Juno announced the deaths today, explaining that both subjects died as a result of cerebral edema, which is a swelling in the brain caused by excess fluid.
The Seattle biotech also said it has halted the trial – known as ROCKET - while it investigates the incidents.
A spokesman told us "The two patients who died earlier this week did not receive fludarabine. They were on a pre-conditioning regimen of cyclophosphamide only."
In July the US FDA ordered Juno to halt the same trial following the death of three patients.
All three patients also died as a result of cerebral edema.
At the time Juno attributed the deaths to chemotherapies the patients received in addition to JCAR015, rather than to the therapy itself.
The firm was allowed to restart the trial a few days later after it removed one of the chemotherapeutic agents – fludarabine – from the protocol.
The trial is a Phase II study examining JCAR015 in the treatment of patients suffering relapsed or refractory B cell acute lymphoblastic leukemia (ALL).
JCAR015 consists of an infusion of the patient's own T cells that have been genetically modified to express a chimeric antigen receptor (CAR) that will bind to leukemia cells that express the CD19 protein on the cell surface.