Former pharmaceutical executive Martin Shkreli departs the U.S. District Court for the Eastern District of New York, August 3, 2017 in the Brooklyn borough of New York City.

Notorious "Pharma Bro" Martin Shkreli on Friday filed suit in federal court in Brooklyn against a Florida man, accusing him of fraudulently inducing Shkreli into signing a promissory note that has left him owing $420,000 to the man's father.

Shkreli is waging his legal battle from a federal prison in Pennsylvania, where he is serving a seven-year fraud sentence for crimes related to hedge funds he previously ran and to a drug company he founded, Retrophin.

Shkreli gained infamy in 2015 for hiking the price of a drug called Daraprim by more than 5,000% while running another drug firm then known as Turing Pharmaceuticals. Turing is now called Phoenixus.

His new lawsuit case stems from a civil judgment against Shkreli last December in New York state court, which said he owed a Massachusetts man named George Yaffe for having failed to pay Yaffe off on a $250,000 promissory note that Shkreli signed in 2012.

The note provided for annual interest compounded at a rate of 8%.

Yaffe's lawsuit said Shkreli signed the note after keeping a $100,000 investment Yaffe had made in one of Shkreli's hedge funds in 2007 at the suggestion of Yaffe's son, Lee, who lives in Florida. Lee Yaffe at the time was friendly with Shkreli.

George Yaffe's suit said that Shkreli in 2007 and 2008 had failed to respond to his requests for information about his investment's performance, and that Shkreli in 2008 told Yaffe in an email that he "might have to give back money to your dad-it's about 125k fyi sorry about the miscommunication or lack of it appreciate your trust."