Multiple-Dose Studies Can Be a More Sensitive Assessment for Bioequivalence Than Single-Dose Studies Abstract and Introduction
Objective: To evaluate the bioequivalence of two enteric-coated formulations of omeprazole, Losec® (reference) and Omepradex® (test). It is hypothesised that formulation differences may be accentuated following multiple-dose administration, and that testing after multiple administration may therefore provide a more sensitive assessment of bioequivalence.Study participants and design: The study comprised two parts: an in vitro dissolution test and an in vivo bioavailability study. The latter was a randomised, two-way crossover comparative study after a single dose and after multiple doses in healthy volunteers. Forty subjects were randomly allocated to receive either test or reference product, once daily in the morning, and blood samples were taken on days 1 and 5. Standard pharmacokinetic analyses were performed, and analysis of variance (ANOVA) was used to compare the log-transformed variables in a model including terms for treatment, subject and period.Results: Although both products meet the formal requirements specified by the United States Pharmacopoeia (USP) for enteric-coated articles, the in vitro dissolution experiments revealed widely differing properties for the two tested products. Less than 10% of the drug content was recovered from the Omepradex® formulation following a pre-exposure to pH 3 or 4, compared with over 90% recovered from the Losec® formulation. These findings were in agreement with the results of the in vivo bioavailability study,which showed that the two products differed in both their rate and extent of absorption after a single dose and following multiple doses. The products failed the bioequivalence test for area under the plasma concentration-time curve (AUC) and maximum plasma drug concentration (Cmax) after a single dose [AUC: test/reference ratio 0.85, 90% confidence interval (0.76-0.95); Cmax: test/reference ratio 0.85, 90% confidence interval (0.75-0.95)], and the difference between the formulations was even more pronounced after multiple doses [AUC: test/reference ratio 0.73, 90% confidence interval (0.65-0.83); Cmax: test/reference ratio 0.71, 90% confidence interval (0.63-0.81)].Conclusions: These data suggest that bioequivalence studies on enteric-coated proton pump inhibitors should include both single- and multiple-dose elements to be fully decisive. The two omeprazole products failed to show bioequivalence, with the observed differences being even more apparent after multiple doses, as postulated. Based on this study, the two products may not be considered either therapeutically equivalent or interchangeable.
Bioequivalence is defined as ‘the absence of a significant difference in the rate and extent to which the active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives become available at the site of drug action when administered at the same molar dose under similar conditions in an appropriately designed study’.[1] Bioequivalence is established by showing that the 90% confidence interval (CI) of the ratio of geometric mean response, usually the area under the plasma concentration-time curve (AUC) and the maximum plasma drug concentration (Cmax), of the two formulations is contained within the limits of 0.8 to 1.25.[2,3] The European Committee for Propriety Medicinal Products (CPMP) recommends that bioequivalence be demonstrated after a single dose. Multiple-dose studies may also be required in the case of dose- or time-dependent pharmacokinetics or when the product has a modified-release profile.[4]
Recent FDA guidance states that single-dose studies are generally more sensitive than multiple-dose studies in assessing release of the drug substance from the drug product into the systemic circulation.[1] This statement is supported by Monte Carlo simulations demonstrating that the probability of failing the bioequivalence test dramatically decreases upon multiple-dose administration, due to the changes in CIs.[5] Consequently, multiple-dose studies are generally not recommended by the FDA for either immediate-release or modified-release products.[1] The present work examines the possibility that a multiple-dose study may be more sensitive than a single-dose study in detecting differences between formulations of a specific drug product, and hence more relevant to bioequivalence assessment.
Omeprazole is a substituted benzimidazole derivative that effectively suppresses gastric acid secretion by inhibiting the H+/K+-ATPase (proton pump) in the parietal cells of the stomach.[6] It is widely used for the treatment of gastric or duodenal ulcers, in the relief of symptoms of gastro-oesophageal reflux disease (GORD), in the healing of erosive oesophagitis and subsequent maintenance treatment, and in the treatment of pathological gastrointestinal hypersecretory conditions. It is also used in combination with antibiotics for eradication therapy of Helicobacter pylori.[7]
Omeprazole has low water solubility and is chemically very unstable in acidic media. This presents a significant challenge for pharmaceutical technologists as the ideal preparation must therefore protect the active ingredient against decomposition by gastric acid, as well as assure an immediate and complete release from the dosage form as soon as it has been transported to the site of absorption in the small intestine. To this end, omeprazole is currently formulated as delayed-release capsules and tablets containing enteric-coated pellets.[7] However, variations in the quality of the enteric coating of different modified-release formulations are thought to have a substantial effect on the in vivo performance of the drug.[8]
The bioavailability of omeprazole has been shown to increase during days 1 to 5 of repeated drug administration,[9,10] after which time both plasma concentrations and the area under the plasma concentration-time curve (AUC) are also greatly increased.[11] This is mainly due to reduced first-pass metabolism during repeated-dose administration.
This study was designed to investigate, in accordance with international regulatory guidelines, the bioequivalence of two omeprazole products following single- and multiple-dose administration, as it was suspected that the differences in the behaviour of the enteric coatings noted in the in vitro test might result in differing bioavailability profiles. Moreover, it was contemplated that any differences would be accentuated following multiple-dose administration.
