A second patient died in a late-stage trial of Zafgen Inc's experimental obesity treatment, highlighting safety concerns associated with drugs that aim to treat a condition that affects one in three Americans.

Shares of Zafgen tumbled more than 50 percent to $8.00 in premarket trading on Wednesday.

The company said a patient receiving its drug, beloranib, died from a blockage in an artery in the lung.

Obesity treatments have been plagued by safety concerns, particularly related to heart risk, increased suicidal thoughts and birth defects, and several have been taken off the market.

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The U.S. Food and Drugs Administration (FDA) has approved drugs made by Vivus Inc, Orexigen Therapeutics Inc and Arena Pharmaceuticals Inc, but has asked for additional studies to check for safety of these drugs.

The treatments suppress appetite by tinkering with nerve signals, telling the brain that the stomach is full.

Zafgen's beloranib takes a different approach. The drug is designed to make the body produce less fat and burn off the excess as fuel.

The drug works by blocking an enzyme called methionine aminopeptidase 2, which plays a key role in the production and use of fatty acids.

Zafgen said on Wednesday that it is discussing with the FDA on the next steps for beloranib, its lead drug candidate.

The company said in October a patient had died in the trial, but the cause of death was unknown. Following the disclosure the FDA placed the trial under a partial clinical hold, citing previously reported blood clotting in ongoing and completed studies.

The Boston, Massachusetts-based drug developer is testing beloranib for a rare genetic eating disorder called Prader-Willi syndrome.

The company is also developing the drug to treat other weight-related disorders.