Johnson & Johnson's (JNJ) recently approved prostate cancer drug Zytiga continues to impress. On Thursday, the company said the drug improved survival and delayed the progression of cancer in patients with less advanced disease, according to a late-stage study.

For now, the drug is approved only for treatment in men with a late-stage form of prostate cancer who have failed to respond to chemotherapy. But the study was in men with earlier stages of the disease and who had not been treated with chemotherapy. Approval to treat this new prospective patient population could significantly grow the drug's market.

Three stocks immediately reacted to the news. J&J shares were up about 1% on the news. Shares of Medivation (MDVN), whose own prostate cancer drug is similar to Zytiga, were up some 15%. And shares of Dendreon (DNDN), which makes the personalized prostate cancer vaccine Provenge, were down about 6% -- coming back from an initial drop of 15%.

The trial results

J&J tested over 1,000 men with prostate cancer who hadn't received chemotherapy, giving some of them Zytiga with the steroid prednisone, and some a placebo and prednisone.

The trial, which began in 2009, was supposed to continue until 2014, according to Dow Jones. But an independent data monitoring committee unanimously recommended to end it early because the interim results showed "evidence of clinical benefit as well as continued evidence of favorable safety" in patients receiving Zytiga. The committee recommended offering the patients taking placebo treatment with Zytiga.

While J&J didn't release specific data, it said that progression-free survival (living without the disease getting worse), showed a statistical difference, while overall survival only showed a "strong trend," a company spokesperson told TheStreet.

J&J plans to apply for regulatory approval beginning in the second half of 2012. JNJ investors, still reeling from years of product recalls, should be happy about this.

What about Dendreon?

No one can say these results came as a surprise. It's been some time now that analysts and pundits have been warning about Zytiga's potential effect on Provenge.

Zytiga, which works by cutting production of a hormone that can stimulate cancer cell growth, is a daily pill that is faster-acting and relives symptoms, such as pain, quicker. Zytiga costs $5,000 a month for eight cycles.

Provenge, which works by making the body's own immune system attack prostate cancer cells, is much more complicated. First, it requires extracting white blood cells from patients, sending it to Dendreon so that they are combined with vaccine components. Then, patients are administered the drug through three infusions in the course of a month. The effects, it appears, are much slower to manifest. Provenge costs over $90,000 for a course of treatment.

With Zytiga's potential expansion to include patients targeted by Provenge, it could easily encroach on Dendreon's market.

Dendreon tried to prepare, attempting to show in ongoing clinical trials that Provenge should be given in sequence with drugs like the ones from J&J and Medivation to improve treatment. A  Deutsche Bank analyst, with a 'buy' on Dendreon certainly thinks that will eventually be the case. Maybe. But what's certain, though, is that Provenge no longer holds the promise it once did.

And while compared to Dendreon's insanely wild ride in the past few years -- before and after Provenge's approval -- Thursday's bump doesn't look like much, it's the overall disappointing trend that's the concern.