Mining Sudanese Medicinal Plants for Natural Compounds against Malaria and Neglected Tropical Diseases

Tropical parasitic diseases such as malaria, human African trypanosomiasis, Chagas disease, mycetoma, and leishmaniasis affect more than a billion people worldwide and have devastating consequences. There is no vaccine for any of these diseases, and the current drugs are problematic given their serious adverse effects and the emergence of drug-resistant parasites. Thus, there is an urgent need for the development of new, efficacious, safe, and cost-effective drugs.
Natural products have in many instances provided new leads to combat neglected tropical diseases. The aim of this work was to systemically evaluate Sudanese medicinal plants for their antiparasitic activity along with their cytotoxicity profile; followed by phytochemical investigation to identify bioactive compounds. A library of 235 plant extracts was prepared from over 60 plants used in Sudanese traditional medicine, and it was assessed for antiprotozoal activity against Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani, and Plasmodium falciparum.
Dereplication was performed for active extracts to enable a rapid identification of known active compounds and prioritization for follow-up isolation. Plants that displayed interesting activities, namely Croton gratissimus, Cuscuta hyalina, and Haplophyllum tuberculatum, were further pursued. HPLC-based activity profiling led to localization of activity and identification of the types of compounds in these plant extracts. Compound isolation and structure elucidation were achieved by a combination of analytical, preparative, and semipreparative chromatographic techniques such as HPLC-PDA-ELSD-MS and microprobe NMR.
HPLC-based activity profiling of Croton gratissimus allowed the identification of flavonoids, mainly quercetin derivatives, as responsible for the antileishmanial activity of the chloroform fraction of the crude ethanolic extract. Of these compounds, quercetin-3,7-dimethylether and ayanin were the most active against the protozoan parasites and with the highest selectivity indices.
Compounds that displayed moderate to higher antitrypanosomatid activity shared structural features, such as Δ2,3 unsaturation, presence of a hydroxyl group at C-3, a carbonyl group at C-4, and a catechol moiety in ring B. Phytochemical characterization of Cuscuta hyalina lead to the isolation of a unique flavonoid, pseudosemiglabrin, for the first time from Cuscuta species.
The antileishmanial activity of Haplophyllum tuberculatum was tracked by HPLC-based activity profiling, and eight compounds were isolated from the chloroform fraction. These included the lignans tetrahydrofuroguaiacin B, nectandrin B, furoguaiaoxidin, and 3,3′-dimethoxy-4,4′-dihydroxylignan-9-ol; and four cinnamoylphenethyl amides, namely dihydro-
feruloyltyramine, N-trans-feruloyltyramine, N,N′-diferuloylputrescine, and 7′-ethoxy-feruloyltyramine. The water fraction yielded steroidal saponins. All these compounds were reported for the first time from Haplophyllum species and the family Rutaceae. Nectandrin B exhibited the highest activity against L. donovani (IC50 4.5 µM) and the highest selectivity index (25.5).
Given the urgent need for better drugs and the fact that mycetoma is the most neglected of the neglected diseases, mycetoma has received special consideration. Different approaches were tackled to ultimately identify potential hits. With regard to antimycetomal natural products, several compounds were selected based on an educated-guess and were assessed accordingly. Of the tested natural compounds, magnolol possessed the highest activity (MIC of 15 µM) and selectivity (SI of 4.9).
In parallel, a drug repurposing (repositioning) strategy was pursued to find more promising hits. A series of nitroimidazole compounds were screened in vitro against the fungus Madurella mycetomatis. From this screening, niclosamide showed interesting activity with a minimal inhibitory concentration ˂5 µM. Furthermore, additional niclosamide analogues were tested for proof of concept. The tested compounds showed similar activity compared to niclosamide, not only against M. mycetomatis but also against the bacteria Actinomadura spp. The finding that a drug like niclosamide, which is on the WHO's list of Essential Medicines, exhibits in vitro activity against both fungal and bacterial mycetoma warrants the consideration of niclosamide or its ethanolamine salt as repurposing candidates for mycetoma.