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Correction: A previous version of this story stated incorrectly that beloranib was the only drug in Zafgen's pipeline. The company is also developing two other preclinical compounds.

A second patient treated with the experimental obesity drug beloranib from Zafgen (ZFGN) - Get Zafgen Inc. Report drug died during a clinical trial, the company said Wednesday.

Zafgen shares plunged 53% to $7.38 in premarket trading on concerns the U.S. Food and Drug Administration might step in to halt all clinical development of beloranib for patient-safety reasons.

The FDA placed a partial clinical hold on beloranib, after the first treated patient died in October.

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Both patients suffered from a rare disease, Prader-Willi Syndrome, causing compulsive overeating and severe obesity. Both patients also died, due to blood clots in the lungs while taking beloranib. A direct tie between the blood clots and beloranib has not been established, but other beloranib-treated patients have also reported blood clots, forcing Zafgen to enact stricter screening methods and safety precautions.

Ever since the first Prader-Willi patient died in October, Zafgen's challenge has been to find a way to prove beloranib to help these patients without putting them at increased safety risk.

Wednesday's report of a second patient death makes Zafgen's job even more difficult, perhaps impossible.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.