Merck diet drug taranabant, which targets the same CB-1 receptor as Sanofi's controversial obesity treatment rimonabant (Acomplia / Zimulti), appears to be effective at a much lower dose, according to results of a Phase III study presented at the scientific meeting of the American College of Cardiology.

Merck said one-year results from the two-year Phase III study showed that patients taking a 2 mg dose of taranabant had a mean weight loss reduction of 14.5 pounds compared to 5.7 pounds for patients on a placebo.

While Merck also tested 4 mg and 6 mg doses of taranabant, it said the incidence of psychiatric side effects -- the problem that has thus far blocked approval of rimonabant for sale in the U.S. -- was greater at the higher doses, and that further development of taranabant would focus on the 2 mg dose.

Acomplia is sold in Europe and a number of other countries at a 20 mg dosage.

The researchers said 57 percent of the patients taking the 2 mg dose of taranabant lost five percent of their body weight, and 28 percent lost 10 percent of their body weight, with maximum weight loss in most patients achieved by the 36th week of the trial.

Researchers reported that the incidence of psychiatric adverse events was 28 percent for patients on taranabant 2 mg, 40 percent on taranabant 4 mg, and 38 percent on taranabant 6 mg.

"Based on the benefit-risk considerations and the lack of a substantial improvement in the efficacy of taranabant at the 4 mg and 6 mg doses seen in our clinical program compared to the 2 mg dose, we have decided to continue to evaluate taranabant in doses up to and including 2 mg in our Phase III studies," said Dr. John Amatruda, Merck's vice president of clinical research in metabolic disorders.

Taranabant, like rimonabant, works by blocking the cannabinoid-1 receptor, which is believed to be responsible for regulating weight by impacting appetite and food intake, satiation (the feeling of satisfaction or fullness after a meal), energy expenditure and cravings.

Dr. Louis Aronne, director of the comprehensive weight control program at New York Presbyterian Hospital, said "the results of this study suggest that taranabant, an investigational therapy, has the potential to be a valuable treatment option, if approved, for patients suffering from obesity and its many complications."