* Agency cites concerns with niacin added to pain drug
* FDA advisory panel to give input on Thursday
* Acura shares fall 21 percent (Adds company comment, updates share price)
WASHINGTON, April 20 U.S. regulators questioned if an experimental pain drug from King Pharmaceuticals KG.N and Acura Pharmaceuticals (ACUR.O) would deter abuse, documents released on Tuesday showed.
Shares of Acura, which is aiming to win approval for its first product, fell 21 percent to $6.23 on Nasdaq.
Acurox pairs an immediate-release form of the opioid painkiller oxycodone with niacin, a B vitamin. Adding niacin was meant to discourage people from taking multiple pills at once as high niacin doses can cause uncomfortable flushing.
Food and Drug Administration staff, in a summary prepared for an advisory panel, said "while the oxycodone component in Acurox is efficacious, the agency has concerns about the use of niacin."
An Acura study suggested "niacin offers little in the way of deterrence to oral abuse," the summary said. Studies showed niacin's negative effects could be reduced by taking it with food.
Adding niacin also "appears to negatively affect the adverse event profile of this drug," the FDA staff said, noting higher reports of flushing in pain patients treated in Acurox compared with a placebo.
The FDA will ask an advisory panel for input on the drug at a public meeting on Thursday.
A key question will be if flushing is an acceptable risk for legitimate patients with pain, according to a separate memo prepared for the panel.
The agency also will ask the committee of outside experts about the ability of food and over-the-counter pain medicines to reduce flushing.
"These are clearly difficult questions for which there are no easy answers," wrote Dr. Bob Rappaport, director of the FDA division that is reviewing Acurox.
Acura, in a separate summary prepared for the panel, said the drug would limit misuse without "without incurring an incremental safety penalty." In addition to niacin, the drug includes ingredients designed to cause burning in the nose if an abuser crushes and snorts it. The drug also turns to a gel if added to water or another solvent to discourage injection.
"It is not a perfect solution to the opioid abuse crisis, but it is a solution that warrants approval of the product," the company said in the summary.
Acura Chief Financial Officer Peter Clemens said in an interview that some patients in the company's late-stage studies of Acura felt minor effects of niacin. But he said none of the effects were severe or caused patients to drop out of the studies.
The FDA usually follows panel recommendations when deciding whether to clear new products for sale.
King shares gained 8 cents to close at $11.50 on the New York Stock Exchange. (Reporting by Lisa Richwine, editing by Gerald E. McCormick)