 Shares of DexCom Inc. DXCM, 5.35 surged 8.1 to pace the SP 500s SPX, -0.37 premarket gainers, after the diabetes care company said it was not currently in active discussions regarding a merger. The stock has been dragged down over the past week, closing at a two-year low on Thursday, after Bloomberg reported that DexCom was in talks to buy Insulet Corp. PODD, -10.93 , a maker of insulin-infusion systems. 