Influence of beta-lactamase inhibitors on the potency of their companion drug with organisms possessing class I enzymes.

The ability of beta-lactamase inhibitors to induce class I beta-lactamases in certain organisms in vitro suggests a potential for antagonism in vivo. Therefore, a study was designed to assess the ability of sulbactam and clavulanate to induce beta-lactamases in two strains each of Enterobacter cloacae, Citrobacter freundii, Serratia marcescens, and Pseudomonas aeruginosa both in vitro and in vivo. Induction in vitro was observed only with S. marcescens and P. aeruginosa and generally only when inhibitor concentrations greater than 2 micrograms/ml were examined. A mouse model of lethal infection, designed to detect in vivo antagonism arising from beta-lactamase induction, was used to determine what effect sulbactam and clavulanate would have on the 50% protective doses (PD50s) of cefoperazone and ticarcillin. Antagonism (a significant increase in the PD50) was observed in only 4 of 32 tests. Three of these involved antagonism of cefoperazone by clavulanate, and one involved antagonism of cefoperazone by sulbactam. In 6 of 32 tests, enhancement of efficacy (a significant decrease in PD50) was observed. In four of these, sulbactam enhanced cefoperazone; in one, sulbactam enhanced ticarcillin; and in one, clavulanate enhanced ticarcillin. Four of the six cases of enhancement occurred when the beta-lactamase inhibitor was given at the time of challenge. None of these positive or negative in vivo effects were predicted by in vitro tests. These data suggest that beta-lactamase inhibitors can influence the in vivo potency of their companion drug in both a beneficial and detrimental fashion against organisms with class I beta-lactamases and that these effects cannot be predicted from in vitro assays.