Merck reiterated on Dec. 12th that it plans to file for FDA approval of diet drug taranabant in 2008 even though some adverse psychiatric side-effects have been observed similar to those found with Sanofi's diet drug Acomplia (rimonabant).

Taranabant, a CB-1 receptor inverse agonist, and rimonabant, a CB-1 receptor antagonist, are in the same class of drugs which operate by blocking receptors in the brain and other organs that when activated by marijuana make pot smokers feel hungry.

While rimonabant was approved for sale in Europe where it is marketed as Acomplia, an FDA advisory panel earlier this year overwhelmingly recommended that it not be approved in the U.S., where it was to be marketed as Zimulti, expressing concern over lack of data on psychiatric side effects.

Sanofi subsequently withdrew the FDA application for rimonabant, but has indicated it intends to refile when it has more data.

Now Merck says that its diet drug, which is nearing the end of phase III trials, has shown that like Acomplia it can help overweight and obese people lose weight, but conceded that researchers also have observed psychiatric side effects.

"We also observed a dose-dependent increase in psychiatric adverse experiences, which we think are mechanism-based," Peter Kim, Merck's president of research, told investors.

In an effort to minimize problems, Merck is studying a 4 mg dose and a 6 mg dose of taranabant in its phase III clinical trial. While Sanofi tested a 5 mg dose of rimonabant, results at that dosage were disappointing and it ultimately settled on the 20 mg dose being sold in Europe and some other countries.

Merck also is including people with controlled depression in its clinical trials unlike Sanofi, which excluded those with a history of depression.