Comparative bioavailability of two commercial preparations of carbamazepine tablets

Biowaiver Monograph for Immediate-Release Solid Oral Dosage Forms: Carbamazepine

Literature relevant to assessing whether BCS-based biowaivers can be applied to immediate release (IR) solid oral dosage forms containing carbamazepine as the single active pharmaceutical ingredient are reviewed. Carbamazepine, which is used for the prophylactic therapy of epilepsy, is a non-ionizable drug that cannot be considered "highly soluble" across the range of pH values usually encountered in the upper gastrointestinal tract. Furthermore, evidence in the open literature suggests that carbamazepine is a BCS Class 2 drug. Nevertheless, the oral absolute bioavailability of carbamazepine lies between 70 and 78% and both in vivo and in vitro data support the classification of carbamazepine as a highly permeable drug. Since the therapeutic and toxic plasma level ranges overlap, carbamazepine is considered to have a narrow therapeutic index. For these reasons, a BCS based biowaiver for IR tablets of carbamazepine cannot be recommended. Interestingly, in nine out of ten studies, USP dissolution conditions (900 mL water with 1% SLS, paddle, 75 rpm) appropriately discriminated among bioinequivalent products and this may be a way forward to predicting whether a given formulation will be bioequivalent to the comparator product.

Exposure-response analysis reveals that clinically important toxicity difference can exist between bioequivalent carbamazepine tablets

AIMS

To assess whether, using the current regulatory criteria, therapeutically important differences can exist between bioequivalent carbamazepine (CBZ) tablets. A secondary goal was to demonstrate quantitatively the relationship between the risk of neurological adverse effects to orally ingested CBZ and the rate of absorption.

METHODS

Results of a bioequivalence study by Olling et al. (Biopharm Drug Dispos 1999; 20: 19-28) were reanalysed. Following an exploratory data analysis step, a mixed-effect pharmacokinetic-pharmacodynamic (PK-PD) model was built to describe the dependence of adverse events on the CBZ concentration.

RESULTS

Rapid development of tolerance was demonstrated for most neurological adverse effects, with a characteristic half-life of 02.29 h and an initial EC50 of 2.33 mg l(-1). The resulting tolerance PK-PD model was characterized further using the tools and terminology of sensitivity analysis. It was demonstrated that the maximum concentration (C(max)) exhibits poor PK and PD sensitivities, and that clinically significant differences can exist between formulations which otherwise comply with the bioequivalence requirements. In contrast, another PK metric, the partial AUC, was a much better marker of the early neurological adverse events observable during the absorption phase of the drug.

CONCLUSIONS

In clinical and regulatory considerations, the development of acute tolerance for adverse effects of CBZ must be taken into account. Partial AUC reflects more sensitively the risk of adverse events than C(max). Instead of the current trend of tightening of the bioequivalence criteria for narrow therapeutic index drugs, the use of alternative, more sensitive PK metrics is proposed.

Pathways of carbamazepine bioactivation in vitro I. Characterization of human cytochromes P450 responsible for the formation of 2- and 3-hydroxylated metabolites

In vitro studies were conducted to identify the cytochromes P450 (P450s) involved in the formation of 2- and 3-hydroxycarbamazepine, metabolites that may serve as precursors in the formation of protein-reactive metabolites. Human liver microsomes (HLMs) converted carbamazepine (30-300 microM) to 3-hydroxycarbamazepine at rates >25 times those of 2-hydroxycarbamazepine. Both the 2- and 3-hydroxylation of carbamazepine appeared to conform to monophasic Michaelis-Menten kinetics in HLMs (apparent K(m) values, approximately 1640 and approximately 217 microM; apparent V(max) values, approximately 5.71 and approximately 46.9 pmol/mg of protein/min, respectively). Rates of carbamazepine 2- and 3-hydroxylation correlated strongly with CYP2B6 activity (r >or= 0.757) in a panel of HLMs (n = 8). Carbamazepine 3-hydroxylation also correlated significantly with CYP2C8 activity at a carbamazepine concentration of 30 microM. Formation of 2- and 3-hydroxycarbamazepine did not correlate significantly with any other P450 activities. The chemical inhibitors ketoconazole (CYP3A) and 7-EFC (CYP2B6) inhibited both 2- and 3-hydroxycarbamazepine formation whereas 4-methylpyrazole (CYP2E1) markedly decreased 2-hydroxycarbamazepine formation. Several recombinant P450s catalyzed carbamazepine 2- and 3-hydroxylation, but after adjustment for relative hepatic abundance, CYP3A4 and CYP2B6 appeared to be the major catalysts of carbamazepine 3-hydroxylase activity, and at least five P450s were significant contributors to 2-hydroxycarbamazepine formation; CYP2E1 made the greatest contribution to the Cl(int) of carbamazepine 2-hydroxylation (approximately 30%), but P450s CYP1A2, 2A6, 2B6, and 3A4 also made significant contributions (approximately 13-18%). These results suggest that CYP2B6 and CYP3A4 are largely responsible for the formation of 3-hyrdoxycarbamazepine, whereas multiple P450s (CYP1A2, 2A6, 2B6, 2E1, and 3A4) contributed to 2-hydroxycarbamazepine formation.

In vitro/in vivo correlations of dissolution data of carbamazepine immediate release tablets with pharmacokinetic data obtained in healthy volunteers

The aim of the study was to select a dissolution test method for carbamazepine (CBZ) immediate release tablets, giving the best in vitro/in vivo correlations (IVIVC) and to determine the potential of this method as an estimate for bioequivalence testing. Four 200 mg CBZ products which are sold on the Dutch market, covering the innovator and three generic products, were selected. They had been tested in a randomised, fourway cross-over bioavailability study in healthy volunteers. Their dissolution rate behaviour in vitro was investigated in two dissolution media: (1) 1% sodium lauryl sulphate in water (SLS), in accordance with the United States Pharmacopeia (USP); (2) 0.1 mol/l Hydrochloric acid in water (HC). In the bioavailability study these products had shown no large differences in the extent of absorption (AUC(0-infinity);) but large differences in absorption rate. The products now also showed large differences in dissolution rate in vitro in both dissolution media, the rank order being the same as for the absorption rate. It was concluded that the absorption rate in vivo depends on the dissolution rate in vivo. 'Level C' IVIVC according to the USP were optimised by plotting percentages dissolved on selected time points (D values) or their reciprocals (1/D values), against several pharmacokinetic parameters primarily related to the absorption phase and against AUC(0-infinity). In this way for each IVIVC the optimum D or 1/D value, was calculated. For both media no meaningful IVIVC were obtained with AUC(0-infinity), but favourable IVIVC were obtained with the parameters primarily related to the absorption phase. In the bioavailability study indicated above it was found that, among the pharmacokinetic characteristics primarily related to the absorption phase, C(max) is the most promising in expressing rate of absorption in bioequivalence testing in single dose studies with CBZ immediate release tablets. Consequently, C(max) was selected for expressing rate of absorption. The most favourable IVIVC were obtained with D(20) in SLS versus C(max). From this IVIVC and the requirements for bioequivalence (AUC(0-infinity): 0.8-1.25 and C(max) : 0.75-1.35; 90% confidence interval), a specification for dissolution testing in SLS was calculated as follows: 'after 20 minutes, 34-99% dissolved'. Owing to the fact that the rate of absorption in vivo depends on i.a. the dissolution rate in vivo, it can be concluded that with this specification bioequivalence with respect to both rate of absorption and extent of absorption is ensured. As this specification is comparable with the USP specification: 'not less than 75% dissolved after 1 h', it is concluded that the USP specification is suitable to ensure bioequivalence of CBZ immediate release tablets.

Bioavailability of carbamazepine from four different products and the occurrence of side effects

The relative bioavailability of four different carbamazepine products, showing large differences in in vitro dissolution profiles, was studied in healthy volunteers to correlate the occurrence of side effects with a measure of the rate of absorption in vivo for bioequivalence testing. Two of the three generic products investigated showed bioequivalence with respect to the extent of absorption with Tegretol. In vivo, the differences found in absorption rate were reflected in the occurrence of side effects, especially dizziness. As a measure for the rate of absorption, the partial AUC did not seem to be a good characteristic to test bioequivalence, as the variability is very high and dependent on the AUC taken. The Cmax/AUCpart seems more promising, especially the partial AUC directly after completion of the absorption process. The variability is low in the case of carbamazepine after a single dose. However, as long as no consensus on the use of other metrics and the objective (clinical or quality control aspects) of bioequivalence testing is reached, and no other pharmacokinetic characteristic is validated, Cmax should be the characteristic of choice for the rate of absorption in single-dose studies with carbamazepine products.

Absolute bioavailability of carbamazepine after oral administration of a 2% syrup

Two healthy male volunteers received 10 mg carbamazepine (CBZ) as a 2-h constant-rate intravenous (i.v.) infusion and 100 mg 15N-labeled CBZ as a 2% oral suspension, concomitantly. The two compounds have identical pharmacokinetics. Their respective concentrations in plasma were determined by gas chromatography-mass spectrometry (GC-MS) for 168 h. The comparison of the areas under the plasma curves (AUC) obtained by the two routes of administration, showed the systemic availability of CBZ given as an oral suspension to be equal to its availability when given intravenously. A two-compartment model was estimated: The apparent volume of distribution of CBZ at the steady-state (Vss) was approximately 1 L/kg.

Statistical analysis of bioavailability studies: parametric and nonparametric confidence intervals

For a two-way cross-over design, which appears to be the most common experimental design in bioavailability studies, 95%-confidence limits for expected bioavailability can be obtained by classical analysis of variance (ANOVA). If symmetry of the confidence interval is desired about zero (differences) or unity (ratios) rather than about the corresponding point estimator, Westlake's modification can be used. Two nonparametric methods and their adaptations to bioavailability ratios are reviewed, one based on Wilcoxon's signed rank test (Tukey), and the other on Pitman's permutation test. The necessary assumptions and the merits of these procedures are discussed. The methods are illustrated by an example of a comparative bioavailability study. A FORTRAN program facilitating the procedures is available from the authors upon request.