In Vivo Pharmacodynamic Target Assessment of Eravacycline against Escherichia coli in a Murine Thigh Infection Model

Abstract

Eravacycline is a novel fluorocycline antibiotic with potent activity against a broad range of pathogens, including strains with tetracycline and other drug resistance phenotypes. The goal of the studies was to determine which pharmacokinetic/pharmacodynamic (PK/PD) parameter and magnitude best correlated with efficacy in the murine thigh infection model. Six Escherichia coli isolates were utilized for the studies. MICs were determined using CLSI methods and ranged from 0.125 to 0.25 mg/liter. A neutropenic murine thigh infection model was utilized for all treatment studies. Single-dose plasma pharmacokinetics were determined in mice after administration of 2.5, 5, 10, 20, 40, and 80 mg/kg of body weight. Pharmacokinetic studies exhibited maximum plasma concentration (C_max) values of 0.34 to 2.58 mg/liter, area under the concentration-time curve (AUC) from time zero to infinity (AUC_0-∞) values of 2.44 to 57.6 mg · h/liter, and elimination half-lives of 3.9 to 17.6 h. Dose fractionation studies were performed using total drug doses of 6.25 mg/kg to 100 mg/kg fractionated into 6-, 8-, 12-, or 24-h regimens. Nonlinear regression analysis demonstrated that the 24-h free drug AUC/MIC (fAUC/MIC) was the PK/PD parameter that best correlated with efficacy (R² = 0.80). In subsequent studies, we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC needed for the efficacy of eravacycline varied among pathogens. Mice were treated with 2-fold increasing doses (range, 3.125 to 50 mg/kg) of eravacycline every 12 h. The mean fAUC/MIC magnitudes associated with the net stasis and the 1-log-kill endpoints were 27.97 ± 8.29 and 32.60 ± 10.85, respectively.

Keywords: Escherichia coli; eravacycline; pharmacodynamics.

FIG 1

Single-dose plasma pharmacokinetics of eravacycline. Six different doses that varied by a 2-fold concentration on a milligram-per-kilogram basis were administered to mice by the i.p. route. Groups of three mice were sampled for each time point. Each symbol represents the mean ± SD for three animals. Shown in the legend is the maximum plasma concentration (C_max), the area under the concentration-time curve from time zero to infinity (AUC_0–∞), and the elimination half-life (t_1/2).

FIG 2

In vivo dose fractionation with eravacycline using a neutropenic murine thigh infection model. Each symbol represents the mean and standard deviation for four thighs infected with E. coli ATCC 25922. The error bars represent the standard deviations. The burden of organisms was measured at the start and end of therapy. Five total drug dose levels (in milligrams per kilogram every 24 h) were fractionated into one of four dosing regimens and are shown on the x axis. The y axis represents the change in organism burden from the start of therapy. The dashed horizontal line represents net stasis over the treatment period. Points above the line represent net growth, and points below the line represent net killing (cidal activity). Q 6h, Q 8h, Q 12h, and Q 24h, dosing every 6, 8, 12, and 24 h, respectively.

FIG 3

Impact of pharmacodynamic regression of the in vivo dose fractionation study with eravacycline against E. coli ATCC 25922. Each symbol represents the mean and standard deviation for four thighs. The dose data are expressed as fAUC/MIC (A), fC_max/MIC (B), and the percentage of time that the plasma free drug concentrations exceed the MIC (Free drug time above MIC) (C). R² is the coefficient of determination. Also shown for each PD index is the maximal effect (E_max), the PD index value associated with 50% of the maximal effect (ED₅₀), and the slope of the relationship, or the Hill coefficient (N). The line drawn through the data points is the best-fit line based upon the sigmoid E_max formula. The dashed horizontal line represents net stasis over the treatment period. Points above the line represent net growth, and points below the line represent net killing (cidal activity).

FIG 4

In vivo dose-effect of eravacycline against six E. coli (EC) strains using a neutropenic murine thigh infection model. Each symbol represents the mean and standard deviation for four thighs. Five total drug dose levels were fractionated into a regimen given every 12 h. The burden of organisms was measured at the start and end of therapy. The study period was 24 h. The horizontal dashed line at 0 represents the burden of organisms in the thighs of mice at the start of therapy. Data points below the line represent killing, and points above the line represent growth.

FIG 5

In vivo dose-effect of eravacycline against six E. coli isolates using a neutropenic murine thigh infection model. Eravacycline exposure is expressed as the free drug 24-h AUC/MIC (fAUC/MIC). R² represents the coefficient of determination. The ED₅₀ represents the AUC/MIC associated with 50% of the maximal effect (E_max), and N is the slope of the relationship, or the Hill coefficient. The line drawn through the data points is the best-fit line based upon the sigmoid E_max formula. The dashed line represents the burden at the start of therapy. Points above the line represent net growth, and those below the line represent killing.