Clinically relevant plasma concentrations of colistin in combination with imipenem enhance pharmacodynamic activity against multidrug-resistant Pseudomonas aeruginosa at multiple inocula

Abstract

The use of combination antibiotic therapy may be beneficial against rapidly emerging resistance in Pseudomonas aeruginosa. The aim of this study was to systematically investigate in vitro bacterial killing and resistance emergence with colistin alone and in combination with imipenem against multidrug-resistant (MDR) P. aeruginosa. Time-kill studies were conducted over 48 h using 5 clinical isolates and ATCC 27853 at two inocula (~10(6) and ~10(8) CFU/ml); MDR, non-MDR, and colistin-heteroresistant and -resistant strains were included. Nine colistin-imipenem combinations were investigated. Microbiological response was examined by log changes at 6, 24, and 48 h. Colistin combined with imipenem at clinically relevant concentrations increased the levels of killing of MDR and colistin-heteroresistant isolates at both inocula. Substantial improvements in activity with combinations were observed across 48 h with all colistin concentrations at the low inoculum and with colistin at 4× and 16× MIC (or 4 and 32 mg/liter) at the high inoculum. Combinations were additive or synergistic against imipenem-resistant isolates (MICs, 16 and 32 mg/liter) at the 10(6)-CFU inoculum in 9, 11, and 12 of 18 cases (i.e., 9 combinations across 2 isolates) at 6, 24, and 48 h, respectively, and against the same isolates at the 10(8)-CFU inoculum in 11, 7, and 8 cases, respectively. Against a colistin-resistant strain (MIC, 128 mg/liter), combinations were additive or synergistic in 9 and 8 of 9 cases at 24 h at the 10(6)- and 10(8)-CFU inocula, respectively, and in 5 and 7 cases at 48 h. This systematic study provides important information for optimization of colistin-imipenem combinations targeting both colistin-susceptible and colistin-resistant subpopulations.

Fig. 1.

Baseline PAPs of the reference strain and all clinical isolates at an initial inoculum of ∼10⁸ CFU/ml. The y axis starts from the limit of detection, and the LOQ is indicated by the dashed horizontal line.

Fig. 2.

(Left) Representative time-kill curves with various clinically relevant concentrations of colistin (Col) and imipenem (Imi) alone and in combination at an inoculum of ∼10⁶ CFU/ml. (Right) PAPs at baseline (0 h) and after 48 h of exposure to colistin monotherapy, colistin-imipenem combination therapy, or neither antibiotic (control). (A) 19147 n/m (colistin resistant, imipenem susceptible, MDR); (B) 20509 n/m (susceptible to colistin and imipenem, non-MDR); (C) 20891 n/m (colistin susceptible, imipenem resistant, MDR). The y axis starts from the limit of detection, and the LOQ is indicated by the dashed horizontal line.

Fig. 3.

(Left) Representative time-kill curves with various clinically relevant concentrations of colistin (Col) and imipenem (Imi) alone and in combination at an inoculum of ∼10⁸ CFU/ml. (Right) PAPs at baseline (0 h) and after 48 h of exposure to colistin monotherapy, colistin-imipenem combination therapy, or neither antibiotic (control). (A) 19147 n/m (colistin resistant, imipenem susceptible, MDR); (B) 20509 n/m (susceptible to colistin and imipenem, non-MDR); (C) 20891 n/m (colistin susceptible, imipenem resistant, MDR). The y axis starts from the limit of detection, and the LOQ is indicated by the dashed horizontal line.