A patient maintained on intravenous morphine for cancer pain is given buprenorphine for breakthrough analgesia. Instead of additional analgesia, his pain control worsens and signs of opioid withdrawal appear. The pharmacodynamic explanation is:
- A Buprenorphine, a partial mu agonist with high affinity, displaces morphine and occupies receptors with lower intrinsic activity ✓
- B Buprenorphine accelerates morphine metabolism through enzyme induction
- C Buprenorphine acts as an inverse agonist at the mu receptor, producing effects opposite to morphine
- D Buprenorphine induces rapid desensitisation and internalisation of mu receptors
Correct answer: A. Buprenorphine, a partial mu agonist with high affinity, displaces morphine and occupies receptors with lower intrinsic activity
Explanation
Buprenorphine binds the mu receptor with very high affinity but has low intrinsic activity (partial agonist). In a patient fully occupied by a full agonist such as morphine, buprenorphine competitively displaces it and lowers net receptor activation, reducing analgesia and precipitating withdrawal. It is not an inverse agonist (option C), enzyme induction takes days not minutes (option B), and receptor internalisation does not explain the acute precipitation of withdrawal (option D).
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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