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Antimicrobial Agents and Chemotherapy, April 1999, p. 882-889, Vol. 43, No. 4
Center for Research in Anti-Infectives and
Biotechnology, Department of Medical Microbiology and Immunology,
Creighton University School of Medicine, Omaha, Nebraska 68178
Received 10 August 1998/Returned for modification 3 December
1998/Accepted 31 January 1999
Although previous studies have indicated that clavulanate may
induce AmpC expression in isolates of Pseudomonas
aeruginosa, the impact of this inducer activity on the
antibacterial activity of ticarcillin at clinically relevant
concentrations has not been investigated. Therefore, a study was
designed to determine if the inducer activity of clavulanate was
associated with in vitro antagonism of ticarcillin at
pharmacokinetically relevant concentrations. By the disk approximation
methodology, clavulanate induction of AmpC expression was observed with
8 of 10 clinical isolates of P. aeruginosa. Quantitative
studies demonstrated a significant induction of AmpC when
clavulanate-inducible strains were exposed to the peak concentrations
of clavulanate achieved in human serum with the 3.2- and 3.1-g doses of
ticarcillin-clavulanate. In studies with three clavulanate-inducible
strains in an in vitro pharmacodynamic model, antagonism of the
bactericidal effect of ticarcillin was observed in some tests with
regimens simulating a 3.1-g dose of ticarcillin-clavulanate and in all
tests with regimens simulating a 3.2-g dose of ticarcillin-clavulanate.
No antagonism was observed in studies with two clavulanate-noninducible
strains. In contrast to clavulanate, tazobactam failed to induce AmpC
expression in any strains, and the pharmacodynamics of
piperacillin-tazobactam were somewhat enhanced over those of
piperacillin alone against all strains studied. Overall, the data
collected from the pharmacodynamic model suggested that induction per
se was not always associated with reduced killing but that a certain
minimal level of induction by clavulanate was required before
antagonism of the antibacterial activity of its companion drug
occurred. Nevertheless, since clinically relevant concentrations of
clavulanate can antagonize the bactericidal activity of ticarcillin,
the combination of ticarcillin-clavulanate should be avoided when
selecting an antipseudomonal
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Clavulanate Induces Expression of the Pseudomonas
aeruginosa AmpC Cephalosporinase at Physiologically Relevant
Concentrations and Antagonizes the Antibacterial Activity of
Ticarcillin
-lactam for the treatment of P. aeruginosa infections, particularly in immunocompromised patients. For piperacillin-tazobactam, induction is not an issue in the
context of treating this pathogen.
*
Corresponding author. Mailing address: Center for
Research in Anti-Infectives and Biotechnology, Department of Medical
Microbiology and Immunology, Creighton University School of Medicine,
2500 California Plaza, Omaha, NE 68178. Phone: (402) 280-1881. Fax: (402) 280-1225. E-mail: pdlister{at}creighton.edu.
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