GlaxoSmithKline has agreed to buy US cancer drug specialist Tesaro for $5.1 billion, marking a major biotech investment by the drugmaker as its seeks to rebuild its pharmaceuticals portfolio.

Britain’s biggest drugmaker is paying $75 a share for the business, a 110 percent premium to the 30-day average price.

News of the lofty valuation sent GSK shares down 7.6 percent, to 38.71, in Monday morning trading. Tesaro shares soared 59 percent on the news, to $73.56.

The deal gives GSK a marketed product for ovarian cancer, Zejula, which belongs to the promising new class of medicines called poly ADP ribose polymerase (PARP) inhibitors. GSK’s UK rival, AstraZeneca, sells the rival PARP drug Lynparza.

Chief Executive Emma Walmsley has made replenishing GSK’s medicine cabinet her top priority and the company signaled its intention to make more acquisitions in April, when it hired a key dealmaker from Roche’s Genentech unit.

GSK has lagged behind rivals in recent years in producing multibillion-dollar blockbusters and it largely sat out a spate of dealmaking by rival drugmakers under previous CEO Andrew Witty.

GSK said buying Tesaro would weigh on adjusted earnings for the first two years by mid to high single-digit percentage rates, but the acquisition should be accretive by 2022.

Zejula’s revenues in its current approved indication as second-line maintenance treatment for ovarian cancer were $166 million in the nine months to September.