The Office of Personnel Management said Tuesday it opposes a bill that aims to increase the transparency of prescription drug costs.

OPM said the bill, HR 4489, would restrict how OPM contracts with pharmacy benefit managers (PBMs) and impose "significant administrative costs" that could be passed on to federal employees. John O'Brien, OPM's director of planning and policy analysis, told lawmakers that OPM wants to write its own methods of oversight, contracting models and pricing into the contracts it strikes with PBMs.

"Requiring the use of specific contracting models and pricing methods via legislation will not allow the program flexibility in an industry where business practices are rapidly evolving," O'Brien said.

PBMs contract with insurers to provide prescription drugs to enrollees in the Federal Employees Health Benefits Program. But critics say PBMs have opaque pricing methods, retain most discounts or rebates prescription drug manufacturers give them, and receive little oversight from OPM.

OPM said nearly 30 percent of the government's $39 billion health plan goes to cover prescription drugs.

The FEHBP Prescription Drug Integrity, Transparency and Cost Savings Act, sponsored by Rep. Stephen Lynch, D-Mass., would require PBMs to return to the government 99 percent of all rebates, market share incentives and other savings they receive. It would place new transparency requirements on PBMs and cap drug prices to make sure the government doesn't pay more than the nationwide average. PBMs would also be prevented from switching federal employees' drugs to cheaper alternatives without their physicians' prior approval.

O'Brien said an OPM working group is now trying to figure out how the agency will mandate transparency in its contracts with PBMs. He said OPM might contractually require PBMs to make their pricing transparent and pass on the full value of discounts, rebates or other credits that PBMs receive from drug manufacturers.

Lynch, the chairman of the House Oversight and Government Reform subcommittee on the federal workforce, Postal Service and the District of Columbia, called O'Brien's testimony "more than a little disappointing."

"Sometimes I feel like I'm pulling you folks along toward the road of reform," Lynch said. "I just wish we were working more closely together to get to the same object. I'm concerned that the agency has become captive to the current system and resistant to change."