While Merck is wrapping up a series of clinical trials of diet drug Taranabant, which it expects to submit for FDA approval later this year, Pfizer has just launched a new Phase III trial of its diet drug CP-945,598, suggesting it may be considerably further away from being brought to market.

Merck has two Phase III trials –one a two-year Phase III study involving 2,400 patients, the second a one-year Phase III study involving 1,000 patients — of taranabant, a CB-1 receptor inverse agonist, that appear to have been completed in the first two months of this year.

To this point, Merck has provided no information on the results of these studies — and for that matter, still has released no final information from a two-year, 2,400-patient Phase II study that was completed in December.

Meanwhile, Pfizer’s clinical trial program for its drug targeting the same receptor — which unlike taranabant, is a CB-1 receptor antagonist like rimonabant (Acomplia) — appears to be lagging significantly behind.

Pfizer seems to still be recruiting participants for its two-year Phase III study of CP-945,598 — which began back in 2006 — and lists the estimated completion date for this study as August 2009.

Moreover, Pfizer has only recently begun recruiting 850 participants for a one-year Phase III trial of CP-945,598 — which like the Merck one-year trial of taranabant is focused not on weight-loss but on the use of the drug to prevent weight-regain — and this trial is not set for completion until Jan. 2010.

The notoriously close-mouthed Pfizer also has provided no information on results of its earlier Phase II study of CP-945,598, which was completed more than two years ago.

Interestingly, however, both Merck and Pfizer have one-year Phase III studies underway of the use of their CB-1 drugs in treating type 2 diabetics.

The treatment of diabetes as well as obesity seems to be at the heart of the hopes of Sanofi-Aventis for a second try next year at U.S. approval of rimonabant. Sanofi withdrew its original application last summer after a U.S. FDA advisory panel expressed concern over the drug’s depressive side-effects.