# Evaluation of ceftazidime/avibactam in combination with colistin against KPC-2-producing Klebsiella pneumoniae in static and dynamic time-kill experiments

## TL;DR

This study tests how well ceftazidime/avibactam and colistin work together against a drug-resistant type of Klebsiella pneumoniae in lab experiments.

## Contribution

The study provides new empirical evidence on the synergy of ceftazidime/avibactam and colistin against KPC-2-producing K. pneumoniae.

## Key findings

- Ceftazidime/avibactam and colistin showed synergy in static experiments against 3 of 5 KPC-2-producing K. pneumoniae strains.
- The combination displayed a bactericidal effect in dynamic experiments over 14–32 hours.
- Colistin alone led to resistance development with 64-fold MIC increases.

## Abstract

Ceftazidime/avibactam is used for severe infections caused by carbapenemase-producing Klebsiella pneumoniae. Combination therapy with older antibiotics is frequently used, but the supporting data are limited. This study aimed to evaluate ceftazidime/avibactam in combination with colistin against KPC-2-producing K. pneumoniae.

Five clinical KPC-2-producing K. pneumoniae strains were characterized by phenotypic antibiotic susceptibility testing and whole-genome sequencing. Single antibiotics and combinations were evaluated in 24-h static time-kill experiments with ceftazidime/avibactam concentrations of 0.5× MICratio and colistin at 0.5× and 1× MIC. One strain was subjected to 32-h dynamic time-kill experiments with ceftazidime/avibactam at concentrations mimicking patient pharmacokinetics in plasma and colistin added to 1 mg/L, i.e. the average free steady-state concentration. Population analysis was performed at 0, 16, and 32 h by plating at 4× and 8× MICratio (ceftazidime/avibactam) or MIC (colistin) to assess resistance development.

All strains were susceptible to ceftazidime/avibactam and colistin, had mutations in ompK35 and ompK36, and carried multiple β-lactamase genes. Ceftazidime/avibactam combined with colistin demonstrated 24-h synergy in static time-kill experiments against 3 of 5 strains. Although ceftazidime/avibactam and colistin alone showed rapid initial killing in the dynamic experiments, regrowth occurred after 4–8 h. The three-drug combination displayed a bactericidal effect and synergy at 14–32 h. Resistance development resulting in 64-fold MIC increases was observed in experiments with colistin alone.

This study showed synergy with ceftazidime/avibactam and colistin against KPC-2-producing K. pneumoniae at clinically relevant concentrations. More studies are warranted to investigate the clinical potential of this combination.