PHENOTHIAZINE INHIBITORS OF TRYPANOTHIONE REDUCTASE AS POTENTIAL ANTITRYPANOSOMAL AND ANTILEISHMANIAL DRUGS

Given the role of trypanothione in the redox defenses of pathogenic tr ypanosomal and leishmanial parasites, in contrast to glutathione for t heir mammalian hosts, selective inhibitors of trypanothione reductase are potential drug leads against trypanosomiasis and leishmaniasis. In the present study, the rational drug design approach was used to disc over tricyclic neuroleptic molecular frameworks as lead structures for the development of inhibitors, selective for trypanothione reductase over host glutathione reductase. From a homology-modeled structure for trypanothione reductase, replaced in the later stages of the study by the X-ray coordinates for the enzyme from Crithidia fasciculata, a se ries of inhibitors based on phenothiazine was designed. These were sho wn to be reversible inhibitors of trypanothione reductase from Trypano soma cruzi, Linearly competitive with trypanothione as substrate and n oncompetitive with NAT)PH, consistent with ping-pong hi bi kinetics. A nalogues, synthesized to define structure-activity relationships for t he active site, included N-acylpromazines, 2-substituted phenothiazine s, and trisubstituted promazines. Analysis of K-1 and I-50 data, on th e basis of calculated log P and molar refractivity values, provided ev idence of a specially favored fit of small 2-substituents (especially 2-chloro and 2-trifluoromethyl), with a remote hydrophobic patch on th e enzyme accessible for larger, hydrophobic a-substituents. There was also evidence of an additional hydrophobic enzymic region available to suitable N-substituents of the promazine nucleus, K-1 data also indic ated that the phenothiazine nucleus can adopt, more than one inhibitor y orientation in its binding site. Selected compounds were tested for in vitro activity against Trypanosoma brucei, T. cruzi, and Leishmania donovani, with selective activities in the micromolar range being det ermined for a number of them.