Shares of Ariad Pharmaceuticals sank Thursday after Sen. Bernie Sanders and Rep. Elijah Cummings sent a letter to the company, demanding more information about its price increases for Iclusig, a drug used to treat chronic myeloid leukemia.

The stock was up slightly before the letter was announced, then sank as much as 7 percent. It later recovered some of its losses and ended the day just slightly lower.

Ariad said it recognizes that "oncology drugs are expensive," but it stands behind "the importance and efficacy" of its products.

"Iclusig is the first drug that we have brought to market after years of risk taking and research, and it serves a very small and seriously ill group of cancer patients," the company said in a statement.

Ariad said it received the letter and plans to respond to Sanders and Cummings' request.

