Evaluating Antimalarial Proteasome Inhibitors for Efficacy in Babesia Blood Stage Cultures

Tick-transmitted Babesia are a major global veterinary threat and an emerging risk to humans. Unlike their Plasmodium relatives, these erythrocyte-infecting Apicomplexa have been largely overlooked and lack specific treatment. Selective targeting of the Babesia proteasome holds promise for drug development. In this study, we screened a library of peptide epoxyketone inhibitors derived from the marine natural product carmaphycin B for their activity against Babesia. Several of these compounds showed activity against both the asexual and sexual blood stages of Plasmodium falciparum. These compounds inactivate β5 proteasome subunit activity in the lysates of Babesia divergens and Babesia microti in the low nanomolar range. Several compounds were tested with the purified B. divergens proteasome and showed IC₅₀ values comparable to carfilzomib, an approved anticancer proteasome inhibitor. They also inhibited B. divergens growth in bovine erythrocyte cultures with solid EC₅₀ values, but importantly, they appeared less toxic to human cells than carfilzomib. These compounds therefore offer a wider therapeutic window and provide new insights into the development of small proteasome inhibitors as selective drugs for babesiosis.