Merck & Co’s new cannabinoid-based obesity therapy taranabant, in the same class as Sanofi-Aventis’ Acomplia, may aid weight loss though there are some concerns that it has similar problematic side effects to the French firm’s treatment.

Data from a 12-week Phase II trial has been published in the January issue of the Cell Metabolism journal and includes 533 obese people treated with either placebo or 0.5mg, 2mg, 4mg or 6mg of taranabant. At the end of the trial, the average weight lost in each group was 2.6lb (for placebo), 6.4lb, 8.6lb, 9lb and 11.6lb respectively.

Researchers of the Merck-sponsored trial noted that the data compared favourably with weight losses seen in late-stage trials of Sanofi’s Acomplia (rimonabant), which is sold in Europe and other markets but received short shrift in the USA.

Last June, Zimulti (as Acomplia would be known in the USA) was rejected unanimously by a Food and Drug Administration advisory panel which said that its benefits do not outweigh the risk of psychiatric adverse effects, including suicide and seizures. Now the possible problem for Merck from this latest study is that similar side effects have been seen with taranabant.

Patients taking the higher dosages of the Merck suffered more from side effects such as nausea, and vomiting. More worrying, however, is that the researchers noted psychiatric problems “reminiscent” of those observed in Acomplia, like depression, anxiety and irritability.

The study’s authors wrote in Cell Medicine that although these trends “were small and not statistically reliable, any increase in incidence would have potentially immense clinical significance when multiplied across a potential patient population of many millions”.

Nevertheless the New Jersey-based firm is encouraged by the findings. Steven Heymsfield of Merck Research Laboratories, a researcher involved with the study, said it was a pleasant surprise to see weight loss even at the lowest loses of taranabant but noted that the studies are at an early stage and a much larger Phase III trial is ongoing. Both taranabant and rimonabant work by blocking cannabinoid type 1 receptors in the brain, which are the same receptors that cause people to get the “munchies” when smoking marijuana.

At its annual investor meeting last month, Merck said that it hopes to file taranabant by the end of the year but acknowledged that the psychiatric side effect problem needs to be addressed if the drug is not going to go the same way as Acomplia.