GlaxoSmithKline is buying US oncology biopharma Tesaro for $5.1 billion, marking the first major purchase by the firm since chief executive Emma Walmsley took charge last year.

The move gives GSK access to Tesaro’s PARP inhibitor Zejula (niraparib), 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.

Clinical trials are currently underway to assess the safety and efficacy of the drug in wider ovarian cancer patient populations, as a monotherapy and in combinations, for the “significantly larger” opportunity of use within the first-line maintenance setting.

The trials are evaluating the potential benefit of Zejula in patients who carry gBRCA mutations as well as in those without whose tumours are HRD-positive and HRD-negative. Results from the first of these studies, PRIMA, are expected to be available in the second half of next year.

The drug is being tested for use as a possible treatment in lung, breast and prostate cancer, both as a monotherapy and in combination with other medicines, including with Tesaro’s own anti-PD-1 antibody (dostarlimab, formerly known as TSR-042).

Tesaro also has several oncology assets in its pipeline, including antibodies directed against PD-1, TIM-3 and LAG-3 targets.

GSK said the proposed transaction will “significantly strengthen” its pharmaceutical business, accelerating the build of its pipeline and commercial capability in oncology. It expects the move to start to be accretive to adjusted earnings per share by 2022.

“This combination will support our aim to deliver long-term sustainable growth and is consistent with our capital allocation priorities,” added Walmsley.

News of the deal came as GSK also announced the sale of its Horlicks nutrition brand to Unilever for $3.8 billion.