Every generic drug on the market had to prove one thing: that it behaves in the body just like the original. That proof is a bioequivalence study, and it all comes down to three numbers read off the concentration–time curve below.
Definitions
- Bioavailability (BA): the rate and extent to which the active drug is absorbed and becomes available in the systemic circulation.
- Bioequivalence (BE): two pharmaceutically equivalent products are bioequivalent if their rate and extent of absorption are not significantly different at the same dose under similar conditions.
- Absolute BA = compared with the intravenous route; relative BA = compared with another (oral) formulation.
The parameters measured
| Parameter | Reflects | Meaning |
|---|---|---|
| Cmax | Rate | Peak plasma concentration. |
| Tmax | Rate | Time to reach the peak. |
| AUC | Extent | Area under the concentration–time curve (total absorption). |
Why bioequivalence matters
- It is the basis for approving generic drugs — a generic must be bioequivalent to the innovator/reference product.
- Pharmaceutical equivalence + bioequivalence = therapeutic equivalence (the products are interchangeable).
Study design
Typically a single-dose, randomised, two-period, two-treatment, two-sequence crossover study in healthy volunteers, with an adequate washout between periods. It compares a Test (T) against a Reference (R) product, usually fasting (sometimes also fed), with blood sampled over time.
BE acceptance criterion: the 90% confidence interval of the ratio (Test/Reference) of the geometric means of Cmax and AUC must lie within 80.00–125.00%.
Biowaiver & the BCS
A biowaiver (no in-vivo BE study needed) may apply through the Biopharmaceutics Classification System (BCS) — typically rapidly dissolving Class I (and some Class III) oral products, plus IV/oral solutions and gases.
| BCS class | Solubility / Permeability |
|---|---|
| Class I | High solubility, high permeability — biowaiver candidate. |
| Class II | Low solubility, high permeability. |
| Class III | High solubility, low permeability. |
| Class IV | Low solubility, low permeability. |
India / regulatory
Under NDCT 2019: BA/BE study permission (decision within 90 working days), BA/BE study-centre approval (Form CT-08) and Ethics Committee approval, all regulated by CDSCO.
Exam tip: BA = rate & extent of absorption; BE = comparable BA of two products. Parameters: Cmax, Tmax (rate); AUC (extent). The criterion to quote is the 90% CI of Test/Reference within 80–125%; biowaivers go via the BCS (Class I/III).
Bioequivalence is the quiet science that makes affordable generics trustworthy. Master the three parameters, the crossover design, and the 80–125% rule, and the topic is yours.