In Silico and In Vitro Drug Repurposing Investigation of FDA-Approved Compounds against Leishmania donovani Targets Relevant to Visceral Leishmaniasis in the Nigerian Context

Background: Visceral leishmaniasis remains a neglected tropical disease of clinical significance in parts of North West and North East Nigeria, yet treatment options are limited by toxicity, cost, and emerging resistance to first-line agents. Drug repurposing, the systematic re-evaluation of approved drugs for new indications, offers a cost-effective pathway to therapeutic discovery. This study employs in silico and in vitro strategies to identify repurposable FDA-approved compounds active against Leishmania donovani. Methods: Structure-based virtual screening of the FDA-approved drug library (approximately 3,000 compounds) was performed against three validated L. donovani drug targets: trypanothione reductase, pteridine reductase 1, and cysteine protease B. Molecular docking was conducted using AutoDock Vina and GLIDE, followed by molecular dynamics simulation of top candidates. Shortlisted compounds were tested in vitro against L. donovani promastigotes and intracellular amastigotes in THP-1-derived macrophages. Selectivity indices against mammalian cells were calculated. Results: In silico screening identified 12 candidate compounds with promising docking scores. Six demonstrated IC50 values below 10 uM against promastigotes. Three candidates (including an approved antifungal and a repurposed kinase inhibitor) showed submicromolar amastigote activity and selectivity indices above 10. Two of these significantly inhibited trypanothione reductase activity in enzyme inhibition assays. Conclusion: FDA-approved drugs targeting L. donovani enzymatic pathways show repurposing potential for visceral leishmaniasis. These candidates provide a rational basis for in vivo efficacy validation and potential clinical development relevant to Nigerian endemic contexts. Keywords: drug repurposing, Leishmania donovani, visceral leishmaniasis, virtual screening, neglected tropical diseases