UK drugmaker GlaxoSmithKline (GSK) has entered into a definitive agreement to acquire Tesaro, a commercial-stage biopharmaceutical company based in Waltham, Massachusetts, USA, for an aggregate cash consideration of approximately $5.1 billion.

GSK said the proposed transaction will significantly strengthen its pharmaceutical business, in particular accelerating the build-up of its pipeline and commercial capability in oncology.

The UK group is especially interested in Tesaro’s major marketed product, Zejula (niraparib), an oral poly ADP ribose polymerase (PARP) inhibitor currently approved for use in ovarian cancer. PARP inhibitors belong to a group of cancer drugs that essentially block an enzyme that cells use to repair DNA.

Currently approved in the US  and Europe as a treatment for adult patients with recurrent ovarian cancer who are in response to platinum-based chemotherapy, regardless of BRCA mutation or biomarker status, Zejula has been seen to demonstrate marked clinical benefit in patients with and without germline mutations in a BRCA gene (gBRCA).

Clinical trials to assess the use the drug as a monotherapy and in combinations for the significantly larger opportunity of first line maintenance treatment of ovarian cancer are also under way, GSK said.

The tests are evaluating the potential benefit of Zejula in patients who carry gBRCA mutations as well as the larger population of patients without gBRCA mutations and whose tumors are HRD-positive and HRD-negative. Results from the first of these studies, PRIMA, are expected in the second half of 2019.

GSK said it believes PARP inhibitors offer significant opportunities for use in the treatment ovarian cancer beyond those who are BRCA-positive as front-line treatment.

"Our strong belief is that PARP inhibitors are important medicines that have been underappreciated in terms of the impact they can have on cancer patients," said Hal Barron, a former Genentech and Calico executive who is now chief scientific officer and president of R&D at the British drugmaker.