Cysteine Proteases play numerous indispensable roles in the biology of parasitic organisms. In general, the enhanced interest in cysteine proteases is reflected in the literature since they are promising chemotherapeutic targets. The development of novel potent and selective inhibitors for cysteine proteases has therefore gained increasing attention in these last few years. The aim of this work is to discover new inhibitors of three different cysteine proteases: caspase-3, falcipain-2 and -3 (FP-2 and FP-3).
To achieve our goal, a series of vinyl sulfones were synthesized and evaluated against caspases-3. Dipeptidyl derivatives were significantly superior to their counterparts containing only Asp at P1, as caspase-3 inhibitors. Fmoc-Val-Asp-trans- CH=CH-SO2Me, 134h, was the most potent inhibitor of caspase-3 in the series, with an IC50 (concentration giving 50% of parasite growth inhibition) of 29 μM and a second-order rate constant of inactivation, kinact/ki, of 1.5 M-1 s-1. Computational studies suggest that the second amino acid occupies the S3 pocket of the enzyme. In addition, Fmoc-Val-Asp-trans-CH=CH-SO2Me, 134h, was inactive for caspase-7 for the tested concentrations showing selectivity for caspase-3.
A series of aurones and benzothiophenes were also synthesized with the aim of optimizing its antiplasmodial activity since the control of malaria continues to be challenged by increased resistance by the parasite Plasmodium falciparum to most available drugs. All the tested compounds showed modest activity, with the most active compound presenting an IC50 of 2.7 μM against P. falciparum strain W2. In the case of aurones the best candidate was 141g with an IC50 against W2 of 4.8 μM and IC50 (FP-2) of 18.6 μM. In the case of benzothiophenes the most active compound, 168d, presented an IC50 value of 2.7 μM and 21.1 μM against P. falciparum strain W2 and FP-2, respectively.
Finally, a virtual screening study was performed in order to find new antimalarial drugs. We screened in silico, the ZINC database, allowing the discovery of novel scaffolds with antiplasmodial activity. A receptor-based approach was successful in retrieving 5 active compounds (185, 187, 195, 197 and 198). The best of them, 187, presented an IC50 of 0.9 μM in vitro against P. falciparum strain W2.
Keywords: Proteases, Cysteine Proteases, Caspase-3, Caspase-7, Falcipain-2, Falcipain-3, Vinyl Sulfone, Aurones, Benzothiophenes, Virtual Screening, Molecular Docking, P. falciparum.