We've known that Medivation's (NASDAQ:MDVN) Xtandi worked in prostate cancer patients who haven't yet received chemotherapy treatment since interim top-line data from the Prevail trial was released in October.

But investors are getting a closer look at how well the drug actually works through the Genitourinary Cancers Symposium that starts Thursday in San Francisco. Abstracts hit the Internet this evening.

Expanding sales
Xtandi is approved for patients who have failed chemotherapy -- typically Sanofi's (NASDAQ:SNY) Taxotere -- but the big money would be as a first-line therapy after the prostate cancer has spread but before patients become asymptomatic.

Dendreon's (OTC:DNDNQ) Provenge was the first drug approved for that indication, but it has struggled, in part because of missteps by Dendreon and in part because of how complex Provenge is to administer.

Both Johnson & Johnson (NYSE:JNJ), which sells Zytiga, and Medivation with marketing partner Astellas went the easier route, gaining FDA approval for their pills in post-chemotherapy patients in April 2011 and August 2012, respectively. Johnson & Johnson received expanded FDA approval for Zytiga to include pre-chemotherapy patients in late 2012, driving last year's U.S. sales 62% higher than 2012.

Solid data
Xtandi reduced the risk of death by 29% and the risk of progression as measured by radiography by a whopping 81% compared to placebo. More than half of patients had a complete or partial response to the drug.

Because the disease progression was delayed, patients could also delay taking Taxotere. The median time to starting chemotherapy was 17 months longer for patients taking Xtandi compared to the placebo group. That's 17 months without having to deal with the nasty side effects that come with chemotherapy.

Pick a winner
In a press conference with reporters, the study author, Tomasz Beer, wasn't willing to pick a winner between Zytiga and Xtandi. It's dangerous to compare across clinical trials with different enrolment criteria.

That's not going to stop investors, though. From what we've seen so far, it appears Xtandi is the better drug, with a 29% reduction in the risk of death compared to 21% for Johnson & Johnson's Zytiga. Dendreon's Provenge is a minor player at this point, but for the record, it reduced the risk of death by 23%.

Zytiga has to be administered with steroids and must be taken on an empty stomach, while Xtandi doesn't have those restrictions.

While the competition between the drugs heats up, it might not last too long. Medivation and Astellas are running a phase 4 trial to see if Zytiga and Xtandi combined are better than Zytiga alone. If it works, the combination could be the standard of care in a few years.