With about $600 million in cash and short-term investments sitting on its balance sheet, ViroPharma (NASDAQ:VPHM) had to make a bet on its future at some point. On Tuesday, it made the move with the acquisition of orphan-drug developer Lev Pharmaceuticals for up to $618 million.

For $443 million up front (part of that in ViroPharma stock), ViroPharma is getting access to Lev's ultra-orphan disease drug Cinryze. The company is awaiting an FDA decision on marketing it as a treatment to prevent flare-ups of a very rare genetic inflammatory disorder.

It's estimated that the disorder, hereditary angioedema (HAE), affects around 10,000 people in the U.S., although only 4,600 patients have been diagnosed with it. Lev had received a rejection letter for Cinryze in January after the FDA asked for "additional analyses of existing efficacy data" and some more manufacturing data on the drug. Lev has tried to resolve the FDA's concerns, and another FDA marketing decision is expected on or around Oct. 14.

ViroPharma thinks Cinryze could be the first approved drug in the U.S. to help with HAE. There are similar competitors in late-stage clinical testing, like Dyax's (NASDAQ:DYAX) DX-88. Another similar compound is approved for use in some European countries, but if Cinryze gets FDA approval, it would have seven years of marketing exclusivity in the U.S., thanks to its orphan-drug status.

ViroPharma expects Cinryze to have a "comparable price to other ultra-orphan drugs," which would make the drug's annualized cost somewhere in the $200,000 to $400,000 range. If it's used by a quarter of the 4,600 identified HAE patients and is priced at $300,000 annually, then it could bring nearly $350 million into ViroPharma's coffers in relatively short order … unless Dyax and others come onto the market and drive the price down.

Many specialty pharmas similar to ViroPharma have been saving up large chunks of cash to make what is hoped to be a transformational acquisition. Not even counting the large-cap drugmakers like Novartis that are addicted to acquisitions, it's likely that those like BioMarin (NASDAQ:BMRN), Endo Pharmaceuticals (NASDAQ:ENDP), and Alpharma (NYSE:ALO) are looking to use their cash to either in-license new drugs or to make wholesale acquisitions.

ViroPharma acquired its only marketed drug, Vancocin, after it bought it from Eli Lilly (NYSE:LLY) on the cheap in 2004, so it has had success with bringing new compounds into its fold. With an FDA decision just around the corner, it looks like we'll find out soon enough if Cinryze is on its way to being another Vancocin, or a value-destroying proposition like OSI Pharmaceuticals (NASDAQ:OSIP) with its horrendous Eyetech deal.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Eli Lilly is an Income Investor pick. The Fool has an A+ disclosure policy.