Juno Therapeutics Inc. said a clinical hold was placed on its trial of a potential leukemia treatment after two patient deaths last week.
Juno shares fell 30% to $28.46 in after-hours trading Thursday.
The Phase 2 trial of JCAR015 tested the drug in patients with relapsed or refractory B cell acute lymphoblastic leukemia. As of 2015, the biopharmaceutical company said JCAR015 was its most advanced product candidate. In June, the company reported favorable JCAR015 results from a Phase 1 study.
The deaths occurred after the addition of fludarabine to the pre-conditioning regimen. Juno has proposed to the U.S. Food and Administration to continue the trial with cyclophosphamide pre-conditioning alone.
Juno will submit a response to the FDA this week that includes revisions to the trial protocol and patient consent forms.
The company said its plans for other CD19-directed CAR-T cell product candidates, including JCAR017, aren’t affected.
Seattle-based Juno launched in 2013 with discoveries by scientists at Fred Hutchinson Cancer Research, Memorial Sloan Kettering Cancer Center and the Seattle Children’s Research Institute.
Last year, Juno announced a 10-year collaboration with Celgene Corp. focused on treatments for cancer and autoimmune diseases. Celgene invested about $1 billion in Juno initially, including an $849.8 million stock purchase and a $150 million payment.
Write to Josh Beckerman at josh.beckerman[a]wsj.com