Synergistic killing of multidrug-resistant Pseudomonas aeruginosa at multiple inocula by colistin combined with doripenem in an in vitro pharmacokinetic/pharmacodynamic model

Abstract

Combination therapy may be required for multidrug-resistant (MDR) Pseudomonas aeruginosa. The aim of this study was to systematically investigate bacterial killing and emergence of colistin resistance with colistin and doripenem combinations against MDR P. aeruginosa. Studies were conducted in a one-compartment in vitro pharmacokinetic/pharmacodynamic model for 96 h at two inocula (~10(6) and ~10(8) CFU/ml) against a colistin-heteroresistant reference strain (ATCC 27853) and a colistin-resistant MDR clinical isolate (19147 n/m). Four combinations utilizing clinically achievable concentrations were investigated. Microbiological response was examined by log changes and population analysis profiles. Colistin (constant concentrations of 0.5 or 2 mg/liter) plus doripenem (peaks of 2.5 or 25 mg/liter every 8 h; half-life, 1.5 h) substantially increased bacterial killing against both strains at the low inoculum, while combinations containing colistin at 2 mg/liter increased activity against ATCC 27853 at the high inoculum; only colistin at 0.5 mg/liter plus doripenem at 2.5 mg/liter failed to improve activity against 19147 n/m at the high inoculum. Combinations were additive or synergistic against ATCC 27853 in 16 and 11 of 20 cases (4 combinations across 5 sample points) at the 10(6)- and 10(8)-CFU/ml inocula, respectively; the corresponding values for 19147 n/m were 16 and 9. Combinations containing doripenem at 25 mg/liter resulted in eradication of 19147 n/m at the low inoculum and substantial reductions in regrowth (including to below the limit of detection at ∼50 h) at the high inoculum. Emergence of colistin-resistant subpopulations of ATCC 27853 was substantially reduced and delayed with combination therapy. This investigation provides important information for optimization of colistin-doripenem combinations.

Fig. 1.

Targeted doripenem (Dor) PK profiles for 25- and 50-mg/liter 8-hourly regimens with measured Dor concentrations.

Fig. 2.

Time-kill curves for colistin monotherapy (A and D), doripenem monotherapy (B and E), and combination therapy (C and F) against ATCC 27853 at the 10⁶-CFU/ml inoculum (left panels) and the 10⁸-CFU/ml inoculum (right panels). The y axis starts from the limit of detection, and the limit of quantification (LOQ) is indicated by the horizontal broken line.

Fig. 3.

Time-kill curves for colistin and doripenem monotherapy (A and C) and combination therapy (B and D) against 19147 n/m at the 10⁶-CFU/ml inoculum (left panels) and the 10⁸-CFU/ml inoculum (right panels). The y axis starts from the limit of detection, and the limit of quantification (LOQ) is indicated by the horizontal broken line.

Fig. 4.

PAPs of ATCC 27853 with colistin monotherapy, colistin-plus-doripenem combination therapy, or neither antibiotic (control) at the 10⁶-CFU/ml inoculum (left panels) and the 10⁸-CFU/ml inoculum (right panels) at 24 h (A and B), 72 h (C and D), and 96 h (E and F). Baseline (0-h) PAPs are shown in all panels. Colonies growing on ≥4 mg/liter colistin are considered resistant. The y axis starts from the limit of detection, and the limit of quantification (LOQ) is indicated by the horizontal broken line.