CHICAGO ( TheStreet) -- Johnson & Johnson's ( JNJ - Get Report) Zytiga improved survival in "pre-chemo" prostate cancer patients by 33% and more than doubled the time before cancer worsened compared to control, according to results from a phase III study to be presented today at the American Society of Clinical Oncology (ASCO) annual meeting.

Zytiga, a pill, was first approved in 2011 as a treatment for prostate cancer patients who have previously been treated with chemotherapy. The new data released for the first time Saturday open a much larger commercial opportunity for J&J because if approved, Zytiga will become a treatment option for men with less advanced prostate cancer.

J&J reported Zytiga sales of $200 million, half of those in the U.S., in the first quarter.

The strong Zytiga results are expected to have positive spillover effects for Medivation ( MDVN), which is developing its own prostate cancer pill MDV3100 that is similar -- and perhaps superior to -- Zytiga.

Both Zytiga and MDV3100 work similarly by interfering with the way prostate cancer cells feed from testosterone produced in the body.

Dendreon ( DNDN), however, may emerge from this year's ASCO meeting with a limp. Based on today's Zytiga results, the pill could steal prostate cancer patients away from its Provenge immunotherapy.

In the J&J study, 1,088 patients with prostate cancer who had not yet received chemotherapy were treated with Zytiga plus the steroid prednisone or a placebo plus prednisone.

The study was stopped early after an interim analysis found that treatment with Zytiga reduced the risk of prostate cancer worsening or death (also known as progression-free survival) by 57% compared to placebo. The result was highly statistically significant, with a p value of 0.0001, meaning there was less than a one-hundredth of 1% chance that the outcome was derived by chance.

Prostate cancer patients treated with placebo reported a median progression-free survival (PFS) of 8.3 months, while the median PFS for Zytiga-treated patients has not yet been reached." Progression events are coming in slowly, which demonstrates the benefit that Zytiga is having for patients," said Dr. Michael Meyers, a Johnson & Johnson vice president in charge of Zytiga's clinical development.

Based on the reported PFS hazard ratio of 0.43 at the time of the interim analysis, however, the median PFS for Zytiga patients could be as high as 19.3 months. That would represent a median PFS improvement of 11 months favoring Zytiga. The actual median PFS for Zytiga-treated patients has not been reported at this ASCO meeting and may be lower.

The overall survival hazard ratio reported Saturday was 0.75, which means Zytiga reduced the risk of death by 25% compared to placebo. Stated another way, Zytiga improved overall survival by 30%. The median overall survival for placebo patients was 27.2 months, the longest survival ever measured in a phase III study. Zytiga-treated patients have yet to reach a median overall survival.

The Zytiga improvement in overall survival, while super-sized, was not statistically significant because the study was stopped early. The p value for overall survival was 0.0097 but the hurdle for statistical significance at the interim analysis was 0.0008.

Again, based on the reported overall survival hazard ratio of 0.75 at the time of the interim analysis, the median overall survival for Zytiga patients could ultimately reach as high as 36.3 months, or a 9-month improvement over placebo. The actual median survival for Zytiga patients won't be known until Johnson & Johnson follows up with additional analysis.

Patients in the control arm were allowed to cross over to receive treatment with Zytiga once their disease progressed, which may confound the survival analysis, Meyers said.

The benefit derived by "pre-chemo" patients from treatment with Zytiga rivals and in some cases exceeds that seen in the pivotal study of Dendreon's Provenge. The FDA approved Provenge based on a 30% improvement in survival in a phase III trial. The median overall survival improvement favoring Provenge was 4 months.

One long-time knock on Provenge, however, is that the personalized prostate cancer "vaccine" has no measureable effect on tumor size or progression, making it difficult for doctors to determine which patients are benefiting from therapy.

In stark contrast, doctors can easily assess whether Zytiga is benefiting patients. The J&J pill delays tumor worsening and the use of pain-killing opiates, delays the need for chemotherapy and slows PSA progression, according to new data from the phase III study released Saturday.

Zytiga was relatively well tolerated by patients. Serious adverse events attributable to the drug included hypertension and elevations of liver enzymes.

A formal presentation of the Zytiga phase III data is scheduled for 9 am EDT at the ASCO meeting. J&J is expected to seek regulatory approval later this year to expand Zytiga's label to include treatment for "pre-chemo" prostate cancer patients.

--Written by Adam Feuerstein in Chicago.

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