Seattle Genetics said it is discontinuing a Phase 3 clinical trial after data showed “a higher rate of deaths, including fatal infections” in acute myeloid leukemia patients receiving its drug than in the study’s other patients.

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Seattle Genetics said it is discontinuing a Phase 3 clinical trial after data showed “a higher rate of deaths, including fatal infections” in acute myeloid leukemia (AML) patients receiving its drug than in the study’s other patients.

As a result of data it received June 16, the company said Monday, Seattle Genetics is suspending patient enrollment and treatment in all of its clinical trials on the drug, called vadastuximab talirine or SGN-CD33A. The drug was also being used in a Phase 1/2 clinical trial in myelodysplastic syndrome (MDS).

Both studies were testing the drug as a so-called front-line treatment, meaning it would be an early therapy rather than a second or third choice for patients who don’t respond to initial treatment. It is pursuing similar studies in various cancers for its leading approved drug, Adcetris.

The Bothell-based company did not disclose the number of patient deaths or other details in its statement early Monday. It said it will review the data and consult with the federal Food and Drug Administration (FDA) to determine future plans for the drug’s development program.

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Three early-stage studies SGN-CD33A in AML patients receiving stem-cell transplants were placed on a clinical hold by the FDA in December, after what the company reported were “four fatal events,” but the hold was lifted in March. The study discontinued Monday was already in progress at that time.

Seattle Genetics said in December that more than 300 patients had been treated with SGN-CD33A in various clinical trials.

The Phase 3 trial discontinued Monday was a double-blind study of SGN-CD33A in combination with either of two hypomethylating agents, compared with those agents alone, in older patients with newly diagnosed AML.

Seattle Genetics has spent more than $100 million over the past five years on developing the drug, making it the company’s second most expensive development program after Adcetris, it said in a regulatory filing last month.

“This is a disappointing and unexpected result,” said Clay Siegall, president and chief executive officer at Seattle Genetics. “Patient safety is our highest priority, and we will closely review the data and evaluate next steps. AML is a devastating disease with a poor prognosis in most patients, and there is a great need for therapeutics against this disease.”

Shares of Seattle Genetics opened the day down 8 percent but recovered somewhat, closing at $61.88, down $2.64 or 4.1 percent.