Use of an Additional Hydrophobic Binding Site, the Z Site, in the Rational Drug Design of a New Class of Stronger Trypanothione Reductase Inhibitor, Quaternary Alkylammonium Phenothiazines
Improved rationally designed lead drug structures against African trypanosomiasis, Chagas disease, and leishmaniasis were obtained against trypanothione reductase from Trypanosoma cruzi. Substituted-benzyl [3-(2-chloro-4a,10a-dihydrophenothiazin-10-yl)propyl]dimethylammonium salts, synthesized by Menschutkin quaternization of the tertiary alkylamine ω-nitrogen atom of chlorpromazine, were linear, competitive inhibitors of recombinant trypanothione reductase from T. cruzi, with either trypanothione disulfide or N-benzyloxycarbonyl-Lcysteinylglycyl 3-dimethylaminopropylamide disulfide as substrate. The permanent positive charge on the distal nitrogen atom of the tricyclic side chain contribution to binding was estimated as g5.6 kcal‚mol-1 by comparison with the analogue with the cationic nitrogen atom of the quaternary replaced by an ether oxygen atom. A further major contribution to improving Ki values and inhibition strength was the hydrophobic natures and structures of the N-benzyl substituents. The strongest inhibitor, the [3-(2-chloro-4a,10a-dihydrophenothiazin-10-yl)propyl]- (3,4-dichlorobenzyl)dimethylammonium derivative (Ki 0.12 µM), was ∼2 orders of magnitude more inhibitory than the parent chlorpromazine. Several of these quaternary phenothiazines completely inhibited T. brucei parasite growth in vitro at < 1 µM). Although active against Leishmania donovani, none of the analogues showed major improvement in this activity relative to chlorpromazine or other nonquaternized phenothiazines. The p-tert-butylbenzyl-quaternized analogue very strongly inhibited (ED50 < 1 µM) growth of the amastigote stage of T. cruzi.
Parveen S, Khan MOF, Austin SE, et al. Antitrypanosomal, antileishmanial, and antimalarial activities of quaternary arylalkylammonium 2-amino-4-chlorophenyl phenyl sulfides, a new class of trypanothione reductase inhibitor, and of N-acyl derivatives of 2-amino-4-chlorophenyl phenyl sulfide. J Med Chem 2005;48:8087-97.