2-Amido-3-(1H-Indol-3-yl)-N-Substitued-Propanamides as a New Class of Falcipain-2 Inhibitors. 1. Design, Synthesis, Biological Evaluation and Binding Model Studies

The Plasmodium falciparum cysteine protease falcipain-2 (FP-2) is an important cysteine protease and an essential hemoglobinase of erythrocytic P. falciparum trophozoites. The discovery of new FP-2 inhibitors is now a hot topic in the search for potential malaria treatments. In this study, a series of novel small molecule FP-2 inhibitors have been designed and synthesized based on three regional optimizations of the lead (R)-2-phenoxycarboxamido-3-(1H-indol-3-yl)-N-benzylpropanamide (1), which was identified using structure-based virtual screening in conjunction with surface plasmon resonance (SPR)-based binding assays. Four compounds – 1, 2b, 2k and 2l –showed moderate FP-2 inhibition activity, with IC(50) values of 10.0-39.4 μM, and the inhibitory activity of compound 2k was ~3-fold better than that of the prototype compound 1 and may prove useful for the development of micromolar level FP-2 inhibitors. Preliminary SAR data was obtained, while molecular modeling revealed that introduction of H-bond donor or/and acceptor atoms to the phenyl ring moiety in the C region would be likely to produce some additional H-bond interactions, which should consequently enhance molecular bioactivity.