S&P Dow Jones Indices announced late Thursday it was booting Nektar Therapeutics /zigman2/quotes/206715615/composite NKTR +1.58% from the S&P 500 index /zigman2/quotes/210599714/realtime SPX +0.06% . Nektar was the worst performing stock on the benchmark index in the past 12 months, dropping more than 70%. Nektar's earnings per share fell 112% in the second quarter, the third worst performance among companies included in the S&P 500. Nektar is set to be replaced by Las Vegas Sands Corp. /zigman2/quotes/208792014/composite LVS -2.26% , which gained more than 5% in the extended session Thursday. The change will take effect Oct. 3. Nektar stock was up 1.9% in after-hours trading Thursday and will move to the S&P MidCap 400. This is the third announced change to the S&P 500 in the past two weeks. Previously, S&P Dow Jones Indices announced that NVR Inc. /zigman2/quotes/209548385/composite NVR +0.03% and CDW Corp. /zigman2/quotes/208546399/composite CDW -0.02% would join the benchmark S&P 500.