NEW YORK (TheStreet) -- Shares of Athersys (ATHX) - Get Report plummeted more than 55% to a 52-week low of 90 cents on Friday after the biotech company's stem-cell therapy to treat strokes failed a mid-stage study.

The therapy called MultiStem, which was company's only product to reach human trials, failed a mid-stage trial that tested it as a treatment for ischemic strokes, which account for approximately 87% of all stroke cases.

The study tested the safety and efficacy of the treatment when given between 24 and 48 hours after an ischemic stroke. The data showed that MultiStem was not better than a placebo when given after 36 hours, but it also demonstrated that the therapy had better efficacy when given before 36 hours.

"Unfortunately, we just didn't have the window right for this study," Athersys COO William Lehmann told Reuters. "We believe investors should see this as a sign that MultiStem works."

The stock's free fall on Friday eliminated more than 40% of the company's market value.

More than 21 million shares had changed hands as of 12:28 p.m., compared to the daily average volume of 1,537,310.

ATHX data by YCharts

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