Merck & Co., which is developing a diet drug that works by blocking the CB-1 receptor like Sanofi's diet pill rimonabant (Acomplia / Zimulti), announced on Dec. 12th it hopes to file for U.S. Food and Drug Administration approval of the weight loss drug in 2008.

If the FDA approval process for Merck's MK-0364 takes the usual six months, Acomplia -- which has been stalled at the FDA for more than a year with no early action in sight -- may wind up with not much more than a one-year head-start on other diet pills that block CB-1 receptors.

Pfizer, like Merck, has a CB-1 receptor antagonist in Phase III trials but has not yet announced when it expects to file for FDA approval. Bristol-Myers Squibb and its partner, Solvay Pharmaceuticals, also have a CB-1-antagonist under development but are thought to be a bit further behind.

Merck said Tuesday that its diet drug is a "highly selective cannabinoid-1 (CB-1) receptor inverse agonist that has shown to be efficacious in weight loss versus placebo in early clinical studies."

The president of Merck's research arm, Peter Kim, provided the new information about MK-0364 to investors and analysts at the Merck Annual Business Briefing.

While Merck seems to be focusing on a 2 mg dose of MK-0364 in its Phase III trial -- a far smaller dosage than the 20 mg rimonabant pills Sanofi is already selling in Europe -- Kim conceded that MK-0364 like Acomplia had been associated in trials with some adverse psychiatric experiences.

But Kim said Merck's drug had been found safe and generally well tolerated in the company's extensive Phase II studies, in which it has been testing 2 mg, 4 mg and 6 mg dosages of the diet drug against a placebo with more than 5,600 participants.

It now has launched a Phase IIb/III trial involving about 1,000 participants -- 400 of whom are getting the 2 mg dose, with 200 getting a 1 mg dose, 200 getting a .5 mg dose, and 200 getting a placebo -- suggesting it has concluded that lower may be better.