Wyeth Pharmaceuticals, a division of Wyeth, and Progenics Pharmaceuticals, Inc. has announced preliminary results from two clinical trials conducted with investigational oral and intravenous formulations of Relistor (methylnaltrexone bromide).

The first of these studies, a phase II trial, evaluated the effects of an oral formulation of Relistor for the treatment of opioid-induced constipation (OIC), in patients with chronic, non-malignant pain. This study showed positive activity.

The second study, a phase III trial, examined the use of an intravenous formulation of Relistor for post-operative ileus (POI). In this study, the drug did not meet its primary or secondary end points.

The study of the Relistor oral formulation was a double-blind, randomized, placebo-controlled phase II trial. In this four-week trial of 122 patients with chronic, non-malignant pain (such as back pain, neuropathic pain or osteoarthritis) who were receiving opioids for pain management, the once daily oral formulation of Relistor showed statistically significant activity as assessed by the occurrence of spontaneous bowel movements and other efficacy measures. This oral formulation was also shown to be generally well tolerated.

"We are pleased by the preliminary findings of this oral formulation," says Paul J. Maddon, M.D., Ph.D., founder, chief executive officer and chief science officer of Progenics Pharmaceuticals, Inc. "We await results within the coming months from a second, ongoing phase II study involving the oral formulation of Relistor that was announced last July to have positive activity in a phase I trial".

Relistor is a peripherally acting mu-opioid receptor antagonist that counteracts the constipating effects of opioid pain medications in the gastrointestinal tract without affecting their ability to relieve pain. Opioids provide pain relief by specifically interacting with mu-opioid receptors within the central nervous system (CNS) - the brain and spinal cord.

However, opioids also interact with mu-opioid receptors found outside the CNS, such as those within the gastrointestinal tract, resulting in constipation that can be debilitating. Relistor selectively displaces opioids from the mu-opioid receptors outside the CNS, including those located in the gastrointestinal tract, thereby decreasing their constipating effects. Because of its chemical structure, Relistor does not affect the opioid-mediated analgesic effects on the CNS.