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Agios Pharmaceutical shares tank amid safety concerns about drug

Shares of Agios Pharmaceutical shed 12 percent Monday amid concern about the safety of one of the molecules in its drug to treat pyruvate kinase deficiency, a rare, genetic anemia.

The company reported that one patient saw a serious adverse event after the data collection cut-off. This patient experienced drug-related cholestatic hepatitis after a 300 mg dose. Agios said it is examining this event further.

CEO David Schenkein explained that safety concerns for the molecule were specific to an "extremely high dose, a dose that we would never use in patients."

"I think some people are missing the big picture, which is that we have two really important medicines that we think are going to change the lives of these children and adults with this disease," Schenkein said on CNBC's "Closing Bell."

Patients with this condition are currently treated with blood transfusions and other supportive care, but there are currently no approved drugs on the market.

With Monday's declines, Agios shares have fallen 23 percent this year.