Irish biopharmaceutical giant Shire said Monday it has agreed to buy rare disease drug maker ViroPharma Inc. for $4.2 billion. Davis, Polk & Wardwell is advising Shire in connection with the acquisition, while Skadden, Arps, Slate, Meagher & Flom is serving as legal counsel to ViroPharma. Fried, Frank, Harris, Shriver & Jacobson, meanwhile, is representing ViroPharma financial adviser Goldman Sachs.

The transaction’s terms call for Shire to pay $50 a share for ViroPharma—a 27 percent premium over the target company’s Friday closing price. At the same time, Shire is terminating its share buyback program in the wake of Monday’s announcement of the transaction.

Shire said it is buying Exton, Pa.–based ViroPharma to help diversify its rare disease drug portfolio. One of ViroPharma’s primary treatments is Cinryze, an injectable drug for people with the immune disorder Hereditary Angioedema (HAE). Shire also produces Firazyr to treat HAE, which blocks the bradykinin protein responsible for HAE’s skin, abdomen, and throat swelling and pain.

“Shire is excited by the prospect of being able to offer two complementary treatments, Firazyr for the treatment of acute HAE attacks and Cinryze for prophylactic treatment of patients suffering from HAE,” said Shire CEO Flemming Ornskov in a statement.

Shire estimates that the acquisition of ViroPharma—which had $428 million in revenue in 2012 and expects to take in as much as $465 million 2013—will generate $2 billion in revenue in 2014.

The transaction is expected to close in the last quarter of 2013 or the first quarter of 2014, pending customary closing conditions.

The Davis Polk team advising Shire on the deal includes corporate partners George Bason and William Chudd, antitrust and competition partner Joel Cohen, executive compensation partner Kyoto Lin, tax partner Michael Mollerus and antitrust and competition counsel Michael Sohn. Also advising are associates Andrew Blau (executive compensation), Daniel Marx and Brian Wolfe (corporate) and Pritesh Shah (intellectual property).

Shire retained Davis Polk in January to serve as counsel on the company’s acquisition of Cambridge, Mass.–based biotechnology company Lotus Tissue Repair. The financial terms of that transaction were not disclosed. The firm also advised Shire in March 2012 on its purchase of Dublin-based pharmaceutical company FerroKin BioSciences for $100 million, with an additional $225 million if FerroKin meets certain milestones.

The Skadden team advising ViroPharma consists of antitrust partner Clifford Aronson, health care enforcement partner Jennifer Bragg, tax partner Jessica Hough, M&A counsel John Meckley, tax counsel Kevin Nichols, M&A partners Eileen Nugent and Michael Rogan, executive compensation and benefits partner Regina Olshan and derivatives and corporate finance partner Yossi Vebman. The Skadden associates working on the matter are Valarie Ney (M&A) and Joseph Penko (executive compensation and benefits).

The Fried Frank lawyers acting as counsel to Goldman Sachs in its capacity as ViroPharma’s financial adviser include corporate partners Abigail Bomba and Philip Richter.