A top Republican senator is pressing the Obama administration for answers on why Medicare is spending more on certain pharmaceuticals.

Sen. Chuck Grassley, R-Iowa, wrote to the Centers for Medicare and Medicaid Services Wednesday asking for information on a spike in spending in Medicare's catastrophic prescription drug program, which covers drug costs for people with expensive chronic conditions.

Grassley pointed to a recent investigation by the Associated Press that showed spending on the drugs increased by 85 percent in three years, from $27 billion in 2013 to $51 billion in 2015.

"Given the increasing costs and the need to ensure that the Medicare benefit, particularly catastrophic coverage, is being appropriately utilized and free from exploitation, it is important to understand the administration's approach to solving these problems," Grassley wrote to CMS Acting Administrator Andy Slavitt.

He asked the agency what is contributing to the dramatic spike and if there is "potential for exploitation of the catastrophic benefit."

Grassley pointed to the approval of new to treat hepatitis C that are extremely expensive.

The cost for catastrophic spending on the hepatitis C drugs Harvoni and Sovaldi in 2014 was $3.5 billion and spiked to $7.5 billion in 2015, Grassley said.

Grassley has pressed for reforms to address high drug prices before. He sponsored legislation that would end pay-for-delay settlements between brand name drug makers and their generic counterparts. Under such agreements, a brand-name drug maker pays generic companies to delay cheaper generics from entering the market.