Carmaphycin B-Based Proteasome Inhibitors to Treat Human African Trypanosomiasis: Structure–Activity Relationship and <i>In Vivo</i> Efficacy

The proteasome is essential for eukaryotic cell proteostasis, and inhibitors of the 20S proteasome are progressing preclinically and clinically as antiparasitics. We screened<i>Trypanosoma brucei</i>, the causative agent of human and animal African trypanosomiasis, <i>in vitro</i> with a set of 27 carmaphycin B analogs, irreversible epoxyketone inhibitors that were originally developed to inhibit the<i>Plasmodium falciparum</i>20S (Pf20S). The structure-activity relationship was distinct from that of the human c20S antitarget by the acceptance of d-amino acids at the P3 position of the peptidyl backbone to yield compounds with greatly decreased toxicity to human cells. For the three most selective compounds, binding to the Tb20S β5 catalytic subunit was confirmed by competition with a fluorescent activity-based probe. For one compound, <b>J-80</b>, with its P3 d-configuration, the differential binding to the parasite's β5 subunit was supported by both covalent and noncovalent docking analysis. Further, <b>J-80</b> was equipotent against both <i>Trypanosoma brucei gambiense</i> and <i>Trypanosoma brucei rhodesiense</i> <i>in vitro</i>. In a mouse model of Stage 1 <i>T. brucei</i> infection, a single intraperitoneal (i.p.) dose of 40 mg/kg <b>J-80</b> halted the growth of the parasite, and when given at 50 mg/kg i.p. twice daily for 5 days, parasitemia was decreased to below the detectable limit, with parasite recrudescence 48 h after the last dose. The <i>in vivo</i> proof of principle demonstrated by a potent, selective, and irreversible inhibitor of Tb20S reveals an alternative path to the development of kinetoplastid proteasome inhibitors that differs from the current focus on allosteric reversible inhibitors.