LONDON--Shire PLC (SHP.LN) is buying rare disease company ViroPharma Inc. (VPHM) for $4.2 billion, or $50 a share, a deal that would enhance its short- and long-term revenue growth profile, the U.K. specialty biopharmaceutical company said Monday.

The purchase price is a 27% premium to ViroPharma's closing share price Friday.

Shire said it expects annual cost synergies of $150 million by 2015, over and above the improved operating leverage already being driven by the company's ongoing "One Shire" reorganization. The deal is expected to be immediately accretive to Shire's non-GAAP earnings per share after completion and to enhance its earnings growth profile, it said.

The acquisition to be affected by a tender offer and funded from Shire's cash resources as well as existing and new bank facilities.

Shire shares closed in London Friday at 2796 pence, valuing the company at 15.49 billion pounds ($24.9 billion).

-Write to Ian Walker at ian.walker@wsj.com

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