[Forbes]-Patients taking an experimental Eli Lilly Alzheimer’s treatment worsened faster than those on placebo. The treatment also apparently caused skin cancer. Lilly says it will stop developing the drug, but will keep working on another, different Alzheimer’s treatment.

The result is another setback for Alzheimer’s research and should probably make scientists and investors think again about how little we know about this disease.

From Lilly’s press release:

“In two pivotal Phase III trials, semagacestat was compared with placebo in more than 2,600 patients with mild-to-moderate Alzheimer’s disease. Lilly has now reviewed data from a pre-planned interim analysis of semagacestat studies. This interim analysis showed that, as expected, cognition and the ability to complete activities of daily living of placebo-treated patients worsened. However, by these same measures, patients treated with semagacestat worsened to a statistically significantly greater degree than those treated with placebo. In addition, data showed semagacestat is associated with an increased risk of skin cancer compared with those who received placebo.”

The idea behind semagacestat was to prevent the creation of beta amyloid, a peptide that forms tangles in the brains of Alzheimer’s patients, by blocking an enzyme called gamma secretase. Amyloid plaque has been fingered by many researchers as a culprit in Alzheimer’s. Pfizer, Elan, and Johnson & Johnson are in the late stages of testing an antibody drug that attacks amyloid, although early results were mixed. Eli Lilly is still developing a similar, but not identical, antibody, and Pfizer has a third in the earlier stages of clinical research.

It may be there was something bad about this compound, or it may be that blocking the gamma secretase enzyme is not a good approach. It’s even possible that the whole idea that amyloid is involved in causing Alzheimer’s is just wrong. Maybe amyloid plaque is a result of the disease, but removing it won’t make things better. If it turns out blocking this protein is not a valid approach, it will set Alzheimer’s drug development back by many years.