U.S. Senator Bernie Sanders wants a drug company to explain how a medicine that used to be free to patients with a rare autoimmune disease now has a list price of $375,000 annually.

Catalyst Pharmaceuticals told investors in December that it would be changing the pricing for Firdapse, used to treat Lambert-Eaton myasthenic syndrome, or LEMS, Sanders wrote Monday in a letter to the company. About one in 100,000 Americans are afflicted with the muscle-weakening disorder.

Those with LEMS have had free access to Firdapse through a Food and Drug Administration program called "compassionate use." But the agency in November gave Catalyst approval to distribute the drug, with the company setting its $375,000 annual price tag the following month.

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"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," wrote the Vermont independent. "I am profoundly concerned that Catalyst's actions will cause patients to suffer or die."

Sanders asked Catalyst to explain the financial and nonfinancial factors that went into its pricing decision, as well as requesting the company relay how much it was paying to buy or manufacture the drug.

"Catalyst's top priority is improving patient care in the LEMS community and potentially elsewhere within the neuromuscular community," the company said in an email to CBS MoneyWatch. "We will respond to Senator Sanders' letter in a timely manner and provide information about Firdapse and the programs that we have in place to raise awareness of LEMS, facilitate accurate and timely diagnosis, and broaden affordable patient access to an FDA-approved treatment."

Escalating prescription drug prices are a political issue. President Donald Trump vowed to tackle the problem during his campaign, and both the House of Representatives and the Senate are holding hearings on the topic this year.