The complaint alleged that PBMs created a "perverse" drug rebate program that excluded lower-priced insulins in favor of "high list price, highly rebated insulin products."

The PBMs' 41-page lawsuit filed in U.S. District Court for the Eastern District of Missouri, said that allowing the FTC's case to play out as an administrative proceeding overseen by an administrative law judge "would subvert bedrock constitutional principles of accountability and fairness in an attempt to transform significant aspects of an entire industry by regulatory fiat."

The three companies have enlisted an army of lawyers to plead their case. CVS is represented by Dechert and Williams & Connolly: Cigna by Husch Blackwell, Wilmer Cutler Pickering Hale and Door, and Rule Garza Howley; and UnitedHealth by Gibson, Dunn & Crutcher.

CVS, Cigna and UnitedHealth are the latest in a throng of companies to have launched challenges to the FTC's power in the wake of the U.S. Supreme Court's June ruling in SEC v. Jarkesy. In a 6-3 decision, the high court found that administrative agencies cannot use their own tribunals to adjudicate private rights, and that defendants have a constitutional right to a jury trial under the Seventh Amendment.

Tuesday's suit alleges that FTC Chair Khan and her two fellow Democratic commissioners on the five-person panel have demonized PBMs, depicting them of carrying out "illegal rebate schemes" that are effectively "bribes." Such loaded language suggests the commission will conclude that the PBMs broke the law "regardless of the evidence or the ALJ's findings or credibility determinations," according to the suit.

Related: In defense of pharmacy benefit managers' rebate strategies: D.C. think tank analyst speaks out

Further, the PBMs argue that the FTC is attempting to apply §5 of the FTC Act, which prohibits unfair methods of competition and unfair business practices, in a novel, overly broad way. In effect, it "gives the commission roving license to define practices as unfair based on its own subjective policy preferences," the suit alleges.

"The commission would upend present-day drug rebate contracts, forcing PBMs to revamp their entire contracting framework and countless contracts with drug manufacturers, health plan sponsors and other private parties," the suit alleges.

The complaint also alleges that the FTC is depriving PBMs of a fair and impartial tribunal in violation of the due process clause in the U.S. Constitution's Fifth Amendment.

Contacted for comment, FTC spokesman Douglas Farrar said: “It has become fashionable for corporate giants to argue that a 110-year-old federal agency is unconstitutional to distract from business practices that we allege, in the case of PBMs, harm sick patients by forcing them to pay huge sums for life saving medicine. It will not work."