Repurposed anticancer drugs disrupt cancer-like traits of Theileria annulata and suppress Leishmania donovani.

Protozoan parasites remain a major threat to global health, affecting both humans and livestock. Theileria annulata, an apicomplexan parasite of cattle, transforms host leukocytes into a cancer-like state, while Leishmania donovani causes visceral leishmaniasis in humans. Given their reliance on host signaling and metabolic pathways, we hypothesized that FDA-approved anticancer drugs could be repurposed as antiparasitic agents. A library of 436 compounds was screened against T. annulata, yielding 130 hits at 10 µM, of which 63 exhibited minimal cytotoxicity toward healthy PBMCs. Secondary screening at 1 µM identified 28 potent inhibitors with > 50% activity. These compounds span diverse scaffolds, including taxanes, nucleoside analogues, antifolates, proteasome inhibitors, and kinase modulators, and target critical pathways such as apoptosis, autophagy, DNA damage response, cytoskeletal dynamics, NF-κB signalling, and tyrosine kinase activity. Notably, floxuridine, masitinib, paclitaxel, cabazitaxel, vinorelbine, maytansine, and bortezomib also suppressed L. donovani, highlighting cross-species efficacy. Target mapping revealed that the majority of active molecules disrupt hallmarks of cancer and parasite-transformed cells, including metabolic reprogramming, genome instability, and invasive phenotypes. Collectively, these findings underscore the value of oncology-derived drug libraries for antiparasitic discovery and identify promising candidates for further evaluation against T. annulata and L. donovani.