Plasma and soft tissue pharmacokinetics of ceftolozane/tazobactam in healthy volunteers after single and multiple intravenous infusion: a microdialysis study

OBJECTIVES

To investigate ceftolozane/tazobactam pharmacokinetics (PK) in plasma and interstitial space fluid (ISF) of muscle and subcutaneous tissue and establish a population PK model.

METHODS

Eight healthy volunteers received four IV doses of 1000/500 mg ceftolozane/tazobactam q8h in a prospective, open-labelled PK study. ISF concentration-time profiles were determined via in vivo microdialysis up to 8 h post-dose and efficacy of unbound ceftolozane and tazobactam was estimated using the time above MIC (%ƒT>MIC) and time above threshold concentration (%T>CT), respectively. A population PK model was established by merging derived plasma and soft tissue PK data.

RESULTS

Ceftolozane reached %ƒT>MIC values of 100% in plasma, muscle and subcutaneous ISF for Enterobacteriaceae and 87%, 89% and 87%, respectively, for Pseudomonas aeruginosa. Tazobactam %T>CT was 21%, 22% and 21% in plasma, muscle and subcutaneous ISF, respectively. Plasma protein binding was 6.3% for ceftolozane and 8.0% for tazobactam. Multiple-dose ceftolozane AUC0-8 ISF/plasma ratios were 0.92 ± 0.17 in muscle and 0.88 ± 0.18 in subcutis, and tazobactam ratios were 0.89 ± 0.25 in muscle and 0.87 ± 0.21 in subcutis, suggesting substantial soft tissue penetration.

CONCLUSIONS

Tazobactam %T>CT values were distinctly below proposed target values, indicating that tazobactam might be underdosed in the investigated drug combination. However, ISF/unbound plasma ratios of ceftolozane and tazobactam support their use in soft tissue infections. A plasma and soft tissue PK model adds important information on the PK profile of ceftolozane/tazobactam. Further investigations in patients suffering from wound infections are needed to confirm these findings.

Target site antimicrobial activity of colistin might be misestimated if tested in conventional growth media

Cation-dependent inhibition of antimicrobial activity was reported for polymyxin antibiotics. Ca(2+) and Mg(2+) concentrations recommended by the Clinical and Laboratory Standards Institute (CLSI) for the supplementation of Müller-Hinton broth (MHB) are markedly lower than interstitial space fluid (ISF) concentrations in vivo. Hence, it was speculated that the antimicrobial activity of colistin might be overestimated if tested using conventional cation-adjusted MHB. The antimicrobial activity of colistin against n = 100 clinical isolates of Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae and Escherichia coli (n = 25 each) was evaluated by broth microdilution and, for selected isolates, by time-kill curves, in MHB without cations (MHB(ONLY)), MHB supplemented with 25 mg/L Ca(2+) and 12.5 mg/L Mg(2+) according to CLSI recommendations (MHB(CLSI)), and in MHB adjusted to 50 mg/L Ca(2+) and 20 mg/L Mg(2+) simulating ISF concentrations (MHB(ISF)). The minimum inhibitory concentration (MIC) values of colistin against the vast majority of isolates of both P. aeruginosa and A. baumannii increased significantly with higher cation concentrations. The susceptibility of K. pneumoniae isolates to colistin did not show significant changes between cation-supplemented media, while the MICs of E. coli decreased with ascending cation concentrations. These findings were confirmed in time-kill studies, where colistin killing against P. aeruginosa 1514 and A. baumannii 1485 declined with increasing cation concentrations. Contrarily, the killing of selected concentrations of colistin against K. pneumoniae 15 and E. coli 16 was enhanced in the presence of increasing cation concentrations. The present data suggest that the clinical antimicrobial activity of colistin might be misestimated in vitro if tested in conventional growth media.