Examkin

Question

Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

A 34-year-old man with severe opioid use disorder presents to an outpatient addiction clinic requesting medication-assisted treatment. He reports that his last intravenous heroin use was approximately 2 hours ago. On examination, he appears comfortable with a heart rate of 78/min, blood pressure of 118/76 mm Hg, and respiratory rate of 14/min. Pupils are 3 mm bilaterally. The physician administers a standard induction dose of sublingual buprenorphine. Thirty minutes later, the patient becomes profoundly agitated, diaphoretic, and develops severe abdominal cramps, bone pain, and yawning. His heart rate increases to 112/min and his pupils dilate to 7 mm. Which of the following pharmacodynamic properties of the administered medication best explains this patient's acute physiological deterioration?

  1. High receptor affinity and low intrinsic activityAnswer
  2. B
    Low receptor affinity and high intrinsic activity
  3. C
    Irreversible, non-competitive receptor antagonism
  4. D
    Rapid hepatic induction of cytochrome P450 enzymes
  5. E
    Acute downregulation of endogenous opioid receptors

Answer

High receptor affinity and low intrinsic activity
The correct answer accurately describes the pharmacodynamic properties of buprenorphine. Affinity refers to the strength of drug-receptor binding, while intrinsic activity (efficacy) refers to the maximum biological response the drug can induce once bound. Buprenorphine has a very high affinity for mu-opioid receptors, allowing it to easily displace full agonists like heroin or methadone. However, as a partial agonist, its intrinsic activity is low compared to full agonists. When it displaces heroin, the overall stimulation of the opioid receptors acutely drops from 100% to a much lower ceiling, instantly triggering severe precipitated withdrawal.

Step-by-Step Solution

1
Analyze the clinical presentation and timing.
The patient used heroin (a full opioid agonist) 2 hours ago and was asymptomatic. After receiving buprenorphine, he rapidly developed classic signs of opioid withdrawal (diaphoresis, abdominal cramps, myalgias, yawning, mydriasis, tachycardia).
Identifying the syndrome as precipitated opioid withdrawal is the critical first step to determining the mechanism.
2
Recall the pharmacologic profile of the administered drug (buprenorphine).
Buprenorphine is a partial agonist at the mu-opioid receptor.
Understanding the drug class allows application of specific pharmacodynamic principles.
3
Apply pharmacodynamic principles (affinity vs. intrinsic activity) to the clinical interaction.
Buprenorphine has a higher affinity for the mu-receptor than heroin, so it competitively displaces the heroin. Because it is a partial agonist, it has a lower intrinsic activity (Emax) than heroin, leading to a sudden, net drop in receptor activation.
This explains why administering a partial agonist to a patient actively under the influence of a full agonist precipitates withdrawal.

Key Concept

Precipitated withdrawal occurs when a partial agonist (high affinity, low intrinsic activity) rapidly displaces a full agonist (high intrinsic activity) from receptors, causing a net decrease in biological effect.
Estimated Time:1m 15s
Rate this question

Topics

Pharmacokinetic and Pharmacodynamic Principles

More questions from this topic