Second update with new information on the Zytiga study plus other background. RARITAN, N.J. ( TheStreet) -- Johnson & Johnson's ( JNJ) prostate cancer drug Zytiga prolonged the lives of patients with less advanced disease, according to results of a late-stage study announced Thursday. Zytiga, a pill, is now in line to treat the same prostate cancer patients targeted by Dendreon ( DNDN) with its Provenge immunotherapy. Dendreon shares fell 79 cents or 7% to $10.12 in Thursday trading on fears that Zytiga competition may further erode already-underperforming Provenge sales. Dendreon was down more sharply earlier Thursday but has rebounded because Zytiga's survival benefit in the study was not statistically significant. Meantime, Medivation ( MDVN) shares jumped $11.81, or 18% to $76 because its prostate cancer drug MDV3100 is a close cousin to Zytiga. Medivation is also conducting a late-stage study of MDV3100 in the same population of prostate cancer patients. J&J shares are up 1% to $64.98. In the J&J study, patients with prostate cancer who had not yet received chemotherapy were treated with Zytiga plus the steroid prednisone or a placebo plus prednisone. An independent data monitoring panel stopped the study early based on findings which showed the Zytiga therapy significantly delayed the growth of prostate cancer and helped patients live longer compared to placebo. All patients in the study will now be offered Zytiga treatment, according to J&J. The Zytiga trial was designed with co-primary endpoints of progression-free survival (PFS) and overall survival. The PFS benefit was statistically significant in favor of Zytiga while the drug showed a "strong trend" in prolonging survival that did not meet statistical significance," said J&J spokeswoman Kellie McLaughlin. The independent data monitors "stopped the trial based on the totality of the data they saw," said McLauglin, which included statistically significant benefit on all secondary endpoints. J&J plans to submit the Zytiga trial for presentation at an upcoming medical meeting, which could be the American Society of Clinical Oncology (ASCO) annual meeting in June. Zytiga was first approved in 2011 as a treatment for prostate cancer patients who have previously been treated with chemotherapy. Thursday's results 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 has not disclosed the extent to which Zytiga prolongs survival in pre-chemo prostate cancer patients, but if the convenient twice-daily pill works equally or better than Provenge, Dendreon could face further difficulties. Determining the real survival benefit of Zytiga could be confounded by the trial stopping early. Provenge is a cancer vaccine custom made for each patient and requires three infusions over the course of one month. Reimbursement issues and wavering demand for Provenge have slowed Dendreon's launch efforts, so a new drug or two entering as a direct competitor won't help.