WO2011095593A1 - Iodo precursor for a pet imaging agent of amyloid plaques - Google Patents

Abstract

The invention relates to a precursor for a [¹⁸F]-labeled PET tracer for imaging of Alzheimer's Disease, its synthesis and the process for preparing the respective [¹⁸F]-labeled PET tracer.

Description

lodo precursor for a PET imaging agent of amyloid plaques

The present invention is directed to a novel precursor of a Positron Emitting Tomography (PET) imaging agent for binding and imaging amyloid deposits and the process for producing said imaging agent.

Background of the Invention

Alzheimer's disease (AD) is a progressive neurodegenerative disorder marked by loss of memory, cognition, and behavioral stability. AD is defined pathologically by extracellular senile plaques comprised of fibrillar deposits of the beta-amyloid peptide (Αβ) and neurofibrillary tangles comprised of paired helical filaments of hyperphosphorylated tau. The 39 to 43 amino acids comprising Αβ peptides are derived from the larger amyloid precursor protein (APP). In the amyloidogenic pathway, Αβ peptides are cleaved from APP by the sequential proteolysis of β- and γ-secretases. Αβ peptides are released as soluble proteins and can be detected at low levels in the cerebrospinal fluid (CSF) in normal aging brains. During the progress of AD the Αβ peptides aggregate and form amyloid deposits in the parenchyma and vasculature of the brain, which can be detected post mortem as diffuse and senile plaques and vascular amyloid during histological examination (for a recent review see: Blennow et al. Lancet. 2006 Jul 29;368(9533):387-403).

Alzheimer's disease is becoming a great health and social economical problem all over the world. There are great efforts being made to develop techniques and methods for the early detection and effective treatment of the disease. Currently, diagnosis of AD in an academic setting of memory-disorder clinics is approximately 85-90% accurate (Petrella JR et al. Radi- ology. 2003 226:315-36). It is based on the exclusion of a variety of diseases causing similar symptoms and the careful neurological and psychiatric examination, as well as neuropsychological testing. However, post mortem histological examination of the brain is still the only definite diagnosis of this disease. Thus the in vivo detection of one pathological feature of the disease - the deposition of amyloid aggregates in the brain - is thought to have a big impact on the early detection of AD and differentiation from other dementias. Additionally, most disease modifying therapies that are under development are aiming at lowering the amyloid load in the brain. Thus imaging the amyloid load in the brain may provide an essential tool for patient stratification and treatment monitoring. In addition, amyloid deposits are also known to play a role in amyloidoses, in which amyloid proteins are abnormally deposited in different organs and/or tissues, causing disease. For a recent review see Chiti et al. Annu Rev Biochem. 2006;75:333-66. Potential ligands for visualizing amyloid aggregates in the brain must show a high binding affinity to amyloid and must cross the blood brain barrier. PET tracers that have been already investigated in humans regarding their binding patterns in brains of AD patients are [F- 18]FDDNP (Shoghi-Jadid et. al, Am J Geriatr Psychiatry 2002; 10:24-35), [C-1 1]PIB (Klunk e. al, Ann Neurol. 2004 55:306-319), [C-1 1]SB-13 (Verhoeff et. al, Am J Geriatr Psychiatry 2004; 12:584-595, [F-18]Bay 94-9172 (Rowe et al. Lancet Neurol 2008, 7: 129-135), [C- 1 1 ]BF227 (Kudo et. al, J Nucl. Med 2007; 49:554-561 ), and [F-18]PIB (Farrar et. al Turku PET Symposium 2007, Abstract 49). For recent reviews see Lockhardt, Drug Discov Today, 2006 1 1 :1093-1099, Henriksen et al., Eur. J. Nucl. Med. Mol. Imaging 2007, Cohen, Mol. Im- aging Biol. 2007 9:204-216, Nordberg, Curr. Opin Biol. 2007, 20:398-402, Small et al., Neurology 2008 7:161 -172, Nordberg, Eur. J. Nucl. Med. Mol. Imaging 2008, 35, S46-S50.

Besides their specific binding to amyloid deposits in the brain, the currently most promising PET tracers show a disadvantageous non-specific accumulation, especially in white matter brain regions in AD patients as well as in healthy controls. Generally, non-specific back- ground binding interferes with the image quality and could e.g. impair the quantification of amyloid and the diagnosis of very early stages of the disease.

Recently, the compound of formula

has been found to be a suitable PET tracer for the detection of amyloid deposits in patients with amyloid-related diseases with high specificity at an early stage of the disease (PCT/EP2009/006406). Hence, there is a need for a commercially useful process for producing the tracer with high yield and for a precursor useful in such method. This problem has been solved by the provision of a precursor using Iodine as a leaving group.

Description of the invention

F-18 labeled pyridine analogues are generally synthesized by heteroaromatic nucleophilic substitution reactions. Described leaving groups are: Chloro, Bromo, lodo, Nitro, and Trimethyl ammonium groups. Wherein this pool Nitro and Trimethyl ammonium are widely considered to be the most reactive in regard to be substituted by F-18 followed by Bromo and Chloro. The lodo leaving group is considered to be the less reactive one. [1] Only in one example Zhang and Horti describe the lodo as beneficial over the Bromo leaving group. [2] They describe poor yields (2-7%) for their Bromopyridine precursor if one uses [¹⁸F]Fluoride/Kryptofix 222/Potassium carbonate (K₂C0₃) in Dimethyl sulfoxide (DMSO). When they used the respective lodo precursor the radiochemical yield was 6 to 8 % higher and reached an absolute level of 8-15% yield.

We surprisingly found the lodo Precursor 1 d to be more reactive than the Bromo precursor 2a and therefore more applicable for said radiofluorination, i.e. the synthesis of the [¹⁸F] Tracer 3. In contrast to Zhang, who uses [¹⁸F]Fluoride/Kryptofix 222/K₂C0₃ in DMSO, we used a different fluorination protocol and we found a much higher fluorination yield than one

[¹⁸F] Tracer 3

Using [¹⁸F]fluoride/Tetrabutylammonium hydroxide (TBAOH) in DMSO for Bromo precursor 2a we achieved a radiochemical yield of -15 % corrected for decay. When we switched to the lodo precursor 1 d the radiochemical yield was 40% higher and reached an absolute value of 56% radiochemical yield corrected for decay.

Thus, one embodiment of the invention is the compound of the formula

or a suitable salt thereof, preferably a pharmaceutically acceptable salt thereof.

In the context of the present invention, preferred suitable salts are pharmaceutically accept- able salts of the compounds according to the invention. The invention also comprises salts which for their part are not suitable for pharmaceutical applications, but which can be used, for example, for isolating or purifying the compounds according to the invention.

Pharmaceutically acceptable salts of the compounds according to the invention include acid addition salts of mineral acids, carboxylic acids and sulphonic acids, for example salts of hy- drochloric acid, hydrobromic acid, sulphuric acid, phosphoric acid, methanesulphonic acid, ethanesulphonic acid, toluenesulphonic acid, benzenesulphonic acid, naphthalene disul- phonic acid, acetic acid, trifluoroacetic acid, propionic acid, lactic acid, tartaric acid, malic acid, citric acid, fumaric acid, maleic acid and benzoic acid. Furthermore, another embodiment of the invention is a method for the preparation of a compound of the formula

is reacted with a radiofluorination agent.

An even more preferred embodiment of the invention is a method for the preparation of a compound according to formula

is reacted with a radiofluorination agent at elevated temperatures in organic solvents or mix- tures thereof.

Temperature ranges can be 140 - 220 °C, preferably 160 - 200 °C, and even more preferably 170 - 190 °C, even more preferably 175 - 185. °C.

Optionally, a microwave reactor may be used. In that case the reaction may be performed at the same or even more elevated temperatures in shorter reaction times.

In a preferred embodiment [¹⁸F]fluoride/TBAOH and a solvent is used as a fluorination agent.

In a preferred embodiment, the fluorination agent is 4,7,13, 16,21 ,24-Hexaoxa-1 , 10 diazabi- cyclo[8.8.8]-hexacosane K¹⁸F (crownether salt Kryptofix K¹⁸F), K¹⁸F, H¹⁸F, KH¹⁸F₂ or tetraal- kylammonium salt of ¹⁸F. More preferably, the fluorination agent is K¹⁸F, H¹⁸F, or KH¹⁸F₂.

The solvents used can be Ν,Ν-Dimethylformamide (DMF), Dimethylsulfoxyde (DMSO), Ace- tonitrile (MeCN), Ν,Ν-Dimethylacetamide (DMA) etc., preferably DMSO, MeCN or DMF. The solvents can also be a mixture of solvents as indicated above.

One further aspect of the invention is a method for the preparation of the compound of the formula

wherein the compound of formula

is reacted with 2-iodopyridine-4-carboxylic acid.

One preferred aspect of the invention is a method for the preparation of the compound of the formula

wherein a salt of the compound of formula

is reacted with 2-iodopyridine-4-carboxylic acid. Preferably the salt is the hydrochloride.

One further aspect of the invention is a method for the preparation of the compound of the formula

wherein the compound of formula

is deprotected and then reacted with 2-iodopyridine-4-carboxylic acid.

F-18 radiolabeling procedures

[F-18] radiolabeling procedures are well known to the person skilled in the art. For example, radiolabeling can be preformed as described in the following.

[F-18]Fluoride can be produced by proton bombardment in a cyclotron using a silver target (1 ml.) filled with [0-18] water for the ¹⁸0 (p,n)¹⁸F reaction. The aqueous [F-18]fluoride can be passed through a cartridge (e.g. QMA-resin cartridge Waters, Sep Pak Light QMA Part.No.: WAT023525 ). The trapped [F-18]fluoride can then be eluted from the cartridge by adding e .g . a Kryptofix K2.2.2/ K₂C0₃ solution (Kryptofix is 4,7, 13, 16,21 ,24-Hexaoxa-1 , 10- diazabicyclo[8.8.8]-hexacosane). The nucleophilic substitution of the precursor works pref- erably in the presence of a base such as Tetrabutylammonium hydroxide (NBu₄OH), Tetrabutylammonium carbonate ((NBu₄)₂C0₃), Tetrabutylammonium hydrogencarbonate (NBu₄HC0₃), K₂C0₃ etc. and at elevated temperatures. The addition of crown ethers such as Kryptofix (K2.2.2) can influence the reaction positively, especially in the presence of K₂C0₃ as the base.

The potassium fluoride Kryptofix complex is preferably dried by repeated azeotropic distillation with sequential addition of acetonitrile. Solvents such as acetonitrile, DMF, DMSO etc. can be used as a reaction solvent. The labeling product can be purified by solid phase extraction using cartridges. Preferred cartridges are Sep-Pak Plus C18 cartridge (Waters, WAT020515). The cartridge can be rinsed with water and the compound can be eluted with acetonitrile. The eluted compound can be diluted with water and can then be subjected to preparative HPLC purification. Preferred HPLC columns are reversed phase columns such as Gemini 5 μ C 18 1 10 A, 250 * 10 mm (Phenomenex, 00G-4435-N0). Mixtures of buffer solution, acids, water etc. with organic solvents such as acetonitrile, methanol, ethanol etc. can be used as mobile.

The solution can then be diluted with e.g. water to be passed through a cartridge for concentration and solvent change. [1] Dolle F.; [¹⁸F]Fluoropyridines: From conventional radiotracers to the labeling of macro- molecules such as proteins and oligonucleotides. Ernst Schering Research Foundation workshop(62): 113-57, 2007, ISBN 978-3-540-32623-6.

[2] Zhang Y.; Synthesis of 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-[¹⁸F]fluoropyridin-4- yl)pyridine ([¹⁸F]NIDA522131 ), a novel potential radioligand for studying extrathalamic nicotinic acetylcholine receptors by PET. J Label Compd Radiopharm 2004; 47: 947-952.

Brief description of the figures

Figure 1 : Preparative HPLC chromatogram for purification of 3 starting from precursor 1 d. Figure 2: Analytical HPLC chromatogram of 3 starting from precursor 1 d (Gamma detection). Figure 3: Analytical HPLC chromatogram of 2b (UV detection) corresponding to Fig. 2. Figure 4: Preparative HPLC chromatogram for purification of 3 starting from precursor 2a. Figure 5: Analytical HPLC chromatogram of 3 starting from precursor 2a (Gamma detection).

Figure 6: Analytical HPLC chromatogram of 2b (UV detection) corresponding to Fig. 5.

Examples

A method for synthesizing and labeling is exemplified in the following Examples. These Examples illustrate certain aspects of the above-described method and advantageous results and are shown by way of illustration and not by way of limitation.

Example 1

N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1 -yl}-2-oxoethyl)-2-iodopyridine-4-carboxamide -Benzyloxy-2-bromo-pyridine

To a solution of 1 0.0 g (57.47 m mol) of 2-bromo-5-hydroxypyridine in 400 ml. N,N- dimethylformamide (DMF) was added 14.75 g (86.21 mmol) of benzyl bromide and 23.82 g (172.4 mmol) of potassium carbonate. The mixture was stirred for 6 h at 60°C and overnight at room temperature. The suspension was filtered off and after evaporation of the solvent the residue was chromatographed on silica gel using a dichloromethane/methanol gradient. Yield: 14.82 g (96.7 %).

MS (ESIpos): m/z = 264, 266 [M+H]⁺

¹H-NMR (300MHz, CHLOROFORM-d): δ [ppm]

6H), 8.14 (d, 1 H).

-(5-Benzyloxy-pyridin-2-yl)-piperazine

All glassware was dried at 100°C. To a solution of 5.27 g (61.22 mmol) of piperazine in 180 ml. toluene was added 561 mg (0.61 mmol) of tris(dibenzylidene acetone) dipalladium(O) and 520 mg (0.83 mmol) of BINAP (2,2'-bis(diphenylphosphino)-1 , 1 -binaphthyl). Then, a solution of 14.7 g (55.66 mmol) of 5-benzyloxy-2-bromo-pyridine (example 1 a) in tetrahydrofuran (THF) was added followed by a suspension of 8.02 g (83.48 mmol) of sodium t-butylate in THF.

The reaction mixture was refluxed for 6 h and stirred at room temperature overnight. After evaporation of the solvents the residue was chromatographed on silica gel using a dichloro- methane/methanol gradient.

Yield: 7.12 g (47.0 %).

MS (ESIpos): m/z = 270 [M+H]⁺

¹H-NMR (300MHz, CHLOROFORM-d): δ [ppm]= 2.97 - 3.07 (m, 4H), 3.36 - 3.46 (m, 4H), 5.04 (s, 2H), 6.63 (d, 1 H), 7.21 (dd, 1 H), 7.29 - 7.48 (m, 5H), 8.00 (d, 1 H). zin-1 -yl}-2-oxoethyl)carbamate

To a solution of 4.63 g (26.43 mmol) i-Butoxycarbonyl-glycine (Aldrich) in 500 mL THF and 5 mL triethyl amine (35.87 mmol) at -15°C, 3.43 mL (26.43 mmol) isobutyl chloroformate were added dropwise and the solution was maintained at this temperature for another 15 min. Then, 7.12 g of 1 -(5-Benzyloxy-pyridin-2-yl)-piperazine (1 b) and 18 mL triethyl amine (129 mmol) in 200 mL THF/dichloromethane (1 : 1 ) were added slowly to this cold solution, the temperature was kept below -10°C for another 15 min and was then allowed to reach room temperature. After stirring overnight the solvent was evaporated and the residue was taken up in ethyl acetate. This solution was washed successively with aqueous sodium carbonate, water, 1 M aqueous hydrochloric acid (HCI) solution, saturated aqueous sodium chloride solution, finally dried over magnesium sulfate and then evaporated. This residue was chroma- tographed on silica gel using a hexane/ethyl acetate gradient.

Yield: 8.04 g (70.6 %).

MS (ESIpos): m/z = 427 [M+H]⁺

¹H-NMR (300MHz, CHLOROFORM-d): δ [ppm]= 1.46 (s, 9H), 3.36 - 3.45 (m, 2H), 3.51 (br. s., 4H), 3.70 - 3.81 (m, 2H), 4.02 (d, 2H), 5.05 (s, 2H), 5.53 (br. s., 1 H), 6.65 (d, 1 H), 7.23 (dd, 1 H), 7.30 - 7.48 (m, 5H), 8.00 (d, 1 H).

d) N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl)-2-iodopyridine-4- carboxamide

8.0 g (1 8.76 mmol) of tert-butyl (2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl) carbamate (1 c) were suspended in 160 mL 2N HCI in diethyl ether and stirred overnight at room temperature. The precipitate was filtered off and washed with ether and dried at 40°C in vacuo.

Yield: 7.4 g (quantitative). The product was used in the next step without further purification. MS (ESIpos): m/z = 327 [M+H]⁺

To a solution of 274 mg (1.10 mmol) of 2-iodopyridine-4-carboxylic acid (Alfa Aesar) and 363 mg (1.0 mmol) of hydrochloride prepared above in 15 mL DMF were added 624 mg (1.2 mmol) Benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP) and 0.70 mL (4 mmol) N-ethyl-N,N-diisopropylamine and the reaction mixture was stirred overnight at room temperature. After evaporation of the solvent the residue was taken up in ethyl acetate. This solution was washed with water and saturated aqueous sodium chloride solution, dried over sodium sulfate and then evaporated. This residue was chromatographed on silica gel using an dichloromethane/methanol gradient and the appropriate fractions were combined and concentrated.

Yield: 320 mg (53.8 %).

MS (ESIpos): m/z = 558 [M+H]⁺

1H-NMR (600MHz, DMSO-d₆): δ [ppm]= 2.52 (m, 2H), 2.60 (m, 2H), 2.74-2.76 (m, 4H), 3.36- 3.37 (m, 2H), 4.24 (s, 2H), 6.02 (d, 1 H), 6.46-6.62 (m, 6H), 6.96 (d, 1 H), 7.12 (d, 1 H), 7.38 (d, 1 H), 7.69 (d, 1 H), 8.17-8.22 (m, 1 H).

Example 2 a) N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl)-2-bromopyridine-4- carboxamide

8.0 g (18.76 mmol) of ierf-butyl (2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1 -yl}-2-oxoethyl) carbamate (1 c) were suspended in 160 mL 2N HCI in diethyl ether and stirred overnight at room temperature. The precipitate was filtered off and washed with ether and dried at 40°C in vacuo.

Yield: 7.4 g (quantitative). The product was used in the next step without further purification. MS (ESIpos): m/z = 327 [M+H]⁺

To a solution of 1.01 g (5.01 mmol) of 2-bromopyridine-4-carboxylic acid (Aldrich) and 2.0 g (5.51 mmol) of hydrochloride prepared above in 160 mL DMF were added 3.13 g (6.0 mmol) PyBOP and 2.75 mL N-ethyl-N,N-diisopropylamine and the reaction mixture was stirred overnight at room temperature. After evaporation of the solvent the residue was chroma- tographed on silica gel using an ethyl acetate/ethanol gradient and the appropriate fractions were combined and concentrated.

Yield: 739 mg (27.7 %).

MS (ESIpos): m/z = 510, 512 [M+H]⁺

¹H-NMR (400MHz, DMSO-d₆): δ [ppm]= 2.72 (s, 1 H), 2.88 (s, 1 H), 3.40 - 3.48 (m, 2H), 3.51 - 3.64 (m, 4H), 4.21 (d, 2H), 5.07 (s, 2H), 6.86 (d, 1 H), 7.25 - 7.48 (m, 6H), 7.81 (dd, 1 H), 7.95 (d, 1 H), 8.02 (s, 1 H), 8.56 (d, 1 H), 9.06 (s, 1 H).

b) N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl)-2-fluoropyridine-4- carboxamide (cold standard)

8.0 g (1 8.76 mmol) of tert-butyl (2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl) carbamate (1_c) were suspended in 160 mL 2N HCI in diethyl ether and stirred overnight at room temperature. The precipitate was filtered off and washed with ether and dried at 40°C in vacuo.

Yield: 7.4 g (quantitative). The product was used in the next step without further purification. MS (ESIpos): m/z = 327 [M+H]⁺

To a solution of 177 mg (1 .25 mmol) of 2-fluoropyridine-4-carboxylic acid (Aldrich) and 501 mg (1 .38 mmol) of hydrochloride prepared above in 40 mL DMF were added 784 mg (1 .5 mmol) PyBOP and 0.80 mL N-ethyl-N,N-diisopropylamine and the reaction mixture was stirred overnight at room temperature. After evaporation of the solvent the residue was chromatographed on silica gel using an ethyl acetate/ethanol gradient.

Yield: 315 mg (50.2 %). MS (ESIpos): m/z = 449 [M+H]⁺

¹H-NMR (400MHz, DMSO-d₆): δ [ppm]= 3.37 (br. s., 2H), 3.44 (br. s., 2H), 3.52 - 3.66 (m, 4H), 4.22 (d, 2H), 5.07 (s, 2H), 6.83 (d, 1 H), 7.27 - 7.47 (m, 6H), 7.53 (s, 1 H), 7.70 - 7.81 (m, 1 H), 7.95 (d, 1 H), 8.39 (d, 1 H), 9.01 (t, 1 H).

Example 3

• [¹⁸F]-N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl)-2-fluoropyridine-4- carboxam ide by label ing of N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1 -yl}-2- -2-bromopyridine-4-carboxamide (Example 2a)

Aqueous [¹⁸F]Fluoride (38.7 GBq) was trapped on a QMA cartridge (Waters) (activated with 5 mL 0.5M K₂C0₃ solution, 10 mL water and 10mL air) and eluted with 2 mL of a TBAOH solution (1.5 mL MeCN, 0.5 mL H₂0 + 8 μί TBAOH sol. (40%)) into the reactor. The solvent was removed by heating at 80°C for 3 min (N₂ stream and vacuum) and at 120 °C for additional 3 min (vacuum). Anhydrous MeCN (1 mL) was added and evaporated as before. A solution of precursor 2a (5 mg) in 500 μΙ anhydrous dimethyl sulfoxide (DMSO) was added. After heating at 1 80 °C for 20 min the crude reaction mixture was diluted with 4 mL water/MeCN (50:50) and purified by preparative HPLC: ACE 5-C18-HL 250mmx10mm; isocratic, 25 % acetonitrile in water with 0.1 % trifluoroacetic acid, flow: 4 mL/min; t_R~22 min. The collected HPLC fraction was diluted with 40 mL water and immobilized on a Sep-Pak plus short tC18 cartridge (Waters), which was washed with 5 mL water and eluted with 1 mL ethanol to deliver the 3.5 GBq of the F-18 labeled product (15.5 % rc. yield, corrected for decay; >96% HPLC) in 1000 μΙ ethanol in a overall synthesis time of -80 min. The desired F-18 labeled product 3 (t_R=3.2 min) was analyzed using analytical HPLC: ACE3-C18 50 mm x 4,6 mm; solvent gradient: start 5 % acetonitrile - 95 % acetonitrile in 0.1 % trifluoroacetic acid in 7 min., flow: 2 mL/min and confirmed by co-injection with the corresponding non-radioactive F- 19 fluoro-standard 2b on the analytical HPLC (t_R=3.1 min).

Alternatively to the method described above the QMA cartridge (Waters) can also be eluted with 2 m L of a tetrabutylammonium hydroxide (TBAOH) solution (1.5 mL acetonitrile (MeCN), 0.3 ml. H₂0 + 8 μΙ_ TBAOH sol. (40 %)) into the reactor. The solvent was then removed by heating the open vial at 120 °C for 10 min in an aluminium heating block under a stream of nitrogen. Anhydrous MeCN (1 ml.) was added and evaporated as before.

[¹⁸F]-N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1-yl}-2-oxoethyl)-2-fluoropyridine-4- carboxam ide by label ing of N-(2-{4-[5-(benzyloxy)pyridin-2-yl]piperazin-1 -yl}-2- -2-iodopyridine-4-carboxamide (Example 1 d)

Aqueous [¹⁸F]Fluoride (1 1 GBq) was trapped on a QMA cartridge (Waters) (activated with 5 ml. 0.5M K₂C0₃ solution, 10 ml. water and 10ml_ air) and eluted with 2 ml. of a TBAOH solution (1.5 ml. MeCN, 0.5 ml. H₂0 + 8 μΙ_ TBAOH sol. (40%)) into the reactor. The solvent was removed by heating at 80°C for 3 min (N₂ stream and vacuum) and at 120 °C for additional 3 min (vacuum). Anhydrous MeCN (1 ml.) was added and evaporated as before. A solution of precursor 1 d (5 mg) in 500 μΙ anhydrous DMSO was added. After heating at 180 °C for 20 min the crude reaction mixture was diluted with 4 ml. water/MeCN (50:50) and purified by preparative HPLC: ACE 5-C18-HL 250mmx10mm; isocratic, 23% acetonitrile in water with 0.1 % trifluoroacetic acid, flow: 4 mL/min; t_R~33 min.

The collected HPLC fraction was diluted with 40mL water and immobilized on a Sep-Pak plus short tC18 cartridge (Waters), which was washed with 10 mL water and eluted with 1 mL ethanol into the product vial to deliver the F-18 labeled product 3 (3.4 GBq) In a overall synthesis time of -95 min and in a radiochemical yield of 56% corrected for decay (radiochemical purity >99% (HPLC).

The desired F-18 labeled product 3 (t_R=3.2 min) was analyzed using analytical HPLC: ACE3- C18 50 mm x 4,6 mm; solvent gradient: start 5 % acetonitrile - 95 % acetonitrile in 0.1 % trifluoroacetic acid in 7 min., flow: 2mL/min and confirmed by co-injection with the corresponding non-radioactive F-19 fluoro-standard 2b on the analytical HPLC (t_R=3.0 min).

Alternatively to the method described above the QMA cartridge (Waters) can also be eluted with 2 mL of a tetrabutylammonium hydroxide (TBAOH) solution (1 .5 mL acetonitrile (MeCN), 0.3 mL H₂0 + 8 μί TBAOH sol. (40 %)) into the reactor. The solvent was then re- moved by heating the open vial at 120 °C for 10 min in an aluminium heating block under a stream of nitrogen. Anhydrous MeCN (1 mL) was added and evaporated as before.

Claims

Claims

. A compound of the formula

or a suitable salt thereof.

2. A method for the preparation of a compound of the formula

is reacted with a radiofluorination agent.

3. A method for the preparation of a compound of the formula

is reacted with a radiofluorination agent at elevated temperatures in organic solvents or mixtures thereof.

4. A method according to claim 2 or 3, wherein [¹⁸F]fluoride/TBAOH is used as a fluorination agent.

5. A method for the preparation of a compound of the formula

6. A method for the preparation of a compound of the formula

wherein the compound of formula

is reacted with 2-iodopyridine-4-carboxylic acid.

7. A method for the preparation of a compound of the formula

wherein a salt of the compound of formula

is reacted with 2-iodopyridine-4-carboxylic acid.

8. A method according to claim 7, wherein the salt is the hydrochloride.

9. A method for the preparation of a compound of the formula

wherein the compound of formula

is deprotected and then reacted with 2-iodopyridine-4-carboxylic acid.