Antiplasmodial potential of South African medicinal plants and phytochemical investigation of Aloe marlothii, Turraea obtusifolia and Artemisia afra for identification of their active compounds

Abstract

The great breakthroughs in modern malaria drug discovery and development were a result of studies of medicinal plants used to treat malaria such as Cinchona by the Incas and Artemisia annua by the Chinese. South Africa is ranked as the third most biodiverse Country. While malaria transmission in South Africa has been restricted to three provinces, many plants have been used traditionally by local populations to treat this disease. A subset of plants used in South African traditional medicine was selected from the University of Pretoria Plant Repository (Biodiscovery Lab). These were extracted with organic solvents and fractionated using a ppSPE workstation following a protocol adopted from National Cancer Institute (USA). Generated extracts and fractions were formatted into 96 deep well plates at a concentration of 5 mg/mL in 100% DMSO using a liquid handler and stored in a robotic freezer as part of the natural product library. Copies of plates (extracts and fractions) were made and screened in vitro against the asexual Plasmodium falciparum NF54 parasites at dual point concentrations of 10 and 20 µg/mL. The screening results indicated that the fractionation was successful in concentrating active compounds into one or sometimes two fractions. Among fractions which displayed good activity (IC50≤10 µg/mL) were fractions from Aloe marlothii, Turraea obtusifolia and Artemisia afra. These were analysed using UPLC-QTOF-MS for tentative identification of compounds. Using mass-directed purification (HPLC), pure compounds were isolated from the three afore mentioned species and their structure were elucidated using NMR and MS. These were anthraquinones, sesquiterpernes, limonoids, pregnane steroids, flavonoids, prostaglandin-like fatty acids. The pure compounds were screened against P. falciparum parasites. Of all screened compounds from A. marlothii, aloesaponarin I displayed good equipotent activity against the asexual P. falciparum NF54 (drug-sensitive) and K1 (multidrug-resistant) strains with IC50 values of 1.58 µg/mL and 1.54 µg/mL, respectively. Aloesaponol IV exhibited pronounced activity against late-stage gametocytes (IC50 = 6.53 µg/mL) demonstrating a 3-fold selective potency towards these sexual stages compared to asexual forms of the parasite. Rubralin B (limonoid from T. obtusifolia) displayed a good activity against the asexual P. falciparum NF54 with an IC50 value of 3.47 µg/mL. As for compounds from A. afra, 1-dehydroartemorin, acacetin and retusin demonstrated good activities against the asexual P. falciparum NF54 with IC50 values of 3.95, 4.18 and 6.03 µg/mL, respectively.