Ever sit in the upper deck of a large ballpark and see a high fly ball that you thought was going out of the park, only to see it die well before the warning track? It's hard to judge balls from up there.

That's kind of where Aastrom Biosciences' (NAS: ASTM) investors are sitting right now. The crack of the bat sounded good, but whether its bone marrow-derived cellular therapy, ixmyelocel-T, will clear the fence remains to be seen.

Aastrom presented data at the American Heart Association Scientific Sessions today for its phase 2 trial testing ixmyelocel-T in patients with critical limb ischemia, a chronic cardiovascular disease that obstructs blood flow and results in open wounds on limbs that often have to be amputated. Investors already knew that the treatment passed its phase 2 trial, but the presentation gave insight into the magnitude of the success.

Data for the primary efficacy endpoint looked good: Relative to placebo, patients getting ixmyelocel-T saw a 62% reduction in the time to first occurrence of treatment failure, a composite endpoint that measurers whether the patient is getting worse.

But that's not the primary endpoint Aastrom plans to use in the phase 3 trial. The patient population enrolled in the phase 2 trial isn't even the same patients it plans to enroll in the phase 3 trial.

Instead, Aastrom plans to look at patients that are furthest along in their disease progression and have already experienced tissue loss. In a subset of phase 2 patients with a wound, ixmyelocel-T reduced the time to treatment failure by 77%, which is better than the 62% seen with the entire patient population.

Aastrom will use amputation-free survival -- amputation or death -- at 12 months as the primary endpoint for the trial, because that's what the Food and Drug Administration wants. The data for a subgroup with wounds showed improvement in amputation-free survival for patients taking ixmyelocel-T, but the improvement wasn't statistically significant given the small size.

Taking a subset of patients from a phase 2 trial to test in phase 3 introduces uncertainty, the bane of investors. It's the reason Aeterna Zentaris (NAS: AEZS) and Keryx Pharmaceuticals (NAS: KERX) remain cheap ahead of the release of results for their phase 3 trial for perifosine despite a solid benefit in the phase 2 trial for the sub-population being tested in the phase 3 trial.

Of course, uncertainty doesn't necessarily mean failure. Exelixis (NAS: EXEL) passed its phase 3 trial for cabozantinib by a wide margin despite guessing completely wrong on how well patients would do.

I think Aastrom is making the right move, given the data, but it certainly makes it less certain that the phase 3 trial will come out positive and will probably hold down its share price until the results are revealed. There is one advantage to sitting in the cheap seats, though: They're cheap, and in biotech that can mean big returns.

At the time this article was published Fool contributor Brian Orelli holds no position in any company mentioned. The Motley Fool owns, and Motley Fool newsletter services have recommended buying, shares of Exelixis. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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