The UK drug company GlaxoSmithKline has agreed a £1.5bn deal to buy a US cancer treatment developer, Sierra Oncology, as it tries to fend off pressure from the activist shareholder Elliott Management. The deal will give Britain’s second-largest pharmaceutical company access to California-based Sierra Oncology’s momelotinib, a drug being tested on anaemic patients with a type of bone marrow cancer called myelofibrosis. GSK said the drug had “significant growth potential” and it expected sales to start next year, with one analyst predicting it could generate peak annual sales of about $1.7bn (£1.3bn). GSK’s chief executive, Dame Emma Walmsley, has been facing mounting calls to shore up its drug pipeline since Elliott, a US-based investor, built up a significant stake in the company last year. The deal comes as GSK prepares to spin off its large consumer healthcare business, which includes brands such as Sensodyne toothpaste and Advil painkillers, in July, the biggest shake-up for the company in two decades. The consumer arm, which will be chaired by the former Tesco boss Sir Dave Lewis, will be called Haleon when it lists on the London Stock Exchange in the summer. That spin-off will give GSK about £7bn in cash it can use to make acquisitions as it tries to find its next blockbuster drug. Elliott has criticised GSK’s leaders for “years of under-management”, and questioned whether Walmsley, who does not have a scientific background, was the right person to lead the company. Shareholders in Sierra, which focuses on targeted therapies for the treatment of rare forms of cancer, will receive $55 per share of common stock in cash, GSK said. That represented a 39% premium to Tuesday’s closing price. GSK’s shares were up nearly 1% by midday on Wednesday. Late-stage clinical trial results in January showed that Sierra’s drug was successful in reducing disease symptoms and cut patients’ dependence on blood transfusions. GSK’s oncology business last year accounted for about 2.8% of total pharmaceutical sales. Its other products include Blenrep, a treatment for another form of blood cancer called multiple myeloma. Last June, GSK agreed to pay up to $2bn to iTeos Therapeutics Inc to develop and sell a potential cancer treatment together. However, GSK has over the past year experienced several trial setbacks on the cancer compounds bintrafusp alfa and feladilimab, which it had touted as potential billion-dollar-sellers. The company will also lose patent exclusivity on the HIV drug dolutegravir at the end of 2027, worth about £3bn in annual sales.