In-vitro evaluation of beta-lactamase inhibition by latamoxef and imipenem

Experimental application of the median-effect principle for in vitro quantification of the combined inhibitory activities of clavulanic acid and imipenem against IRT-4 beta-lactamase

Inhibitor-resistant TEM beta-lactamases (IRT) have been identified in Enterobacteriaceae which, however, remained susceptible to cephalosporins. We evaluated the combined inhibitory activity of clavulanic acid and imipenem at ratios of 1:1 and 1:3 against IRT-4, using the median effect principle of Chou and Talalay. The combination of the two drugs, which produced a nearly additive effect, meant their concentrations could be lowered 1.3-4.9-fold, while maintaining a 50% inhibitory effect against the IRT-4 in comparison with each drug alone. From a therapeutic point of view, such a combination is not efficient but this method of Chou and Talalay, used for the first time to assay combined inhibitory activity of beta-lactamase inhibitors, could be used to evaluate new molecules and/or strategies to inactivate beta-lactamase.

In vitro antibacterial activity and mechanism of action of J-111,225, a novel 1beta-methylcarbapenem, against transferable IMP-1 metallo-beta-lactamase producers

IMP-1 beta-lactamase, a class B zinc metallo-enzyme encoded by the transferable bla(IMP) gene, is known to confer high-level resistance to carbapenems as well as to penicillins and cephalosporins. J-111, 225 is a novel 1beta-methylcarbapenem with a structurally unique side chain comprising a trans-3,5-disubstituted pyrrolidinylthio moiety at the C2 position. It inhibited 17 Serratia marcescens and two Pseudomonas aeruginosa IMP-1-producing clinical isolates at a concentration of 32 mg/L (range 4-32 mg/L). It showed synergy with imipenem against IMP-1-producing S. marcescens BB5886 and P. aeruginosa GN17203 with minimal FIC indices of 0.38 and 0.5, respectively. J-111,225 was more resistant than imipenem to hydrolysis by class B metallo-beta-lactamases. In kinetic studies, J-111,225 inhibited the IMP-I enzyme with a K(i) of 0.18 microM when imipenem was used as a substrate. In contrast, J-111,225 was the substrate for hydrolysis by other class B beta-lactamases such as Bacteroides fragilis CcrA, Stenotrophomonas maltophilia L1 and Bacillus cereus type II enzyme with respective K(m) values of 11, 10 and 148 microM. The greater antibacterial activity of J-111,225 against IMP-1-producing bacteria may result from its unique interaction with the beta-lactamase.