A review of methylnaltrexone, a peripheral opioid receptor antagonist, and its role in opioid-induced constipation.

OBJECTIVES

Opioid medications are frequently used in pain and palliative care patients with malignancy to manage symptoms such as pain and dyspnea. However, opiates are associated with various side effects. Constipation is a particularly problematic and common side effect of opioid pharmacology. Opioid antagonists have been studied in the management of opioid-induced constipation. Methylnaltrexone (MNTX) is a peripheral opioid antagonist currently under clinical investigation. It offers the potential to reverse undesirable side effects without reversing analgesia.

METHODS

This article attempts to review existing clinical data, focusing on antagonism of opioid-induced adverse effects on the gastrointestinal system.

RESULTS

MNTX seems to be well tolerated with limited or transient side effects. MNTX has been shown to improve oral-cecal transit times in opioid treated patients, induce laxation in chronic opioid users, and neither reverses the analgesic effects of morphine nor cause withdrawal symptoms.

SIGNIFICANCE OF RESULTS

Larger clinical trials of MNTX are still necessary to support its use as a standard for treatment of opioid-induced constipation.