Synthesis and evaluation of peptidyl α,β-unsaturated carbonyl derivatives as anti-malarial calpain inhibitors

Malarial calpain is a cysteine protease believed to be a central mediator essential for parasitic activities. N-Acetyl-L-leucyl-L-leucyl-L-norleucinal (ALLN), a calpain inhibitor, showed an excellent inhibitory effect on the erythrocytic stages of Plasmodium falciparum . However the aldehyde group of ALLN makes it susceptible to metabolism. Therefore, we designed α,β-unsaturated carbonyl peptides that could serve as electrophiles for cysteine residues in calpain. Among the synthetic analogs based on the structure of ALLN, peptidyl esters 7 , 8 and 9 showed the most potent anti-malarial effects, with the same IC50 values of 5.0 μM. Also they showed the high selective toxicity for the malaria versus Hela cell with 40.6, 69.2 and 24.3 fold for 7 , 8 and 9 , respectively. Dipeptidyl α,β-unsaturated carbonyl derivatives consisting of two amino acids gave better anti-malarial effects than those consisting with one amino acid. The fluctuation in anti-malarial activity with small changes in chemical structure indicates the possibilities of improving synthetic analogs.