Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Shire (NASDAQ: SHPG), a Irish-based global pharmaceutical company perhaps best known for its line of attention deficit hyperactivity disorder drugs, surged as much as 20%, or $37, after the company confirmed that rival AbbVie (NYSE:ABBV) had made an unsolicited bid to buy Shire.

So what: According to Shire's press release, AbbVie approached Shire with an unsolicited bid on May 30 that would have totaled about $46 billion in a mix of cash and stock, valuing Shire 23% higher than its previous day's closing price. However, the proposal was rebuffed by Shire with its board of directors believing it undervalued Shire and its pipeline. Shire's board notes that it anticipates doubling its annual revenue from $10 billion in 2013 to $20 billion by 2020, and believes the purchase price would deny current shareholders the benefits of Shire executing on its strategies.

Now what: What this is really all about is the continuing theme of mergers and acquisitions in foreign markets to escape high U.S. corporate taxes, whether the companies involved seem to want to admit it or not. AbbVie does have the current best-selling drug in the world, Humira, set to lose its first patents within a few years, so it'll have a gaping $10 billion-plus per year hole in revenue to soon fill. Shire would have certainly diversified AbbVie's product line and helped its cause. Not to mention, cost synergies from combining research department likely would have saved millions of dollars.

However, the real allure here is Shire's Irish address. If AbbVie were to successfully purchase Shire it could move its corporate headquarters to Ireland where corporate marginal taxes top out at 12.5% compared to a top-end corporate marginal tax rate of 40% in the U.S. – the second highest in the world. With deductions, most Irish multinationals can get their corporate tax rate into the single-digits, which for a company their size could mean hundreds of millions in tax savings. At its currently inflated price I'm not too particularly fond of Shire and would wait for a significant pullback before considering it as an investment, though AbbVie could make for an intriguing play if its hepatitis C direct-acting antiviral is approved by the Food and Drug Administration before the end of the year and launches successfully.