The Trump administration on Wednesday sued the drug manufacturer Gilead Sciences, which sells a landmark HIV drug touted as one of the most important mechanisms to stop the spread of the virus across the nation.

The U.S. government, on behalf of the Department of Health and Human Services, filed a lawsuit alleging Gilead, which makes the pre-exposure prophylaxis (PrEP) drug Truvada, had infringed on patents owned by the federal government.

“HHS recognizes Gilead’s role in selling Truvada and Descovy to patients for prevention of HIV. Communities have put these drugs to use in saving lives and reducing the spread of HIV,” Alex Azar, the secretary of HHS, said in a statement. “However, Gilead must respect the U.S. patent system, the groundbreaking work by CDC researchers, and the substantial taxpayer contributions to the development of these drugs.”

Researchers at the Centers for Disease Control and Prevention spent years working on a treatment regimen using Truvada to stop the spread of HIV, funded by about $50 million in taxpayer money, according to a March report from The Washington Post. Gilead donated Truvada to the government to use in its testing.

The drug was approved in 2004 to treat people already infected with HIV. About the same time, the CDC began studying if it could also be used to prevent infection, filing its patent applications in 2006 after the drug was shown to work as a prophylactic in primates. Truvada was approved for use in humans as PrEP in 2012, and the government was granted several patents for the treatment regimens it developed using Truvada in 2015.

Truvada now costs about $20,000 a year in the U.S. When used correctly, the CDC noted, PrEP reduces the risk of contracting HIV from sex by about 99%.

However, Gilead has rebuffed attempts by the government to request that the company license the treatment regimen and contended that the government’s patent is invalid. In August, Gilead also filed a petition with the U.S. Patent and Trademark Office challenging the government’s patent.

The drugmaker has reaped billions in sales from PrEP sales in recent years, according to HHS, including about $3 billion in 2018 alone. The U.S. government has not received any royalties from those sales.

“The United States Patent and Trademark Office granted four patents to HHS that protect CDC’s work and the taxpayers’ investment,” HHS said in a statement Wednesday. “These patents entitle HHS to license CDC’s PrEP regimens and receive a reasonable royalty for their use. Two other companies that manufacture generic equivalents of Truvada for PrEP in foreign countries have agreed to licenses with HHS.”

Public health advocates have lambasted the high cost of the drug, although it’s unclear if the government could leverage its patents in order to see the prices lowered. A generic version of Truvada is expected to be available in 2020.

One advocacy group, PrEP4All Collaboration, tweeted that it sees the suit as a “first step” toward lowered costs for patients.

President Donald Trump has made stopping the spread of HIV and AIDS by 2030 a key goal of his administration. Health officials said the use of PrEP was an important component of that goal, and Azar announced in May that Gilead had agreed to donate Truvada for as many as 200,000 people annually for up to 11 years to reduce the risk of HIV infection.