Rational design of selective submicromolar inhibitors of Tritrichomonas foetus hypoxanthine-guanine-xanthine phosphoribosyltransferase

All parasitic protozoa lack the ability to synthesize purine nucleotides de novo, relying instead on purine salvage enzymes for their survival. Hypoxa nthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) from the protozo an parasite Tritrichomonas foetus is a rational target for antiparasitic dr ug design because it is the primary enzyme the parasite uses to salvage pur ine 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 de sign compounds that bind tightly to the purine pocket of HGXPRT. The goal o f the current project was to improve the affinity and selectivity of previo usly identified HGXPRT inhibitor TF1 [4-(3 -nitroanilino)phthalic anhydride ]. A virtual library of substituted 4-phthalimidocarboxanilides was constru cted using methods of structure-based drug design, and was implemented synt hetically on solid support. Compound 20 [(4'-phthalimido)carboxamido-3 -ben zyloxybenzene] was then used as a secondary lead for the second round of co mbinatorial chemistry, producing a number of low-micromolar inhibitors of H GXPRT. One of these compounds, TF2 [(4'-phthalimido) carboxamido-3-(4-bromo benzy loxy)benzene], was further characterized as a competitive inhibitor o f T. foetus HGXPRT with respect to guanine with a K-I of 0.49 mu M and a 30 -fold selectivity over the human HGPRT. TF2 inhibited the growth of culture d T. foetus cells in a concentration-dependent manner with an ED50 of 2.8 m u M, and this inhibitory effect could be reversed by addition of exogenous hypoxanthine. These studies underscore the efficiency of combining structur e-based drug design with combinatorial chemistry to produce effective speci es-specific enzyme inhibitors of medicinal importance.