BUZZ-Valeant Pharmaceuticals: CEO summoned to testify before U.S. Senate panel
** Canadian drugmaker's U.S.-listed shares down 8 pct at $28.53; among top percentage losers on NYSE
** Outgoing chief executive of Valeant, which is under scrutiny for dramatically hiking price of older drugs, has been summoned to testify at U.S. congressional hearing on April 27
** Valeant said last week CEO Michael Pearson would step down and billionaire William Ackman, whose Pershing Square Capital Management owns 9 percent stake in Valeant, has joined board
** Company's Toronto-listed stock was down 8.6 pct at C$37.61
** Up to Thursday's close, Valeant's U.S.-listed stock had fallen 68 pct this year
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