Optimal piperacillin-tazobactam dosing strategies against extended-spectrum-lactamase-producing Enterobacteriaceae

Piperacillin-tazobactam has been proposed as an alternative to carbapenems for the treatment of infections caused by extended-spectrum-lactamase (ESBL)-producing Enterobacteriaceae. However, limited understanding of optimal dosing strategies for this combination may curtail its utility. In this study, we correlated various exposures of piperacillin-tazobactam to efficacy, using a modified pharmacokinetic/pharmacodynamic index. Using a clinical Klebsiella pneumoniae isolate expressing CTX-M-15, piperacillin MIC values were determined with increasing tazobactam concentrations and fitted to a sigmoid inhibitory maximum effect (E max ) model. A hollow-fiber infection model (HFIM) was used to evaluate the efficacy of escalating tazobactam dosing with a fixed piperacillin exposure. Simulated drug concentrations from the HFIM were incorporated in the E max model to determine the percentage of free time above instantaneous MIC (%fTMICi) associated with each experimental exposure. The target %fTMICi associated with growth suppression was prospectively validated using an SHV-12-producing isolate of Escherichia coli and 2 other CTX-M-15-producing K. pneumoniae isolates. Based on our reference isolate, piperacillin-tazobactam exposures of %fTMICi of 55.1% were associated with growth suppression. Despite underlying differences, these findings were consistent with prospective observations in 3 other clinical isolates. Our modeling approach can be applied relatively easily in the clinical setting, and it appeared to be robust in predicting the effectiveness of various piperacillin-tazobactam exposures. This modified pharmacokinetic/pharmacodynamic index could be used to characterize response to other -lactam/-lactamase inhibitor combinations

In Vivo Resistance to Ceftolozane/Tazobactam in Pseudomonas aeruginosa Arising by AmpC- and Non-AmpC-Mediated Pathways

Two pairs of ceftolozane/tazobactam susceptible/resistant P. aeruginosa were isolated from 2 patients after exposure to β-lactams. The genetic basis of ceftolozane/tazobactam resistance was evaluated, and β-lactam-resistant mechanisms were assessed by phenotypic assays. Whole genome sequencing identified mutations in AmpC including the mutation (V213A) and a deletion of 7 amino acids (P210–G216) in the Ω-loop. Phenotypic assays showed that ceftolozane/tazobactam resistance in the strain with AmpCV213A variant was associated with increased β-lactamase hydrolysis activity. On the other hand, the deletion of 7 amino acids in the Ω-loop of AmpC did not display enhanced β-lactamase activity. Resistance to ceftolozane/tazobactam in P. aeruginosa is associated with changes in AmpC; however, the apparent loss of β-lactamase activity in AmpC∆7 suggests that non-AmpC mechanisms could play an important role in resistance to β-lactam/β-lactamase inhibitor combinations