Development of a new class of inhibitors for the malarial aspartic protease plasmepsin II based on a central 7-azabicyclo[2.2.1]heptane scaffold

Plasmepsin II (PMII), a malarial aspartic protease involved in the catabolism of hemoglobin in parasites of the genus Plasmodium, and renin, a human aspartic protease, share 35% sequence identity in their mature chains. Structures of 4-arylpiperidine inhibitors complexed to human renin were reported by Roche recently. The major conformational changes, compared to a structure of renin, with a peptidomimetic inhibitor were identified and subsequently modeled in a structure of PMII (Fig. 1). Ibis distorted structure of PMII served as active-site model for a novel class of PMII inhibitors, according to a structure-based de novo design approach (Fig. 2). These newly designed inhibitors feature a rigid 7-azabicyclo[2.2.1]heptane scaffold, which, in its protonated form, is assumed to undergo ionic H-bonding with the two catalytic Asp residues at the active site of PMII. Two substituents depart from the scaffold for occupancy of either the S1/S3 or S2'-pocket and the hydrophobic flap pocket, newly created by the conformational changes in PM. The inhibitors synthesized starting from N-Boc-protected 7-azabicyclo[2.2.1]hept-2-ene (6; Schemes 1-5) displayed up to single-digit micromolar activity (IC50 values) toward PMII and good selectivity towards renin. The clear structure-activity relationship (SAR; Table) provides strong validation of the proposed conformational changes in PMII and the occupancy of the resulting hydrophobic flap pocket by our new inhibitors.