A palliative care patient on long-term oral morphine develops opioid-induced constipation (OIC) that does not respond to traditional laxatives. The physician considers methylnaltrexone. What is the pharmacological basis for methylnaltrexone's selective peripheral action without reversing analgesia?

• A Methylnaltrexone is a selective kappa-opioid receptor antagonist; since OIC is mediated by kappa receptors in the gut and analgesia by mu receptors in the CNS, it reverses constipation without reversing analgesia
• B Methylnaltrexone is a quaternary nitrogen derivative of naltrexone; the charged quaternary nitrogen prevents passage across the blood-brain barrier, restricting action to peripheral opioid receptors in the gut, thus reversing OIC without CNS penetration or antagonism of central analgesia ✓
• C Methylnaltrexone is preferentially metabolized in intestinal tissue before reaching systemic circulation (first-pass gut effect), confining its action to the GI tract
• D Methylnaltrexone binds opioid receptors with higher affinity than morphine in the GI tract due to lower pH, but has equal affinity in the CNS, effectively competing only peripherally

Explanation

Methylnaltrexone is naltrexone's N-methyl quaternary ammonium derivative. The quaternary nitrogen carries a permanent positive charge at physiological pH, which dramatically reduces the drug's lipid solubility and ability to cross the blood-brain barrier via passive diffusion (unlike tertiary amine opioid antagonists like naloxone/naltrexone that readily cross the BBB). This peripherally restricted mechanism allows methylnaltrexone to selectively antagonize mu-opioid receptors in the enteric nervous system of the gastrointestinal tract (the primary site of opioid-induced constipation) without affecting centrally-mediated analgesia or precipitating central opioid withdrawal. Naloxegol and alvimopan are other peripherally acting mu-opioid receptor antagonists (PAMORAs) used for OIC.

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

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Written and medically reviewed by the StethoPrep medical team.