The Centers for Medicare & Medicaid Services says a large health insurer will have to sit on the Medicare drug plan market sidelines until it resolves CMS compliance concerns.

CMS intends to make Aetna Inc., Hartford (NYSE:AET), suspend new sales of Medicare Advantage plans with drug coverage and stand-alone Medicare Part D prescription drug plan coverage starting April 21, officials say.

Aetna has about 400,000 enrollees in Medicare Advantage plans with prescription benefits and 600,000 in prescription drug plans.

The marketing suspension will not affect current plan members, officials say.

The suspension also will not have much immediate effect on marketing efforts, because the next open enrollment will not start until October.

CMS has accused Aetna of changing its Medicare plan drug formulary, or approved drug list, to a closed list, from an open list, and then failing to provide the required level of continued access to the drugs taken off the formulary.

Aetna also violated coverage determination processing rules and appeal rules, and it applied prior authorization and step therapy drug requirements that had not been approved by Medicare, officials say.

Some violations of appeals rules occurred as recently as late March, Brenda Tranchida, director of the CMS program compliance and oversight group, writes in a letter to Aetna.

Tranchida notes that Aetna has until April 16 to file a written rebuttal and until April 21 to request a hearing.

The marketing suspension “will remain in effect until Aetna demonstrates to CMS that it has corrected its deficiencies and they are not likely to recur,” officials say.

“Aetna is cooperating fully with CMS on its review, and is working to resolve the issues CMS has raised as soon as possible,” Aetna says.

“Compliance problems are unacceptable to Aetna,” Aetna President Mark Bertolini says in a statement. “The issues raised to us by CMS have our utmost attention. Aetna takes our obligations to our Medicare beneficiaries seriously, and our priority is to help ensure they have access to high-quality care, excellent service and needed medications. We are working with CMS to resolve these matters, and we also will be doing proactive outreach to impacted members to resolve these issues.”