A second patient participating in a late-stage trial of Zafgen's experimental obesity drug died from a blockage in an artery in the lung, highlighting safety concerns on obesity treatments, which have long been associated with unwanted health implications such as heart risks and birth defects.

In October, Zafgen said that that a patient had died during the clinical trial of beloranib, but the cause of death was not known. The disclosure prompted the U.S. Food and Drug Administration (FDA) to place the study under partial clinical hold, citing earlier reports of blood clotting in completed and ongoing studies.

The second fatality resulted in the federal agency placing the study testing on complete hold, which means that the biopharmaceutical company has to stop the extension phase of the trial wherein all participating patients take the drug, and all other clinical works on the treatment are done.

Beloranib is being tested in the treatment of Prader-Willi syndrome (PWS), a rare genetic disorder marked by a constant sense of hunger. Individuals who have the condition want to eat all the time because they do not feel full.

PWS causes patients to suffer from problematic hunger-related behaviors. Many of those affected with this disorder become morbidly obese.

The experimental drug, which has been tested in two other types of obesity, works by helping the body improve its ability to metabolize fat.

Zafgen said that it is working with experts to have a better understanding of the fatal incident and the occurrence of artery blockages in patients with PWS. It likewise said on Wednesday, Dec. 2 that it still anticipates the results from the randomized portion of the study which will come out in the first quarter of 2016.

"We are working diligently to assemble and analyze the data from our ZAF-311 clinical trial to be able to provide a clear view of the safety and efficacy of beloranib in the PWS population," said Zafgen CEO Thomas Hughes.

Hughes explained that Zafgen is working with thrombosis and PWS experts in developing an all-inclusive approach in order to have a better grasp of the occurrence and causes of thromboembolic disease in relation to PWS.

"We remain committed to advancing beloranib as a potential new therapy for this underserved patient population," Hughes added.