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.