Biopharmaceutic classifications system parameters

Bioavailability depends not only on having the drug in solution, but also on the drug s permeability. A jejunal permeability of at least 2-4 x 10 4cm/s, measured in human subjects by intubation, is considered high [97]. For many drugs and other substances, this permeability corresponds to a fraction absorbed of 90% or better. Amidon et al. [97] thus proposed a Biopharmaceutics Classification System (BCS) for drugs based on the above definitions of these two parameters. Table 3 defines the BCS and includes some drugs representative of each class. 

Aqueous solubility of ionizable molecules at different pH values is an important characteristic because it indicates the potential substance behavior in the stomach and intestinal tract and its potential impact on bioavailability. Moreover, it also provides important information for formulation scientists to define the class of a drug substance in the Biopharmaceutics Classification System (BCS), a regulatory guidance for bioequivalence studies. The BCS is a scientific framework proposed by the FDA to classify drug substances based on their aqueous solubility and intestinal permeability and defines important parameters in the selection of drug candidates into development. According to the BCS, drug substances are classified as shown in Table 12-4. 

Figure 9.5 The Biopharmaceutics Classification System provides a scientific basis for predicting intestinal drug absorption and identifying the rate-limiting step based on primary biopharmaceutical properties such as solubility and Pefr.The BCSdividesdrugsintofourdifferent classes based on these two parameters. Drug regulation aspects related to in vivo performance...

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