NEW YORK (Reuters) - U.S. House Democrats said on Thursday they were launching an investigation into why prices for Multiple Sclerosis (MS) treatments have nearly quintupled since 2004, and they sent letters requesting information from seven drugmakers.

U.S. Representatives Elijah Cummings and Peter Welch sent the letters to Bayer AG, Biogen Inc, Merck KGaA’s EMD Serono unit, Novartis AG, Roche Holding AG, Sanofi SA, and Teva Pharmaceutical Industries Inc.

The lawmakers on the U.S. House Committee on Oversight and Government Reform said in the letters that they were looking to determine whether drug companies were raising prices for MS drugs in lockstep with their competitors.

The average annual cost of MS therapy rose to $78,000 in 2016 from $16,000 in 2004, according to the National Multiple Sclerosis Society.

Drugmakers have faced a global pushback from regulators and lawmakers about high prices.

President Donald Trump has said the companies are “getting away with murder” and has pledged to lower the cost of prescription drugs. He met with Representative Cummings to discuss the issue in March.

Previous congressional investigations and hearings into drug pricing practices have had far-reaching effects. Mylan NV sharply reduced the cost of EpiPen after the Oversight Committee began looking into the increases that drove the list price of the life saving allergy treatment to around $600 a pair.

Valeant Pharmaceuticals International and Turing Pharmaceuticals have also faced congressional pressure over high prices.

Roche said it plans to work with the lawmakers to address their questions, and said the industry needs to reverse the trend of sharp increases in MS drug prices.

It said its treatment Ocrevus is priced around 20 percent below the current average cost of treatment and does not plan to aggressively raise its price.

A Bayer spokeswoman said the company had received the letter and planned to reply directly to the congress members. Novartis said it was reviewing the letter to determine how to address the request.

Sanofi did not immediately have a comment on the probe. Biogen, EMD Serono, and Teva Pharmaceutical Industries Inc could not immediately be reached for comment.