Synthesis, Inhibition Potency, Binding Mode, and Antiprotozoal Activities of Fluorescent Inhibitors of Trypanothione Reductase Based on Mepacrine-Conjugated Diaryl Sulfide Scaffolds

Trypanothione reductase (TR) is a flavoenzyme unique to trypanosomatid parasites and a target for lead discovery programs. Various inhibitor scaffolds have emerged in the past, exhibiting moderate affinity for the parasite enzyme. Herein we show that the combination of two structural motifs of known TR inhibitors - diaryl sulfides and mepacrine - enables the simultaneous addressing of two hydrophobic patches in the active site. The binding efficacy of these conjugates is enhanced over that of the respective parent inhibitors. They show K-ic values for the parasite enzyme down to 0.9 +/- 0.1 mu M and exhibit high selectivity for TR over human glutathione reductase (GR). Despite their considerable molecular mass and in some cases permanent positive charges, in vitro studies revealed IC50 values in the low micromolar to sub-micromolar range against Trypanosoma brucei rhodesiense and Trypanosoma cruzi, as well as the malaria parasite Plasmodium falciparum, which lack trypanothione metabolism. The inhibitors exhibit strong fluorescence due to their aminoacridine moiety. This feature allows visualization of the drugs in the parasite where high accumulation was observed by fluorescence microscopy even after short exposure times.