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Differences between ceftizoxime and its stereoisomer in antibacterial activity and affinity for penicillin-binding proteins

A new cephalosporin derivative, ceftizoxime (syn FK 749), and its anti isomer, FR 14060, were compared in antibacterial activity, outer membrane permeability, stability to beta-lactamases, and affinity for penicillin-binding proteins (PBPs), using Escherichia coli NIHJ JC-2 and Enterobacter cloacae 58-5 as the test organisms. Although ceftizoxime was superior in antibacterial activity to FR 14060, no marked differences between the two agents were found in outer membrane permeability and stability to cephalosporinase. However, the affinity for PBPs and stability to penicillinase of ceftizoxime and FR 14060 differed significantly. Concentrations of ceftizoxime required to reduce [14C]penicillin G binding by 50% were below 1 microgram/ml for PBPs 1a and 1bs of E. cloacae 58-5 and below 3.2 microgram/ml for PBPs 1a and 1bs of E. coli NIHJ JC-2. A more than 10-fold-higher concentration of FR 14060 was required for 50% reduction of (14C]penicillin G binding to PBP 1bs of strains tested. Ceftizoxime was severalfold more stable than FR 14060 to penicillinase, but the antibacterial activity of both drugs against penicillinase-producing E. coli was as strong as against non-penicillinase-producing E. coli. These results indicate that the difference between the two compounds in antibacterial activity is likely to be due to differences in their abilities to inhibit peptidoglycan polymerization.