LONDON—GlaxoSmithKline has agreed to buy U.S. cancer specialist Tesaro for $5.1 billion, a costly investment to rebuild the pharmaceuticals business by new Chief Executive Emma Walmsley that unnerved investors.
GSK has lagged 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. That is a situation Walmsley wants to change.
