If AstraZeneca (NYSE: AZN) had to have a drug get held up in the regulatory process, it certainly lucked out with the right one.

The Food and Drug Administration sent the drugmaker back to the drawing board for the second time yesterday when it rejected motavizumab, AstraZeneca's newest treatment for respiratory syncytial virus (RSV). In 2008, the company said it didn't think a new trial would be necessary, and resubmitted the marketing application about a year later. This time around, it appears the FDA really does want a new trial before it'll approve motavizumab.

On one hand, this is fairly disappointing, especially because the course of events seems to show a communication breakdown somewhere between AstraZeneca and the FDA, something Roche and ImmunoGen's (Nasdaq: IMGN) investors know all too well. But AstraZeneca's rejection isn't as big a deal as it was for Roche and Immunogen, since motavizumab would compete with its own Synagis, which brought in more than $1 billion last year.

Alnylam Pharmaceuticals (Nasdaq: ALNY) and Cubist Pharmaceuticals (Nasdaq: CBST) are working on a treatment for RSV -- Synagis and motavizumab are just prophylactics that prevent infection -- but it's still in phase 2 development, so AstraZeneca can enjoy its Synagis sales for a while.

AstraZeneca hasn't decided exactly what it's going to do, but I wouldn't be surprised if the company just put motavizumab on the shelf and called it a day. There's no guarantee that results of a new trial will help convince doctors and the FDA that motavizumab works better than Synagis. Eli Lilly (NYSE: LLY) followed that path when it dropped development of its osteoporosis drug arzoxifene, after a clinical trial suggested it wouldn't work better than its current offerings.

Not every drug will get through the marathon of drug development. It's crucial for investors to keep track of which ones are important, and which are fairly disposable. Fortunately, motavizumab lands squarely in the latter category.

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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.