Xtandi's new Prosper trial win has Pfizer and Astellas eyeing a bigger market

Steve Benner, head of oncology at Astellas, said competing with J&J's Zytiga “is an example where we just need to get additional data out that will help show the benefits of Xtandi in additional patient populations."

After Johnson & Johnson’s Zytiga put up impressive prostate cancer data at the American Society of Clinical Oncology meeting in June, Astellas and Pfizer needed a win for its rival medicine Xtandi. And now it has one.

Thanks to a protocol change that sped up results by two years, the two companies on Thursday unveiled top-line data from the Prosper trial, which tested Xtandi in patients whose cancer hadn’t yet spread through the body. The drug met its primary goal—heading off metastases longer than anti-hormone treatments alone.

Details won’t be available until they’re presented at an upcoming medical conference. But the results were good enough to prompt conversations with regulators about a new approval in all patients with castration-resistant prostate cancer (CPRC), the companies said. The drug is currently only approved for patients with metastatic disease.

It would be “a significant expansion for the drug,” Astellas SVP and oncology chief Steven Benner said in an interview last week ahead of the Prosper release. “If it works out as expected, it’s a way to bring Xtandi to a new patient population.”

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Astellas hasn’t yet estimated how many patients fit into the category of pre-metastatic CPRC, partly because patients are likely to end up in the metastatic setting eventually. The drug brought in $2.27 billion in sales last year.

The Prosper trial—and the new indication it may bring—could be a needed boost in Xtandi’s competition with J&J’s Zytiga. After Xtandi won approval in 2014, the drug quickly caught up with Zytiga in prescription-number terms, but at the end of last year, J&J started gaining while Xtandi began to lose ground, according to QuintilesIMS data quoted by Leerink in a recent investor note.

In fact, Xtandi sales fell in the first quarter, a fact that Pfizer blamed on increasing use of prescription assistance programs, an effect expected to moderate as the year progressed. But still, as oncology chief Albert Bourla acknowledged during the company’s first-quarter earnings call, “Today, Xtandi is performing below our expectations in the U.S.”

And then there’s the ASCO data. Two trials supported broader Zytiga use, one in patients still sensitive to hormones, where patients taking Zytiga, androgen-deprivation therapy and prednisone cut the risk of disease progression or death by 53%. That translated into a median of 33 months for the Zytiga group, compared with 14.8 months for patients on prednisone and hormone therapy alone.

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A different trial, dubbed Stampede, added Zytiga to standard therapy in newly diagnosed, low-risk patients. The three-year overall survival rate was 83% in the Zytiga group versus 76% in the standard-therapy group, and Zytiga lowered the relative chance of treatment failure by 71%. As Leerink Partners analyst Geoffrey Porges pointed out, the new studies could put many more patients in line for Zytiga treatment.

“This study result, with the huge overall survival benefit and improvement in failure-free survival and time to skeletal events, establishes the Zytiga-plus-ADT combination as the new standard of care for men starting long-term ADT for metastatic or recurrent prostate cancer,” Porges said. That assessment had analysts questioning anew the $14 billion price Pfizer paid for Medivation, a deal with Xtandi at its center.

But with Xtandi now posting a success in Prosper, it could answer Zytiga’s expansion with one of its own. The drug is also under study in hormone-sensitive disease, in two different trials, one focused on patients whose cancer has metastasized, called Arches, and the other, dubbed Embark, on patients whose cancer hasn’t yet spread.

“This is an example where we just need to get additional data out that will help show the benefits of Xtandi in additional patient populations,” Benner said.

Tyler Marciniak, Astellas head of product communications, said if those studies succeed, too, Xtandi would be looking at three potential indications to add to its label. “When you look at the entire population of men with prostate cancer,” Marciniak said, “the earlier we can move it, the more men we can potentially help with Xtandi.”

Editor's note: This story was modified to fix an inaccurate quote.