(Bloomberg) — The most valuable part of tiny biotech Knight Therapeutics Inc.’s new treatment for a rare tropical disease isn’t the drug. It’s a $125 million piece of paperwork that came with its approval from the FDA.

When Knight’s drug, Impavido, was approved in March, the U.S. Food and Drug Administration awarded one of its first priority review vouchers — the cornerstone of an seven-year-old program to entice drugmakers into developing medicines for diseases that sicken millions but are never likely to be big moneymakers. Knight agreed to sell the voucher six months later.

The voucher, a ticket to move any drug to the front of the line at the FDA, could be worth billions to a company that uses it to jump ahead of a competitor — or takes it off the market so a rival can’t use it. The payment Knight fetched from Gilead Sciences Inc. was about double the only previous voucher sale on record, and prices are poised to balloon further as drugmakers seek even a brief head start for new products.

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The typical FDA review takes about 10 months, and priority reviews cut the wait to about six months. “If you have a multibillion-dollar drug, four months is worth a lot,” said Howard Liang, an analyst at Leerink Partners.

For small pharmaceutical companies like Knight, vouchers are a way to help turn a profit in a field where taking a drug from invention to use can cost about $1 billion, according to a 2010 study in the journal Nature. Knight had about $200 million in cash before the deal with Gilead.

See also: Cost to develop a drug more than doubles.

The FDA’s voucher program was first proposed in a 2006 academic paper and became law the following year, to be used for diseases like malaria, cholera and dengue fever.

Four grants

The market for the VIP passes has taken a while to pick up speed, with only four granted so far. Novartis AG received one in 2009 for its Coartem to treat malaria, which the company used to accelerate review of a gout drug that failed. Johnson & Johnson hasn’t yet used a voucher it got in 2012 for Sirturo, which treats tuberculosis.

Regeneron Pharmaceuticals Inc. and Sanofi were the first to buy a voucher in the market, paying $67.5 million for the one BioMarin Pharmaceutical Inc. was awarded last February. They’ve said they’ll use the pass to eliminate rival Amgen Inc.’s lead in bringing to market a potential blockbuster drug to treat ultra-high bad cholesterol levels. Amgen submitted its drug in August, while Sanofi and Regeneron planned to submit theirs late last year. The voucher could draw them even.

Gilead is still deciding how it will use the voucher it bought from Knight, said Cara Miller, a spokeswoman. The company may have acquired it simply to keep out of the hands of Merck & Co., which is developing a rival hepatitis C drug, according to RBC Capital Markets. Gilead declined to comment further.

Unnoticed diseases

While pharmaceutical giants use the vouchers to jockey for position in lucrative markets, the smaller drugmakers that have sold the passes are treating diseases that often go unnoticed in the developed world.

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BioMarin got its voucher after getting FDA approval for Vimizim, for patients with Morquio A syndrome, a rare ailment that can lead to heart disease, vision and hearing loss and even death. Knight’s Impavido treats leishmaniasis, a sometimes-fatal parasite-borne illness that can cause symptoms from skin sores to swelling in the spleen or liver.

Knight’s case shows how much a voucher can mean to a small pharmaceutical company. Its tale begins in 2013, when CEO Jonathan Goodman agreed to sell Paladin Labs Inc., the drugmaker he founded in 1995. Goodman said Paladin’s buyer, Dublin-based Endo International Plc, wouldn’t raise its bid to include the value of a potential voucher, so he kept Impavido and created Knight purely to house the drug.

New business

Since selling the voucher, Goodman has fielded so many calls from other drugmakers that he may make part of Knight’s business to specialize in partnerships to develop voucher-eligible drugs.

“I think it’s going to look more promising now because someone has shown success,” Goodman said in an interview. He sees potential for two or three vouchers a year to be granted and sold over the next decade.

One risk to the program seen by both Goodman and David Ridley, one of three Duke University professors who first proposed the vouchers, is a glut in the market that drives down prices. That could happen if many vouchers are issued at the same time or if the program is expanded to include treatments that drug companies would invent anyway, without the extra incentive.

An FDA spokeswoman declined to comment on the agency’s plans for the future of the program.

For now, the vouchers have given Eugene Seymour, chief executive officer of NanoViricides Inc., a way to work on treatments for dengue and Ebola with more confidence that he can make money on the effort.

“We really wanted to do dengue for humanitarian purposes,” Seymour said in an interview. “I am a physician. I am interested in humanity. I also run a New York Stock Exchange-listed company. I have to go after big targets. I have to be able to show my board that there is a potential return.”

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