Drugmaker AbbVie is raising its takeover offer for Irish pharmaceutical company Shire for a fourth time as it attempts to secure a tax inversion deal. Under the new offer, AbbVie raised its cash-and-stock bid by 11% to $51.6 billion. Shire already rejected AbbVie's previous bid of $46 billion, saying it significantly undervalued the company. As part of the deal, AbbVie would acquire Shire and move the combined company's tax home to the U.K., which has a main corporate tax rate of 21%, below the U.S. federal corporate tax rate of 35%. Shire did not immediately reject the offer, saying its board would meet to consider the proposal.