Bill Doyle is leaving Mr Ackman's hedge fund, Pershing Square, after less than two years, as the fund tries to claw its way back from losses on Valeant shares, according to its latest letter to investors.
Mr Doyle is a former Harvard classmate of Mr Ackman and introduced him to Michael Pearson, the chief executive who was building Valeant into a pharmaceuticals giant using an innovative business model combining debt-funded acquisitions and raising drug prices.
Mr Doyle and Mr Pearson had been colleagues at McKinsey, the consulting firm, and the pair helped persuade Mr Ackman to fund an audacious takeover bid by Valeant for Allergan. "Bill vouched for Mike," Mr Ackman said of Mr Doyle after that ultimately doomed bid was announced in 2014. "There was a lot of trust there."
In Pershing Square's letter to investors on Wednesday, Mr Ackman said that Mr Doyle would be leaving to concentrate on his role as executive chairman of a brain cancer company Novocure.