First pass metabolism determines what fraction of an oral dose will reach the circulation - the bioavailable fraction. Intravenous drugs don’t experience this first pass effect and are, by definition, 100% bioavailable. Drugs administered orally or inhaled demonstrate less than 100% bioavailability.
The first pass effect (also known as first-pass metabolism or presystemic metabolism) is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of drug lost during the process of absorption which is generally related to the liver and gut wall. Notable drugs that experience a significant first-pass ...
A first-pass effect is defined as a low systemic availability of the drug as a result of significant metabolism. Although a first-pass effect can occur in a variety of tissues, including the intestines Doherty and Pang (1997) and uterus De Ziegler et al (1997), it is most often observed with the liver.
When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors.
Because the liver is the primary organ of metabolism, the consequence of first-pass metabolism is the break down of significant amounts of a drug before it can reach the systemic circulation, so those drugs never reach the site of action!
first-pass effect: the intestinal and hepatic degradation or alteration of a drug or substance taken by mouth, after absorption, removing some of the active substance from the blood before it enters the general circulation. Synonym(s): first-pass effect
Understanding First Pass Metabolism : Drug Absorption. The drug is absorbed from the GI tract and passes via the portal vein into the liver where some drugs are metabolised. Sometimes the result of first pass metabolism means that only a proportion of the drug reaches the circulation. First pass metabolism can occur in the gut and the liver.
In this article in the series of ‘bite sized’ pharmacology, we will look at the concept of first pass metabolism. All drugs given by the oral route undergo a degree of first pass metabolism either in the gut or the liver, with some drugs being destroyed before they reach the systemic circulation.
The first-pass hepatic metabolism of a number of important therapeutic agents is inconsistent with traditional models that assume that the hepatic extraction ratio of a drug is constant in each ...