(Sharecast News) - GlaxoSmithKline has agreed to buy US-based oncology-focused pharmaceutical group Tesaro Inc for around $5.1bn (£4bn).Under the terms of the deal, which has been unanimously approved by Tesaro's board, shareholders will receive $75 per share in cash, which represents a premium of 110% to the 30-day volume-weighted average price.Glaxo expects the deal - due to complete in the first quarter of next year - to accelerate the build of its pipeline and commercial capability in oncology.Tesaro is a commercial-stage biopharmaceutical company, with a major marketed product, Zejula (niraparib), an oral poly ADP ribose polymerase (PARP) inhibitor currently approved for use in ovarian cancer.Hal Barron, chief scientific officer and president of research and development at GSK, said: "Our strong belief is that PARP inhibitors are important medicines that have been under appreciated in terms of the impact they can have on cancer patients. We are optimistic that Zejula will demonstrate benefit in patients with ovarian cancer beyond those who are BRCA-positive as front-line treatment."We are also very excited that through this transaction, we will have the opportunity to work with an outstanding Boston-based oncology group with deep clinical development expertise and together we will explore Zejula's efficacy beyond ovarian cancer into multiple tumour types to help many more patients."Zejula's revenues in its current approved indication as second-line maintenance treatment for ovarian cancer were $166m for the nine months ended 30 September 2018.At 1245 GMT, GSK shares were down 4% to 1,557p and Tesaro shares were up a whopping 60% in pre-market trade at $73.78.