Congress just fired a shot against its next drug-pricing target - and its shares took a hit

Congress is going after another drug maker's pricing tactics.

In a letter to the National Institutes of Health and the Department of Health and Human Services, 12 congressmen asked the government agencies to bring down the price of Xtandi, a drug used to treat prostate cancer that.

The drug costs $129,000 for a course of treatment in the US, but just one-third of that in countries like Japan and Sweden.

Xtandi was developed by California-based biopharmaceutical company Medivation and Japan's Astellas Pharma. Medivation was down about 5% in after-hours trading Monday after the letter was made public.

The congressmen, including Sen. Bernie Sanders (I-VT) and Rep. Elijah Cummings (D-MD), asked the NIH and HHS to hold a public hearing that would determine whether they should use "march-in rights" that would allow them to lower the price of the drug, even though it's under patent protection.

Cummings and Sanders have gone after other pharmaceutical companies in the past, in particular Turing Pharmaceuticals and Valeant Pharmaceuticals, which are known for buying up old drugs and jacking up their prices. Xtandi is different from other such targets, because it's still on patent and hasn't drastically risen in price since its approval.

These "march-in rights" to set new prices have not been used since their inception in 1980 in the context of high pharmaceutical prices, the letter said.

Xtandi, otherwise called enzalutamide, is used to treat late-stage prostate cancer. It was originally approved by the FDA in 2012 for its ability to increase how long the patients survived. The congressman argued that while the drug is priced at $129,000 in the US, it costs about a third of that price in other countries like Japan and Sweden.