The combination of colistin and doripenem is synergistic against Klebsiella pneumoniae at multiple inocula and suppresses colistin resistance in an in vitro pharmacokinetic/pharmacodynamic model

Abstract

Multidrug-resistant (MDR) Klebsiella pneumoniae may require combination therapy. We systematically investigated bacterial killing with colistin and doripenem mono- and combination therapy against MDR K. pneumoniae and emergence of colistin resistance. A one-compartment in vitro pharmacokinetic/pharmacodynamic model was employed over a 72-h period with two inocula (∼10(6) and ∼10(8) CFU/ml); a colistin-heteroresistant reference strain (ATCC 13883) and three clinical isolates (colistin-susceptible FADDI-KP032 [doripenem resistant], colistin-heteroresistant FADDI-KP033, and colistin-resistant FADDI-KP035) were included. Four combinations utilizing clinically achievable concentrations were investigated. Microbiological responses were examined by determining log changes and population analysis profiles (for emergence of colistin resistance) over 72 h. Against colistin-susceptible and -heteroresistant isolates, combinations of colistin (constant concentration regimens of 0.5 or 2 mg/liter) plus doripenem (steady-state peak concentration [C(max)] of 2.5 or 25 mg/liter over 8 h; half-life, 1.5 h) generally resulted in substantial improvements in bacterial killing at both inocula. Combinations were additive or synergistic against ATCC 13883, FADDI-KP032, and FADDI-KP033 in 9, 9, and 14 of 16 cases (4 combinations at 6, 24, 48, and 72 h) at the 10(6)-CFU/ml inoculum and 14, 11, and 12 of 16 cases at the 10(8)-CFU/ml inoculum, respectively. Combinations at the highest dosage regimens resulted in undetectable bacterial counts at 72 h in 5 of 8 cases (4 isolates at 2 inocula). Emergence of colistin-resistant subpopulations in colistin-susceptible and -heteroresistant isolates was virtually eliminated with combination therapy. Against the colistin-resistant isolate, colistin at 2 mg/liter plus doripenem (C(max), 25 mg/liter) at the low inoculum improved bacterial killing. This investigation provides important information for optimization of colistin-doripenem combinations.

Fig 1

(Left) Time-kill curves with various clinically relevant dosage regimens of colistin (Col) and doripenem (Dor) alone and in combination at an inoculum of ∼10⁶ CFU/ml. (Right) PAPs at baseline and after 72 h of exposure to colistin monotherapy, colistin-doripenem combination therapy, or neither antibiotic (growth control). Doripenem monotherapy regimens were not included in colistin-PAP examination. (A) ATCC 13883 (colistin heteroresistant, doripenem susceptible, MDR); (B) FADDI-KP033 (colistin heteroresistant, doripenem susceptible, MDR); (C) FADDI-KP032 (colistin susceptible, doripenem resistant, MDR); (D) FADDI-KP035 (colistin resistant, doripenem susceptible, MDR). The y axis starts at the limit of detection, and the limit of quantification is indicated by the dashed horizontal line.

Fig 2

(Left) Time-kill curves with various clinically relevant dosage regimens of colistin (Col) and doripenem (Dor) alone and in combination at an inoculum of ∼10⁸ CFU/ml. (Right) PAPs at baseline and after 72 h of exposure to colistin monotherapy, colistin-doripenem combination therapy, or neither antibiotic (growth control). Doripenem monotherapy regimens were not included in colistin-PAPs examination. (A) ATCC 13883 (colistin heteroresistant, doripenem susceptible, MDR); (B) FADDI-KP033 (colistin heteroresistant, doripenem susceptible, MDR); (C) FADDI-KP032 (colistin susceptible, doripenem resistant, MDR); (D) FADDI-KP035 (colistin resistant, doripenem susceptible, MDR). The y axis starts at the limit of detection, and the limit of quantification is indicated by the dashed horizontal line.