Regarding the pharmacology of opioids, all of the following statements are TRUE EXCEPT:

Explanation

## Opioid Pharmacology: Metabolism and Mechanism ### Analysis of Each Statement **Key Point:** Tramadol's analgesic efficacy depends on BOTH mu-receptor agonism AND monoamine reuptake inhibition—neither mechanism alone is sufficient. | Opioid | Metabolism | Key Pharmacokinetic Feature | |--------|-----------|-----------------------------| | Morphine | Hepatic glucuronidation (Phase II) | M3G and M6G formed; M6G is active and accumulates in renal failure | | Fentanyl | Hepatic N-dealkylation (CYP3A4) | Highly lipophilic; rapid redistribution causes short duration despite long t½ | | Tramadol | Hepatic O- and N-demethylation | Dual mechanism: mu agonism + SNRI (serotonin-norepinephrine reuptake inhibition) | | Remifentanil | Plasma/tissue esterase hydrolysis | Context-sensitive half-time independent of infusion duration | ### Why the Distractor is Wrong **High-Yield:** Tramadol's analgesic potency relies on **both** mechanisms working synergistically: - Mu-receptor agonism accounts for ~10% of analgesia - Monoamine reuptake inhibition (inhibition of serotonin and norepinephrine reuptake) accounts for ~90% of the analgesic effect - This dual mechanism explains why naloxone does not fully reverse tramadol analgesia (unlike pure opioids) - The statement claiming "negligible contribution" from monoamine reuptake inhibition is factually incorrect ### Correct Statements Explained 1. **Morphine glucuronidation:** M6G is indeed active and can cause prolonged effects in renal failure [cite:KD Tripathi 8e Ch 12] 2. **Fentanyl redistribution:** Lipophilicity → rapid CNS entry and exit → short clinical duration despite 3–4 hour elimination half-life 3. **Remifentanil esterase metabolism:** Allows use in prolonged infusions without accumulation; context-sensitive half-time remains ~3–4 min regardless of infusion length **Clinical Pearl:** Tramadol is contraindicated in patients on SSRIs/SNRIs due to serotonin syndrome risk—this reflects its significant monoaminergic activity.