Structure-Activity Relationships of Styrylquinoline and Styrylquinoxaline Derivatives as α-Synuclein Imaging Probes.

pubs.acs.org/acsmedchemlett Letter Structure−Activity Relationships of Styrylquinoline and Styrylquinoxaline Derivatives as α‑Synuclein Imaging Probes Kohei Nakagawa, Hiroyuki Watanabe,* Sho Kaide, and Masahiro Ono* Cite This: ACS Med. Chem. Lett. 2022, 13, 1598−1605 ACCESS Metrics & More Read Online Article Recommendations sı Supporting Information * ABSTRACT: Synucleinopathies are characterized by the deposition of α-synuclein (α-syn) aggregates before the onset of clinical symptoms. Therefore, in vivo imaging of α-syn may contribute to early diagnosis of these diseases and has attracted much attention in recent years. However, no clinically useful probes have been reported. In the present study, 16 quinoline/quinoxaline derivatives with different styryl and fluorine groups were evaluated in order to develop α-syn imaging probes. Among them, SQ3, which is a quinoline analogue with a p-(dimethylamino)styryl group and fluoroethoxy group at the 2- and 7- positions of the skeleton, displayed moderate selectivity for α-syn aggregates over β-amyloid (Aβ) aggregates (Ki = 230 nM), while maintaining high binding affinity for α-syn aggregates (Ki = 39.3 nM). In a biodistribution study, [18F]SQ3 exhibited high uptake (2.08% ID/g at 2 min after intravenous injection) into a normal mouse brain. Taken together, we demonstrate that [18F]SQ3 has basic properties as a lead compound for the development of a useful α-syn imaging probe. KEYWORDS: α-Synuclein, PET probe, Styrylquinoline, Styrylquinoxaline I permeability. The second problem is selectivity for α-syn aggregates over β-amyloid (Aβ) aggregates. It is well-known that α-syn aggregates are colocalized with Aβ�which is a major biomarker of Alzheimer’s disease (AD)�aggregates in some synucleinopathy patients’ brains.6 Since both proteins form aggregates with β-sheet structures, most of the probes with high affinity for α-syn aggregates also exhibit high affinity for Aβ aggregates. In addition, the concentration of α-syn aggregates is much lower than that of Aβ aggregates in the brain.7 Taken together, α-syn imaging probes must show selectivity for α-syn aggregates versus Aβ aggregates. Therefore, it is necessary to identify a compound showing three properties: high binding affinity for α-syn aggregates, high brain uptake, and selective binding for α-syn aggregates. Various kinds of quinoline and quinoxaline analogues were reported as amyloid imaging probes.8−13 Some reports suggested that binding affinities for Aβ aggregates change, depending on the position of the substitution group on the quinoxaline scaffold.11,13 Furthermore, there was a report that the quinoline scaffold with a styryl moiety at the 2-position displayed a high binding affinity for α-syn aggregates ([18F]14: n recent years, with the advent of an aging society, the increase in the number of patients suffering from synucleinopathies, including Parkinson’s disease, dementia with Lewy bodies, and multiple-system atrophy, has been a concern. However, even the method of definite diagnosis has not been established, let alone radical treatment. Abnormal depositions of Lewy bodies, Lewy neurites, and glial cytoplasmic inclusions are observed in the brains of patients suffering from these diseases before the onset of clinical symptoms.1 α-Synuclein (α-syn) aggregates are major constituents of these hallmarks and have been gathering attention as biomarkers of synucleinopathies. However, the association between the progress of synucleinopathies and amount of α-syn aggregates in the brain is unclear. Therefore, in vivo imaging of α-syn is considered to contribute to early diagnosis and elucidation of the pathophysiology of synucleinopathies. Among several imaging methods, positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are excellent tools for non-invasive and quantitative imaging of biomolecules with high sensitivity. Based on this, several kinds of nuclear medicine imaging probes targeting α-syn aggregates have been reported over the past few years.2−5 However, the detection of α-syn aggregates in vivo remains elusive. There are two major problems regarding the in vivo imaging of α-syn aggregates. The first problem is low brain permeability. Many α-syn imaging probes with high binding affinity for α-syn aggregates generally have a large molecular size (molecular weight (MW) > 430) and high lipophilicity (CLogP > 4.0), markedly decreasing brain © 2022 American Chemical Society Received: June 15, 2022 Accepted: September 21, 2022 Published: September 26, 2022 1598 https://doi.org/10.1021/acsmedchemlett.2c00279 ACS Med. Chem. Lett. 2022, 13, 1598−1605 ACS Medicinal Chemistry Letters pubs.acs.org/acsmedchemlett Letter Scheme 1. Synthesis Route of Quinoxaline Derivativesa a Reagents and conditions: (a) pyruvic aldehyde, EtOH, 25 °C; (b) p-dimethylaminobenzaldehyde, piperidine, AcOH, toluene, reflux; (c) Bu4Sn, Pd(Ph3)4, toluene, reflux; (d) I2, CHCl3, 25 °C; (e) (1) Cu(acac)2, LiOH·H2O, N1,N2-bis(4-hydroxy-2,6-dimethylphenyl)oxalamide, dimethyl sulfoxide (DMSO)/H2O, 80 °C, (2) 2-fluoroethyl p-toluenesulfonate, Cs2CO3, DMF, 95 °C. Scheme 2-1. Synthesis Routes of Quinoline, Aldehyde, and Fluorine Scaffoldsa a Reagents and conditions: (a) ethyl vinyl ether, AcOH, 25 °C → 100 °C; (b) N-bromosuccinimide (NBS), conc. H2SO4, 25 °C; (c) NHMe2, H2O, 80 °C; (d) 1,2,3-triazole, K2CO3, DMF, 100 °C; (e) tetrabutylammonium fluoride (TBAF), tetrahydrofuran (THF), 70 °C; (f) TREAT HF, 130 °C. inhibition constant Ki = 18 nM, dissociation constant Kd = 79 nM) in vitro.14 This report also suggested that the double bond between quinoline and the aromatic ring may be important to enhance the affinity for α-syn aggregates. This study was focused on the styrylquinoline/quinoxaline backbone, and structure−activity relationship studies were performed on 16 derivatives for the development of α-syn imaging probes. Quinoxaline derivatives were synthesized according to Scheme 1. After a mixture of 1 and 1′ was synthesized according to a method reported previously,15 styrylquinoxaline scaffolds were obtained by a condensation reaction. In the case of 3 (ISQ), the 7-tributyltin quinoxaline scaffold was prepared from a mixture of bromo compounds using a bromo-totributyltin exchange reaction catalyzed by Pd(0) and isolated. Thereafter, the 7-tributyltin scaffold was reacted with I2 in chloroform at 25 °C to give 3. In the case of 4 (SQ1) and 5 (SQ2), crude phenol scaffolds were reacted with 2-fluoroethyl p-toluenesulfonate in N,N-dimethylformamide (DMF) to give 4 and 5 after preparing the crude phenol scaffolds from a mixture of bromo compounds using a method reported previously.16 Compounds 4 (SQ1) and 5 (SQ2) could be separated by column chromatography. Finally, the structures of quinoxaline derivatives were determined by X-ray crystallography. The total yields from the materials were 6−16%. Next, quinoline derivatives were synthesized acc (...truncated)