What happened

It's tough when any onetime highflier falls precipitously from its height. Such was the case with ambitious biotech Sesen Bio (SESN -2.52%), which saw its share price decline by a queasy 42% on Monday following continued fallout from a very discouraging communication from the FDA.

So what

A few hours short of market close on Friday, Sesen Bio announced that it received a Complete Response Letter from the healthcare regulator about its leading drug candidate, Vicineum. The FDA was the bearer of bad news, informing that company that it could not approve Vicineum "in its present form."

A researcher studying a sample in a petri dish.

Image source: Getty Images.

The regulator provided recommendations for additional data and analysis that might bolster the chances for the BLA to succeed in the future.

Sesen Bio had filed a Biologics License Application for its medication in the treatment of non-muscle-invasive bladder cancer (NMIBC) that does not initially respond to the first-line Bacillus Calmette-Guerin (BCG) therapy.

Now what

As typically happens with FDA rejections in the biotech world, Sesen Bio put a hopeful spin on the regulator's response. "We remain dedicated to our mission to save and improve the lives of patients by bringing new treatment options to patients, and we intend to work closely with the FDA to understand next steps," it quoted CEO Thomas Cannell as saying.

The company added that it aims to request a Type A meeting as soon as possible with the regulator, in order to discuss how it might increase Vicineum's chances for approval. Regardless, the FDA's decision is a stinging defeat for the company, particularly since Vicineum had performed well in late-stage clinical testing.