methylnaltrexone

(me-thil-nal-trex-one) methylnaltrexone,

Relistor

(trade name)

Classification

Therapeutic: laxatives
Pharmacologic: opioid antagonists
Pregnancy Category: B

Indications

Treatment of constipation caused by opioid use in patients being treated palliatively, when laxative therapy has failed.

Action

Acts peripherally as mu-opioid receptor antagonist, blocking opioid effects on the GI tract.

Therapeutic effects

Blocks constipating effects of opioids on the GI tract without loss of analgesia.

Pharmacokinetics

Absorption: Rapidly absorbed after subcutaneous administration.Distribution: Moderate tissue distribution, does not cross the blood-brain barrier.Metabolism and Excretion: Some metabolism; 85% excreted unchanged in urine.Half-life: 8 hr.

Time/action profile

ROUTE	ONSET	PEAK	DURATION
Subcut	rapid	0.5 hr	24–48 hr

Contraindications/Precautions

Contraindicated in: Known/suspected mechanical GI obstruction.Use Cautiously in: Known/suspected lesions of GI tract (↑ risk for GI perforation); Obstetric / Lactation: Use in pregnancy only if clearly needed; use cautiously during lactation; Pediatric: Safety and efficacy not established.

Adverse Reactions/Side Effects

Central nervous system

dizziness

Gastrointestinal

GI perforation (life-threatening)
abdominal pain (most frequent)
flatulence (most frequent)
nausea (most frequent)
diarrhea

Dermatologic

hyperhidrosis

Interactions

Drug-Drug interaction

None noted.

Route/Dosage

Subcutaneous (Adults) 38–<62 kg—8 mg every other day, not to exceed every 24 hr; 62–114 kg—12 mg every other day, not to exceed every 24 hr; other weights—0.15 mg/kg every other day, not to exceed every 24 hr.

Renal Impairment

(Adults) CCr <30 mL/min—use 50% of recommended dose based on weight.

Availability

Solution for subcutaneous injection (single use vial): 12 mg/0.6 mL Cost: $500.22 / 7 vialsSolution for subcutaneous injection (prefilled syringes): 8 mg/0.4 mL, 12 mg/0.6 mL Cost: All strengths $71.46 / syringe

Nursing implications

Nursing assessment

Assess bowel sounds and frequency, quantity, and consistency of stools periodically during therapy.
Monitor pain intensity during therapy. Methylnaltrexone does not affect pain or effects of opioid analgesics on pain control.

Potential Nursing Diagnoses

Constipation (Indications)
Diarrhea (Adverse Reactions)

Implementation

Subcutaneous: Pinch skin and administer in upper arm, abdomen, or thigh at a 45° angle using a 1-mL syringe with a 27-gauge needle inserted the full length of the needle. Do not rub the injection site. Solution is clear and colorless to pale yellow. Do not administer solutions that are discolored or contain a precipitate. Solution is stable for 24 hr at room temperature. Protect vials from light. Do not freeze. Do not use single-use vials for more than 1 dose.

Patient/Family Teaching

Instruct patient on administration of methylnaltrexone and disposal of supplies. Usual schedule is one dose every other day, as needed, but no more than one dose in a 24-hr period. Advise patient to read the Patient Information prior to starting therapy and with each Rx refill.
Advise patient that laxation may occur within 30 min, so toilet facilities should be available following administration.
Advise patient to continue taking other medications for constipation unless told not to by health care professional.
May cause dizziness. Caution patient to avoid driving and other activities requiring alertness until response to medication is known.
Advise patient to notify health care professional and discontinue therapy if severe or persistent diarrhea occurs or if abdominal pain, nausea, or vomiting persists or worsens.
Instruct patient to stop taking methylnaltrexone if they stop taking opioid medications.
Advise patient to consult health care professional prior to taking other Rx, OTC, or herbal products.
Advise female patients to notify health care professional if pregnancy is planned or suspected or if breast feeding.

Evaluation/Desired Outcomes

Laxation and relief of opioid-induced constipation.

methylnaltrexone

A proprietary quaternary amine and mu opioid receptor antagonist, which reverses opioid-induced constipation, without impacting on central opioid analgesia.
Indication
Patients in palliative care for whom usual laxatives have failed.
Adverse effect
Abdominal pain, flatulence.