Slaughter and May and Ropes & Gray have taken the lead for Irish-headquartered drug maker Shire as it makes an unsolicited $30bn (£19.2bn) approach for rival drug company Baxalta.

Rare drug maker Baxalta was spun off from its parent company Baxter International in July. It is listed on the New York Stock Exchange and reported revenue of $6bn (£3.84bn) in 2014.

Long-standing adviser Slaughter and May is acting for Shire with a team led by corporate partners Martin Hattrell and Adam Eastell. They are being supported by tax partner Tony Beare and competition partner Claire Jeffs.

In January Hattrell represented Shire in its acquisition of US company NPS Pharmaceuticals for $5.2bn (£3.34bn).

Hattrell also acted for Shire when it was the subject of an attempted takeover by US drugmaker AbbVie in June 2014.

AbbVie made an initial offer of £27bn for Shire which later rose to £32bn. The Shire board recommended the offer, however the deal fell apart after US lawmakers introduced a law targeting "tax inversions", defined as US corporations acquiring overseas companies and moving their headquarters offshore for tax reasons.

Ropes & Gray is advising Shire on the US aspects of the deal. Its team is being led by Boston M&A partner Christopher Comeau. He is being supported by Boston securities partner Paul Kinsella and Boston tax partners David Saltzman and Christopher Leich.

Baxalta has drafted in the services of US firm Kirkland & Ellis for advice on the takeover offer. The firm's team is being led by Chicago corporate partner Scott Falk and New York corporate partner Daniel Wolf.

In June Shire hired Reckitt Benckiser's group general counsel, Bill Mordan, as its general counsel.