Global pharmaceutical giant AbbVie (NYSE:ABBV), perhaps best-known for producing the current best-selling drug in the world, arthritis treatment Humira, outlined a new proposal this morning that would boost its previous offer price to purchase Irish drugmaker Shire (NASDAQ:SHPG) by 11%.

AbbVie's new proposal, which was specifically pitched to Shire's largest shareholders, values Shire at 51.15 British pounds per share, or $51.3 billion, up from its prior unsolicited bid of $46.2 billion, which was originally proposed on May 30. The new proposal boosts the per-share cash component by 10% to 22.44 British pounds and offers 0.8568 shares of new AbbVie common stock to Shire shareholders. As noted by AbbVie, this would boost Shire's stake in the new company to 24%.

According to AbbVie CEO Richard Gonzalez,

AbbVie will bring greater financial strength and R&D experience to this combination that will enable both companies to reach their full potential for their shareholders and patients in need across the globe.

AbbVie has made a compelling offer to Shire that creates immediate and long-term value to shareholders of both companies. We think its shareholders should strongly encourage the Shire board to engage in constructive dialogue with AbbVie.

Not to mention, Ireland's corporate marginal tax rate peaks at 12.5% compared to a 40% peak corporate marginal tax rate in the U.S., which could bring substantial tax savings if AbbVie relocated its headquarters.

AbbVie has encouraged Shire's largest investors to consider its new potential offer and take their ideas to Shire's board of directors. Under U.K. takeover laws, AbbVie needs to either submit a formal proposal by July 18 or walk away from its attempt to purchase Shire.

Shire's board of directors has confirmed that it will meet to discuss the new proposal, but advises existing shareholders not to take any action based on AbbVie's as-of-now informal proposal.

Shire, known for its rare-disease drugs, has rejected three AbbVie offers to date. Shire has said that AbbVie's overtures undervalue the company and its prospects. The drugmaker has also said that its board had concerns about AbbVie's interest in making the move for tax purposes.

As of midday trading, both companies were moving modestly lower with both AbbVie and Shire shares down around 3%.

-- Material from The Associated Press was used in this report.

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