influence of TEM-1 beta-lactamase on the pharmacodynamic activity of simulated total versus free-drug serum concentrations of cefditoren (400 milligrams) versus amoxicillin-clavulanic acid (2,000/125 milligrams) against Haemophilus influenzae strains exhibiting an N526K mutation in the ftsI gene

Abstract

The aim of this study was to explore bactericidal activity of total and free serum simulated concentrations after the oral administration of cefditoren (400 mg, twice daily [bid]) versus the oral administration of amoxicillin-clavulanic acid extended release formulation (2,000/125 mg bid) against Haemophilus influenzae. A computerized pharmacodynamic simulation was performed, and colony counts and beta-lactamase activity were determined over 48 h. Three strains were used: ampicillin-susceptible, beta-lactamase-negative ampicillin-resistant (BLNAR) (also resistant to amoxicillin-clavulanic acid) and beta-lactamase-positive amoxicillin-clavulanic acid-resistant (BLPACR) strains, with cefditoren MICs of < or =0.12 microg/ml and amoxicillin-clavulanic acid MICs of 2, 8, and 8 microg/ml, respectively. Against the ampicillin-susceptible and BLNAR strains, bactericidal activity (> or =3 log(10) reduction) was obtained from 6 h on with either total and free cefditoren or amoxicillin-clavulanic acid. Against the BLPACR strain, free cefditoren showed bactericidal activity from 8 h on. In amoxicillin-clavulanic acid simulations the increase in colony counts from 4 h on occurred in parallel with the increase in beta-lactamase activity for the BLPACR strain. Since both BLNAR and BLPACR strains exhibited the same MIC, this was due to the significantly lower (P < or = 0.012) amoxicillin concentrations from 4 h on in simulations with beta-lactamase positive versus negative strains, thus decreasing the time above MIC (T>MIC). From a pharmacodynamic point of view, the theoretical amoxicillin T>MIC against strains with elevated ampicillin/amoxicillin-clavulanic acid MICs should be considered with caution since the presence of beta-lactamase inactivates the antibiotic, thus rendering inaccurate theoretical calculations. The experimental bactericidal activity of cefditoren is maintained over the dosing interval regardless of the presence of a mutation in the ftsI gene or beta-lactamase production.

FIG. 1.

Diagram of the in vitro computerized device. 1, Central compartment (spinner flask, tubing, and lumen of capillary); 2, fresh broth reservoir; 3, elimination; 4, peristaltic pumps; 5, syringe pump; 6, PTFE tubing; 7, silicone tubing; 8, infection compartment (extracapillary space); 9, inoculation port; 10, sample port, 11, RS-232 connection; 12, GSIOC connection; 13, temperature probe; 14, air filter; 15, incubator.

FIG. 2.

Target (dotted lines) and experimental (solid lines) profiles in β-lactamase-negative simulations of total (black) and free (gray) concentrations in a 400-mg cefditoren dose simulation and 2,000 mg of amoxicillin and 125 mg of clavulanic acid from the sustained release simulation.

FIG. 3.

Colony counts over 48 h in antibiotic-free simulations (black), simulations with total cefditoren (red solid line), free cefditoren (red dotted line), total amoxicillin-clavulanic acid (blue solid line), and free amoxicillin-clavulanic acid (blue dotted line). The regimens simulated were cefditoren (400 mg bid) and amoxicillin-clavulanic acid (2,000/125 mg bid) sustained release.

FIG. 4.

β-Lactamase activity (absorbance units) of the BLPACR in simulations with total cefditoren (red line), free cefditoren (dotted red line), total amoxicillin-clavulanic acid (blue line), and free amoxicillin-clavulanic acid (dotted blue line) versus antibiotic-free simulations (black line).