Kerstin Andrae-Marobela

Total synthesis, antiprotozoal and cytotoxicity activities of rhuschalcone VI and analogs

The total synthesis of a potent antiplasmodial natural bichalcone, rhuschalcone VI, is described starting from simple and available resorcinol and 4-hydroxybenzaldehyde. Key steps include the solvent-free Aldol syntheses of chalcones, and the successful application of the Suzuki-Miyaura coupling reaction in the synthesis of bichalcones. The present work constitutes a general method for the rapid syntheses of a number of bichalcones related to rhuschalcone VI. Some of the bichalcones showed moderate antiprotozoal activities against Bodo caudatus, a preliminary screening system for antitrypanosomal activities, most of them with little or no cytotoxicity.

Virtualizing the p-ANAPL Library: A Step towards Drug Discovery from African Medicinal Plants

Background Natural products play a key role in drug discovery programs, both serving as drugs and as templates for the synthesis of drugs, even though the quantities and availabilities of samples for screening are often limitted. Experimental approach A current collection of physical samples of > 500 compound derived from African medicinal plants aimed at screening for drug discovery has been made by donations from several researchers from across the continent to be directly available for drug discovery programs. A virtual library of 3D structures of compounds has been generated and Lipinski’s “Rule of Five” has been used to evaluate likely oral availability of the samples. Results A majority of the compound samples are made of flavonoids and about two thirds (2/3) are compliant to the “Rule of Five”. The pharmacological profiles of thirty six (36) selected compounds in the collection have been discussed. Conclusions and implications The p-ANAPL library is the largest physical collection of natural products derived from African medicinal plants directly available for screening purposes. The virtual library is also available and could be employed in virtual screening campaigns.

Determination of potentially toxic heavy metals in traditionally used medicinal plants for HIV/AIDS opportunistic infections in Ngamiland District in Northern Botswana.

The determination of four potentially toxic heavy metals, arsenic, chromium, lead and nickel in twelve plant species used for the treatment of perceived HIV and AIDS-associated opportunistic infections by traditional healers in Ngamiland District in Northern Botswana, a metal mining area, was carried out using atomic absorption spectrometry. The medicinal plants; Dichrostachys cinerea, Maerua angolensis, Mimusops zeyheri, Albizia anthelmintica, Plumbago zeylanica, Combretum imberbe, Indigofera flavicans, Clerodendrum ternatum, Solanum panduriforme, Capparis tomentosa, Terminalia sericea and Maytenus senegalensis contained heavy metals in varying quantities: arsenic 0.19-0.54 μg g(-1), chromium 0.15-1.27 μg g(-1), lead 0.12-0.23 μg g(-1) and nickel 0.09-0.21 μg g(-1) of dry weight. Chromium was found to be the most abundant followed by arsenic and lead. Nickel was undetectable in nine plant species. M. senegalensis contained the largest amounts of arsenic, chromium and lead. All metals determined were below the WHO permissive maximum levels. The possible maximum weekly intakes of the heavy metals following treatment regimes were insignificant compared to the provisional tolerable weekly intake levels recommended by WHO and the Joint FAO/WHO Expert Committee on Food Additives. This suggests that heavy metal exposure to patients originating from consumption of traditional medicinal plant preparations is within non health-compromising limits.

Screening of the pan-African Natural Product Library identifies ixoratannin A-2 and boldine as novel HIV-1 inhibitors

The continued burden of HIV in resource-limited regions such as parts of sub-Saharan Africa, combined with adverse effects and potential risks of resistance to existing antiretroviral therapies, emphasize the need to identify new HIV inhibitors. Here we performed a virtual screen of molecules from the pan-African Natural Product Library, the largest collection of medicinal plant-derived pure compounds on the African continent. We identified eight molecules with structural similarity to reported interactors of Vpu, an HIV-1 accessory protein with reported ion channel activity. Using in vitro HIV-1 replication assays with a CD4+ T cell line and peripheral blood mononuclear cells, we confirmed antiviral activity and minimal cytotoxicity for two compounds, ixoratannin A-2 and boldine. Notably, ixoratannin A-2 retained inhibitory activity against recombinant HIV-1 strains encoding patient-derived mutations that confer resistance to protease, non-nucleoside reverse transcriptase, or integrase inhibitors. Moreover, ixoratannin A-2 was less effective at inhibiting replication of HIV-1 lacking Vpu, supporting this protein as a possible direct or indirect target. In contrast, boldine was less effective against a protease inhibitor-resistant HIV-1 strain. Both ixoratannin A-2 and boldine also inhibited in vitro replication of hepatitis C virus (HCV). However, BIT-225, a previously-reported Vpu inhibitor, demonstrated antiviral activity but also cytotoxicity in HIV-1 and HCV replication assays. Our work identifies pure compounds derived from African plants with potential novel activities against viruses that disproportionately afflict resource-limited regions of the world.

Old plants newly discovered: Cassia sieberiana D.C. and Cassia abbreviata Oliv. Oliv. root extracts inhibit in vitro HIV-1c replication in peripheral blood mononuclear cells (PBMCs) by different modes of action.

ETHNOPHARMACOLOGICAL RELEVANCE Despite advances in anti-retroviral therapy which has transformed HIV/AIDS from a fatal to a manageable chronic disease, increasing viral drug resistance, side effects and uneven access to anti-retroviral drugs remain considerable therapeutic challenges. Partly as a consequence of these shortcomings and partly based on the fact that HIV/AIDS gives rise to opportunistic infections whose symptoms have been managed in Africa in an HIV/AIDS-independent context by traditional healers for centuries, many HIV/AIDS patients use herbal medicines. The aim of this study was to screen selected medicinal plants from Botswana, used by traditional healers to treat/manage HIV/AIDS, for inhibitory activities on HIV replication. MATERIALS AND METHODS Based on an ethnomedical survey, ethanolic tannin-containing and tannin-free extracts from 10 medicinal plants were tested for inhibitory properties against a clone of HIV-1c (MJ(4)) measuring cytopathic effect protection and levels of viral p24 antigen in infected PBMCs. RESULTS Cassia sieberiana D.C., Cassia abbreviata Oliv. Oliv. and Plumbago zeylanica L. extracts showed significant inhibition of HIV-1c (MJ(4)) replication. The inhibitory activity of the Plumbago zeylanica extract could be attributed to its tannin content. Anti-HIV activity of Cassia sieberiana root and bark extracts, and Cassia abbreviata root extracts occurred in a concentration-dependent manner with an effective concentration (EC(50)) of 65.1μg/ml, 85.3μg/ml and 102.8μg/ml, respectively. Experiments to elucidate possible mechanism(s) of action revealed that Cassia sieberiana root and bark extracts blocked HIV replication at its binding- (EC(50)=70.2μg/ml and 90.8μg/ml, respectively) and entry stage (EC(50)=88.9μg/ml and 100.5μg/ml, respectively) while Cassia abbreviata extracts did not. CONCLUSIONS We report here for the first time a direct inhibitory effect on HIV-1c replication of extracts from two extremely popular medicinal plants, Cassia sieberiana and Cassia abbreviata. Considering the traditional uses of both Cassia species, our findings strongly suggest pilot clinical observational studies involving traditional healers to further evaluate the therapeutic potential of the Cassia extracts.

A new approach for anthelmintic discovery for humans

Natural product-based drug discovery has been deemphasized by the pharmaceutical industry. This situation is discordant with the fact that most people in developing countries rely on traditional medicines derived from local biodiversity for healthcare. Despite economic growth in the past 10 years, Africa remains plagued by parasitic infections, out of reach of eradication. Limited regional funding for drug discovery complicates the situation. Novel models are needed to bring sustainability to local drug discovery programs. This Opinion describes an innovative partnership that promotes local leadership to harness a recombinant yeast-based assay to screen for novel anthelmintic candidates in collections of African natural products. Implementation of this strategy in biodiversity-rich but resource-constrained settings can help build sustainable local capacity for drug discovery.

“Now I Heal with Pride”—The Application of Screens-to-Nature Technology to Indigenous Knowledge Systems Research in Botswana: Implications for Drug Discovery

The combination of indigenous knowledge systems (IKS) with techniques of modern drug discovery platforms has a great potential to overcome the current innovation deficits in drug discovery. We describe the adaptation of a set of field-suitable bioassays, the “Screens-to-Nature” (STN) system, to a participatory research tool suitable for overcoming some of the difficulties in establishing cooperation between indigenous knowledge holders and scientists. Extracts from 621 plant samples representing 214 species from 71 plant families in Botswana have been qualitatively screened for antibacterial, antifungal, and antiprotozoal activities, as well as for α-glucosidase and protease inhibitory properties. The results provide a first bioactivity profile of medicinal plants in Botswana. Close to half of the samples (47%) were provided by traditional healers and community members from two regions in Botswana, the Kweneng and Ngamiland Districts. Screening results were consistently shared and discussed with indigenous knowledge holders who participated in the STN project. A survey conducted among 28 traditional healers revealed that a large majority (93%) perceived the STN approach as beneficial to indigenous knowledge holders and expressed their desire to continue contributing to natural product research. In that way, the STN approach provided a basis for the establishment of an indigenous knowledge-guided drug discovery platform in Botswana.

The dialectics of indigenous knowledge: Community views from Botswana

Indigenous knowledge (IK) has become a popular subject in the last decade. IK has been recognized by global institutions as a valuable source for scientific innovation and significant in promoting development for poorer communities. However, little attention has been paid to how communities themselves particularly conceptualize IK and how they situate this knowledge in everyday life context. This paper summarizes perceptions of IK from 20 rural communities in the Ghanzi (4) and North-East (16) districts in Botswana. Our findings indicate that IK cannot be defined by contrasting it to other knowledge systems. Rather, IK is abstract and concrete, individual and collective, specific and holistic in a dialectical manner. This inherent “unity of opposites” promotes constant change of IK. Applied to the challenge of linking IK to development, community perceptions inform us that a dialectical process-oriented approach to IK is necessary to harness it for a sustainable development and meaningful education of indigenous people in Botswana.

Identification of Bichalcones as Sirtuin Inhibitors by Virtual Screening and In Vitro Testing

Sirtuins are nicotinamide adenine dinucleotide (NAD+)-dependent class III histone deacetylases, which have been linked to the pathogenesis of numerous diseases, including HIV, metabolic disorders, neurodegeneration and cancer. Docking of the virtual pan-African natural products library (p-ANAPL), followed by in vitro testing, resulted in the identification of two inhibitors of sirtuin 1, 2 and 3 (sirt1–3). Two bichalcones, known as rhuschalcone IV (8) and an analogue of rhuschalcone I (9), previously isolated from the medicinal plant Rhus pyroides, were shown to be active in the in vitro assay. The rhuschalcone I analogue (9) showed the best activity against sirt1, with an IC50 value of 40.8 µM. Based on the docking experiments, suggestions for improving the biological activities of the newly identified hit compounds have been provided.