Assessing Bioequivalence Using Genomic Data

Abstract

For approval of a generic drug product, the assessment of bioequivalence in drug absorption is usually considered as a surrogate for evaluation of drug efficacy and safety in clinical studies. For some drug products, the United States Food and Drug Administration indicates that the assessment of similarity between dissolution profiles may be used as a surrogate for assessment of bioequivalence. Along this line, we propose assessing bioequivalence using genomic data collected from the same individuals, assuming that there is an established relationship between pharmacokinetic and genomic data. Because there may be a bias in the prediction of pharmacokinetic data using genomic data and the variations in these two types of data are different, we propose to assess bioequivalence based on sensitivity analysis of prediction bias and variation difference within some predetermined limits. Our methods are derived for average, population, and individual bioequivalence.     

Bioequivalence studies: biometrical concepts of alternative designs and pooled analysis.

A bioequivalence study compares the bioavailability between a test and a reference drug product in terms of the rate and extent of drug absorption. Area under the plasma concentration-time curve (AUC) and maximum plasma concentration (Cmax) are the pharmacokinetic parameters that serve as characteristics for the assessment of the extent and rate of absorption, respectively. The experimental design of a bioequivalence study is usually a crossover and rarely a parallel or a paired comparative. The statistical assessment of bioequivalence is based on the 90% confidence interval for the ratio of the test mean to the reference mean for AUC and Cmax The aims of this paper are to: (i) investigate alternative designs to a crossover design for conducting bioequivalence studies; (ii) propose the statistical analysis of different designs for bioequivalence studies on the same products; and (iii) discuss their usefulness for the approval of new generic drug products. For this purpose, three case studies are illustrated and analysed. The first case study concerns the investigation of the merits of a crossover design relative to a parallel group design for highly variable drugs using as an example a bioequivalence study of tamoxifen products. The second case study concerns the pooled statistical analysis of two bioequivalent studies of the same levodopa products. The analyses of the individual studies failed to meet the regulatory criteria for bioequivalence. The one study design was a paired comparative and the other one a crossover. Under some assumptions the crossover design may be considered as a paired comparative and the data from the two studies may be analysed together as a paired comparative design. The third case study concerns the statistical pooled analysis of two bioequivalent studies of the same clodronate products. The one study was a three-period crossover pilot study and it was used to identify the variability of the active substance. Then, this variability was used to determine the number of subjects for the main pivotal study which was a two-period crossover. The pilot study design was converted into a two-period crossover design and the data from the two studies were analysed together as a two-period crossover design. The original data of the studies were modified accordingly.     

Bridging bioequivalence studies

In some new regions, an innovative drug of the original region was not marketed. However, after the patent of the innovative drug is expired, a generic copy of the innovative drug from the original region was introduced and approved for marketing in the new region. Another generic copy manufactured by the local sponsor of the new region is seeking for approval in the new region. Despite unavailability of the innovative drug, the regulatory authority of the new region still wants to approve the local generic copy based on assessment of bioequivalence between the local generic drug and the innovative drug. Following the bridging concept suggested by the ICH E5 guidance, we propose a method to evaluate average bioequivalence between the generic copy of the new region and the innovative drug of the original region using the generic copy of the original region as the bridging reference formulation. Sample size required by the bioequivalence study in the new region is also provided. Numerical examples illustrate the proposed method.     

Bayesian approach to average bioequivalence using Bayes' factor

In recent years, bioavailability studies for assessment of bioequivalence between two or more drug formulations have become very popular in drug development. However the current practice for the assessment of bioequivalence suffers from certain serious drawbacks. For example, sometimes these tests fail to control the consumer's risk. In this article a new methodology based on the application of Bayes' factor for solving the average bioequivalence problem is proposed. We compare our approach with the existing methods by using real data sets from the Food and Drug Administration (FDA). Results are further explored using simulation studies.     

Assessment of cutaneous drug delivery using microdialysis

During the last decade microdialysis has been successfully applied to assess cutaneous drug delivery of numerous substances, indicating the large potential for bioequivalence/bioavailability evaluation of topical formulations. The technique has been shown to be minimally invasive and supply pharmacokinetic information directly in the target organ for cutaneous drug delivery with high temporal resolution without further intervention with the tissue after implantation. However, there are a few challenges that need to be addressed before microdialysis can be regarded as a generally applicable routine technique for cutaneous drug delivery assessments. Firstly, the technique is currently not suitable for sampling of highly lipophilic compounds and, secondly, more studies are desirable for elucidation of the variables associated with the technique to increase reproducibility. The present literature indicates that the condition of the skin at the individual assessment sites is the main variable, but also variables associated with relative recovery, differentiation between the pharmacokinetic parameters (i.e., lag time, distribution, absorption and elimination rate) can influences the reproducibility of the technique. Furthermore, it has been indicated that cutaneous microdialysis in rats may be useful for prediction of dermal pharmacokinetic properties of novel drugs/topical formulations in man.     

Bioequivalence review for drug interchangeability.

To monitor the performance of the approved generic copies of a brand-name drug, we propose some methods in assessing bioequivalence among generic copies and the brand-name drug, and among generic copies themselves, using data from several bioequivalence studies adopting the standard 2 x 2 crossover design without carryover effects. We propose a meta-analysis method that increases statistical power when the between-subject variability is not large. A nonmeta-analysis is also considered. A numerical example of applying both methods is presented for illustration.     

Use of Bayesian Methods for Multivariate Bioequivalence Measures

In this paper, we introduce a Bayesian analysis for bioequivalence data assuming multivariate pharmacokinetic measures. With the introduction of correlation parameters between the pharmacokinetic measures or between the random effects in the bioequivalence models, we observe a good improvement in the bioequivalence results. These results are of great practical interest since they can yield higher accuracy and reliability for the bioequivalence tests, usually assumed by regulatory offices. An example is introduced to illustrate the proposed methodology by comparing the usual univariate bioequivalence methods with multivariate bioequivalence. We also consider some usual existing discrimination Bayesian methods to choose the best model to be used in bioequivalence studies.     

Prediction of Pharmacokinetic Parameters and the Assessment of Their Variability in Bioequivalence Studies by Artificial Neural Networks

Purpose. The methodology of predicting the pharmacokinetic parameters (AUC, c_max, t_max) and the assessment of their variability in bioequivalence studies has been developed with the use of artificial neural networks. Methods. The data sets included results of 3 distinct bioequivalence studies of oral verapamil products, involving a total of 98 subjects and 312 drug applications. The modeling process involved building feedforward/backpropagation neural networks. Models for pharmacokinetic parameter prediction were also used for the assessment of their variability and for detecting the most influential variables for selected pharmacokinetic parameters. Variables of input neurons based on logistic parameters of the bioequivalence study, clinical-biochemical parameters, and the physical examination of individuals. Results. The average absolute prediction errors of the neural networks for AUC, c_max, and t_max prediction were: 30.54%, 39.56% and 30.74%, respectively. A sensitivity analysis demonstrated that for verapamil the three most influential variables assigned to input neurons were: total protein concentration, aspartate aminotransferase (AST) levels, and heart-rate for AUC, AST levels, total proteins and alanine aminotransferase (ALT) levels, for c_max, and the presence of food, blood pressure, and body-frame for t_max. Conclusions. The developed methodology could supply inclusion or exclusion criteria for subjects to be included in bioequivalence studies.     

Meta-analysis for bioequivalence review

The problem of drug interchangeability among a brand-name drug and its generic copies is considered. Under current Food and Drug Administration (FDA) regulation, a patient may switch from the brand-name drug to a generic drug if the generic drug is shown to be bioequivalent to the brand-name drug based on bioequivalence testing. After the patent of a brand-name drug is expired, usually there will be a number of generic copies available on the market. The FDA does not indicate that a patient may switch from a generic to another even though both of the generic drugs are bioequivalent to the brand-name drug. As a result, drug interchangeability among the brand-name and its generic copies is a safety concern. In this paper, we propose to perform a meta-analysis for an overview of bioequivalence. The proposed meta-analysis provides an assessment of bioequivalence among generic copies of a brand-name that can be used as a tool to monitor the performance of the approved generic copies of the brand-name drug. In addition, it provides more accurate estimates of inter- and intrasubject variabilities of the drug product.     

Determining Equivalence for Generic Locally Acting Drug Products

For regulatory purposes, bioequivalence (BE) 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 becomes available at the site of drug action when administered at the same molar dose under similar conditions in an appropriately designed study. The U.S. Food and Drug Administration may accept evidence from various approaches for determining the bioavailability (BA) or BE of a drug product, including in vivo pharmacokinetic studies, certain in vitro studies that are correlated with and predictive of human in vivo BA, and well-controlled clinical endpoint studies. In this article, we describe some of the statistical approaches used in clinical endpoint studies of bioequivalence for generic drugs. We outline a conventional design and statistical analysis for such studies, including the equivalence criteria. We exemplify this approach with a more detailed discussion of studies of nasal spray products for allergic rhinitis, pointing out some of the statistical issues particular to this area. Finally, we describe a new statistical approach to evaluating BE for topical, locally acting dosage forms (e.g., creams, ointments, and gels) studied in vitro using excised human skin, with a specific focus on unique issues in the design and statistical analysis of data from such studies.     

Profile analysis for assessing in vitro bioequivalence

For locally acting drug products such as nasal aerosols and nasal sprays, therapeutic equivalence between two drug products may be established by in vitro bioequivalence studies based on measurements intended to reflect the rate and extent to which the active ingredient becomes available at the site of action. For cascade impaction or multistage liquid impinger for particle size distribution, profile analysis is required. However, we find that the analysis procedure described in the 1999 FDA guidance lacks statistical justification. In this article, we explain why FDA's approach is incorrect and propose a correct statistical method for profile analysis using the basic ideas in the FDA guidance.     

Chiral bioequivalence: effect of absorption rate on racemic etodolac

BACKGROUND: For many racemic drugs, bioequivalence assessment based on isomer-nonspecific assays is appropriate because enantiomeric area under the concentration-time curve (AUC) exposure ratios are close to unity. Use of nonspecific methods in cases in which the ratio is substantially greater or less than 1, however, may obscure real therapeutic differences among formulations, especially if the enantiomers exhibit differing pharmacological potencies. OBJECTIVE: To examine the influence of absorption rate on etodolac bioequivalence as measured by total [(R,S)-] and (S)-etodolac. DESIGN: Single dose, 3-period, crossover, pharmacokinetic study in 24 healthy volunteers in which the administration rate of etodolac was varied. METHODS: Participants received etodolac 400mg in solution, given as a single dose over 1 minute or as divided doses over 30 and 90 minutes. Unresolved and enantiomer concentrations of etodolac were measured by a validated HPLC assay. The enantiomer ratio was similarly measured by HPLC. RESULTS: Bioequivalence parameters derived for both unresolved and (S)etodolac indicate that peak plasma drug concentration (Cmax) was not bioequivalent. By delaying absorption, bioequivalence was lost. CONCLUSIONS: Collectively, these data demonstrate that bioequivalence between 2 products of etodolac based on enantiomerically nonspecific criteria alone may not generalise to the pharmacologically relevant (S)-enantiomer. This suggests that enantiospecific assays are necessary for bioequivalence assessments.     

PROFILE ANALYSIS FOR ASSESSING IN VITRO BIOEQUIVALENCE

For locally acting drug products such as nasal aerosols and nasal sprays, therapeutic equivalence between two drug products may be established by in vitro bioequivalence studies based on measurements intended to reflect the rate and extent to which the active ingredient becomes available at the site of action. For cascade impaction or multistage liquid impinger for particle size distribution, profile analysis is required. However, we find that the analysis procedure described in the 1999 FDA guidance lacks statistical justification. In this article, we explain why FDA's approach is incorrect and propose a correct statistical method for profile analysis using the basic ideas in the FDA guidance.     

Role of stereoselective assays in bioequivalence studies of racemic drugs: have we reached a consensus?

The existence of stereoselectivity in metabolism and drug disposition, coupled with the existence of genetic polymorphisms and modulation of enantiomeric kinetics via special delivery systems, provides some compulsion to assess bioequivalence using stereoselective data. However, examination of the literature suggests that nonstereoselective data are commonly used for the bioequivalence assessment of drug racemates.     

Individual bioequivalence revisited.

For decades, the establishment of bioequivalence has generally relied on the comparison of population averages between the test and reference formulations. In the early 1990s, individual bioequivalence was proposed to ensure that an individual could be switched from the reference product to the test product with unchanged efficacy and safety. Since 1997, the US Food and Drug Administration (FDA) has published three guidance documents on the proposed criterion and statistical methodology for the individual bioequivalence approach. From a scientific stand-point, the individual bioequivalence criterion appears to offer several advantages for some drug products compared with the average criterion. It allows comparison of intraindividual variances, scaling the bioequivalence criterion to the reference variability and detection of an important subject-by-formulation interaction if it exists. Based on these considerations, the FDA has recently recommended replicate study designs for modified release dosage forms and highly variable drug products. The new criterion also promotes inclusion of a heterogeneous population of volunteers in bioequivalence studies. Despite all the advantages of the individual bioequivalence approach, questions remain on the optimal use of replicate study designs and the proposed criterion for evaluation of bioequivalence between formulations. In the finalised guidance documents, therefore, the FDA maintains the average bioequivalence criterion while allowing other criteria under certain circumstances. Collection and analysis of bioequivalence data from replicate study designs may permit further assessment and resolution of these questions.     

Application of a generic physiologically based pharmacokinetic model to the estimation of xenobiotic levels in human plasma.

Estimation of xenobiotic kinetics in humans frequently relies upon extrapolation from experimental data generated in animals. In an accompanying paper, we have presented a unique, generic, physiologically based pharmacokinetic model and described its application to the prediction of rat plasma pharmacokinetics from in vitro data alone. Here we demonstrate the application of the same model, parameterized for human physiology, to the estimation of plasma pharmacokinetics in humans and report a comparative evaluation against some recently published predictive methods that involve scaling from in vivo animal data. The model was parameterized through an optimization process, using a training set of in vivo data taken from the literature, and validated using a separate test set of published in vivo data. On average, the vertical divergence of the predicted plasma concentrations from the observed data, on a semilog concentration-time plot, was 0.47 log unit. For the training set, more than 80% of the predicted values of a standardized measure of the area under the concentration-time curve were within 3-fold of the observed values; over 70% of the test set predictions were within the same margin. Furthermore, in terms of predicting human clearance for the test set, the model was found to match or exceed the performance of three published interspecies scaling methods, all of which showed a distinct bias toward overprediction. We conclude that the generic physiologically based pharmacokinetic model, as a means of integrating readily determined in vitro and/or in silico data, is potentially a powerful, cost-effective tool for predicting human xenobiotic kinetics in drug discovery and risk assessment.     

Analytical methods validation: bioavailability, bioequivalence and pharmacokinetic studies. Conference report.

This is a summary report of the conference on Analytical Methods Validation: Bioavailability, Bioequivalence and Pharmacokinetic Studies. The conference was held from December 3 to 5, 1990 in the Washington, DC area and was sponsored by the American Association of Pharmaceutical Scientists, US Food and Drug Administration, Federation International Pharmaceutique, Health Protection Branch (Canada) and Association of Official Analytical Chemists. The purpose of the report is to represent our assessment of the major agreements and issues discussed at the conference. The report is also intended to provide guiding principles for validation of analytical methods employed in bioavailability, bioequivalence and pharmacokinetic studies in man and animals. The objectives of the conference were: 1. To reach a consensus on what should be required in analytical methods validation and the procedures to establish validation; 2. To determine processes of application of the validation procedures in the bioavailability, bioequivalence and pharmacokinetic studies; 3. To develop a report on analytical methods validation (which may be referred to in developing future formal guidelines). Acceptable standards for documenting and validating analytical methods with regard to processes, parameters or data treatments were discussed because of their importance in assessment of pharmacokinetic, bioavailability and bioequivalence studies. Other topics which were considered essential in the conduct of pharmacokinetic studies or in establishing bioequivalency criteria, including measurement of drug metabolites and stereoselective determinations, were also deliberated.     

Assessment of selection bias in estimates of relative bioavailability and intrasubject variability from bioequivalence evaluations

The outcome selection of bioequivalence evaluations for abbreviated new drug applications results in bias towards unity of test-reference ratio estimates, and underestimation of intrasubject variability for the test drug product. In this study, the selection bias in the estimates of testreference ratio and intrasubject variability as function of the true testreference ratio, intrasubject variability, and sample sizes was derived, and the relationship of the selection bias with the true test-reference ratio, intrasubject variability, and sample sizes was evaluated using the derived functions for the selection bias. It was shown in this study that the selection bias decreases with the true test-reference ratio approaching unity, with decreasing intrasubject variability, or with increasing sample sizes of bioequivalence trials. As demonstrated in this study, the selection bias can reach such high levels that it can result in misleading optimism about interchangeability between the bioinequivalent generic versions of a reference drug product, especially for highly variable drug products. As demonstrated in this study, trial repeating raises the chances for test products of low test-reference ratio to meet the bioequivalence requirements, and as such inflates value of the expected selec tion bias. The a priori knowledge of intrasubject variability for the reference drug product may be used to infer the intensity of outcome selection, and as such to predict selection bias in test-reference ratio estimates for the test drug product.     

Challenges and Opportunities in Achieving Bioequivalence for Fixed-Dose Combination Products

Fixed-dose combination (FDC) products are becoming a popular treatment option because of increased patient compliance and convenience, improved clinical effectiveness, and reduced cost to the patient, among several other reasons. A commonly applied approach for approval of a FDC product is demonstrating bioequivalence between the FDC and co-administration of individual mono-products, provided that there is adequate safety and efficacy data for co-administration of the individual agents. However, achieving bioequivalence between the FDC and individual mono-products can be very challenging, and sometimes not possible since combining multiple active ingredients, especially insoluble molecules, in a single drug product could complicate its biopharmaceutical and pharmacokinetic behavior. In this review, some of the major challenges often encountered while assessing bioequivalence during FDC development will be presented along with discussion of future opportunities to facilitate FDC development and approval.