Which of the following opioids undergoes glucuronidation to form active metabolites that accumulate in renal failure?

Explanation

## Morphine Metabolism and Renal Failure Complications ### Morphine Glucuronidation Pathway **Key Point:** Morphine is metabolized by hepatic glucuronidation to form two major active metabolites: morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). Both are renally excreted and accumulate in renal failure, causing prolonged analgesia and respiratory depression. ### Active Metabolites of Morphine | Metabolite | Activity | Clinical Consequence | |-----------|----------|---------------------| | **Morphine-6-glucuronide (M6G)** | Highly analgesic (more potent than morphine) | Accumulates → prolonged analgesia, respiratory depression | | **Morphine-3-glucuronide (M3G)** | Neurotoxic (hyperalgesia, allodynia) | Accumulates → paradoxical pain, myoclonus, seizures | | **Unchanged morphine** | Original activity | Minimal renal excretion (10%) | **High-Yield:** In renal failure (eGFR < 30 mL/min), morphine metabolites accumulate to toxic levels, necessitating: - Dose reduction (50% of normal) - Increased dosing intervals (q8h → q12h) - Alternative opioids (fentanyl, methadone) preferred **Clinical Pearl:** M6G is actually MORE analgesic than morphine itself but is renally cleared. This is why morphine causes prolonged analgesia in renal failure—not just from the parent drug, but from active metabolite accumulation. **Mnemonic:** **FARM** — Fentanyl, Alfentanil, Remifentanil, Methadone (opioids safe in renal failure). Morphine is NOT on this list. ### Why Other Opioids Are Safer in Renal Failure - **Fentanyl:** Hepatic metabolism to inactive metabolites; no active renal metabolites - **Methadone:** Hepatic metabolism; enterohepatic recirculation; minimal renal excretion - **Remifentanil:** Rapid ester hydrolysis by plasma and tissue esterases; independent of renal function