Irish drugmaker Shire PLC has successfully completed its $4.2 billion tender offer for the rare disease treatment developer ViroPharma Inc. and said Friday it hopes to quickly complete the deal.

Shire had offered to buy shares of ViroPharma for $50 apiece, and said Friday that nearly 80 percent of the Exton, Pa., company's outstanding shares had been validly tendered as of the offer's Thursday night expiration.

Most of the company's remaining shares will be converted into the right to receive $50 per share in cash.

ViroPharma focuses on serious diseases with few, if any, available therapies. Its products include Cinryze, which is used to prevent and treat attacks of hereditary angioedema, a rare genetic disorder that can cause dangerous swelling of the throat or larynx. It also is developing maribavir, a potential treatment for a virus that can lead to serious disease or death in people with compromised immune systems.

Shire announced its offer in November, and the per-share price represented a 27 percent premium on ViroPharma's closing price the last trading day before the deal was announced.

U.S.-traded shares of Shire fell 2.5 percent, or $3.72, to $146.24 in morning trading Friday, while broader trading indexes dropped about 1 percent. Shire's share price is still up about 9 percent since the day before it announced the ViroPharma deal.