Leading Swiss firms Homburger and Lenz & Staehelin have scooped the lead roles advising on German drug company Merck's E11.5bn (£7.7bn) acquisition of Swiss biotech firm Serono.

The deal, which is understood to be the biggest takeover of a Swiss company by a foreign investor, was carried out under Swiss law.

Homburger advised Merck on the deal, with corporate head Heinz Schaerer leading the team. While the Zurich firm has advised Merck in the past, this is the first time they have acted as its counsel on an acquisition.

Lenz advised Serono, a regular client of the firm, with corporate partner Andreas von Planta leading the team.

US M&A giant Skadden Arps Slate Meagher & Flom also bagged a role giving additional advice to the Bertarelli family, which owns a majority stake in Serono.

German lawyers also managed to grab a slice of the action. Magic circle giant Allen Overy's Frankfurt arm advised Merck on the formation of a credit facility to finance the deal while Slaughter and May best friend Hengeler Mueller advised the banks Dresdner Kleinwort, Bear Stearns and Societe Generale on a financing package.