** Drugmaker's shares down 10 pct at $19.50 premarket
** Two U.S. senators launched a bipartisan investigation into pharmaceutical drug pricing on Wednesday, requesting documents from four drugmakers, including Retrophin and Valeant Pharmaceuticals International Inc
** Valeant's U.S.-listed shares up 1.2 pct at $99.10
** The other companies being investigated are privately held Turing Pharmaceuticals and Rodelis Therapeutics
** Shares of many drugmakers have slumped since Democratic presidential hopeful Hillary Clinton last month proposed ways to prevent industry "profiteering.
** Up to Tuesday's close, Retrophin's stock had fallen about 24 pct since Sept. 21 when Clinton tweeted her intent to tackle unnecessarily high prices in some drug markets