How does a 20 percent drop sound? That’s what happened to shares in the fledgling biopharma today after one of three studies showed its experimental cancer drug, ipilimumab, missed its primary endpoint for treating melanoma in 10 percent of patients. The drug is supposed to be a back-up therapy to be used when so-called first line meds don’t work.

Despite the bad news, Medarex and its partner, Bristol-Myers Squibb earlier today held a briefing for analysts and vowed to push ahead with plans to meet with FDA officials and file for marketing approval in the first half of 2008. (Here is the Bristol statement). The thinking is that melanoma sufferers have few treatment options, potentially raising the odds that the experimental med will somehow pass muster.

But Wall Street doesn’t think so. Mark Monane, an analyst at Needham, lowered his rating today on Medarex. “While the company has announced plans to discuss potential approval of the drug with the FDA based on the encouraging secondary outcomes and the overall totality of the data, we are not optimistic about a favorable outcome in 2nd line melanoma,” he writes in an investor note.

This isn’t good news for Bristol either, of course. The drugmaker, which was once an oncology powerhouse and is now undergoing a big downsizing to save $1.5 billion, recently gave up the rights to a bladder cancer drug.