What happened

Juno Therapeutics (NASDAQ:JUNO) ended down 10.6% on Thursday after announcing Wednesday after the bell that the biotech was discontinuing development of its lead treatment JCAR015.

So what

This shouldn't have come as much of a shock to investors; JCAR015 has had a rough couple of months. The phase 2 ROCKET trial testing JCAR015 in patients with acute lymphoblastic leukemia (ALL) was put on hold because patients died from cerebral edema. Juno hypothesized that the problem was with one part of the preconditioning treatment, so it was removed and the trial was resumed.

Unfortunately, more patients treated with the new protocol died from cerebral edema , so the trial was put on hold again.

Micrographic view of stained leukemia cells

Image source: Getty Images.

After studying the situation, management thinks it might be possible to modify the protocol to avoid the deadly side effect, but that would require the company to take a step back into phase 1 development. Considering the delay and that Juno and its partner, Celgene (NASDAQ:CELG), have other drugs they could use their resources to develop, Juno and Celgene decided to stop development of JCAR015.

Now what

While losing a lead candidate is a big deal for a development-stage company, Juno has other CAR T treatments in development. JCAR014 and JCAR017 target the same B-lymphocyte antigen, CD19, as JCAR015, but they're being tested in other blood cancers, so investors should know soon whether the side effect is a problem with the target, JCAR015 specifically, or a more general problem of treating ALL patients with CAR T therapy.

Beyond those two treatments, Juno has JCAR018 targeting CD22 and JTCR016, a T cell receptor (TCR) cell product that targets WT-1, an intracellular protein that is overexpressed in a number of cancers. Investors should keep an eye on JTCR016 since it's the first TCR cell product in the clinic, which should give a preview of how well that technology works.

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