A second patient has died while taking part in a late-stage trial assessing an anti-obesity drug being developed by US biotech Zafgen for the genetic disorder Prader-Willi syndrome.

Back in October clinical trials of beloranib were temporarily placed on hold by US regulators after the first patient died while taking the drug. While the incident remains under investigation, the death certificate has listed respiratory failure due to pulmonary emboli as the cause.

Following a review the company decided to close the randomised portion of its Phase III ZAF-311 clinical trial of beloranib in patients with Prader-Willi syndrome, but continue with the six-month open label extension study. It is in this arm that a second patient fatality occurred, with the cause of death bilateral pulmonary emboli.

Beloranib is a novel, first-in-class injectable small molecule therapy with a unique mechanism of action that reduces hunger while stimulating the use of stored fat as an energy source. The drug is a potent inhibitor of MetAP2, an enzyme that modulates the activity of key cellular processes that control metabolism.  

Shares in the firm plummeted 60% on the news of the second death, but Reuters has reported SunTrust Robinson Humphrey analysts as saying there’s still at least a 40% chance of success for beloranib as it has not been proven the blood clot was caused by the drug.

The Boston-based biotech said it is currently in discussions with the FDA to determine the next steps for the beloranib programme.