Yesterday, Adolor (Nasdaq: ADLR) and marketing partner GlaxoSmithKline (NYSE: GSK) got a bit of good news from an FDA advisory panel after it gave a mostly positive report on Adolor's lead drug, Entereg.

Entereg is set for a Feb. 10 FDA decision on its marketing application as a treatment for post-operative ileus (POI). Currently, there are no approved drugs available for the painful post-abdominal-surgery constipation-like condition. In a close 9-6 vote, the advisory panel agreed that the benefits of Entereg's use in POI outweighed its risks, and it was agreed 14-0 that Adolor and GSK needed to develop a better risk-management plan to prevent off-label usage of the drug.

More importantly, the FDA briefing documents presented before the panel hearing indicate that the FDA will approve Entereg once Adolor and Glaxo better develop this risk-management plan. This is something that should take a few months at most, so expect an FDA approvable letter around Feb. 10, with a quick class 1 response from the duo, and final approval coming a couple of months later.

With some previous safety issues in its long-term use, the FDA is going to do its best to prevent off-label usage of Entereg outside of its proposed indication of helping prevent POI for bowel resection surgeries.

The question thus becomes: What is the size of this market? Some journal articles have estimated that the total number of annual abdominal-related surgeries -- like hernia repairs or hysterectomies -- in the U.S. to be nearly 1.7 million surgeries.

The label for Entereg will likely indicate the drug can be used only in large and small-bowel resection surgeries, though, since this is where it showed the most effectiveness in preventing POI. The size of this patient group is approximately 300,000 annually, and Entereg will be given for up to eight days to these patients. Almost anyway you slice it, and even if you give Entereg credit for some use in other abdominal-related surgeries, this puts the market potential for Entereg at most in the low couple of hundred million dollars in the U.S., barring further label expansions into the nearly 460,000 annual abdominal hysterectomies.

Glaxo will be doing all the marketing of Entereg worldwide. Adolor and GSK will be splitting the profits from all sales, with Adolor getting 45% of all net sales. By my rough estimations, this puts Adolor's peak income from Entereg with its current label in the high double-digit millions of dollars or low one hundred million dollar range, depending on how the competition shakes out.

Speaking of competition, Adolor and GSK will have the whole POI market to themselves for at least a year unless Progenics Pharmaceuticals (Nasdaq: PGNX) and Wyeth (NYSE: WYE) can get their rival drug approved for use in POI. Progenics is about a year behind Adolor with its POI drug and expects to file for FDA approval in the middle of 2008 for its compound. Progenics hasn't had the safety issues with its POI drug that Adolor has had in longer-term dosing, but compared to Entereg's tablet form, Progenics' compound is a much less convenient intravenous infusion.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. GlaxoSmithKline is an active Income Investor pick. The Fool has an A+ disclosure policy.