(CNN) Sen. Bernie Sanders sent a blistering letter to a pharmaceutical company on Monday, demanding answers about its decision to charge $375,000 for a formerly low-cost drug and calling it corporate greed at its worst.

"Catalyst's decision to set the annual list price at $375,000 is not only a blatant fleecing of American taxpayers, but is also an immoral exploitation of patients who need this medication," the independent senator from Vermont wrote. "Simply put, it is corporate greed.

"I am profoundly concerned that Catalyst's actions will cause patients to suffer or die."

Sanders sent the letter to Patrick McEnany, president and CEO of Calayst Pharmaceuticals, about the pricing of its drug Firdapse, which is used to treat a rare neuromuscular disease called Lambert-Eaton myasthenic syndrome , also known as LEMS.

Sanders asked the pharmaceutical company to describe the financial and non-financial factors that led to the decision to raise the price so dramatically.

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