Merck ($MRK) is racing to be first on the market among contenders in the PD-1 field of cancer therapy, and the European Medicines Agency has agreed to review its pembrolizumab as a treatment for advanced melanoma. The drug, formerly MK-3475, is an immunotherapy designed to galvanize the body's T cells against tumors and spur the self-destruction of cancer cells. Merck has already filed for FDA approval in the same indication, and the company leads a pack of drugmakers developing similar therapies, including Bristol-Myers Squibb ($BMY), Roche ($RHHBY) and AstraZeneca ($AZN). PD-1 therapies have charted excellent clinical activity across a fleet of trials in various cancers, and analysts expect the market for such drugs to reach as high as $35 billion a year. More