RATIONAL DESIGN OF SELECTIVE LIGANDS FOR TRYPANOTHIONE REDUCTASE FROMTRYPANOSOMA-CRUZI - STRUCTURAL EFFECTS ON THE INHIBITION BY DIBENZAZEPINES BASED ON IMIPRAMINE

Trypanothione reductase, the enzyme which in trypanosomal and leishman ial parasites catalyses the reduction of trypanothione disulphide to t he redox-protective dithiol and has been identified as a potential tar get for rational antiparasite drug design, has been found to be strong ly inhibited by tricyclic compounds containing the saturated dibenzaze pine (imipramine) nucleus, with K-i values in the low micromolar range . This drug lead structure was designed by molecular graphics analysis of a three-dimensional homology model, focussing on the active-site, Inhibition studies were carried out to determine the effect of inhibit or structure on the inhibitory strength towards recombinant trypanothi one reductase from Trypanosoma cruzi. Hansch analysis showed that inhi bitory strength depended on terms in pi, pi(2) and sigma(m) indicating dependence on both lipophilicity and inductive effect for ring-substi tuted analogues of imipramine. The side-chain omega-aminoalkyl chain h ad to be longer than 2-carbon units for inhibition. The effect on inhi bition strength of the substituent at the omega-amino position on the side-chain of the central ring nitrogen atom depended markedly on the detailed substitution pattern of the rest of the molecule. This provid es kinetic evidence studies of multiple binding modes within a single, blanket binding site for the inhibitor with the tricyclic ring system in the general region of the hydrophobic pocket lined by Trp21, Tyr11 0, Met113 and Phe114. This aspect of the structural sensitivity of the precise active-site triangulation adopted by the inhibitor is probabl y a function of the use of hydrophobic interactions of low directional specificity in this pocket combined with an electrostatic anchoring b y the omega-N+HMe2 function of the inhibitor, presumably with a glutam ate side-chain, such as Glu-18, Glu-466' and/or Glu-467'.