Two prominent Democratic lawmakers are requesting that the Federal Trade Commission investigate whether price hikes for two blockbuster blood thinners were illegally coordinated by Janssen and Bristol Myers Squibb.

Sen. Amy Klobuchar (D-Minn.), the chair of the Senate Judiciary Committee’s subcommittee on antitrust enforcement, and Rep. Katie Porter (D-Calif.), a rising star in the Democratic Party and a member of the House’s oversight panel, wrote to the FTC on Monday regarding what they called “parallel” price hikes on Xarelto and Eliquis, drugs that patients take to prevent heart attacks, strokes, and blood clots.

“We are principally concerned that the prices for these products, which are close substitutes for each other, have been raised in lockstep in a manner that seems coordinated to maintain pricing parity,” the lawmakers wrote.

Eliquis and Xarelto are the first and third most expensive pharmacy drugs for the Medicare program, according to the most recent available spending data. Traditional Medicare plans spent $9.9 billion on Eliquis in 2020 and $4.7 billion on Xarelto. The drugs cost Medicare on average $3,761 and $3,968 per patient per year, respectively.

The prices for the drugs have more than doubled over the last decade, the lawmakers wrote, and cost several multiples of the prices in other countries.

Price fixing, or when competitors coordinate to raise prices together, is illegal under antitrust law.

The drugs are a part of Medicare’s pharmacy benefit, where patients pay a percentage of a drug’s price at the counter, meaning price hikes increase patient spending. Right now, there’s no limit on how much seniors can pay for expensive medications.

Congressional Democrats reached a compromise on drug pricing reform legislation in November that would allow the secretary of Health and Human Services to negotiate lower prices for the drugs that Medicare spends the most on, and also cap what seniors pay per month on prescriptions.

However, the legislation has stalled amid tense negotiations over other policy areas, and it’s unclear whether Democrats will find a way to advance it.