Virtual screening of combinatorial libraries across a gene family: in search of inhibitors of Giardia lamblia guanine phosphoribosyltransferase

Parasitic protozoa lack the ability to synthesize purine nucleotides de nov o, relying instead on purine salvage enzymes for their survival. Guanine ph osphoribosyltransferase (GPRT) from the protozoan parasite Giardia lamblia is a potential target for rational antiparasitic drug design, based on the experimental evidence, which indicates the lack of interconversion between adenine and guanine nucleotide pools. The present study is a continuation o f our efforts to use three-dimensional structures of parasitic phosphoribos yltransferases (PRTs) to design novel antiparasitic agents. Two micromolar phthalimide-based GPRT inhibitors were identified by screening the in-house phthalimide library. A combination of structure-based scaffold selection u sing virtual library screening across the PRT gene family and solid phase l ibrary synthesis led to identification of smaller (molecular weight, < 300) ligands with moderate to low specificity for GPRT; the best inhibitors, GP 3 and GP5, had K-i values in the 23 to 25 muM range. These results represen t significant progress toward the goal of designing potent inhibitors of pu rine salvage in Giardia parasites. As a second step in this process, alteri ng the phthalimide moiety to optimize interactions In the guanine-binding p ocket of GPRT is expected to lead to compounds with promising activity agai nst G. lamblia PRT.