Yesterday, Motley Fool Rule Breakers pick Millennium Pharmaceuticals (Nasdaq: MLNM) got a bit of expected -- but still good -- news when it received word that the FDA had given priority review status to its application to expand the use of lead drug Velcade into a front-line treatment against multiple myeloma.

Velcade is already approved to treat multiple myeloma (a form of cancer) after patients have failed other treatments. It filed the expanded marketing application last month. Currently, only Celgene's (Nasdaq: CELG) rival compound Thalomid is approved as a front-line drug for multiple myeloma patients, but another Celgene drug, Revlimid, is also used heavily in this indication.

The FDA grants a priority review to compounds that would be significant improvements over currently approved drugs. Most drugs receiving a priority review get approved the first go around with the FDA; one study done for the agency found 62% of priority review marketing applications got approved without being subjected to an approvable letter or other delays.

Millennium expects to hear back from the FDA by June 20 on its front-line Velcade marketing application. It has guided for sales of Velcade to be up 20% to 30%, to $320 million to $345 million, in the U.S. this year. Getting formal approval for Velcade to be used as a front-line treatment for multiple myeloma is undoubtedly important for Millennium and partner Johnson & Johnson (NYSE: JNJ) in their efforts to reach these higher Velcade sales levels.

But cancer-treating drugs like Velcade often start to show increased use well ahead of any formal FDA approval, because many insurers and doctors rely on treatment guidelines compiled in compendiums -- like a bulletin put out by the Association of Community Cancer Centers, for instance -- in determining when to use a compound, and don't stick strictly to a drug's FDA-approved use. In most compendiums that I've seen, Velcade's use in multiple myeloma is already on a level equal to Thalomid and Revlimid, so the benefits of the June FDA label expansion would not be as beneficial as a label expansion can be to other types of drugs.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Johnson & Johnson is an active Income Investor selection. The Fool has an A+ disclosure policy.