Johnson & Johnson's (NYSE:JNJ) sold over $1 billion worth of its prostate cancer drug Zytiga in the first half of the year, putting it on pace to be a blockbuster two times over .

But not if Medivation (NASDAQ:MDVN) and partner Astellas Pharma have anything to say about it. The duo just gained FDA approval for an expanded indication for their rival drug Xtandi.

Johnson & Johnson has had the pre-chemotherapy market essentially all to itself. Dendreon's (NASDAQOTH:DNDNQ) Provenge is approved for those patients, but hasn't been able to compete with Zytiga because it's easier for doctors to prescribe the oral medication. The patient-specific Provenge treatment requires patients to go to an infusion center to get their immune cells collected and then a second trip to get them infused back in after they're trained to attack the tumor.

Source: Medivation and Astellas

Xtandi was approved as a post-chemotherapy regimen for prostate cancer in 2012, but that's a smaller market. U.S. sales of Xtandi were $83.3 million in the second quarter versus $235 million for Zytiga. By comparison Dedreon only sold $82 million in the second quarter, despite having a pre-chemo designation .

New label 
The expanded approval for Xtandi comes with a new label that, quite frankly, knocks the socks off of Zytiga.

The clinical trial to support the approval, dubbed PREVAIL, showed that Xtandi extended survival by a statistically significant amount. The 2.2-month increase over placebo isn't all that great, but it beats Zytiga since the difference in overall survival between Zytiga and placebo wasn't statistically significant .

Just as important as overall survival, Xtandi delayed the time until chemotherapy was necessary by more than 17 months. Given how toxic chemotherapy is, the longer a patient can go without having to deal with the side effects, the better. Zytiga only delayed chemotherapy use by 8.4 months.

Source: Medivation and Astellas

Data from the trial have been available since October, so doctors could have prescribed it off label before today if they were seeking out the data on their own. The FDA approval allows Medivation and Astellas to market the drug for patients that haven't had chemotherapy yet and tell doctors about the stellar data. And insurers are more likely to cover the therapy before chemotherapy now that it has an FDA endorsement.

Instant blockbuster

In the long term, I think Xtandi knocks off Zytiga and becomes the go-to drug for patients to use before chemotherapy. But don't expect Johnson & Johnson to go down without a fight. Being first to market in the pre-chemotherapy space is a big plus. Doctors are used to prescribing Zytiga, and it'll take a little while for Medivation and Astellas to knock out Johnson & Johnson.

It'll happen sooner or later though. Don't expect this fight to go the full 12 rounds.

Brian Orelli has no position in any stocks mentioned. The Motley Fool recommends Johnson & Johnson and owns shares of the company. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.