Pharmacodynamics of the new des-f(6)-quinolone garenoxacin in a murine thigh infection model

Abstract

Garenoxacin is a new des-F(6)-quinolone with broad-spectrum activity against both gram-positive cocci and gram-negative bacilli. We used the neutropenic murine thigh infection model to characterize the time course of antimicrobial activity of garenoxacin and determine which pharmacokinetic-pharmacodynamic (PK-PD) parameter best correlated with efficacy. Serum drug levels following three fourfold-escalating single-dose levels of garenoxacin were measured by microbiologic assay. In vivo postantibiotic effects (PAEs) were determined after doses of 16 and 64 mg/kg of body weight. Mice had 10(6.5) to 10(6.7) CFU of Streptococcus pneumoniae strain ATCC 10813 or Staphylococcus aureus strain ATCC 33591 per thigh when they were treated for 24 h with garenoxacin at a dose of 4 to 128 mg/kg/day fractionated for 3-, 6-, 12-, and 24-hour dosing regimens. Nonlinear regression analysis was used to determine which PK-PD parameter best correlated with the measurement of CFU/thigh at 24 h. Pharmacokinetic studies yielded peak/dose values of 0.2 to 0.3, area under the concentration-time curve (AUC)/dose values of 0.1 to 0.5, and half-lives of 0.7 to 1.6 h. Garenoxacin produced in vivo PAEs of 1.4 to 8.2 h with S. pneumoniae ATCC 10813, 7.6 to >12.4 h with S. aureus ATCC 25923, and 0 to 1.5 h with Klebsiella pneumoniae ATCC 43816. The 24-h AUC/MIC ratio was the PK-PD parameter that best correlated with efficacy (R2=71 to 90% for the two organisms compared with 43 to 56% for the peak/MIC ratio and 47 to 75% for percent time above the MIC [% T>MIC]). In subsequent studies we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC ratio needed for efficacy of garenoxacin varied among pathogens (including resistant strains). Mice had 10(5.9) to 10(7.2) CFU of 6 strains of S. aureus (2 methicillin resistant), 11 strains of S. pneumoniae (5 penicillin susceptible, 1 penicillin intermediate, and 5 penicillin resistant, and of the resistant strains, 3 were also ciprofloxacin resistant), and 4 gram-negative strains per thigh when treated for 24 h with 1 to 64 mg of garenoxacin per kg every 12 h. A sigmoid dose-response model was used to estimate the doses (mg/kg/24 h) required to achieve a net bacteriostatic effect over 24 h. MICs ranged from 0.008 to 4 microg/ml. The free drug 24-h AUC/MIC ratios for each static dose (2.8 to 128 mg/kg/day) varied from 8.2 to 145. The mean 24-h AUC/MIC ratios +/- standard deviations for S. pneumoniae, S. aureus, and gram-negative strains were 33 +/- 18, 81 +/- 37, and 33 +/- 30, respectively. Methicillin, penicillin, or ciprofloxacin resistance did not alter the magnitude of the AUC/MIC ratio required for efficacy.

FIG. 1.

Serum garenoxacin concentrations after administration of single doses of 16, 64, and 256 mg/kg of body weight in infected neutropenic mice. Each symbol represents the mean ± standard deviation of the levels in the sera of three mice. t_1/2, serum elimination half-life in hours; C_max, peak serum level; AUC, serum area under the concentration time curve.

FIG. 2.

In vivo PAE of garenoxacin after administration of single doses of 16 and 64 mg/kg against S. pneumoniae ATCC 10813, S. aureus ATCC 6538p, and K. pneumoniae ATCC 43816. Each symbol represents the mean ± standard deviation for two mice. The widths of hollow bars represent the duration of time total serum levels exceeded the MIC for the infecting pathogen. The widths of the solid bars represent the duration of time free drug serum levels exceeded the MIC for the infecting pathogen.

FIG. 3.

(a) The relationship between the garenoxacin dosing interval and efficacy against S. pneumoniae ATCC 10813 in a murine thigh infection model. Each symbol represents the mean data per mouse from two thighs. (b) The relationship between the garenoxacin dosing interval and efficacy against S. aureus ATCC 33591 in a murine thigh infection model. Each symbol represents the mean data per mouse from two thighs.

FIG. 4.

The relationships of the garenoxacin free drug 24-h AUC/MIC ratio, the percentage of the dosing interval that levels in serum remained above the MIC, and the peak/MIC ratio for S. pneumoniae ATCC 10813 with the log₁₀ CFU/thigh after 24 h of therapy. Each symbol represents the mean data per mouse from two thighs. R², coefficient of determination.

FIG. 5.

The relationships of the garenoxacin free drug 24-h AUC/MIC ratio, the percentage of the dosing interval that levels in serum remained above the MIC, and the peak/MIC ratio for S. aureus ATCC 33591 with the log₁₀ CFU/thigh after 24 h of therapy. Each symbol represents the mean data per mouse from two thighs. R², coefficient of determination.

FIG. 6.

The relationship between the garenoxacin free drug 24 h AUC/MIC and efficacy against 11 S. pneumoniae, 5 S. aureus, and 4 gram-negative bacilli. Each symbol represents the mean data per mouse from two thighs. R², coefficient of determination.