Bioequivalence Studies to Support Abbreviated New Drug Applications

Key Considerations for Generic Drug Development

Generic drugs play a significant role in modern healthcare by providing safe, effective, and lower cost alternatives to their corresponding brand-name drugs. Indeed, the profile of generic drugs has only increased in recent years, as public and political pressure across the ideological spectrum has mounted to help combat what some consider to be out-of-control drug prices. In response, FDA has made facilitating the development and approval of new generic drugs one of their highest stated priorities. One way that FDA has encouraged the timely approval of generic drugs is by reducing the burden of information required for the marketing application. Instead of a full New Drug Application (NDA) as required for most new drugs, an abbreviated application, known as an ANDA, is used for generic drugs.

What is Bioequivalence?

Perhaps the most critical requirement for an ANDA submission is that an applicant must demonstrate that the proposed generic drug product is bioequivalent to the Reference Listed Drug (RLD) or Reference Product. The definition of bioequivalence is contained within the Food, Drug, and Cosmetic Act (FD&C), with FDA’s current thinking on bioequivalence studies provided in a guidance entitled “Bioequivalence Studies with Pharmacokinetic Endpoints for Drugs Submitted Under an ANDA.” According to Section 505(j)(8)(B)(i) in the FD&C Act, a generic drug is bioequivalent to the listed drug if the rate and extent of absorption of the generic drug do not show a significant difference from the rate and extent of absorption of the listed drug when administered at the same molar dose under similar experimental conditions in either a single dose or in multiple doses.

Key Considerations for Bioequivalence Studies

For most products, the focus of bioequivalence studies is on the release of the drug substance from the drug product into the systemic circulation and specifically, on how the resulting systemic exposure profile of the test drug product compares to that of the RLD. Below, we discuss five considerations for a successful bioequivalence study.

1. Appropriate Study Design

For any drug, appropriate study design is critical to a successful development program and the ultimate approvability of the product. There are a myriad of study designs that can be employed depending upon the goals of the study, but in general, controlled studies may be categorized as either parallel (non-crossover) or crossover. In a parallel bioequivalence study, subjects are divided into two groups (A and B) and receive only the treatment assigned to their group (either A or B; i.e., the RLD or the generic drug). This stands in contrast to the crossover study, in which one group of patients receives treatment A followed later by treatment B and the other group receives treatment B followed later by treatment A. Among the advantages of crossover studies is that they typically require fewer subjects than parallel studies and offer increased statistical power. Within the basket of crossover studies, a study may be classified as either nonreplicate or replicate. Nonreplicate designs involve administering each treatment only once per subject, while replicate designs involve administration of one or both treatments at least twice per subject. The replicate design has the advantage of using fewer subjects than the nonreplicate design and is especially useful for highly variable drugs. For bioequivalence studies, a replicate crossover design often represents an attractive alternative to other study designs.

2. Parent Drug Versus Metabolites

The foundation of a bioequivalence study is adequate sampling of appropriate body fluids for the presence of the drug. The parent drug in the dosage form should always be measured in these fluids, unless accurate assay quantification is not possible. In general, only parent drug levels are measured, rather than those of the metabolites, because the concentration-time profile of the parent drug is more sensitive to changes in formulation performance than that of a metabolite, which is more reflective of metabolite formation, distribution, and elimination. That being said, primary metabolite(s), which are formed directly from the parent compound, should be measured if they are both.

  • Formed substantially through pre-systemic metabolism (i.e., first-pass, gut wall, or gut lumen metabolism); and
  • Contribute significantly to the safety and/or efficacy profile of the product.

This approach should be used for all drug products, including pro-drugs, with the drug expressed using confidence intervals (CI). Metabolite data can also be used to provide supportive evidence of a comparable therapeutic outcome.

3. First Point Cmax

Concentration-time profiles provide an indication of the absorption and elimination of a drug from the body fluid or tissue being assessed. When the first point of a concentration-time curve is the highest point, it raises significant questions of bias in the estimation of the maximum observed concentration (Cmax) because it is may be indicative of insufficient early sampling times. A carefully conducted pilot study can help avoid this problem by informing an adequately robust sampling schedule. In the main bioequivalence study, collection of blood samples at a sufficiently early time point, often between 5 and 15 minutes after dosing, as well as at other appropriate time points (generally 2-5 collections) during the first hour post-dose is critical; indeed, FDA may not accept data from patients with insufficient early sampling, and this could detrimentally affect the robustness and interpretability of the pharmacokinetics analysis.

4. Effects of Alcohol on Modified Release Drug Products

In addition to appropriate study design and sampling schedules, consideration must be given to the desired release characteristics of the test drug and how these characteristics might impact the performance of the formulation when administered to patients outside of the controlled environment of a clinical trial. Importantly, the consumption of alcoholic beverages can affect the release of a drug substance from a modified release (MR) formulation. For example, the formulation can lose its modified release characteristics, leading to more rapid drug release and altered systemic exposure that can have deleterious effects on the safety and/or efficacy of the drug. FDA recommends that Sponsors developing certain extended release solid oral dosage forms conduct in vitro studies to determine the potential for dose dumping in alcohol. Assessments using media containing various alcohol concentrations may be recommended and a human bioequivalence study in which the drug product is co-administered with alcohol may also be appropriate.

5. Endogenous Compounds

Endogenous compounds are drugs that are already present in the body either because the body produces them or they are present in the normal diet. Because these compounds are identical to the drug that is being administered, determining the amount of drug released from the dosage form and absorbed by each subject can be difficult. Sponsors should measure baseline endogenous levels in the matrix of interest (e.g., blood or plasma), subtracting these levels from the total concentrations measured from each subject after the drug product has been administered. In this way, an estimate of the actual drug availability can be determined. Depending upon whether the endogenous compound is naturally produced by the body or is present in the diet, the recommended approaches for determining bioequivalence are as follows:

  • When the body produces the compound, multiple baseline concentrations should be determined in the time period before study drug administration. These baseline concentrations should be taken into account during the PK analysis in a manner consistent with the pharmacokinetic properties of the drug.
  • When there is dietary intake of the compound, intake should be strictly controlled both before and during the study. Subjects should be housed at the clinic before the study and served standardized meals containing an amount of the compound similar to that in the meals to be served on the PK sampling day.

For both of these approaches, Sponsors should determine baseline concentrations for each dosing period. If a baseline correction results in a negative plasma concentration value, the value should be set equal to 0 before calculating the baseline-corrected area under the concentration-time curve (AUC). Pharmacokinetic and statistical analysis should be performed on both uncorrected and corrected data. Determination of bioequivalence should be based on the baseline-corrected data.


Sponsors interested in seeking FDA approval of a generic drug should consider several factors when planning their bioequivalence study. Careful consideration of the appropriate study design, sampling schedule, desired release characteristics, and any confounding by endogenous compounds will go a long way toward promoting a successful development program. Allucent’s experts have considerable experience with generic drug development, and we welcome the opportunity to help guide your program toward approval. Contact us about how to effectively design and execute your next bioequivalence study. 

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