pharmaceutical giant glaxosmithkline plc said wednesday that it is buying biopharmaceutical company sierra oncology inc for 19 billion and backed its guidance glaxosmithkline will pay 55 per share in cash a 39 premium to its closing price of 3952 on tuesday the deal has the support of sierras board as well as shareholders owning 28 of its stock glaxo reaffirmed its guidance it expects 20212026 sales growth of more than 5 and for 10 adjusted operating profit the deal is expected to close in the third quarter of this year