Analysts Not Sold On Medivation’s Xtandi as Breast Cancer Drug

December 10, 2014

By Riley McDermid, BioSpace.com Breaking News Sr. Editor

Despite recent data presented at the San Antonio Breast Cancer Symposium, Wall Street analysts remain divided over whether or not Medivation, Inc.’s cancer drug Xtandi will actually wind up being effective against breast cancer, Geoffrey Porges, a biotech analyst with Sanford Bernstein said this week.

In a note titled “Medivation: Can Xtandi Really Work in Breast Cancer? Conflicting Science, Limited Data, Small Population,” Porges said that while initial abstracts do seem attractive, he remains concerned about initial Xtandi data released that do not compare favorably with early data from the PD1/PD-L1 antibodies in triple negative breast cancer.

“To us the most interesting abstract is P5-19-09 showing initial results of the first study of Xtandi (enzalutamide) in breast cancer,” wrote Porges in a note to investors, since the study showed clinical activity for Xtandi in a “very difficult-to-treat breast cancer subpopulation.”

The study presented by Medivation has two stages. In the first stage, 26 patients with androgen receptor positive, triple negative breast cancer (AR+ TNBC) were evaluable, said Porges.

“At 16 weeks, 11 of 26 patients, or 42 percent, achieved clinical benefit, as defined by complete response plus partial response plus stable disease for 16 weeks. At 24 weeks, clinical benefit rate was 35 percent (or 9 of 26 patients), and includes 1 CR and 1 PR (or 8 percent response rate),” notes Porges.

Data from Stage 1 were sufficiently positive to advance to Stage 2 with 118 patients enrolled.

“In Stage 2, 13 additional patients had clinical benefit at week 16, including 3 PRs, as of the September cutoff (it is unclear what the denominator is for these responders). Additional data are expected to be presented during Poster Session 5 on Friday,” he said.

But multiple indicators remain that don’t seem to support Xtandi as an eventual TNBC therapy, said Porges. Among these are the fact that to date, there are no targeted therapies approved for triple negative breast cancer and that Xtandi’s data doesn’t seem to stack up to other companies involved in studying the disease.

“For example, Roche's PD-L1 antibody MPDL3280A showed 33 percent ORR (3/9 patients responding including 1 CR and 2 PRs),” he said. “The Roche data as well as data from the MRK PD1 antibody in TNBC are slated for presentation at SABCS as well. The unfavorable comparison probably contributed to weakness in Medivation today (down 3 to 4 percent).”

As such, Porges said Sanford Bernstein has low expectation for Xtandi's breast cancer indications and do not forecast any revenue for breast cancer in their Medivation model.

“AR+ TNBC represents a relatively small revenue opportunity as it accounts for up to 5 percent of all breast cancer. There are roughly 100,000 incident cases of metastatic breast cancer in the U.S., multiplied by 5 percent sub-population, [or around] 4 months duration of therapy at $8,000/month suggests revenue opportunity in the $160 million range,” calculated Porges in his note.

Xtandi is undergoing two additional signal-finding phase II studies, one in ER+/PR+/Her2- breast cancer, and the other in AR+ Her2 breast cancer. “These populations are larger but preclinical and clinical evidence is even more limited,” he said.