title: Presystemic extraction: mechanisms and consequences.
abstract: Many drugs have incomplete systemic availability after oral dosage. This can be attributed to incomplete absorption from the gastrointestinal tract, or to presystemic extraction, in which a fraction of an orally administered dose is biotransformed before reaching the systemic circulation. Presystemic extraction can occur either via biotransformation by gastrointestinal mucosa or enteric flora, or via metabolism during the "first-pass" through the liver. For drugs with low oral bioavailability due to high presystemic extraction, impaired clearance leads to increased peak plasma levels and greater area under the concentration-time curve, but minimal change in elimination half-life.
authors.affiliation: Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine, Boston, Massachusetts.
authors.forename: D J
authors.initials: DJ
authors.lastname: Greenblatt
mesh_terms: D000284:Administration, Oral; D001682:Biological Availability; D001711:Biotransformation; D004064:Digestive System / Q000378:metabolism* / Q000382:microbiology; D006207:Half-Life; D006801:Humans; D007275:Injections, Intravenous; D007408:Intestinal Absorption; D008099:Liver / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D004364:Pharmaceutical Preparations / Q000008:administration & dosage / Q000378:metabolism*; D010599:Pharmacokinetics
chemical_list: D004364:Pharmaceutical Preparations