In an announcement that cannot fail to have reverberations for Acomplia (rimonabant), Merck & Co. announced on Oct. 2 that it has halted development of its experimental antiobesity drug taranabant because of concern over side-effects.

Merck said taranabant, which like rimonabant acts by blocking certain cannabinoid receptors (CB-1 receptors) that regulate appetite, was associated with increased risk of psychiatric events -- the factor that has kept Acomplia off the U.S. market.

Merck said in its announcement that both effectiveness and side effects are dependent on dose levels, with higher doses producing greater weight-loss but more adverse events.

"Available Phase III data showed that both efficacy and adverse events were dose related, with greater efficacy and more adverse events in the higher doses," said John Amatruda, Merck's senior vice president and research head for diabetes and obesity.

"Therefore, after careful consideration, we determined that the overall profile of taranabant does not support further development for obesity.," he added.

Five clinical trials of the drug are being halted, said Merck spokesperson Amy Rose.

While Merck was testing its drug at a variety of lower dose levels than the dosage approved for Acomplia, which is available in the European Union and a number of other countries, Merck essentially conceded that dose levels that minimize risk do not adequately help people lose weight.

While Acomplia continues to pursue development of Acomplia, Merck's halt to development of taranabant suggests that this class of drugs -- if submitted for new indications such as diabetes -- is likely to face tough scrutiny from both U.S. and European regulators.

Pfizer still has a CB-1 receptor antagonist, CP-945598 (otenabant), in late-stage clinical trials, but given that the company said this week it is abandoning early-stage research in obesity, the future of this drug appears murky.