Buprenorphine is used for both pain management and opioid use disorder (OUD). Its ceiling effect on respiratory depression (but not analgesia) is explained by which receptor pharmacology?

• A Buprenorphine is a partial agonist at μ-opioid receptors with very high receptor binding affinity; partial agonism limits maximal efficacy (Emax) for respiratory depression while its high-affinity competitive antagonism resists reversal by full-agonist opioids ✓
• B Buprenorphine is a full agonist at κ-opioid receptors but a partial agonist at μ-receptors; κ-agonism contributes to analgesia without respiratory depression ceiling
• C Buprenorphine activates δ-opioid receptor more than μ-receptor at high doses, and δ-receptors do not mediate respiratory depression
• D Buprenorphine's active metabolite norbuprenorphine is a μ-receptor antagonist that limits the respiratory depressant effects of the parent drug

Explanation

Buprenorphine is a partial agonist at μ-opioid receptors, meaning it activates the receptor but cannot produce the same maximal response (Emax) as full agonists like morphine. Respiratory depression is a G-protein-mediated effect that is particularly sensitive to high Emax — with partial agonism, respiratory depression reaches a ceiling even as the dose increases. Analgesia, being mediated by both spinal and supraspinal pathways with different receptor reserve thresholds, is still clinically effective at partial agonist efficacy. Norbuprenorphine is actually a full μ-agonist metabolite but is restricted from CNS entry by P-glycoprotein. Buprenorphine's high receptor affinity (Kd ~0.2 nM) also explains why naloxone reversal requires high doses.

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

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Written and medically reviewed by the StethoPrep medical team.