* FDA raises concerns of effectiveness of pain patch
* NeurogesX seeks Qutenza approval for pain in HIV patients
* US FDA panel to review proposed new use on Thursday
* Shares fall 23 percent (Recasts first sentence with stock fall, adds details on approval process)
WASHINGTON, Feb 7 (Reuters) - NeurogesX Inc's pain treatment derived from chili peppers had only mixed success at treating pain in HIV patients, U.S. health regulators said on Tuesday, sending shares of the tiny company down 23 percent.
NeurogesX won FDA approval in 2009 for its Qutenza patch as a treatment for pain related to shingles. The product's active ingredient is a synthetic form of the agent that makes chili peppers hot, known as capsaicin.
The company now hopes to get the nod to sell the product for peripheral neuropathic pain that afflicts as many as 40 percent of HIV sufferers.
An FDA committee of outside experts will meet to discuss Qutenza's use among HIV patients on Thursday. The FDA is expected to make a decision by March 7.
On Tuesday, Food and Drug Administration reviewers said in a report that the Qutenza patch produced statistically significant pain reduction among people with HIV.
But that success was due to a 90-minute application. FDA staff said company studies failed to demonstrate the efficacy of the 30-minute treatment the company proposed for use in treating HIV-related pain.
Statistical concerns including a lack of evidence that the results can be repeated "have raised the question of whether evidence of substantial efficacy has been demonstrated for this proposed treatment regimen," said Dr. Bob Rappaport, director of the FDA's Division of Anesthesia, Analgesia and Addition Products.
FDA staff said studies with HIV patients showed no new safety issues.
European Union regulators have already approved Qutenza for controlling pain in nondiabetic adults including people with HIV.
Shares in the San Mateo, California-based biopharmaceutical company were down more than 23 percent at 89 cents on Tuesday morning on the Nasdaq. The exchange has threatened to delist the stock unless it can be sustained at $1 a share or above between now and July 28. (Reporting By David Morgan; Editing by Gerald E. McCormick, Derek Caney and Matthew Lewis)