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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.
In particular, the oral route is subject to a high first-pass effect, which results in high levels of estradiol and consequent estrogenic effects in the liver and low potency due to first-pass hepatic and intestinal metabolism into metabolites like estrone and estrogen conjugates. [10]
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
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]
By definition, when a medication is administered intravenously, its bioavailability is 100%. [2] [3] However, when a medication is administered via routes other than intravenous, its bioavailability is lower due to intestinal epithelium absorption and first-pass metabolism.
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 ...
In addition, the rectal route bypasses around two-thirds of the first-pass metabolism as the rectum's venous drainage is two-thirds systemic (middle and inferior rectal vein) and one-third hepatic portal system (superior rectal vein). This means the drug will reach the circulatory system with significantly less alteration and in greater ...
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 ...