Seven patients in the trial, which was in the final stages, developed a severe infection called sepsis, and five died.
The company said it would stop adding new patients to the trial.
It said it has no hypothesis yet for how the patients died, but four of the patients that developed sepsis also developed a condition called agranulocytosis, where white-blood cells disappear from the body, which can be linked to the drug.
Shares were up 50% for the year before the drop on Wednesday.
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