Pharmacokinetics · Short notes

Bioequivalence Studies: Short Notes & Cheat Sheet

Pharmaceutical and therapeutic equivalence, crossover study design, AUC and Cmax acceptance limits, biowaivers and dissolution similarity.

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Short notes

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A standard crossover bioequivalence study

Each volunteer receives both products in random order, separated by a washout.

Definitions

Pharmaceutical equivalents have the same active ingredient, strength, dosage form and route.
Pharmaceutical alternatives have the same active moiety but a different salt, ester or dosage form.
Bioequivalent products show no significant difference in the rate and extent of absorption.
Therapeutic equivalents are pharmaceutical equivalents that are also bioequivalent, so they can be substituted.

Study design

Usually a randomised, two-period, two-sequence crossover in healthy volunteers (at least 12), given single doses under fasting conditions (fed studies when food matters).
The washout between periods is usually at least 5 half-lives.
Blood is sampled often enough to define Cmax and to capture at least 80% of the total AUC.
The comparator is the reference (innovator) product.
Studies follow GCP, with ethics committee approval and informed consent.

Acceptance and biowaivers

Primary parameters are AUC (extent) and Cmax (rate), analysed after log-transformation.
The 90% confidence interval of the test/reference geometric mean ratio must fall within 80.00–125.00%.
Narrow-therapeutic-index drugs often need tighter limits (e.g. 90.00–111.11%). For highly variable drugs, regulators may allow wider Cmax limits using reference scaling.
BCS class I (and some class III) immediate-release products may get a biowaiver if they dissolve rapidly (≥ 85% in 30 minutes at pH 1.2, 4.5 and 6.8).
Dissolution profiles are compared with the similarity factor f2; f2 of 50–100 means similar.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Key numbers

Item	Value
90% CI for AUC and Cmax	80.00–125.00%
Narrow TI drugs	Often 90.00–111.11%
Minimum volunteers	12
Washout	≥ 5 half-lives
AUC coverage	≥ 80% of AUC0–∞
Similar dissolution	f2 ≥ 50
Rapid dissolution (biowaiver)	≥ 85% in 30 min

Equivalence terms

Term	Meaning
Pharmaceutical equivalent	Same API, strength, form, route
Pharmaceutical alternative	Same moiety, different salt or form
Bioequivalent	Same rate and extent of absorption
Therapeutic equivalent	Pharmaceutical equivalent + bioequivalent

Memory aids

80 to 125: The 90% confidence interval must lie within 80–125% for both AUC and Cmax.

Exam traps

It is the 90% confidence interval, not the point estimate alone, that must fall within 80–125%.
Tmax is not usually a primary bioequivalence parameter.
Crossover designs let each volunteer act as their own control, reducing variability.
An f2 below 50 means the dissolution profiles are not similar.

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