A Juno Therapeutics Inc trial of its experimental cancer therapy, known as JCAR015, was put on hold by U.S. regulators after the deaths last week of two leukemia patients, the company said on Thursday.

Shares of Juno fell 30 percent to $28.50 in after-hours trading on the Nasdaq after they closed at $40.82.

A third trial patient died in May, Juno Chief Executive Officer Hans Bishop said during a conference call. All three patients were in their 20s, and the deaths were linked to swelling in the brain, the company said.

JCAR015 is a chimeric antigen receptor T-Cell, or CAR-T, therapy, involving a complicated process of extracting immune system T cells from an individual patient, altering their DNA to sharpen their ability to spot and kill cancer cells, and infusing them back into the same patient.

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Seattle-based Juno said the deaths occurred after the chemotherapy agent fludarabine was added to the trial program. The company said it has proposed to the U.S. Food and Drug Administration that the trial be continued using JCAR015 with just cyclophosphamide as a "pre-conditioning agent."

Patients receiving CAR-T therapies typically receive doses of chemotherapy beforehand, meant to make the tumor more vulnerable to the CAR-T cell.

In the results so far of some early-stage clinical trials, CAR-T therapies eliminated all trace of leukemia and lymphoma in 40 percent to 90 percent of patients who had run out of other options. But the experimental medicines can cause cytokine release syndrome, a dangerous buildup of toxic debris from killed tumor cells, and damage to healthy tissue. The inflammation is typically controlled, however, with steroids and other treatments.

Juno said the FDA asked that the company submit a revised patient consent form, investigator brochure, trial protocol, and a copy of the presentation made to the agency on Wednesday. Juno said it plans to submit the requested information to the FDA this week.

CEO Bishop said the company will have more clarity in the next few weeks on timing of the halted trial. Juno had planned to seek approval for JCAR015 for adult patients with acute lymphoblastic leukemia by early 2017. The company is testing other CAR-T cells for pediatric ALL, non-Hodgkins lymphoma and other cancers.

He said Juno's development of other products, including CAR-T therapy JCAR017, are not affected by the trial hold.

Other companies developing CAR-T therapies include Kite Pharma Inc, Bluebird Bio Inc, Celgene Corp and Novartis AG.