Resistance to Pyrimethamine, an antimalarial medicine has been reported to be due to mutations in Dihydrofolate reductase-thymidylate synthase (DHFR-TS). Phytochemicals, particularly from plants that been used in ethnomedicine, have been reported to have privileged structures that might bind strongly to the mutants of DHFR-TS. The aim of this study is to identify phytochemicals of Acalypha wilkesiana, Cymbopogon citratus, Azadirachta indica, and Morinda lucida with high binding affinities for the Plasmodium falciparum DHFR-TS. The three-dimensional structures of the phytochemicals, wide type and mutant forms of DHTR-TS were obtained from PubChem and Protein Databank (PDB) respectively. They were appropriately prepared and molecular docking simulations was implemented to predict binding affinities of the phytochemicals to the wildtype and mutant forms of DHTR-TS. Druglikeness assessment was implemented to triage the top binding phytochemicals and molecular dynamics simulations was done to establish the stability of the interaction of the top-ranked phytochemical with one of the mutants of DHFR-TS. Nineteen phytochemicals showed higher binding affinities to both the wild type and mutant forms DHFR-TS than Pyrimethamine. Molecular dynamics revealed that the receptor-ligand binding of luteolin, the top-ranked, drug like phytochemicals, to the quadruple mutant was stable.

Malaria, leishmaniasis, and African trypanosomiasis are protozoan diseases that constitute a major global health problem, especially in developing countries; however the development of drug resistance coupled with the toxicity of current treatments have hindered their management. The implication of certain enzymes (dihydrofolate reductase, DHFR) or proteins (potassium channels) in the pathogenesis of these protozoan diseases is undeniable. In this study, a series of three DHFR inhibitors (6-5 fused heterocyclic derivatives X, Y and Z) and one K+ channel blocker (E4031) were screened for their inhibitory effect on Leishmania donovani, Plasmodium falciparum, and Trypanosoma brucei. Resazurin assay was used to assess antitrypanosomal and antileishmanial activities of test compounds, whereas the antimalarial activity was appraised through SYBR Green I test. Moreover, cytotoxicity of test compounds was evaluated on Vero, Raw 264.7 and HepG-2 cells using the resazurin-based test, whilst their pharmacokinetic properties were predicted using the online tool pkCSM. As a result, compound Y revealed a selective (selectivity index range: 2.69 - >61.4; Vero, Raw 264.7 and HepG-2 cells) and broad-spectrum antiprotozoal activity against L. donovani promastigotes (IC50: 12.4 µM) and amastigotes (IC50: 4.28 µM), P. falciparum (IC50: 0.028 µM), and T. brucei brucei (IC50: 0.81 µM). In addition, compound X inhibited the growth P. falciparum (IC50: 0.0052 µM) and T. brucei brucei (IC50: 6.49 µM). In silico screening of the active antiprotozoal compounds revealed positive drug-likeness scores as none of Lipinski's rule of five’s criteria was violated by these compounds. However, in-depth pharmacokinetic and mechanistic studies are warranted to support discovery of novel antiprotozoal agents against malaria, leishmaniasis, and African trypanosomiasis by repurposing K+ channel blocker and DHFR inhibitors.