Sun14November1246PM 34

Are the following true or false of opioids?

True / False

Remifentanil has a context sensitive half life of approximately 3 minutes Correct

Buprenorphine has higher potency than morphine Correct

Fentanyl is 1000 times more potent than morphine Correct

Alfentanil has a higher lipid solubility than fentanyl Incorrect answer selected

Morphine has an oral bioavailability of 90% Correct

Explanation

Alfentanil has lower lipid solubility than fentanyl. However, it has a pKa of 6.5 and therefore at physiological pH (7.4), 89% of the drug exists in unionised form and therefore readily available to cross plasma membranes. This is in comparison to fentanyl, which has a pKa of 8.4 and at physiological pH, only 9% is unionised and available to cross plasma membranes. This explains alfentanil's faster onset of action in comparison to fentanyl despite having lower lipid solubility.

Buprenorphine is a partial opioid agonist however has greater affinity at the MOP receptor than morphine (about 30 times). Given that potency is a measure of activity at the receptor, buprenorphine has greater activity as it has a higher affinity despite only being a partial agonist.

Fentanyl is 100 times more potent than morphine. Potency is a measure of drug activity at a receptor and defines the amount of drug required to produce a given effect. Therefore a lower concentration of fentanyl is required (micrograms) to produce its effect in comparison to morphine (milligrams).

Bioavailability is the fraction of administered dose of drug entering systemic circulation following administration by a specified route. Morphine has oral bioavailability of 30% due to hepatic first pass metabolism. Morphine is the reference opioid against which all others are compared.

Remifentanil undergoes rapid metabolism by readily available plasma esterases. Its duration of action is therefore linked to metabolism and not distribution. Context-sensitive half-life or context sensitive half-time is defined as the time taken for blood plasma concentration of a drug to decline by one half after an infusion designed to maintain a steady state (i.e. a constant plasma concentration) has been stopped. The "context" is the duration of infusion. It has a context sensitive half-time of 3 minutes.

Reference:

Anaesthesia UK. Pharmacology of Opioids I.

Anaesthesia UK. Pharmacology of Opioids II.

Answer Statistics

Not enough data to display the statistics

Average score: 64.35%

Times answered: 239