Configuration of the methoxyimino group and penetration ability of cefotaxime and its structural analogues

Among the different mechanisms by which bacteria are resistant to beta-lactam antibiotics, three are of major interest: production of beta-lactamase, modifications of the target penicillin-binding proteins (PBPs) and decreased permeability. Cefotaxime (CTX) is a recently synthesized cephalosporin derivative, active against beta-lactamase producing Gram-negative bacteria and possessing in the acylamino side chain a methoxyimino group in the syn-configuration. It has been compared for affinity to PBPs and penetration ability with its isomer possessing the same group in the anti-configuration and the corresponding demethoxyimino derivative. The anti-isomer, although resistant to beta-lactamase, is devoid of antibacterial activity (MIC for Escherichia coli higher than 500 micrograms/ml). The affinities of CTX and its analogues for the PBPs of several strains of E. coli have been determined in vivo and in vitro by competition experiments using intact cells and bacterial envelopes, respectively. Only minor differences in the affinity for the target PBPs were detected in vitro. However, in vivo studies proved that the 50% saturating concentrations for the PBPs were more than 100-fold higher for the anti-isomer than for CTX. The reported results suggest that a very simple structural modification of the CTX molecule greatly decreases the penetration ability of the antibiotic through the outer cell layers, thus dramatically affecting its antibacterial properties.