Partial resistance of E. coli mutants against 2, 4-diamino-5-benzylpyrimidines by interactions with bacterial membrane lipopolysaccharides. Derivation of quantitative structure-binding relationships.

A series of previously synthesized 2,4-diamino-5-benzylpyrimidines, inhibitors of bacterial dihydrofolate reductase (DHFR) showed decreased inhibition of E. coli cultures, despite increased inhibitory activity against DHFR. Preliminary studies using E. coli mutants with different degrees of outer membrane deficiencies suggested that the decrease in activity was partly due to inactivation because of binding to outer membrane constituents. In the present study antibacterial activities of the benzylpyrimidines have been systematically determined as a function of cell membrane defects in E. coli using bacterial growth kinetic techniques. It has been shown that the observed differences in activity were not due to different binding affinities to the target enzyme of the mutants. Lipopolysaccharides have been extracted from the mutants and used in binding studies by ultrafiltration, photometric and NMR techniques. The observed differences in binding affinity to the lipopolysaccharides have been related to the differences in the lipophilic properties and molecular weight of the substituents. Quantitative structure-activity relationships have been derived. The results of the study show the importance of drug-membrane interactions for the rational development of antibacterials.