Activity of serum concentrations of daptomycin vs. vancomycin against vancomycin-susceptible and resistant Enterococcus faecium in the presence of albumin physiological concentrations: an in vitro pharmacodynamic simulation

Abstract

Killing curves with daptomycin (56 μg/ml, the Cmax after 4 mg/kg) in media containing albumin physiological concentrations showed bactericidal activity against Enterococcus faecium, but due to its high protein binding (91.7%), cidal activity was delayed when compared to that in media without albumin [1]. This study explores the activity over 24 h of simulated serum concentrations of daptomycin (after 6 mg/kg od regimen) vs. vancomycin (1,000 mg bid regimen) against two vancomycin-susceptible but tolerant and one resistant E. faecium in an in vitro pharmacodynamic device using media containing albumin physiological concentrations. A one-compartment dynamic model was used to expose bacteria to changing study drug concentrations [2] using Mueller Hinton broth supplemented with albumin (4 g/dl). In daptomycin simulations, media was calcium-adjusted by adding 100 μg/ml of Ca [3]. Albumin concentrations were determined by the bromcresol green method, ionized calcium concentrations were measured using the AU2700 system (Olympus Diagnostic, Barcelona, Spain), and the total Ca by the Arsenazo III method (BioSystems, Barcelona, Spain). The distribution volume of vancomycin (66.5 ml) and daptomycin (100 ml) were calculated using the expression Vd=Cl/Ke to simulate the half-lives of vancomycin [4] and daptomycin [5]. Pharmacokinetic profiles in serum after 1,000 mg vancomycin [4] bid and 6 mg/kg daptomycin [5] od were simulated over 24 h. The target pharmacokinetic parameters [4, 5] were: Cmax=43.05 μg/ml, Cmin=10.8 μg/ml, and t1/2=6 h for vancomycin; and Cmax=86.4 μg/ml, Cmin=10.7 μg/ml, and t1/2=8 h for daptomycin. The initial inoculum was 8.9×10–1.8×10 cfu/ml. Samples (0.5 ml) were collected at 0, 1, 2, 3, 4, 6, 8, 10, 12, and 24 h, serially diluted in 0.9% sodium chloride, washed in normal saline, vacuum-filtered through 0.45-micron membranes to avoid the carry-over effect, and the filters plated for colony counting (detection limit=50 cfu/ml). Experiments were performed in triplicate. Differences between initial inocula and colony counts at the different timepoints (expressed as % reduction) and the times to obtain 90% (T90%), 99% (T99%), and 99.9% (T99.9%) reductions were determined. Aliquots (0.5 ml) were taken at 1, 2, 3, 4, 6, 8, 10, 12, and 24 h for the measurement of concentrations by bioassay [2] (detection limit: 0.5μg/ml daptomycin, 4μg/ml vancomycin). The total and extrapolated free-drug concentrations (adjusting by 91.7% protein binding of daptomycin and 36.9% of vancomycin) were analyzed by a non-compartmental approach using the WinNonlin Professional program (Pharsight, Mountain View, CA). The area under the concentration–time curve (AUCall) and AUC0–24h/MIC for the total and free drugs were calculated. Changes in viable colony counts at 12 h and 24 h were compared by the two-tailed t-test. The experimental pharmacokinetic parameters were: Cmax=44.5±2.6 μg/ml, Cmin=10.6±1.0 μg/ml, t1/2=6.0±0.5 h and AUCall=286.5±13.1 μg/ml×h for vancomycin; and Cmax=85.7±5.8 μg/ml, Cmin=9.4±1.1 μg/ml, t1/2=8.1±0.2 h and AUCall=852.7±15.8 μg/ml×h for daptomycin. The measured albumin concentrations were 3.9±0.21 g/dl, meaEur J Clin Microbiol Infect Dis (2008) 27:1009–1011 DOI 10.1007/s10096-008-0525-3