(Adds further details, competition, shares)
By Ben Hirschler
LONDON, Dec 3 (Reuters) - GlaxoSmithKline has agreed to buy U.S. 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, an 110 percent premium to the 30-day average price. News of the lofty valuation sent GSK shares down 4 percent on Monday.
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 medicines cabinet her top priority and the company signalled its intention to make more acquisitions in April, when it hired a key deal-maker 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 deal-making 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. (Reporting by Ben Hirschler Editing by Edmund Blair and Andrew Heavens)