Abstract
Oral drugs, which are superior in terms of convenience and safety, are the most preferred dosage form from the perspective of a patient’s quality of life. In both clinical practice and drug development, an important aspect of handling oral drugs is the understanding of gastrointestinal（GI） absorption. Drug absorption from the GI tract includes disintegration of the drug product, dissolution of the drug substance, and membrane permeation through the epithelial cells of the GI tract. All of these processes are easily affected by the conditions of administration and the physiological environment in the GI tract and are influenced by various factors that can change the physiological environment in the GI tract, such as disease/pathology, food/beverage, concomitant medications, age, and gender. This complex and highly sensitive physiological environment of the GI tract is thought to be the cause of the large inter-individual variability observed in the pharmacokinetics of oral drugs, but it is not fully understood. Our research group has recently found that the osmotic environment in the GI tract alters luminal fluid volume, leading to changes in drug absorption by altering drug concentration. In this article, we review the physiological environment and functions in the GI tract that should be understood as the basis for predicting drug absorption in individual patients, focusing on the fluid absorption/secretion behavior caused by osmotic pressure in the GI tract and its effect on drug absorption dynamics.
