Amyloid cascade in Alzheimer's disease: Recent advances in medicinal chemistry

Alzheimer's disease is of major concern all over the world due to a number of factors including (i) an aging population (ii) increasing life span and (iii) lack of effective pharmacotherapy options. The past decade has seen intense research in discovering disease-modifying multitargeting small molecules as therapeutic options. The pathophysiology of Alzheimer's disease is attributed to a number of factors such as the cholinergic dysfunction, amyloid/tau toxicity and oxidative stress/mitochondrial dysfunction. In recent years, targeting the amyloid cascade has emerged as an attractive strategy to discover novel neurotherapeutics. Formation of beta-amyloid species, with different degrees of solubility and neurotoxicity is associated with the gradual decline in cognition leading to dementia. The two commonly used approaches to prevent beta-amyloid accumulation in the brain include (i) development of beta-secretase inhibitors and (ii) designing direct inhibitors of beta-amyloid (self-induced) aggregation. This review highlights the amyloid cascade hypothesis and the key chemical features required to design small molecules that inhibit lower and higher order beta-amyloid aggregates. Several recent examples of small synthetic molecules with disease-modifying properties were considered and their molecular docking studies were conducted using either a dimer or steric-zipper assembly of beta-amyloid. These investigations provide a mechanistic understanding on the structural requirements needed to design novel small molecules with anti-amyloid aggregation properties. Significantly, this work also demonstrates that the structural requirements to prevent aggregation of various amyloid species differs considerably, which explains the fact that many small molecules do not exhibit similar inhibition profile toward diverse amyloid species such as dimers, trimers, tetramers, oligomers, protofibrils and fibrils.