Biopharmaceutical Classification of Drugs.

Class 1: High Solubility - High Permeability
Class 2: Low Solubility - High Permeability
Class 3: High Solubility - Low Permeability
Class 4: Low Solubility - Low Permeability

Class I drugs exhibit a high absorption number and a high dissolution number. The rate limiting step is drug dissolution and if dissolution is very rapid then gastric emptying rate becomes the rate determining step.Rate of absorption is higher than rate of excretion. e.g. Metoprolol, Diltiazem, Verapamil, Propranolol.

Class II drugs have a high absorption number but a low dissolution number. In vivo drug dissolution is then a rate limiting step for absorption except at a very high dose number. The absorption for class II drugs is usually slower than class I and occurs over a longer period of time. In vitro- In vivo correlation (IVIVC) is usually excepted for class I and class II drugs. e.g. Phenytoin, Danazol, Ketoconazole, Mefenamic acid, Nifedipine.

Class III drugs, permeability is rate limiting step for drug absorption. These drugs exhibit a high variation in the rate and extent of drug absorption. Since the dissolution is rapid, the variation is attributable to alteration of physiology and membrane permeability rather than the dosage form factors. e.g. Cimetidine, Acyclovir, Neomycin B, Captopril.

Class IV drugs exhibit a lot of problems for effective oral administration. Fortunately, extreme examples of class IV compounds are the exception rather than the rule and are rarely developed and reach the market. Nevertheless a number of class IV drugs do exist. e.g. Taxol, Griseofulvin.

Note:
- Absorption no is ratio of mean residence time to mean absorption time.
- Dissolution no is ratio of mean residence time to mean dissolution time.