A 45-year-old patient with chronic cancer pain on oral morphine develops opioid-induced constipation (OIC) that does not respond to lactulose. Which drug specifically targets the peripheral mechanism of OIC without reversing central analgesia?
- A Naloxone (IV)
- B Methylnaltrexone ✓
- C Naltrexone (oral)
- D Buprenorphine
Explanation
Methylnaltrexone (and naloxegol, alvimopan) are peripherally acting mu-opioid receptor antagonists (PAMORAs). They selectively block opioid receptors in the enteric nervous system without crossing the blood-brain barrier (due to quaternary ammonium structure or P-glycoprotein substrate properties), thereby restoring gastrointestinal motility without antagonizing central analgesia. Full-dose IV naloxone would reverse both peripheral and central opioid effects, precipitating pain crisis. Oral naltrexone at standard doses has significant CNS penetration. Buprenorphine is a partial mu-agonist and would not treat constipation.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.