1 Institute for Anaesthesiology, University Medical Centre Sint Radboud, PO Box 9101, 6500 HB Nijmegen; 2 DADA Consultancy, Dennenstraat 109, 6543 JR Nijmegen; 3 Disphar International, Winkelskamp 6, 7522 PZ Hengelo (Gld), The Netherlands
Received 26 October 2001; returned 20 June 2002; revised 30 October 2002; accepted 6 November 2002
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Abstract |
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Keywords: amoxicillin-clavulanate, co-amoxiclav, amoxiclav, pharmacokinetics, variable absorption, AUC, amoxicillin/clavulanate ratio
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Introduction |
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The standard dose of co-amoxiclav (amoxicillin/clavulanic acid) for adults with lower respiratory tract infection has, for many years, been 500/125 mg orally three times a day. However, it is generally recognized that patient compliance improves with a reduced dosing frequency, and co-amoxiclav has now been licensed for use at a dose of 875/125 mg orally twice daily. The efficacy of this twice-daily dosage regimen is supported by pharmacokinetic, microbiological and clinical data.610
The dosage ratio of amoxicillin/clavulanic acid in oral co-amoxiclav formulations has changed since the combination has been in use, from 2:1 initially, to 4:1, and more recently to 7:1. The amoxicillin/clavulanic acid ratio was increased in order to reduce the incidence of side effects of clavulanic acid seen with the 2:1 ratio.11 As the efficacies of all of these combinations appear to be similar, this would suggest that the amount of clavulanic acid necessary to inhibit bacterial ß-lactamases may not be that critical.
The aims of this investigation were to calculate the pharmacokinetic parameters, and to identify parameters that may influence the observed differences in absorption of clavulanic acid and amoxicillin, following administration of four different dosage formulations.
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Materials and methods |
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The evaluation was based on the data from plasma concentrationtime curves obtained from four different open, randomized, two-treatment, two-period, two-sequence, crossover Phase I bioequivalence studies, each involving 36 male subjects. Subjects were treated with any of the following co-amoxiclav formulations, given as a single dose: tablets 250/125, 500/125 and 875/125 mg, or 10 mL of an oral suspension 250/62.5 mg per 5 mL, and data from 144 subjects and 288 drug administrations were available for evaluation. Treatments were separated by a 1 week washout period, and each subject participated in one study only.
The clinical trials were performed by the Gesellschaft für Therapeutische Forschung (GTF, Nürnberg-Heroldsberg, Germany). The study protocols and written volunteer information sheets were granted approval by the Ethics committee of the University of Köln (Köln, Germany) and all volunteers gave written informed consent.
Subjects were not allowed to consume beverages/food containing methylxanthines, grapefruit products and/or alcohol for 24 h before until 24 h after dosing. Smoking was allowed, except for 2 h before until 4 h after dosing.
Trial course
For each study, the subjects were divided randomly into two groups. Randomization was carried out by following the procedure PLAN of the SAS Institute (Cary, NC, USA).
Group 1 was assigned to treatment sequence III (formulation I followed by formulation II). Group 2 was assigned to sequence III. Subjects received each of the following two formulations following an overnight fast.
Study 1. Formulation A1 = single oral dose of one co-amoxiclav 250/125 mg tablet [Losan Pharma, Neuenburg, Germany (287 mg amoxicillin trihydrate and 148.9 mg potassium salt of clavulanic acid)], and formulation B1 = single oral dose of one Augmentin 375 mg film-coated tablet (Beecham Research, UK). Thirty-six healthy Caucasian male volunteers without any co-medication (age 26 ± 5 years, height 182.1 ± 5.8 cm, body weight 78.6 ± 8.4 kg).
Study 2. Formulation A2 = single oral dose of one co-amoxiclav 500/125 mg tablet (Losan Pharma; 574 mg amoxicillin trihydrate and 148.9 mg potassium salt of clavulanic acid), and formulation B2 = single oral dose of one Augmentin 625 mg film-coated tablet (Beecham Research). Thirty-six healthy Caucasian male volunteers without any co-medication (age 26 ± 5 years, height 183.3 ± 6.9 cm, body weight 78.9 ± 8.6 kg).
Study 3. Formulation A3 = one single oral dose of 10 mL co-amoxiclav 250/62.5 mg/5 mL oral suspension, equivalent to 500 mg amoxicillin and 125 mg clavulanic acid (Losan Pharma; 574 mg amoxicillin trihydrate and 148.9 mg potassium salt of clavulanic acid), and formulation B3 = 10 mL suspension of Augmentin 250/62 SF oral suspension (Beecham Research). Thirty-six healthy Caucasian male volunteers without any co-medication (age 28 ± 4 years, height 181.8 ± 7.1 cm, body weight 79.7 ± 8.2 kg).
Study 4. Formulation A4 = single oral dose of one co-amoxiclav 875/125 mg tablet [Cimex AG Pharmaceutika, Liesberg, Switzerland (1004 mg amoxicillin trihydrate and 148.9 mg potassium salt of clavulanic acid)], and formulation B4 = single oral dose of one Augmentin 1 g tablet (SmithKline Beecham, Austria). Thirty-six healthy Caucasian male volunteers without any co-medication (age 28 ± 4 years, height 181.8 ± 7.1 cm, body weight 79.7 ± 8.2 kg).
Drug administration
Before drug administration, subjects fasted for at least 10 h. Fasting was continued until 4 h after dosing. Subjects were free to drink water, low-fat milk, apple juice, diluted orange juice, coffee and tea after the initial 4 h. All subjects received the same standardized low-fat lunch (6 h) and light snack (9 h) after drug administration.
Blood sampling
On the day of dosing, a physician inserted an indwelling Venflon 2 intravenous cannula in a forearm vein of each subject. The cannula was removed after withdrawal of the 12 h post-dosing sample.
Blood samples (5 mL) were collected in heparinized glass tubes just before dosing and at 0.25, 0.50, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 3, 4, 5, 6, 8, 10 and 12 h after dosing. The blood samples were centrifuged at 3280g for 10 min and plasma was stored at 70°C until analysis.
Bioanalysis
Plasma amoxicillin and clavulanic acid concentrations were determined using validated methods such as LC/MS/MS analysis (GTF).12,13 Lower limit of quantification (LOQ) values were, respectively, 20.0 and 50.0 ng/mL.
In brief, mass spectrometric positive ion detection (selected reaction monitoring, SRM) was achieved by precursor [M+H]+product ion for amoxicillin m/z 366
208, and for the internal standard m/z 350
160. For clavulanic acid, the precursor [M+H]+
product ions were m/z 198
108, and for the internal standard m/z 232
140.
The inter-day precision and accuracy (relative error) of the back-calculated calibration standards of amoxicillin ranged from 1.1% to 6.5% and from 5.6% to 4.4%, respectively. For the spiked quality control standards of amoxicillin, the inter-day precision ranged from 3.6% to 9.2% with an inter-day accuracy (relative error) between 4.3% and 6.7%. The intra-day precision and relative error of the amoxicillin assay ranged from 2.1% to 9.1% and between 6.1% and 5.0%.
The inter-day precision and accuracy (relative error) of the back-calculated calibration standards of clavulanic acid ranged from 0.7% to 4.6% and from 3.1% to 6.5%, respectively. For the spiked quality control standards of clavulanic acid, the inter-day precision ranged from 2.7% to 5.4%, with an inter-day accuracy (relative error) between 1.3% and 6.2%. The intra-day precision and relative error of the clavulanic acid assay ranged from 0.7% to 2.6% and between 0.5% and 6.8%.
Pharmacokinetics
Based on the plasma amoxicillin and clavulanic acid concentrations of individual subjects, the following pharmacokinetic parameters were determined by non-compartmental analysis using WinNonlin Professional (Version 2.0, Pharsight Corporation, copyright 19941998, Palo Alto, CA, USA): maximum plasma drug concentration (Cmax, mg/L); time to reach Cmax (Tmax, h); area under the plasma concentrationtime curve until the last measurable concentration (Ct), calculated by the linear trapezoidal method (AUCt, mg·h/L); elimination half-life associated with the terminal slope of a semi-logarithmic concentrationtime curve (t1/2z, h), calculated as ln2/z, where
is the elimination rate constant; means of the individual AUCt data of clavulanic acid were calculated over the intervals of 1 AUCt unit (mg·h/L) and correlated for variation in amoxicillin AUCt and demographic data.
Statistical analysis
Analysis of variance (ANOVA) was carried out, and significance was defined at P 0.05.
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Results |
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Discussion |
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The AUC observed for amoxicillin in this study did not show significant variation either between subjects, based on demographic data, or between formulations, once corrected for the dose. In contrast, high variability was seen between subjects in the AUC of clavulanic acid, with values ranging over a five-fold difference. All subjects in this study were healthy males (with normal renal function), and it is difficult to explain the high variability seen in the clavulanic acid AUC on patient factors. However, similar findings to these have been reported in other studies.15,1820
With the highest dosage of amoxicillin (875 mg) used in this study, the Tmax broadened, which would suggest a rate-limiting step in the absorption process, in line with other studies.21,22 Of interest, in Figure 3 the AUCamox/AUCclav regression curve of amoxicillin 875 mg has a negative slope, which may indicate that the saturable absorption seen for amoxicillin is influenced by the presence of clavulanic acid. This effect is reported to be bigger with higher dosages of amoxicillin, of up to 3 g in humans,21,22 or at even higher doses of 25 mg/kg in cats23 and dogs.24 In contrast, the slopes of the regression lines of the 250/125 and 500/125 mg dosages were all positive, indicating no influence of clavulanic acid administration on the absorption of amoxicillin at these dosages.
In this study, we have been able to show that four different co-amoxiclav formulations each gave a five-fold variation in the absorption, or in the AUCt value, of clavulanic acid for the same 125 mg dose. This would suggest that in some patients only a 20:1 amoxicillin/clavulanic acid AUCt ratio is achieved after the oral dose of 500/125 mg co-amoxiclav, yet they still appear to benefit from the presence of clavulanic acid (as therapy failures are rarely reported). For patients receiving the 875/125 mg dosage, our findings would indicate that the individual AUCt ratio may vary up to 35:1. However, as the clinical efficacy of co-amoxiclav has been maintained for >10 years with an apparent excess of clavulanic acid in fixed-dose combination preparations, it is probable that the absolute amount of clavulanic acid administered to patients is more important than maintaining a minimal plasma concentration ratio of the two agents.2528
In conclusion, the observed variations in the AUCt ratio of amoxicillin/clavulanic acid (210:1) highlight the variable nature of clavulanic acid absorption. However, clinical data would suggest that such variability does not affect the efficacy of co-amoxiclav and that the current dosage ratio of 4:1 may be considered as conservative.
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Footnotes |
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References |
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