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. 2010 Jul 8;1(9):598–607. doi: 10.1021/cn100042d

Synthesis and Evaluation of Novel Chalcone Derivatives with 99mTc/Re Complexes as Potential Probes for Detection of β-Amyloid Plaques

Masahiro Ono †,‡,*, Ryoichi Ikeoka †, Hiroyuki Watanabe †, Hiroyuki Kimura ‡, Takeshi Fuchigami †, Mamoru Haratake †, Hideo Saji ‡, Morio Nakayama †,*
PMCID: PMC3368688  PMID: 22778849

Abstract

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Four 99mTc-labeled chalcone derivatives and their corresponding rhenium analogues were tested as potential probes for imaging β-amyloid plaques. The chalcones showed higher affinity for Aβ(1−42) aggregates than did 99mTc complexes. In sections of brain tissue from an animal model of AD, the four Re chalcones intensely stained β-amyloid plaques. In biodistribution experiments using normal mice, 99mTc-BAT-chalcone ([99mTc]17) displayed high uptake in the brain (1.48% ID/g) at 2 min postinjection. The radioactivity washed out from the brain rapidly (0.17% ID/g at 60 min), a highly desirable feature for an imaging agent. [99mTc]17 may be a potential probe for imaging β-amyloid plaques in Alzheimer’s brains.

Keywords: Alzheimer’s disease, β-amyloid plaque, 99mTc, single photon emission computed tomography (SPECT) imaging

Alzheimer’s disease (AD), the most common senile dementia, is characterized by β-amyloid (Aβ) plaques, vascular amyloid, neurofibrillary tangles, and progressive neurodegeneration. The formation of plaques composed of β-amyloid peptides (Aβ) in the brain is considered the initial neurodegenerative event in AD (1). Thus, the detection of individual plaques in vivo by single photon emission tomography (SPECT) or positron emission tomography (PET) should improve diagnosis and also accelerate the discovery of effective therapeutic agents for AD (2−4).

Many radiolabeled probes for imaging β-amyloid based on Congo Red, thioflavin T, and DDNP have been reported. Among them, [11C]PIB (5,6), [11C]SB-13 (7,8), [18F]BAY94-9172 (9,10), [11C]BF-227 (11), [18F]FDDNP (12,13), [123I]IMPY (14−16), and [18F]AV-45 (17,18) have been tested clinically and demonstrated potential utility.

Recently, while searching for novel imaging probes, we found that 125I-, 11C-, and 18F-labeled chalcone derivatives showed high affinity for Aβ aggregates and good uptake into and rapid clearance from the brain (19−21). In this study, we planned the development of novel chalcone derivatives labeled with technetium-99m (99mTc). 99mTc (T1/2 = 6.01 h, 141 keV) has become the most commonly used radionuclide in diagnostic nuclear medicine for several reasons: it is readily produced by a 99Mo/99mTc generator, the medium gamma-ray energy it emits is suitable for detection, and its physical half-life is compatible with the biological localization and residence time required for imaging. It is ready availability, essentially 24 h/day, and ease of use make it the radionuclide of choice. New 99mTc-labeled imaging agents will provide simple, convenient, and widespread SPECT-based imaging methods for detecting and eventually quantifying β-amyloid plaques in living brain tissue.

Kung et al. reported that a dopamine transporter imaging agent, [99mTc]TRODAT-1, is useful to detect the loss of dopamine neurons in the basal ganglia associated with Parkinson’s disease. This is the first example of a 99mTc imaging agent that can penetrate the blood−brain barrier via a simple diffusion mechanism and localize at sites in the central nervous system. Based on this success, efforts were made to search for comparable 99mTc imaging agents that target binding sites on β-amyloid plaques in the brain of AD patients. Several 99mTc-labeled imaging probes have been developed (Figure 1), but no clinical study of them has been reported (22−25).

Figure 1.

Figure 1

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Chemical structure of 99mTc-labeled Aβ imaging probes reported previously.

In the present study, to develop more useful 99mTc-labeled probes for clinical diagnosis, we synthesized four chalcone derivatives with monoamine−monoamide dithiol (MAMA) and bis-amino-bis-thiol (BAT). MAMA and BAT were selected as chelation ligands taking into consideration the permeability of the blood−brain barrier, because they form an electrically neutral complex with 99mTc (26). We then evaluated their biological potential as probes by testing their affinity for Aβ aggregates and β-amyloid plaques in sections of brain tissue from Tg2576 mice and their uptake by and clearance from the brain in biodistribution experiments using normal mice. To our knowledge, this is the first time chalcones with 99mTc/Re complexes have been proposed as probes for the detection of β-amyloid plaques in the brain.

Results and Discussion

The synthesis of the 99mTc/Re chalcone derivatives is outlined in Schemes 1−3. The chelation ligands (MAMA and BAT) were synthesized according to methods reported previously with some slight modifications (Scheme 1) (26). The most useful method of preparing chalcones is the condensation of acetophenones with benzaldehydes19. In this process, 4-hydroxybenzaldehyde was reacted with 4-dimethylaminobenzaldehyde in the presence of a basic catalyst (10% KOH) in ethanol at room temperature to form 4-dimethylamino-4′-hydroxy-chalcone (4) in a yield of 70%. The reaction of dibromo with 4 produced two chalcone derivatives (5 and 6) with alkyl groups (n = 3 or 5) of different lengths. Then, 5 (n = 5) or 6 (n = 3) was joined to 2 (Tr-MAMA) or 3 (Tr-Boc-BAT) to generate compounds 7 (n = 5) and 13 (n = 3) (Tr-MAMA-chalcones) or compounds 10 (n = 5) and 16 (n = 3) (Tr-Boc-BAT). Compounds 8, 11, 14, and 17 (the precursors for 99mTc labeling) were obtained by deprotection of the thiol groups in 7, 10, 13, and 16, respectively. The Re complexes (9, 12, 15, and 18) were prepared by the reaction of 7, 11, 13, and 17 with (PPh3)2ReOCl3. The corresponding 99mTc complexes, [99mTc]8, [99mTc]11, [99mTc]14, and [99mTc]17, were prepared by a ligand exchange reaction employing the precursor 99mTc-glucoheptonate (GH). The resulting mixture was analyzed by reversed-phase HPLC, showing that a single radioactive complex formed with radiochemical purity higher than 95% after purification by HPLC. The identity of the complex was established by comparative HPLC using the corresponding Re complexes as a reference (Table 1). The retention times for [99mTc]8, [99mTc]11, [99mTc]14, and [99mTc]17 on HPLC (radioactivity) were 14.2, 20.3, 9.3, and 12.4 min, respectively. The retention times of the corresponding Re complexes on HPLC (UV detection) were 13.5, 18.4, 8.6, and 11.2 min, respectively.

Scheme 1. Synthesis of Chelation Ligands.

Scheme 1

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Scheme 3. 99mTc Labeling of Chalcone Derivatives.

Scheme 3

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Table 1. HPLC Retention Times of 99mTc-Labeled Chalcones and Their Re Analogues and log P Values of 99mTc-Labeled Chalcones.

99mTc compound retention time (min)a Re compound retention time (min)a log P of 99mTc compoundsb
[99mTc]8 14.2 9 13.5 2.55 ± 0.19
[99mTc]11 20.3 12 18.4 2.73 ± 0.16
[99mTc]14 9.3 15 8.6 1.51 ± 0.09
[99mTc]17 12.4 18 11.2 2.51 ± 0.05
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a

Reversed-phase HPLC using a mixture of H2O/acetonitrile (2/3) as a mobile phase.

b

The measurement was done in triplicate and repeated three times. Each value represents the mean ± SD for three independent experiments.

Scheme 2. Synthesis of Chalcone Derivatives.

Scheme 2

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In vitro binding experiments to evaluate the affinity of the 99mTc-labeled chalcones for Aβ aggregates were carried out in solutions (Figure 2). To make quantitative comparisons with 99mTc-BAT and 99mTc-MAMA, we showed the binding affinity as total Aβ(1−42) aggregate-bound radioactivity (%) at different concentrations of Aβ aggregates. In this assay, we also confirmed that the nonspecific Aβ(1−42) aggregate-bound radioactivity (%) was 1.9−3.2% in the four 99mTc-labeled chalcones, indicating that nearly all of the radioactivity occupied the specific binding site for Aβ aggregates. The percent radioactivity of the chalcones bound to aggregates increased dependent on the dose of Aβ(1−42). In terms of Aβ(1−42) aggregate-bound radioactivity, the derivatives ranked in the following order: [99mTc]11 > [99mTc]8 > [99mTc]14 > [99mTc]17. 99mTc complexes (99mTc-MAMA and 99mTc-BAT) showed no marked binding to Aβ(1−42) aggregates. This result suggests that the length of the alkyl chain between 99mTc complexes and the chalcone backbone played an important role in the binding of Aβ(1−42) aggregates, and the difference in ligand (MAMA and BAT) did not affect the affinity.

Figure 2.

Figure 2

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Binding assay of 99mTc-labeled chalcone derivatives (◆, blue) [99mTc]8, (◼) [99mTc]11, (▲) [99mTc]14, (●, orange) [99mTc]17, (●, gray) 99mTc-MAMA, (◆, gray) 99mTc-BAT with Aβ(1−42) aggregates. Values are the mean ± standard error of the mean for three independent experiments.

To confirm the affinity for β-amyloid plaques in the mouse brain, neuropathological fluorescent staining with Re-chalcone derivatives (9, 12, 15, and 18) was carried out using Tg2576 mouse brain sections (Figure 3). Many β-amyloid plaques were clearly stained with the derivatives (Figure 3A,C,E,G), as reflected by the high affinity for Aβ aggregates in binding assays in vitro, while only minimum labeling was observed in the wild-type mouse brain (Figure 3I). The labeling pattern was consistent with that observed with thioflavin S (Figure 3B,D,F,H). These results suggest that the 99mTc-chalcones bind to β-amyloid plaques in the mouse brain in addition to having affinity for synthetic Aβ(1−42) aggregates.

Figure 3.

Figure 3

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Fluorescent staining of chalcone derivatives 9 (A), 12 (C), 15 (E), and 18 (G) in 10-μm Tg2576 mouse brain sections. Labeled plaques were confirmed by staining of the adjacent sections with thioflavin-S (B, D, F, H). No apparent staining of 18 (I) was observed in the age-matched control mouse brain section.

Four 99mTc-labeled chalcones ([99mTc]8, [99mTc]11, [99mTc]14, [99mTc]17) were examined as to their biodistribution in normal mice (Table 2). A biodistribution study provides important information on brain uptake. The ideal β-amyloid imaging probe should have good blood−brain barrier penetration to deliver a sufficient dose into the brain while achieving rapid clearance from normal regions to result in a higher signal-to-noise ratio in the AD brain. Previous studies suggested that the optimal lipophilicity for entry into the brain is obtained with log P values of between 1 and 3 (27). All four ligands examined in the present study displayed optimal lipophilicity as reflected by their log P values (Table 1). Among the four 99mTc-labeled chalcones, [99mTc]17 showed the highest uptake (1.48% ID/g at 2 min postinjection), but [99mTc]8, [99mTc]11, and [99mTc]14 did not show sufficient uptake (0.32−0.78% ID/g at 2 min postinjection) despite their lipophilicity. The reason for the low uptake remains unknown. The uptake of [99mTc]17 was greater than that of 99mTc-labeled Aβ imaging agents reported previously, supporting the feasibility of developing 99mTc-labeled probes for the detection of β-amyloid plaques in vivo. Thereafter, the radioactivity of [99mTc]17 that accumulated in the brain was rapidly eliminated (0.17% ID/g at 60 min postinjection), a highly desirable property for imaging agents. The pharmacokinetics of [99mTc]17 in the brain appears superior to that of any other 99mTc-labeled probe reported previously, indicating that this chalcone should be investigated further for the imaging of β-amyloid. [99mTc]8, [99mTc]11, and [99mTc]14 showed no marked initial uptake, 0.32−0.78% ID/g, and were washed out from the brain relatively slowly (0.11−0.16% ID/g at 60 min). The uptake of [99mTc]8, [99mTc]11, and [99mTc]14 appears insufficient for the imaging of β-amyloid plaques in the brain, and additional structural changes are needed to further improve the properties of these chalcone derivatives.

Table 2. Biodistribution of Radioactivity after Injection of 99mTc-Labeled Chalcone Derivatives in Normal Micea.

  time after injection (min)
organ 2 10 30 60
[99mTc]8
blood 1.85(0.31) 0.91(0.15) 0.62(0.16) 0.28(0.02)
liver 18.92(2.03) 24.48(1.34) 26.63(5.57) 17.05(1.52)
kidney 9.45(1.24) 7.62(3.79) 9.85(1.35) 5.25(0.87)
intestine b 4.71(0.63) 12.45(2.90) 34.90(3.01) 36.49(6.04)
spleen 4.16(0.52) 3.17(0.52) 2.37(0.48) 1.28(0.09)
lung 16.13(1.34) 6.59(1.23) 3.61(1.00) 1.44(0.17)
stomach b 0.76(0.10) 1.31(0.15) 2.06(0.65) 1.67(0.29)
pancreas 3.78(0.49) 5.28(0.33) 4.71(0.96) 2.27(0.14)
heart 11.05(1.99) 5.10(1.00) 2.16(0.63) 0.87(0.22)
brain 0.22(0.05) 0.32(0.14) 0.19(0.030) 0.11(0.01)
[99mTc]11
blood 2.49(0.24) 0.92(0.05) 0.50(0.11) 0.35(0.13)
liver 23.89(2.51) 24.03(4.51) 23.18(3.67) 21.95(4.58)
kidney 11.26(0.62) 9.66(0.58) 7.64(0.88) 6.39(1.51)
intestine b 6.27(0.31) 15.99(0.87) 37.18(2.54) 54.09(10.94)
spleen 3.15(0.22) 1.64(0.33) 0.69(0.16) 0.35(0.15)
lung 15.71(4.59) 4.54(0.57) 1.89(0.24) 1.28(0.49)
stomach b 0.95(0.14) 1.37(0.19) 1.77(0.62) 2.41(0.99)
pancreas 4.93(0.87) 3.94(0.84) 1.71(0.30) 0.80(0.32)
heart 13.17(1.42) 3.03(0.36) 1.30(0.35) 0.78(0.10)
brain 0.78(0.16) 0.55(0.06) 0.28(0.09) 0.16(0.09)
[99mTc]14
blood 7.84(2.85) 5.37(2.42) 1.55(0.62) 0.41(0.19)
liver 18.35(1.93) 24.89(2.27) 21.29(3.34) 12.96(3.14)
kidney 10.50(1.36) 11.03(2.66) 9.62(1.54) 4.65(0.75)
intestine b 3.70(0.55) 9.60(1.35) 31.67(4.85) 41.40(8.54)
spleen 5.87(2.07) 4.44(0.69) 2.71(0.92) 1.42(0.85)
lung 16.33(4.74) 7.87(1.60) 3.16(0.68) 4.12(4.25)
stomach b 0.85(0.21) 1.42(0.28) 2.29(0.61) 2.32(0.81)
pancreas 3.14(1.32) 5.30(2.14) 4.64(0.76) 1.63(0.37)
heart 13.26(2.46) 4.96(1.83) 2.54(0.56) 1.23(0.28)
brain 0.62(0.27) 0.47(0.13) 0.38(0.11) 0.16(0.08)
[99mTc]17
blood 2.81(0.76) 0.95(0.45) 0.56(0.30) 0.29(0.13)
liver 21.26(2.50) 27.33(2.45) 22.08(3.93) 14.34(0.60)
kidney 11.21(1.46) 8.54(0.64) 4.18(0.52) 1.92(0.41)
intestine b 6.22(0.40) 21.95(2.50) 42.24(3.78) 53.39(4.78)
spleen 2.91(0.61) 2.37(0.55) 0.74(0.16) 0.30(0.04)
lung 10.33(1.80) 5.92(1.38) 2.47(0.62) 0.73(0.27)
stomach b 1.14(0.26) 1.80(0.22) 1.93(0.19) 1.68(0.67)
pancreas 6.91(2.23) 4.45(0.54) 1.44(0.33) 0.47(0.06)
heart 11.71(2.13) 3.01(0.70) 0.98(0.26) 0.44(0.10)
brain 1.48(0.44) 1.09(0.20) 0.35(0.14) 0.17(0.06)
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a

Each value represents the mean (SD) for 3−6 mice at each interval. Expressed as % injected dose per gram.

b

Expressed as % injected dose per organ.

In conclusion, we successfully designed and synthesized novel chalcone derivatives conjugated with 99mTc or Re complexes for the detection of β-amyloid plaques in the brain. When in vitro plaque labeling was carried out using sections of brain from Tg2576 mice, four Re-chalcones intensely stained β-amyloid plaques. In addition, [99mTc]17 displayed good uptake into and a rapid washout from the brain after its injection into normal mice. The combination of affinity for β-amyloid plaques, good uptake, and rapid clearance makes [99mTc]17 a promising probe for the detection of β-amyloid plaques in the brain. The results of the present study should provide information useful to the development of 99mTc-labeled probes for the imaging of β-amyloid plaques in the brain.

Methods

General

All reagents were obtained commercially and used without further purification unless otherwise indicated. 1H NMR spectra were obtained on a Varian Gemini 300 spectrometer with TMS as an internal standard. Coupling constants are reported in hertz. Multiplicity was defined by s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), and m (multiplet). Mass spectra were obtained on a JEOL IMS-DX. HPLC was performed with a Shimadzu system (a LC-10AT pump with a SPD-10A UV detector, λ = 254 nm) using a Cosmosil C18 column (Nakalai Tesque, 5C18-AR-II, 4.6 mm × 150 mm) and acetonitrile/H2O (3/2) as the mobile phase at a flow rate of 1.0 mL/min. All key compounds were proven by this method to show >95% purity.

Chemistry

2-(Tritylthio)ethanamine Hydrochloride (1)

A solution of 2-mercaptoethylamine hydrochloride (1.14 g, 10 mmol) and triphenylmethyl chloride (2.79 g, 10 mmol) in DMF (10 mL) was stirred at room temperature for 72 h. After the evaporation of DMF, the residue was redissolved in ethyl acetate and added to a 5% NaHCO3 solution in an ice bath. The white precipitate that formed was filtered to yield 1 (3.50 g, 98% yield). 1H NMR (300 MHz, CDCl3) δ 2.31 (t, J = 6.8 Hz, 2H), 2.60 (t, J = 6.8 Hz, 2H), 7.18−7.31 (m, 9H), 7.43 (d, J = 7.2 Hz, 6H).

2-(2-(Tritylthio)ethylamino)-N-(2-(tritylthio)ethyl)acetamide (Tr-MAMA) (2)

To a stirring solution of 1 (2.50 g, 7.02 mmol) in CHCl3 (20 mL) and triethylamine (5 mL) was slowly added bromoacetyl bromide (0.5 mL, 5.76 mmol) in an ice bath, and the mixture was stirred at 0 °C for 3 h. The mixture was washed with a diluted H2SO4 (pH 3), a saturated NaHCO3, and a saturated NaCl solution, sequentially. The organic layers were combined and dried with Na2SO4. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography using CHCl3 and then ethyl acetate to give 1.39 g of 2 (58% yield). 1H NMR (300 MHz, CDCl3) δ 2.35 (q, J = 6.6 Hz, 4H), 2.45 (t, J = 5.9 Hz, 2H), 3.02 (s, 2H), 3.07 (q, J = 6.3 Hz, 2H), 7.18−7.29 (m, 18H), 7.38−7.42 (m, 12H). MS m/z 679 (MH+).

tert-Butyl-2-(2-(tritylthio)ethylamino)ethyl-2-(tritylthio)ethylcarbamate (Tr-BAT) (3)

To a solution of N1,N2-bis(2-(tritylthio)ethyl)ethane-1,2-diamine (3.33 g, 5 mmol) and DIPEA (0.86 mL, 5 mmol) in CH2Cl2 (80 mL) was added dropwise a solution of (BOC)2O (1.09 g, 5 mmol) in CH2Cl2 (20 mL) in an ice bath. The mixture was stirred at 0 °C for 1 h, concentrated, and purified by column chromatography (CHCl3/CH3OH = 99:1) to give 3.59 g of 3 (94%). 1H NMR (300 MHz, CDCl3) δ 1.37 (s, 9H), 1.54 (s, broad, 1H), 2.46−2.28 (m, 8H), 2.96 (s, broad, 4H), 7.17−7.30 (m, 18H), 7.37−7.43 (m, 12H).

(E)-3-(4-(Dimethylamino)phenyl)-1-(4-hydroxyphenyl)prop-2-en-1-one (4)

p-Hydroxyacetophenone (1.36 g, 10 mmol) and p-dimethylaminobenzaldehyde (1.49 g, 10 mmol) were dissolved in EtOH (15 mL). A 30-mL aliquot of a 10% aqueous KOH solution was then slowly added dropwise to the reaction mixture. The mixture was stirred for 12 h at 100 °C and then poured into a 1 M aqueous HCl solution and extracted with ethyl acetate. The organic layers were combined and dried over Na2SO4. Evaporation of the solvent afforded a residue, which was purified by silica gel chromatography (CHCl3/CH3OH = 49:1) to give 1.86 g of 4 (70%). 1H NMR (300 MHz, CDCl3) δ 3.05 (s, 6H), 6.70 (d, J = 8.7 Hz, 2H), 6.91 (d, J = 9.0 Hz, 2H), 7.34 (d, J = 15.3 Hz, 1H), 7.55 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 16.2 Hz, 1H), 7.99 (d, J = 9.0 Hz, 2H).

(E)-1-(4-(5-Bromopentyloxy)phenyl)-3-(4-(dimethylamino)phenyl)prop-2-en-1-one (5)

To a solution of 4 (1.51 g, 5.64 mmol) in acetonitrile (15 mL) were added 1,5-dibromopentane (1.54 mL, 11.4 mmol) and K2CO3 (1.5 g). The reaction mixture was heated to reflux for 4 h. After the acetonitrile was evaporated, the residue was poured into water and extracted with CHCl3. The organic layer was combined and dried with Na2SO4. After the mixture was concentrated, the residue was redissolved in DMSO and washed using hexane. The DMSO layer was concentrated and purified by silica gel chromatography using CHCl3 to give 1.48 g of 5 (63% yield). 1H NMR (300 MHz, CDCl3) δ 1.62−1.70 (m, 2H), 1.83−1.90 (m, 2H), 1.91−2.01 (m, 2H), 3.05 (s, 6H), 3.56 (t, J = 6.8 Hz, 2H), 4.06 (t, J = 6.3 Hz, 2H), 6.70 (d, J = 9.0 Hz, 2H), 6.95 (d, J = 8.7 Hz, 2H), 7.36 (d, J = 15.3 Hz, 1H), 7.55 (d, J = 8.7 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 8.02 (d, J = 9.0 Hz, 2H).

(E)-1-(4-(3-Bromopropoxy)phenyl)-3-(4-(dimethylamino)phenyl)prop-2-en-1-one (6)

To a solution of 4 (890 mg, 3.33 mmol) in acetonitrile (25 mL) were added 1,3-dibromopropane (0.679 mL, 6.66 mmol) and K2CO3 (1.0 g). The reaction mixture was heated to reflux for 4 h. After the acetonitrile was evaporated, the residue was poured into a saturated NaCl solution and extracted with CHCl3. The organic layer was combined and dried with Na2SO4. After the mixture was concentrated, the residue was redissolved in DMF and washed using hexane. The DMF layer was concentrated and purified by silica gel chromatography (CHCl3/CH3OH = 199:1) to give 981 mg of 6 (73% yield). 1H NMR (300 MHz, CDCl3) δ 2.35 (quin, J = 6.1 Hz, 2H), 3.04 (s, 6H), 3.61 (t, J = 6.5 Hz, 2H), 4.18 (t, J = 5.7 Hz, 2H), 6.69 (d, J = 9.0 Hz, 2H), 6.97 (d, J = 8.7 Hz, 2H), 7.35 (d, J = 15.3 Hz, 1H), 7.55 (d, J = 8.7 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 8.02 (d, J = 9.0 Hz, 2H).

Compound 7 (Tr-MAMA-C5-CH)

To a solution of 5 (485 mg, 1.13 mmol) and 2 (830 mg, 1.22 mmol) in DMF (10 mL) was added DIPEA (1 mL). The reaction mixture was heated to reflux for 12 h. After the evaporation of the solvent, water was added. The mixture was extracted with CHCl3. The organic layers were combined and dried with Na2SO4. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography (CHCl3/CH3OH = 199:1) to give 330 mg of 7 (29% yield). 1H NMR (300 MHz, CDCl3) δ 1.39 (s, 4H), 1.69−1.71 (m, 2H), 2.27−2.41 (m, 8H), 2.87 (s, 2H), 2.95−3.01 (m, 2H), 3.04 (s, 6H), 3.93 (t, J = 6.5 Hz, 2H), 6.70 (d, J = 9.0 Hz, 2H), 6.89 (d, J = 8.7 Hz, 2H), 7.14−7.27 (m, 18H), 7.32−7.39 (m, 13H), 7.55 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 7.99 (d, J = 9.0 Hz, 2H). MS m/z 1014 (MH+).

Compound 8 (MAMA-C5-CH)

To a solution of a little 7 in TFA (200 mL) was added triethylsilane (10 mL), and the solution was mixed. The solvent was removed under a stream of nitrogen gas to give 8. MS m/z 530 (MH+).

Compound 9 (Re-MAMA-C5-CH)

To a solution of 7 (82 mg, 0.081 mmol) in TFA (4 mL) was added triethylsilane (0.2 mL), and the solution was mixed, and then the solvent was removed under a stream of nitrogen gas. The residue was resolved in a mixed solvent (22 mL, CH2Cl2/CH3OH = 10:1), to which were added (Ph3P)2ReOCl3 (135 mg, 0.162 mmol) and 1 M sodium acetate in methanol (4 mL). The reaction mixture was heated to reflux for 6 h and, after cooling to room temperature, mixed with ethyl acetate (60 mL) and filtered. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography (CHCl3/CH3OH = 49:1) and then RP-HPLC (acetonitrile/H2O = 4:1) to give 20 mg (35%) of 9. 1H NMR (300 MHz, CDCl3) δ 1.60−1.67 (m, 2H), 1.89−1.92 (m, 4H), 2.82−2.91 (m, 1H), 3.05 (s, 6H), 3.14−3.27 (m, 3H), 3.33−3.42 (m, 1H), 3.52−3.65 (m, 1H), 3.95−4.16 (m, 6H), 4.56−4.62 (m, 1H), 4.67 (d, J = 16.5 Hz, 1H), 6.70 (d, J = 8.7 Hz, 2H), 6.95 (d, J = 8.7 Hz, 2H), 7.35 (d, J = 15.6 Hz, 1H), 7.55 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 8.03 (d, J = 8.7 Hz, 2H). HRMS m/z C28H37N3O4ReS2 found 730.1748, calcd 730.1783 (MH+).

Compound 10 (Tr-BAT-C5-CH)

To a solution of 5 (630 mg, 1.51 mmol) and 3 (855 mg, 1.12 mmol) in DMF (20 mL) was added DIPEA (2 mL). The reaction mixture was heated to reflux for 12 h. After the evaporation of the solvent, the residue was purified by silica gel chromatography using CHCl3 and then ethyl acetate/hexane (1:2) to give 126 mg of 10 (10% yield). 1H NMR (300 MHz, CDCl3) δ 1.32−1.37 (m, 12H), 1.70−1.77 (m, 2H), 2.20−2.35 (m, 10H), 2.85−3.01 (m, 4H), 3.04 (s, 6H), 3.98 (t, J = 6.5 Hz, 2H), 6.69 (d, J = 9.3 Hz, 2H), 6.93 (d, J = 8.7 Hz, 2H), 7.15−7.29 (m, 18H), 7.33−7.42 (m, 13H), 7.55 (d, J = 8.7 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 8.01 (d, J = 9.0 Hz, 2H). MS m/z 1100 (MH+).

Compound 11 (BAT-C5-CH)

To a solution of 10 in TFA (200 mL) was added triethylsilane (10 mL), and the solution was mixed. The solvent was removed under a stream of nitrogen gas to give 11. MS m/z 516 (MH+).

Compound 12 (Re-BAT-C5-CH)

To a solution of 10 (110 mg, 0.100 mmol) in TFA (4 mL) was added triethylsilane (0.2 mL), and the solution was mixed, and then the solvent was removed under a stream of nitrogen gas. The residue was resolved in a mixed solvent (22 mL, CH2Cl2/CH3OH = 10:1), to which were added (Ph3P)2ReOCl3 (167 mg, 0.200 mmol) and 1 M sodium acetate in methanol (4 mL). The reaction mixture was heated to reflux for 6 h and, after cooling to room temperature, mixed with ethyl acetate (60 mL) and filtered. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography (CHCl3/CH3OH = 49:1) and then RP-HPLC (acetonitorile/H2O = 4:1) to give 44 mg (61%) of 12. 1H NMR (300 MHz, CDCl3) δ 1.60−1.73 (m, 2H), 1.87−1.92 (m, 4H), 2.72−2.78 (m, 1H), 2.96−3.00 (m, 2H), 3.05 (s, 6H), 3.21−3.41 (m, 4H), 3.53−3.63 (m, 1H), 3.76−3.92 (m, 3H), 4.06−4.17 (m, 5H), 6.70 (d, J = 8.7 Hz, 2H), 6.96 (d, J = 9.0 Hz, 2H), 7.36 (d, J = 15.6 Hz, 1H), 7.56 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 8.03 (d, J = 9.0 Hz, 2H). HRMS m/z C28H39N3O3ReS2 found 716.1990, calcd 716.1990 (MH+).

Compound 13 (Tr-MAMA-C3-CH)

To a solution of 6 (981 mg, 2.53 mmol) and 2 (1.72 g, 2.53 mmol) in DMF (30 mL) was added DIPEA (3 mL). The reaction mixture was heated to reflux for 12 h. After the evaporation of the solvent, water was added. The mixture was extracted with CHCl3. The organic layers were combined and dried with Na2SO4. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography (ethyl acetate/hexane = 3:2) to give 714 mg of 13 (29% yield). 1H NMR (300 MHz, CDCl3) δ 1.80−1.87 (m, 2H), 2.28 (t, J = 6.3 Hz, 2H), 2.33−2.43 (m, 4H), 2.51 (t, J = 6.9 Hz, 2H), 2.91 (s, 2H), 2.99−3.03 (m, 2H), 3.05 (s, 6H), 3.99 (t, J = 5.9 Hz, 2H), 6.70 (d, J = 9.0 Hz, 2H), 6.86 (d, J = 9.0 Hz, 2H), 7.14−7.27 (m, 18H), 7.29−7.38 (m, 13H), 7.54 (d, J = 8.7 Hz, 2H), 7.78 (d, J = 15.3 Hz, 1H), 7.93 (d, J = 9.0 Hz, 2H). MS m/z 986 (MH+).

Compound 14 (MAMA-C3-CH)

To a solution of 13 in TFA (200 mL) was added triethylsilane (10 mL), and the solution was mixed. The solvent was removed under a stream of nitrogen gas to give 14. MS m/z 502 (MH+).

Compound 15 (Re-MAMA-C3-CH)

To a solution of 13 (99 mg, 0.100 mmol) in TFA (4 mL) was added triethylsilane (0.2 mL), and the solution was mixed, and then the solvent was removed under a stream of nitrogen gas. The residue was resolved in a mixed solvent (22 mL, CH2Cl2/CH3OH = 10:1), to which were added (Ph3P)2ReOCl3 (167 mg, 0.200 mmol) and 1 M sodium acetate in methanol (4 mL). The reaction mixture was heated to reflux for 6 h and, after cooling to room temperature, mixed with ethyl acetate (60 mL) and filtered. Evaporation of the solvent gave a residue, which was purified with silica gel chromatography (CHCl3/CH3OH = 49:1) and then RP-HPLC (acetonitorile/H2O = 4:1) to give 20 mg (29%) of 15. 1H NMR (300 MHz, CDCl3) δ 1.65−1.75 (m, 1H), 2.32−2.37 (m, 2H), 2.90−2.98 (m, 1H), 3.05 (s, 6H), 3.15−3.34 (m, 3H), 3.40−3.52 (m, 1H), 3.81−3.86 (m, 1H), 4.09−4.19 (m, 5H), 4.58−4.64 (m, 1H), 4.74 (d, J = 16.2 Hz, 1H), 6.70 (d, J = 9.0 Hz, 2H), 6.96 (d, J = 8.7 Hz, 2H), 7.40 (d, J = 15.3 Hz, 1H), 7.56 (d, J = 8.7 Hz, 2H), 7.79 (d, J = 15.3 Hz, 1H), 8.03 (d, J = 9.0 Hz, 2H). HRMS m/z C26H33N3O4ReS2 found 702.1476, calcd 702.1470 (MH+).

Compound 16 (Tr-BAT-C3-CH)

To a solution of 6 (243 mg, 0.626 mmol) and 3 (479 mg, 0.626 mmol) in DMF (10 mL) was added DIPEA (1 mL). The reaction mixture was heated to reflux for 12 h. After the evaporation of the solvent, a saturated NaCl solution was added. The mixture was extracted with CHCl3. The organic layers were combined and dried with Na2SO4. Evaporation of the solvent gave a residue, which was purified by silica gel chromatography using CHCl3 and then ethyl acetate/hexane (1:2) to give 128 mg of 16 (19% yield). 1H NMR (300 MHz, CDCl3) δ 1.37 (s, 9H), 1.73 (t, J = 6.0 Hz, 2H), 2.24−2.38 (m, 10H), 2.88−2.95 (m, 4H), 3.04 (s, 6H), 3.97 (t, J = 6.2 Hz, 2H), 6.70 (d, J = 9.0 Hz, 2H), 6.89 (d, J = 8.7 Hz, 2H), 7.15−7.31 (m, 19H), 7.37−7.41 (m, 12H), 7.54 (d, J = 9.0 Hz, 2H), 7.78 (d, J = 15.6 Hz, 1H), 7.96 (d, J = 8.7 Hz, 2H). MS m/z 1072 (MH+).

Compound 17 (BAT-C3-CH)

To a solution of 16 in TFA (200 mL) was added triethylsilane (10 mL), and the solution was mixed. The solvent was removed under a stream of nitrogen gas to give 17. MS m/z 488 (MH+).

Compound 18 (Re-BAT-C3-CH)

To a solution of 16 (104 mg, 0.097 mmol) in TFA (4 mL) was added triethylsilane (0.2 mL), and the solution was mixed, and then the solvent was removed under a stream of nitrogen gas. The residue was resolved in a mixed solvent (22 mL, CH2Cl2/CH3OH = 10:1), to which were added (Ph3P)2ReOCl3 (167 mg, 0.200 mmol) and 1 M sodium acetate in methanol (4 mL). The reaction mixture was heated to reflux for 6 h and, after cooling to room temperature, mixed with ethyl acetate (60 mL) and filtered. Evaporation of the solvent gave a residue, which was purified with silica gel chromatography (CHCl3/CH3OH = 49:1) and then RP-HPLC (acetonitrile/H2O = 4:1) to give 44 mg (66%) of 18. 1H NMR (300 MHz, CDCl3) δ1.75−1.85 (m, 1H), 2.31−2.38 (m, 2H), 2.76−2.82 (m, 1H), 2.91−3.08 (m, 2H), 3.05 (s, 6H), 3.24−3.49 (m, 4H), 3.76−4.00 (m, 3H), 4.12−4.33 (m, 5H), 6.70 (d, J = 9.0 Hz, 2H), 6.95 (d, J = 9.0 Hz, 2H), 7.35 (d, J = 15.3 Hz, 1H), 7.56 (d, J = 9.0 Hz, 2H), 7.79 (d, J = 15.3 Hz, 1H), 8.03 (d, J = 8.7 Hz, 2H). HRMS m/z C26H35N3O3ReS2 found 688.1631, calcd 688.1677 (MH+).

99mTc Labeling Reaction

To a solution of sodium heptonate dehydrate (2.0 g, 7.04 mmol) in nanopure water (25 mL) was added a 0.75 mL aliquot of a SnCl2·2H2O solution [12 mg of Tin(II) chloride dehydrate (53.2 mmol) was dissolved in 0.1 M HCl (15 μL)]. This solution was adjusted to pH 8.5−9.0 with a small amount of 0.1 M NaOH and then lyophilized to give tin glucoheptonate (SnGH) kit. SnGH kit (1 mg) was added to a Na99mTcO4 solution (200 μL) and reacted at room temperature for 10 min to give a 99mTcGH solution. To solutions of 7, 10, 13, and 16 (0.5 mg) in TFA (200 μL) was added triethylsilane (10 μL), and the solutions were mixed, and then the solvents were removed under a stream of nitrogen gas. The residues were resolved in acetonitrile (200 μL), to which were added a 0.1 M HCl solution (15 μL) and the 99mTcGH solution (200 mL). The reaction mixtures were heated to 80−90 °C for 10 min. The residue taken up in 100 μL of acetonitrile was purified by a reversed-phase HPLC system with an isocratic solvent of acetonitrile/H2O (3/2) as the mobile phase at a flow rate of 1.0 mL/min. After cooling to room temperature, the mixtures were purified with RP-HPLC to give [99mTc]8, [99mTc]11, [99mTc]14, and [99mTc]17. The desired fraction was collected in a flask and evaporated dry. The radiochemical identity of [99mTc]8, [99mTc]11, [99mTc]14, and [99mTc]17 was verified with the corresponding Re-complex from the HPLC profiles. The final 99mTc-complexes showed a single peak of radioactivity at retention times of 14.2, 20.3, 9.3, and 12.4 min, respectively, close to those of the Re complexes. The four 99mTc-complexes were obtained in 46−95% radiochemical yields with a radiochemical purity of >95% after HPLC.

Partition Coefficient

Partition coefficients were measured by mixing the [99mTc]tracer with 3 mL each of 1-octanol and buffer (0.1 M phosphate, pH 7.4) in a test tube. The test tube was vortexed for 3 min at room temperature, then centrifuged for 5 min. Two weighed samples (0.5 mL) from the 1-octanol and buffer layers were measured in a well counter. The partition coefficient was determined by calculating the ratio of counts per minute per gram of 1-octanol to that of buffer. Samples from the 1-octanol layer were repartitioned until consistent partitions of coefficient values were obtained. The measurement was done in triplicate and repeated three times.

Binding Assays Using the Aggregated Aβ Peptides in Solution

A solid form of Aβ(1−42) was purchased from Peptide Institute (Osaka, Japan). Aggregation was carried out by gently dissolving the peptide (0.25 mg/mL) in a buffer solution (pH 7.4) containing 10 mM sodium phosphate and 1 mM EDTA. The solutions were incubated at 37 °C for 42 h with gentle and constant shaking. The binding assay was performed by mixing 50 μL of Aβ(1−42) aggregates (0−100 μg/mL), 50 μL of an appropriate concentration of the 99mTc-labeled form ([99mTc]8, [99mTc]11, [99mTc]14, [99mTc]17), and 900 μL of 30% ethanol in 12 mm × 75 mm borosilicate glass tubes. Nonspecific binding was defined in the presence of 1 μM of chalcone derivative (4-dimethylamino-4′-iodo-chalcone) (20). After incubation for 3 h at room temperature, the mixture was filtered through GF/B filters (Whatman, Kent, U.K.) using an M-24 cell harvester (Brandel, Gaithersburg, MD). Filters containing the bound 99mTc-labeled form were examined in a γ counter (Perkin-Elmer, WIZARD2 2470).

Biodistribution in Normal Mice

The experiments with animals were conducted in accordance with our institutional guidelines and approved by the Nagasaki University Animal Care Committee. A saline solution (100 μL) of 99mTc-chalcones (1.0 × 107 cpm/mL) containing ethanol (30 μL) was injected intravenously directly into the tail of ddY mice (5 weeks old, 22−25 g). The mice were sacrificed at various time points postinjection. The organs of interest were removed and weighed, and radioactivity was measured with an automatic γ counter (Perkin-Elmer, WIZARD2 2470).

Neuropathological Staining of Mouse Brain Sections

The Tg2576 transgenic (female, 30-month-old) and wild-type mice (female, 30-month-old) mice were used as the Alzheimer’s model and control, respectively. After the mice were sacrificed by decapitation, the brains were immediately removed and frozen in powdered dry ice. The frozen blocks were sliced into serial sections, 10 μm thick. Each slide was incubated with a 50% EtOH solution (2.5−10 μM) of 9, 12, 15, and 18 for 2−9 h. The sections were washed in 50% EtOH for 5 min two times and examined for 9, 12, 15, and 18 with excitation of 458 nm using a microscope (Carl Zeiss, LSM710). Thereafter, the serial sections were also stained with thioflavin S, a pathological dye commonly used for staining β-amyloid plaques in the brain.

Note Added after ASAP Publication

This paper was published on the Web on July 8, 2010, with an error in Figure 3. The corrected version was reposted on July 13, 2010.

This study was supported by the Program for Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO), a Health Labour Sciences Research Grant, and a Grant-in-aid for Young Scientists (A) and Exploratory Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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