Amikacin activity against carbapenem-resistant Klebsiella pneumoniae without phenotypic aminoglycoside resistance: impact of aminoglycoside-modifying enzyme (AME) gene aac(6′)-Ib-cr in vitro and in vivo

Abstract Objectives Aminoglycosides are often combined with β-lactams for the treatment of carbapenem-resistant Enterobacterales, but the impact of aminoglycoside-modifying enzyme (AME) genes on treatment efficacy in phenotypically susceptible isolates remains unclear. We evaluated whether aac(6′)-Ib-cr, the AME gene with recognized activity against amikacin, attenuates amikacin efficacy in KPC-producing Klebsiella pneumoniae. Methods Thirteen clinical isolates of KPC-producing K. pneumoniae without phenotypic aminoglycoside resistance (amikacin MIC ≤4 mg/L; median 2, range 0.5–3 mg/L) were studied. Three were AME-negative; 10 were AME-positive, carrying aac(6′)-Ib-cr, aac(3)-IIa and strA/strB. Amikacin activity was assessed by time-kill assays and a Galleria mellonella survival model, compared with ceftazidime–avibactam. Results In time-kill assays, amikacin killing at 24 h was greater in AME-negative isolates than in AME-positive isolates (mean 4.28 ± 1.30 versus 2.41 ± 2.16 log10 cfu/mL), with more regrowth by 72 h in AME-positive isolates (median 1.94 [0–5.82] versus 0 [0–3.32] log10). In the survival model, amikacin reduced larval mortality versus saline (HR, 0.32; 95% CI, 0.24–0.43; P < 0.001). The interaction between AME-positive status and amikacin (HR, 2.11; 95% CI, 1.54–2.90; P < 0.001) indicated reduced efficacy that persisted after adjustment for amikacin MIC (adjusted HR, 2.09; 95% CI, 1.52–2.86; P < 0.001). Ceftazidime–avibactam reduced mortality (HR, 0.24; 95% CI, 0.18–0.33; P < 0.001) with no significant interaction with AME gene status (HR, 1.03; 95% CI, 0.71–1.48; P = 0.894). Conclusion Carriage of aac(6′)-Ib-cr was associated with reduced amikacin efficacy despite phenotypic susceptibility, independently of amikacin MIC, consistent with a treatment effect modifier rather than a binary resistance determinant. Genotyping of aac(6′)-Ib-cr may help guide aminoglycoside use in carbapenem-resistant Klebsiella pneumoniae infections.