Pfizer and Allergan said they have put to an end to their merger negotiations by mutual agreement. The decision came after the U.S. Treasury and the Internal Revenues Service (IRS) issued a proposal to introduce new rules that would limit the ability of U.S.-based companies to move their headquarters overseas to lower their taxes. After the planned merger, the combined company would have been based in Ireland, where Allergan has its fiscal and executive head office. Pfizer will pay a break-up fee of $150 million to Allergan to reimburse expenses related to the planned $160 billion merger.