The largest U.S. manager of prescription benefits is telling drugmakers that the current pricing model is broken, and it is taking aim at Amgen Inc and other makers of new migraine medicines to try and fix it.

Express Scripts told Reuters it is pressing them to forego the usual strategy of setting a high U.S. list price, then lowering the cost for health plans through hefty rebates. It is also seeking a refund if the drugs don’t work within a defined timeframe. The shift could help Express Scripts and other pharmacy benefits managers (PBMs) bring prices down, and deflect growing criticism of their role as “middlemen” in the drug supply chain.

Express Scripts is advising drugmakers to take that shift into account as they launch a new class of migraine drugs.

“If your expectation is that you are not going to actually get that high list price, then don’t do that to patients who have high co-pays,” Chief Medical Officer Steve Miller said in an interview, describing his message to Amgen and its rivals. “Let’s be more balanced. Let’s get back to where gross-to-net is not so different.” Amgen’s Aimovig is expected to be approved next month, followed by similar drugs from Teva Pharmaceutical Industries Ltd and Eli Lilly & Co that the companies say could benefit up to 4 million people in the United States.

Wall Street analysts expect Amgen to announce a list price of up to $10,000 per year for Aimovig once it is approved, setting the tone for competitors.

But Express Scripts and other PBMs restrict access to new drugs they deem too expensive, asking doctors to provide detailed evidence of why a specific patient may benefit, requiring the use of other drugs for a period of time or favoring cheaper rivals when available.

Express Scripts is also pushing Amgen and its peers to refund two-thirds of the cost of a migraine drug if a patient stops treatment within 90 days because it didn’t work or caused major side effects.

Such guarantees are becoming more prevalent for older drugs with competing products on the market, including diabetes and hepatitis C therapies. It would be unusual to introduce them for the first drug in an entirely new class of therapy. Amgen, which will market Aimovig in partnership with Novartis, declined to comment on talks with Express Scripts. Research chief Sean Harper said in a recent interview that payers share blame for the current pricing model.

Their demands for ever-larger rebates has forced manufacturers to use higher list prices as a benchmark, he said. “It is a ridiculous situation that we are in,” Harper said. “No one is paying the list price, but patients are exposed for a period of time” until they pay off their insurance deductible.

In interviews with Reuters, smaller PBM Abarca Health and insurer Highmark Health said they were adopting similar tactics to Express Scripts in negotiating coverage for the migraine drugs. CVS declined comment. Teva said it was evaluating the pricing environment for its migraine drug. Officials at Lilly did not respond to a request for comment.