Shire is not quite to the finish line with ViroPharma

Shire ($SHPG) had hoped to pretty much have all of the shares of ViroPharma ($VPHM), the maker of rare disease drugs, pledged to its $4.2 billion buyout offer by the end of the year. But it didn't happen, and so it extended its $50 a share offer until last night. Now, it is extending the deadline again.

The company said today that it will leave the offer on the table until Jan. 23. It said it had about 78% of shares committed already for its buyout of the Exton, PA-based company.

Ireland-based Shire surprised the market in November by beating out other suitors, which reportedly included Sanofi ($SNY), to snatch up ViroPharma. Often reported as a potential takeover target itself, Shire is trying to diversify itself away from its reliance on its ADHD franchise. The company is studying its top-selling drug, Vyvanse, for a new indication for patients with binge eating disorder, but Shire is looking for new areas that will help it keep growing.

CEO Flemming Ornskov has said he wants to grow Shire's orphan drug portfolio. ViroPharma's lead drug is Cinryze, a $350,000-a-year treatment for hereditary angioedema. That's a rare disease, with about 18,000 patients in the U.S. and Europe. One of Shire's non-ADHD drugs is Firazyr, its own treatment for hereditary angioedema, and Ornskov sees the two as a matched set. Firazyr treats acute attacks of swelling in patients with the disorder, while Cinryze is designed to prevent them.

To help pay the hefty sum, Shire intends to squeeze about $150 million in savings from the combined company. It has said that ViroPharma will add to Shire's bottom line immediately. Shire has taken steps in advance of closing the deal to find other savings, and it said it would lay off 180 employees as it discontinues some R&D programs outside of rare diseases.

- here's the announcement