A series of chloro- and aminoalkylamino-substituted neocryptolepine (5-methyl-5H-indolo[2,3-b]quinoline) derivatives were synthesized and evaluated as antiplasmodial agents. The evaluation also included cytotoxicity (MRC5 cells), inhibition of beta-hematin formation, and DNA interactions (DNA-methyl green assay). Introduction of aminoalkylamino chains increased the antiplasmodial activity of the neocryptolepine core substantially. The most efficient compounds showed antiplasmodial activities in the nanomolar range. N(1),N(1)-Diethyl-N(4)-(5-methyl-5H-indolo[2,3-b]quinolin-8-yl)pentane-1,4-diamine 11c showed an IC(50) of 0.01 microM and a selectivity index of 1800.

Cysteine proteases of the papain superfamily are present in nearly all eukaryotes and also play pivotal roles in the biology of parasites. Inhibition of cysteine proteases is emerging as an important strategy to combat parasitic diseases such as sleeping sickness, Chagas disease, and leishmaniasis. Inspired by the in vivo antiparasitic activity of the vinylsulfone-based cysteine protease inhibitors, a series of α-ketoheterocycles were developed as reversible inhibitors of a recombinant L. mexicana cysteine protease, CPB2.8. Three isoxazoles and especially one oxadiazole compound are potent reversible inhibitors of CPB2.8; however, in vitro whole-organism screening against a panel of protozoan parasites did not fully correlate with the observed inhibition of the cysteine protease.

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