In vitro efficacy of ceftazidime-avibactam in combination with aztreonam, meropenem, and amikacin against carbapenem-resistant Klebsiella pneumoniae strains according to carbapenemase types

Carbapenem resistant Klebsiella pneumoniae (CRKP) isolates are major causative agents of severe nosocomial infections associated with high mortality, representing a significant global health concern due to limited therapeutic options. This study aimed to evaluate the in vitro efficacy of ceftazidime-avibactam (CZA) in combination with aztreonam (AZT), meropenem (MEM), and amikacin (AK), and to investigate the relationship between the observed effects and carbapenemase genotypes, thereby providing guidance for rational combination therapy choices in CRKP infections. Between January and November 2024, 30 CRKP isolates obtained from a tertiary care hospital were included. Identification and routine susceptibility testing were performed using VITEK-2. Minimum inhibitory concentrations (MICs) of CZA, AZT, MEM, and AK were determined by E-test, and combination efficacy was assessed using the E-test cross synergy method. ΣFIC thresholds were defined as follows: ≤0.5 synergy; >0.5-≤1 additive; >1-≤4 indifferent; >4, antagonism. Carbapenemase genes (blaKPC, blaNDM, blaOXA-48, blaVIM, blaIMP) were investigated by polymerase chain reaction (PCR). Statistical analyses were performed with IBM SPSS Statistics v21, using chi-square and/or Fisher's Exact tests, with a significance threshold of p<0,05. CZA-AZT exhibited 100% synergy in all isolates. For CZA-MEM, synergy was 36,7% (11/30), additive 16,6%, and indifferent 46,7%; for CZA-AK, synergy was 20,0%, additive 23,3%, and indifferent 56,7%. None of the isolates showed any signs of antagonism. The distribution of effects differed significantly among the combinations (chi-square=42,90; df=4; p<0,001). At the genotypic level, CZA-MEM synergy was 13,6% (3/22) in NDM positive isolates, while it was 100% (8/8) in NDM negative isolates (Fisher p<0,001). For CZA-AK, synergy was 9,1% (2/22) in NDM positive and 50,0% (4/8) in NDM negative isolates (Fisher p=0,029). In OXA-48 positive isolates, CZA-MEM synergy was 19,0% (4/21), whereas it was 77,8% (7/9) in OXA-48 negative isolates (Fisher p=0,004). No significant association was found between OXA-48 status and CZA-AK synergy (p=1,000). In NDM+OXA-48-positive isolates, CZA-MEM synergy was 5,6% (1/18) compared to 100% (7/7) in isolates without this genotype (Fisher p<0,001). For CZA-AK, synergy was 11,1% (2/18) in NDM+OXA-48-positive isolates versus 40,0% (2/5) in negative ones; this difference was not statistically significant (Fisher p=0,194). In conclusion, when planning combination therapy for high risk CRKP isolates, the findings indicate that CZA-AZT represents a strong candidate for first line consideration. In contrast, the efficacy of CZA-MEM and CZA-AK combinations varies according to the genotypic and phenotypic profiles. These results suggest that incorporating both genotypic and phenotypic resistance characteristics, supported by in vitro combination testing, can guide clinicians toward more rational therapeutic strategies in the management of CRKP infections. Keywords: Klebsiella pneumoniae, ceftazidime-avibactam, aztreonam, meropenem, amikacin, synergy, NDM, OXA-48.