Nature-Inspired Gallinamides Are Potent Antischistosomal Agents: Inhibition of the cathepsin B1 Protease Target and Binding Mode Analysis

Schistosomiasis,\ncaused by a parasitic blood fluke of\nthe genus <i>Schistosoma,</i> is a global health problem\nfor which new chemotherapeutic\noptions are needed. We explored the scaffold of gallinamide A, a natural\npeptidic metabolite of marine cyanobacteria that has previously been\nshown to inhibit cathepsin L-type proteases. We screened a library\nof 19 synthetic gallinamide A analogs and identified nanomolar inhibitors\nof the cathepsin B-type protease SmCB1, which is a drug target for\nthe treatment of schistosomiasis mansoni. Against cultured S. mansoni schistosomula and adult worms, many of\nthe gallinamides generated a range of deleterious phenotypic responses.\nImaging with a fluorescent-activity-based probe derived from gallinamide\nA demonstrated that SmCB1 is the primary target for gallinamides in\nthe parasite. Furthermore, we solved the high-resolution crystal structures\nof SmCB1 in complex with gallinamide A and its two analogs and describe\nthe acrylamide covalent warhead and binding mode in the active site.\nQuantum chemical calculations evaluated the contribution of individual\npositions in the peptidomimetic scaffold to the inhibition of the\ntarget and demonstrated the importance of the P1′ and P2 positions.\nOur study introduces gallinamides as a powerful chemotype that can\nbe exploited for the development of novel antischistosomal chemotherapeutics.