ABSTRACT
Biopharmaceutics is the science of the factors determining drug biological availability in humans and animals. Biological availability is the percentage of the therapeutic substance which is absorbed from the dosage form to the general circulation or to the site of action. The preparation of a prolonged-action formulation should ensure adequate drug concentrations by the receptor site to maintain therapeutic action and should also prolong the duration of such concentrations. Absorption is the net transfer of a compound (drug) from the site of absorption into the circulation. In the case of gastrointestinal absorption, it is the net transfer of a compound (drug) from the gastrointestinal contents into the circulation. Absorption from the small intestine is similarly dependent upon the degree of dissociation. In the blood, a drug can be in the free form or bound to plasma proteins. The bound protein varies depending on the chemical character of the drug.
