After winning Food and Drug Administration (FDA) approval for use in castration-resistant metastatic prostate cancer in 2012, Pfizer's (NYSE:PFE) Xtandi quickly gained ground on Johnson & Johnson's (NYSE:JNJ) multibillion-dollar Zytiga. However, Xtandi's momentum has slowed, leaving some to question if Pfizer's $14 billion acquisition of Medivation to land Xtandi was a mistake.

This week, Pfizer may have alleviated some of that concern by reporting new data from a trial showing Xtandi delays the spread of this cancer in non-metastatic patients.

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Two titans battling

Zytiga won FDA approval in 2011 and since then, it's gone on to become a widely used therapy in castration-resistant metastatic prostate-cancer patients. Sales quickly eclipsed the billion-dollar blockbuster threshold, and despite competition from Xtandi, Zytiga's annual sales pace remains impressive, with annualized revenue of $2.2 billion in Q2.

Like Zytiga, Xtandi also achieved significant commercial success. Initially approved for patients with castration-resistant metastatic disease following chemotherapy, an expanded label has since allowed it to chip away at an increasingly bigger share of Zytiga's sales. As a result, Xtandi's sales were about $2 billion last year.

Xtandi's momentum, though, has been waning recently. In the first quarter, Pfizer said Xtandi sales slipped, in part because of greater use of its patient-assistance program. In Q2, sales made some headway higher, but optimism for future growth became cloudy when Johnson & Johnson rolled out new Zytiga data in June.

Specifically, Johnson & Johnson evaluated Zytiga in newly diagnosed metastatic prostate cancer patients and they found that adding it to androgen deprivation therapy reduced the risk of disease progression by 53%. A second trial also showed that using Zytiga in low-risk, newly diagnosed non-metastatic prostate-cancer patients significantly improved the likelihood of treatment success.

These results suggest Zytiga may be used earlier in treatment than Xtandi. If so, it could treat far more of the 177,000 prostate-cancer patients who are diagnosed with the disease in the U.S. every year. Though that's true, Zytiga's advantage over Xtandi could be short-lived because Pfizer and Astellas (Pfizer's partner on Xtandi) reported results this week suggesting Xtandi can be used earlier in treatment, too.

In patients, without metastatic prostate cancer who are on hormone therapy, yet have rising PSA levels -- an early signal of disease progression -- Xtandi delayed prostate cancer's progression to metastatic disease. Pfizer and Astellas are saving the specific data for release at an upcoming conference, but the results must be pretty good given that management plans to speak with regulators soon about expanding Xtandi's label to include all patients who are castration resistant.

Looking ahead

Prostate cancer is incredibly common, and that means these companies are battling over billions of dollars in annual sales. The market-share winner will depend heavily on how early in treatment each drug is used, so in both instances, use in non-metastatic prostate-cancer patients is commercially significant. It remains to be seen how oncologists view the competing data so far, and how these drugs will change standard care in castration-resistant patients.

Ongoing trials at both companies are likely to further blur the lines in terms of addressable patients. Overall, perhaps the most important takeaway is that both Zytiga and Xtandi could see revenue growth in the coming years because of earlier use in non-metastatic patients.

Todd Campbell owns shares of Pfizer. His clients may have positions in the companies mentioned. The Motley Fool owns shares of and recommends Johnson & Johnson. The Motley Fool has a disclosure policy.