A deal struck between drugmakers AbbVie and United Therapeutics Wednesday set a record price for a voucher that can be redeemed for a fast-track review of a new medicine by the Food and Drug Administration.

AbbVie, marketer of Humira and AndroGel, has agreed to pay $350 million to United Therapeutics, a company specializing in treatments for rare diseases, for a ticket to the regulatory fast lane.

The tickets, or priority review vouchers as they're known, were created by Congress in 2007 to encourage the development of drugs for neglected tropical disease and rare illnesses affecting children.

Drugs like those aren't usually lucrative for companies. The priority review voucher was conceived of as a prize that would be given to companies that brought medicines for certain overlooked diseases to market.

The voucher entitles its holder to move a drug through the review line faster. FDA has to make a decision about a voucher drug in about six months rather than the 10 or so for a drug submitted without one.

There's no guarantee the FDA's decision will be an approval; the agency could just reject a drug faster.

Oh, there's one other important feature of the vouchers: They can be kept by the companies that won them or they can be sold to the highest bidder.

United Therapeutics got its voucher in March when the FDA approved a drug called Unituxin to treat pediatric neuroblastoma, a rare cancer in kids. Rather than keep the ticket, the company sold it. "We are very pleased to monetize our [voucher], and hope that this transaction will encourage others to join us in focusing development efforts on rare pediatric diseases," Roger Jeffs, president and co-CEO of United Therapeutics, said in a statement.

AbbVie, for its part, confirmed the terms of the deal in an email to Shots but declined to say what the company's plans are for the voucher.

Duke University's David Ridley, with two colleagues helped lay the foundation for the vouchers, told Shots in an email that the prices fetched by vouchers in 2015 "were in line with our expectations." A 2006 paper co-authored by Ridley pegged the value of a voucher at more than $300 million for a drug with the potential for blockbuster sales.

In May, French drugmaker Sanofi paid $245 million for a voucher. Last November, Gilead Sciences paid $125 million to Knight Therapeutics for a voucher that was awarded for the approval of a drug to treat leishmaniasis. All told, four vouchers have been sold.

Ridley said that "while the value of some vouchers might be around $300 million, the price might be lower if there are many sellers." He added, "United Therapeutics was lucky and/or smart that it was the only seller at a time when a buyer was eager to have a voucher."

On a related note, the FDA is expanding the illnesses that can earn companies a voucher. Chagas' disease and neurocysticercosis, a parasitic infection that can lead to epilepsy, have been added to the list of eligible conditions.

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