Verastem, a biopharmaceutical company focused on developing small molecule drugs that target cancer stem cells, filed on Thursday with the SEC to raise up to $50 million in an initial public offering. The Cambridge, MA-based company was founded in 2010 and plans to list on the NASDAQ under the symbol VSTM. UBS Investment Bank, Leerink Swann are the lead underwriters on the deal. Terms and timing are still to be determined.