William Doyle, a key figure in Pershing Square Capital Management's controversial investment in battered drug company Valeant Pharmaceuticals (VRX.TO), is leaving the hedge fund, its founder, William Ackman, told investors on Wednesday.

Doyle joined Pershing Square's investment team in October 2014, but had also been involved in other projects including advising cancer therapy company Novocure, which listed its stock last year.

"The demands of overseeing Novocure and managing its relationship with shareholders and other stakeholders have made it infeasible for Bill to continue as a member of the investment team," Ackman wrote in a letter to investors that was seen by Reuters.

Novocure said on Wednesday that Doyle will be the company's executive chairman after having already been a board member. Ackman said Doyle will also spend time at Table Management, an entity that oversees private investments for Ackman's family, Ackman wrote in his letter.

Doyle introduced Michael Pearson, the former CEO of Valeant, and Ackman, the billionaire activist investor, in early 2014 and a year later Ackman's Pershing Square invested in the drugmaker.

In 2015, Ackman called Valeant his best idea but it has become the firm's biggest ever loser. Valeant's stock has tumbled 85 percent amid scrutiny over its accounting and pricing practices, ultimately prompting Ackman to join Valeant's board and forcing Pearson to leave his position as CEO.

Doyle was friends with Pearson from their days as consultants at McKinsey and Doyle knew Ackman through Harvard Business School. Doyle joined Pershing Square's investment team shortly after brokering the initial meeting and helped oversee science-oriented investments.

Last month, Pershing Square said Doyle would not stand for reelection as a board member at animal health company Zoetis Inc. Earlier this week the fund sold a chunk of its investment in the company.

Doyle's exit marks the second departure of a Pershing Square investment team member in five months. In January, Paul Hilal, who has been friends with Ackman since college, left the firm.