Biopharmaceutics Classification System characterization

Following oral administration, dissolution of the drug molecule in the intestinal milieu is a prerequisite for the absorption process. According to the Biopharmaceutical Classification System (BCS) [2], poor water-soluble compounds (i.e., aqueous solubility less than 100 pg/ mL) are class 2 or class 4 compounds. For class 2 compounds, the absorption level is dictated by the dissolution properties of the molecule in the gastrointestinal (GI) fluids. BCS class 4 compounds, which are characterized by both low solubility and poor intestinal wall permeability, are generally poor drug candidates (unless the dose is very low). 

Figure 6.16 The Quantitative Biopharmaceutics Classification System (QBCS) utilizes specific cutoff points for drug classification in the solubility-dose ratio (1/0), apparent permeability (Papp) plane. Each class of the QBCS can be characterized on the basis of the anticipated values for the fraction of dose absorbed, Fa and the fraction of dose dissolved, at the end of the dissolution process assuming no interplay between dissolution and uptake. In essence the classification system is static in nature.

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