Evaluation of Pharmacokinetic/Pharmacodynamic Model-Based Optimized Combination Regimens against Multidrug-Resistant Pseudomonas aeruginosa in a Murine Thigh Infection Model by Using Humanized Dosing Schemes

Abstract

We previously optimized imipenem and tobramycin combination regimens against a double-resistant clinical Pseudomonas aeruginosa isolate by using in vitro infection models, mechanism-based pharmacokinetic/pharmacodynamic modeling (MBM), and Monte Carlo simulations. The current study aimed to evaluate these regimens in a neutropenic murine thigh infection model and to characterize the time course of bacterial killing and regrowth via MBM. We studied monotherapies and combinations of imipenem with tobramycin in vivo against the double-resistant clinical P. aeruginosa isolate by using humanized dosing schemes. Viable count profiles of total and resistant populations were quantified over 24 h. Tobramycin monotherapy (7 mg/kg every 24 h [q24h] as a 0.5-h infusion) was ineffective. Imipenem monotherapies (continuous infusion of 4 or 5 g/day with a 1-g loading dose) yielded 2.47 or 2.57 log₁₀ CFU/thigh killing at 6 h. At 24 h, imipenem at 4 g/day led to regrowth up to the initial inoculum (4.79 ± 0.26 log₁₀ CFU/thigh), whereas imipenem at 5 g/day displayed 1.75 log₁₀ killing versus the initial inoculum. The combinations (i.e., imipenem at 4 or 5 g/day plus tobramycin) provided a clear benefit, with bacterial killing of ≥2.51 or ≥1.50 log₁₀ CFU/thigh compared to the respective most active monotherapy at 24 h. No colonies were detected on 3×MIC agar plates for combinations, whereas increased resistance (at 3×MIC) emerged for monotherapies (except imipenem at 5 g/day). MBM suggested that tobramycin considerably enhanced the imipenem target site concentration up to 2.6-fold. The combination regimens, rationally optimized via a translational modeling approach, demonstrated substantially enhanced bacterial killing and suppression of regrowth in vivo against a double-resistant isolate and are therefore promising for future clinical evaluation.

Keywords: imipenem; mathematical modeling; neutropenic thigh infection model; population pharmacokinetics and pharmacodynamics; tobramycin.

FIG 1

Observed (markers) and population-fitted (lines) viable counts for optimized imipenem (IPM)-plus-tobramycin (TOB) dosage regimens in a mouse thigh infection model with FADDI-PA088 (A1 and B1). The changes in log₁₀ CFU/thigh at 24 h relative to the start of treatment are also presented for all regimens (A2 and B2). Each symbol and bar represents the mean ± the standard deviation of four thighs from two mice.

FIG 2

The 24-h viable counts of total (on drug-free plates) and resistant (on imipenem [IPM]- and tobramycin [TOB]-containing [3×MIC] agar plates) subpopulations for each treatment regimen. Each bar represents the mean ± the standard deviation of four thighs from two mice. Panel D contains two bars for tobramycin, reflecting experiments 1 and 2.

FIG 3

The structure of the mechanism-based model of bacterial growth and killing by imipenem (IPM) and tobramycin (TOB) in monotherapies and optimized combination regimens. The first population, i.e., IPM^s/TOB^r, was susceptible to imipenem and resistant to tobramycin. The second population, i.e., IPMⁱ/TOB^r (imipenem intermediate and tobramycin resistant), is not shown. A life cycle growth model (63, 64) was utilized to describe the underlying biology of bacterial replication via two states for each of the two populations. The maximum killing rate constants (K_max) and the associated antibiotic concentrations (KC₅₀s) causing 50% of K_max are explained in Table 1. The permeabilizing effect of tobramycin on the bacterial outer membrane (i.e., tobramycin enhancing the target site penetration of imipenem) was applied to both populations. The parameters describing the outer membrane effect (I_max,OM,TOB and IC_50,OM,TOB) are explained in Table 1.

FIG 4

The outer membrane effect of tobramycin (i.e., tobramycin enhancing the target site penetration of imipenem) resulted in a decrease in the imipenem concentration required for half-maximal bacterial killing (KC_50,IPM) of both bacterial populations. The decrease in the KC_50,IPM for both populations (SR in panel A and IR in panel B) was estimated up to the maximal clinically achievable plasma unbound tobramycin concentrations following a dose of 7 mg/kg q24h administered as a 0.5-h infusion. Parameters are explained in Table 1.