WASHINGTON (AP) " Juno Therapeutics again halted its study of an experimental leukemia treatment Wednesday after two more patients died of complications.
It was the second setback for the closely watched study, which was previously halted after two patient deaths in July. Company shares plunged more than 27 percent in frenzied trading.
The Seattle company said that the latest deaths occurred earlier this week after patients suffered a severe form of brain swelling, similar to the previous two fatalities. Company executives in July blamed the deaths on chemotherapy drugs used in a pre-treatment regimen, rather than on the company's experimental drug.
The Food and Drug Administration granted researchers permission to resume the study several days later.
Juno said Wednesday it's working with regulators to determine what to do next.
The company's therapy, known only as JCAR015, is in mid-stage testing, a make-or-break phase of development that usually focuses on effectiveness.
Juno's approach is part of a promising, but still unproven, approach that reengineers patients' immune systems to attack cancer.
The company filters patients' blood to remove white blood cells called T-cells and genetically alters them in the lab so they can target cancer cells. They then return the modified cells to the patient.
The company is studying several other gene-altering immunotherapies and said those trials will continue.
Shares of Juno Therapeutics Inc. fell $7.96, or 27 percent, to $21.92 in afternoon trading.
This story has been automatically published from the Associated Press wire which uses US spellings