Pharmacodynamics of ceftazidime/avibactam against extracellular and intracellular forms of Pseudomonas aeruginosa

Buyck, Julien [FUCAM] Luyckx, C. [UCL] Muccioli, Giulio [UCL] Krause, K.M. Nichols, W. W. Tulkens, Paul M. [UCL] Van Bambeke, Françoise [UCL]

Objectives: When tested in broth, avibactam reverses ceftazidime resistance in many Pseudomonas aeruginosa that express ESBLs. We examined whether similar reversal is observed against intracellular forms of P. aeruginosa. Methods: Strains: reference strains; two engineered strains with basal non-inducible expression of AmpC and their isogenic mutants with stably derepressed AmpC; and clinical isolates with complete, partial or no resistance to reversion with avibactam. Pharmacodynamic model: 24 h concentration-response to ceftazidime [0.01-200 mg/L alone or with avibactam (4 mg/L)] of bacteria in broth or bacteria phagocytosed by THP-1 monocytes, with calculation of ceftazidime relative potency ( Cs : concentration yielding a static effect) and maximal relative effect [ Emax : cfu decrease at infinitely large antibiotic concentrations (efficacy in the model)] using the Hill equation. Cellular content of avibactam: quantification by LC-MS/MS. Results: For both extracellular and intracellular bacteria, ceftazidime Cs was always close to its MIC. For ceftazidime-resistant strains, avibactam addition shifted ceftazidime C s to values close to the MIC of the combination in broth. Emax was systematically below the detection limit (-5 log 10 ) for extracellular bacteria, but limited to -1.3 log10 for intracellular bacteria (except for two isolates) with no effect of avibactam. The cellular concentration of avibactam reflected extracellular concentration and was not influenced by ceftazidime (0-160 mg/L). Conclusions: The potential for avibactam to inhibit beta-lactamases does not differ for extracellular and intracellular forms of P. aeruginosa , denoting an unhindered access to its target in both situations. The loss of maximal relative efficacy of ceftazidime against intracellular P. aeruginosa was unrelated to resistance via avibactam-inhibitable beta-lactamases.