Methylnaltrexone
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| Clinical data | |
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| Trade names | Relistor |
| AHFS/Drugs.com | Monograph |
| MedlinePlus | a608052 |
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| Routes of administration |
Oral, intravenous, subcutaneous |
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| Pharmacokinetic data | |
| Protein binding | 11-15.3% |
| Metabolism | Hepatic |
| Biological half-life | 8 hours |
| Excretion | Urine (50%), faeces (50%) |
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| Synonyms | MNTX, naltrexone-methyl-bromide |
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| ChEMBL | |
| ECHA InfoCard | 100.122.861 |
| Chemical and physical data | |
| Formula | C21H26NO4 |
| Molar mass | 356.44 g/mol |
| 3D model (Jmol) | |
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Methylnaltrexone (MNTX, brand name Relistor), used in form of methylnaltrexone bromide (INN, USAN, BAN), is one of the newer agents of peripherally-acting μ-opioid antagonists that act to reverse some of the side effects of opioid drugs such as constipation without affecting analgesia or precipitating withdrawals. Because MNTX is a quaternary ammonium cation, it cannot cross the blood–brain barrier, and so has antagonist effects throughout the body, counteracting effects such as itching and constipation, but without affecting opioid effects in the brain such as analgesia. However, since a significant fraction (up to 60%) of opioid analgesia can be mediated by opioid receptors on peripheral sensory neurons, particularly in inflammatory conditions such as arthritis, traumatic or surgical pain, MNTX may increase pain under such circumstances.
Methylnaltrexone is approved for the treatment of opioid-induced constipation (OIC). It is generally only to be used when ordinary laxatives have failed.
Methylnaltrexone binds to the same receptors as opioid analgesics such as morphine, but it acts as an antagonist, blocking the effects of those analgesics, specifically the constipating effects on the gastrointestinal tract. Furthermore, as methylnaltrexone cannot cross the blood–brain barrier, it does not reverse the pain-killing properties of opioid agonists or cause withdrawal symptoms. Methylnaltrexone is unable to enter the brain primarily because it carries a positive charge on its nitrogen atom. This is the primary difference that makes methylnaltrexone behave differently from naltrexone.
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