Effects of 500 mg Immediate Release and Extended Release Methylnaltrexone on Loperamide-induced Delay of the Oro-cecal and Whole-gut Transit Time in Healthy Subjects

The increasing prevalence of opioid use and consequently, opioid-induced bowel dysfunction has prompted interest in identifying effective treatment options. Until now, the treatment of opioid-induced constipation (OIC) has been viewed as an extension of constipation in general. Traditional therapies for constipation such as bulking agents, stool softeners, stimulant laxatives, and osmotic agents are commonly utilized, but the effects of such therapies are nonspecific and are often generating diarrhea or cramps and some of these drugs cause severe side effects. Furthermore, these conventional measures are sometimes insufficient in some patients, especially those requiring increasing doses of opioids. Opioid-induced constipation is predominantly mediated by gastrointestinal μ-opioid receptors. Selective blockade of these peripheral receptors might relieve constipation without compromising centrally mediated effects of opioid analgesia or precipitating withdrawal. Naloxone is a competitive antagonist of opioid receptors inside and outside the central nervous system used as a solution for injection in the treatment of opioid overdose. When administered orally, it can reduce opioid-induced constipation due to a local action in the gut. Another way to prevent central actions is the use of opioid antagonists which can't penetrate the blood-brain barrier such as methylnaltrexone and alvimopan. Their antagonism of μ-opioid receptors in the gastrointestinal tract seems to reverse opioid-induced gut hypomotility. It is assumed that methylnaltrexone after oral administration influences intestinal motility by local blockade of opioid receptors along the luminal surface of the gut. Because methylnaltrexone seems to have an absorption window in the proximal small intestine as caused by lower activity of P-glycoprotein in that region (similar to other quaternary compounds, eg. trospium chloride), immediate release (uncoated) methylnaltrexone is better absorbed form the small intestine and might therefore be less active than the enteric-coated drug. However, the pharmacokinetic and pharmacodynamic data on oral methylnaltrexone are very preliminarily so far. Furthermore, intestinal transit time has been measured using lactulose as a probe compound that has an own laxative effect. Therefore, the following clinical study was initiated to proof the concept in a controlled clinical trial in healthy subjects, whether methylnaltrexone antagonizes the loperamide induced delay of oro-cecal and whole-gut transit time after oral administration of immediate and extended release capsules in comparison to subcutaneous administration. Loperamide is an opioid agonist and acts on the µ-receptors in the myenteric plexus. It does not affect the central nervous system like other opioids. Loperamide significantly prolongs the mouth-to-cecum transit time as evaluated by the lactulose hydrogen breath test. This effect may be antagonized by the concomitant administration of naloxone.