Juno Therapeutics said that two patients died during its clinical trial for a cancer therapy and that the U.S. Food and Drug Administration has placed the study on hold.

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A cancer-treatment trial conducted by Juno Therapeutics was halted Wednesday after two patients died last week.

The publicly held biotechnology company said Thursday the U.S. Food and Drug Administration has placed a clinical hold on the trial after the deaths, which resulted from swelling in the patients’ brain.

The JCAR015, or “Rocket,” trial involves using engineered T-cells designed to attack cancer cells to treat adult patients who have relapsed acute lymphoblastic leukemia. More than 20 people are enrolled in the trial.

Juno executives said in a conference call Thursday that the hold will likely delay the company’s forecast of bringing the treatment to market by 2017.

The deaths occurred after the company added a new drug as part of the treatment administered before the engineered cells are introduced.

Juno proposed to the FDA it would eliminate the new drug and continue with the trial. The FDA asked Juno to submit new documents, including a new patient consent form and a new protocol.

Juno CEO Hans Bishop said on the conference call that the company would submit the revised documents this week and that the FDA has agreed to expedite its review.

The deaths came after Juno added a chemotherapy drug called fludarabine to the treatment plan. Patients received the fludarabine as part of a process to prepare their bodies for the engineered cells to be added.

Juno added the drug in the second quarter to its existing treatment plan. It previously used only cyclophosphamide, a different drug, to achieve similar effects.

After studying the treatment since the deaths, Juno concluded that the fludarabine triggered the increased neurotoxicity, or brain damage caused by toxic substances.

A third patient died earlier in the trial, in May. At the time, Juno and the FDA found there were other factors involved in the death, and the trial wasn’t halted.

After the first death last week, Juno launched an internal review that would eventually point to fludarabine. The drug is used in two of Juno’s other trials, which test similar therapies on children and adults with different cancers, and has caused some neurotoxicity, but not death.

The company was prepared for some neurotoxicity to occur with the addition of fludarabine, but not with the intensity that occurred in these cases.

None of Juno’s other trials is affected by the hold.

Juno previously halted a trial in 2014 after two patient deaths, for unrelated causes.

Bishop was hopeful that the trial will be able to continue successfully by removing fludarabine and going back to using only cyclophosphamide in its place.

Juno’s stock price dropped 30 percent after the trial’s halt was announced and was trading down about 28 percent to $29.14 per share in after-hours trading Thursday.

Juno made a big splash in the market in December 2014 when the company went public with the biggest biotech IPO of 2014. Its stock price rose 45 percent on the first day of trading, bolstered by people who were encouraged by early successful results of Juno’s research.

The first phase of the Rocket trial showed more than 77 percent of patients had a “complete response” to the treatment.

Juno Therapeutics has about 450 employees and is headquartered in Seattle’s South Lake Union neighborhood.