Valeant Pharmaceuticals International Inc. Chief ExecutiveOfficer Mike Pearson will step down ina board and management shakeup after a series of controversies thatculminated in a 61 percent drop in the shares last week.

The drugmaker has started a search to replace Pearson, who willstay CEO until a successor is found, according to a statementMonday. Bill Ackman, the billionaire investor whose Pershing SquareCapital Management LP is one of the drugmaker’s biggest investors,will join the board. Valeant said it asked former Chief FinancialOfficer Howard Schiller, who took over for Pearson during atwo-month medical leave, to resign from the board following“improper conduct.” Schiller, in a statement via his law firm, saidhe never engaged in any improper conduct and refused to step downas director.

The latest developments cap three chaotic weeks since Pearson’sreturn from his leave, raising even more questions about Valeant’saccounting practices. Since the end of February, Valeant hasdelayed filing of its annual report pending an internalinvestigation by an ad hoc committee, disclosed an Securities and Exchange Commission probe andsaid it would restate several quarters worth of results. Thecompany, which has been under scrutiny for months for its business practices, saidMonday it has discovered “one or more material weaknesses” ininternal controls.