Shares of Biohaven Pharmaceutical Holding Company Ltd. BHVN, +0.10% tumbled 16% in premarket trading on Monday after the company said its experimental treatment for generalized anxiety disorder failed in a clinical trial. Patients taking the investigational drug, troriluzole, did not have a different result than those using the placebo. Biohaven said it will continue to test troriluzole as a treatment for obsessive compulsive disorder, Alzheimer's disease and spinocerebellar ataxia, a rare brain disease. Biohaven's stock is up 19% over the past year. The S&P 500 SPX, -1.25% has gained about 22%.