BASIS OF SELECTIVITY OF ANTIBACTERIAL DIAMINOPYRIMIDINES

The basis for the high affinity and selectivity of trimethoprim 4-diam ino-5-(3',4',5'-trimethoxybenzyl)pyrimidine, TMP] and several close st ructural analogues is reviewed. Methoxy group substitution on the benz yl group of 2,4-diaminobenzylpyrimidine markedly affects both Escheric hia coli dihydrofolate reductase (DHFR) K-i values and in vitro antiba cterial activity. TMP is several hundred-fold more potent than the uns ubstituted benzylpyrimidine, and the monomethoxy and dimethoxy analogu es are of intermediate activity. However, equilibrium dissociation con stants determined in the absence of cofactor (NADPH) show that the bin ding of these diaminobenzylpyrimidines in the enzyme-inhibitor binary complex is considerably weaker and does not vary among the compounds. Thus, the TMP binding affinity of E. coli DHFR is increased by NADPH i n the ternary complex, and this increased affinity (cooperativity) var ies with methoxy group substitution. In contrast, mouse DHFR has a wea ker binding affinity for diaminobenzylpyrimidines, and none of the ana logues show strong NADPH cooperative effects. The difference in the ma gnitude of NADPH/TMP cooperativity between bacterial and mammalian DHF R is an important factor in selectivity. The E. coli enzyme binds TMP more avidly in binary complex, and an additional selectivity factor of 30-fold arises from differences in cooperativity. Although the X-ray crystal structures of bacterial and vertebrate DHFR have been studied extensively, no single hypothesis convincingly explains the molecular basis of TMP selectivity. However, information on the three-dimensiona l structure of the enzyme has been used to rationally design novel, hi gh-affinity inhibitors.