CONSHOHOCKEN, Pa. ( TheStreet) -- U.S. regulators rejected NuPathe's ( PATH) migraine patch Zelrix, the company said Tuesday.

NuPathe shares were down 51% to $1.99 in pre-market trading.

NuPathe's Zelrix is a patch that uses electric current (via a small battery and chip) to actively push the commonly used migraine medicine sumatriptan through the skin and into the patient's bloodstream.

The U.S. Food and Drug Administration rejected Zelrix, citing lingering questions and concerns about manufacturing and safety, NuPathe said, adding that additional studies may be conducted to address the issues raised by FDA.

NuPathe plans to meet with FDA to discuss ways it can address FDA's concerns, but the company offered no timeline, only saying that Zelrix will not be launched in the first half of 2012, as previously expected.

NuPathe shares were halted Monday afternoon at $4.05 a share. The company had $36 million, or $2.46 a share, in cash at the end of the second quarter.

--Written by Adam Feuerstein in Boston.

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