Within hours of Sanofi's decision to pull the plug on diet drug Acomplia (rimonabant), Pfizer announced it was also killing a similar diet pill, otenabant (CP-945,598), which had been in Phase III clinical trials.

Otenabant, like the far higher profile Acomplia, worked by blocking the CB-1 receptors in the brain -- the cannabinoid receptors believed to play a critical role in the regulation of food intake and energy expenditure.

Curiously, Pfizer said it was killing otenabant -- which it had been expected to submit for U.S. FDA approval next year -- even though it believed it had the potential to be both safe and effective.

"Based on all currently available information and an ongoing review of the phase 3 program by an independent Data Monitoring Committee, Pfizer believes that the CP-945,598 compound has the potential to be a safe and effective treatment for weight management," the company said in a statement.

"However, the company has decided to discontinue the development program based on changing regulatory perspectives on the risk/benefit profile of the CB-1 class and likely new regulatory requirements for approval.

“While confident in the safety of the compound, we believe that this is the appropriate decision based on all available information regarding this class of agents, as well as recent discussions with regulatory authorities,” said Martin Mackay, president, Pfizer Global Research and Development.

The Sanofi and Pfizer decisions come just over a month after Merck & Co. announced that it was halting development of a similar experimental antiobesity drug taranabant, which also acted on CB-1 receptors, because of concern over side-effects.

While that clearly means no CB-1 receptor blockers, with the exception of the rimonabant look-a-likes being produced in India, will be available for some number of years, several CB-1 weight-loss drugs remain in clinical trials.

The most advanced so-called CB- 1 receptor antagonist now in development is ibipinabant, being developed by Bristol-Myers Squibb and Solvay, and a developmental drug called AZD-2207 being developed by AstraZeneca.

Both of these continue to be in mid-stage Phase II tests, according to Thomson Pharma data. A second AstraZeneca drug, AZD-1175, and a drug known as V-23434 from Vernalis are in Phase I development.