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.