Rational Design of Selective Submicromolar Inhibitors of Tritrichomonas foetus Hypoxanthine-Guanine-Xanthine Phosphoribosyltransferase

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Abstract
All parasitic protozoa lack the ability to synthesize purine nucleotides de novo, relying instead on purine salvage enzymes for their survival. Hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) from the protozoan parasite Tritrichomonas foetus is a rational target for antiparasitic drug design because it is the primary enzyme the parasite uses to salvage purine bases from the host. The study presented here is a continuation of our efforts to use the X-ray structure of the T. foetus HGXPRT−GMP complex to design compounds that bind tightly to the purine pocket of HGXPRT. The goal of the current project was to improve the affinity and selectivity of previously identified HGXPRT inhibitor TF1 [4-(3-nitroanilino)phthalic anhydride]. A virtual library of substituted 4-phthalimidocarboxanilides was constructed using methods of structure-based drug design, and was implemented synthetically on solid support. Compound 20 [(4‘-phthalimido)carboxamido-3-benzyloxybenzene] was then used as a secondary lead for the second round of combinatorial chemistry, producing a number of low-micromolar inhibitors of HGXPRT. One of these compounds, TF2 [(4‘-phthalimido)carboxamido-3-(4-bromobenzyloxy)benzene], was further characterized as a competitive inhibitor of T. foetus HGXPRT with respect to guanine with a KI of 0.49 μM and a 30-fold selectivity over the human HGPRT. TF2 inhibited the growth of cultured T. foetus cells in a concentration-dependent manner with an ED50 of 2.8 μM, and this inhibitory effect could be reversed by addition of exogenous hypoxanthine. These studies underscore the efficiency of combining structure-based drug design with combinatorial chemistry to produce effective species-specific enzyme inhibitors of medicinal importance.