In vitro effect of the presence of human albumin or human serum on the bactericidal activity of daptomycin against strains with the main resistance phenotypes in Gram-positives

Objectives Bactericidal activity depends on antibiotic–bacteria couples, resistance phenotype and theoretically on protein binding. This work explores the influence of protein binding on the bactericidal activity of two antibiotics, daptomycin versus vancomycin, that exhibit, respectively, different Cmax (56 versus 25.5 mg/L), protein binding (91.7% versus 36.9%) and thus theoretical free-drug fractions (4.7 versus 16.1 mg/L). Methods The effect of the presence of physiological concentrations of human albumin (4 g/dL) or human serum (90%) on the bactericidal activity of daptomycin was studied against Gram-positive isolates with troublesome resistance phenotypes [multidrug-resistant Streptococcus pneumoniae (MDRSP), methicillin-resistant Staphylococcus aureus (MRSA), heterogeneous vancomycin-intermediate MRSA (MRSA-hVI) and vancomycin-resistant Enterococcus faecium]. Killing curves (final inocula of ∼107 cfu/mL) were performed using daptomycin and vancomycin concentrations similar to the Cmax obtained in serum. Results Daptomycin was rapidly bactericidal (≥3 log10 initial inocula reduction) against S. pneumoniae and S. aureus, regardless of the strain tested or the presence of albumin or human serum (that slightly delayed bactericidal activity). Against vancomycin-susceptible or -resistant enterococci, daptomycin exhibited rapid bactericidal activity, delayed to 8 and 24 h, respectively, by human albumin. Vancomycin exhibited much slower bactericidal activity against MDRSP and methicillin-susceptible or -resistant S. aureus, but was never bactericidal against MRSA-hVI and vancomycin-susceptible or -resistant E. faecium. Conclusions Daptomycin exhibited rapid bactericidal activity against the strains of the three Gram-positive species tested, regardless of resistance phenotype or the presence of physiological concentrations of albumin.