Is this retroactive or just going forward??? A U.S. Treasury Department crackdown is demonstrating the risks of joining the wave of corporate tax inversions too late. Another sign came as Illinois drugmaker AbbVie Inc. (ABBV) said this week that itβs on the verge of canceling its $51 billion takeover of Shire Plc (SHP), which would be the largest inversion on record. Two other U.S. companies had already dropped plans to shift their legal addresses to lower-tax countries since the Treasury Department announced tougher rules on Sept. 22.