Juno Therapeutics Inc said one patient had died and another was not expected to recover after suffering cerebral edema during a trial testing its experimental leukemia drug.

The company's shares tumbled nearly 45 percent to $15.90 in premarket trading on Wednesday.

Juno has said in July that three leukemia patients died from side effects during the a trial of the same genetically engineered cancer therapy, known as JCAR015.

The company said on Wednesday that it had voluntarily put the mid-stage study on hold earlier this week and also informed the U.S. Food and Drug Administration.

JCAR015 is an experimental therapy requiring a complicated process of extracting immune system T cells from a patient, altering their DNA to sharpen their ability to spot and kill cancer cells, and infusing them back into the same patient.

The drug is being evaluated in adult patients with relapsed or refractory B cell acute lymphoblastic leukemia, a type of blood cancer.

Seattle-based Juno had said that the three patient deaths in July occurred after the chemotherapy agent fludarabine was added to the trial program.

The FDA had lifted its hold after Juno reverted to its original drug regimen, excluding fludarabine.