Specific inhibitors of Plasmodium falciparum thioredoxin reductase as potential antimalarial agents.

Plasmodium falciparum thioredoxin reductase (PfTrxR: NADPH + Trx(S) 2  + H +  ↔ NADP +  + Trx(SH) 2 ) is a high M r flavin-dependent TrxR that reduces thioredoxin (Trx) via a CysXXXXCys pair located penultimately to the C-terminal Gly. In this respect, PfTrxR differs significantly from its human counterpart which bears a Cys-Sec redox pair at the same position. PfTrxR is essentially involved in antioxidant defense and redox regulation of the parasite and has been previously validated by knock-out studies as a potential drug target for malaria chemotherapy. Moreover, human TrxR is present in most cancer cells at levels tenfold higher than in normal cells. Here we report the discovery of a series of potent inhibitors of PfTrxR. The three most promising inhibitors, 3 ( IC 50 PfTrxR = 2 μ M and IC 50 hTrxR = 50 μ M ) , 7 ( IC 50 PfTrxR = 2 μ M and IC 50 hTrxR = 140 μ M ) , and 11 ( IC 50 PfTrxR = 0.5 μ M and IC 50 hTrxR = 4 μ M ) were selective for the parasite enzyme. Detailed mechanistic characterization of the effects of these compounds on the PfTrxR-catalyzed reaction showed clear uncompetitive inhibition with respect to both substrate and cofactor. For the most specific PfTrxR inhibitor 7 , an alkylation mechanism study based on a thiol conjugation model was performed. Furthermore, all three compounds were active in the lower micromolar range on the chloroquine-resistant P. falciparum strain K1 in vitro.