Making a drug more convenient to take by producing an extended-release version can help drive sales. But things become more complicated when the extended-release version has to compete with a generic immediate-release version. Popping twice as many pills may be an acceptable trade-off for patients if the copay for the drug is cheaper.

Impax Laboratories (Nasdaq: IPXL) is in that situation with its extended-release version of carbidopa-levodopa, called IPX066. Generic versions of the combo drug to treat Parkinson's disease have been on the market for years.

But Impax has one big advantage: IPX066 appears to work much better than immediate-release carbidopa-levodopa. In phase 3 data released yesterday afternoon, IPX066 substantially improved symptoms of Parkinson's disease patients compared to the immediate-release version. Reducing the number of times the drug had to be taken from five times a day down to about 3.5 times per day was just an added bonus.

There are already generic versions of the extended-release version of the combo, Sinemet CR, which was originally developed by Merck (NYSE: MRK), but it doesn't work as quickly as IPX066 and is generally only used at night. Impax should have no problem competing with Sinemet CR or the immediate-release generic, although it will have to compete with other Parkinson's disease drugs like Novartis' (NYSE: NVS) Stalevo.

Impax plans to file its marketing application to the Food and Drug Administration in the fourth quarter. It'll market the drug on its own in the U.S., where it already has a sales force in place because it has a co-promotion agreement with Pfizer (NYSE: PFE) to sell Lyrica. Outside of the U.S. and Taiwan, Impax has already licensed the drug to GlaxoSmithKline (NYSE: GSK).

We'll get a better look at the data at upcoming medical meetings, but considering the data we've seen so far, it doesn't look like Impax should have any problems getting IPX066 approved.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.