In pre-market trading, shares of Juno Therapeutics (JUNO) are down 30% after the company announced that one its leukemia cancer trials has been halted due to two patient deaths. This pivotal trial, 'ROCKET,' was one that was treating patients with Leukemia. The trial was a crucial one because it was the lead clinical program of the company. 

The clinical drug put on hold was the company's Leukemia drug JCAR015, which uses CAR-T cells -- Chimeric antigen receptor T cells. This is form of immunotherapy where patients' cells are extracted from their bodies and loaded with engineered T-cells that are put back into those bodies. The T-cells are then designed to only target and kill cancerous cells, and leave healthy cells alone. 

The reason the FDA has placed the clinical hold is because two patients died as the result of cerebral edema following treatment with JCAR015. The FDA has placed this trial on hold until it determines whether or not the drug caused the patients' death. A chemotherapy agent, fludarabine, was recently added to the preconditioning portion of treatment, and Juno believes it caused the problem.

Juno is quickly moving to amends its clinical trial protocol to no longer combine the drug together with this chemotherapy agent. The ball is now in the FDA's court on whether or not it will allow this amendment to go through. It will also determine if the clinical hold, pending a positive investigation, should be lifted. If the FDA finds fault with JCAR015 itself, then it could spell even more trouble for Juno's investors. 

Other CAR-T cell companies that are falling in sympathy with Juno as they use similar technologies are Kite Pharma (KITE), Bluebird Bio (BLUE), and Ziopharm Oncology (ZIOP).