Two in vitro studies assessed the potential of daptomycin (Cubicin), a newly marketed antibiotic, to affect the cytochrome P450 (CYP450) isoforms in primary cultured human hepatocytes. Both induction and inhibition of isoforms 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4 were evaluated. The highest concentrations of daptomycin used in both the induction and inhibition assays were approximately eight-fold higher than the peak total drug concentration (50-60 microg/mL), or the peak free drug concentration (estimated 5-6 microg/mL), in plasma at the clinical dose regimen of 4 mg/kg qd. Results in primary human hepatocytes indicate that daptomycin, at concentrations up to 400 microg total drug/mL, demonstrated no biologically significant induction of any of the CYP450 isoform activities in comparison with the negative control or known inducers. At daptomycin concentrations up to 40 microg free drug/mL, no biologically significant inhibition of the activities of these CYP450 isoforms was observed as compared with known inhibitors. The human hepatocyte results demonstrate that daptomycin has no effects on hepatic CYP450-mediated drug metabolism and, therefore, suggest that daptomycin is unlikely to show potential for pharmacokinetic interactions with concomitantly administered drugs that are metabolized by CYP450 isoforms.