Pharmacy benefit managers are taking their turn in the Capitol Hill spotlight on prescription drug prices.

The Senate Finance Committee on Tuesday held a hearing with executives from several of the biggest companies that manage prescription drug plans for insurers, employers and other clients. Pharmacy benefit managers, or PBMs, have taken criticism as middlemen who add costs to an already expensive system for prescription drugs.

But the companies say they play a key role in restraining prices, partially through the rebates or discounts they negotiate for some medications. Executives from CVS Health, UnitedHealth Group's OptumRx and Cigna are scheduled to testify. Lawmakers have already grilled drugmakers about prescription drug prices and heard testimony from people affected by the soaring cost of insulin.