The drug, vadastuximab talirine, had already previously received a clinical hold order from US regulator, the Food and Drug Administration, after six patients developed liver toxicity and four died.
The hold was subsequently lifted and Seattle said that its decision to stop the trial was not related to liver toxicity.
The CASCADE trial was for older sufferers of acute myeloid leukemia (AML) and the number of deaths was not disclosed.
A Phase 1/2 clinical trial in frontline high risk myelodysplastic syndrome (MDS) has also been halted while the company reviews the vadastuximab talirine data with the FDA.
“This is a disappointing and unexpected result for the CASCADE trial. 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” said