Effect of protein binding of daptomycin on MIC and antibacterial activity

Abstract

A higher rate of clinical failures in patients treated with daptomycin (2 mg/kg of body weight, given once daily) compared with rates in patients treated with conventional regimens caused early termination of this comparative clinical trial. One explanation for these failures could be that daptomycin is highly protein bound and that the concentration of the unbound active drug is too low for antibacterial activity. To assess this explanation, we studied the binding of daptomycin to proteins by using an ultrafiltration method. pH (7.0 to 7.4), temperature (25 or 37°C), or daily freezing and thawing over 2 months had no effect on binding of daptomycin to proteins. We found that daptomycin was bound to albumin (90%) at 4 g/100 ml. Binding of daptomycin was not concentration dependent (2.5 to 80 ,ug/ml). In human serum samples spiked with daptomycin, average binding was 94% 2.4%. In 6 subjects given an intravenous infusion of daptomycin (3 mg/kg), average binding was 90% + 2.1%. Susceptibility studies showed that a concentration in serum 20 times the unbound concentration was needed to equal the MIC of the total drug. These results indicate that daptomycin is highly bound (90 to 94%) to albumin and that clinical failure to daptomycin can in part be explained by the low concentration of the unbound drug.

Vancomycin-Resistant Enterococci

“…The inoculum effect was small, the agent was effectively bactericidal alone (unlike teicoplanin and vancomycin) (25,261), and there was no crossresistance with vancomycin and teicoplanin. Addition of serum to the medium impaired the in vitro activity (25,157,170,240). Variable but quite encouraging results were obtained in animal models.…”

Daptomycin or teicoplanin in combination with gentamicin for treatment of experimental endocarditis due to a highly glycopeptide-resistant isolate of Enterococcus faecium

“…A similar observation has been reported previously (2) in a rabbit endocarditis caused by a glycopeptide-susceptible Enterococcus faecalis isolate treated with daptomycin at a dose close to that used in our LoD regimen. Furthermore, failures of daptomycin to cure staphylococcal human endocarditis have been reported recently (8,14), suggesting that higher doses should be used to obtain adequate unbound concentrations of daptomycin in serum.…”

Importance of Relating Efficacy Measures to Unbound Drug Concentrations for Anti-Infective Agents

“…First, protein supplements, such as human or bovine serum albumin or human serum, may be added to the growth media to mimic physiological binding conditions in in vitro settings. Although there is no general consensus on the amount of protein to be added, 4 g/dl is typically regarded as the target concentration, as it resembles normal physiological conditions (16,17,20,21,26,27,(118)(119)(120)(121)(122). Selection of the serum concentration is even more difficult, as bacterial growth is often inhibited once the serum content exceeds 70% of the growth medium (16).…”

“…In vitro, the impact of protein binding on antimicrobial activity is often investigated through determination of the MIC, time-kill curves, and cell culture assays (13,16). Numerous in vitro studies have been published, which evaluated the impact of protein binding on antimicrobials (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28), antivirals (29,30), and antifungals (31,32) by using protein supplements and/or serum to mimic in vivo conditions. In the majority of studies, free drug concentrations are not measured directly in the experimental setting, and the extent of protein binding is accounted for by the using binding values reported in the literature.…”