Development of sulphonamide-trimethoprim combinations for urinary tract infections. Part I: Comparison of the antibacterial effect of sulphonamides alone and in combination with trimethoprim.

Plasma half life and in vitro activity were major criteria for selection of sulphonamides which are likely to give a strong synergistic action with trimethoprim in vivo. On the basis of literature data six sulphonamides, sulphadiazine, sulphachloropyridazine, sulphamethoxazole, sulphaisodimidine, sulphamerazine and sulphamethomidine appeared particularly suitable for combination with trimethoprim. An investigation of the activity in vitro of these compounds and their combinations with the latter against clinically isolated, sulphonamide-sensitive Klebsiella-Enterobacter and Escherichia coli strains showed optimal synergy at trimethoprim-sulphonamide ratios between 1:10 and 1:40, but that appreciable mutual potentiation occurred within a rather broad range of concentration ratios. Limited experiments indicated that synergy occurs less frequently and is less pronounced against sulphonamide resistant bacteria. The different sulphonamides behaved rather similarly in their combinations with trimethoprim, and in order to find the best sulphonamide, detailed comparisons of the pharmacokinetic properties of the different combinations are necessary.