UPDATE 2-Labopharm sees FDA rejecting pain formulation

* Labopharm's pain combination misses key endpoint

* Says New Drug Application likely in jeopardy

* Plans to discuss with FDA

* Shares drop 25 percent after the open. (Adds details)

TORONTO, April 2 (Reuters) - Shares of Labopharm Inc (DDS.TO) tumbled on Thursday after it said a twice-daily pain combination missed a key goal in a late-stage trial and U.S. regulators would likely reject its application for the formulation's approval.

The study related to its twice-daily formulation of tramadol and acetaminophen. It showed results that were not statistically significant when judging pain-intensity differences, an important goal of the study.

The Montreal-based drugmaker said it planned to discuss the findings with the U.S. Food and Drug Administration to determine "the path forward."

Labopharm said that the study results would have no bearing on the upcoming U.S. launch of its once-daily tramadol product which received regulatory approval late last year.

The shares dropped 25 percent to C$1.23 soon after the Toronto Stock Exchange opened. ($1=$1.25 Canadian) (Reporting by Scott Anderson, editing by Frank McGurty)