Latham & Watkins and Wachtell Lipton Rosen & Katz have taken the lead roles on highly-acquisitive pharmaceutical company Actavis' $25bn (£15bn) purchase of Forest Laboratories.

As a result of the deal – the second largest announced corporate acquisition of 2014 so far -Irish-American drugmaker Actavis will expand into higher-margin treatments for Alzheimer's, hypertension and other disorders.

Actavis expects combined annual revenues in the company to reach $15bn (£9bn) in 2015.

Latham acted for Actavis, the fourth time it has led for the client on a multi-billion cross-border transaction in the last three years.

The firm's deal team was led by Orange County partners Scott Shean and Charles Ruck and New York partner Stephen Amdur, and included capital markets partner Wesley Holmes, finance partner Daniel Seale, tax partners Laurence Stein and Nicholas DeNovio, and benefits and compensation partner James Barrall, all of whom are based in the US.

Many of the partners in the deal team also advised Actavis on its $8.5bn (£5.1bn) acquisition of Warner Chilcott, which completed last year. Arthur Cox, which took a role for Warner Chilcott on that deal, is understood to have advised Actavis on Irish law matters this time round.

Skadden Arps Slater Meagher & Flom has also taken a role advising Actavis on antitrust matters, with Washington DC-based global antitrust and competition head Steven Sunshine leading a team alongside partner Ingrid Vandenborre in Brussels and New York associate Maria Raptis.

Forest Labs turned to Wachtell, with corporate co-chair Andy Brownstein and corporate partner Igor Kirman leading. Brownstein previously advised Forest Labs in successive proxy fights with activist shareholder Carl Icahn, who has been one of the main beneficiaries of the sale to Actavis.

Dublin-based corporate firm William Fry also took a role for Forest Labs on Irish law matters, with corporate partner David Fitzgibbon and tax partner Sonya Manzor leading a team.

The firm first started working for Forest Labs on other Irish law matters in the last 12 months.

On antitrust matters, Forest Labs turned to Weil Gotshal & Manges, with Washington DC partners Steven Newborn, Ann Malester, Steven Bernstein advising alonsgide London partner Douglas Nave. Stikemans also provided advice to Forest Labs on Canadian competition matters.