Depressed? Merck's research neuroscientists would have reason to be. The pharma giant bet a fortune over the past decade that a novel compound that blocks a neurotransmitter called "Substance P" would become an effective new treatment for depression-a multibillion-dollar global market profitably dominated by Prozac, Paxil, and Zoloft. Merck bet wrong. Its aspiring antidepressant performed well in early experiments but flunked the phase III clinical trials required by the U.S. Food and Drug Administration. Not good.

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