clinical pharmacology
Drug Bioavailability
Causes of low bioavailability
Orally administered drugs must pass through the intestinal wall and then the portal circulation to the liver; both are common sites of first-pass metabolism (metabolism that occurs before a drug reaches systemic circulation). Thus, many drugs may be metabolized before adequate plasma concentrations are reached. Low bioavailability is most common with oral dosage forms of poorly water-soluble, slowly absorbed drugs. Insufficient time for absorption in the gastrointestinal (GI) tract is a common cause of low bioavailability. If the drug does not dissolve readily or cannot penetrate the epithelial membrane (eg, if it is highly ionized and polar), time at the absorption site may be insufficient. In such cases, bioavailability tends to be highly variable as well as low. Age, sex, physical activity, genetic phenotype, stress, disorders (eg, achlorhydria, malabsorption syndromes), or previous GI surgery (eg, bariatric surgery) can also affect drug bioavailability. Chemical reactions that reduce absorption can decrease bioavailability. They include formation of a complex (eg, betweentetracyclineand polyvalent metal ions), hydrolysis by gastric acid or digestive enzymes (eg, penicillin andchloramphenicolpalmitate hydrolysis), conjugation in the intestinal wall (eg, sulfoconjugation ofisoproterenol), adsorption to other drugs (eg,digoxintocholestyramine), and metabolism by luminal microflora. 