Structure-activity relationships for a class of inhibitors of purine nucleoside phosphorylase

Values of inhibition constants, K-i, for a family of structurally related, competitive inhibitors of calf spleen purine nucleoside phosphorylase (PNP) have been determined employing both inosine as substrate and a manual assay and 2-amino-6-mercapto-7-methylpurine ribonucleoside (MESG) as substrate and a robot-based enzyme kinetics facility. Several of the values determined robotically were confirmed employing the same substrate and a manual assay. Surprisingly, for many of the inhibitors examined, values of K-i determined with MESG as substrate are smaller than those obtained employing inosine as substrate by a factor that varies from less than 2 to 10. Values of concentrations required for 50% inhibition of PNP, IC50, have also been determined for the same family of inhibitors employing inosine as substrate. Values of IC50ino and those for K-i(ino) and K-i(mesg) for subsets of the inhibitors have been employed as training sets to create quantitative structure-activity relationships (QSAR) which have substantial power to predict values of IC50 and K-i for inhibitors outside the training set. These QSAR models should be useful in guiding future medicinal chemistry efforts designed to discover inhibitors of PNP having increased potency.