Endogenous Opioids and the Gastrointestinal Tract

Peripherally acting mu-opioid receptor antagonists and postoperative ileus: mechanisms of action and clinical applicability

Postoperative ileus (POI), a transient cessation of coordinated bowel function after surgery, is an important health care problem. The etiology of POI is multifactorial and related to both the surgical and anesthetic pathways chosen. Opioids used to manage surgical pain can exacerbate POI, delaying gastrointestinal (GI) recovery. Peripherally acting mu-opioid receptor (PAM-OR) antagonists are designed to mitigate the deleterious effects of opioids on GI motility. This new class is investigational for POI management with the goal of accelerating the recovery of upper and lower GI tract function after bowel resection. In this review, we summarize the mechanisms by which POI occurs and the role of opioids and opioid receptors in the enteric nervous system, discuss the mechanism of action of PAM-OR antagonists, and review clinical pharmacology and Phase II/III POI trial results of methylnaltrexone and alvimopan. Finally, the role of anesthesiologists in managing POI in the context of a multimodal approach is discussed.

Alvimopan accelerates gastrointestinal recovery after bowel resection regardless of age, gender, race, or concomitant medication use

BACKGROUND

Postoperative ileus is a transient cessation of bowel motility that occurs after bowel resection (BR). Alvimopan, a peripherally acting mu-opioid receptor antagonist accelerated gastrointestinal (GI) recovery in 5 randomized, double-blind, phase III postoperative ileus trials.

METHODS

Individual covariates (age, gender, race) were assessed separately using Cox proportional hazards models that included the main effects of treatment and covariate factor. Time-to-GI recovery (GI-3 [first toleration of solid food and first bowel movement or flatus]; GI-2 [first toleration of solid food and first bowel movement]) for patients who underwent open laparotomy for BR in the absence of epidural anesthesia and received alvimopan (12 mg) or placebo was analyzed within subgroups (age, gender, race, concomitant medication use) using Cox proportional hazards models to generate hazard ratios (HRs). P values were calculated with the Wald chi2 test.

RESULTS

Elderly (>or=65 years), male, and nonwhite patients achieved GI-3 recovery later than younger (<65 years), female, and white patients (HR > 1 and P < .05 for all). Overall, alvimopan (12 mg) accelerated GI-3 recovery by 12 hours and GI-2 recovery by 17 hours compared with placebo. Within subgroups, regardless of covariate effect, patients who received alvimopan (12 mg) achieved GI-2 and GI-3 recovery sooner than patients who received placebo (HR > 1 and P < .05 for all).

CONCLUSIONS

These post hoc analyses support that alvimopan (12 mg) accelerates GI recovery across various patient populations.

Treatment of opioid-induced gut dysfunction

Opioid analgesics are the mainstay in the treatment of moderate-to-severe pain, yet their use is frequently associated with adverse effects, the most common and debilitating being constipation. Opioid-induced motor stasis results from blockade of gastrointestinal peristalsis and fluid secretion, and reflects the action of the endogenous opioid system in the gut. Methylnaltrexone and alvimopan are new investigational drugs that selectively target peripheral mu-opioid receptors because they are poorly absorbed in the intestine and do not enter the brain. Clinical studies have proved the concept that these drugs prevent opioid-induced bowel dysfunction without interfering with analgesia. As reviewed in this article, opioid receptor antagonists with a peripherally restricted site of action also hold therapeutic promise in postoperative ileus and chronic constipation due to the fact that they have been found to stimulate intestinal transit.