JAC Advance Access originally published online on March 6, 2007
Journal of Antimicrobial Chemotherapy 2007 59(4):652-657; doi:10.1093/jac/dkm015
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Are ß-lactam breakpoints adequate to define non-susceptibility for all Haemophilus influenzae resistance phenotypes from a pharmacodynamic point of view?
1 Microbiology Department, School of Medicine, Universidad Complutense, Avda.Complutense s/n, 28040 Madrid, Spain 2 Scientific Department, Tedec-Meiji Farma SA, Ctra. M-300, Km. 30 500, 28802 Alcalá de Henares, Madrid, Spain
* Corresponding author. Tel/Fax: +34-91-3941511; E-mail: laguilar{at}med.ucm.es
Received 26 September 2006; returned 21 November 2006; revised 24 November 2006; accepted 16 January 2007
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Abstract |
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Objectives: To investigate the bactericidal activity, against Haemophilus influenzae strains exhibiting different resistance phenotypes, of simulated serum concentrations obtained in humans after administration of 400 mg of cefditoren twice daily, 500 mg of cefuroxime twice daily, 875/125 mg of co-amoxiclav twice daily or 875/125 mg of co-amoxiclav three times daily.
Methods: An in vitro computerized pharmacodynamic simulation was carried out and colony counts determined over 24 h. Four H. influenzae strains were used, one ampicillin-susceptible strain (Strain 1) and three ampicillin-resistant strains following CLSI and BSAC breakpoints: one ß-lactamase-positive strain with an MIC of co-amoxiclav of 0.5 mg/L (Strain 2), one ß-lactamase-negative ampicillin-resistant strain (BLNAR; ampicillin MIC = 16 mg/L) (Strain 3) and one ß-lactamase-positive strain with an MIC of co-amoxiclav of 4 mg/L (Strain 4). All strains were susceptible to cefuroxime and co-amoxiclav according to current CLSI breakpoints, but Strains 3 and 4 were resistant according to BSAC breakpoints. All strains exhibited cefditoren MIC 
0.12 mg/L.
Results: Bacterial counts of Strains 1 and 2 were 
6 log10 reduced with all antibiotics tested at 12 and 24 h. Against Strains 3 and 4, log10 reductions at 12 and 24 h were significantly higher for cefditoren versus cefuroxime (P < 0.01) (although both exhibited bactericidal activity, i.e. 3 log10 reduction) and versus the two co-amoxiclav regimens (P < 0.001) (that exhibited negligible initial inocula reductions).
Conclusions: Cefditoren exhibited the highest bactericidal activity maintained over time against ampicillin-resistant H. influenzae, regardless of ß-lactamase production and/or BLNAR phenotype. From the pharmacodynamic perspective, BSAC breakpoints seem more adequate to define or detect BLNAR strains.
Keywords: cefditoren , co-amoxiclav , H. influenzae
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Introduction |
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Despite the use of anti-Haemophilus influenzae b vaccine that has decreased the incidence in invasive disease, H. influenzae remains a key aetiological agent of upper and lower respiratory tract infections in humans (its natural exclusive host). This change in H. influenzae ecology has been accompanied by changes in resistance phenotypes: the ß-lactamase production phenotype that has been related to antibiotic consumption,1,2 the ß-lactamase-negative ampicillin-resistant phenotype (BLNAR) that has appeared and increased worldwide in the last decade3–7 and the ß-lactamase-positive amoxicillin/clavulanate-resistant (BLPACR) phenotype that is nowadays anecdotal.8–10
In Spain, although ß-lactamase production in H. influenzae was found at a constant rate of 
20–25% in successive nationwide surveillance studies (SAUCE Program) carried out in 1996–97,11 1998–9912 and 2001–02,13 the BLNAR phenotype prevalence was 9% and 4.5% in the 1998–99 and 2001–02 surveillances, respectively, and the BLPACR phenotype was not detected in the first surveillance and anecdotal (0.1%) in the other two.
The increase and/or appearance of these new BLNAR and BLPACR phenotypes raises the question of the clinical activity of old and new ß-lactams against them.
Cefditoren is a third-generation oral cephalosporin with an intrinsic activity similar to cefotaxime against penicillin-resistant Streptococcus pneumoniae14 and ampicillin-resistant H. influenzae,15 which has demonstrated clinical and bacteriological efficacy in the treatment of lower respiratory tract infections.16,17
Since efficacy clinical trials will not include enough patients infected by BLNAR or BLPACR strains to reach conclusions, an experimental pharmacodynamic approach mimicking physiological concentrations after standard doses can be used to explore the antibiotic activity and the adequacy of breakpoints predicting bactericidal activity and/or eradication. In this study, a pharmacodynamic simulation in a computerized device was carried out with the new third-generation cephalosporin cefditoren in comparison with two well-known ß-lactams (cefuroxime and co-amoxiclav) against four H. influenzae strains with different ampicillin resistance phenotypes (one susceptible strain, one BLNAR and two ß-lactamase-producing strains) susceptible to study drugs.
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Materials and methods |
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Strains
Four H. influenzae isolates were used throughout this study. MICs of ampicillin were 0.03 mg/L for Strain 1, 4 mg/L for Strain 2, 16 mg/L for Strain 3 (BLNAR phenotype) and 4 mg/L for Strain 4. Strains 2 and 4 were ß-lactamase producers, as tested by the chromogenic cephalosporin method (Nitrocefin; Oxoid Ltd, Basingstoke, UK). Strain 3 (BLNAR phenotype) presented an N526K mutation in the ftsI gene encoding PBP3, determined by PCR amplification and direct sequencing.18
Prior to and after the simulation process, MICs were determined three times by the microdilution method following CLSI (formerly NCCLS) recommendations.19 Modal values were considered.
Cefditoren reference standard was supplied by Tedec-Meiji Farma, Madrid, Spain. Ampicillin, cefuroxime, amoxicillin trihydrate and lithium clavulanate laboratory reference standards were supplied by GlaxoSmithKline (Worthing, UK).
A previously described two-compartment dynamic model was used to expose bacteria to changing study drug concentrations, avoiding the dilution of the bacterial inoculum together with the drug.20 The central compartment consists of a spinner flask, the lumina of the capillaries within the dialyser (FX50 class; Fresenius Medical Care S.A., Barcelona, Spain) and the tubing in-between. The infectious site was represented by the extra-capillary space of the dialyser unit combined with the intra-dialyser circulating tubing. The high surface-area-to-volume ratio of the dialysis unit (>200 cm2/mL) guarantees a rapid equilibration of the concentration of the antimicrobial agent between the two compartments.
Before each experiment, the central compartment was filled with growth medium consisting of Mueller-Hinton broth supplemented with 15 mg/L nicotinamide adenine dinucleotide (NAD), 15 mg/L haemin and 5 mg/mL yeast extract (Difco Laboratories, Detroit, MI, USA). The exponential decay of antibiotic concentrations was achieved by a continuous dilution–elimination process using computerized peristaltic pumps (Masterflex; Cole-Parmer Instrument Co., Chicago, IL, USA) set to simulate half-lives of co-amoxiclav,20 cefditoren21,22 and cefuroxime23 in human serum. In control drug-free simulations, the rate of peristaltic pumps was fixed to 1 mL/min. The flow rates in the peristaltic pumps were controlled using Win Lin software v.2 (Masterflex; Cole-Parmer Instrument Co.). Additional pumps circulated the antimicrobial/medium mixture at a rate of 50 mL/min between the central and peripheral compartments and at 25 mL/min within the extra-capillary space through external tubing. A computer-controlled syringe pump (402 Dilutor Dispenser; Gilson S.A., Villiers-le-Bel, France) allowed the simulation of drug concentrations by infusion of the drug into the central compartment until the maximum concentration achieved in serum (Cmax) was reached. Both compartments were maintained at 37°C during the simulation process.
Pharmacokinetic profiles in serum after oral cefditoren-pivoxil 400 mg twice daily,21,22 cefuroxime-axetil 500 mg twice daily,23,24 co-amoxiclav 875/125 mg twice daily25–28 and co-amoxiclav 875/125 mg three times daily25–28 were simulated over 24 h. The target total drug pharmacokinetic parameters, based on mean values reported in humans, were Cmax = 4.20 mg/L, Tmax (time to obtained Cmax) = 2.75 h and t1/2 (half-life) = 1.55 h for cefditoren,21,22 and Cmax = 7.70 mg/L, Tmax = 3.0 h and t1/2 = 1.10 h for cefuroxime.23 For co-amoxiclav combinations, the half-life and Tmax of clavulanic acid were employed: Tmax = 1.0 h, and t1/2 = 1.1 h25,26; the target Cmax for amoxicillin was 10.30 mg/L25,27,28 and that for clavulanic acid was 2.40 mg/L.25,26
Measurement of antibacterial effect
A bacterial suspension in Mueller-Hinton broth supplemented with 15 mg/L NAD, 15 mg/L haemin and 5 mg/mL yeast extract from an overnight culture in chocolate agar was allowed to grow to a density of 108 cfu/mL, as measured by a UV spectrophotometer (Hitachi U-1100). Sixty millilitres of this inoculum was introduced into the peripheral compartment. Samples (0.5 mL) from the peripheral compartment were collected at 0, 1, 2, 3, 4, 6, 8, 10, 12 and 24 h and serially diluted in 0.9% sodium chloride. At least four dilutions of each sample were spread onto chocolate agar and incubated overnight for colony counting. The limit of detection was 50 cfu/mL. Each experiment was performed in triplicate.
For the measurement of simulated antimicrobial concentrations, additional aliquots (0.5 mL) were taken from the peripheral compartment at 0, 1, 2, 3, 4, 6, 8, 10, 12 and 24 h, and at the time corresponding to Tmax of each antimicrobial. All samples were stored at – 50°C until use. Concentrations were determined in triplicate by bioassay using Morganella morganii ATCC 8076H as indicator organism for cefditoren, Bacillus subtilis ATCC 6633 for cefuroxime, Micrococcus luteus ATCC 9341 for amoxicillin and Klebsiella pneumoniae NCTC 11228 for clavulanic acid concentrations.29 Plates were inoculated with an even lawn of indicator organism and incubated for 18–24 h at 37°C. Standards and dilutions were prepared in the same broth employed in the pharmacokinetic simulation. Intra-day and inter-day coefficients of variation were 2.3% and 8.2%, respectively, for cefditoren at a concentration of 0.75 mg/L, 8% and 1.5%, respectively, for amoxicillin at a concentration of 0.3 mg/L, 3% and 4.2%, respectively, for clavulanic acid at a concentration of 0.75 mg/L and 1.4% and 1%, respectively, for cefuroxime at a concentration of 3 mg/L.
Antimicrobial concentrations were analysed by a non-compartmental approach using the WinNonlin Professional program (Pharsight, Mountain View, CA, USA). Cmax and Tmax were obtained directly from observed data. The AUC was calculated by the trapezoidal rule. The time at which concentrations exceeded the MIC (t > MIC) for each strain was calculated graphically by plotting mean concentrations at each timepoint versus time and expressed as percentage of the dosing interval.
Measurement of ß-lactamase activity
ß-Lactamase activity was measured by modification of a previously described method.30,31 In brief, 0.025 mL of a 500 mg/L solution of nitrocefin was added to 225 µL of samples collected at sampling times for colony counting and incubated for 30 min at 37°C. Afterwards, 0.5 mL of 0.05 M phosphate buffer was added and absorbance at 486 nm (A486) was spectrophotometrically read, using broth without inoculum as baseline absorbance.
Differences in log10 colony counts at each sampling time with respect to initial inocula were calculated. Differences between antibiotics for each strain or between strains for each antibiotic were determined at 12 and 24 h by the Student's t-test. P < 0.01 was considered statistically significant.
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Figure 1 shows target and experimental concentrations of study drugs. Experimental half-life (h), Tmax (h) and Cmax (mg/L) were 1.7 ± 0.2, 3.0 ± 0.0 and 4.1 ± 0.3 for cefditoren, 1.2 ± 0.1, 3.0 ± 0.0 and 7.7 ± 0.2 for cefuroxime, 1.1 ± 0.0, 1.0 ± 0.0 and 10.4 ± 0.4 for amoxicillin and 1.2 ± 0.2, 1.0 ± 0.0 and 2.4 ± 0.5 for clavulanic acid, respectively. AUC0–12 (h x mg/L) was 16.4 ± 1.4 for cefditoren, 22.2 ± 1.1 for cefuroxime, 24.4 ± 1.3 for amoxicillin twice daily and 5.5 ± 1.1 for clavulanic acid twice daily. AUC0–8 was 24.1 ± 1.3 for amoxicillin three times daily and 5.5 ± 1.1 for clavulanic acid three times daily.
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Susceptibilities of strains to study drugs are shown in Table 1. The same MIC values were found prior to and after the simulations were carried out.
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Table 1 shows t > MIC (%), log10 cfu/mL and ß-lactamase production (absorbance units) at selected timepoints (0, 4, 8 and 12 h). Initial inocula in control curves (antibiotic-free simulations) increased to 8.1–8.4 log10 cfu/mL for the four strains at 12 and 24 h with ß-lactamase activity of
0.5 absorbance units for ß-lactamase-producing strains (Strains 2 and 4). No ß-lactamase activity was detected with Strains 1 and 3 all throughout controls and experiments.
Against Strains 1 (ampicillin-susceptible) and 2 (ß-lactamase-producer with ampicillin MIC = 4 mg/L and susceptible to co-amoxiclav), no significant differences (P > 0.05) were found between the four antibiotic regimens with log10 reductions 6 at 12 h and maintained at 24 h, because t > MIC was 49% dosing interval for the four regimens. Bactericidal activity (3 log10 reduction) at 12 and 24 h was obtained with all antibiotic regimens. ß-Lactamase activity was non-detectable at these sampling times for the ß-lactamase-producing strain (Strain 2).
Log10 reductions at 12 and 24 h were significantly higher for cefditoren versus cefuroxime (P < 0.01) and versus the two co-amoxiclav regimens (P < 0.001) against Strain 3 (BLNAR) and Strain 4 (ß-lactamase-producer with ampicillin and co-amoxiclav MIC of 4 mg/L). Bactericidal activity (3 log10 reduction) was obtained at 12 and 24 h with cefditoren and cefuroxime, but not with both co-amoxiclav regimens (reductions of 0.8 log10 and high final colony counts of 7 log10 at 24 h). More than 5 log10 reductions were obtained with t > MIC 100% (cefditoren), whereas variable results were obtained with t > MIC < 35%: low reductions for co-amoxiclav regimens (1.6 log10), but of 3–4 log10 for cefuroxime with only 18% t > MIC values. ß-Lactamase activity of Strain 4 was non-detectable at 12 and 24 h with cefditoren and cefuroxime (antibiotics exhibiting bactericidal activity) and reached at 24 h values similar to those at time 0 with both co-amoxiclav regimens (0.4 ± 0.0 absorbance units, despite non-detectable ß-lactamase activity at 12 h for the three times daily regimen and 0.4 ± 0.0 for the twice daily regimen).
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Discussion |
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Antibiotic activity against H. influenzae needs to be assessed, facing the continuous increase in the BLNAR phenotype and the emergence of the BLPACR phenotype and its potential subsequent dissemination. The evaluation should include the study of the pharmacodynamic profile of oral ß-lactams, in commonly used regimens, to see the adequacy of breakpoints defining susceptibility to validate their use in daily practice.
Amoxicillin and co-amoxiclav resistance of the above-mentioned phenotypes depends on changes in PBP3,32 whereas the ß-lactamase production in the BLPACR phenotype seems similar to that of the co-amoxiclav-susceptible ß-lactamase-producing strains,32 owing to the higher prevalence of the TEM-1 enzyme.
In this study, four strains were used, one ampicillin-susceptible strain and three ampicillin-resistant strains following CLSI33 and BSAC34 breakpoints: one BLNAR strain (ampicillin MIC = 16 mg/L, while BLNAR has been defined by ampicillin MIC values 1 mg/L18 or 2 mg/L13) and two ß-lactamase- positive strains (with an ampicillin MIC = 4 mg/L and co-amoxiclav MIC = 0.5 and 4 mg/L). All strains were susceptible to cefuroxime and co-amoxiclav following current CLSI breakpoints (4 mg/L),33 but Strains 3 and 4 were resistant to the two antibiotics if BSAC breakpoints (2 mg/L) are considered. There are no defined CLSI or BSAC breakpoints for cefditoren, but following the suggested breakpoint of 0.5– 1 mg/L,35–37 all strains can be considered susceptible because they exhibited MIC values 0.12 mg/L.
Despite the susceptibility (following CLSI breakpoints) of the four isolates to study drugs, statistically significant differences were found in the activity of cefditoren versus the other antibiotic regimens tested against Strains 3 and 4, owing to 5 log10 reductions in initial inocula with cefditoren, 3.5–4 log10 with cefuroxime and 0.5–1.5 log10 with co-amoxiclav. Considering these results, BSAC breakpoints seem more adequate from the pharmacodynamic perspective, since these two strains are considered resistant. Similar results with resistant strains (MIC = 5 mg/L) were obtained in a previous study after 12 h exposure to a simulated co-amoxiclav 875/125 mg twice daily regimen.38 Bacterial counts of Strains 1 and 2 (susceptible strains according to both CLSI and BSAC breakpoints) were 6 log10 reduced with all antibiotics tested at 12 and 24 h.
The cut-off of ampicillin concentration defining BLNAR is crucial because mutations of the ftsI gene encoding PBP3 have been found in strains exhibiting ampicillin MICs as low as 1 mg/L,15 which are considered ampicillin-susceptible by CLSI and BSAC criteria. These mutations have also been found in the BLPACR phenotype together with the TEM-1 ß-lactamase.32 The pharmacodynamic adequacy of antibiotic regimens against the BLNAR and BLPACR phenotypes should be re-assessed because of the decreased activity of classical anti-Haemophilus formulations such as amoxicillin regimens (875 mg two or three times daily) with or without clavulanic acid (125 mg two or three times daily). However, these ampicillin-resistant phenotypes exhibited MIC50/MIC90 values as low as 0.015/0.03 for third-generation cephalosporins such as cefditoren or cefotaxime.15
The results of this study show that cefditoren exhibited the highest bactericidal activity maintained over time against ampicillin-resistant H. influenzae, regardless of ß-lactamase production and/or BLNAR phenotype. From the pharmacodynamic perspective, BSAC breakpoints seem more adequate to define or detect BLNAR strains. Detecting these strains may be important in clinical practice because of the possible increasing prevalence of this phenotype and the lower bacteriological responsiveness of these strains to some common antibiotics. Continuous surveillance exploring whether the incidence of these phenotypes is increasing, and monitoring the presence of ftsI mutation in strains with different MIC values, as well as the comparative testing of the pharmacodynamic activity of different antibiotic regimens against these emerging or increasing number of strains reported in the literature should give us a complete picture of the problem and the strategies to counter it.
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P. C. is an employee (Scientific Director) of Tedec-Meiji Farma S.A., Madrid, Spain. Rest of the authors: none to declare.
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Acknowledgements |
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We thank J. García-de-Lomas (Instituto Valenciano de Microbiología; IVAMI) for the supply of the study strains and for sequencing the ftsI genes. We also thank M. Gimeno, L. Valdés and J. E. Martín for their critical review of the manuscript. This study was funded by an unrestricted grant from Tedec-Meiji Farma, S.A., Madrid, Spain and GlaxoSmithKline S.A., Madrid, Spain.
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References |
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