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Experts Unravel the Top 5 Myths Related to Bioequivalence

Bioequivalence enables bridging safety and effectiveness of oral pharmaceutical dosage forms using pharmacokinetics. The concepts behind bioequivalence are technical and cross-functional in nature (a collaboration between biopharmaceutics and pharmaceutical sciences). Thus, interpreting the guidelines available as well as navigating the outcomes of such trials is challenging. The Code of Federal Regulations (CFR) Subchapter D § 320.1 provides a definition:

Bioequivalence means 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.

This definition is complex and includes the following underlined terms:

Bioequivalence means 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.

These terms and phrases are confusing! In this blog, we present and dispel 5 myths associated with bioequivalence.

Myth 1: I have a highly variable drug, and therefore I need to conduct a BE study with 200 subjects.

This is a profound assumption regarding a BE study for a highly variable drug. The higher the variability, the higher the sample size. Safe assumption, right? Well, not quite. Let’s examine the regulatory guidance for highly variable drugs.

Highly variable drug products have larger than 30% intra-subject variability for a parameter. This must be evident in the BE study being performed. The agencies consider the role of variability in assessing impact on confidence intervals. If your drug is highly variable but has a wide therapeutic window, you may be able to apply much less stringent bioequivalence criteria using the reference-scaling method.

Let’s suppose a wider difference in Cmax, which is usually the parameter most subject to high variability, is considered clinically insignificant based on a robust justification. The agencies would consider a widened acceptance range. For example, the acceptance criteria for Cmax can be widened to 69.84 – 143.19% from 80-125% (EU BE Guidance). For the acceptance interval to be widened, the BE study should employ a replicate design where the within-subject variability for Cmax of the reference compound in the study is >30%. Regulators will accept either a 3-period or a 4-period crossover scheme in the replicate design study.

So, what is this replicate design?

Traditional BE studies are simple 2-treatment, 2-period crossover studies (the test-reference/reference-test; “TR/RT” design). If you run such a simple design for your high variability product, the standard design can require relatively large sample sizes (“200”), especially if the mean ratio is not expected to be 1.0.

In replicate crossover designs, we add an extra reference period arm to the study, such that the sequences are TRR/RRT/RTR. This can allow the sample size to be reduced by 25% compared to a standard design. Adding more periods and sequences can further reduce subjects needed by up to 50% (e.g., an TRRT/RTTR design). By testing the reference product in two periods, one can assess the reference variance and determine if it exceeds 30%, allowing for reference scaling analysis (US FDA). Such designs are accepted by regulatory agencies since they too do not want to see unnecessary and unwarranted exposure of healthy subjects.

Myth 2: Misunderstanding the differences between passing bioequivalence, failed bioequivalence, and an inconclusive outcome (missing BE bounds, but 90% CI still part way between 80 and 125%).

Once you get the data from a BE study, you will likely see one of the following scenarios.

You will likely see a clear bioequivalent or bio-inequivalent outcome. However, more common scenarios are the ones within the red box above, which might mean inconclusive weight of evidence. Formulations may not show bioequivalence for many reasons. These include statistical reasons such as that the estimated µT − µR lies very far from zero, or that the variability is greater than anticipated. This results in too wide a confidence interval. But the more common reason is sometimes inadequate sample size. If the issue relates to variability and uncertainty around variability, you might design a second larger sample size study now that you understand the variability. Alternatively, you can prospectively design a 2-stage BE study or a group sequential study design to assess the variability. One would conclude bio-inequivalence in situations where there is >90% power, the resulting sample size is very large, and when the point estimates lie outside the acceptance bounds. Failure to show bioequivalence will necessitate reformulation to ensure critical product quality attributes are met.  However, it may be possible to show that small deviations are not clinically meaningful based on the pharmacokinetics and exposure/response in phase 3 trials.

Myth 3: I need to use the identical dose of test and reference products for showing bioequivalence.

Let’s go back to the CFR definition of bioequivalence. What does it say for dose? It says, “same molar dose,” right? But does that mean the dose has to be identical to what is being used in a dosage form (e.g., labeled 100 mg)? Or does it mean the bioavailable dose? Well, it’s open to interpretation.

Let’s look at an important precedent that changed the way we look at “same molar dose”.

Abbott’s Tricor (fenofibrate) was developed in the early 2000s, and the sponsor filed a series of formulation switches using the 505(b)(2) pathway (FDA approval dossier). Ring a bell? Let’s look at hard evidence. Looking at the summary basis of approval, we see that “Tricor fenofibrate test “EZ” tablets (1 x 145 mg, or 3 x 48 mg) was bioequivalent to Tricor micronized fenofibrate capsule (1 x 200 mg)”. So, are 145 mg (new formulation) and 200 mg (old formulation) considered same molar doses? Or do they mean same “bioavailable” dose?  In any case, the full story behind Abbott’s approach and the changes to regulations that followed is captured in the paper by Downing et al (2012).

So, myth dispelled? Not sure. Dose differences between test and reference are not that common.

Myth 4: I have a fixed-dose combination product (FDC) with a new molecule and an already approved drug product. If I run a BE study in the US with that product, I can use the same BE study for worldwide registration in all markets.

If you are the sponsor of the innovator product with a globally single compositional image dosage form, then you need not run BE studies to gain approvals within regional markets. For your generic component (already approved reference), however, it depends on what image is being used in that regional market and applicable Reference Listed Drug or their equivalents. Now, you are subject to regionally applicable guidances and availability of the product locally. Typically, depending on how the reference formulation appears in that market and how different is from the one you studied, you may end up needing to run the BE study per market. The problem is you do not know how different or similar it is between, let us say, a US sourced reference vs EU sourced reference. You do not have access to the compositional details other than what is contained within the regulatory dossiers, and such sensitive information is usually redacted. Suffice it to say, you may need to prepare for a BE study, at least for the generic component, in every market you intend to register that FDC.

Myth 5: The intended marketed dose of my drug is 1000 mg, and I intended to use 5 tablets of 200 mg to create the dose. I am running a BE study, and I will be using the clinical dose.

BE studies typically consider the highest strength tablet for the study. In this case, you would use a 200 mg strength. The BE study tests the adequacy of the product performance and not the dose. If you have difficulty establishing bioanalytical sensitivity at the 200 mg strength, you could argue for using a higher dose. BE guidances will indicate that it is preferable to study the highest marketed strength as a single unit.

Bioequivalence is a heavily researched subject within biopharmaceutics. We hope we have highlighted some common themes where there is sufficient confusion around the purpose and necessity behind these concepts. Hopefully, we have cleared the air up a bit. If you have a formulation, it will likely undergo changes during its product lifecycle. Paying attention to concepts such as bioequivalence and the emerging quantitative approaches to demonstrate similarity can help to reduce the uncertainty around formulation switches, whether they are pre- or post-approval.

Certara scientists have expertise in the use of model-informed drug development tools as well as the use of physiologically-based pharmacokinetic modeling as it pertains to biopharmaceutics.

To learn more about using reference-scaled average bioequivalence (RSABE) methodology to demonstrate bioequivalence for highly variable drugs, please watch this webinar.


How to Use Phoenix RSABE Templates


Downing NS, et al (2012). How Abbott’s Fenofibrate Franchise Avoided Generic Competition Arch Intern Med. 14; 172(9): 724–730.

Food and Drug Administration (1992). Guidance for industry: statistical procedures for bioequivalence studies using a standard two treatment crossover design. US Department of Health and Human Services , Rockville, MD.

Food and Drug Administration (2001). Guidance for industry: statistical approaches to establishing bioequivalence . US Department of Health and Human Services, Rockville, MD.

Food and Drug Administration (2003). Guidance for industry: bioavailability and bioequivalence studies for orally administered drug products—general considerations. US Department of Health and Human Services, Rockville, MD.

Food and Drug Administration (1999). Center for Drug Evaluation and Research (CDER). Statistical Information from the June 1999 Draft Guidance and Statistical Information for In Vitro Bioequivalence Data, Posted on August 18, 1999. US Department of Health and Human Services, Rockville, MD.

Food and Drug Administration. Drug Approval Package: Tricor (fenofibrate) Tablets; Applications No.: 021203 and 021656.

Julious S et al (2010). An Introduction to Statistics in Early Phase Trials, John Wiley & Sons, NY.

US Code of Federal Regulations. (2000). Bioavailability and bioequivalence requirements. US Government Printing Office, Washington, DC, Vol. 21, Part 320. Also see: https://www.govinfo.gov/content/pkg/CFR-2009-title21-vol5/pdf/CFR-2009-title21-vol5-sec320-1.pdf.

About the authors

Rajesh Krishna, PhD
By: Rajesh Krishna, PhD

Rajesh Krishna, PhD, FAAPS is Senior Director, clinical pharmacology and lead of the integrated practice area on rare diseases at Certara Strategic Consulting.  With ~25 years of combined pharmaceutical industry and consulting experience, he has contributed to over 40 INDs; over 200 Phase 1/1b studies; and to several NDAs/BLAs.  He is an author of Raj’s clinical pharmacology blogs.

Paul Fackler
By: Paul Fackler

Dr. Fackler joined Alvogen in 2018 and directs both their clinical and non-clinical studies. Prior to Alvogen, he was at Merck Research Laboratories from 2010 to 2018 managing a clinical group to support development of second-generation products such as new fixed dose combinations, products with improved PK profiles, and those with new delivery routes. He also worked at Teva Pharmaceuticals as Vice President Generic R&D, 1998 to 2010, Springborn Laboratories, and Johnson Matthey. He has served on the FDA Advisory Committee for Clinical Pharmacology and has been on two USP expert committees.

By: Rajesh Krishna PhD and Paul Fackler