Pharmacodynamic Hill-parameters (E_max, EC₅₀, H) for both strains at varying human serum fractions (0% to 70%).

<p>Pharmacodynamic Hill-parameters (E_max, EC₅₀, H) for both strains at varying human serum fractions (0% to 70%).</p

Defining the Active Fraction of Daptomycin against Methicillin-Resistant <i>Staphylococcus aureus</i> (MRSA) Using a Pharmacokinetic and Pharmacodynamic Approach

<div><p>Our objective was to study the pharmacodynamics of daptomycin in the presence of varying concentrations of human serum (HS) <i>in vitro</i> to quantify the fraction of daptomycin that is ‘active’. Time kill experiments were performed with daptomycin (0 to 256 mg/L) against two MRSA strains at log-phase growth, in the presence of HS (0%, 10%, 30%, 50%, 70%) combined with Mueller-Hinton broth. Daptomycin ≥ 2 mg/L achieved 99.9% kill within 8 h at all HS concentrations; early killing activity was slightly attenuated at higher HS concentrations. After 1 h, bacterial reduction of USA300 upon exposure to daptomycin 4 mg/L ranged from -3.1 to -0.5 log₁₀CFU/mL in the presence of 0% to 70% HS, respectively. Bactericidal activity was achieved against both strains at daptomycin ≥ 4 mg/L for all fractions of HS exposure. A mechanism-based mathematical model (MBM) was developed to estimate the active daptomycin fraction at each %HS, comprising 3 bacterial subpopulations differing in daptomycin susceptibility. Time-kill data were fit with this MBM with excellent precision (r² >0.95). The active fraction of daptomycin was estimated to range from 34.6% to 25.2% at HS fractions of 10% to 70%, respectively. Despite the reported low unbound fraction of daptomycin, the impact of protein binding on the activity of daptomycin was modest. The active fraction approach can be utilized to design <i>in vitro</i> experiments and to optimize therapeutic regimens of daptomycin in humans.</p></div

Structural mathematical model for bacterial growth and killing by daptomycin showing both states of the susceptible population (intermediate and ‘resistant’ population not shown).

<p>Structural mathematical model for bacterial growth and killing by daptomycin showing both states of the susceptible population (intermediate and ‘resistant’ population not shown).</p

Mechanism based mathematical model parameter descriptions, symbols, units, and estimates (standard error (SE)%), characterizing the pharmacodynamics of daptomycin.

<p>Mechanism based mathematical model parameter descriptions, symbols, units, and estimates (standard error (SE)%), characterizing the pharmacodynamics of daptomycin.</p

Pharmacodynamic relationship between total daptomycin concentration and the log ratio area at each condition of human serum exposure against USA300.

<p>All r² values were >0.97. Similar results were obtained for Mu50.</p

Time kill data (symbols) and model fitted predictions (solid lines) for each condition of human serum exposure for daptomycin against USA300.

<p>Each panel represents increasing v/v ratios of human serum and MHB as follows: 0% human serum (panel A), 10% human serum (panel B), 30% human serum (panel C), 50% human serum (panel D) and 70% human serum (panel E).</p

Observed relationship between each concentration of human serum exposure and model estimated active fraction values.

<p>Observed relationship between each concentration of human serum exposure and model estimated active fraction values.</p