Design, Selection, and Characterization of Thioflavin-Based Intercalation Compounds with Metal Chelating Properties for Application in Alzheimer’s Disease

Abstract

Metal chelation is considered a rational therapeutic approach for interdicting Alzheimer's amyloid pathogenesis. At present, enhancing the targeting and efficacy of metal-ion chelating agents through ligand design is a main strategy in the development of the next generation of metal chelators. Inspired by the traditional dye Thioflavin-T, we have designed new multifunctional molecules that contain both amyloid binding and metal chelating properties. In silico techniques have enabled us to identify commercial compounds that enclose the designed molecular framework (M1), include potential antioxidant properties, facilitate the formation of iodine-labeled derivatives, and can be permeable through the blood-brain barrier. Iodination reactions of the selected compounds, 2-(2-hydroxyphenyl)benzoxazole (HBX), 2-(2-hydroxyphenyl)benzothiazole (HBT), and 2-(2-aminophenyl)-1H-benzimidazole (BM), have led to the corresponding iodinated derivatives HBXI, HBTI, and BMI, which have been characterized by X-ray diffraction. The chelating properties of the latter compounds toward Cu(II) and Zn(II) have been examined in the solid phase and in solution. The acidity constants of HBXI, HBTI, and BMI and the formation constants of the corresponding ML and ML2 complexes [M = Cu(II), Zn(II)] have been determined by UV-vis pH titrations. The calculated values for the overall formation constants for the ML2 complexes indicate the suitability of the HBXI, HBTI, and BMI ligands for sequestering Cu(II) and Zn(II) metal ions present in freshly prepared solutions of beta-amyloid (Abeta) peptide. This was confirmed by Abeta aggregation studies showing that these compounds are able to arrest the metal-promoted increase in amyloid fibril buildup. The fluorescence features of HBX, HBT, BM, and the corresponding iodinated derivatives, together with fluorescence microscopy studies on two types of pregrown fibrils, have shown that HBX and HBT compounds could behave as potential markers for the presence of amyloid fibrils, whereas HBXI and HBTI may be especially suitable for radioisotopic detection of Abeta deposits. Taken together, the results reported in this work show the potential of new multifunctional thioflavin-based chelating agents as Alzheimer's disease therapeutics.

Design of small molecules that target metal-Aβ species and regulate metal-induced Aβ aggregation and neurotoxicity

“…From these solution speciation diagrams, the concentrations of unchelated Cu 2þ and Zn 2þ (pM ¼ − log½M unchelated ) at a specific pH value can be calculated (pCu ¼ 9.9 and 10.2 at pH 6.6 and pH 7.4, respectively; pZn ¼ 6.1 at pH 7.4). The pM values represent the metal binding affinity of L2-b at a specific pH value as well as an estimate of its ability to sequester metal ions bound to Aβ peptide (21,35). The pCu and pZn values for L2-b, corresponding to approximate dissociation constants (K d ), indicated that these K d values were comparable to the reported ones of Cu-Aβ (picomolar to nanomolar) and Zn-Aβ (micromolar) (2,3,8,9,21,(35)(36)(37).…”

“…The pM values represent the metal binding affinity of L2-b at a specific pH value as well as an estimate of its ability to sequester metal ions bound to Aβ peptide (21,35). The pCu and pZn values for L2-b, corresponding to approximate dissociation constants (K d ), indicated that these K d values were comparable to the reported ones of Cu-Aβ (picomolar to nanomolar) and Zn-Aβ (micromolar) (2,3,8,9,21,(35)(36)(37). Thus, the affinities of L2-b for Cu 2þ and Zn 2þ at the relevant pHs [for metal-induced Aβ aggregation (2-4, 7-9)] suggest that this compound is able to chelate metal ions from soluble Aβ species, which could support the results from reactivity studies of L2-b with metal-treated Aβ species (vide infra).…”

“…To date, only limited efforts have been made toward this goal (2,13,(19)(20)(21)(22)(23)(24). Recently, we reported small molecules having bifunctionality (metal chelation and Aβ interaction) (22).…”

Insights into antiamyloidogenic properties of the green tea extract (−)-epigallocatechin-3-gallate toward metal-associated amyloid-β species

“…The involvement of metal-Aβ species in AD pathogenesis, however, has not been clearly elucidated. To advance our understanding of the potential neurotoxicity of metal-Aβ species, efforts to develop chemical tools capable of interacting directly with metal-Aβ species and modulating their reactivity in vitro and in biological systems are under way (1,(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). In particular, novel bifunctional compounds that contain elements for metal chelation and Aβ interaction have recently been prepared or identified via rational structure-based design strategies or systematic selection of natural products.…”

Selective Acetylcholinesterase Inhibitor Activated by Acetylcholinesterase Releases an Active Chelator with Neurorescuing and Anti-Amyloid Activities

“…Indeed, numerous new chelators with potentially better profiles have been developed, including chelators with specific transporters or nanoparticles as chelator carriers, chelators with amyloid-binding activity, and site-activated chelators with antioxidant or anti-acetylcholinesterase (AChE)/monoamine oxidase (MAO) activity (26)(27)(28)(29)(30)(31)(32).…”