It has been difficult to develop drugs for brain disorders including AD. For example, many groups have tried to target γ-secretase.
A. Why is γ-secretase a good target for Alzheimer's treatment?
B. What is one reason, so far, that drugs targeting γ-secretase have not been successful?

A.γ-secretase is the enzyme that produces the Aβ peptide after APP-β has been cleaved from the APP peptide. This includes Aβ₄₀ and Aβ₄₂, peptides that aggregate to form plaques in Alzheimer's. If this enzyme could be altered it could reduce the production of Aβ₄₂, which would reduce the potential for aggregates to form and slow the progression of the disease.
B. There are many side effects to the drugs probably because γ-secretase targets many proteins other than APP and so the drugs have very broad and damaging effects.