Bristol-Myers Squibb wants to dominate an emerging field of medicine that unleashes the immune system against tumors. Its melanoma drug ipilimumab is one of the stars of the American Society of Clinical Oncology meeting in Chicago.

"We intend to be at the leading edge" of the new field, says Bristol-Myers chief scientific officer Elliott Sigal, speaking from his 31st story hotel room overlooking the sprawling convention center. "This is just the first step." Bristol has more immune-boosting cancer drugs in early stages of testing, including one called PD1 antibody.

But getting the melanoma drug approved could prove surprisingly tricky. Its main trial included an unconventional control arm, which could raise eyes at the FDA. Instead of comparing Bristol's ipilimumab to a placebo or to standard treatments (there are few options in melanoma), the trial compared ipilimumab to a vaccine called gp10o. Patients who got ipilimumab lived longer.

The interpretation most melanoma researchers are making here is that ipilimumab is effective, and the vaccine essentially acted as a placebo.

But the FDA may ask: How do researchers know that the vaccine is not toxic, and that this--not effects of ipilimumab--accounted for the difference in survival between the two patient groups?

"That is a credible scientific question. It is a question that I asked when I first saw the data: how do we know for sure that gp100 is not causing some [negative] change," says Sigal.

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