The chief executive of Valeant Pharmaceuticals International Inc, which is under scrutiny for dramatically hiking the price of older drugs, has been summoned to testify at a U.S. congressional hearing on April 27, the panel said on Monday. The Senate Special Committee on Aging hearing comes as the Canadian-based company is coping with a variety of federal investigations into its accounting practices that led to a restatement and delays in the filing of its annual report.