Affinity of carumonam for penicillin-binding proteins

Direct labeling experiments with [14C]-carumonam as well as competition binding assays with [14C]-benzylpenicillin and [14C]-ceftriaxone have demonstrated that penicillin-binding protein (PBP) 3 of Escherichia coli, Enterobacter cloacae and Pseudomonas aeruginosa has the highest affinity for carumonam (Ro 17-2301, AMA-1080). PBP 1a is inhibited only at significantly higher concentration whereas other PBPs display poor affinity. The IC50 values for binding to PBP 3 in E. coli and E. cloacae are in the range of the MIC-values (0.1-0.4 micrograms/ml), whereas those for P. aeruginosa are much lower than the MIC obtained in vitro. In accordance with the PBP affinity pattern observed for carumonam, filament formation resulted after exposure to concentrations greater than 1/2 MIC. Although PBP 1 and 2 of Staphylococcus aureus could be titrated with radioactive carumonam, very high concentrations were required and in line with the poor affinity for all four PBPs in S. aureus, this organism is resistant to carumonam.