Biopharmaceutics Classification System (BCS) Classes
C) Four classes is the correct answer.
BCS Framework
The Biopharmaceutics Classification System (BCS) categorizes drugs into four distinct classes based on two fundamental biopharmaceutical properties: aqueous solubility and intestinal membrane permeability 1, 2.
The Four BCS Classes
The classification scheme divides drugs as follows 1, 3:
- Class I: High solubility, high permeability
- Class II: Low solubility, high permeability
- Class III: High solubility, low permeability
- Class IV: Low solubility, low permeability
Clinical Significance
The BCS captures the two most significant factors influencing oral drug absorption and has become a cornerstone for drug discovery, product development, and regulatory sciences since its introduction in 1995 1, 2.
Distribution Among Essential Medicines
Analysis of the World Health Organization's Model List of Essential Medicines revealed the following distribution among classified drugs 3:
- 84% belong to Class I (highly soluble, highly permeable)
- 17% belong to Class II (poorly soluble, highly permeable)
- 39% belong to Class III (highly soluble, poorly permeable)
- 10% belong to Class IV (poorly soluble, poorly permeable)
Regulatory Impact
The majority of approved immediate-release oral drug products can have their clinical performance assured with in vitro dissolution testing rather than empirical in vivo human studies, particularly for Class I drugs 1. Drug regulatory agencies worldwide have effectively implemented the BCS in setting bioavailability/bioequivalence standards for immediate-release oral drug product approval 1.
Important Refinements
While the original BCS defines four classes, sub-classifications have been proposed for Classes II and IV to account for acid (a), base (b), and neutral (c) compounds, as the pKa significantly impacts dissolution behavior in the variable pH environment of the gastrointestinal tract 2. However, the fundamental system remains four classes 1, 2, 3.