The Trump administration on Friday called on Congress to pass its proposed ban on "backdoor deals" that pharmaceutical companies cut with middlemen who get preferred status for their drugs in Medicare's prescription drug plans.

The proposal, unveiled Thursday, would pass an estimated $29 billion in rebates paid to so-called pharmacy benefits managers to consumers. Drug manufacturers pay PBMs the rebates for getting their drugs covered by Medicare's Part D prescription plan. Instead, PBMs would get a flat fee for including drugs on their plans. The rule would also create a new so-called safe harbor for drug discounts to be passed on to patients at the pharmacy counter.

Health and Human Services Secretary Alex Azar also urged Congress to extend the rule to employer-sponsored and other private insurance plans.

"Today's rebate system is set up in the shadows, to serve entrenched interests: drug companies who set these prices so high, and the pharmacy benefit managers who receive billions of dollars in rebates without patients ever knowing where the money goes," Azar said in a statement. "Congress has an opportunity to follow through on their calls for transparency, too, by passing our proposal into law immediately and extending it into the commercial drug market."

Azar, who is scheduled to deliver the remarks at the Bipartisan Policy Center in Washington on Friday, said the Trump administration plans to end "the era of backdoor deals in the drug industry."

Futures prices for shares of the parent companies of the nation's largest PBMs plunged. The nation's four largest PBMs are now all integrated with health insurance firms, following the recent completion of CVS Health's merger with Aetna and Cigna's acquisition of Express Scripts. UnitedHealth Group owns the OptumRX PBM and Anthem will be launching its own unit IngenioRx in the second quarter.