Labopharm Inc.'s chief executive said on Friday he was "pretty confident" the company's once-daily version of the pain-killer tramadol would receive U.S. approval despite a letter from the U.S. Food and Drug Administration that said some issues still must be resolved.

"We're pretty confident. You're not done until you're done," James Howard-Tripp, Labopharm president and CEO, told Reuters.

"We have got it approved in other jurisdictions. We believe it's a good product. We believe in our data package, so we would be pretty optimistic."

Labopharm shares dropped almost 25 percent on Friday after the company said it had received an approvable letter from the U.S. Food and Drug Administration for its version of the tramadol pain killer, but that the FDA said there are some unresolved issues.

The shares were down C$1.69, or 20.8 percent, at C$6.42 on the Toronto Stock Exchange, after dropping as low as C$6.09 earlier in the day. On Nasdaq, the shares were off $1.56, or 21.3 percent, at $5.75 after earlier falling as low as $5.42.

Howard-Tripp said the company plans to discuss the letter with the FDA "fairly rapidly" and believes it can address the issues raised without the need for additional data.

Howard-Tripp refused to say what issues were flagged.

"We are not trying to be deliberately obscure. We need to fully understand quite what it is they want, and until we fully understand, we obviously can't tell the marketplace what it is," he said.

"If we can't understand it, it doesn't make sense to try to put information out."

Analysts said the news was a setback for the company and raised doubts that it would meet its earlier targeted approval date.

"I think this is more of a setback in timing, but the problem is, we don't know for how long," said Laurence Terrisse-Rulleau, a biotechnology analyst at Blackmont Capital in Montreal.

"I'm still confident in the product. I'm still confident that they will get approved, I just don't know when."

Terrisse-Rulleau said it is unlikely that the company will meet the approval target of first quarter 2007. She said it will now "most likely" be in the third quarter of 2007.

"The problem is it depends if the FDA is going to take 50 days or six months," she said. "And it is going to take weeks before we find out."

Labopharm's new drug application for tramadol was submitted to the FDA in November 2005.

Once-daily tramadol, Labopharm's key product, has received regulatory approval in 22 European countries and commercial launch of the product across Europe is under way.