The U.S. Federal Trade Commission has opened a probe into how big pharmacy benefit managers like CVS Caremark affect pricing and patients' access to prescription drugs at a time when costs of some medicines, even older ones like insulin, have skyrocketed, the agency said on Tuesday.

As part of the probe, the FTC is sending demands for information to CVS Health Corp.'s Caremark, Humana Inc., Cigna Corp.'s Express Scripts and UnitedHealth Group's OptumRx, among others.

Pharmacy benefit managers are usually hired to negotiate with drug makers for rebates and lower fees and to reimburse pharmacies for prescriptions that they dispense.

"This study will shine a light on these companies' practices and their impact on pharmacies, payers, doctors, and patients," FTC Chair Lina Khan said in a statement.

Some of the biggest PBMs are owned by big health insurers or own mail order or specialty pharmacies.

The FTC said in a statement that it will ask about fees charged to independent pharmacies and reimbursements that are then clawed back from them, efforts to steer consumers to PBM-owned pharmacies and their specialty drug policies.