Michael Acceptor Based Antiplasmodial and Antitrypanosomal Cysteine Protease Inhibitors with Unusual Amino Acids

New peptidic Michael acceptor based cysteine protease inhibitors displaying antiparasitic activity were identified by testing a broad series of 45 compounds in total, containing Asn, Gln, or Phe. As target enzymes, falcipain-2 and -3 from P. falciparum and rhodesain from T. b. rhodesiense were used. In the case of the Asn/Gln containing compounds, the trityl-protected, diastereomeric E-configured vinylogous dipeptide esters 16 (Boc-(S)-Phg-(R/S)-vGln(Trt)-OEt) were discovered as most active inhibitors concerning both protease inhibition and antiparasitic acitivity, with inhibition constants in the submicromolar range. The compounds were shown to display time-dependent and competitive inhibition. In the case of the Phe containing compounds, the maleic acid derivatives 42 and 43 (BnO-Phe←Mal-Phe-OBn, BnO-Phe←Mal-Phe-Ala-OBn, Mal = maleic acid) displayed good inhibition of rhodesain as well as good antitrypanosomal activity, while the fumaric acid derived E-analogue 14 (BnO-Phe←Fum-Phe-OBn) only displayed inhibition of the target enzymes but no antiparasitic activity. Inhibition by these Phe derivatives was shown to be time-independent and competitive.