Opioids in renal failure: morphine metabolites versus fentanyl
When the kidneys fail, morphine's active breakdown products (metabolites) can build up and cause toxicity, whereas fentanyl has no active product that depends on the kidneys for removal.
Remember
Choose fentanyl rather than morphine in renal impairment (reduced kidney function), because morphine's active metabolites build up when the kidneys cannot clear them. Fentanyl is safer, not risk-free, so doses still need careful adjustment.
Morphine is a strong opioid, a type of painkiller. The liver converts it by glucuronidation (attaching a sugar-like acid to make the drug water-soluble). This makes two main metabolites: morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G).
M3G is neuroexcitatory (it can over-stimulate nerve cells) and is thought to cause myoclonus (sudden, brief, shock-like muscle jerks) and hyperalgesia (an abnormal increase in sensitivity to pain). M6G is analgesic and mainly adds to sedation (drowsiness) and respiratory depression (dangerously slow, shallow breathing).
Both metabolites are removed by the kidneys, which is why toxicity can develop when the kidneys fail.
Fentanyl is a very strong opioid, about 100 times as potent as morphine, so it is given in much smaller doses. It is broken down in the liver by the enzyme CYP3A4 (one of the enzymes that metabolise drugs) and has no active renally cleared metabolite, that is, no active metabolite that depends on the kidneys for removal. That is why it is safer than morphine in renal impairment.
Fentanyl is safer, not risk-free: its effects can still be prolonged, so the dose needs careful adjustment.
Key facts
- M3G
- Neuroexcitatory, and thought to cause myoclonus and hyperalgesia.
- M6G
- Analgesic and more potent than morphine itself, and it mainly adds to sedation and respiratory depression.
- Removal of the morphine metabolites
- Both M3G and M6G are removed by the kidneys, so they build up in renal impairment.
- Fentanyl breakdown
- Metabolised by the liver enzyme CYP3A4, and it has no active renally cleared metabolite.
- Lipophilicity
- Fentanyl is highly lipophilic (fat-soluble) and morphine is relatively hydrophilic (water-loving); this explains fentanyl's fast onset, not its advantage in kidney disease.
- Caution
- Fentanyl is safer, not risk-free, so doses are still adjusted to response.
Exam traps
- A drug cleared unchanged by the kidneys would build up in kidney failure. Fentanyl is not that kind of drug, because it is mainly broken down by the liver enzyme CYP3A4.
- Build-up in kidney failure depends on whether the drug or its active metabolites are cleared by the kidneys, not on potency. Fentanyl is about 100 times more potent than morphine, not less.
- M6G does build up in renal failure, but it is an analgesic metabolite that mainly adds to sedation and respiratory depression. The neuroexcitatory effects, such as myoclonus, are linked to M3G.
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