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First-pass metabolism may occur in the liver (for propranolol, lidocaine, clomethiazole, and nitroglycerin) or in the gut (for benzylpenicillin and insulin). [4] The four primary systems that affect the first pass effect of a drug are the enzymes of the gastrointestinal lumen, [5] gastrointestinal wall enzymes, [6] [7] [8] bacterial enzymes [5] and hepatic enzymes.
This is known as the first pass effect. As a consequence, certain drugs can only be taken via certain routes. For example, nitroglycerin cannot be swallowed because the liver would deactivate the medication, but it can be taken under the tongue or transdermally (through the skin) and thus is absorbed in a way that bypasses the portal venous system.
Whether a drug is taken with or without food will also affect absorption, other drugs taken concurrently may alter absorption and first-pass metabolism, intestinal motility alters the dissolution of the drug and may affect the degree of chemical degradation of the drug by intestinal microflora.
Furthermore, after absorption from the gastrointestinal tract, such drugs must pass to the liver, where they may be extensively altered; this is known as the first pass effect of drug metabolism. Due to the digestive activity of the stomach and intestines, the oral route is unsuitable for certain substances, such as salvinorin A
Although FMO5 was the first distinct FMO, it is not clear what function it serves since it does not oxygenate the typical FMO substrates involved in first-pass metabolism. Analyses of FMO genes across several species have shown extensive silent DNA mutations, which indicate that the current FMO gene family exists because of selective pressure ...
Conversely, if it is absorbed by the upper portion of the rectum, progesterone is subject to hepatic first-pass metabolism due to entry into the hepatic portal system via the superior rectal vein. [18] As such, although rectal administration is a parenteral route, it may still be subject to some first-pass metabolism similarly to oral ...
News of the Supreme Court ruling that affirmative action in higher education is unconstitutional has catapulted the policy that was legal for at least 45 years to the forefront.
In particular, the oral route is subject to a high first-pass effect, which results in high levels of testosterone in the liver and consequent hepatic androgenic effects, as well as low potency due to first-pass metabolism in the intestines and liver into metabolites like dihydrotestosterone and androgen conjugates. [2]