The Food and Drug Administration will review a constipation drug marketed by Salix Pharmaceuticals (Nasdaq: SLXP) to consider adding a new indication for the product.

Relistor is an injectible drug approved in 2008 to to treat constipation in seriously-ill patients taking pain-killing opioid medication. Opioids relieve pain by acting on receptors in the spinal cord and brain. But opioids also interact with receptors in the gastrointestinal tract, which causes constipation.

Relistor can reduce the constipating effects of opioids without affecting their pain-killing abilities.

Salix and drug partner Progenics Pharmaceuticals (Nasdaq: PGNX) want to expand Relistor’s use to treat opioid-induded constipation in patients taking the drugs for non-cancer pain. Robert Israel, Progenics senior vice president of medical affairs said that the supplemental new drug application includes results from 31 clinical studies involving more than 4,000 patients. The FDA has set an April 27, 2012 target date to take action on the sNDA.

Salix secured licensing rights to Relistor in a February deal that paid Progenics $60 million up front. As a specialty pharma company focused on GI drugs, Relistor fits with a Salix sales force that already markets GI drugs such as traveler’s diarrhea treatment Xifaxan as well as ulcer drug Pepcid, whose marketing rights Salix acquired from Merck (NYSE: MRK) in 2007.

If Relistor reaches development milestones in the United States, Progenics is due another $90 million. Sales-based milestones are valued at up to an additional $200 million.

Salix has marketing rights to Relistor globally except for Japan, where Ono Pharmaceuticals holds rights to the drug.

Before Salix licensed Relistor’s rights, the drug was marketed by Wyeth, which is now part of Pfizer (NYSE: PFE). Worldwide Relistor sales totaled $16 million in 2010.

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