Towards Improved Peptidic α-Ketoamide Inhibitors of the Plasmodial Subtilisin-Like SUB1: Exploration of N-Terminal Extensions and Cyclic Constraints.

After more than 15 years of decline, the Malaria epidemy has increased again since 2017, reinforcing the need to identify drug candidates active on new targets involved in at least two biological stages of the Plasmodium life cycle. The SUB1 protease, which is essential for parasite egress in both hepatic and blood stages, would meet these criteria. We previously reported the structure-activity relationship analysis of α-ketoamide-containing inhibitors encompassing positions P4-P2'. Despite compounds with high inhibitory potencies were identified, their antiparasitic activity remained limited, probably due to insufficient cell permeability. Here, we present our efforts to improve it through the N-terminal introduction of basic or hydrophobic moieties and/or cyclization. Compared to our previous reference compounds 1/2 (Ac-Ile/Cpg-Thr-Ala-AlaCO-Asp-Glu(Oall)-NH2), we identified analogues with improved Pf-/PvSUB1 inhibition (IC50 values in the 10-20 nM  range) and parasite growth inhibition (up to 98% at 100 μM). The increase in potency was mainly observed when increasing the overall hydrophobicity of the compounds. Conjugation to the cell penetrating peptide octa-arginine was also favorable. Finally, the crystal structure of PvSUB1 in complex with compound 15 has been determined at 1.6 Å resolution. Compared to compound 1, this structure extended to the P5 residue and revealed two additional hydrogen bonds.