Drug absorption is a complicated process in which many physiological and phys-iochemical factors are involved. understanding the principles of drug absorption benefits the designing of formulation strategies to enhance the bioavailability and in vivo drug activity. in summary, drug absorption mechanisms include passive diffusion and active transport. Permeability, solubility, and dissolution, Gi physiological conditions, and dosage forms can influence the drug absorption rate. in general, if a drug has high water solubility and low membrane permeability (hydrophilic drugs), permeability usually limits absorption, unless it is carrier mediated or paracellular absorption dependent. Strategies which can enhance the drug permeability in dosage design could be used to increase this permeability controlled drug absorption. In contrast, if a drug has low solubility and high permeability (lipophilic drugs), solubility (and dissolution) usually limits absorption. Formulation strategies should be optimized in the dosage form to enhance the solubility (and dissolution) controlled drug absorption. If neither of the above two properties limits the absorption such as for drugs with high solubility and high permeability, then gastric emptying rate limits the drug absorption. Both in vivo and in vitro methods have been explored to assess drug absorption in human. Rat and human show similar drug absorption profiles and similar transporter expression patterns in the small intestine, while the two species exhibit distinct expression levels and patterns for metabolizing enzymes in the intestine. These data provide the molecular mechanisms for the similarity and correlation of drug absorption (Fa) in the small intestine between rat and human. Therefore, rat can be used to predict oral drug absorption (Fa) in the small intestine of human, but not to predict drug metabolism ( Fg and Fh) and oral bioavailability

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