AbbVie Inc. and Shire plc agreed to terminate what would have been the biggest U.S. tax inversion after AbbVie pulled its support for the deal in the wake of proposed changes to U.S. rules governing such transactions.

AbbVie, based in North Chicago, Illinois, planned to buy Shire for an estimated $52 billion, then move the combined company’s legal address to the U.K. to lower its tax bill and access cash trapped overseas. After confirming the deal was dead, the drugmaker announced a $5 billion share buyback over the next several years and increased its quarterly dividend by 17 percent, to 49 cents per share.

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