NEW YORK (TheStreet) -- Shares of Juno Therapeutics (JUNO) are plunging 27.85% to $29.45 in after-hours trading Thursday after the FDA put a cancer drug on a phase 2 clinical hold after two patients involved in the trial died last week.

The Seattle-based biopharmaceutical company said the deaths followed the recent addition of chemotherapy drug fludarabine to the pre-conditioning regimen.

Juno has proposed to the FDA that the clinical trial known as "ROCKET" continue using its JCAR015 drug with cyclophosphamide pre-conditioning alone.

The agency asked the company to submit a complete response to the clinical hold, including a revised patient informed consent form and trial protocol.

Juno said it will submit the requested information to the FDA this week.

The trial involved adult patients with relapsed or refractory B cell acute lymphoblastic leukemia.

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