Antimicrobial and antimalarial activity of Tabernaemontana elegans Stapf. (Apocynaceae)

Abstract

Medicinal plants play a crucial role in the healthcare treatment of many infections and also serve as the basis for the synthesis of novel chemotherapeutic agents. Included in these plants is the Tabernaemontana genus (Apocynaceae) comprising of about 18 species in Africa of which two (Tabernaemontana elegans and T. ventricosa) are indigenous to South Africa. These plant species are used by indigenous people to treat various diseases including venereal diseases, malaria cancer, tuberculosis, stomachache and infertility. The aim of the study was to validate some of the ethnomedicinal uses of T. elegans as well as to isolate and identify the compounds responsible for their activity. In this study, different plant parts of T. elegans were collected based on their medicinal uses for venereal diseases and malaria by Vha-Venda people residing in Mutale Municipality, Limpopo Province. The plant parts were extracted sequentially using hexane, dichloromethane, methanol, water and decoctions were prepared separately. The recovered crude extracts were tested against Neisseria gonorrhoeae, Candida albicans, human immunodeficiency virus (HIV) and Plasmodium falciparum. Additionally, the plant extracts were subjected to an antiproliferative bioassay against mammalian skeletal myoblast cells. Based on the in vitro screening results, the plant extracts were then subjected to 1H-NMR-based metabolomics in an attempt to discriminate between selective and non-selective antiplasmodial plant extracts. The best antiplasmodial selective plant extract (stem bark) was further subjected to phytochemical analysis using conventional chromatographic and spectroscopic techniques. For the antigonoccocal assay, the positive control was ciprofloxacin with MIC value <0.05 mg/ml. The best antigonoccocal activity was observed from the methanol extract of roots with MIC value of 0.10 ± 0.04 mg/ml and methanol extract of the stem bark with MIC value of 0.20 ± 0.06 mg/ml. Root decoction also inhibited bacterial growth with MIC value of 0.80 ± 0.37 mg/ml. An activity of 6 mg/ml was observed in the water extract of the stem bark. This was the first study to report on significant antigonoccocal activity of T. elegans. Ciprofloxacin was also used as the positive control for the anticandidal assay and displayed an MIC value of <0.05 mg/ml. The extracts displayed an activity of >6 mg/ml for the anticandidal assay. The root extracts (water and dichloromethane) showed a moderate inhibition of 60.78 ± 0.16% and 50.24 ± 0.11% when tested against HIV 1 reverse transcriptase (RT) enzyme. The decoction and water extracts of leaves showed moderate inhibition activity of 58.32 ± 0.05% and 54.00 ± 0.12% and the stem bark also showed moderate inhibition activity in the decoction and water extracts with activity of 57.06 ± 0.03% and 51.29 ± 0.13%, respectively. This was the first study to report on the anti-HIV activity of T. elegans. The positive control (doxorubicin) displayed 95.83 ± 0.005% inhibition of the tested enzyme. Six plant extracts exhibited significant antiplasmodial activity ranging from 0.16 ± 0.01 – 4.62 ± 0.55 μg/ml with methanol extracts of stem bark (IC50 = 0.31 ± 0.01 μg/ml) and methanol root (IC50 = 0.16 ± 0.01 μg/ml) being the extracts with the best activity. The positive control, chloroquine, displayed an antiplasmodial activity of 0.004 ± 0.002 μg/ml. The plant extracts were subjected to 1H NMR-based metabolomics to discriminate between the selective and non-selective antiplasmodial plant extracts. An activity profile that could be used to predict activity of unknown samples was observed. Phytochemical analysis was conducted on the combined non-polar extracts (hexane and dichloromethane) of the stem bark which were the most active plant extracts. Four compounds were isolated and were subjected to further NMR analysis in an attempt to predict antiplasmodial activity and for the corroboration of their respective structures. Two of the compounds (2 and 3) were predicted to have significant antiplasmodial activity based on activity profile generated from metabolomics. The study supported some (malaria and gonorrhoea) of the ethnomedicinal uses of T. elegans by indigenous South African people and demonstrated that metabolomics could be used in predicting antiplasmodial activity of unknown samples. Further studies will focus on identification and in vitro screening of the four compounds as well as isolating more compounds.