Clinical status of methylnaltrexone, a new agent to prevent and manage opioid-induced side effects.

Opioids are associated with a number of adverse effects, constipation being the most common long-term adverse effect in patients with advanced cancer. Significant progress has been made over the past several decades in understanding the mechanisms of action of opioid compounds; however, these advances have yielded few new treatments for the bowel dysfunction caused by opioids. Methylnaltrexone, the first peripheral opioid receptor antagonist and currently under clinical investigation, has the potential to prevent or treat opioid-induced peripherally mediated side effects, such as constipation, without interfering with analgesia. This article reviews existing clinical data on methylnaltrexone, focusing on the antagonism of opioid-induced adverse effects in the gut.