The two companies, Zinc Health Services and Ascent Health Services, are group purchasing organisations, that negotiate after-market discounts or rebates with drug manufacturers on behalf of PBMs and hold the contracts that govern those rebates.

Zinc Health negotiates rebates for CVS Health Corp and Ascent Health for Cigna Group’s Express Scripts unit and Prime Therapeutics, which is a privately held PBM, the FTC said on Wednesday.

PBMs act as middlemen and negotiate rebates and fees with drug manufacturers, create lists of medications that are covered by insurance, and reimburse pharmacies for patients’ prescriptions.

The inquiry aims to shed light on unfair PBM practices, which include directing patients towards PBM-owned pharmacies, using “complicated and opaque” reimbursement methods, as well as negotiating rebates and fees with drug manufacturers that may impact the cost of prescription drugs to payers and patients, the agency said.

The FTC last year demanded information from the six largest PBMs in the US, including CVS’ Caremark, Humana Inc, Express Scripts and UnitedHealth Group Inc’s OptumRx.