Buprenorphine's ceiling effect on respiratory depression but not on analgesia is explained by which receptor pharmacology concept?

• A Buprenorphine is a full mu agonist but metabolized differently in brainstem vs. spinal cord
• B Buprenorphine is a partial mu-opioid receptor agonist with high receptor affinity but low intrinsic efficacy; respiratory depression requires higher receptor occupancy than analgesia, hitting the ceiling at lower doses ✓
• C Buprenorphine's kappa receptor antagonism counteracts mu-mediated respiratory depression
• D Buprenorphine induces beta-arrestin recruitment causing biased signaling only for respiratory depression pathways

Explanation

Buprenorphine is a partial agonist at mu-opioid receptors (MOP) with very high receptor affinity (sub-nanomolar) but submaximal intrinsic efficacy. Analgesia in the spinal cord requires lower fractional receptor occupancy than respiratory depression (which requires high occupancy in the brainstem respiratory centers). As a partial agonist, buprenorphine reaches its maximal effect (ceiling) for respiratory depression at relatively low doses but still produces meaningful analgesia at those doses. Its high affinity also explains why naloxone must be given at higher doses or as repeated doses to reverse buprenorphine overdose compared to full agonists.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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