A U.S. drug company is taking the Canadian government to court for its attempt to lower the price of what has been called the world's most expensive drug.

Alexion Pharmaceuticals has filed a motion in Federal Court, arguing that Canada's drug price watchdog has no authority to force the company to lower its price for Soliris.

This is the single greatest threat to pricing of drugs in Canada ever. - Amir Attaran , health law expert

The company says in the court documents that the price of Soliris has not changed since it went on the market about six years ago and that the price difference between the two countries reflects the difference in exchange rates between the U.S. and Canada.

The medication is approved to treat two rare blood diseases that affect about one in every one million people. A 12-month treatment costs about $700,000 in Canada, while in the U.S. it costs about $669,000.

Both diseases — paroxysmal nocturnal hemoglobinuria (PNH) and atypical haemolytic uremic syndrome (AHUS) — prompt the immune system to kill red blood cells, causing anemia, blood clots, organ failure and, eventually, death.

While Soliris is not a cure, it can stop the assault on the body's tissues and organs. Since patients typically need to take the medication indefinitely, it can cost tens of millions of dollars over a lifetime.

Due to the high cost, some patients in Canada can't get the drug. Only some provinces will cover the cost of treatment and there are different criteria to qualify for coverage in various jurisdictions.

Soliris is the only drug Alexion produces, but it's earned the firm revenues of more than $6 billion over eight years.

Canada's Patented Medicine Prices Review Board is challenging the cost of the drug, saying the price could be considered excessive and that it costs more in Canada than anywhere elsewhere in the world.

The review board launched hearings in June to force Alexion to lower its price. That could force the company to reimburse Ottawa for past overpayments and provinces that have covered the drug costs could apply to recoup some of that money.

Alexion fired back on Sept. 11 by filing a motion in Federal Court, asking for the review board to be prohibited from going ahead with its hearing — or from making any order that would affect the price of Soliris.

'Greatest threat' to drug pricing

A University of Ottawa professor who specializes in health law said he was shocked that Alexion would challenge Canada's authority to regulate drug prices. If Alexion's case is successful, it could end Ottawa's ability to control the cost of patented drugs, Amir Attaran told CBC News.

"This is the single greatest threat to pricing of drugs in Canada ever," he said Thursday.

The company has not yet returned calls for comment.