plant lead compounds (just as virtual and experimental HTS have accelerated lead identification and changed drug discovery), it has also introduced a large number of pan-assay interference compounds (PAINS) which turn out to be dead-ends after a great deal of time and resources have been spent (Sink et al., 2010Sink, R., Gobec, S., Pecar, S., Zega, A., 2010. False positives in the early stages of drug discovery. Curr. Med. Chem. 17, 4231-4255.; Baell and Nissink, 2017Baell, J.B., Nissink, J.W.M., 2017. Seven year itch: Pan-assay Interference compounds (PAINS) in 2017: utility and limitations. ACS Chem. Biol. 13, 36-44.). PAINS are frequent hitters (promiscuous compounds) that render efforts to develop novel HIV-1 RT drugs from plant-derived compounds fruitless. They represent poor choices for drug development and can furnish misleading data that in isolation may be suggestive of a selective and optimizable anti-HIV RT hit. Compounds are regarded as false positives if they interfere in binding interactions by forming aggregates, those that are protein-reactive, sometimes light induced, or those that directly interfere in assay signalling (Baell and Holloway, 2010Baell, J.B., Holloway, G.A., 2010. New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays. J. Med. Chem. 53, 2719-2740.). Putative PAINS include azo compounds, cyanopyridones, divinylketones, certain indoles, quinones, rhodanines and other compounds of cyclic and heterocyclic nature (Baell and Holloway, 2010Baell, J.B., Holloway, G.A., 2010. New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays. J. Med. Chem. 53, 2719-2740.). PAINS have contributed to the rapidly growing body of anti-HIV-1 RT literature, yet very few screening hits have progressed to the level of candidate drugs (Baell and Holloway, 2010Baell, J.B., Holloway, G.A., 2010. New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays. J. Med. Chem. 53, 2719-2740.).

Although it is beyond the scope of this review to categorically state which anti-HIV-1 RT phytochemicals are PAINS, active compounds in this review should be screened to ascertain whether they are false positive hits which may present challenges in anti-HIV-1 RT drug development. Suffice to state many studies have been devoted to understanding the origins of PAINS and the findings have been incorporated in filters and methods that can predict and eliminate problematic molecules from further consideration. There are electronic filters which recognize and exclude PAINS from further analysis (Baell and Nissink, 2017Baell, J.B., Nissink, J.W.M., 2017. Seven year itch: Pan-assay Interference compounds (PAINS) in 2017: utility and limitations. ACS Chem. Biol. 13, 36-44.). While many PAINS are usually highly promiscuous compounds, others may be important compounds which may be further explored for polypharmacology in general and multi-target anti-HIV-1 RT activities in particular (Gilberg et al., 2016Gilberg, E., Jasial, S., Stumpfe, D., Dimova, D., Bajorath, J., 2016. Highly promiscuous small molecules from biological screening assays include many pan-assay interference compounds but also candidates for polypharmacology. J. Med. Chem. 59, 10285-10290.). Stated differently, the practice of excluding PAINS may be fraught with danger because such black box segregation, based entirely on inputs and outputs, without knowledge of internal workings, is as simplistic as it lacks drug-like complexity (Baell and Nissink, 2017Baell, J.B., Nissink, J.W.M., 2017. Seven year itch: Pan-assay Interference compounds (PAINS) in 2017: utility and limitations. ACS Chem. Biol. 13, 36-44.). Still, screening for PAINS can help avoid undertaking wasteful studies such as full efficacy, pharmacokinetics, tissue distribution, and metabolism experiments destined for failure (Baell and Holloway, 2010Baell, J.B., Holloway, G.A., 2010. New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays. J. Med. Chem. 53, 2719-2740.).

Analysis of structure–activity relationships

Structural modification of plant-derived active principles may provide a continuous source of potential anti-HIV-1 RT drug candidates. Certain structural features of plant products are responsible for their anti-HIV-1 RT biological activities. Identifying such special scaffolds is of great importance because laboratory synthesis of molecules containing similar scaffolds can serve as an effective strategy for new drug synthesis (Zhao et al., 2018aZhao, W.Z., Wang, H.T., Huang, H.J., Lo, Y.L., Lin, A.M.Y., 2018. Neuroprotective effects of baicalein on acrolein-induced neurotoxicity in the nigrostriatal dopaminergic system of rat brain. Mol. Neurobiol. 55, 130-137.,bZhou, J., Ci’an Zhang, Y.S., Wang, L., Zhang, J., Li, F., Mao, W., 2018. Corilagin attenuates allergy and anaphylactic reaction by inhibiting degranulation of mast cells. Med. Sci. Monit. 24, 891-896.). Already, a pharmaceutical formulation which consists of an arylnaphthalene lignan compound, based on justiprocumin A isolated from the plant J. gendarussa, Acanthaceae, has been patented as an effective treatment for HIV/AIDS (Zhang et al., 2014aZhang, H., Soejarto, D., Rong, L., Fong, H.H., Rumschlag-Booms, E., 2014. U.S. Patent Application No. 14/235,870.,bZhang, H.J., Rumschlag-Booms, E., Guan, Y.F., Wang, D.Y., Liu, K.L., Li, W.F., Nguyen, V.H., Cuong, N.M., Soejarto, D.D., Fong, H.H.S., Rong, L., 2017. Potent inhibitor of drug-resistant HIV-1 strains identified from the medicinal plant Justicia gendarussa. J. Nat. Prod. 80, 1798-1807.). In structure–activity relationship studies, patentiflorin A, a novel arylnaphthalene lignan from J. gendarussa plants in Vietnam, showed potential as an anti-HIV-1 RT drug. Patentiflorin A was more effective than AZT in inhibiting four different HIV-1 isolates, IC₅₀ values in the range 14–32 nM (Zhao et al., 2018aZhao, C., Rakesh, K.P., Mumtaz, S., Moku, B., Asiri, A.M., Marwani, H.M., Manukumar, H.M., Qin, H.L., 2018. Arylnaphthalene lactone analogues: synthesis and development as excellent biological candidates for future drug discovery. RSC Adv. 8, 9487-9502.,bZhao, Q., Chen, X.Y., Martin, C., 2016. Scutellaria baicalensis, the golden herb from the garden of Chinese medicinal plants. Sci. Bull. 61, 1391-1398.).

Phenylethylthiazolylthiourea (PETT) derivatives have been identified as a new series of non-nucleoside inhibitors of HIV-1 RT (Cantrell et al., 1996Cantrell, A.S., Engelhardt, P., Högberg, M., Jaskunas, S.R., Johansson, N.G., Jordan, C.L., Kangasmetsä, J., Kinnick, M.D., Lind, P., Morin, J.M., Muesing, M.A., Noreén, R., Obergm, B., Pranc, P., Sahlberg, C., Ternansky, R.J., Vasileff, R.T., Vrang, L., West, S.J., Zhang, H., 1996. Phenethylthiazolylthiourea (PETT) compounds as a new class of HIV-1 reverse transcriptase inhibitors. 2. Synthesis and further structure–activity relationship studies of PETT analogs. J. Med. Chem. 39, 4261-4274.). Structure–activity relationship studies of this class of compounds resulted in the identification of N-[2-(2-pyridyl)ethyl]-N′-[2-(5-bromopyridyl)]thiourea hydrochloride (trovirdine; LY300046·HCl) as a highly potent anti-HIV-1 RT agent (Cantrell et al., 1996Cantrell, A.S., Engelhardt, P., Högberg, M., Jaskunas, S.R., Johansson, N.G., Jordan, C.L., Kangasmetsä, J., Kinnick, M.D., Lind, P., Morin, J.M., Muesing, M.A., Noreén, R., Obergm, B., Pranc, P., Sahlberg, C., Ternansky, R.J., Vasileff, R.T., Vrang, L., West, S.J., Zhang, H., 1996. Phenethylthiazolylthiourea (PETT) compounds as a new class of HIV-1 reverse transcriptase inhibitors. 2. Synthesis and further structure–activity relationship studies of PETT analogs. J. Med. Chem. 39, 4261-4274.). Trovirdine is currently in phase one clinical trials for potential use in the treatment of AIDS. Comparative studies with flavonoids (hydroxyflavones, dihydroxyflavones and polyhydroxyflavones and flavanones), carried out to clarify the structure–activity relationships, revealed that the presence of both the unsaturated double bond between positions 2 and 3 of the flavonoid pyrone ring, and the three hydroxyl groups introduced on positions 5, 6 and 7 were a prerequisite for the inhibition of HIV-1 RT activity (Ono et al., 1990Ono, K., Nakane, H., Fukushima, M., Chermann, J.C., Barre-Sinoussi, F., 1990. Differential inhibitory effects of various flavonoids on the activities of reverse transcriptase and cellular DNA and RNA polymerases. Eur. J. Biochem. 190, 469-476.).

Analysis of structure–activity relationships has been helpful in understanding the mechanisms of quercetagetin which contains the structures of both baicalein (1) and quercetin, and myricetin which has the structure of quercetin with an additional hydroxyl group on the 5′ position. Quercetagetin and myricetin are strong inhibitors of HIV-1 RT activity (Ono et al., 1990Ono, K., Nakane, H., Fukushima, M., Chermann, J.C., Barre-Sinoussi, F., 1990. Differential inhibitory effects of various flavonoids on the activities of reverse transcriptase and cellular DNA and RNA polymerases. Eur. J. Biochem. 190, 469-476.). The RT inhibition by baicalein is highly specific whereas quercetin and quercetagetin are also strong inhibitors of DNA polymerase β and DNA polymerase I, respectively. Myricetin is a potent inhibitor of both DNA polymerase α and DNA polymerase I (Ono et al., 1990Ono, K., Nakane, H., Fukushima, M., Chermann, J.C., Barre-Sinoussi, F., 1990. Differential inhibitory effects of various flavonoids on the activities of reverse transcriptase and cellular DNA and RNA polymerases. Eur. J. Biochem. 190, 469-476.). While the anti-RT activity of the alkaloid castanospermine was high, IC₅₀ 1.1 µM (Whitby et al., 2004Whitby, K., Taylor, D., Patel, D., Ahmed, P., Tyms, A.S., 2004. Action of celgosivir (6 O-butanoyl castanospermine) against the pestivirus BVDV: implications for the treatment of hepatitis C. Antivir. Chem. Chemother. 15, 141-151.), several of its O-acyl derivatives are 20 times more active than AZT. However, O-acyl derivatives were highly toxic and unsuitable for further development into anti-HIV drugs (Hamburger and Hostettmann, 1991Hamburger, M., Hostettmann, K., 1991. 7. Bioactivity in plants: the link between phytochemistry and medicine. Phytochemistry 30, 3864-3874.). Advances in synthetic chemistry can also generate derivatives with improved efficacies that are orders of magnitude greater than the parent plant product-derived compounds. A mono-methyl substitution enhanced the potency of coumarin analogues. A 4-methyl substituted lactam derivative had potent anti-RT activity in acutely infected H9 lymphocytes and was about 225-fold more active than AZT in the same assay (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.).

Further studies including in silico molecular docking simulations and X-ray diffraction should be done to investigate the binding affinities of plant active compounds to HIV-1 RT. Future studies should also focus on the elaboration of new derivatives with better anti-RT activities than original plant-derived active ingredients. The discovery of potent anti-RT plant-derived compounds can also be enhanced by computational and virtual screening of compound library databases. In this approach, electronic compound libraries are probed for a series of chemical pharmacophores shared by compounds known to exhibit a known pharmacological property (Andersen et al., 2018Andersen, R.J., Ntie-Kang, F., Tietjen, I., 2018. Natural product-derived compounds in HIV suppression, remission, and eradication strategies. Antivir. Res. 158, 63-77.). The performance of computer algorithms in generating reliable 3D conformers of compound models and the speed of docking algorithms have improved tremendously over the past decade, thus making these approaches accessible even to small, remote, or resource-limited laboratories (Andersen et al., 2018Andersen, R.J., Ntie-Kang, F., Tietjen, I., 2018. Natural product-derived compounds in HIV suppression, remission, and eradication strategies. Antivir. Res. 158, 63-77.). Several computer programmes that utilize statistically-derived structure–toxicity/metabolism relationships have been developed for the prediction of toxicity and metabolism, so as to combat the main drawbacks (time and financial costs) in traditional drug discovery methods as well as to promote animal welfare (Onguéné et al., 2018Onguéné, P.A., Simoben, C.V., Fotso, G.W., Andrae-Marobela, K., Khalid, S.A., Ngadjui, B.T., Meva’aMbaze, L., Ntie-Kang, F., 2018. In silico toxicity profiling of natural product compound libraries from African flora with anti-malarial and anti-HIV properties. Comput. Biol. Chem. 72, 136-149.).

Barreca et al. (2002)Barreca, M.L., Balzarini, J., Chimirri, A., Clercq, E.D., Luca, L.D., Höltje, H.D., Rao, A., 2002. Design, synthesis, structure- activity relationships, and molecular modeling studies of 2, 3-diaryl-1, 3-thiazolidin-4-ones as potent anti-HIV agents. J. Med. Chem. 45, 5410-5413. performed the design, synthesis, and the structure–activity relationship studies of a series of 2,3-diaryl-1,3-thiazolidin-4-ones and found that some derivatives were highly effective in inhibiting HIV-1 RT at nanomolar concentrations with minimal cytotoxicity. Thus, computational studies can be used to delineate the ligand-RT interactions and to probe the binding of the ligands to HIV-1 RT (Barreca et al., 2002). Hannongbua et al. (2001)Hannongbua, S., Nivesanond, K., Lawtrakul, L., Pungpo, P., Wolschann, P., 2001. 3D-quantitative structure–activity relationships of HEPT derivatives as HIV-1 reverse transcriptase inhibitors, based on Ab initio calculations. J. Chem. Inf. Comp. Sci. 41, 848-855. applied comparative molecular field analysis (CoMFA) to a large set of 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT) analogues. The starting geometry of HEPT was obtained from crystallographic data of HEPT/HIV-1 RT complexes. The structures of 101 HEPT derivatives were considered and fully optimized by ab initio molecular orbital calculations at the HF/3-21G level. Hannongbua et al. (2001)Hannongbua, S., Nivesanond, K., Lawtrakul, L., Pungpo, P., Wolschann, P., 2001. 3D-quantitative structure–activity relationships of HEPT derivatives as HIV-1 reverse transcriptase inhibitors, based on Ab initio calculations. J. Chem. Inf. Comp. Sci. 41, 848-855. found that the best CoMFA model was satisfactory in both statistical significance and predictive ability of 0.858. Their derived model showed the importance of steric contributions (64.4%) and electrostatic interactions for HIV-1 RT inhibition.

In addition, steric and electrostatic contour maps from their analysis were in conformity with the experimentally observed trend that there were steric interactions between the side chain of HEPT and an aromatic ring of Tyr181 (Hannongbua et al., 2001Hannongbua, S., Nivesanond, K., Lawtrakul, L., Pungpo, P., Wolschann, P., 2001. 3D-quantitative structure–activity relationships of HEPT derivatives as HIV-1 reverse transcriptase inhibitors, based on Ab initio calculations. J. Chem. Inf. Comp. Sci. 41, 848-855.). A moderately sized group at C5 enhances contact with Tyr181 enough to push it into a position which renders the protein non-functional, but a smaller group has insufficient steric requirements to do this and a larger group renders the ligand too large for the cavity. These results not only help to explain the mutation-induced resistance of HIV-1 RT but also lead to a better understanding of structural requirements of HEPT analogues for the inhibition RT. This is important for the design of novel and more potent anti-RT drugs.

Application of structure-based drug design strategies may aid in the development of novel HIV-1 RT inhibitors (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.). Detailed analysis of the conformational changes among the various HIV-1 RT structures is important. It may reveal additional sites for binding new plant-derived inhibitors or show sites where plant agents can interfere with the polymerization and/or flexibility of the RT enzyme. The considerable physical and genetic flexibility of HIV-1 RT also suggests that more effective anti-RT drugs should be designed to target the conserved portions of HIV-1 RT that the virus cannot easily afford to change. Therefore, an important issue in analyzing the structure–activity relationship of HIV is the genetic flexibility of the RT enzyme, and this makes the development of novel plant-derived RT inhibitors a difficult task (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.).

Binding affinity is a measure of the tightness with which an active plant compound attaches to HIV-1 RT. Besides efficacy, binding is one of the factors that influences potency (Uzochukwu et al., 2016Uzochukwu, I.C., Olubiyi, O.O., Ezebuo, F.C., Obinwa, I.C., Ajaegbu, E.E., Eze, P.M., Orji, C.E., Onuoha, M.C., Ikegbunam, M.N., 2016. Ending the Ebola virus scourge: a case for natural products. J. Pharm. Res. 1, 1-6.). The pleiotropic ability of plant active compounds to bind concurrently to more than one target may be a great advantage because of the expected synergistic effect of such multi-target inhibitors (Uzochukwu et al., 2016Uzochukwu, I.C., Olubiyi, O.O., Ezebuo, F.C., Obinwa, I.C., Ajaegbu, E.E., Eze, P.M., Orji, C.E., Onuoha, M.C., Ikegbunam, M.N., 2016. Ending the Ebola virus scourge: a case for natural products. J. Pharm. Res. 1, 1-6.). Thus, a possible drug-design strategy would be to devise compounds that can interfere with the binding of the metal ions (Mg²⁺ or Mn²⁺⁾ at the polymerase active site (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.). It is also attractive to consider developing agents that bind to HIV-1 RT polymerase active site but are not nucleoside analogues (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.).

As new knowledge on mechanisms of polymerization, drug inhibition and drug resistance is emerging from studies on structure–function relationships, it may be possible to develop new or improved plant-derived HIV-1 RT inhibitors that may circumvent current patterns of drug resistance mutations (Ding et al., 2018Ding, J., Das, K., Hsiou, Y., Zhang, W., Arnold, E., Yadav, P.N., Hughes, S.H., 2018. Structural studies of HIV-1 reverse transcriptase and implications for drug design. In:Veerapandian, P. (Ed.), Structure-Based Drug Design. Routledge, New York, pp.41–82.). On the other hand, binding alone does not determine the overall potency phytochemical compounds because it does not always correlate with inactivation of HIV-1 RT (Uzochukwu et al., 2016Uzochukwu, I.C., Olubiyi, O.O., Ezebuo, F.C., Obinwa, I.C., Ajaegbu, E.E., Eze, P.M., Orji, C.E., Onuoha, M.C., Ikegbunam, M.N., 2016. Ending the Ebola virus scourge: a case for natural products. J. Pharm. Res. 1, 1-6.).

More clinical trials are urgently needed

In terms of drug development, there has been chequered progress with plant-derived anti-HIV-1 RT compounds. For example, some coumarins from plants and their analogues have unique mechanisms of action against HIV-1 replication and can serve as potent RT inhibitors (Kostova, 2006Kostova, I., 2006. Coumarins as inhibitors of HIV reverse transcriptase. Curr. HIV Res. 4, 347-363.). Yu et al. (2003)Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345. discovered a dicamphanoyl-khellactone (DCK) analogue, a 3-hydroxymethyl-4-methyl khellactone coumarin derivative, a modified form of suksdorfin isolated from methanol extracts of the plant Lomatium suksdorfii (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). DCK (PA-334B) is a nanomolar inhibitor of drug resistant HIV-1 isolates. Panacos pharmaceutical completed the required DCK preclinical studies for IND filing (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.), but there is no information whether DCK has been taken to clinical trial.

Based on in vitro and in vivo studies, calanolides are attractive candidates for therapeutic use because they do not have antagonistic anti-HIV-1 drug interactions or synergistic toxicity, in addition to being active against other infectious agents (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). Calanolide A proceeded into preclinical and clinical trials (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). It also has activity against all Mycobacterium tuberculosis strains thus may allow more efficient treatment of patients co-infected with HIV and tuberculosis (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Synergy of calanolides with AZT was also confirmed in vivo along with its significant anti-HIV-1 RT activity (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). According to Yu et al. (2003)Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345., phase IA studies showed that calanolide A was generally well tolerated in doses up to 600 mg. In human subjects, plasma levels of calanolide A were higher than those predicted from animal studies.

In phase IB studies, calanolide A was administered orally to HIV infected subjects. Clinical and laboratory assessment on viral load and CD4 count indicated that anti-HIV-1 effects of calanolide A appeared to be dose-dependent and maximized on day 14 or 16 (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). Calanolide A was also in phase II clinical trials (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Sarawak Medichem Pharmaceuticals screened volunteers for combination therapy of calanolide A for treating HIV/AIDS, and hoped to progress to phase III clinical trials in late 2002. However, its development was on hold and its fate was dependent on the Sarawak government who owned Sarwak Medichem Pharmaceuticals and its HIV therapeutic candidates (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Excellent results in preclinical studies and early clinical trials with calanolides demonstrate that coumarin compounds have significant activity and unique HIV inhibitory mechanisms with potential for future drug development and therapy.

In phase I trials was 3,5-di-O-caffeoylquinic acid, isolated from the plant Inula britannica (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). China's Academy of Military Sciences carried out clinical trials for the compound which acts as an irreversible inhibitor of HIV-1 RT (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). QS-21 based on saponins derived from the South American tree Quillaja saponaria, Rosaceae, was an integral part of experimental vaccines evaluated in phase II and III trials for HIV and other infectious diseases (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Crofelemer (CAS 148465-45-6) was in phase III trials (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). It is an oligomeric proanthocyanidin developed by Napo's partners, Trine Pharmaceuticals Inc. and AsiaPharm Group Ltd. Derived from the latex of Croton lecheri, Euphorbiaceae, to treat different types of HIV-induced diarrhoea, Crofelemer was in various stages of clinical development for four distinct product indications including CRO-HIV for AIDS diarrhoea in Phase III (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Napo obtained Special Protocol Assessment (SPA) agreement from U.S. FDA for Crofelemer in HIV/AIDS Diarrhoea Assessment (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.).

An anti-RT inhibitor from plants mentioned in this review, betulinic acid is usually found as free aglycon or as glycosyl derivatives. Some betulinic acid derivatives such as Bevirimat (lacking the carboxyl C-28) have higher anti-HIV-1 potency than the parent compound (Ríos and Máñez, 2018Ríos, J.L., Máñez, S., 2018. New pharmacological opportunities for betulinic acid. Planta Med. 84, 8-19.). PA-457 (Bevirimat) was in phase IIb trials (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Panacos Pharmaceutical developed PA-457 as an anti-HIV drug (maturation inhibitor). The antiretroviral activity of PA-457 is pleiotropic as it also blocks a late step in the processing of HIV group-specific antigen (Gag) protein (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). The resulting HIV particles are structurally defective and incapable of spreading infection in the body (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). Due to its poor solubility in water, betulinic acid has low gastrointestinal absorption which never exceeds 1% (Ríos and Máñez, 2018Ríos, J.L., Máñez, S., 2018. New pharmacological opportunities for betulinic acid. Planta Med. 84, 8-19.). Therefore, the main limitation to the clinical use of betulinic acid is its poor hydrosolubility. For HIV therapy, further studies are needed to develop novel administration formulations and methods for betulinic acid.

Isolation of pure compounds and the poor water-solubility of active plant-derived RT inhibitors and their active analogues can limit their further development as drug candidates (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). Therefore, introducing polar functional groups into the structure may improve water solubility and provide the possibility of prodrugs (Yu et al., 2003Yu, D., Suzuki, M., Xie, L., Morris-Natschke, S.L., Lee, K.H., 2003. Recent progress in the development of coumarin derivatives as potent anti-HIV agents. Med. Res. Rev. 23, 322-345.). Notwithstanding, derivatives of betulinic acid are promising compounds for treating HIV/AIDS (Ríos and Máñez, 2018Ríos, J.L., Máñez, S., 2018. New pharmacological opportunities for betulinic acid. Planta Med. 84, 8-19.). Succinyl and 3′-substituted glutaryl betulin derivatives show stronger anti-HIV activity and higher therapeutic index values than their dihydrobetulin counterparts (Sun et al., 1998Sun, I.C., Wang, H.K., Kashiwada, Y., Shen, J.K., Cosentino, L.M., Chen, C.H., Yang, L.M., Lee, K.H., 1998. Anti-AIDS agents. 34. Synthesis and structure–activity relationships of betulin derivatives as anti-HIV agents. J. Med. Chem. 41, 4648-4657.). Due to its pleiotropic biological effects, especially its outstanding anti-HIV RT activity, betulinic acid isolated from various medicinal plants continues to attract the attention of many scientists (Huang et al., 2018Huang, Q.X., Chen, H.F., Luo, X.R., Zhang, Y.X., Yao, X., Zheng, X., 2018. Structure and anti-HIV activity of betulinic acid analogues. Curr. Med. Sci. 38, 387-397.). By June 2018, some derivatives of betulinic acid with inhibitory activities at nanomolar concentrations had entered phase II clinical trials (Huang et al., 2018Huang, Q.X., Chen, H.F., Luo, X.R., Zhang, Y.X., Yao, X., Zheng, X., 2018. Structure and anti-HIV activity of betulinic acid analogues. Curr. Med. Sci. 38, 387-397.).

Celgosivir synthesized by selective C-acylation of castanospermine is rapidly converted to castanospermine in vivo (Sung et al., 2016Sung, C., Wei, Y., Watanabe, S., Lee, H.S., Khoo, Y.M., Fan, L., Rathore, A.P., Chan, K.W., Choy, M.M., Kamaraj, U.S., Sessions, O.M., Aw, P., de Sessions, P.F., Lee, B., Connolly, J.E., Hibberd, M.L., Vijaykrishna, D., Wijaya, L., Ooi, E.E., Low, J.G., Vasudevan, S.G., 2016. Extended evaluation of virological, immunological and pharmacokinetic endpoints of CELADEN: a randomized, placebo-controlled trial of celgosivir in dengue fever patients. PLoS Negl. Trop. Dis. 10, e0004851.). Celgosivir is a 6-O-butanoyl prodrug of castanospermine (Low et al., 2014Low, J.G., Sung, C., Wijaya, L., Wei, Y., Rathore, A.P., Watanabe, S., Tan, B.H., Toh, L., Chua, L.T., Hou, Y., Chow, A., Howe, S., Chan, W.K., Tan, K.H., Chung, J.S., Cherng, B.P., Lye, D.C., Tambayah, P.A., Ng, L.C., Connolly, J., Hibberd, M.L., Leo, Y.S., Cheung, Y.B., Ooi, E.E., Vasudevan, S.G., 2014. Efficacy and safety of celgosivir in patients with dengue fever (CELADEN): a phase 1b, randomised, double-blind, placebo-controlled, proof-of-concept trial. Lancet Infect. Dis. 14, 706-715.). Combination therapy of 6-O-butanoyl castanospermine (Celgosivir) with Peginterferon and/or Ribavirin was tested in phase II clinical trials for the treatment of patients with chronic hepatitis C virus (Wojtowicz et al., 2016Wojtowicz, K., Januchowski, R., Sosińska, P., Nowicki, M., Zabel, M., 2016. Effect of brefeldin A and castanospermine on resistant cell lines as supplements in anticancer therapy. Oncol. Rep. 35, 2896-2906.). Although generally safe and well tolerated, Celgosivir did not reduce viral loads (Sung et al., 2016Sung, C., Wei, Y., Watanabe, S., Lee, H.S., Khoo, Y.M., Fan, L., Rathore, A.P., Chan, K.W., Choy, M.M., Kamaraj, U.S., Sessions, O.M., Aw, P., de Sessions, P.F., Lee, B., Connolly, J.E., Hibberd, M.L., Vijaykrishna, D., Wijaya, L., Ooi, E.E., Low, J.G., Vasudevan, S.G., 2016. Extended evaluation of virological, immunological and pharmacokinetic endpoints of CELADEN: a randomized, placebo-controlled trial of celgosivir in dengue fever patients. PLoS Negl. Trop. Dis. 10, e0004851.). After tests in phase 1 and 2 trials as a possible treatment for HIV infection, Celgosivir's efficacy was not superior to existing treatments; hence testing was discontinued (Low et al., 2014Low, J.G., Sung, C., Wijaya, L., Wei, Y., Rathore, A.P., Watanabe, S., Tan, B.H., Toh, L., Chua, L.T., Hou, Y., Chow, A., Howe, S., Chan, W.K., Tan, K.H., Chung, J.S., Cherng, B.P., Lye, D.C., Tambayah, P.A., Ng, L.C., Connolly, J., Hibberd, M.L., Leo, Y.S., Cheung, Y.B., Ooi, E.E., Vasudevan, S.G., 2014. Efficacy and safety of celgosivir in patients with dengue fever (CELADEN): a phase 1b, randomised, double-blind, placebo-controlled, proof-of-concept trial. Lancet Infect. Dis. 14, 706-715.).

A large number of peptides such as ascalin interrupt HIV-1 replication. Natural and synthetic peptides are being studied for their possible clinical applications in preventing HIV-1 infection due to their low systemic toxicity; this is their main advantage (Ng et al., 2015Ng, T.B., Cheung, R.C.F., Wong, J.H., Chan, W.Y., 2015. Proteins, peptides, polysaccharides, and nucleotides with inhibitory activity on human immunodeficiency virus and its enzymes. Appl. Microbiol. Biotechnol. 99, 10399-10414.). Although lupeol binds to an HIV-1 RT allosteric pocket (Esposito et al., 2017Esposito, F., Mandrone, M., Del Vecchio, C., Carli, I., Distinto, S., Corona, A., Lianza, M., Piano, D., Tacchini, M., Maccioni, E., Cottiglia, F., Saccon, E., Poli, F., Parolin, C., Tramontano, E., 2017. Multi-target activity of Hemidesmus indicus decoction against innovative HIV-1 drug targets and characterization of lupeol mode of action. Pathog. Dis. 75, http://dx.doi.org/10.1093/femspd/ftx065.
http://dx.doi.org/10.1093/femspd/ftx065... ) and has high bioavailability, factors such as effective dose and the duration of treatment limit its therapeutic applications (Siddique and Saleem, 2011Siddique, H.R., Saleem, M., 2011. Beneficial health effects of lupeol triterpene: a review of preclinical studies. Life Sci. 88, 285-293.). Clinical trials with lignans also produced disappointing outcomes (Hamburger and Hostettmann, 1991Hamburger, M., Hostettmann, K., 1991. 7. Bioactivity in plants: the link between phytochemistry and medicine. Phytochemistry 30, 3864-3874.).

Only 1 in 250,000 plant samples may directly lead to a commercial drug (Macilwain, 1998Macilwain, C., 1998. When rhetoric hits reality in debate on bioprospecting. Nature 392, 535.). Unsurprisingly, there are few plant-derived anti-HIV-1 RT agents in clinical trials. In fact, there were no plant-based HIV/AIDS drugs approved or launched during the period of 2000–2006 (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). In 2008, plant-derived compounds which were in clinical trials were 3,5-di-O-caffeoylquinic acid, calanolide A, castanospermine, and betulinic acid (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.). From 2012 to 2017, only twelve new antivirals were approved by the FDA in the USA; eight are for the treatment of pathologies related to hepatitis C virus and two are combinations of anti-HIV drugs (Mercorelli et al., 2018Mercorelli, B., Palù, G., Loregian, A., 2018. Drug repurposing for viral infectious diseases: how far are we?. Trends Microbiol. 6, 865-876.). There are few scientific reports on plant compounds in clinical trials for the treatment of HIV/AIDS because this field is not well researched (Salehi et al., 2018Salehi, B., Kumar, N.V.A., Şener, B., Sharifi-Rad, M., Kılıç, M., Mahady, G.B., Vlaisavljevic, S., Iriti, M., Kobarfard, F., Setzer, W.N., Ayatollahi, S.A., Ata, A., Sharifi-Rad, J., 2018. Medicinal plants used in the treatment of human immunodeficiency virus. Int. J. Mol. Sci. 19, .
https://doi.org/10.3390/ijms19051459... ). Further, most of the studies are in vitro and very few investigations have been performed in vivo or in humans (Salehi et al., 2018Salehi, B., Kumar, N.V.A., Şener, B., Sharifi-Rad, M., Kılıç, M., Mahady, G.B., Vlaisavljevic, S., Iriti, M., Kobarfard, F., Setzer, W.N., Ayatollahi, S.A., Ata, A., Sharifi-Rad, J., 2018. Medicinal plants used in the treatment of human immunodeficiency virus. Int. J. Mol. Sci. 19, .
https://doi.org/10.3390/ijms19051459... ). Since only 10–15% of plant diversity is explored for pharmaceutical purposes (Saklani and Kutty, 2008Saklani, A., Kutty, S.K., 2008. Plant-derived compounds in clinical trials. Drug Discov. Today 13, 161-171.), the transition from natural product to anti-HIV-1 RT drug prototypes is a daunting prospect.

Limitations in current research on HIV-1 RT inihibitors from plants

The main weakness is that majority of the studies are based on in vitro models and merely report broad phytochemical groups such as phenolic acids, flavonoids and terpenes as being the active molecules against HIV-1 RT. Therefore, there is an urgent need to pinpoint exact novel plant chemical compounds that inhibit HIV-1 RT. The lack of clinical data to support the reported in vitro biological activity is also a major shortcoming. This makes it difficult to depict any novel plant compounds with real promise for clinical application. Anti-HIV-1 RT plant extracts and active compounds should be rigorously tested in animal and human trials. But at the moment there is a paucity of clinical data. Clinical research data may provide more concrete information on the efficacy of medicinal plants as potent inhibitors of HIV-1 RT. Although many plants possess anti-HIV-1 RT properties, their extracts and active compounds should be evaluated for human cytotoxicity and dosage. Further studies on pharmacovigilance and interactions between synthetic cART drugs and plant chemical compounds are also needed.

While crude extracts and some isolated compounds are active against HIV-1 RT, others are toxic to human cells. Such toxic plant compounds should be redirected into different pipelines including the possibilities to be used anticancer agents. Toxic compounds should also be modified through the use of computational chemistry. More data on the structure–activity relationships for plant active compounds and their derivatives are still needed. Screening for anti-HIV-1 RT activity is commendable but current approaches are cul-de-sac scientific expeditions because they have not yielded patentable molecules and commercial products or clinical drugs. Many compounds described in this review reportedly display off-target activity and/or are vulnerable to facile excretion upon metabolism, thereby limiting their appeal as clinically useful HIV-1 RT inhibitors. This challenge could be addressed by the relevant experiments on absorption–distribution–metabolism–excretion (ADME). There is also need for more data on the chemical stability of the active compounds and their susceptibility to oxygen, light and aqueous environment.

Conclusion

This review provides an insightful snapshot of species, genera and families of plants that contain chemical compounds that work as inhibitors of HIV-1 RT. Chemical diversity and anti-HIV-1 RT inhibitory profiles of the various plants have been presented. Anti-RT inhibitors from plants have a plurality of functions. Many of the active compounds against HIV-1 RT are promiscuous with known pleiotropic pharmacological effects. The multiplicity of pharmacological effects was initially heralded as being advantageous but it seems to have become the lightning rod that has drawn to it the negative impulses not least the lack of commercial interest to develop the active compounds into new antiretroviral drugs. Therefore, despite current optimism emanating from their reported anti-RT activities, the phytochemicals have limited high-end clinical applications. Their therapeutic potential is still a distant goal as the molecules are still at the screening stage and initial phases of the drug discovery pipeline. The manufacture of anti-HIV herbal medicines may be a viable option in resource-poor settings. Current anti-HIV-1 RT inhibitors from plants can be repurposed to treat other diseases. To accelerate drug discovery and development, there is an urgent need to tap into the rich vein of indigenous knowledge of putative anti-HIV/AIDS medicinal plants (reverse pharmacology), determine pan-assay interference compounds, analyze structure–activity relationships, and conduct more clinical trials.

Acknowledgements

Moola Nyambe is thanked for drawing the chemical sctructures. Editors and anonymous reviewers are thanked for their comments which improved the original manuscript.

References

Publication Dates

History