Juno Therapeutics announced that it is putting its ROCKET clinical trial on hold at the direction of the Food and Drug Administration (FDA) after a total of three patients deaths. The patients in the trial are adults with relapsed or refractory B cell acute lymphoblastic leukemia. One patient died this past May, while two more died this past week.

Juno’s CEO, Hans Bishop, stated the patients died as a result of a cerebral edema or swelling in the brain caused by excess fluid. The likely cause of the cerebral edemas was the use of fludarabine in the preconditioning regimen. A preconditioning regimen is the administration of drugs like fludarabine that helps the immune system fight the cancerous cells to pave the way for immunotherapy treatment.

During the ROCKET trial, patients were given high doses of fludarabine to kill cancer cells and make room for cancer-killing cells produced by the immunotherapy. While fludarabine has been shown to greatly increase efficacy in previous trials, it negatively interacted with Juno’s immunotherapy treatment leading to cerebral edemas in three patients.

While immunotherapy holds great potential to save many lives, it is far from understood. Most immunotherapy treatments are still in the experimentation phase, and Juno’s ROCKET trial is no exception. The company will stop using fludarabine in the preconditioning regimen, and has asked the FDA to continue with the ROCKET trial. The FDA will allow the trial to continue with revised protocols and additional consent forms from patients.

While the news is a disappointment to cancer patients seeking new treatment options, the ROCKET trial could still yield a valuable cancer treatment. The FDA will no doubt be closely observing the study and its results, but it has given many patients hope that new cancer treatments are on the horizon.