Dose Optimization of Combined Linezolid and Fosfomycin against Enterococcus by Using an In Vitro Pharmacokinetic/Pharmacodynamic Model

Abstract

The rapid spread of antibiotic resistance among Enterococcus has prompted considerable interest in determining the dosage regimen of linezolid combined with fosfomycin. A checkerboard assay was employed to evaluate whether linezolid combined with fosfomycin had a synergistic effect on Enterococcus isolates from the hospital, including three drug-resistant strains (MIC of linezolid [MIC_LZD], ≥8 mg/L; MIC of fosfomycin [MIC_FOF], ≥256 mg/L). The in vitro static time-kill assay, dynamic pharmacokinetic (PK)/pharmacodynamic (PD) model, and semimechanistic PK/PD model were used to explore and predict effective combined dosage regimens. The checkerboard assay and in vitro static time-kill assay demonstrated that linezolid combined with fosfomycin has a synergistic effect on drug-resistant and sensitive Enterococcus. In the in vitro PK/PD model, the dosage regimen of linezolid (8 mg/L or 12 mg/L, steady-state concentration) combined with fosfomycin (6 g or 8 g) via a 0.5-h infusion every 8 h effectively suppressed bacterial growth at 24 h with a 3 log₁₀ CFU/mL decrease compared with the initial inocula against two resistant and one sensitive Enterococcus isolates. The semimechanistic PK/PD model predicted that linezolid (more than 16 mg/L) combined with fosfomycin (6 g or 10 g) via a 0.5-h infusion every 8 h was required to achieve a 4 log₁₀ CFU/mL decrease at 24 h against Enterococcus isolates (MIC_LZD ≥ 8 mg/L and MIC_FOF ≥ 256 mg/L). According to the prediction of the semimechanical PK/PD model, the effect of the combination was driven by linezolid, with fosfomycin enhancing the effect. Our study is the first to explore the synergistic effects of these two drugs from a qualitative and quantitative perspective and provides a simulation tool for future studies. IMPORTANCE In this study, we found that linezolid combined with fosfomycin could kill Enterococcus in vitro and that the administered dose was significantly lower after the combination treatment, which could reduce adverse effects and the development of drug resistance. The potential mechanism of the two-drug combination against Enterococcus was revealed from a quantitative perspective, which is an important step toward dose optimization in simulated humans. We hope that our research will help build a better relationship between clinicians and patients as we work together to address the challenges of antibiotic resistance in the 21st century.

Keywords: Enterococcus; PK/PD model; combination therapy; fosfomycin; linezolid.

FIG 1

Static time-kill curves show the bactericidal effect of linezolid (orange), fosfomycin (blue), and their combination (green) against Enterococcus isolates (mean ± standard deviation [SD], n = 3). LZD, linezolid; FOF, fosfomycin.

FIG 2

Dynamic in vitro PK/PD killing kinetics of strains ATCC 29212 (A), no. 1 (B), no. 2 (C), and no. 6 (D) in different dosage regimens. Fosfomycin doses of 4, 6, and 8 g were infused for 0.5 h every 8 h.

FIG 3

The values of ΔlogCFU_0-24 for strains ATCC 29212, no. 1, no. 2, and no. 6 after each monotherapy and the combination.

FIG 4

Visual predictive check of each PK/PD model. Solid points represent observed bacterial counts (the values of the same batch were tested three times). Points linked by a line are from the same arm. Different colors represent different dosage regimens. Green lines are the model-predicted 5th, 50th, and 95th percentiles of bacterial counts. From left to right, the four columns of the figure represent bacterial strains no. 1, no. 2, ATCC 29212, and no. 6, respectively. FOF+LZD, fosfomycin combined with linezolid; Control, no drug.

FIG 5

Observed (symbols) and model fitted (lines) viable counts for the dynamic in vitro PK/PD model experiments with fosfomycin or linezolid alone and the combination against Enterococcus strains ATCC 29212 (A), no. 1 (B), no. 2 (C), and no. 6 (D). Fosfomycin doses of 4, 6, and 8 g were infused for 0.5 h every 8 h.

FIG 6

Validation of the PK/PD modeling for the regimen of 12 mg/L linezolid in combination with 8 g fosfomycin every 8 h with a 0.5 h infusion.

FIG 7

Pharmacodynamic predictions of linezolid and fosfomycin mono and combination therapy against Enterococcus strains no. 2 (A), no. 6 (B), and no. 1 (C). The units for linezolid and fosfomycin are milligrams per liter at steady state and grams with a 0.5-h infusion, respectively.