The lead CAR-T program of Juno Therapeutics (JUNO) has been placed on a clinical hold by U.S. regulators following the deaths of two patients in a pivotal clinical trial.

Juno shares are down 29% to $29.01. Shares of Kite Pharma (KITE) , Ziopharm (ZIOP) and Bluebird Bio (BLUE - Get Report) -- competitors developing their cancer-killing CAR-T therapies -- are all trading lower.

CAR-T, or chimeric antigen receptor T-cells, is a new form of cancer immunotherapy in which a patient's own T cells are removed and then engineered to identify and kill malignant blood cancer cells.

The U.S. Food and Drug Administration ordered a clinical hold after two leukemia patients died from cerebral edema following treatment with Juno's product JCAR015 in a phase II clinical trial. A third patient in the same study died earlier this year, also from cerebral edema.

Juno believes the deaths were caused by the use of the chemotherapy agent fludarabine interacting with JCAR015. Fludarabine is used to clear out a patient's old T cells in order for the re-engineered T cells in the CAR-T to grow and take hold.

The company intends to quickly submit paperwork to the FDA to amend its study so that fludarabine is no longer used. Juno hopes the FDA concerns are satisfied and the clinical hold can be lifted but there are not guarantees. Previous guidance from Juno that JCAR015 could be approved in 2017 is now off the table.

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