D3 receptor radioligands [11C]fallypride and [18F]fallypride

ARTICLE IN PRESS Applied Radiation and Isotopes 68 (2010) 1079–1086

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An improved synthesis of dopamine D2/D3 receptor radioligands [11C]fallypride and [18F]fallypride Mingzhang Gao, Min Wang, Bruce H. Mock, Barbara E. Glick-Wilson, Karmen K. Yoder, Gary D. Hutchins, Qi-Huang Zheng  Department of Radiology, Indiana University School of Medicine, 1345 West 16th Street, L-3 Room 202, Indianapolis, IN 46202, USA

a r t i c l e in fo

abstract

Article history: Received 2 February 2009 Received in revised form 11 June 2009 Accepted 7 September 2009

Improved syntheses of dopamine D2/D3 receptor radioligands [11C]Fallypride and [18F]Fallypride are reported. The phenolic precursor (9) for C-11 labeling and the Fallypride (10) reference standard were synthesized from the starting material 2-hydroxy-3-methoxy-5-(2-propenyl)benzoic acid methyl ester (1) in 7 and 8 steps with 16% and 5% overall chemical yields, respectively. The tosylated precursor (15) for F-18 labeling was synthesized from compound 1 in 5 steps with 32% overall chemical yield. An alternate synthetic approach for Fallypride has been developed using the same starting material 1 in 5 steps with 26% overall chemical yield. [11C]Fallypride ([11C]10) was prepared by O-[11C]methylation of the phenolic precursor with [11C]methyl triflate and purified with a semi-preparative HPLC method in 50–60% radiochemical yield, decay corrected to end of bombardment (EOB), based on [11C]CO2, and 3707185 GBq/mmol specific radioactivity at EOB. [18F]Fallypride ([18F]10) was prepared by nucleophilic substitution of the tosylated precursor with K[18F]F/ Kryptofix 2.2.2 and HPLC combined with solid-phase extraction (SPE) purification in variable (up to 50%) decay corrected radiochemical yield from K[18F]F and 111–222 GBq/mmol specific activity at EOB. & 2010 Elsevier Ltd. All rights reserved.

Keywords: [11C]Fallypride [18F]Fallypride Positron emission tomography (PET) Imaging Dopamine D2/D3 receptor

1. Introduction Fallypride ((S)-N-((1-allylpyrrolidin-2-yl)methyl-5-(3-fluoropropyl)-2,3-dimethoxybenzamide) is a well-known dopamine D2/D3 antagonist with high affinity (Ki 30 pM) for D2 receptor sites (Mukherjee et al., 2004). The fluorine-18 labeled version of Fallypride is a commonly used radioligand for positron emission tomography (PET) examination of dopamine D2/D3 receptors in human brain and used extensively by many PET centers (Elsinga et al., 2006; Mukherjee et al., 1999, 1995). The carbon-11 labeled version of Fallypride has some advantages in back-to-back same-day PET studies, such as avoiding movement of the subject from the PET scanner and performing repeat studies within 2–3 h of the first study to explore drug effects. When pharmacological or behavioral challenges are being studied, these advantages will become very valuable (Mukherjee et al., 2004). Radiolabeled Fallypride is a useful PET brain imaging agent, and a broader research investigation to explore and validate the utility of [18F]Fallypride-PET and/or [11C]Fallypride-PET is important. However, the limited commercial availability, complicated synthetic procedures, and high costs of starting materials and precursors can present obstacles to more widespread evaluation of this intriguing agent.

Wishing to study this compound in our PET center, we decided to make our own material by following the literature methods. Although several papers dealing with the synthesis of [11C]Fallypride and [18F]Fallypride have appeared (Mukherjee et al., 2004, 1999, 1995), there are gaps in synthetic detail among them, and certain key steps gave poor yields or were difficult to repeat in our hands. Consequently, we investigated alternate approaches and modifications that eventually resulted in an improved synthesis of [11C]Fallypride and [18F]Fallypride starting from 2-hydroxy-3-methoxy-5-(2-propenyl)benzoic acid methyl ester (1). In this paper we provide more complete experimental procedures, yields, analytical details and new findings for the improved syntheses of Fallypride reference standard 10, phenolic precursor 9 (for carbon-11 labeling) and tosylated precursor 15 (for fluorine-18 labeling). In addition, we describe new, simplified radiolabeling and purification procedures for the production of [11C]Fallypride ([11C]10) using [11C]methyl triflate ([11C]CH3OTf) (Jewett, 1992; Mock et al., 1999), and for the production of [18F]Fallypride using K[18F]F/Kryptofix 2.2.2.

2. Results and discussion 2.1. Chemistry

 Corresponding author. Department of Radiology, Indiana University School of

Medicine, 1345 West 16th Street, L-3 Room 202, Indianapolis, IN 46202-2111, USA. Tel.: + 1 317 278 4671; fax: + 1 317 278 9711. E-mail address: [email protected] (Q.-H. Zheng). 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.09.071

The Fallypride standard compound (10) and its phenolic precursor (9) were prepared as shown in Scheme 1 using modifications of the literature procedures (Mukherjee et al., 2004).

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CO2CH3

CO2CH3

OH

1

OMEM

MEMCl, NaH, THF

OCH3 2 92%

1) B2H6, THF 2) 30% H2O2, 1 N NaOH

OCH3 CO2CH3

CO2CH3

OMEM

OMEM Bu4NF, THF

TsCl, DMAP, CH2Cl2 TsO 4 81%

HO

OCH3

3 66%

CO2CH3

CO2CH3

OMEM

OH

KOH, MeOH, HCl

TFA, CH2Cl2 F 5 80%

OCH3

F

OCH3

6 98% H2 N

OCH3

OCH3

H N

OH H NH

F

CO2H

8 N

O 9 43%

OH

BOP, Et3N, CH3CN F 7 96%

OCH3

CH3I/K2CO3 CH3COCH3 OCH3 OCH3 H NH

F

N O Fallypride, 10 31%

Scheme 1. Synthesis of the phenolic precursor 9 for C-11 labeling and the Fallypride 10 reference standard.

Commercially available compound 1 was protected with (2-methoxyethoxy)methyl (MEM) chloride to give the MEM ether compound 2 in 92% yield. The allyl group of 2 was converted to the corresponding alcohol 3 by treatment with dihydroborane followed by hydrogen peroxide and sodium hydroxide in 66% yield. Compound 3 was reacted with p-toluenesulfonyl chloride (TsCl) under basic conditions using 4-dimethylaminopyridine (DMAP) to provide tosylate 4 in 81% yield. The literature tosylation procedure using triethylamine (Mukherjee et al., 2004) gave very poor yields in our hands. Therefore, we used DMAP instead of triethylamine as a base for the tosylation reaction. Fluorination of compound 4 with tetrabutylammonium fluoride (TBAF) afforded fluoride 5 in 80% yield. Then, we reversed the reaction sequences of the literature procedures (Mukherjee et al., 2004). Compound 5 was deprotected by trifluoroacetic acid (TFA) to remove the MEM group to give phenol 6 in 98% yield first, which was then followed by saponification of the ester group to yield acid 7 in 96% yield. This modification significantly improved the yields and simplified the work-up procedures. Compound 7 was coupled with the commercially available amine 8 in an amide linkage using the coupling reagent benzotriazol-1-yloxy-tris(dimethoxyamino) phosphonium hexafluorophosphate (BOP) to give the phenolic precursor 9 in 43% yield. The direct methylation of compound 9 with methyl iodide (CH3I) afforded the standard compound Fallypride 10 in 31% yield. It is important to note no synthetic

procedure for Fallypride was given in the literature (Mukherjee et al., 2004). The overall chemical yields for the phenolic precursor 9 and Fallypride 10 were 16% (7 steps) and 5% (8 steps) from the starting material 1, respectively. In comparison with the literature (Mukherjee et al., 2004), the overall chemical yield for compound 9 was significantly improved from approximately 10% to 16% in seven steps. The synthesis of the tosylated precursor 15 is outlined in Scheme 2 using modifications of the literature procedures (Bishop et al., 1991; Mukherjee et al., 1995). The methylation of compound 1 with CH3I provided compound 11 in 95% yield. The allyl group of 11 was converted to the corresponding alcohol 12 by treatment with dihydroborane followed by hydrogen peroxide and base in 69% yield. The hydrolysis of the ester group of 12 afforded acid 13 in 97% yield. Compound 13 was coupled with the amine 8 in an amide linkage using the coupling reagent dicyclohexylcarbodiimide (DCC) to give the alcohol 14 in 58% yield. Compound 14 was reacted with TsCl and base DMAP to provide the tosylated precursor 15 in 86% yield. Likewise, using DMAP instead of pyridine (Mukherjee et al., 1995) as a base for the key step tosylation reaction significantly increased the yield of the tosylated precursor 15 from 70% to 86%. The overall chemical yield for the tosylated precursor 15 was 32% (5 steps) from compound 1. In order to obtain significant amount of authentic standard, an alternate synthetic approach for Fallypride was developed as

ARTICLE IN PRESS M. Gao et al. / Applied Radiation and Isotopes 68 (2010) 1079–1086

1

CO2CH3

CH3I, K2CO3 CH3COCH3

CO2CH3

1) B2H6, THF 2) 30% H2O2, 1 N NaOH

OCH3

11 95%

1081

OCH3

HO

OCH3

OCH3

12 69% KOH, MeOH, HCl

OCH3 CO2H

OCH3 H

OCH3

8 DCC, pyridine

NH

HO

N

HO

O 14 58%

OCH3

13 97%

TsCl, DMAP CH2Cl2 OCH3 OCH3 H NH

TsO

N O 15 86% Scheme 2. Synthesis of the tosylated precursor 15 for F-18 labeling.

CO2CH3 12

CO2H

OCH3

DAST, CH2Cl2, -78 oC

OCH3

KOH, MeOH, HCl F 16 75%

F

OCH3

17 98%

OCH3

8, BOP, Et3N Fallypride, 10 53% Scheme 3. Alternate synthetic approach for the Fallypride 10 reference standard.

shown in Scheme 3. The alcohol 12 was converted to the corresponding fluoride 16 by treatment with diethylaminosulfur trifluoride (DAST) in 75% yield. Saponification of the ester group of 16 gave the acid 17 in 98% yield. Compound 17 was coupled with the amine 8 using BOP provided Fallypride in 53% yield. The overall chemical yield for Fallypride was 26% (5 steps) from compound 1 using this alternate approach.

OCH3 O11CH3

11

9

[ C]CH3OTf, 2 N NaOH CH3COCH3, 70 oC

H F

NH N O [11C]Fallypride, [11C]10 50-60%

Scheme 4. Synthesis of [11C]Fallypride ([11C]10).

2.2. Radiochemistry 11

11

Synthesis of the tracer [ C]Fallypride ([ C]10) is indicated in Scheme 4. The phenolic precursor 9 was labeled using [11C]CH3OTf (Jewett, 1992; Mock et al., 1999) through O-[11C]methylation (Zheng et al., 2003) and isolated by a semipreparative HPLC method (Wang et al., 2007) to produce the corresponding pure radiolabeled compound [11C]10 in 50–60% radiochemical yield, decay corrected to end of bombardment (EOB), based on [11C]CO2. [11C]CH3OTf is a proven methylation reagent with greater reactivity than commonly used [11C]methyl

iodide ([11C]CH3I) (Mukherjee et al., 2004; Fei et al., 2004a), and thus, the radiochemical yields for [11C]Fallypride in our method is much higher 25–40% than that reported previously, decay corrected from [11C]CH3I (Mukherjee et al., 2004). The radiosynthesis was performed in an home-built automated multipurpose 11C-radiosynthesis module, allowing measurement of specific radioactivity during synthesis (Mock et al., 2005a, b). The overall synthesis, purification and formulation time was 25–30 min from EOB. The specific radioactivity was in a range of 185–555 GBq/mmol at EOB. Semi-preparative HPLC for C-11

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labeling was performed using a YMC-Pack ODS-A, S-5 mm, 12 nm, 10  250 mm reverse phase column; 30% CH3CN:70% 20 mM H3PO4 (pH 2.5) mobile phase; 5.0 mL/min flow rate; UV (280 nm) and g-ray (PIN diode) flow detectors. This HPLC system (acidic mobile phase) differed from that (basic mobile phase, 56% CH3CN:0.1% Et3N:43.9% H2O) reported in the literature (Mukherjee et al., 2004), as it gave better separation of [11C]Fallypride from the phenolic precursor and more accurate measurement of specific radioactivity of [11C]Fallypride. Chemical purity and radiochemical purity were determined by analytical HPLC (Zheng and Mock, 2005). Analytical HPLC was performed using a Prodigy (Phenomenex) 5 mm C-18 column, 4.6  250 mm; 65% CH3CN:35% 20 mM phosphate buffer (pH 6.7) mobile phase; flow rate 1.0 mL/min; and UV (280 nm) and g-ray (PIN diode) flow detectors. The chemical purity of the precursor and reference standard was 496%. The radiochemical purity of the target tracer was 499% determined by radio-HPLC through g-ray (PIN diode) flow detector, and the chemical purity of the target tracer was determined by reverse-phase HPLC through UV flow detector, which showed there was still a tiny amount of phenolic precursor (0.2–0.4 mg/mL) contaminated in the [11C]Fallypride tracer solution. Synthesis of the tracer [18F]Fallypride ([18F]10) is outlined in Scheme 5. The tosylated precursor 15 was labeled with K[18F]F/ Kryptofix 2.2.2 through nucleophilic substitution and isolated by a semi-preparative HPLC method (C-18 column) (Fei et al., 2004b) and a solid-phase extraction (SPE) method (C-18 Plus Sep-Pak cartridge) (a second purification or isolation process) to produce in the corresponding radiolabeled compound [18F]10 variable (up to 50%) decay-corrected radiochemical yield from K[18F]F. The most reproducible decay-corrected radiochemical yield was  20%. The radiosynthesis was performed using an automated multipurpose 18F-radiosynthesis module of our own design. The overall synthesis purification and formulation time was 60–70 min from EOB. The specific radioactivity was 111–222 GBq/mmol at EOB. No-carrier-added [18F]fluoride ion in [18O]water was trapped with or without a QMA cartridge. If the cyclotron-produced [18F]fluoride ion was trapped without the use of a cartridge, it would significantly increase the specific activity of the prepared [18F]Fallypride (Lu et al., 2009). The amounts of tosylated precursor used were 1–3 mg. The large amount of precursor would increase the radiochemical yield of [18F]Fallypride, but decrease the chemical purity of the [18F]Fallypride tracer solution due to precursor contamination. The reaction solvent and temperature were either CH3CN/120 1C or DMSO/150 1C. Using DMSO as the reaction solvent and running radiolabeling reaction at 150 1C would increase radiochemical yield (Kuhnast et al., 2009). Semi-preparative HPLC for F-18 labeling was performed using a YMC-Pack ODS-A, S-5 mm, 12 nm, 10  250 mm reverse phase column; 60% CH3CN:40% 0.1 M NaHCO3 (pH 10.0) mobile phase; 4.0 mL/min flow rate; UV (280 nm) and g-ray (PIN diode) flow detectors. Compared to the literature HPLC method (acidic mobile phase, 60% CH3CN:40% 0.01 M H3PO4) (Mukherjee et al., 1995), this HPLC system (basic mobile phase) gave better separation of [18F]Fallypride from the

OCH3

non-reacted K[18F]F/Kryptofix 2.2.2 and tosylated precursor, and non-radiolabeled undesired side-products, and a more accurate measurement of specific radioactivity of [18F]Fallypride. Chemical purity and radiochemical purity were determined by analytical HPLC (Zheng and Mock, 2005) using the same HPLC system aforementioned. The chemical purity of the precursor and reference standard was 496%. The radiochemical purity of the target tracer was 499% determined by radio-HPLC through g-ray (PIN diode) flow detector, and the chemical purity of the target tracer was determined by reverse-phase HPLC through UV flow detector, very tiny amount of tosylated precursor (0–0.3 mg/mL) contaminating the [18F]Fallypride tracer solution. Dual HPLC purification (Mukherjee et al., 1999, 1995) failed to remove most of non-radiolabeled undesired side-products. Therefore, we used a C-18 Plus Sep-Pak cartridge for this purpose, which significantly improved the chemical purity of the tracer solution. The chemical purity of the [18F]Fallypride tracer solution with Sep-Pak purification was increased higher 10–20% than that without Sep-Pak purification. In comparison with the results reported in the literature (Mukherjee et al., 2004, 1999, 1995), several improvements in the synthetic methodology for Fallypride, its phenolic and tosylated precursors, [11C]Fallypride and [18F]Fallypride have been made. The published synthetic approaches have been modified with moderate to excellent chemical yields. A convenient and reliable gas-phase production (GPP) method for the most commonly used radiolabeled precursor [11C]CH3OTf (Mock et al., 1999) has been developed. Automated multi-purpose 11C- and 18F-radiosynthesis modules of our own design for fully automated syntheses of [11C]Fallypride and [18F]Fallypride have been built, featuring HPLC combined with SPE for dual purification of [18F]Fallypride, high C-11 labeling yield and F-18 labeling yield, high specific radioactivity and radiochemical purity, and shortened synthesis time. New and efficient HPLC systems for the purification and quality control (QC) method of the target tracers [11C]Fallypride and [18F]Fallypride have also been developed.

3. Conclusion Improved and efficient syntheses of Fallypride, its phenolic precursor (for C-11 labeling) and its tosylated precursor (for F-18 labeling) have been developed. The phenolic precursor was labeled with [11C]CH3OTf, and isolated by a semi-preparative HPLC purification procedure to provide [11C]Fallypride in high radiochemical yields, short overall synthesis time, and good specific radioactivity. The tosylated precursor was labeled with K[18F]F/Kryptofix 2.2.2 through nucleophilic substitution and isolated by a semi-preparative HPLC combined with SPE method to produce [18F]Fallypride in high radiochemical yield and good specific radioactivity. The modifications and new findings in the synthetic methodology, radiolabeling, preparative separation and analytical details for Fallypride, phenolic precursor and tosylated precursor, [11C]Fallypride and [18F]Fallypride have been addressed. This improved method is efficient and convenient. It is anticipated that the approaches and improvements described here can be applied with advantage to the synthesis of other 11 C-and 18F-radioligands for PET imaging.

OCH3 H

18

15

K[ F]F, K2.2.2/K2CO3 18 F DMSO, 150 oC

NH N O [18F]Fallypride, [18F] 10 10-50%

Scheme 5. Synthesis of [18F]Fallypride ([18F]10).

4. Experimental 4.1. General All commercial reagents and solvents from Sigma-Aldrich and Fisher Scientific were used without further purification ‘hexanes’

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contains a mixture of isomers. Compound 1 was purchased from Sigma-Aldrich. [11C]CH3OTf was prepared according to a literature procedure (Mock et al., 1999). Melting points were determined on a MEL-TEMP II capillary tube apparatus and were uncorrected. 1 H NMR spectra were recorded on Varian Gemini 2000 200 MHz FT-NMR and Bruker Avance II 500 MHz NMR spectrometers using tetramethylsilane (TMS) as an internal standard. Chemical shift data for the proton resonances were reported in parts per million (ppm, d scale) relative to internal standard TMS (d 0.0), and coupling constants (J) were reported in hertz (Hz). Low resolution mass spectra (LRMS) were obtained using a Bruker Biflex III MALDI-Tof mass spectrometer. Chromatographic solvent proportions are indicated as volume: volume ratio. TLC was run using Analtech silica gel GF uniplates (5  10 cm). Plates were visualized under UV light. Preparative TLC was run using Analtech silica gel UV 254 plates (20  20 cm). Normal phase flash column chromatography was carried out on EM Science silica gel 60 (230–400 mesh) with a forced flow of the indicated solvent system in the proportions described below. All moisture- and/or air-sensitive reactions were performed under a positive pressure of nitrogen maintained by a direct line from a nitrogen source. QMAcarbonate and C18 Plus Sep-Pak cartridges were obtained from Waters Corporation (Milford, MA). Sterile Millex-GS and MillexFG 0.22 mm filter units were obtained from Millipore Corporation (Bedford, MA). 4.2. 3-Methoxy-2-[(2-methoxyethoxy)methoxy]-5-(2propenyl)benzoic acid methyl ester ð2Þ Compound 1 (12.0 g, 54.0 mmol), dissolved in THF (40 mL) was added dropwise to a suspended mixture of sodium hydride (60% dispersion in mineral oil, 4.0 g, 100 mmol) in THF (80 mL) at 0 1C, and the reaction was allowed to stir for 30 min at 0 1C. Then (2-methoxyethoxy)methyl chloride (9.41 g, 75.6 mmol) was added slowly. The reaction mixture was allowed to warm to room temperature (rt) for 3 h. After that the reaction was quenched by slow addition of water at 0 1C. The THF was removed by evaporation, and the mixture was extracted by CH2Cl2 (100 mL  3), dried with MgSO4, filtered, and concentrated. The crude product was purified by column chromatography on silica gel with eluent (1:2 EtOAc/hexanes) to afford 2 (15.4 g, 92%) as a yellow oil, Rf =0.70 (1:1 EtOAc/hexanes). 1H NMR (CDCl3): d 3.33 (d, J= 8.0 Hz, 2H, CH2), 3.37 (s, 3H, OCH3), 3.56 (t, J=4.6 Hz, 2H, CH2), 3.82 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.95 (t, J =4.6 Hz, 2H, CH2), 5.06 (dd, J= 1.4, 13.0 Hz, 2H,QCH2), 5.19 (s, 2H, OCH2O), 5.82–5.99 (m, 1H,QCH), 6.86 (d, J =1.4 Hz, 1H, Ph-H), 7.14 (s, 1H, Ph-H). MS (ESI): 333 ([M+ Na] + , 12%), 235 (100%). 4.3. Methoxy-2-[(2-methoxyethoxy)methoxy]-5-(3hydroxypropyl)benzoic methyl ester ð3Þ B2H6 (1 M, 30 mL) was added to a cooled solution of compound 2 (6.00 g, 19.4 mmol) in THF (50 mL), the mixture was stirred at 0 1C for 40 min followed by 1 h at rt. To the reaction mixture was carefully added 1 N NaOH (50 mL) at 0 1C, followed by 30% aqueous hydrogen peroxide (40 mL), and the reaction was continued to stir for 1.5 h. The THF was removed by rotary evaporation, and the aqueous solution was extracted with CH2Cl2 (80 mL  3). The organic layer was dried (MgSO4), filtered, and evaporated in vacuo to give crude product 3, which was purified by column chromatography on silica gel with eluent (2:3 EtOAc/ hexanes) to obtain pure 3 (4.19 g, 66%) as a clear oil, Rf = 0.22 (1:1 EtOAc/hexanes). 1H NMR (CDCl3): d 1.76 (s, 1H, OH), 1.79–1.90 (m, 2H, CH2), 2.67 (t, J= 7.6 Hz, 2H, CH2), 3.37 (s, 3H, OCH3), 3.57 (dd, J= 4.4, 6.0 Hz, 2H, CH2), 3.66 (t, J= 6.4 Hz, 2H, CH2), 3.83

1083

(s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.95 (dd, J= 4.4, 5.0 Hz, 2H, CH2), 5.19 (s, 2H, OCH2O), 6.88 (d, J=1.8 Hz, 1H, Ph-H), 7.16 (d, J= 1.8 Hz, 1H, Ph-H). MS (ESI): 351 ([M+Na] + , 100%). 4.4. 3-Methoxy-2-[(2-methoxyethoxy)methoxy]-5-(3-[(4methoxyphenyl)sulfonyloxy]propyl) benzoic acid methyl ester ð4Þ TsCl (3.24 g, 17.0 mmol) was slowly added to compound 3 (4.66 g, 14.4 mmol) in CH2Cl2 (100 mL) at 0 1C, followed by DMAP (2.49 g, 20.4 mmol). The reaction mixture was stirred at 0 1C for 1 h then at rt for 3 h. The reaction mixture was mixed with silica gel and directly purified by column chromatography on silica gel (20% EtOAc/hexanes) to provide 4 (5.54 g, 81%) as a colorless oil, Rf = 0.60 (1:1 EtOAc/hexanes). 1H NMR (CDCl3): d 1.88–2.01 (m, 2H, CH2), 2.44 (s, 3H, CH3), 2.63 (t, J =7.6 Hz, 2H, CH2), 3.37 (s, 3H, OCH3), 3.56 (t, J= 4.6 Hz, 2H, CH2), 3.82 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.94 (t, J= 4.6 Hz, 2H, CH2), 4.03 (t, J= 6.0 Hz, 2H, CH2), 5.19 (s, 2H, OCH2O), 6.83 (d, J= 1.8 Hz, 1H, Ph-H), 7.08 (d, J= 1.8 Hz, 1H, Ph-H), 7.32 (d, J= 8.0 Hz, 2H, Ph-H), 7.76 (d, J= 8.0 Hz, 2H, Ph-H). MS (ESI): 505 (M + Na, 40%), 483 ([M+H] + , 10%), 407 (100%). 4.5. 3-Methoxy-2-[(2-methoxyethoxy)methoxy]-5-(2fluoropropyl)benzoic acid methyl ester ð5Þ TBAF in THF (1 M, 17.7 mL) was added to a solution of compound 4 (5.14 g, 10.7 mmol) in THF (80 mL) at 0 1C and the reaction was stirred for 5 h. The solution was concentrated by evaporation under reduced pressure, and water (20 mL) was added. The mixture was extracted with CH2Cl2 (60 mL  3), dried with MgSO4 and concentrated, and then purified by column chromatography on silica gel (30% EtOAc/hexanes) to give 5 (2.81 g, 80%) as a brownish oil, Rf = 0.66 (1:1 EtOAc/hexanes). 1H NMR (CDCl3): d 1.88–2.12 (m, 2H, CH2), 2.72 (t, J= 7.6 Hz, 2H, CH2), 3.37 (s, 3H, OCH3), 3.56 (dd, J= 3.6, 5.6 Hz, 2H, CH2), 3.84 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.94 (dd, J= 3.6, 5.6 Hz, 2H, CH2), 4.36 (dt, J= 5.8, 47.2 Hz, 2H, CH2F), 5.19 (s, 2H, OCH2O), 6.87 (d, J= 2.0 Hz, 1H, Ph-H), 7.16 (d, J= 2.0 Hz, 1H, Ph-H). MS (ESI): 353 ([M+Na] + , 13%), 255 (100%). 4.6. Methyl 5-(3-fluoropropyl)-2-hydroxy-3-methoxybenzoate ð6Þ TFA (1.35 g, 10.9 mmol) was slowly added to a solution of compound 5 (1.20 g, 3.63 mmol) in CH2Cl2 (25 mL) at 0 1C. This mixture was allowed to stir for 45 min at 0 1C then at rt for 1 h. The solvent was evaporated under reduced pressure and a solution of NaHCO3 (5%, 20 mL) was added, extracted by CH2Cl2, washed with brine, dried with MgSO4, and concentrated in vacuo to afford 6 (0.86 g, 98%) as a yellow oil, which is pure enough for next step reaction without further purification, Rf = 0.72 (1:1 EtOAc/hexanes). 1H NMR (CDCl3): d 1.88–2.11 (m, 2H, CH2), 2.69 (t, J= 7.6 Hz, 2H, CH2), 3.89 (s, 3H, OCH3), 3.94 (s, 3H, OCH3), 4.36 (dt, J=5.8, 47.0 Hz, 2H, CH2F), 6.88 (d, J= 1.8 Hz, 1H, Ph-H), 7.25 (d, J= 1.8 Hz, 1H, Ph-H), 10.85 (s, 1H, OH). MS (ESI): 243 ([M+H] + , 12%), 211 (100%). 4.7. 2-Hydroxy-3-methoxy-5-(3-fluoropropyl)benzoic acid ð7Þ Compound 6 (0.84 g, 3.47 mmol) was dissolved in methanol (60 mL), and KOH (1.55 g, 27.8 mmol) was added, then the solution was stirred at rt overnight. Methanol was removed and a solution of HCl (2 N) was added to adjust pH 5 to produce a white precipitate, which was filtered, and dried in air to afford 7 (0.76 g, 96%) as a pure white solid, mp 114–116 1C, Rf = 0.22 (1:9 MeOH/CH2Cl2). 1H NMR (CDCl3): d 1.90–2.10 (m, 2H, CH2),

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2.71 (t, J=7.6 Hz, 2H, CH2), 3.91 (s, 3H, OCH3), 4.38 (dt, J=5.8, 47.4 Hz, 2H, CH2F), 6.94 (d, J=1.8 Hz, 1H, Ph-H), 7.34 (d, J= 1.8 Hz, 1H, Ph-H). MS (ESI): 229 ([M+H] + , 11%), 211 (100%).

4.8. (S)-N-[(1-allyl-2-pyrolidinyl)methyl]-2-hydroxy-3-methoxy-5(3-fluoropropyl)benzamide ð9Þ BOP (0.428 g, 0.97 mmol) and Et3N (0.2 mL) were added to a solution of compound 7 (0.20 g, 0.88 mmol) and (S)-(1-allylpyrrolidin-2-yl)methanamine 8 (0.14 g, 1.1 mmol) in acetonitrile (20 mL) at 0 1C. After stirring the reaction mixture at rt for 12 h, the solvent was removed in vacuo and to the residue was added CH2Cl2. It was washed with water and brine. The organic layer was dried with MgSO4, filtered, evaporated, and purified by preparative TLC (silica gel, 1:9 MeOH/CH2Cl2) to afford 9 (132 mg, 43%) as a yellow oil, Rf = 0.50 (1:9 MeOH/CH2Cl2). 1H NMR (CDCl3): d 1.76–1.97 (m, 4H, 2  CH2), 2.00–2.07 (m, 2H, CH2), 2.47–2.67 (m, 3H), 2.96–3.25 (m, 3H), 3.41–3.69 (m, 3H), 3.88 (s, 3H, OCH3), 4.36 (dt, J =5.8, 47.4 Hz, 2H, CH2F), 5.18–5.34 (m, 2H,QCH2), 5.80–6.00 (m, 1H,QCH), 6.81 (s, 1H, Ph-H), 6.91 (s, 1H, Ph-H), 7.64 (s, 1H). MS (ESI): 351 ([M+ H] + , 100%).

4.9. (S)-N-((1-Allylpyrrolidin-2-yl)methyl)-5-(3-fluoropropyl)-2,3dimethoxybenzamide (Fallypride, 10) Method A: Compound 9 (100 mg, 0.286 mmol), CH3I (53 mg, 0.37 mmol), and K2CO3 (51 mg, 0.37 mmol) were dissolved in acetone (5 mL) and the mixture was stirred at rt for 10 h. Then the mixture was filtered, evaporated, and purified by preparative TLC (silica gel, 1:9 MeOH/CH2Cl2) to yield 10 (32 mg, 31%) as a brownish oil. Method B: Compound 17 (0.31 g, 1.28 mmol) was dissolved in acetonitrile (40 mL) and to this solution was added compound 8 (0.18 g, 1.28 mmol), BOP (0.68 g, 1.54 mmol) and Et3N (0.16 g, 1.54 mmol). The reaction mixture was stirred at rt for 20 h. Solvents were removed in vacuo and the residue was dissolved in EtOAc, and washed with water. The organic layer was dried with MgSO4, filtered, evaporated, and purified by preparative TLC (silica gel, 1:9 MeOH/CH2Cl2) to yield 10 (247 mg, 53%) as a brownish oil, Rf =0.55 (1:9 MeOH/CH2Cl2). 1H NMR (CDCl3): d 1.87–2.11 (m, 5H), 2.16–2.27 (m, 2H, CH2), 2.73 (t, J=7.8 Hz, 2H, CH2), 2.80–2.92 (m, 1H), 3.34–3.49 (m, 2H), 3.74–3.79 (m, 3H), 3.88 (s, 3H, OCH3), 3.93 (s, 3H, OCH3), 4.36 (dt, J =5.8, 47.2 Hz, 2H, CH2F), 5.35–5.52 (m, 2H,QCH2), 5.87–6.04 (m, 1H,QCH), 6.92 (d, J= 2.0 Hz, 1H, Ph-H), 7.45 (d, J =2.0 Hz, 1H, Ph-H), 8.80 (s, 1H, CONH). MS (ESI): 365 ([M+ H] + , 100%).

4.10. Methyl 5-allyl-2,3-dimethoxybenzoate ð11Þ K2CO3 (8.29 g, 60 mmol) and CH3I (21.3 g, 150 mmol) were added to a solution of compound 1 (11.11 g, 50 mmol) in acetone (120 mL). The reaction mixture was stirred at rt for 3 h and heated at reflux for 15 h. After the mixture was cooled to rt, the salts were filtered off and washed with acetone, and the solution was evaporated to provide a crude oily product, which was purified by column chromatography on silica gel with eluent (1:5 EtOAc/ hexanes) to afford 11 (11.2 g, 95%) as a pale yellow oil, Rf =0.33 (1:3 EtOAc/hexanes). 1H NMR (CDCl3): d 3.33 (d, J =6.6 Hz, 2H, CH2), 3.86 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.89 (s, 3H, OCH3), 5.06 (s, 1H,QCHH), 5.12 (dd, J= 1.6, 3.4 Hz, 1H,QCHH), 5.83–6.04 (m, 1H, CH), 6.87 (d, J= 1.8 Hz, 1H, Ph-H), 7.13 (d, J=1.8 Hz, 1H, PhH). MS (ESI): 259 ([M+ Na] + , 15%), 205 (100%).

4.11. Methyl 5-(3-hydroxypropyl)-2,3-dimethoxybenzoate ð12Þ Compound 12 was prepared using the procedure described for compound 3 as a clear oil in 69% yield, Rf = 0.16 (1:3 EtOAc/ hexanes). 1H NMR (CDCl3): d 1.80–1.94 (m, 2H, CH2), 2.16 (s, 1H, OH), 2.68 (t, J= 7.6 Hz, 2H, CH2), 3.67 (t, J=6.4 Hz, 2H, CH2), 3.86 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.89 (s, 3H, OCH3), 6.88 (d, J=1.8 Hz, 1H, Ph-H), 7.14 (d, J= 1.8 Hz, 1H, Ph-H). MS (ESI): 261 ([M+ Li] + , 38%), 205 (100%). 4.12. 5-(3-Hydroxypropyl)-2,3-dimethoxybenzoic acid ð13Þ Compound 13 was prepared using the procedure described for compound 7 as a colorless oil in 97% yield, Rf = 0.15 (1:9 MeOH/ CH2Cl2). 1H NMR (CDCl3): d 1.81–2.16 (m, 2H, CH2), 2.71 (t, J= 7.6 Hz, 2H, CH2), 3.67 (t, J =6.4 Hz, 2H, CH2), 3.90 (s, 3H, OCH3), 4.04 (s, 3H, OCH3), 6.99 (d, J= 1.8 Hz, 1H, Ph-H), 7.25 (d, J= 1.8 Hz, 1H, Ph-H). MS (ESI): 263 ([M+ Na] + , 11%), 223 (100%). 4.13. (S)-N-((1-Allylpyrrolidin-2-yl)methyl)-5-(3-hydroxypropyl)2,3-dimethoxybenzamide ð14Þ Compound 13 (0.36 g, 1.5 mmol), compound 8 (0.21 g, 1.5 mmol) and pyridine (0.126 g, 1.6 mmol) were dissolved in dichloromethane (40 mL). The mixture was cooled to 0 1C, and DCC (0.33 g, 1.6 mmol) was added. The reaction mixture was allowed to stir at 0 1C for 3 h and subsequently at rt for 15 h. The precipitated diclohexylurea was filtered and the filtrate was washed with water. The organic layer was dried over MgSO4, filtered, and evaporated. The residue was purified by column chromatography (silical gel, 3% MeOH/CH2Cl2) to afford 14 (0.315 g, 58%) as a light yellow oil, Rf =0.21 (1:9 MeOH/CH2Cl2). 1 H NMR (CDCl3): d 1.71 (dd, J= 7.8, 13.0 Hz, 2H, CH2), 1.84 (dd, J=6.4, 8.4 Hz, 2H, CH2), 2.16–2.31 (m, 1H), 2.86 (t, J= 7. 4 Hz, 2H, CH2), 2.89 (dd, J= 7.8, 13.6 Hz, 2H, CH2), 3.10–3.14 (m, 1H), 3.36 (ddd, J= 3.0, 4.6, 13.6 Hz, 1H), 3.45 (dd, J= 5.0, 13.6 Hz, 1H), 3.65 (t, J=6.4 Hz, 2H, CH2), 3.75 (ddd, J =2.6, 7.2, 14.0 Hz, 1H), 3.87 (s, 3H, OCH3), 3.88 (s, 3H, OCH3), 5.06–5.23 (m, 2H,QCH2), 5.80–6.01 (m, 1H,QCH), 6.88 (d, J= 2.0 Hz, 1H, Ph-H), 7.52 (d, J= 2.0 Hz, 1H, Ph-H), 8.43 (d, J= 4.8 Hz, 1H, CONH). MS (ESI): 363 ([M+H] + , 100%). 4.14. (S)-3-(3-((1-Allylpyrrolidin-2-yl)methylcarbamoyl)-4,5dimethoxyphenyl)propyl 4-methylbenzenesulfonate ð15Þ Compound 15 was prepared using the procedure described for compound 4 as a brownish oil in 86% yield, Rf = 0.44 (1:9 MeOH/ CH2Cl2). 1H NMR (CDCl3): d 1.71–1.78 (m, 2H, CH2), 1.96 (t, J= 7.2 Hz, 2H, CH2), 2.08–2.34 (m, 2H), 2.44 (s, 3H, CH3), 2.65 (t, J= 7.6 Hz, 2H, CH2), 2.73 (t, J= 7.2 Hz, 1H), 2.92 (dd, J =7.6, 13.6 Hz, 1H), 3.19 (s, 1H), 3.34 (d, J =13.8 Hz, 1H), 3.49 (dd, J =6.5, 13.0 Hz, 2H, CH2), 3.72–3.82 (m, 1H), 3.87 (s, 3H, OCH3), 3.88 (s, 3H, OCH3), 4.02 (t, J=6.2 Hz, 2H, CH2), 5.09–5.26 (m, 2H,QCH2), 5.81–6.01 (m, 1H,QCH), 6.83 (d, J=2.0 Hz, 1H, Ph-H), 7.32 (d, J =8.2 Hz, 2H, Ph-H), 7.49 (d, J =2.0 Hz, 1H, Ph-H), 7.76 (d, J=8.2 Hz, 2H, Ph-H), 8.44 (d, J= 4.0 Hz, CONH). MS (ESI): 517 ([M+H] + , 100%). 4.15. Methyl 5-(3-fluoropropyl)-2,3-dimethoxybenzoate ð16Þ A solution of compound 12 (5.28 g, 23.78 mmol) in CH2Cl2 (60 mL) was cooled to  78 1C, and a solution of DAST (4.6 g, 28.53 mmol) in CH2Cl2 (10 mL) was added dropwise over 15 min. The reaction mixture was allowed to warm to rt and then stirred at rt for 16 h. After the mixture was cooled to 0 1C, 30 mL of water

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was added to quench the reaction. The layers were separated, and extracted with additional CH2Cl2 (30 mL  2). The combined organic layer was washed by aqueous NaHCO3 (20 mL), dried with Na2SO4, filtered, and the resultant residue was purified by column chromatography (silica gel, 30% EtOAc/hexanes) to give 16 (3.99 g, 75%) as a yellowish oil, Rf =0.61 (1:3 EtOAc/hexanes). 1 H NMR (CDCl3): d 1.87–2.16 (m, 2H, CH2), 2.72 (t, J =7.6 Hz, 2H, CH2), 3.87 (s, 3H, OCH3), 3.88 (s, 3H, OCH3), 3.90 (s, 3H, OCH3), 4.37 (dt, J= 6.0, 47.0 Hz, 2H, CH2F), 6.88 (d, J= 1.8 Hz, 1H, Ph-H), 7.14 (d, J =1.8 Hz, 1H, Ph-H). MS (ESI): 279 ([M+ Na] + , 12%), 225 (100%). 4.16. 5-(3-Fluoropropyl)-2,3-dimethoxybenzoic acid ð17Þ Compound 17 was prepared using the procedure described for compound 7 as a yellowish oil in 98% yield, Rf = 0.30 (1:9 EtOH/ CH2Cl2). 1H NMR (CDCl3): d 1.88–2.14 (m, 2H, CH2), 2.75 (t, J= 7.6 Hz, 2H, CH2), 3.91 (s, 3H, OCH3), 4.05 (s, 3H, OCH3), 4.36 (dt, J=7.6, 47.2 Hz, 2H, CH2F), 6.98 (d, J=1.8 Hz, 1H, Ph-H), 7.54 (d, J= 1.8 Hz, 1H, Ph-H). MS (ESI): 265 ([M+Na] + , 10%), 225 (100%).

1085

loop for introduction to the semipreparative reverse-phase (C18) HPLC column for initial purification of the labeled product. Using a mobile phase of 60% CH3CN:40% 0.1 M NaHCO3 (pH 10.0) and a flow rate of 4 mL/min, the fraction containing [18F]10 eluted from the column with a retention time of 9 min, and was automatically diverted to a second reaction vial for evaporation and dilution with 10 mL water. The diluted tracer solution was then passed through a C-18 Sep-Pak Plus cartridge, and washed with water (5 mL  3). The cartridge was eluted with EtOH (1 mL  2) to release [18F]10. The eluted product was then sterile-filtered through a Millex-FG 0.22 mm membrane into a sterile vial and formulated with 10 mL saline. Total radioactivity was assayed and total volume was noted for tracer dose dispensing. Retention times in the semi-preparative HPLC system were: tR 15= 23.0 min, tR 10 = 14.0 min, tR [18F]10 =14.0 min. Retention times in the analytical HPLC system were: tR 15= 8.7 min, tR 10= 5.4 min, tR [18F]10= 5.4 min. The overall decay-corrected radiochemical yield varied from 10% up to 50% from K[18F]F. The typical decaycorrected radiochemical yield was  20% from K[18F]F.

Acknowledgments 4.17. [11C]Fallypride ð½11 C10Þ [11C]CO2 (  37 GBq) was produced by the 14N(p,a)11C nuclear reaction on ultra high purity nitrogen ( + 1% O2) in the small volume (9.5 cm3) aluminum gas target of the Siemens Eclipse RDS-111 cyclotron. Precursor 9 (0.1 mg) was dissolved in acetone (300 mL) and added to the 5 mL reaction vial of the methylation module, along with 2 mL of 2 N NaOH. [11C]CH3OTf was passed into the reaction vial at rt until radioactivity reached a maximum, and then the reaction vial was isolated and heated at 70 1C for 3 min. The reaction mixture was cooled to  50 1C, diluted with 1 mL of 0.1 M NaHCO3 and injected onto the semi-preparative HPLC column through a 2 mL injection loop. Using a mobile phase of 30% CH3CN:70% 20 mM H3PO4 (pH 2.5) mobile phase and flow rate 5.0 mL/min flow rate, the fraction containing [11C]10 eluted from the column with a retention time of 9 min. The product fraction was collected, the solvent was removed by rotatory evaporation under vacuum, and the final product, [11C]10, was formulated in saline, sterile-filtered through a sterile vented Millex-GS 0.22 mm cellulose acetate membrane, and collected into a sterile vial. Total radioactivity was assayed and total volume was noted for tracer dose dispensing. Retention times in the semipreparative HPLC system were: tR 9= 6.70 min, tR 10 =8.98 min, tR [11C]10= 8.98 min. Retention times in the analytical HPLC system were: tR 9 =4.35 min, tR 10= 5.42 min, tR [11C]10 =5.42 min. The radiochemical yields were 50–60% decay corrected to EOB, based on [11C]CO2. 4.18. [18F]Fallypride ð½18 F10Þ No-carrier-added (NCA) aqueous H[18F]F was produced by O(p,n)18F nuclear reaction using a Siemens Eclipse RDS-111 cyclotron by irradiation of H2 18 O (2.5 mL). [18F]Fluoride (7.4–18.5 GBq) in [18O]water plus 0.1 mL K2CO3 solution (1.7 mg) and Kryptofix 2.2.2 (10 mg) in 1.0 mL CH3CN were placed in the fluorination reaction vial (5-mL V-vial) and repeated azeotropic distillation was performed at 110 1C to remove water and form the anhydrous K[18F]F-Kryptofix 2.2.2 complex. Tosylated precursor 15 (1 mg) dissolved in DMSO (1.0 mL) was introduced to the reaction vessel and heated at 150 1C for 20 min to affect radiofluorination. After cooling to  90 1C, 1 mL DMSOH2O (2:8 v/v) was added to the reaction vessel for dilution, at which time the reaction mixture was loaded into a 3-mL injection 18

This work was partially supported by the Indiana Genomics Initiative (INGEN) of Indiana University, which is supported in part by Lilly Endowment Inc. 1H NMR spectra were recorded on a Bruker Avance II 500 MHz NMR spectrometer in the Department of Chemistry and Chemical Biology at Indiana University Purdue University Indianapolis (IUPUI), which is supported by a NSF-MRI grant CHE-0619254. The referees criticisms and editor’s comments for the revision of the manuscript are greatly appreciated.

References Bishop, J.E., Mathis, C.A., Gerdes, J.M., Whitney, J.M., Eaton, A.M., Mailman, R.B., 1991. Synthesis and in vitro evaluation of 2,3-dimethoxy-5-(fluoroalkyl)substituted benzamides: high-affinity ligands for CNS dopamine D2 receptors. J. Med. Chem. 34, 1612–1624. Elsinga, P.H., Hatano, K., Ishiwata, K., 2006. PET tracers for imaging of the dopaminergic system. Curr. Med. Chem. 13, 2139–2153. Fei, X., Mock, B.H., DeGrado, T.R., Wang, J.-Q., Glick-Wilson, B.E., Sullivan, M.L., Hutchins, G.D., Zheng, Q.-H., 2004a. An improved synthesis of PET dopamine D2 receptors radioligand [11C]raclopride. Syn. Commun. 34, 1897–1907. Fei, X., Wang, J.-Q., Miller, K.D., Sledge, G.W., Hutchins, G.D., Zheng, Q.-H., 2004b. Synthesis of [18F]Xeloda as a novel potential PET radiotracer for imaging enzymes in cancers. Nucl. Med. Biol. 31, 1033–1041. Jewett, D.M., 1992. A simple synthesis of [11C]methyl triflate. Appl. Radiat. Isot. 43, 1383–1385. Kuhnast, B., Hinnen, F., Dolle, F., 2009. Production of [18F]Fallypride on a TRACERLab FXFN synthesizer. J. Label. Compd. Radiopharm. 52, S286. Lu, S., Giamis, A.M., Pike, V.W., 2009. Synthesis of [18F]Fallypride in a microreactor: Rapid optimization and multiple-production in small doses for microPET studies. Cur. Radiopharm. 2, 49–55. Mock, B.H., Mulholland, G.K., Vavrek, M.T., 1992. Convenient gas phase bromination of [11C]methane and production of [11C]methyl triflate. Int. J. Rad. Appl. Instrum. A 43, 1383–1385. Mock, B.H., Zheng, Q.-H., DeGrado, T.R., 2005a. A multi-purpose 11C-radiosynthesis system. J. Label. Compd. Radiopharm. 48, S225. Mock, B.H., Glick-Wilson, B.E., Zheng, Q.-H., DeGrado, T.R., 2005b. Automated measurement of specific activity of radiolabeled ligands during synthesis. J. Label. Compd. Radiopharm. 48, S224. Mukherjee, J., Shi, B., Christian, B.T., Chattopadhyay, S., Narayanan, T.K., 2004. 11CFallypride: radiosynthesis and preliminary evaluation of a novel dopamine D2/D3 receptor PET radiotracer in non-human primate brain. Bioorg. Med. Chem. 12, 95–102. Mukherjee, J., Yang, Z.-Y., Brown, T., Lew, R., Wernick, M., Ouyang, X., Yasillo, N., Chen, C.-T., Mintzer, R., Cooper, M., 1999. Preliminary assessment of extrastriatal dopamine D-2 receptor binding in the rodent and nonhuman primate brains using the high affinity radioligand, 18F-fallypride. Nucl. Med. Biol. 26, 519–527. Mukherjee, J., Yang, Z.-Y., Das, M.K., Brown, T., 1995. Fluorinated benzamide neuroleptics-III. Development of (S)-N-[(1-allyl-2-pyrrolidinyl)methyl]-5-(3[18F]fluoropropyl)-2,3-dimethoxybenzamide as an improved dopamine D-2 receptor tracer. Nucl. Med. Biol. 22, 283–296.

ARTICLE IN PRESS 1086

M. Gao et al. / Applied Radiation and Isotopes 68 (2010) 1079–1086

Wang, M., Lacy, G., Gao, M., Miller, K.D., Sledge, G.W., Zheng, Q.-H., 2007. Synthesis of carbon-11 labeled sulfonanilide analogues as new potential PET agents for imaging of aromatase in breast cancer. Bioorg. Med. Chem. Lett. 17, 332–336. Zheng, Q.-H., Liu, X., Fei, X., Wang, J.-Q., Ohannesian, D.W., Erickson, L.C., Stone, K.L., Hutchins, G.D., 2003. Synthesis and preliminary biological evaluation of

radiolabeled O6-benzylguanine derivatives, new potential PET imaging agents for the DNA repair protein O6-alkylguanine-DNA alkyltransferase in breast cancer. Nucl. Med. Biol. 30, 405–415. Zheng, Q.-H., Mock, B.H., 2005. Purification of carbon-11 PET radiotracers from unlabeled precursors by preparative HPLC and SPE. Biomed. Chromatogr. 19, 671–676.