Weil Gotshal & Manges and Wachtell Lipton Rosen & Katz are taking the key advisory roles as Sanofi-Aventis's multibillion-dollar bid for US biotechnology company Genzyme goes hostile.

Weil Gotshal is acting for French pharma company Sanofi on its $18.5bn (£11.7bn) bid with a team including corporate partners Claude Serra in Paris and Michael Aiello in New York.

Wachtell is advising Genzyme, with corporate partners Andrew Brownstein and Greg Ostling taking the key roles.

Sanofi has offered Genzyme shareholders, who own more than 50% of the company, $69 (£44) in cash per share in a deal that will expire on 10 December.

Sanofi chief executive officer Christopher Viehbacher said: "Sanofi is committed to a transaction with Genzyme, and we believe that our offer reflects both Genzyme's upside potential and its current operational challenges."

Genzyme refuted Sanofi's attempts to negotiate a deal last month, which followed the rejection of an earlier offer in July this year. Genzyme deems the bid undervalues the company.

The underwriters on the deal are JP Morgan, BNP Paribas and Societe Generale.

Genzyme produces treatments for multiple sclerosis, cholesterol and rare genetic diseases.