Three U.S. senators are asking why a 40-year-old cancer drug has increased in price by 1,400 percent since 2013.

The drug in question, lomustine, was introduced in 1976 to treat brain tumors and Hodgkin lymphoma.

Lomustine, which has no generic competition, cost $50 for a capsule with the highest dose in 2013. Now a capsule with the same dose costs $768.

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Sens. Susan Collins Susan Margaret CollinsGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (R-Maine,) Catherine Cortez Masto Catherine Marie Cortez MastoDemocratic Senate campaign arm raised nearly M in August VA problems raise worries about mail slowdown, prescriptions Cortez Masto touts mail-in voting in convention speech MORE (D-Nev.) and Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) asked Tri-Source Pharma CEO Robert DiCrisci in a letter to explain the price increase and how much of a profit it makes from the drug.

Tri-Source's subsidiary NextSource Biopharmaceuticals supplies the product in a deal with a drug manufacturer, and has raised the price nine times since acquiring it in 2013 from a different company, according to The Wall Street Journal.

The newspaper compared the situation to a move made by Turing Pharmaceuticals in 2015, when CEO Martin Shkreli acquired an off-patent drug and increased the price from about $14 a pill to $750 a pill.

The senators also asked the company for information regarding its expenses related to the sales of the drug, all documents and communications referring or relating to the drug's cost estimates and profit projections, and a list of all drugs sold by the company that have increased in price by at least 200 percent in a single year.