Arylbenzamide and Arylcarboxamide Derivatives as Modulators of Amyloid-Beta Aggregation

Abstract

Alzheimer’s disease (AD) is a complex neurodegenerative disease with increasing incidence and prevalence globally. The current AD therapies based on small molecules offer only symptomatic relief and are not curative therapies. The recently launched anti-amyloid monoclonal antibodies hold promise although these are new to the market and their long-term benefits and potential disease-modifying effects are unknown. The global increases in the aging population and increasing life span mandate the need to understand the mechanisms of AD and discover effective and safe therapies. Over the past several decades, few hypotheses have been proposed to explain the pathophysiology of AD, among which the amyloid beta (Aβ) cascade is now considered as one of the initiating factors that drives the progression and other pathological factors of AD. The aggregation of Aβ into oligomers and fibrils together with its downstream signaling pathway are neurotoxic. Thus, small molecule modulators that could reduce the overall toxic burden of Aβ aggregates are thought to be beneficial in treating AD. In this thesis, a library of 72 small molecule derivatives were designed based on the chemical structure of chalcone and curcumin, two bioactive natural compounds that are able to modulate Aβ aggregation and reduce their neurotoxicity. The derivatives reported in this thesis encompass four different templates, namely, N-benzyl (Chapter 2), N-phenethyl (Chapter 3), N-benzyloxy (Chapter 4), and N-phenyl (Chapter 5) benzamide and carboxamides. These compounds were synthesized by coupling the amine substrates with either acid halides or carboxylic acids to obtain the target compounds in 72-93.6% yields. A number of biophysical and biochemical experiments were carried out to determine the ability of these small molecules to modulate the aggregation properties of Aβ42. The experiments carried out include i) thioflavin T based fluorescence aggregation kinetics experiments; ii) transmission electron microscopy studies; iii) 8-anilino-1-naphthalenesulfonic acid based fluorescence spectroscopy; iv) antioxidant assay by fluorescence spectroscopy; iv) cell viability studies in mouse hippocampal HT22 neuronal cells and Aβ42-induced neurotoxicity assay; v) fluorescence microscopy studies to assess the neurotoxicity using Proteostat dye, and vi) computational modelling studies to determine the interactions of small molecules with Aβ42 aggregates. From this library, 51 aggregation inhibitors were identified (inhibition of Aβ42 ranging from 7-53.1% at 25 µM). These derivatives were able to provide significant neuroprotection from Aβ42-induced cytotoxicity in mouse hippocampal HT22 cells (cell viability ranging from 80.8-96.8% versus 38.7% for Aβ42-treated control). Molecular docking studies indicate that these derivatives were able to interact with the hydrophobic domains of the Aβ42 oligomer and fibril through hydrophobic interactions. In a striking and unusual finding, 8 derivatives were identified as Aβ42 aggregation promotors with the ability to promote the aggregation by 1.2-5.1 folds. Two lead promotors 14b (N-benzylbenzofuran-2-carboxamide) and 14c (N-benzylbenzo[b]thiophene-2-carboxamide) were identified. These two compounds were able to rescue HT22 cells from Aβ42-induced cytotoxicity (cell viability 73.8% and 73.9% for 14b and 14c versus 19.7% for Aβ42-treated control). These two compounds have the ability to increase the surface hydrophobicity of Aβ42 aggregates and promote fibrillogenesis. Molecular docking studies suggested that Aβ42 aggregates might undergo conformational change upon binding and thus transit to much more stable and less toxic/nontoxic fibrils. Further structure-activity relationship study indicated that the hydroxy- and methoxy-disubstituted phenyl moiety was required to possess Aβ42 inhibition activity, where the presence of bicyclic aromatic rings such as benzofuran and benzothiophene, and 4-methoxyphenyl moiety is required for pro-aggregation activity. The results show that these benzamides and carboxamides possessing N-benzyl, N-phenethyl, N-benzyloxy, and N-phenyl templates hold promise in the design and development of novel small molecules as Aβ42 aggregation modulators. Remarkably 14b (N-benzylbenzofuran-2-carboxamide) and 14c (N-benzylbenzo[b]thiophene-2-carboxamide) were able to accelerate Aβ42 aggregation and remodel the aggregation pathway to form less toxic/nontoxic aggregates suggesting their application as novel chemical tools to understand the mechanisms of Aβ42 aggregation cascade.