Pyrazolo[1,5-c]quinazoline-based heterocycles exhibit potent antileishmanial activity: Mechanistic insights and toxicity profile.

Leishmaniasis remains a major global health challenge, with current chemo-therapeutics limited by high cost, toxicity, and poor bio-availability. In this study, seventeen new pyrazolo[1,5-c]quinazoline derivatives were designed and synthesized and evaluated for their in vitro antileishmanial activity against Leishmania donovani promastigotes. Compounds 19e and 20k demonstrated superior potency, exhibiting IC50 values lower than miltefosine, while showing minimal cytotoxicity in HeLa and RAW cells (CC50), resulting in favorable selectivity indices. Mechanistic investigations revealed that both compounds significantly elevated intracellular reactive oxygen species (ROS) levels and induced pronounced cell cycle arrest at the sub-G0/G1 phase, indicative of programmed-like parasite death. Importantly, in-depth in vivo toxicity profiling in BALB/c mice revealed an excellent safety profile, with no mortality observed up to 20 mg/kg, and only mild, transient histopathological changes noted at the highest tested dose. These findings collectively highlight the potent antileishmanial activity, mechanistic specificity, and favorable safety profile of 19e and 20k. Overall, these pyrazoloquinazoline derivatives represent promising, non-toxic lead candidates for further optimization and development as potential therapeutic agents against visceral leishmaniasis. Their combination of high efficacy, low mammalian cytotoxicity, and predictable mechanistic action underscores their potential to address the unmet need for safer and more effective treatments for leishmaniasis, supporting their advancement toward preclinical evaluation and future clinical development.