Pfizer (NYSE: PFE) had a mixed bag of clinical trial results at the end of last week. A trial testing apixaban, its heart drug that's partnered with Bristol-Myers Squibb (NYSE: BMY), was stopped early because there was an increase in bleeding in the patients taking it. On the good side, Pfizer said its experimental cancer compound, axitinib, passed its clinical trial in kidney cancer.

I have to think the company would have rather seen the results flip-flopped.

The potential market for apixaban is huge. Pfizer and Bristol-Myers are trying to enter a megablockbuster showdown with Boehringer Ingelheim and a compound from Bayer and Johnson & Johnson (NYSE: JNJ).

This trial was testing apixaban in patients with a heart condition called acute coronary syndrome (ACS). The stoppage won't affect the trials testing apixaban in atrial fibrillation -- irregular heartbeat -- or as a treatment of venous thromboembolism -- blood clots that can form after surgery -- but investors should be at least a little worried that the ACS trial was stopped because of side effects.

Blood thinners are tricky because thinning out the blood too much can cause uncontrolled bleeding. Whether apixaban will be able to strike the right balance in atrial fibrillation and venous thromboembolism remains to be seen. Early results sure looked promising.

The positive results for cancer drug axitinib are certainly good news, but Pfizer already has two drugs, Sutent and Torisel, approved for use in kidney cancer patients. If axitinib is a rock star, it might take some market share away from Bayer and Onyx Pharmaceuticals' (Nasdaq: ONXX) Nexavar, but it'll also plunder sales of the company's current drugs, which doesn't add much value.

Drug development is a tough business, and a 50% success rate isn't all that bad. Unfortunately, not all wins are equal, and Pfizer could use a big one right about now.

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