BACE Inhibitor Clinical Trials for Alzheimer's Disease

The amyloid hypothesis posits that the amyloid-(3 aggregates in the brain initiate a cascade of events that eventually lead to neuron loss and Alzheimer's disease. Recent clinical trials of passive immunotherapy with anti-amyloid-(3 antibodies support this hypothesis, because clearing plaques led to better cognitive outcomes. Orally available small molecule BACE1 inhibitors are another approach to slowing the buildup of plaques and thereby cognitive worsening by preventing the cleavage of amyloid-(3 protein precursor (A(3PP) into amyloid-(3 peptide, the major component of plaques. This approach is particularly attractive because of their ease of use, low cost, and advanced clinical stage. However, although effective in preventing amyloid-(3 production in late-stage clinical trials, BACE inhibitors have been associated with early, non-progressive, likely reversible, cognitive decline. The clinical trials tested high levels of BACE inhibition, greater than 50%, whereas genetics suggest that even a 30% inhibition may be sufficient to protect from Alzheimer's disease. Aside from A(3PP, BACE1 cleaves many other substrates in the brain that may be contributing to the cognitive worsening. It is important to know what the cause of cognitive worsening is, and if a lower level of inhibition would sufficiently slow the progress of pathology while preventing these unwanted side effects. Should these side effects be mitigated, BACE inhibitors could rapidly move forward in clinical trials either as a primary prevention strategy in individuals that are at risk or biomarker positive, or as a maintenance therapy following amyloid clearance with an anti-amyloid antibody.