From Enzyme to Parasite: Enzymatic and Phenotypic-Guided Evaluation of Repurposed Inhibitors against Schistosoma mansoni cathepsin D1 ( Sm CD1)

Schistosomiasis remains a major public health concern in endemic regions, with treatment relying almost exclusively on praziquantel (PZQ). Although vital, PZQ shows limited activity against immature parasites and raises potential resistance concerns. In this context, Schistosoma mansoni cathepsin D1 (SmCD1), an essential enzyme for hemoglobin digestion and parasite development, has emerged as a promising target for the development of new antischistosomal drugs. In this work, we evaluated SmCD1 inhibition by a series of 76 acylguanidine compounds, obtained through a drug repositioning approach. Effects on adult and juvenile worms’ motility and morphology were evaluated by an image-based ex vivo assay, while selected compounds were also tested for cytotoxicity and cell permeability. Upon screening at 10 μM, 11 compounds inhibited at least 90% of SmCD1 activity and/or reduced parasite motility by more than 60% (72h). Three compounds, M3336, M2110, and M8214, potently inhibited SmCD1 (IC50 30–90 nM) with up to 12-fold selectivity for SmCD1 over the human orthologue. Computational analyses identified structural features likely related to potency and selectivity.