Today the US Chamber of Commerce and the Texas Association of Business filed suit against the IRS and Treasury in the US District Court for the Western District of Texas over the inversion regulations (T.D. 9761) issued on April 4. The suit alleges that the “multiple domestic entity acquisition rule” contained in the regulations harms members of the plaintiffs, citing the abandoned proposed merger between Pfizer Inc. and Allergan Plc.  The suit argues that the regulations violate the Administrative Procedure Act and constitute “arbitrary and capricious rulemaking.”  In addition, the suit argues that Treasury failed to provide sufficient notice and opportunity for comment with respect to the regulations.