Towards disease-modifying treatment of Alzheimer's disease: Drugs targeting β-amyloid

Pathological, genetic, biochemical and pharmacological studies support the hypothesis that brain accumulation of oligomeric species of β-amyloid (Aβ) peptides may cause Alzheimer's disease (AD). Drugs currently used for the treatment of AD produce limited clinical benefits and do not treat the underlying causes of the disease. In the last 10 years, new therapeutic approaches targeting Aβ have been discovered and developed with the hope of modifying the natural history of the disease. Several active and passive immunotherapy approaches are under investigation in clinical trials with the aim of accelerating Aβ clearance from the brain of the AD patients. The most advanced of these immunological approaches is bapineuzumab, composed of humanized anti-Aβ monoclonal antibodies, that is being tested in two large late-stage trials. Compounds that interfere with proteases regulating Aβ formation from amyloid precursor protein (APP) are also actively pursued. Unfortunately, the most biologically attractive of these proteases, β-secretase, that regulates the first step of the amyloidogenic APP metabolism, was found to be particularly problematic to block and only one compound (CTS21166) has reached clinical testing so far. Conversely, several inhibitors of γ-secretase, the protease that regulates the last metabolic step generating Aβ, have been identified, the most advanced being LY-450139 (semagacestat), presently in Phase III clinical development. Compounds that stimulate α-secretase, the enzyme responsible for the nonamyloidogenic metabolism of APP, are also being developed one of them, EHT-0202, has recently started a Phase II study. Furthermore, brain penetrant inhibitors of Aβ aggregation have been identified and one of such compounds, PBT-2, has produced encouraging neuropsychological results in a recently completed Phase II study. With all these anti-Aβ approaches in clinical testing, we will know in few years if the Aβ hypothesis of AD is correct.