Allen & Overy (A&O) and Slaughter and May are among a line-up of top law firms advising on Japanese pharma group Shionogi's purchase of a 10% stake in Pfizer and GlaxoSmithKline's (GSK) joint venture ViiV Healthcare.

The HIV and AIDS venture, formed in 2009, is 85% owned by GSK, with Pfizer holding the other 15%. Shionogi's purchase will see those stakes fall to 76.5% and 13.5% respectively. Financial terms of the deal were not disclosed.

A&O's London office was instructed by Pfizer to revise ViiV's terms, with corporate partner Ian Stanley leading the team alongside intellectual property (IP) partner Colleen Keck and senior associate Elvan Hussein. 

Slaughters advised GSK with a City team featuring M&A partners David Johnson and Simon Nicholls, and corporate partner Richard Smith, alongside a team from Cleary Gottlieb Steen & Hamilton led by New York IP partner Leonard Jacoby.

Skadden Arps Slate Meagher & Flom represented Shionogi with a team headed up by New York-based M&A partner Neil Stronski. A&O and Slaughters also advised Pfizer and GSK on the joint venture that created ViiV in 2009, alongside Clifford Chance, Jones Day and Morgan Lewis & Bockius.

A&O's Keck told Legal Week: "Although the amendments to the shareholders' agreement were very complicated, the discussions went smoothly."