Drugmaker AbbVie has clinched a deal worth about $55 billion to acquire British rival Shire in the biggest tax inversion deal by a U.S. company. AbbVie and Shire said the new company will be incorporated in the UK, which will effectively drop AbbVie's tax rate by 2016 to 13% from 22.6% last year. In addition, AbbVie will expand its product portfolio, gaining access to Shire's rare-disease drugs. Shire shareholders will receive about 53 pounds a share in cash and stock. After AbbVie had four previous offers rejected, the announcement of the deal comes on the last day the companies had to reach an agreement, according to British takeover rules. TheStreet's Brittany Umar reports from New York.