Sometimes, early stage data from a clinical trial can help inform investors about the prospects of a drug, and sometimes the data just makes a drug's future even harder to gauge. On Tuesday, Elan (NYSE:ELN) and partner Wyeth (NYSE:WYE) released new mid-stage data for their potential blockbuster bapineuzumab (AAB-001) for Alzheimer's disease, and it doesn't make things any clearer.

It's important to remember that the hurdles for a successful phase 2 clinical study are not the same as for a phase 3 trial. In the bapineuzumab study, patients taking the drug didn't show a statistically significant improvement in the symptoms, but that can be OK in some cases.

After the study was completed, though, Elan and Wyeth did notice that a large group of Alzheimer's patients taking bapineuzumab and lacking a specific genetic characteristic did fare better on both effectiveness and safety issues. All we got was this headline information, and next month at the International Conference on Alzheimer's Disease, we'll get more information about how well this specific patient group performed compared with placebo and the other patients treated with bapineuzumab.

What makes Alzheimer's disease clinical studies and studies for drugs to treat other forms of cognitive impairment so tough is that there are often no definitive or irrefutable goals, like survival, on which to gauge a compound's effectiveness. This is why the success of Alzheimer's disease compounds is more unpredictable than normal and why many predict that these compounds will fail at a higher rate than other drugs. Compounds like Myriad Genetics' (NASDAQ:MYGN) Flurizan are in a similar murky situation, although at a much later stage in development.

That being said, the goal of any phase 2 study is to (at a minimum) help inform a drugmaker on how to design the more important phase 3 studies and what sort of data these phase 3 studies must produce to be a success. In this respect, preliminarily at least, I don't think the bapineuzumab study was a success, unless something more definitive comes out later this month -- especially considering that the companies have already begun multiple phase 3 trials.

Elan and Wyeth also noted that "there were imbalances" in this phase 2 trial that could have skewed its results. When you combine this with a small study size, murky trial endpoints, and unexpected post-hoc subgroup data, investors should feel just as ambiguous about bapineuzumab's prospects as they were before the phase 2 study data came out. Therefore, I think the risk of bapineuzumab failing in the clinic has not gone down. Invest accordingly.

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