Johnson & Johnson (JNJ 0.84%) and Medivation (MDVN) , with its partner Astellas Pharma, are locked in a high-stakes battle for prostate cancer treatment market share. Both companies have highly successful prostate cancer drugs in Zytiga and Xtandi, respectively. But Johnson & Johnson's Zytiga appears to be losing steam as oncologists embrace Xtandi.

I wrote about that dynamic last quarter, and based on J&J's fourth-quarter earnings release, it appears this quarter may have played out similarly.

Growing, but slowing
Zytiga remains an unquestionable success for J&J. That drug's sales jumped 85% from a year ago in the fourth quarter to $211 million. Zytiga's growth was even better overseas, with sales up 89% from last year to $284 million.

The fourth-quarter results brought Zytiga's full year sales to $1.7 billion, which is 77% higher than the fourth quarter of 2012. Roughly $750 million of those sales were generated in the United States, up 62%, while overseas sales jumped 90% to $948 million.

But turning attention to sequential growth throws some cold water on those seemingly rock-solid results. For a second consecutive quarter, Zytiga's sequential growth rate in the United States was in the single digits. After losing U.S. prostate cancer patient market share early in the fourth quarter, J&J's market share rebounded into the year's end. However, J&J's market share remains below its peak, and given that Xtandi's market share climbed steadily in the fourth quarter, Zytiga's year-end push doesn't appear to have come at Medivation and Astellas' expense.

Source: Johnson & Johnson Q4 Earnings Presentation

Instead, that share growth probably came from other treatments, including the most commonly prescribed treatment for metastatic castration resistant prostate cancer: bicalutamide. That was sold as AstraZeneca's billion-dollar-a-year drug Casodex before losing patent protection in 2008. AstraZeneca still generates a few hundred million dollars in sales from Casodex, and that revenue may be shifting to J&J and Medivation. 

Market share may also have come at the expense of Sanofi's Jevtana. It was supposed to succeed Sanofi's blockbuster prostate cancer drug Taxotere when it lost patent protection in 2010, but Jevtana hasn't lived up to sales expectations.

Dendreon's (NASDAQ: DNDN) struggling drug Provenge may have lost more ground, too. Despite being the only immunotherapy treatment available for prostate cancer, Provenge never gained significant traction, and sales of the drug slumped 13% in the third quarter from a year ago, prompting Dendreon to put itself up for sale. 

Fool-worthy final thoughts
Zytiga's 3.4% sequential fourth-quarter sales growth in the United States suggests Xtandi is winning business more quickly in both previously treated chemotherapy patients, for which Xtandi is already approved, and in pre-chemotherapy patients, for which Xtandi is awaiting FDA approval and is used off-label.

Medivation and Astellas may also start cutting into J&J's overseas sales, too.  Xtandi has just started competing with Zytiga in many of those international markets, and international sequential growth for Zytiga slipped to 9.2% in the fourth quarter, down from 17.6% in the third quarter.

There's plenty of opportunity for both J&J and Medivation to win business away from AstraZeneca's and Sanofi's legacy drugs and their competing generics. The two companies will probably continue to eat away at Dendreon's sales, too. But Zytiga's sequential slowing suggests Xtandi may be the prostate cancer drug to watch in 2014.

Editor's Note: A previous version of this article did not mention that Astellas Pharma is Medication's partner on Xtandi. The Fool regrets the error.