Drug development can be a roller coaster -- sometimes with just a single drug.

Last month, Pfizer (NYSE:PFE) stopped a clinical trial early because Sutent was helping pancreatic cancer patients so well that it was inhumane to keep giving some patients a placebo. Yesterday, Pfizer stopped another trial testing Sutent in breast cancer patients, but for the exact opposite reason: Continuing the trial was futile, since it was clear that the drug wouldn't be able to beat Roche's Xeloda.

That's a lot different from saying Sutent failed -- it just couldn't do better than what's already available. Sutent still has a chance to enter the breast cancer market, since the drug is still being tested in three phase 3 trials and two phase 2 trials. In one, Pfizer is going the "if you can't beat 'em, join 'em" route by testing Sutent in combination with Xeloda. It's also challenging Roche's Avastin and sanofi-aventis' (NYSE:SNY) Taxotere in head-to-head trials.

Even if it fails to show an effect in breast cancer, Sutent is a mighty fine drug for Pfizer. Sutent is already approved for treating gastrointestinal and kidney cancers and managed $847 million in sales last year. A likely approval for the rare pancreatic cancer should push the drug into the magical blockbuster status.

However, Sutent's failure is a reminder to investors with high hopes for drugs such as Novartis' (NYSE:NVS) Afinitor and Onyx Pharmaceuticals' (NASDAQ:ONXX) Nexavar. It's not impossible for drugs to work in multiple cancers -- Eli Lilly's (NYSE:LLY) Gemzar is approved for four different types, for example -- but just because a drug works well in one cancer, that doesn't mean it's guaranteed to succeed in treating all cancers.