Johnson & Johnson's (NYSE: JNJ) purchase of Cougar Biotechnology looks like a roaring success. The company stopped a phase 3 trial of abiraterone, which Johnson & Johnson got in the purchase, because interim results showed the drug worked better than placebo in prostate cancer patients.

The company didn't say how long abiraterone was able to extend survival -- it's saving the full data for a scientific meeting in October -- but beating placebo by a statistically significant amount should be enough to get the drug approved, given the lack of options for patients who have failed sanofi-aventis' (NYSE: SNY) Taxotere.

Sanofi recently reported data on this patient population for its own second-line drug, cabazitaxel, but the comparison with abiraterone could be difficult. Cabazitaxel produced a median survival of 15.1 months, but the drug was compared to EMD Serono and OSI Pharmaceuticals' (Nasdaq: OSIP) Novantrone. Johnson & Johnson compared an abiraterone-and-prednisone regimen to prednisone alone.

Whichever drug ends up coming out on top, it's clear that Dendreon's (Nasdaq: DNDN) Provenge won't be affected. Provenge competes directly with Taxotere for patients whose tumors have become resistant to hormone-reducing therapies. Without further testing, both of these drugs will primarily be used in patients after they've failed Taxotere, and perhaps Provenge as well.

Johnson & Johnson is testing abiraterone in patients earlier in their disease progression, but it's only in a phase 2 trial right now. For the immediate future, Provenge will only have to worry about Taxotere, to which it appears to be superior.

Prostate cancer has generally been considered a tough disease to treat, but recent positive results look like good news for patients. For investors, the results highlight one of the challenges of investing in drugmakers: looking in the rear view mirror -- where you'll find Medivation (Nasdaq: MDVN) and OncoGenex Pharmaceuticals (Nasdaq: OGXI) with their own prostate-cancer drugs -- is a necessary evil.

Jim Mueller suggests bypassing branded drugs in favor of this generic-drug maker.

Johnson & Johnson is a Motley Fool Income Investor pick. Motley Fool Options has recommended a diagonal call position on Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. True to its name, The Motley Fool is made up of a motley assortment of writers and analysts, each with a unique perspective; sometimes we agree, sometimes we disagree, but we all believe in the power of learning from each other through our Foolish community.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.