What happened

Shares of  Juno Therapeutics Inc. (NASDAQ:JUNO), a clinical-stage biotech developing new cancer therapies, are losing ground after the company presented data from a study with its lead candidate. Although JCAR017's efficacy data was impressive, toxicity issues took center stage. The stock has given up some recent gains, having fallen 11.1% lower as of 3:55 p.m. during Monday's session.

So what 

Today the company presented data showing impressive responses to its new lead candidate, JCAR017, but toxicity issues were troubling in light of Juno's recent past. It's been less than a year since disclosure of patient deaths during studies with Juno's former lead candidate, JCAR015, crashed the stock, but today's mishap isn't nearly as severe.

Frustrated businessman looking at laptop.

Image source: Getty Images.

Within the core group of 44 patients treated with JCAR017, eight experienced severe neurotoxicity, one experienced severe cytokine release syndrome, and one patient died. The deceased patient was suffering from a low white blood cell count and being treated for fungal and bacterial infections. The 82-year-old also refused a ventilator following lung failure.

Now what

The investigator attributed the cause of death to a preconditioning regimen that helps JCAR017 do its job, and not the candidate itself. That said, today's safety data is still troubling.

The non-Hodgkin lymphoma patients in the study have relapsed following several lines of treatment and are generally frail. The study is expected to enroll 274 patients and run a few more years. Given the rates of severe cytokine release syndrome and neurotoxicity presented today, the odds that future events will end the program before it enters the commercialization stage aren't as long as anyone would like them to be.

Cory Renauer has no position in any stocks mentioned. The Motley Fool recommends Juno Therapeutics. The Motley Fool has a disclosure policy.