Which of the following statements about first-pass metabolism is true?
A.
It occurs in the liver after a drug has been absorbed into the bloodstream
B.
It increases the bioavailability of a drug
C.
It only occurs with intravenous drug administration
D.
It can be bypassed by administering a drug sublingually
The Correct Answer and Explanation is:
The correct answer is D: It can be bypassed by administering a drug sublingually.
Explanation:
First-pass metabolism, also known as first-pass effect or hepatic first-pass effect, refers to the metabolic process in which the concentration of a drug is significantly reduced before it reaches systemic circulation. This occurs primarily in the liver, which is the body’s main site of drug metabolism. Here’s a detailed explanation of the concept and how different routes of administration can influence it:
- What is First-Pass Metabolism? When a drug is administered orally, it is absorbed through the gastrointestinal tract and enters the portal circulation. From there, it travels to the liver via the hepatic portal vein. The liver metabolizes a significant portion of the drug before it enters the systemic circulation, thus reducing the amount of active drug that reaches the bloodstream and, consequently, the therapeutic effect. This process is known as first-pass metabolism.
- Impact on Drug Bioavailability: First-pass metabolism typically decreases the bioavailability of a drug. Bioavailability refers to the proportion of the drug that reaches systemic circulation unchanged. Because the liver metabolizes a portion of the drug, the amount available to exert therapeutic effects is less than what was originally administered.
- Routes of Administration and First-Pass Metabolism: Different routes of drug administration can influence the extent of first-pass metabolism. Oral administration is most affected because the drug is absorbed from the gastrointestinal tract and passes through the liver before reaching systemic circulation.
- Intravenous (IV) Administration: Bypasses the first-pass effect because the drug is directly introduced into systemic circulation, so the entire dose is available to exert its effects.
- Sublingual Administration: This method involves placing the drug under the tongue, where it dissolves and is absorbed directly into the bloodstream through the mucous membranes. This route avoids the gastrointestinal tract and the hepatic portal circulation, thereby bypassing first-pass metabolism. This allows more of the active drug to reach systemic circulation compared to oral administration.
- Incorrect Statements:
- A is incorrect because first-pass metabolism occurs in the liver before the drug reaches the bloodstream.
- B is incorrect because first-pass metabolism decreases, rather than increases, the bioavailability of a drug.
- C is incorrect because first-pass metabolism is a concern with oral administration, not intravenous.
In summary, administering a drug sublingually can bypass the first-pass metabolism, thereby allowing more of the drug to reach systemic circulation and be effective.