Alternatively non-human systems capable of predicting drug absorption in humans can be used (such as in-vitro culture methods). Permeability class boundaries are based indirectly on the extent of absorption of a drug substance in humans and directly on the measurement of rates of mass transfer across human intestinal membrane. The volume estimate of 250 ml is derived from typical bioequivalence study protocols that prescribe administration of a drug product to fasting human volunteers with a glass of water. A drug is considered highly soluble when the highest dose strength is soluble in 250 ml or less of aqueous media over the pH range of 1 to 7.5.

Solubility class boundaries are based on the highest dose strength of an immediate release product. The drugs are classified in BCS on the basis of solubility, permeability, and dissolution. Usually they are not well absorbed over the intestinal mucosa and a high variability is expected.

Those compounds have a poor bioavailability.

Class IV - low permeability, low solubility.

If the formulation does not change the permeability or gastro-intestinal duration time, then class I criteria can be applied.

The absorption is limited by the permeation rate but the drug is solvated very fast.

Class III - low permeability, high solubility.

A correlation between the in vivo bioavailability and the in vitro solvation can be found.

The bioavailability of those products is limited by their solvation rate.Example: glibenclamide, bicalutamide, ezetimibe, aceclofenac.Class II - high permeability, low solubility.Those compounds are well absorbed and their absorption rate is usually higher than excretion.Class I - high permeability, high solubility.According to the Biopharmaceutical Classification System (BCS) drug substances are classified to four classes upon their solubility and permeability:

Bcs class 2 drug list pdf