The companies said the Food and Drug Administration has requested additional overall survival data on ipilimumab. They are revising timelines and said the application will not be submitted in 2008.

But the firms also said they remain committed to developing ipilimumab.

Medarex's shares fell 11% to $7.10 in after-hours trading while Bristol-Myers' were flat at $22.15.

Earlier this month, Medarex lost 15% of its value because of its licensing agreement to receive double-digit royalties on sales of Pfizer Inc.'s (PFE) tremelimumab, which failed in a late-stage trial for treating advanced melanoma and now has an uncertain future.

The Princeton, N.J., biotech said ipilimumab differed from the skin cancer treatment on which Pfizer halted clinical trials after a review of interim data showed that the drug was not superior to standard chemotherapy.

The two drugs belong to a class of drugs called CTLA-4 antagonists because they are antibodies that bind to protein, CTLA-4, which may block the body's immune response to certain cancers. Theoretically, immobilizing the protein would induce the body to attack the disease.

Medarex says the two antibodies are different molecules; three studies of its Phase II program were suggestive of ipilimumab's potential for clinical anti- tumor activity; and its Phase III program is designed differently from Pfizer's trial.

Twelve abstracts evaluating ipilimumab in melanoma will be presented at the American Society of Clinical Oncology annual meeting in June.