Mersana Therapeutics Inc.’s stock MRSN cratered 73% premarket Thursday, after the company announced that the U.S. Food and Drug Administration had placed a partial clinical hold on two trials of a treatment for ovarian cancer after five participants died. The agency has directed to company to pause new patient enrollment in the trials dubbed Up-Next and Upgrade-A, which are evaluating UpRi in platinum-sensitive ovarian cancer. “The partial clinical hold follows a submission by Mersana of a recent aggregate safety report of all patients dosed with UpRi (approximately 560 patients) evaluating bleeding events,” the Cambridge, Mass.-based company said in a statement. While most of those cases were deemed to be low-grade, five fatal events occurred. The cause remains under observation. The stock was up 63% in the year through Wednesday’s close, while the S&P 500 SPX has gained 14%.
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