A 70-year-old patient with chronic kidney disease (eGFR 15 mL/min) is prescribed morphine for cancer pain. The treating physician is concerned about metabolite accumulation. Which metabolite of morphine is responsible for opioid neurotoxicity in renal failure?

• A Morphine-3-glucuronide (M3G), which activates mu receptors causing respiratory depression
• B Morphine-6-glucuronide (M6G), an active metabolite more potent than morphine that accumulates in renal failure and causes CNS toxicity including myoclonus, seizures, and respiratory depression ✓
• C Normorphine (N-demethylated morphine), which accumulates and causes neuroexcitatory effects
• D Morphine-3-sulphate, which acts as a proconvulsant by GABA-A antagonism

Explanation

Morphine is metabolised primarily by UGT2B7 to morphine-3-glucuronide (M3G, ~55%) and morphine-6-glucuronide (M6G, ~10%). M6G is a potent mu-opioid receptor agonist — approximately 13 times more potent than morphine. Both M3G and M6G are renally excreted; in renal failure, M6G accumulates to levels that cause progressive opioid toxicity including respiratory depression, sedation, and myoclonus. M3G has been proposed to cause neuroexcitation but does not bind mu receptors meaningfully. In renal failure, opioids with no active metabolites (e.g., fentanyl, buprenorphine, methadone) are preferred.

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

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Written and medically reviewed by the StethoPrep medical team.