Arena Pharmaceuticals Inc. saw its shares falling over 20 percent Monday following the announcement that its trial for its obesity treatment met its primary endpoints, but a key statistic on weight loss might not be impressive enough.

 

The company said lorcaserin met the study’s weight-reducing goals in a late-stage study but not the Food and Drug Administration’s benchmark which call for at least a 5 percentage point difference in weight loss between the test drug and placebo.

 

Patients given lorcaserin lost on average about 5.8 percent of their weight or 12.7 pounds, while patients on placebo lost 2.2 percent of 4.7 pounds, which means a percentage-point difference of only 3.6.

 

On a conference call with analysts Monday morning, Jack Lief, the chief executive of Arena, said only one of the two standards had to be met. “There’s no doubt that we’ve met the FDA guidance in terms of both safety and efficacy.”

 

However, analysts appeared not so optimistic about the future of the drug if approved by the agency.

 

“Although the results technically satisfy the FDA requirements for approvability, we consider the weight loss benefits to be underwhelming at best,” an analyst at J.P. Morgan, Cory Kasimov wrote in a note to clients.

 

The company is waiting for the results of a second trial, due around September, before seeking for the FDA approval.