First pass effect
The first pass effect is a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, 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 effect are imipramine, morphine, propranolol, buprenorphine, diazepam, midazolam, pethidine, tetrahydrocannabinol, ethanol, cimetidine, lidocaine, and nitroglycerin. In contrast some drugs are enhanced in potency: for example, the effect of the most commonly considered active ingredient in cannabis, THC, is enhanced by transformation of a significant portion into 11-hydroxy-THC that more readily crosses the blood-brain barrier and thus achieves greater potency than the original THC.
First pass metabolism may occur in the liver or in the gut.
After a drug is swallowed, it is absorbed by the digestive system and enters the hepatic portal system. It is carried through the portal vein into the liver before it reaches the rest of the body. The liver metabolizes many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of the circulatory system. This first pass through the liver thus may greatly reduce the bioavailability of the drug.
The four primary systems that affect the first pass effect of a drug are the enzymes of the gastrointestinal lumen, gut wall enzymes, bacterial enzymes, and hepatic enzymes.
In drug design, drug candidates may have good druglikeness but fail on first-pass metabolism because it is biochemically selective.
Alternative routes of administration like suppository, intravenous, intramuscular, inhalational aerosol, transdermal, and sublingual avoid the first-pass effect because they allow drugs to be absorbed directly into the systemic circulation.
Drugs with high first pass effect typically have a considerably higher oral dose than sublingual or parenteral dose. There is marked individual variation in the oral dose due to differences in the extent of first pass metabolism, frequently among several other factors. Oral bioavailability of many vulnerable drug appears to be increased in patients with compromised liver function. Bioavailability is also increased if another drug competing for first pass metabolism enzymes is given concurrently e.g. propranolol and chlorpromazine.
