Davis Polk & Wardwell has won the lead role on a $43.7bn (£21.9bn) biotech takeover bid, as Swiss healthcare group Roche offers to buy the remaining shares in US biotech company Genentech.

Roche already owns 55.9% on Genentech, and the deal to purchase the remaining 44.1% would form the seventh-largest drug maker in the US, with sales of $15bn (£7.5bn) in the country, according to Roche.

Davis Polk's New York-based litigation partner Arthur Golden and M&A partner Christopher Mayer are representing Roche on the proposed deal, while Genentech is yet to announce legal advisors.

Roche has offered to buy the shares for $89 (£45) each, at a one-day premium to Genentech shareholders of 8.8% and a one-month premium of 19.0%

Roche acquired its majority stake in Genentech in 1990. Listing its reasons for wanting to fully take over the biotech company, Roche said it expects the combination to generate annual pre-tax cost synergies of around $750m-$850m (£375m-£425m). The company said the savings will enable the new company to increase investment in innovating new drugs.

Roche's chairman Franz Humer commented: "The transaction will create a unique opportunity to evolve Roche's hub-and-spoke model into a structure that allows us to strengthen the focus on innovation."

Genentech's South San Francisco base would become Roche's US headquarters if the deal is completed.

Last week it emerged that Davis Polk has taken a role alongside Slaughter and May advising Spanish banking giant Banco Santander on its £1.25bn bid for Alliance & Leicester. Allen & Overy is advising the UK bank.