Wednesday's 9% rise in Adolor (NASDAQ:ADLR) shares owed to the increased possibility of its lead drug's approval, after the FDA designated an advisory panel to review it.

Adolor's opioid-induced and post-operative ileus treatment Entereg, partnered with GlaxoSmithKline (NYSE:GSK), has had a long and winding development history. Adolor has already received two approvable letters for Entereg in 2005 and 2006. The FDA first decided not to approve the drug in 2005 for lack of evidence of efficacy, then asked for more safety data in the 2006 letter.

Earlier in the year, Adolor's hopes of fixing the FDA's safety concerns appeared dashed, after new adverse events popped up in a study intended to assuage the agency's safety concerns about the drug. Entereg-treated patients experienced a previously unseen increased rate of neoplasms (abnormal cell growth that can sometimes lead to cancer) compared to placebo patients.

Despite the new safety issues from this long-term-use study, Adolor persisted in filing another Entereg marketing application for post-operative ileus (POI). POI patients will only be taking Entereg for a short time, and may not experience the same risks as with its long-term use.

It's hard to see Entereg getting FDA approval after the negative safety data popped up. An advisory panel hearing means that the fate of Entereg for POI may be slightly less negative than before, though.

Investors in shares of Progenics Pharmaceuticals (NASDAQ:PGNX) and its partner Wyeth (NYSE:WYE) will also benefit from following the panel hearing, since they are awaiting a Jan. 30 PDUFA date on a similarly targeted compound.

Investors should mark their calendars; the Entereg advisory committee is scheduled to meet Jan. 23. With a PDUFA date less than three weeks later on Feb. 10, the meeting results will help investors make a better -- but by no means sure -- bet on which way the FDA is leaning.

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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.