Recent Development of Bifunctional Small Molecules to Study Metal-Amyloid-β Species in Alzheimer's Disease

Abstract

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease related to the deposition of aggregated amyloid-β (Aβ) peptides in the brain. It has been proposed that metal ion dyshomeostasis and miscompartmentalization contribute to AD progression, especially as metal ions (e.g., Cu(II) and Zn(II)) found in Aβ plaques of the diseased brain can bind to Aβ and be linked to aggregation and neurotoxicity. The role of metal ions in AD pathogenesis, however, is uncertain. To accelerate understanding in this area and contribute to therapeutic development, recent efforts to devise suitable chemical reagents that can target metal ions associated with Aβ have been made using rational structure-based design that combines two functions (metal chelation and Aβ interaction) in the same molecule. This paper presents bifunctional compounds developed by two different design strategies (linkage or incorporation) and discusses progress in their applications as chemical tools and/or potential therapeutics.

Figure 1

Chemical structures of EDTA (N,N ^'-1,2-ethanediylbis[N-(carboxymethyl)]glycine), clioquinol (CQ, 5-chloro-7-iodo-8-hydroxyquinoline), and cyclen (cyc, 1,4,7,10-tetraazacyclododecane).

Figure 2

Chemical structures of ThT (thioflavin-T), a neutral ThT derivative, Pittsburgh Compound B, the KLVFF peptide, curcumin, IMPY, and p-I-stilbene (ThT = 2-[4-(dimethylamino)phenyl]-3,6-dimethylbenzothiazolium; neutral ThT derivative = 2-(4-(dimethylamino)phenyl)-6-methylbenzothiazole; Pittsburgh Compound B = 2-[4-(methyl-¹¹C-amino)phenyl]-6-benzothiazolol; curcumin = ((1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione; IMPY = 4-(7-iodoimidazo[1,2-a]pyridin-2-yl)-N,N-dimethylaniline; p-I-stilbene = N,N-dimethyl-4-[(1E)-2-(4-iodophenyl)ethenyl]benzenamine).

Figure 3

Overview of the two rational structure-based design strategies currently used to develop small molecules having bifunctionality (metal chelation and Aβ interaction). The first approach is based on the connection of metal chelation and Aβ recognition structural moieties (Approach I: linkage). The second approach is direct incorporation of a metal chelation site into an Aβ interacting framework (Approach II: incorporation).

Figure 4

Chemical structures of small molecules XH1 (N,N-bis[2-[[2-[[4-(2-benzothiazolyl)phenyl]amino]-2-oxoethyl](carboxymethyl)amino]ethyl]glycine), cyc-KLVFF, and cyc-Curcumin that were designed by Approach I (Figure 3).

Figure 5

Chemical structures of small molecules (a) HBX/HBXI, HBT/HBTI, and BM/BMI, (b) 1, and (c) 2 that were constructed employing Approach II (Figure 3) (HBX = 2-(2-hydroxyphenyl)benzoxazole; HBT = 2-(2-hydroxyphenyl)benzothiazole; BM = 2-(2-aminophenyl)benzimidazole; 1 = 2-[4-(dimethylamino)phenyl]imidazo[1,2-a]pyridine-8-ol; 2 = N ¹,N ¹-dimethyl-N ⁴-(pyridin-2-ylmethylene)benzene-1,4-diamine).