WASHINGTON (Reuters) - Two top U.S. senators launched an investigation into rising insulin prices on Friday, sending letters to the three leading manufacturers seeking answers as to why the nearly 100-year-old drug’s cost has rapidly risen, causing taxpayers to spend millions of dollars a year.

Republican Senator Chuck Grassley, chairman of the Senate Finance Committee, and Senator Ron Wyden, the committee’s top Democrat, sent letters to the heads of Eli Lilly and Co, Novo Nordisk A/S and Sanofi SA, the long-time leading manufacturers of insulin.

The senators pointed to similar, large insulin price increases at all three companies. Eli Lilly’s Humalog, for instance, rose from $35 to $234 per dose between 2001 and 2015, a 585 percent increase, they wrote. Insulin has been available since the early 20th century.

The senators asked for information on the process used to determine list prices and the process used to determine net prices after negotiations with pharmacy benefits managers (PBMs) and health insurance plans. Their letters also asked for information about the cost of research and development, production, revenues and gross margins from insulin sales.

“These hardships can lead to serious medical complications that are entirely preventable and completely unacceptable for the world’s wealthiest country,” the senators wrote in their letters.

“We are concerned that the substantial increases in the price of insulin over the past several years will continue their upward drive and pose increasingly severe hardships not only on patients that require access to the drug in order to stay alive but also on the taxpayer,” they wrote.