Weil Gotshal & Manges is advising Sanofi-Aventis on its $18.5bn (£12bn) takeover bid for US biopharmaceutical company Genzyme, reports The Am Law Daily.

The French pharma giant approached Genzyme earlier this summer for talks about a possible merger, before making an offer in late July, the value of which was revealed this Sunday (29 August).

Genzyme rejected Sanofi's bid, with a letter from Genzyme CEO Henri Termeer to Sanofi chief executive Chris Viehbacher (pictured) stating that the offer "fails to establish a basis for engagement by the Genzyme board."

In an earlier letter to Sanofi, Genzyme had stated: "Each member of the Genzyme board believes this is not the right time to sell the company, because your opportunistic takeover proposal does not begin to recognise the significant progress underway to rectify our manufacturing challenges or the potential for our new-product pipeline."

Sanofi's offer of $69 (£45) a share is 38% more than Genzyme's share price before news of a possible deal emerged in late July. Sunday's move opens the way for the bid to turn hostile, should Genzyme not react favourably to an offer originally extended on 29 July.

The Weil Gotshal team is being led by New York-based corporate partner Michael Aiello. Weil has represented the drugmaker in the past, including late last year on its $1.9bn (£1.2bn) acquisition of consumer products company Chattem.

Wachtell Lipton Rosen & Katz has also won a role to represent Sanofi. The elite US firm advised on the mega-deal which created the company in 2004 – the $68bn (£44bn) acquisition of Aventis by Sanofi Synthelabo.

Genzyme has instructed Ropes & Gray, after the firm last year advised on its acquisition of the rights to three cancer drugs from Bayer.

The Am Law Daily is a blog on law.com, Legal Week's US affiliate title.