1. J Antimicrob Chemother. 2017 May 1;72(5):1400-1409. doi: 10.1093/jac/dkw587.

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

Buyck JM(1), Luyckx C(1), Muccioli GG(2)(3), Krause KM(4), Nichols WW(5), Tulkens
PM(1), Van Bambeke F(1).

Author information: 
(1)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain, Brussels, Belgium.
(2)MASSMET Platform, Louvain Drug Research Institute, Université catholique de
Louvain, Brussels, Belgium.
(3)Bioanalysis and Pharmacology of Bioactive Lipids Research Group, Louvain Drug 
Research Institute, Université catholique de Louvain, Brussels, Belgium.
(4)Cerexa Inc., Oakland, CA, USA.
(5)AstraZeneca Pharmaceuticals, Waltham, MA, USA.

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 ( C s : concentration yielding a static effect) and 
maximal relative effect [ E max : 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 C s 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. E
max was systematically below the detection limit (-5 log 10 ) for extracellular
bacteria, but limited to -1.3 log 10 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 β-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 β-lactamases.

DOI: 10.1093/jac/dkw587 
PMID: 28137941