Synergy of an Investigational Glycopeptide, LY333328, with Once-Daily Gentamicin against Vancomycin-Resistant Enterococcus faecium in a Multiple-Dose, In Vitro Pharmacodynamic Model

The pharmacodynamics of an investigational glycopeptide, LY333328 (LY), alone and in combination with gentamicin, against one vancomycin-susceptible and two vancomycin-resistant Enterococcus faecium strains were studied with a multiple-dose, in vitro pharmacodynamic model (PDM). Dose-range data for the PDM studies were obtained from static time-kill curve studies. In PDM experiments conducted over 48 h, peak LY concentrations of 0.1× and 1× the MIC every 24 h and peak gentamicin concentrations of 18 μg/ml every 24 h (Gq24h) and 6 μg/ml every 8 h (Gq8h) were studied alone and in the four possible LY-gentamicin combinations. Compared to either antibiotic alone, LY-gentamicin combination regimens produced significantly higher apparent killing rates (KRs) calculated during the initial 2 h postdosing. The mean KRs for LY or gentamicin alone versus those for the LY-gentamicin combination regimens were 0.35 ± 0.55 log(10) CFU/ml/h (95% confidence interval [CI(95%)], 0 to 0.70) and 1.46 ± 0.71 log(10) CFU/ml/h (CI(95%), 1.01 to 1.91), respectively (P < 0.0001). Bacterial killing at 48 h (BK(48)), which was calculated by subtracting the bacterial counts at 48 h from the initial inoculum, with a negative value indicating net growth, was also significantly greater. The mean BK(48)s were −0.69 ± 0.44 log(10) CFU/ml (CI(95%), −0.41 to −0.97) and 3.72 ± 2.28 log(10) CFU/ml (CI(95%), 2.28 to 5.17) for LY or gentamicin alone versus LY-gentamicin combination regimens, respectively (P < 0.0001). None of the 12 regimens with LY or gentamicin alone but 75% (9 of 12) of the LY-gentamicin combination regimens were bactericidal. Eighty-three percent (10 of 12) of the LY-gentamicin combination regimens also demonstrated synergy. No significant differences between the pharmacodynamics of LY-gentamicin combination regimens containing Gq24h versus those containing Gq8h were detected.