A remorseful Martin Shkreli promised the judge who's going to sentence him next week that he'll be "more careful, open and honest" if she doesn't impose a long prison term, acknowledging "I was a fool. I should have known better."

"I assure you that any mercy shown at sentencing will be met with a strict adherence to this oath and I hope to make your honour proud of me in the years ahead," Shkreli said in a letter penned from the Brooklyn lockup where he's been since September.

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It's a turnaround for Shkreli, who was dubbed the most-hated-man in America after raising the price of a life-saving drug by 5,000 per cent. He blasted members of a congressional panel who had quizzed him about the price hike, calling them "imbeciles" on Twitter.