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Design, synthesis, and preliminary evaluation of SPECT probes for imaging #-amyloid in Alzheimer’s disease-affected brain Yuki Okumura, Yoshifumi Maya, Takako Onishi, Yoshinari Shoyama, Akihiro Izawa, Daisaku Nakamura, Shigeyuki Tanifuji, Akihiro Tanaka, Yasushi Arano, and Hiroki Matsumoto ACS Chem. Neurosci., Just Accepted Manuscript • DOI: 10.1021/acschemneuro.8b00064 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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ACS Chemical Neuroscience

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Design, synthesis, and preliminary evaluation of

2

SPECT probes for imaging β-amyloid in

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Alzheimer’s disease-affected brain

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Yuki Okumura,*,†, ‡ Yoshifumi Maya,† Takako Onishi ,† Yoshinari Shoyama,† Akihiro Izawa,†

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Daisaku Nakamura,† Shigeyuki Tanifuji,† Akihiro Tanaka,† Yasushi Arano,‡ and Hiroki

6

Matsumoto†

7

†

8

0266, Japan

9

‡

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Research Centre, Nihon Medi-Physics Co., Ltd., 3-1, Kitasode, Sodegaura City, Chiba 299-

Department of Molecular Imaging and Radiotherapy, Graduate School of Pharmaceutical

Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan

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ACS Paragon Plus Environment

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ACS Chemical Neuroscience 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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In this study, we synthesized of a series of 2-phenyl- and 2-pyridyl-imidazo[1,2-a]pyridine

3

derivatives and examine their suitability as novel probes for single-photon emission computed

4

tomography (SPECT)-based imaging of β-amyloid (Aβ). Among the 11 evaluated compounds,

5

10 showed moderate affinity to Aβ(1−42) aggregates, exhibiting half-maximal inhibitory

6

concentrations (IC50) of 14.7 ± 6.07–87.6 ± 39.8 nM. In vitro autoradiography indicated that 123I-

7

labeled triazole-substituted derivatives displayed highly selective binding to Aβ plaques in the

8

hippocampal region of Alzheimer’s disease (AD)-affected brain. Moreover, biodistribution

9

studies performed on normal rats demonstrated that all

123

I-labeled probes featured high initial

10

uptake into the brain followed by a rapid washout and were thus well suited for imaging Aβ

11

plaques, with the highest selectivity observed for a 1H-1,2,3-triazole-substituted 2-pyridyl-

12

imidazopyridine derivative, [123I]ABC577. This compound showed good kinetics in rat brain as

13

well as moderate in vivo stability in rats and is thus a promising SPECT imaging probe for AD in

14

clinical settings.

15 16

KEYWORDS

17

123

18

imaging

I-ABC577, single-photon emission computed tomography, Alzheimer’s disease, β-amyloid,

19


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INTRODUCTION

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Alzheimer's disease (AD), the most common progressive neurodegenerative disorder

3

accounting for 60–70% of all dementia cases,1 is characterized by the presence of senile plaques

4

(containing β-amyloid (Aβ) peptides) and neurofibrillary tangles (NFTs; containing

5

hyperphosphorylated tau protein) in the brain. Among these pathological markers, Aβ plaques

6

are known as one of the earliest ones2-4 and are believed to play an important role in the

7

progression of AD.5–7 Therefore, non-invasive diagnostic imaging of these plaques using

8

methods such as positron emission tomography (PET) and single-photon emission computed

9

tomography (SPECT) could be useful for tracking amyloid pathology. Importantly, pre-

10

symptomatic changes in the brain of AD patients can be determined using certain promising PET

11

imaging agents such as [18F]florbetapir,8,9 [18F]flutemetamol,10–12 and [18F]florbetaben3,13,14

12

(Figure 1). Corresponding clinical studies show that PET can visualize Aβ present in the brain

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with high sensitivity and specificity, thus being useful for discriminating non-AD-related

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dementia. Based on these results, the aforementioned PET agents have been approved by the

15

United States Food and Drug Administration (FDA) and the European Medicines Agency

16

(EMA). In contrast, despite much effort devoted to their development, the corresponding SPECT

17

imaging agents are currently unavailable. Although the spatial resolution and quantitative

18

accuracy of SPECT are generally inferior to those of PET, the former modality is more available

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and affordable than the latter, especially in developing countries, highlighting the need to

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develop clinically useful SPECT imaging probes for Aβ plaques. Recently, we have developed

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6-iodo-2-[2-(1H-1,2,3-triazole-1-yl)imidazo[1,2-a]pyridine (28, [123I]ABC577, Figure 1)15 as a

22

clinically useful SPECT imaging probe and reported the results from the initial clinical studies.


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Herein, we describe the design, synthesis, and preclinical evaluation of [123I]ABC577 and its

2

analogs.

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RESULTS AND DISCUSSION

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Design. Suitable Aβ imaging probes should exhibit high permeability through the blood-brain

5

barrier (BBB), high specific binding to Aβ, low non-specific binding, and rapid clearance from

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normal brain regions. Based on these requirements, several Aβ imaging probes have been

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designed from fluorescent dye compounds such as Congo Red (CR) and Thioflavin-T (ThT) and

8

evaluated for in vivo imaging of Aβ plaques in an AD brain. The first-generation [11C]PIB

9

amyloid probe (Figure 1B),16–18 frequently used as a benchmark for PET amyloid imaging

10

agents, was developed as a neutral ThT derivative with improved brain uptake, whereas [11C]SB-

11

13 (Figure 1B)19,20 was designed to reduce the molecular weight of CR. Since the use of these

12

11

13

PET probes have been developed, with three of them approved by the FDA as

14

radiopharmaceuticals. Since ThT has a lower molecular weight than CR, thus exhibiting better

15

BBB penetration, some research groups have worked on developing ThT-based SPECT imaging

16

agents (Figure 1A).21–25 Among these species, [123I]IMPY (Figure 1B)26–28 is the only SPECT

17

probe tested in humans. However, this compound exhibits unsatisfactory signal-to-noise ratios,

18

having certain undesirable characteristics such as high lipophilicity and in vivo instability.26,29

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Since the increased lipophilicity of probes enhances their nonspecific uptake in the subcortical

20

white matter of the brain,30 18F-labeled amyloid imaging probes are commonly inferior to the less

21

lipophilic [11C]-PIB.31 Thus, since the lipophilicity of

22

to be much higher than that of

23

practical applications is correspondingly much more challenging.

C-labeled probes is limited by their short half-lives of 20.3 min, several

18

123

18

F-labeled amyloid

I-labeled SPECT imaging probes tends

F-labeled PET probes, the development of the former for


4

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In the light of these findings, we attempted to develop novel radioiodinated imidazopyridine 123

2

derivatives as probes for SPECT amyloid imaging. The structures of

3

2-pyridyl-imidazo[1,2-a]pyridines that were designed and evaluated in this study are shown in

4

Figure 2. The IMPY-derived 2-phenyl-imidazo[1,2-a]pyridine scaffold was chosen as a core

5

structure for the developed probes due to its lower lipophilicity compared to these of 2-phenyl-

6

benzothiazole and stilbene scaffolds used in the approved agents (Figure 1). Most of the

7

previously reported probes feature electron-donating groups such as N-methylamino- or N,N-

8

dimethylaminophenyl on at least one end of their molecules. Therefore, we replaced these groups

9

with an alkoxy group or electron-rich five-membered heteroaromatic ring and compared the

10

lipophilicity of the resulting compounds with that of IMPY by calculating their cLogP (Table 1).

11

The replacement of a phenyl group by a pyridyl group is also reported to improve the

12

pharmacokinetics of Aβ imaging probes because it decreases their lipophilicity.32–34 Thus, based

13

on previously reported results, we synthesized and evaluated 2-pyridyl-imidazo[1,2-a]pyridine

14

derivatives in addition to their 2-phenyl analogs.

15

Chemistry. The synthesis of 2-phenyl-imidazo[1,2-a]pyridine derivatives is outlined in Schemes

16

1 and 2. The core structure, 2-phenyl-imidazo[1,2-a]pyridine, was obtained by condensation of

17

the corresponding para-substituted phenacyl bromides with 2-amino-5-iodopyridine in

18

acetonitrile. The 2-phenyl-imidazo[1,2-a]pyridine derivatives bearing alkoxy groups were

19

synthesized from intermediate 2 (Scheme 1) prepared from the previously reported phenacyl

20

bromide 135 in 78% yield and subsequently converted to 3 (53% yield), 4 (95% yield), 7 (61%

21

yield), and 8 (78% yield) by O-alkylation with trimethylsilyldiazomethane in methanol/1,4-

22

dioxane

23

dimethylformamide (DMF). The silyl group was cleaved by tetra-n-butylammonium fluoride

or

with

1-fluoro-2-(tosyloxy)ethane

under

basic

I-labeled 2-phenyl- and

conditions

(K2CO3)

in


5


Page 6 of 47

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(TBAF) to afford alcohols 9 and 10 in yields of 80 and 84%, respectively. Also, 1,2,3-triazole

2

derivative 14 (28% yield over three steps) was prepared by the reaction of 4-

3

azidophenacylbromide with 2-amino-5-iodopyridine followed by 1,3-dipolar addition to

4

trimethylsilylacetylene and trimethylsilyl group cleavage (Scheme 2).

5

Next, 2-(3-pyridyl)imidazo[1,2-a]pyridine derivatives were prepared as shown in Scheme 3.

6

The methoxy derivative 18 was synthesized by cyclization of bromide 16 (prepared by

7

bromination of 5-acetyl-2-bromopyridine by CuBr2) with 2-amino-5-iodopyridine and a

8

subsequent reaction with sodium methoxide (20% yield over three steps). Five-membered

9

heteroaromatics (27, 28, 29, and 30) were also synthesized from 5-acetyl-2-bromopyridine. In

10

their syntheses, 5-acetyl-2-bromopyridine was reacted in DMF under basic conditions with the

11

required five-membered heteroaromatics to afford 5-acetylpyridine derivatives 19, 20, 21, and 22

12

in yields of 81, 30, 17, and 85%, respectively. These were subsequently converted to silyl enol

13

ethers during the reaction of trimethylsilyl bromide/triethylamine and brominated with N-

14

bromosuccinimide in CH2Cl2 to afford β-bromoketone derivatives 23, 24, 25, and 26 in yields of

15

72, 82, 45, and 97%, respectively. Then, the thus-obtained 23, 24, 25, and 26 were reacted with

16

2-amino-5-iodopyridine to obtain 27, 28, 29, and 30 in yields of 55, 46, 35, and 71%,

17

respectively.

18

Tributylstannyl precursors 31–40 were prepared from the corresponding iodo compounds (3,

19

4, 9, 10, 14, 18, 27, 28, 29, and 30) through an iodo-to-tributylstannyl exchange reaction

20

catalyzed by Pd(Ph3P)4/triethylamine (Scheme 4) in yields of 41, 55, 73, 42, 61, 41, 36, 31, 22,

21

and 33%, respectively. Purities of key compounds were shown to be higher than 95%, as

22

determined by high-performance liquid chromatography (HPLC).


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Radiolabeling. Radioiodinated compounds [123I]3, [123I]4, [123I]9, [123I]10, [123I]14, [123I]18,

2

[123I]27, [123I]28, [123I]29, and [123I]30 were prepared from the corresponding tributylstannyl

3

precursors dissolved in CH3CN by an iodo-destannylation reaction using hydrogen

4

peroxide/hydrochloric acid as an oxidant under no-carrier-added conditions (Scheme 4). The

5

labeled compounds were purified by HPLC and solid-phase extraction, and their radiochemical

6

purities were determined to be above 95% by radio-thin layer chromatography (radio-TLC).

7

In vitro Binding Assay to Aβ(1–42) Aggregates. The affinities of the imidazo[1,2-a]pyridine

8

derivatives to the abovementioned aggregates were evaluated based on the inhibition of

9

[123I]IMPY binding to synthetic human Aβ(1–42) fibrils and expressed as the half-maximal

10

inhibitory concentration (IC50) values (nM). The IC50 value of IMPY also measured using the

11

same system for comparison (Table 2). As for the compounds bearing an alkoxy group,

12

fluoroethoxy group-bearing 4 showed an affinity (20.0 ± 7.77 nM) comparable to that of IMPY

13

(24.8 ± 1.19 nM), and the other alkoxy-group-bearing compounds exhibited moderate affinities

14

(47.2–87.6 nM). The two compounds with a five-membered heterocyclic system registered

15

promising binding affinities with the highest affinity (IC50 = 14.7 ± 6.07 nM) observed for 28

16

featuring a 1H-1,2,3-triazole ring at the 2-position of the pyridine ring, followed by 27 bearing a

17

pyrazole ring (IC50 = 18.2 ± 1.11 nM). The increase in the A-binding affinities was also

18

observed with compounds bearing a similar ring system,36 suggesting that the incorporation of an

19

electron-donating group would be promising to the development of radiolabeled probes with

20

high binding affinities to Aβ instead of amino groups used in most of the previously reported

21

probes. On the other hand, the affinity of 2H-1,2,3-triazole derivative 29 was much lower than

22

that of its isomers, namely 1H-1,2,3-triazole derivative 28 and 1H-1,2,4-triazole derivative 30.

23

These findings indicated that the position of the heteroatoms in the five-membered ring


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Page 8 of 47

1

significantly affects the affinity of the Aβ aggregates. The IC50 values of the imidazo[1,2-

2

a]pyridine derivatives (except for 29) were close to that of IMPY (24.8 ± 1.19 nM), which was

3

sufficient for the in vivo visualization of Aβ aggregates.

4

Autoradiography of Postmortem AD Brain Sections. The binding of 123I-labeled probes to Aβ

5

plaques in the human brain was evaluated by in vitro autoradiography using tissue sections from

6

postmortem AD brains (Figure 3 and Table 3). The spots of radioactivity derived from

7

labeled probes in the cortex were confirmed by immunostaining the same brain sections with an

8

anti-Aβ antibody (Figure 4), and the white matter area in these sections was outlined by a

9

Klüver-Barrera staining (Figure 5). When AD brain sections were incubated with

123

123

I-

I-labeled

10

imidazo[1,2-a]pyridine derivatives, the obtained images revealed the visualization of the Aβ

11

plaques with minimal background noise. [123I]IMPY, which was used for comparison,

12

accumulated in the hippocampal region of the AD brain section, which was enriched with Aβ

13

plaques, in preference to the healthy control region (gray matter signal = 137 photostimulated

14

luminescence per square millimeter (PSL/mm2)). However, the non-specific accumulation of

15

[123I]IMPY in brain white matter was also high (Figure 3A), with the gray/white matter signal

16

ratio being as low as 1.2. Imidazo[1,2-a]pyridine derivatives bearing an alkoxy group exhibited a

17

lower amyloid affinity than [123I]IMPY (14.6–26.2 PSL/mm2), although their gray/white matter

18

signal ratio was slightly higher than that of [123I]IMPY (1.3–2.2). In contrast, imidazo[1,2-

19

a]pyridine derivatives with a five-membered heterocyclic ring system exhibited high affinities

20

(52.2–128 PSL/mm2), except for [123I]29 (9.82 PSL/mm2, IC50 = 451 ± 250 nM). All triazole

21

derivatives ([123I]14, [123I]28, [123I]29, and [123I]30) showed low non-specific binding to white

22

matter, with their notably high gray/white matter signal ratios (2.7–6.8) attributed to the lower

23

lipophilicity of these compounds due to the presence of a polar triazole moiety. Comparison of


8

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1

the 2-phenyl ([123I]3 and [123I]14) and 2-pyridyl ([123I]18 and [123I]28) derivatives showed that

2

the Aβ binding affinity was not affected by the replacement of a benzene ring with a pyridine

3

ring (26.2, 78.1, 23.0, and 128 PSL/mm2, respectively). However, the gray/white matter signal

4

ratio was notably improved by the abovementioned replacement (1.3, 2.7, 1.6, and 6.8,

5

respectively).

6

In vivo Kinetics in Normal Rat Brain. The biodistribution of radiolabeled probes in normal

7

rats is widely used for selecting candidates appropriate from a biological point of view, i.e., those

8

featuring facile penetration through the intact BBB and rapid washout from the normal brain.

9

Table 4 summarizes the time−activity data in the brain of normal rats after intravenous 123

I-labeled probes, showing that fluoroethoxy-group-bearing [123I]4,

10

administration of

11

hydroxyalkyl-group-bearing [123I]9 and [123I]10 showed low uptakes at 2 min (0.72, 0.97, and

12

1.05% injected dose per gram (ID/g), respectively) and thus exhibited low penetration ability

13

through the intact BBB. In contrast, the rest of the radiolabeled probes showed good uptake into

14

the brain (~1.09–2.09% ID/g brain). In general, optimal BBB penetration is observed for LogP =

15

1.5–2.7, with mean = 2.1.37 Since the LogPoctanol/water values of these ligands (Table 1) indicated

16

facile uptake into normal rat brains, the

17

were rapidly washed out. The brain2 min/brain60 min ratio is commonly used as an important index

18

to select probes with appropriate in vivo kinetics. The corresponding values for the

19

probes are shown in Table 5. It was found that all evaluated probes exhibited higher ratios than

20

[123I]IMPY (18.8–48.5 vs. 18.2). Moreover, the obtained results indicated that [123I]28 was best

21

suited for use as an Aβ imaging probe among the compounds tested in this study because it

22

exhibited high initial uptake into the brain followed by quick washout (brain2 min/brain60 min =

123

I-labeled probes that initially penetrated the rat brain

123

I-labeled


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Page 10 of 47

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41.0). The favorable in vivo kinetics of the abovementioned species may be attributable to their

2

reduced lipophilicity achieved by introducing a 1H-1,2,3-triazole moiety.

3

In vivo Stability Studies in Normal Rats. Since [123I]14, [123I]28, and [123I]30 showed suitable

4

characteristics for Aβ imaging, we compared the in vivo stability of these probes in rat plasma

5

with that of [123I]IMPY. The results showed that the former exhibited higher stability than the

6

latter (Table 6). Thin layer chromatography analyses of 28 and 30 showed a few unidentified

7

metabolites with post-injection time. While [123I]IMPY also registered a few metabolites with

8

time, a metabolite in this case was generated as early as 2 min post-injection at an Rf value close

9

to that of [123I]IMPY, suggesting demethylation of the N-dimethyl group (Supplemental Figure

10

S1), which may account for its insufficient signal-to-noise ratios observed in the clinical

11

studies.26,29 These results also suggested that the compounds without N-alkyl groups such as

12

[123I]14, [123I]28, and [123I]30 would be more appropriate for in vivo imaging of Aβ.

13

The autoradiography results revealed that 1H-1,2,3-triazole derivatives [123I]14 and [123I]28

14

and 1H-1,2,4-triazole derivative [123I]30 displayed excellent binding affinities to Aβ plaques in

15

the hippocampal region of an AD brain, exhibiting a higher selectivity than [123I]IMPY and

16

higher in vivo stability in normal rats. Furthermore, all these probes displayed good kinetics with

17

brain2

18

prepared 2-phenyl- and 2-pyridyl-imidazo[1,2-a]pyridine derivatives containing an alkoxy group

19

or five-membered aromatic heteroring system are well suited for in vivo imaging of Aβ plaques.

20

In

21

([123I]ABC577) exhibited the highest gray/white matter selectivity according to the in vitro

22

autoradiography analysis, as well as good kinetics in rat brain, and moderate in vivo stability in

23

rats.

min/brain60 min

particular,

ratios of 18.8–48.5. Thus, the obtained results suggested that the newly

1H-1,2,3-triazole

2-pyridyl-imidazo[1,2-a]pyridine

derivative

[123I]28


10

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1


CONCLUSION

2

In this study, 2-phenyl- and 2-pyridyl-imidazo[1,2-a]pyridine derivatives containing an

3

alkoxy group or a five-membered aromatic heteroring system were successfully synthesized as

4

novel SPECT imaging probes for Aβ plaques. An in vitro binding assay to Aβ(1–42) aggregates

5

confirmed that these species possess high binding affinities (IC50) sufficient for in vivo

6

applications. A high performance was observed for some of the ligands containing a five-

7

membered heterocyclic system. The autoradiography results revealed that 1H-1,2,3-triazole

8

derivatives [123I]14 and [123I]28, and 1H-1,2,4-triazole derivative [123I]30 displayed excellent

9

binding affinities to Aβ plaques in the hippocampal region of an AD brain, exhibiting a higher

10

selectivity than [123I]IMPY and higher in vivo stability in normal rats. Furthermore, all these

11

probes displayed good kinetics, with brain2

12

suggested that the newly prepared 2-phenyl- and 2-pyridyl-imidazo[1,2-a]pyridine derivatives

13

containing an alkoxy group or five-membered aromatic heteroring system would be useful for

14

the in vivo imaging of Aβ plaques. In particular, 1H-1,2,3-triazole 2-pyridyl-imidazo[1,2-

15

a]pyridine derivative [123I]28 ([123I]ABC577) exhibited the highest gray/white matter selectivity

16

according to in vitro autoradiography analysis, good kinetics in rat brain, and moderate in vivo

17

stability in rats. In our previous report,15 [123I]ABC577 demonstrated high affinity to AD

18

homogenate, low off-target binding, and the possibility to be a useful SPECT imaging tool to

19

identify Aβ in the human brain in preliminary clinical studies．The current work could pave the

20

way to the development of further SPECT imaging agents for AD brain imaging, with further

21

testing and clinical studies expected to be reported in due course.

min/brain60 min

ratios of 18.8–48.5. These findings

22 23

METHODS


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Page 12 of 47

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General Remarks. All utilized reagents were commercial products that were used without

2

further purification unless otherwise indicated. 1H NMR and 13C NMR spectra were recorded on

3

a JEOL 500-MHz or Bruker 500-MHz NMR spectrometer in CDCl3 or dimethyl sulfoxide

4

(DMSO)-d6, utilizing tetramethylsilane (TMS) as an internal standard. The coupling constants

5

were reported in Hertz (Hz) and the multiplicity was denoted as s (singlet), d (doublet), t (triplet),

6

q (quartet), quin (quintet), and m (multiplet). Mass spectra were acquired using a Waters Xevo

7

QTofMS (Waters Corporation, Milford, MA) instrument. Radiochemical purity was determined

8

by radio-TLC (Rita Star, Raytest, Straubenhardt, Germany) on silica gel 60 F254 plates using

9

EtOAc/CH3OH/Et2NH (100:4:1, v/v/v) as a mobile phase. HPLC was performed using an

10

Alliance 2695 system (Waters Corporation, Milford, MA) with Waters 2487 Detector (λ = 254

11

nm) using a phenyl column (Waters Corporation Phenyl, 4.6 mm × 150 mm, 5 μm) and

12

acetonitrile/H2O containing 0.1 vol% trifluoroacetic acid as the mobile phase at a flow rate of 1.0

13

mL/min. All key compounds showed >95% purity when tested with this method. Wistar rats

14

were purchased from Japan SLC, Inc. (Hamamatsu, Japan) and were housed under conditions of

15

controlled temperature (18–28 °C) and lighting (12:12 h light–dark cycle) with free access to

16

food and water. The protocols for all animal experiments were approved by the committee on

17

animal welfare at Nihon Medi-Physics Co., Ltd. All reagents and solvents were obtained

18

commercially from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), Tokyo Chemical

19

Industry Co., Ltd. (Tokyo, Japan), Nacalai Tesque, Inc. (Kyoto, Japan), and Sigma-Aldrich Co.

20

LLC. (St. Louis, MO).

21

Chemistry. 2-(4-Hydroxyphenyl)-6-iodoimidazo[1,2-a]pyridine (2). 2-Amino-5-iodopyridine

22

(449 mg, 2.00 mmol) was added to a solution of 1 (441 mg, 2.00 mmol) in CH3CN (15 mL), and

23

the reaction mixture was heated under reflux for 5 h. After the reaction was complete, the


12

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1

mixture was allowed to cool down to room temperature and filtered. The precipitate was washed

2

with CH3CN (10 mL), dried under reduced pressure, suspended in a mixture of water (10.0 mL)

3

and methanol (10.0 mL), and treated with saturated aqueous NaHCO3 (~10 mL), followed by a

4

5-min sonication. The resulting mixture was filtered, and the precipitate was washed with water

5

(10 mL) and dried under reduced pressure to afford 2 (526 mg, 78.0%). 1H NMR (500 MHz,

6

DMSO-d6):  8.86–8.84 (m, 1H), 8.14 (s, 1H), 7.78–7.74 (m, 2H), 7.40–7.35 (m, 2H), 6.86–6.82

7

(m, 2H);

8

118.1, 116.1, 108.0, 75.9.

9

2-(4-Methoxyphenyl)-6-iodoimidazo[1,2-a]pyridine

13

C NMR (125 MHz, DMSO-d6):  158.1, 145.9, 143.9, 132.5, 131.7, 127.7, 125.0,

(3).

A

solution

of

10

trimethylsilyldiazomethane in hexane (2 M, 1.2 mL, 2.4 mmol) was added to a solution of 2 (400

11

mg, 1.20 mmol) in a mixture of methanol (30 mL) and 1,4-dioxane (50 mL). The reaction

12

mixture was stirred for 24.5 h at room temperature and subsequently concentrated under reduced

13

pressure. The residue was recrystallized from EtOAc to afford 3 (223 mg, 53.0%). 1H NMR (500

14

MHz, CDCl3):  8.37–8.34 (m, 1H), 7.88–7.84 (m, 2H), 7.72 (s, 1H), 7.40 (d, 1H, J = 9.4 Hz),

15

7.31 (dd, 1H, J = 9.4, 1.6 Hz), 6.99–6.95 (m, 2H), 3.86 (s, 3H); 13C NMR (125 MHz, DMSO-d6):

16

 159.9, 146.3, 144.2, 132.3, 130.3, 127.4, 125.9, 118.3, 114.2, 106.88, 74.8, 55.3; Accurate

17

mass (ES+) m/z 350.9998 [M+H]+ (calcd. for C14H12IN2O: 350.9994).

18

2-[4-(2-Fluoroethoxy)phenyl]-6-iodoimidazo[1,2-a]pyridine (4). K2CO3 (5.50 g, 41.4 mmol)

19

and 1-fluoro-2-(tosyloxy)ethane (4.52 g, 20.7 mmol) were added to a solution of 2 (4.65 g, 13.8

20

mmol) in DMF (70 mL) and the reaction mixture was heated to 90 °C for 1.5 h. The reaction was

21

quenched by the addition of saturated aqueous NaHCO3, and the mixture was extracted with

22

EtOAc (100 mL × 2). The extract was washed with water and brine, dried over anhydrous


13


Page 14 of 47

1

Na2SO4, and concentrated to dryness. The crude product was purified by flash silica gel column

2

chromatography (hexane:EtOAc = 1:2, v/v) to afford 4 (5.00 g, 94.9%). 1H NMR (500 MHz,

3

CDCl3):  8.36 (s, 1H), 7.86 (d, 2H, J = 8.2 Hz), 7.71 (s, 1H), 7.39 (d, 1H, J = 8.7 Hz), 7.32–7.30

4

(m, 1H), 6.99 (d, 2H, J = 8.25 Hz), 4.84–4.71 (dd, 2H, JHH = 4.1, 2JHF = 47.2 Hz), 4.30–4.22 (dd,

5

2H, JHH = 4.1, 3JHF = 27.5 Hz); 13C NMR (125 MHz, CDCl3):  158.6, 146.1, 144.2, 132.4, 130.3,

6

127.4, 126.5, 118.3, 114.9, 107.0, 81.9 (d, 2JCF = 170.8 Hz), 74.8, 67.2 (d, 3JCF = 20.6 Hz); 19F

7

NMR (470 MHz, CDCl3):  –223.7 (dd, 2JHF = 47.4 Hz, 3JHF = 27.7 Hz); Accurate mass (ES+)

8

m/z 383.0042 [M+H]+ (calcd. for C15H13FIN2O: 383.0057).

9

2-[4-(2-t-Butyldiphenylsiloxyethoxy)phenyl]-6-iodoimidazo[1,2-a]pyridine (7). Compound 5

10

(51.8 g, 142 mmol) was added to a solution of 2 (40.0 g, 119 mmol) and K2CO3 (49.3 g, 357

11

mmol) in DMF (320 mL). The reaction mixture was stirred at 86 °C for 3 h, quenched with the

12

addition of brine, and extracted with EtOAc (500 mL × 2). The combined organic layers were

13

dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was

14

purified by silica gel column chromatography (heptane:EtOAc = 1:1, v/v) to afford 7 (53.3 g,

15

60.7%). 1H NMR (500 MHz, CDCl3):  8.29 (s, 1H), 7.82 (d, 2H, J = 8.2 Hz), 7.72 (d, 4H, J =

16

6.0 Hz), 7.44–7.37 (m, 8H), 7.27 (dd, 1H, J = 9.4, 1.6 Hz), 6.93 (d, 2H, J = 6.9 Hz), 4.12 (t, 2H,

17

J = 5.1 Hz), 4.01 (t, 2H, J = 5.1 Hz), 1.07 (s, 9H).

18

2-[4-(3-t-Butyldiphenysiloxypropoxy)phenyl]-6-iodoimidazo[1,2-a]pyridine (8). Compound 6

19

(52.2 g, 138 mmol) was added to a solution of 2 (31.0 g, 92.2 mmol) and K2CO3 (38.2 g, 277

20

mmol) in DMF (260 mL). The reaction mixture was stirred at 88 °C for 3 h, quenched with the

21

addition of brine, and extracted with EtOAc (500 mL × 2). The combined organic layers were

22

dried over anhydrous MgSO4 and concentrated under reduced pressure. The crude product was


14

Page 15 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

purified by silica gel column chromatography (CHCl3:CH3OH = 50:1, v/v) to afford 8 (45.5 g,

2

78.0%). 1H NMR (500 MHz, CDCl3):  8.36 (d, 1H, J = 0.8 Hz), 7.85 (d, 2H, J = 9.2 Hz), 7.72

3

(s, 1H), 7.65 (dd, 4H, J = 6.4, 1.2 Hz), 7.42–7.29 (m, 8H), 6.95 (d, 2H, J = 9.2 Hz), 4.16 (t, 2H, J

4

= 6.0 Hz), 3.87 (t, 2H, J = 6.0 Hz), 2.04 (tt, 2H J = 6.0, 6.0 Hz), 1.05 (s, 9H).

5

2-[4-(2-Hydroxyethoxy)phenyl]-6-iodoimidazo[1,2-a]pyridine (9). A 1.0 M TBAF solution in

6

tetrahydrofuran (THF, 101 mL) was added to a solution of 7 (52.0 g, 84.0 mmol) in THF (150

7

mL). The reaction mixture was stirred at room temperature for 2 h, followed by quenching with

8

saturated aqueous NH4Cl and the addition of CH3CN (50 mL). The obtained precipitate was

9

filtered and washed with water (200 mL) and CH3CN (200 mL) to afford 9 (25.5 g, 79.9%). 1H

10

NMR (500 MHz, DMSO-d6):  8.86 (s, 1H), 8.19 (s, 1H), 7.86 (d, 2H, J = 11.2 Hz), 7.39 (s, 2H),

11

7.00 (d, 2H, J = 11.2 Hz), 4.81 (t, 1H, J = 5.5 Hz), 4.04 (t, 2H, J = 5.0 Hz), 3.74 (dt, 2H, J = 5.5,

12

5.0 Hz);

13

116.7, 114.8, 108.1, 76.7, 69.5, 59.4; Accurate mass (ES+) m/z 381.0094 [M+H]+ (calcd. for

14

C15H14IN2O: 381.0100); Anal. C, 47.29; H, 3.45; N, 7.21 (calcd. for C15H13IN2O: C, 47.39; H,

15

3.45; N, 7.37).

16

2-[4-(3-Hydroxypropoxy)phenyl]-6-iodoimidazo[1,2-a]pyridine (10). A 1.0 M TBAF solution

17

in THF (85.2 mL) was added to a solution of 8 (44.9 g, 71.0 mmol) in THF (142 mL). The

18

reaction mixture was stirred at room temperature for 1.5 h, quenched with saturated aqueous

19

NH4Cl, and diluted with water (142 mL) and CH3CN (142 mL). The precipitate was filtered and

20

washed with water (200 mL) and CH3CN (200 mL) to afford 10 (23.5 g, 83.9%). 1H NMR (500

21

MHz, DMSO-d6):  8.86 (s, 1H), 8.19 (s, 1H), 7.85 (d, 2H, J = 8.7 Hz), 7.39 (s, 2H), 6.99 (d, 2H,

22

J = 8.7 Hz), 4.49 (br s, 1H), 4.08 (t, 2H, J = 6.4 Hz), 3.57 (dt, 2H, J = 6.4 Hz), 1.88 (tt, 2H, J =

13

C NMR (125 MHz, DMSO-d6):  158.9, 143.0, 142.4, 133.5, 131.5, 127.1, 124.4,


15


Page 16 of 47

1

6.4, 6.4 Hz); 13C NMR (125 MHz, DMSO-d6):  158.7, 144.6, 143.2, 132.1, 131.2, 127.0, 125.7,

2

117.6, 114.6, 107.8, 75.7, 64.6, 57.3, 32.1; Accurate mass (ES+) m/z 395.0252 [M+H]+ (calcd.

3

for C16H16IN2O2: 395.0257); Anal. C, 48.54; H, 3.83; N, 7.00 (calcd. for C16H15IN2O: C, 48.75;

4

H, 3.84; N, 7.11).

5

2-(Azidophenyl)-6-imidazo[1,2-a]pyridine (12). A solution of 11 (218 mg, 0.909 mmol) and 2-

6

amino-5-iodopyridine (200 mg, 0.909 mmol) in CH3CN (1.0 mL) was heated to 80 °C for 3 h.

7

The mixture was allowed to cool down to room temperature and filtered, and the obtained

8

precipitate was washed with CH3CN (20 mL) and dried under reduced pressure. The crude

9

product was suspended in a mixture of water (3.0 mL) and methanol (3.0 mL), and treated with

10

saturated aqueous NaHCO3 (~4 mL), followed by a 5-min sonication. The obtained precipitate

11

was filtered, washed with water (10 mL), and dried under reduced pressure to afford 12 (214 mg,

12

65.2%). 1H NMR (500 MHz, CDCl3):  8.89 (s, 1H), 8.31 (s, 1H), 7.99 (d, 2H, J = 8.7 Hz), 7.42

13

(s, 1H), 7.42 (s, 1H), 7.19 (d, 2H, J = 8.7 Hz).

14

6-Iodo-2-[4-(1H-1,2,3-triazol-1-yl)phenyl]imidazo[1,2-a]pyridine (14).

15

Trimethylsilylacetylene (0.164 mL, 1.18 mmol) and copper(II) sulfate pentahydrate (29.6 mg,

16

0.118 mmol) were added to a solution of 12 (214 mg, 0.593 mmol) in DMF (3.0 mL), and the

17

reaction mixture was stirred at 80 °C for 3 h. After the reaction was complete, the mixture was

18

allowed to cool down to room temperature and diluted with water (20 mL). The precipitate was

19

filtered, washed with water (10 mL), and dried under reduced pressure to obtain crude 13 (137

20

mg). A 1.0 M TBAF solution in THF (0.3 mL) was added to a suspension of crude 13 (137 mg)

21

in THF (3.0 mL), and the mixture was refluxed for 4 h. The solution was then allowed to cool

22

down to room temperature, and the obtained precipitate was filtered, successively washed with

23

THF (5.0 mL) and diethyl ether (5.0 mL), and dried under reduced pressure to afford 14 (97.8


16

Page 17 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

mg, 42.5%). 1H NMR (500 MHz, DMSO-d6):  8.92 (s, 1H), 8.84 (d, 1H, J = 0.9 Hz), 8.41 (s,

2

1H), 8.15 (d, 2H, J = 8.7 Hz), 7.98 (d, 2H, J = 8.7 Hz), 7.96 (d, 1H, J = 0.9 Hz), 7.45 (s, 1H); 13C

3

NMR (125 MHz, DMSO-d6):  144.2, 144.1, 136.7, 134.9, 134.3, 133.3, 132.1, 127.5, 123.6,

4

121.1, 118.6, 110.1, 76.6; Accurate mass (ES+) m/z 388.0060 [M+H]+ (calcd. for C15H11IN5:

5

388.0059). Anal. C, 46.46; H, 2.73; N, 17.83 (calcd for C15H10IN5: C, 46.53; H, 2.60; N, 18.09).

6

2-(2-Bromopyridin-5-yl)-6-iodoimidazo[1,2-a]pyridine (17). CuBr2 (3.57 g, 16.0 mmol) was

7

added to a solution of 15 (1.50 g, 7.61 mmol) in EtOAc (30 mL), and the mixture was refluxed

8

for 5 h, allowed to cool down to room temperature, and concentrated under reduced pressure.

9

The obtained residue was purified by flash silica gel column chromatography (hexane:EtOAc =

10

7:1, v/v) to obtain crude 16 (2.23 g), which was used to prepare 17 following the protocol

11

described above for 2 (1.26 g, 52.6%). 1H NMR (500 MHz, DMSO-d6):  8.91–8.90 (d, 1H, J =

12

2.3 Hz), 8.41 (s, 1H), 8.20 (dd, 1H, J = 8.3, 2.3 Hz), 7.67 (d, 1H, J = 8.3 Hz), 7.42 (s, 2H).

13

2-(2-Methoxypyridin-5-yl)-6-iodoimidazo[1,2-a]pyridine (18). Sodium methoxide (171 mg,

14

3.17 mmol) was added to a solution of 17 (200 mg, 0.634 mmol) in CH3OH (3.0 mL) and DMSO

15

(3.0 mL) at 0 °C. The reaction mixture was stirred for 3 h, quenched with saturated aqueous

16

NH4Cl, and extracted with EtOAc (30 mL × 2). The combined organic layers were washed with

17

water and brine, dried over anhydrous MgSO4, and concentrated under reduced pressure. The

18

crude product was purified by silica gel column chromatography (hexane:EtOAc = 1:1, v/v) to

19

afford 18 (86.0 mg, 38.6%). 1H NMR (500 MHz, DMSO-d6):  8.89 (s, 1H), 8.73 (s, 1H), 8.27 (s,

20

1H), 8.20 (d, 1H, J = 8.5 Hz), 7.42 (s, 2H), 6.89 (d, 1H, J = 8.5 Hz), 3.89 (s, 3H); 13C NMR (125

21

MHz, DMSO-d6):  163.9, 144.8, 144.1, 142.8, 137.1, 133.1, 123.7, 118.4, 111.2, 109.0, 76.4,

22

55.4, 53.9; Accurate mass (ES+) m/z 351.9938 [M+H]+ (calcd. for C13H11IN3O: 351.9947).


17


Page 18 of 47

1

1-(2-(Pyrazol-1-yl)pyridin-5-yl)ethane-1-one (19). Compound 15 (400 mg, 1.99 mmol) and

2

K2CO3 (828 mg, 5.99 mmol) were added to a solution of pyrazole (408 mg, 5.99 mmol) in DMF

3

(10 mL), and the reaction mixture was heated to 100 °C for 5 h. After the reaction was complete,

4

the reaction mixture was allowed to cool down to room temperature, quenched with saturated

5

aqueous NH4Cl, and extracted with CH2Cl2 (50 mL × 2). The combined organic fractions were

6

washed with water and brine, dried over anhydrous MgSO4, and concentrated under reduced

7

pressure. The crude product was purified by silica gel column chromatography (CH2Cl2:EtOAc =

8

50:1, v/v) to afford 19 (304 mg, 81.4%). 1H NMR (500 MHz, CDCl3):  8.98 (d, 1H, J = 2.3 Hz),

9

8.63 (d, 1H, J = 2.7 Hz), 8.36 (dd, 1H, J = 8.5, 2.3 Hz), 8.08 (d, 1H, J = 8.5 Hz), 7.79 (d, 1H, J =

10

1.4 Hz), 6.51 (dd, 1H, J = 2.7, 1.4 Hz), 2.65 (s, 3H).

11

1-(2-(1H-1,2,3-Triazol-1-yl)pyridin-5-yl)ethan-1-one (20). Compound 20 (57.2 mg, 30.4%)

12

was prepared as described above for 19. 1H NMR (500 MHz, CDCl3):  9.05 (d, 1H, J = 2.3 Hz),

13

8.65 (d, 1H, J = 1.1 Hz), 8.46 (dd, 1H, J = 8.7, 2.3 Hz), 8.34 (d, 1H, J = 8.7 Hz), 7.86 (d, 1H, J =

14

1.1 Hz), 2.68 (s, 3H).

15

1-(2-(2H-1,2,3-Triazol-2-yl)pyridin-5-yl)ethan-1-one (21). Compound 21 (31.9 mg, 17.0%)

16

was prepared as described above for 19. 1H NMR (500 MHz, CDCl3):  9.13 (d, 1H, J = 2.3 Hz),

17

8.45 (dd, 1H, J = 8.6, 2.3 Hz), 8.21 (d, 1H, J = 8.6 Hz), 7.97 (s, 2H), 2.69 (s, 3H).

18

1-(2-(1H-1,2,4-Triazol-1-yl)pyridin-5-yl)ethan-1-one (22). Compound 22 (480 mg, 85.0%)

19

was prepared as described above for 19. 1H NMR (500 MHz, CDCl3):  9.25 (s, 1H), 9.02 (d, 1H,

20

J = 2.0 Hz), 8.44 (dd, 1H, J = 8.5, 2.0 Hz), 8.14 (s, 1H), 8.02 (d, 1H, J = 8.5 Hz), 2.68 (s, 3H).

21

1-(2-(Pyrazol-1-yl)pyridin-5-yl)-2-bromoethan-1-one (23). Triethylamine (230 μL) and

22

bromotrimethylsilane (140 μL, 1.07 mmol) were added to a solution of 19 (100 mg, 0.534 mmol)


18

Page 19 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

in CH2Cl2 (4.0 mL) at 0 °C. The reaction mixture was stirred overnight at room temperature,

2

quenched with water, and extracted with CH2Cl2. (20 mL × 2). The combined organic layers

3

were washed with water and brine, dried over anhydrous MgSO4, and concentrated under

4

reduced pressure. The residue was dissolved in THF (4.0 mL) and treated with N-

5

bromosuccinimide (94.3 mg, 0.530 mmol) at 0 °C, and the obtained mixture was stirred at room

6

temperature for 1 h. After the reaction was complete, the solvent was evaporated, and the residue

7

was purified by silica gel column chromatography (CH2Cl2:EtOAc = 50:1, v/v) to afford 23 (100

8

mg, 71.7%). 1H NMR (500 MHz, CDCl3):  9.02 (d, 1H, J = 2.3 Hz), 8.62 (d, 1H, J = 2.3 Hz),

9

8.39 (dd, 1H, J = 8.7, 1.9 Hz), 8.10 (d, 1H, J = 8.7 Hz), 7.79 (s, 1H), 6.52 (t, 1H, J = 1.9 Hz),

10

4.41 (s, 2H).

11

1-(2-(1H-1,2,3-Triazol-1-yl)pyridin-5-yl)-2-bromoethan-1-one (24). Compound 24 (66.4 mg,

12

81.9%) was prepared similarly to compound 23. 1H NMR (500 MHz, CDCl3):  9.12 (d, 1H, J =

13

2.2 Hz), 8.67 (d, 1H, J = 1.1 Hz), 8.51 (dd, 1H, J = 8.7, 2.2 Hz), 8.38 (d, 1H, J = 8.7 Hz), 7.87 (d,

14

1H, J = 1.1 Hz), 4.44 (s, 2H).

15

1-(2-(2H-1,2,3-Triazol-2-yl)pyridin-5-yl)-2-bromoethan-1-one (25). Compound 25 (20.2 mg,

16

44.7%) was prepared similarly to compound 23. 1H NMR (500 MHz, CDCl3):  9.18 (d, 1H, J =

17

2.1 Hz), 8.49 (dd, 1H, J = 8.5, 2.1 Hz), 8.24 (d, 1H, J = 8.5 Hz), 7.98 (s, 2H), 4.45 (s, 2H).

18

1-(2-(1H-1,2,4-Triazol-1-yl)pyridin-5-yl)2-bromoethan-1-one (26). Compound 26 (657 mg,

19

96.5%) was prepared similarly to compound 23. 1H NMR (500 MHz, CDCl3):  9.27 (br s, 1H),

20

9.07 (d, 1H, J = 2.2 Hz), 8.48 (dd, 1H, J = 8.5, 2.2 Hz), 8.15 (s, 1H), 8.06 (d, 1H, J = 8.5 Hz),

21

4.43 (s, 2H).


19


Page 20 of 47

1

6-Iodo-2-(2-pyrazol-1-yl)pyridin-5-yl])imidazo[1,2-a]pyridine (27). 2-Amino-5-iodopyridine

2

(83.6 mg, 0.380 mmol) was added to a solution of 23 (100 mg, 0.376 mmol) in CH3CN (3.8 mL),

3

and the reaction mixture was refluxed for 4 h. After the reaction was complete, the mixture was

4

allowed to cool down to room temperature and filtered. The precipitate was washed with CH3CN

5

(5.0 mL) and dried under reduced pressure. The crude crystalline solid was suspended in a

6

mixture of water (50 mL) and CH3OH (50 mL), treated with saturated aqueous NaHCO3 (~50

7

mL), and sonicated for 5 min. The obtained precipitate was filtered, washed with water (10 mL),

8

and dried under reduced pressure to afford 27 (80.2 mg, 54.5%). 1H NMR (500 MHz, DMSO-

9

d6):  9.04 (d, 1H, J = 2.3 Hz), 8.97 (s, 1H), 8.66 (d, 1H, J = 2.3 Hz), 8.50 (dd, 1H, J = 8.3, 2.3

10

Hz), 8.46 (s, 1H), 8.01 (d, 1H, J = 8.3 Hz), 7.86–7.85 (m, 1H), 7.48 (s, 2H), 6.61–6.60 (m, 1H);

11

13

12

126.4, 117.4, 111.5, 109.0, 107.7, 75.7, Accurate mass (ES+) m/z 388.0064 [M+H]+ (calcd. for

13

C15H11IN5: 388.0059).

14

6-Iodo-2-[2-(1H-1,2,3-triazol)-1-yl]pyridin-5-yl]imidazo[1,2-a]pyridine (28). Compound 28

15

(44.0 mg, 45.5%) was prepared similarly to compound 27. 1H NMR (500 MHz, DMSO-d6): 

16

9.11 (d, 1H, J = 2.3 Hz), 8.92 (br s, 1H), 8.82 (d, 1H, J = 0.9 Hz), 8.57 (dd, 1H, J = 8.5, 2.3 Hz),

17

8.47 (s, 1H), 8.17 (d, 1H, J = 8.5 Hz), 7.95 (d, 1H, J = 0.9 Hz), 7.45 (br s, 2H); 13C NMR (125

18

MHz, DMSO-d6):  148.3, 146.4, 144.3, 141.2, 137.1, 134.9, 133.7, 132.2, 130.3, 122.5, 118.6,

19

114.5, 110.7, 77.0, Accurate mass (ES+) m/z 389.0007 [M+H]+ (calcd. for C14H10IN6: 389.0012);

20

Anal. C, 43.32; H, 2.34; N, 21.44 (calcd. for C14H9IN6: C, 43.32; H, 2.34; N, 21.65).

21

6-Iodo-2-[2-(2H-1,2,3-triazol-1-yl)pyridin-5-yl]imidazo[1,2-a]pyridine (29). Compound 29

22

(10.4 mg, 35.3%) was prepared similarly to compound 27. 1H NMR (500 MHz, DMSO-d6): 

C NMR (125 MHz, DMSO-d6):  149.7, 144.9, 143.1, 141.7, 140.7, 135.7, 132.3, 131.0, 127.0,


20

Page 21 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

9.13 (d, 1H, J = 2.3 Hz), 8.96 (br s, 1H), 8.56 (dd, 1H, J = 8.7, 2.3 Hz), 8.50 (s, 1H), 8.17 (s, 2H),

2

8.10 (d, 1H, J = 0.9 Hz), 7.48 (br s, 2H); 13C NMR (125 MHz, DMSO-d6):  149.1, 145.3, 143.1,

3

140.3, 136.7, 135.7, 132.5, 131.1, 128.5, 117.4, 113.6, 109.5, 76.0, Accurate mass (ES+) m/z

4

389.0004 [M+H]+ (calcd. for C14H10IN6: 389.0012).

5

6-Iodo-2-[2-(1H-1,2,4-triazol-1-yl)pyridin-5-yl]imidazo[1,2-a]pyridine (30). Compound 30

6

(675 mg, 70.7%) was prepared similarly to compound 27. 1H NMR (500 MHz, DMSO-d6): 

7

9.38 (br s, 1H), 9.10 (d, 1H, J = 2.1 Hz), 8.96 (br s, 1H), 8.57 (dd, 1H, J = 8.5, 2.1 Hz), 8.49 (s,

8

1H), 8.31 (s, 1H), 7.96 (d, 1H, J = 8.5 Hz), 7.49 (br s, 2H); 13C NMR (125 MHz, DMSO-d6): 

9

153.5, 148.7, 146.2, 144.3, 142.4, 141.4, 137.2, 133.6, 132.2, 129.7, 118.5, 113.6, 110.5, 76.8,

10

Accurate mass (ES+) m/z 389.0008 [M+H]+ (calcd. for C14H10IN6: 389.0012).

11

2-(4’-Methoxyphenyl)-6-tributylstannylimidazo[1,2-a]pyridine (31). Triethylamine (2.0 mL),

12

bis(tributyltin) (190 μL, 0.375 mmol), and tetrakis(triphenylphosphine)palladium (20.0 mg,

13

catalytic amount) were added to a solution of 3 (87.6 mg, 0.250 mmol) in dioxane (10.0 mL).

14

The reaction mixture was heated to 90 °C for 21.5 h and subsequently concentrated. The

15

obtained residue was purified by flash silica gel column chromatography (hexane:EtOAc = 3:1,

16

v/v) to afford 31 (53.0 mg, 41.2%). 1H NMR (500 MHz, CDCl3):  8.00–7.94 (m, 1H), 7.89 (d,

17

2H, J = 8.7 Hz), 7.74 (s, 1H), 7.58 (d, 1H, J = 8.7 Hz), 7.14 (d, 1H, J = 8.7 Hz), 6.97 (d, 2H, J =

18

8.7 Hz), 3.86 (s, 3H), 1.59–1.48 (m, 6H), 1.39–1.32 (m, 6H), 1.18–1.05 (m, 6H), 0.90 (t, 9H, J =

19

7.3 Hz); 13C NMR (125 MHz, CDCl3):  159.5, 145.6, 145.0, 131.0, 130.0, 127.2, 126.7, 121.7,

20

116.8, 114.1, 106.2, 55.2, 28.9, 27.2, 13.6, 9.7, Accurate mass (ES+) m/z 515.2076 [M+H]+

21

(calcd. for C26H39N2OSn: 515.2084).


21


Page 22 of 47

1

2-[4’-(2-Fluoroethoxy)phenyl]-6-tributylstannylimidazo[1,2-a]pyridine (32). Compound 32

2

(298 mg, 55.4%) was prepared similarly to compound 31. 1H NMR (500 MHz, CDCl3):  7.99–

3

7.93 (m, 1H), 7.89 (d, 2H, J = 8.5 Hz), 7.74 (s, 1H), 7.58 (d, 1H, J = 8.7 Hz), 7.14 (d, 1H, J = 8.7

4

Hz), 6.99 (d, 2H, J = 8.7 Hz), 4.76 (dt, 2H, 2JHF = 47.2 Hz, JHH = 4.1 Hz), 4.25 (dt, 2H, 3JHF =

5

28.0 Hz, JHH = 4.1 Hz), 1.64–1.47 (m, 6H), 1.38–1.31 (m, 6H), 1.18–1.04 (m, 6H), 0.90 (t, 9H, J

6

= 7.3 Hz); 13C NMR (125 MHz, CDCl3):  158.4, 145.7, 144.9, 131.3, 130.1, 127.4, 122.0, 117.0,

7

114.9, 106.5, 82.0 (d, 2JCF = 170.8 Hz), 67.2 (d, 3JCF = 21.1 Hz), 29.1, 27.4, 13.7, 9.9; Accurate

8

mass (ES+) m/z 547.2152 [M+H]+ (calcd. for C27H40N2OFSn: 547.2147).

9

2-[4’-(2-Hydroxyethoxy)phenyl]-6-tributylstannylimidazo[1,2-a]pyridine (33). Compound

10

33 (73.0 mg, 73.1%) was prepared similarly to compound 31. 1H NMR (500 MHz, CDCl3): 

11

7.99–7.93 (m, 1H), 7.87 (d, 1H, J = 8.3 Hz), 7.74 (s, 1H), 7.57 (d, 1H, J = 8.7 Hz), 7.13 (d, 1H, J

12

= 8.7 Hz), 6.96 (d, 1H, J = 8.3 Hz), 4.11 (t, 2H, J = 4.6 Hz,), 3.97 (t, 2H, J = 4.6 Hz), 1.63–1.47

13

(m, 6H), 1.38–1.31 (m, 6H), 1.17–1.04 (m, 6H), 0.90 (t, 9H, J = 7.1 Hz);

14

CDCl3):  158.6, 145.7, 145.1, 131.2, 130.0, 127.4, 127.3, 121.9, 116.9, 114.8, 106.4, 69.3, 61.5,

15

29.1, 27.4, 13.7, 9.9; Accurate mass (ES+) m/z 545.2192 [M+H]+ (calcd. for C27H41N2O2Sn:

16

545.2190); Anal. C, 59.48; H, 7.26; N, 4.98 (calcd. for C27H40N2O2Sn: C, 59.69; H, 7.42; N,

17

5.16).

18

2-[4’-(2-Hydroxypropoxy)phenyl]-6-tributylstannylimidazo[1,2-a]pyridine (34). Compound

19

34 (68.0 mg, 42.2%) was prepared similarly to compound 31. 1H NMR (500 MHz, CDCl3): 

20

7.99–7.93 (m, 1H), 7.87 (d, 2H, J = 8.9 Hz), 7.74 (s, 1H), 7.57 (d, 1H, J = 8.9 Hz), 7.13 (d, 1H, J

21

= 8.9 Hz), 6.97 (d, 2H, J = 8.9 Hz), 4.17 (t, 2H, J = 6.0 Hz), 3.89 (t, 2H, J = 6.0 Hz), 2.07 (tt, 2H,

22

J = 6.0, 6.0 Hz), 1.60–1.52 (m, 6H), 1.38–1.31 (m, 6H), 1.17–1.04 (m, 6H), 0.90 (t, 9H, J = 7.3

13

C NMR (125 MHz,


22

Page 23 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13

C NMR (125 MHz, CDCl3):  158.8, 145.7, 145.1, 131.2, 130.1, 127.4, 127.0, 121.9,

1

Hz);

2

117.0, 114.8, 106.4, 65.9, 60.5, 32.1, 29.1, 27.4, 13.7, 9.9; Accurate mass (ES+) m/z 559.2358

3

[M+H]+ (calcd. for C28H43N2O2Sn: 559.2347); Anal. C, 60.08; H, 7.47; N, 4.87 (calcd. for

4

C28H42N2O2Sn: C, 60.34; H, 7.60; N, 5.03).

5

2-[4-(1H-1,2,3-Triazol-1-yl)phenyl]-6-tributylstannylimidazo[1,2-a]pyridine (35).

6

Compound 35 (43.0 mg, 60.5%) was prepared similarly to compound 31. 1H NMR (500 MHz,

7

CDCl3):  8.13 (d, 2H, J = 8.7 H z), 8.05 (d, 1H, J = 0.9 Hz), 8.01 (s, 1H), 7.90 (s, 1H), 7.87 (d,

8

1H, J = 0.9 Hz,), 7.82 (d, 2H, J = 8.7 Hz), 7.62 (d, 1H, J = 8.7 Hz), 7.20 (d, 1H, J = 8.7 Hz),

9

1.65–1.49 (m, 6H), 1.40–1.32 (m, 6H), 1.20–1.06 (m, 6H), 0.91 (t, 9H, J = 7.3 Hz);

13

C NMR

10

(125 MHz, CDCl3):  145.9, 143.7, 136.3, 134.8, 134.5, 131.8, 131.2, 127.2, 122.6, 121.6, 120.9,

11

117.2, 107.7, 29.0, 27.3, 13.7, 9.9; Accurate mass (ES+) m/z 552.2153 [M+H]+ (calcd. for

12

C27H38N5Sn: 552.2149); Anal. C, 58.90; H, 6.82; N, 12.85 (calcd. for C27H37N5Sn: C, 58.93; H,

13

6.78; N, 12.73).

14

2-(2-Methoxypyridin-5-yl)-6-tributylstannylimidazo[1,2-a]pyridine (36). Compound 36 (32.0

15

mg, 41.1%) was prepared similarly to compound 31. 1H NMR (500 MHz, CDCl3):  8.69 (d, 1H,

16

J = 2.3 Hz), 8.19 (dd, 1H, J = 8.7, 2.3 Hz), 7.99 (s, 1H), 7.77 (s, 1H), 7.59 (d, 1H, J = 8.7 Hz),

17

7.17 (d, 1H, J = 8.7 Hz), 6.83 (d, 1H, J = 8.7 Hz), 3.99 (s, 3H), 1.60–1.53 (m, 6H), 1.41–1.32 (m,

18

6H), 1.19–1.05 (m, 6H), 0.91 (t, 9H, J = 7.3 Hz); 13C NMR (125 MHz, CDCl3):  164.0, 145.9,

19

144.6, 142.6, 136.7, 131.5, 130.1, 123.8, 122.3, 117.0, 110.9, 106.5, 53.6, 29.1, 27.4, 13.7, 9.9,

20

Accurate mass (ES+) m/z 516.2042 [M+H]+ (calcd. for C25H38N3OSn: 516.2037).

21

2-[2-(Pyrazol-1-yl)pyridin-5-yl]-6-tributylstannylimidazo[1,2-a]pyridine (37). Compound 37

22

(25.1 mg, 35.7%) was prepared similarly to compound 31. 1H NMR (500 MHz, CDCl3):  8.94


23


Page 24 of 47

1

(d, 1H, J = 2.3 Hz), 8.66 (d, 1H, J = 2.3 Hz), 8.40 (dd, 1H, J = 8.7, 2.3 Hz), 8.05 (d, 1H, J = 8.7

2

Hz), 8.01 (s, 1H), 7.89 (s, 1H), 7.75 (s, 1H), 7.61 (d, 1H, J = 8.7 Hz), 7.20 (d, 1H, J = 8.7 Hz),

3

6.48 (s, 1H), 1.59–1.53 (m, 6H), 1.39–1.32 (m, 6H), 1.14–1.09 (m, 6H), 0.90 (t, 9H, J = 7.4 Hz);

4

13

5

127.1, 122.7, 117.1, 112.4, 107.8, 107.4, 29.0, 27.3, 13.7, 9.9, Accurate mass (ES+) m/z

6

552.2150 [M+H]+ (calcd. for C27H38N5Sn: 552.2149).

7

2-[2-(1H-1,2,3,-Triazol-1-yl)pyridin-5-yl]-6-tributylstannylimidazo[1,2-a]pyridine (38).

8


9

CDCl3):  9.05 (d, 1H, J = 1.4 Hz), 8.63 (d, 1H, J = 0.9 Hz), 8.48 (dd, 1H, J = 8.4, 2.3 Hz), 8.27

10

(d, 1H, J = 8.4 Hz), 8.02 (s, 1H), 7.93 (s, 1H), 7.85 (d, 1H, J = 0.9 Hz), 7.61 (d, 1H, J = 9.2 Hz),

11

7.22 (d, 1H, J = 9.2 Hz), 1.65–1.48 (m, 6H), 1.39–1.32 (m, 6H), 1.20–1.06 (m, 6H), 0.91 (t, 9H,

12

J = 7.1 Hz);

13

130.5, 130.2, 123.1, 121.1, 117.3, 114.0, 107.9, 29.0, 27.3, 13.6, 10.0; MS (FAB+) m/z 553

14

[M+H]+; Accurate mass (ES+) m/z 553.2110 [M+H]+ (calcd. for C26H37N6Sn: 553.2102); Anal.

15

C, 56.65; H, 6.51; N, 15.25 (calcd. for C26H36N6Sn: C, 56.64; H, 6.58; N, 15.24).

16


17


18

CDCl3):  9.09 (s, 1H), 8.52 (d, 1H, J = 8.5 Hz), 8.16 (d, 1H, J = 8.5 Hz), 8.03 (s, 1H), 7.97 (s,

19

1H), 7.92 (s, 2H), 7.62 (d, 1H, J = 8.9 Hz), 7.23 (d, 1H, J = 8 .9 Hz), 1.60–1.50 (m, 6H), 1.40–

20

1.34 (m, 6H), 1.15–1.12 (m, 6H), 0.91 (t, 9H, J = 7.1 Hz); 13C NMR (125 MHz, CDCl3):  150.1,

21

146.3, 146.1, 141.1, 136.5, 136.2, 132.0, 130.2, 129.9, 122.9, 117.2, 113.9, 107.9, 29.0, 27.3,

22

13.7, 9.9, Accurate mass (ES+) m/z 553.2113 [M+H]+ (calcd. for C26H37N6Sn: 553.2102).

C NMR (125 MHz, CDCl3):  150.8, 146.0, 145.6, 142.1, 141.7, 136.2, 131.9, 130.1, 128.2,

13

C NMR (125 MHz, CDCl3):  148.2, 146.2, 146.1, 141.2, 136.4, 134.2, 132.1,


24

Page 25 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1


2


3

CDCl3):  9.21 (s, 1H), 8.99 (d, 1H, J = 1.8 Hz), 8.47 (dd, 1H, J = 8.4, 1.8 Hz), 8.12 (s, 1H), 8.03

4

(s, 1H), 7.98 (d, 1H, J = 8.4 Hz),7.93 (s, 1H), 7.62 (d, 1H, J = 8.7 Hz), 7.22 (d, 1H, J = 8.7 Hz),

5

1.65–1.49 (m, 6H), 1.36–1.49 (m, 6H), 1.21–1.07 (m, 6H), 0.91 (t, J = 7.3 Hz, 9H);

6

(125 MHz, CDCl3):  152.8, 148.5, 146.1, 146.0, 141.5, 141.1, 136.5, 132.1, 130.2, 129.9, 123.0,

7

117.2, 113.0, 107.7, 29.0, 27.3, 13.7, 9.9, Accurate mass (ES+) m/z 553.2091 [M+H]+ (calcd. for

8

C26H37N6Sn: 553.2102).

9

Radiolabeling.

123

13

C NMR

I-labeled probes, namely [123I]3, [123I]4, [123I]9, [123I]10, [123I]14, [123I]18,

10

[123I]27, [123I]28, [123I]29, [123I]30, and [123I]IMPY were prepared from the corresponding

11

tributyltin precursors via iododestannylation. Typically, a solution of the tributylstannyl

12

precursor in CH3CN (1 mg/mL, 15–90 μL) was treated with hydrochloric acid (1 or 2 M, 17–170

13

μL), carrier-free [123I]sodium iodide (30 μL, 66.6–1127 MBq), and 30 wt% hydrogen peroxide

14

(5–10 μL). After standing at 40 °C for 10 min, the reaction mixture was subjected to HPLC

15

(YMC PackPro C8, 4.6×150 mm, 5 μm), and the fractions containing the 123I-labeled probe were

16

isolated. The radiochemical yields were in the range of 7.3−49.2% (uncorrected for radioactive

17

decay), and the radiochemical purities of all probes exceeded 95%.

18

LogPoctanol Measurement. Typically, the

19

octanol saturated with water, and a 10-μL aliquot of the thus-obtained solution was added to a

20

microtube containing water-saturated 1-octanol and 1-octanol-saturated water (100 L each).

21

The reaction mixture was stirred and subsequently shaken for 5 min (20 to 25 °C, 20 rpm). After

22

a 20-min centrifugation at 3000 rpm and 23 °C, the octanol and water layers were sampled (50

123

I-labeled probe was diluted to ~1 MBq/mL with 1-


25


Page 26 of 47

1

μL each), and their radioactivities were determined using an autowell gamma counter. The

2

logPoctanol values were calculated as: log P = log[count1-octanol/countwater].

3

In vitro Binding Assay. The binding of

4

filtration techniques described elsewhere.38 A solid form of Aβ(1–42) was purchased from

5

Peptide Institute (Osaka, Japan). Aggregation was carried out by gently dissolving the peptide

6

(0.25 mg/mL) in a buffered solution (pH 7.4) containing 10 mM sodium phosphate and 1 mM

7

ethylenediaminetetraacetic acid (EDTA). The solution was incubated at 37 °C for 42 h upon

8

gentle and constant shaking. Subsequently, solutions of the tested compound (50 μL, 0.064–5

9

μM in 10% ethanol), [123I]IMPY (50 μL, 0.508–1.19 MBq/mL), Aβ aggregates (50 μL) and 10%

10

ethanol (100 μL) were mixed, and the resulting solution was incubated at 37 °C for 3 h.

11

Subsequently, the bound and free radioactivities were separated by filtration under reduced

12

pressure (MultiScreen HTS Vacuum Manifold; Merck Millipore), followed by three washes with

13

10% ethanol. The radioactivity retained on the filters was quantified by an automatic gamma

14

counter (ARC-7001; Hitachi Aloka Medical, Ltd.), and the IC50 values were determined using

15

GraphPad Prism (GraphPad Software Inc., La Jolla, USA).

16

In vitro Autoradiography. Frozen brain samples from a control subject and AD patients (Figure

17

4A: 86-year-old female, Figure 4B: 79-year-old male) were cut into 5-μm sections using a

18

cryostat (Leica Microsystems, CM3050S, Nussloch, Germany). The obtained sections were

19

dipped into phosphate-buffered saline (PBS) for 15, 5, and 5 min each, and then dipped in PBS

20

containing 1% bovine serum albumin (BSA). The thus-treated sections were incubated with 123I-

21

labeled probes (10 kBq/mL) for 30 min at room temperature, washed with PBS containing 1%

22

BSA for 5 min, and subjected to two 5-min rinses with PBS. The non-specific binding was

23

determined in the presence of 5 μM non-labeled compounds using an identical procedure. After

123

I-labeled probes to Aβ(1–42) was evaluated by


26

Page 27 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

drying, the sections were exposed to imaging plates (Fujifilm) overnight. Autoradiographic

2

images were obtained using a BAS-2500 instrument (Fujifilm Co. Ltd., Tokyo, Japan). Adjacent

3

sections were immunostained using the anti-Aβ monoclonal antibody 82E1.

4

In vivo Kinetics in Normal Rat Brain. Each

5

non-anesthesia into the iodine-pretreated tail veins of male Wistar rats (eight weeks old). The

6

rats were sacrificed at 2, 5, 15, 30, and 60 min after injection (three animals at each time). The

7

brains of the sacrificed rats were removed and weighed, and their radioactivity was measured

8

using a single-channel analyzer (Ohyo Koken Kogyo Co. Ltd., SP-20, Tokyo, Japan). Data were

9

expressed as the percent injected dose per gram tissue (%ID/g). 123

123

I-labeled probe (4 MBq) was injected under

10

In vivo Stability. Each

I-labeled probe (4 MBq) was injected under non-anesthesia into the

11

iodine-pretreated tail veins of male Wistar rats (eight weeks old). The rats were sacrificed at 2, 5,

12

15, and 30 min after the injection (three animals at each time), and their blood samples were

13

centrifuged at 3,000 rpm for 10 min at 4 °C to separate the plasma. The plasma samples (400 μL)

14

were mixed with methanol (1 mL) and centrifuged at 4,400 rpm for 10 min at 4 °C to remove the

15

plasma proteins. The supernatant was analyzed by radio-TLC (Silica Gel 60F 254, mobile phase:

16

acetonitrile-water (10:1, v/v)) using a Raytest Rita 68000 linear analyzer (Raytest, Straubenhardt,

17

Germany).

18


27


1 2

Page 28 of 47

FIGURES

3 4

Figure 1. Structures of SPECT and PET Aβ imaging agents evaluated in humans. (A) Agents

5

approved by FDA and EMA; (B) first-generation imaging agents; (C) ABC577 (present study).

6


28

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1 2

Figure 2. Structures of the developed

3

a]pyridines.

123

I-labeled 2-phenyl- and 2-pyridyl-imidazo[1,2-

4

5 6

Figure 3. Comparison of in vitro autoradiography images of Alzheimer’s disease brain sections

7

obtained using [123I]IMPY (A), [123I]3 (B), [123I]4 (C), [123I]9 (D), [123I]10 (E), [123I]14 (F),

8

[123I]18 (G), [123I]27 (H), [123I]28 (I15 and J), [123I]29 (K), and [123I]30 (L).

9


29


Page 30 of 47

1 2

Figure 4. AD brain sections immunohistochemically stained using a monoclonal Aβ antibody.

3

(A) Used for [123I]IMPY, [123I]3, [123I]4, [123I]9, [123I]10, [123I]14, [123I]18, and [123I]28; (B) used

4

for [123I]27, [123I]28, [123I]29, and [123I]30. (C, D) High-magnification images corresponding to

5

the framed areas.

6


30

Page 31 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 5. AD brain sections stained using the Klüver-Barrera method. (A) Used for [123I]IMPY,

3

[123I]3, [123I]4, [123I]9, [123I]10, [123I]14, [123I]18, and [123I]28; (B) used for [123I]27, [123I]28,

4

[123I]29, and [123I]30.

5


31


Page 32 of 47

1

SCHEMES

2 3

Scheme 1. Synthetic route to 2-phenyl-imidazo[1,2-a]pyridine derivatives bearing alkoxy groupsa

4 5

a

6

methanol, 1,4-dioxane; (c) 1-fluoro-2-(tosyloxy)ethane, K2CO3, DMF; (d) K2CO3, DMF; (e)

7

TBAF, THF.

Reagents and conditions: (a) 2-amino-5-iodopyridine, CH3CN; (b) trimethylsilyldiazomethane,

8 9 10

Scheme 2. Synthetic route to 2-phenyl-imidazo[1,2-a]pyridine derivatives bearing 1,2,3triazole groupsa

11


32

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1

a

2

CuSO4·5H2O, DMF; (c) TBAF, THF.

3

Scheme 3. Synthetic route to 2-(3-pyridyl)imidazo[1,2-a]pyridine derivativesa

Reagents and conditions: (a) 2-amino-5-iodopyridine, CH3CN; (b) trimethylsilylacetylene,

4


33


Page 34 of 47

1

a

2

MeOH; (d) R1H, K2CO3, DMSO; (e) (1) TMSBr, Et3N, CH2Cl2; (2) NBS, THF; (f) 2-amino-5-

3

iodopyridine, CH3CN.

4 5

Scheme 4. Preparation of tributylstannyl precursors and subsequent radiosynthesis of 123Ilabeled compounds

Reagents and conditions: (a) CuBr2, EtOAc; (b) 2-amino-5-iodopyridine, CH3CN; (c) NaOMe,

6 7

a

Reagents and conditions: (a) Pd(PPh3)4, (Bu3Sn)2, Et3N, 1,4-dioxane; (b) Na123I, H2O2, HCl,

8

H2O, CH3CN.

9


34

Page 35 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


TABLES Table 1. Comparative lipophilicities of prospective SPECT imaging agents. Entry

Compound

cLogPa

LogPoctanol/water

1

3

4.65

1.73

2

4

4.90

2.05

3

9

3.77

1.84

4

10

4.17

2.08

5

14

4.29

1.98

6

18

4.02

-b

7

27

3.63

-b

8

28 (ABC577)

3.43

2.10

9

29

3.58

2.05

10

30

2.28

2.23

11

IMPY

4.91

3.5839

2

a

3

ChemBioDraw Ultra 14.0 software (CambridgeSoft, Cambridge, MA).40 bNot determined.

The calculated logarithms of water/octanol partition coefficients (cLogP) were obtained using

4


35


Page 36 of 47

Table 2. Affinities of prospective SPECT imaging agents to Aβ(1–42) aggregates.

1

a

Entry

Compound

IC50 (nM)a

1

3

55.8 ± 24.0

2

4

20.0 ± 7.77

3

9

64.5 ± 18.5

4

10

47.2 ± 11.5

5

14

70.9 ± 16.3

6

18

87.6 ± 39.8

7

27

18.2 ± 1.11

8

28 (ABC577)

14.7 ± 6.07

9

29

451 ± 250

10

30

44.6 ± 11.5

11

IMPY

24.8 ± 1.19

Values are cited as mean ± standard error obtained in three independent experiments.

2


36

Page 37 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Gray and white matter signals and gray/white matter signal ratios of 123I-labeled compounds. Gray matter signal

White matter signal

Gray/white matter

PSL/mm2

PSL/mm2

signal ratio

[123I]3

26.2

20.2

1.3

[123I]4

19.5

11.5

1.7

[123I]9

9.92

4.51

2.2

[123I]10

14.6

9.13

1.6

[123I]14

78.1

28.9

2.7

[123I]18

23.0

14.4

1.6

[123I]27

52.2

40.2

1.3

[123I]28 (ABC577)b

128

18.8

6.8

[123I]28 (ABC577)c

83.2

15.4

5.4

[123I]29

9.82

2.81

3.5

[123I]30

78.9

18.3

4.3

[123I]IMPY

134

112

1.2

Compound

1 2

a

Determined by autoradiography. bFor in vitro autoradiography image I (Figure 3). cFor in vitro autoradiography image J (Figure 3).

3


37


Page 38 of 47

1 Table 4. Biodistribution of radioactivity after injection of 123I-labeled compounds into normal rats.a Time after injection (min) Tissue 2

5

15

30

60

[123I]3 Brain

2.09 ± 0.88

1.20 ± 0.20

0.39 ± 0.03

0.15 ± 0.02

0.06 ± 0.01

Blood

0.52 ± 0.06

0.41 ± 0.08

0.31 ± 0.02

0.24 ± 0.04

0.20 ± 0.00

[123I]4 Brain

0.72 ± 0.07

0.49 ± 0.05

-b

0.08 ± 0.01

0.03 ± 0.00

Blood

0.46 ± 0.00

0.37 ± 0.00

-b

0.19 ± 0.04

0.11 ± 0.01

[123I]9 Brain

0.97 ± 0.20

0.62 ± 0.08

0.21 ± 0.03

0.07 ± 0.01

0.02 ± 0.00

Blood

0.71 ± 0.10

0.53 ± 0.05

0.29 ± 0.02

0.22 ± 0.01

0.15 ± 0.02

[123I]10 Brain

1.05 ± 0.17

0.58 ± 0.01

0.24 ± 0.02

0.09 ± 0.02

0.03 ± 0.00

Blood

0.66 ± 0.06

0.49 ± 0.03

0.45 ± 0.02

0.38 ± 0.03

0.28 ± 0.02

[123I]14 Brain

1.09 ± 0.21

0.70 ± 0.08

0.26 ± 0.04

0.10 ± 0.01

0.03 ± 0.01

Blood

0.50 ± 0.06

0.32 ± 0.04

0.24 ± 0.02

0.18 ± 0.02

0.12 ± 0.02

0.08 ± 0.01

0.04 ± 0.00

[123I]18 Brain

1.35 ± 0.20

0.64 ± 0.07

0.24 ± 0.03


38

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Blood

0.40 ± 0.03

0.29 ± 0.01

0.21 ± 0.01

0.13 ± 0.01

0.11 ± 0.01

[123I]27 Brain

1.50 ± 0.27

1.19 ± 0.07

0.49 ± 0.03

0.20 ± 0.00

0.08 ± 0.01

Blood

0.39 ± 0.05

0.28 ± 0.01

0.15 ± 0.01

0.11 ± 0.00

0.08 ± 0.01

[123I]28: ABC577 Brain

1.23 ± 0.13

0.81 ± 0.12

0.29 ± 0.05

0.09 ± 0.01

0.03 ± 0.00

Blood

0.38 ± 0.07

0.28 ± 0.03

0.19 ± 0.02

0.12 ± 0.01

0.08 ± 0.01

[123I]30 Brain

1.29 ± 0.21

0.86 ± 0.16

0.40 ± 0.01

0.14 ± 0.02

0.06 ± 0.01

Blood

0.30 ± 0.02

0.22 ± 0.02

0.14 ± 0.01

0.08 ± 0.00

0.06 ± 0.00

[123I]IMPY

1 2

Brain

1.64 ± 0.33

1.19 ± 0.09

0.55 ± 0.07

0.23 ± 0.03

0.09 ± 0.01

Blood

0.50 ± 0.08

0.36 ± 0.01

0.32 ± 0.01

0.26 ± 0.01

0.21 ± 0.01

a

Expressed as % injected dose per gram. Each value represents the mean ± standard error for three animals at each interval. bNot determined.

3


39


Page 40 of 47

Table 5. Brain2 min/brain60 min ratios of 123I-labeled compounds obtained by in vivo biodistribution analysis. Compound

Brain2 min/brain60 min

[123I]3

34.8

[123I]4

24.0

[123I]9

48.5

[123I]10

35.0

[123I]14

36.3

[123I]18

33.8

[123I]27

18.8

[123I]28 (ABC577)

41.0

[123I]30

21.5

[123I]IMPY

18.2

1


40

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1

Table 6. Fraction of the intact form of 123I-labeled compounds in rat blood plasma after injection.a

Compound

Time after injection (min) 2

5

15

30

90.6 ± 1.1

75.3 ± 3.6

46.2 ± 3.0

19.8 ± 2.1

81.4 ± 4.8

61.0 ± 3.2

37.8±4.1

19.0 ± 0.4

[123I]30

85.3 ± 2.3

75.0 ± 3.9

54.8 ± 2.5

35.7 ± 0.7

[123I]IMPY

63.2 ± 0.5

40.1 ± 5.1

21.1 ± 2.0

12.2 ± 0.0

[123I]14 [123I]28 (ABC577)

2

a

3

represents the mean ± standard error for three animals at each interval.

Expressed as % of intact compound in plasma at each post-injection time point. Each value

4


41


Page 42 of 47

1

ASSOCIATED CONTENT

2

Supporting Information: Includes a figure describing the metabolic stability of

3

compounds in rat blood plasma after injection, 1H NMR spectra of key target compounds, and

4

details of radiolabeling procedures. This material is available free of charge via the Internet at

5

http://pubs.acs.org.

6

ABBREVIATIONS

7

ABC577, 6-iodo-2-[2-(1H-1,2,3-triazole-1-yl]imidazo[1,2-a]pyridine; BBB, blood-brain barrier;

8

CR, Congo Red; EDTA, ethylenediaminetetraacetic acid; EMA, the European Medicines

9

Agency; FAB, fast atom bombardment; ID/g, injected dose per gram; IMPY, 2-(4’-

10

dimethylaminophenyl)-6-imidazo[1,2-a]pyridine; NFTs, neurofibrillary tangles; PBS, phosphate-

11

buffered saline; PSL/mm2, photostimulated luminescence per square millimeter; ThT, thioflavin-

12

T.

123

I-labeled

13 14

AUTHOR INFORMATION

15

Corresponding Author

16

*E-mail: [email protected]; tel.: +81 438 62 7611; fax: +81 438 62 5911

17

Notes

18

The authors declare no competing financial interest.

19

ACKNOWLEDGMENT

20

The authors would like to thank Ms. Chihiro Usui, Mr. Tadashi Iwasaki, and Ms. Yukie

21

Tomizawa for their assistance in this study.


42

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1 2

3 4 5 6 7 8 9 10 11 12 13 14 15


For Table of Contents Use Only

Design, synthesis, and preliminary evaluation of SPECT probes for imaging β-amyloid in Alzheimer’s disease-affected brain Yuki Okumura,*,†, ‡ Yoshifumi Maya,† Takako Onishi ,† Yoshinari Shoyama,† Akihiro Izawa,† Daisaku Nakamura,† Shigeyuki Tanifuji,† Akihiro Tanaka,† Yasushi Arano,‡ and Hiroki Matsumoto† †Research Centre, Nihon Medi-Physics Co., Ltd., 3-1, Kitasode, Sodegaura City, Chiba 2990266, Japan ‡Department of Molecular Imaging and Radiotherapy, Graduate School of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba 260-8675, Japan


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Design, Synthesis, and Preliminary Evaluation of ... - ACS Publications

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