Drug Absorption and Bioavailability

Abstract

Absorption is the process of drug movement from the site of drug administration to the systemic circulation. Various processes underlie the successful absorption of drugs. They include passive diffusion, facilitated diffusion, active transport, and endocytosis. Drug absorption is quantified in terms of bioavailability. Bioavailability is the extent to which absorption occurs. In other words, bioavailability is the fraction of the administered drug that reaches the systemic circulation in the unchanged form. Various factors impede or enhance absorption. The lipid solubility, pH of the medium and the presence of and the density of membrane transporters have a greater effect on the rate of absorption. Various routes of drug administration are employed to maximize the amount of drug absorbed and hasten the onset of action of drugs. The intravenous route lacks a phase of absorption as the drug is directly injected into the systemic circulation. Quantification of the bioavailability by studying the structure and the presence of chemical groups is called Quantitative structure-bioavailability relationship (QSBR). Various novel models have been proposed to improve drug absorption and increase systemic exposure to drugs with low oral bioavailability.