The health insurance company said the use of Sarepta's therapy is "considered investigational and not medically necessary for all indications," including DMD.

In April, the Peripheral and Central Nervous System Drugs Advisory Committee of the FDA held a meeting and voted against approval of Eteplirsen. The therapy was however approved on September 19 as an orphan drug status.

Related Link: Sarepta's Deal With Summit Could Be More About European Eteplirsen Approval Expectations

According to Anthem, Sarepta secured the FDA's approval based on an increase in dystrophin seen in the sekeltal muscle of some of the boys were were treated with the drug. However, the insurance company added that "uncertainty exists regarding whether the small observed increase in dystrophin will confer a clinically meaningful benefit."

Anthem said the FDA labeled indications include a statement that a clinical benefit has not be established and continued approval is contingent upon verification of a clinical benefit in ongoing confirmatory clinical trials.

The stock traded recently at $58.58, down 5.1 percent.