Potent inhibitors of malarial aspartic proteases, the plasmepsins, by hydroformylation of substituted 7-azanorbornenes

The increasing prevalence of multidrug-resistant strains of the malarial parasite Plasmodium falciparum requires the urgent development of new therapeutic agents with novel modes of action. The vacuolar malarial aspartic proteases plasmepsin (PM) I, II, and IV are involved in hemoglobin degradation and play a central role in the growth and maturation of the parasite in the human host. We report the structure-based design, synthesis, and in vitro evaluation of a new generation of PM inhibitors featuring a highly decorated 7-azabicyclo[2.2.1]heptane core. While this protonated central core addresses the catalytic Asp dyad, three substituents bind to the flap, the S1/S3, and the S1' pockets of the enzymes. A hydroformylation reaction is the key synthetic step for the introduction of the new vector reaching into the S1' pocket. The configuration of the racemic ligands was confirmed by extensive NMR and X-ray crystallographic analysis. In vitro biological assays revealed high potency of the new inhibitors against the three plasmepsins (IC(50) values down to 6 nM) and good selectivity towards the closely related human cathepsins D and E. The occupancy of the S1' pocket makes an essential contribution to the gain in binding affinity and selectivity, which is particularly large in the case of the PM IV enzyme. Designing non-peptidic ligands for PM II is a valid route to generate compounds that inhibit the entire family of vacuolar plasmepsins.