Source of information: Drugbank (External Link). Last updated on: 3rd July 18
*Trade Name used in the content below may not be the same as the HSA-registered product.
Active Ingredient / Synonyms
MNTX | Methylnaltrexone |
Methylnaltrexone is a pheriphally-acting μ-opioid antagonists that acts on the gastrointestinal tract to decrease opioid-induced constipation without producing analgesic effects or withdrawal symptoms. It is also a weak CYP2D6 inhibitor. FDA approved in 2008.
Treatment of opioids induced constipation in palliative patients that are inadequately responding to laxative therapy.
Mechanism of Action
Methylnaltrexone is a pheriphally-acting μ-opioid antagonists that acts on the gastrointestinal tract inhibit opioid-induced decrease in gastric motility and transit time. Because methylnaltrexone is a quaternary derivative of naltrexone, it produces its gastrointestinal effects without producing analgesic effects or withdrawal symptoms as it does not cross the blood-brain-barrier.
Use of opioids induces slowing of gastrointestinal motility and transit. Following remifentanil administration, the methylnaltrexone and placebo groups showed no change in pupiliary constriction while the naloxone group showed a marked change over the time interval tested.
Methylnaltrexone is rapidly absorbed. Tmax (SubQ): 30 minutes (regardless of dose); Cmax, 0.15 mg/kg SubQ dose = 117 ng/mL; AUC24, 0.15 mg/kg SubQ dose = 175 ng·hr/mL;
Volume of distribution, steady state = 1.1 L/kg
60% of the dose is metabolized. Conversion to methyl-6-naltrexol isomers (5% of total dose) and methylnaltrexone sulfate (1.3% of total dose) appear to be the primary pathways of metabolism. N‑demethylation of methylnaltrexone to produce naltrexone is not significant.
Most of the drug is eliminated as unchanged drug (85% of administered radioactivity). Approximately half of the dose is excreted in the urine and somewhat less in feces.
terminal: 8.89 ± 2.59 h (intravenous) terminal: 6.14- 8.83 h (subcutaneous)
10.5 ± 1.5 ml/min/kg (IV)
LD50: 50 mg/kg (primates); Orthostatic hypotension at plasma levels in excess of 1.400 ng/mL. The most common (>5%) adverse reactions reported with methylnaltrexone bromide are abdominal pain, flatulence, nausea, dizziness, diarrhea and hyperhidrosis.
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