Alfentanil onset is rapid due to what pharmacokinetic feature?

Alfentanil onset is rapid due to what pharmacokinetic feature?

• A. It has rapid onset due to rapid effect-site equilibrium.
• B. It has slower onset due to slow transport.
• C. It has a large volume of distribution.
• D. It is not subject to hepatic metabolism.

Answer: (A)

Onset speed is determined by how fast the drug moves from plasma into the brain, i.e., how quickly it reaches the site of action. Alfentanil has a very rapid effect-site equilibration, so after IV administration the brain concentration rises quickly and analgesia appears promptly. This rapid CNS entry is aided by its chemical properties that favor fast transfer to the CNS (such as a favorable fraction un-ionized at physiologic pH), leading to a short effect-site equilibration time (short ke0). This isn’t about having a large volume of distribution, which mainly affects distribution and duration rather than how quickly the effect site is reached. Nor is it due to being exempt from hepatic metabolism; alfentanil is metabolized by the liver, so its rapid onset is not explained by the absence of metabolism.

Onset speed is determined by how fast the drug moves from plasma into the brain, i.e., how quickly it reaches the site of action. Alfentanil has a very rapid effect-site equilibration, so after IV administration the brain concentration rises quickly and analgesia appears promptly. This rapid CNS entry is aided by its chemical properties that favor fast transfer to the CNS (such as a favorable fraction un-ionized at physiologic pH), leading to a short effect-site equilibration time (short ke0).

This isn’t about having a large volume of distribution, which mainly affects distribution and duration rather than how quickly the effect site is reached. Nor is it due to being exempt from hepatic metabolism; alfentanil is metabolized by the liver, so its rapid onset is not explained by the absence of metabolism.