Method for preparing a marked purine derivative, said derivative and uses thereof

Abstract

The present invention relates to a method for preparing a 2-fluoropurine marked with the radioisotope .sup.18F comprising a fluorination step for a 2-nitropurine derivative. The present invention comprises a 2-fluoropurine derivative marked with the radioisotope .sup.18F which can be obtained by or during a method according to the invention and its various uses.

Claims

1. A method for performing PET imaging studies in a subject having chronic lymphoid leukemia or a related illness, comprising: synthesizing a .sup.18F-labeled fludarabine corresponding to the formula (B) from a protected 2-nitropurine derivative having the formula (1c) ##STR00019## in which R and R′ are identical or different protecting groups; R.sub.2 is hydrogen, an optionally substituted alkyl group, an optionally substituted aryl group, an optionally substituted furanose group, or an optionally substituted pyranose group; R.sub.3 is hydrogen, an optionally substituted alkyl group, an optionally substituted aryl group, a halogen, a —OR.sub.8 group or a —SR.sub.8 group, with R.sub.8 being hydrogen, an optionally substituted alkyl group or an optionally substituted aryl group, wherein said synthesizing includes the following successive steps of (i) reacting the protected 2-nitropurine derivative having the formula (1c) with a source of [.sup.18F] labeled fluoride ions in a solvent of CH.sub.3CN at a temperature between 55° C. and 60° C. for a duration of between 1 min and 30 min to obtain a protected 2-[.sup.18F] fluoropurine derivative and optionally a partially unprotected 2-[.sup.18F] fluoropurine derivative, and (ii) unprotecting said protected 2-[.sup.18F] fluoropurine derivative and said partially unprotective 2-[.sup.18F] fluoropurine derivative to obtain said .sup.18F-labeled fludarabine corresponding to the formula (B) with a yield of 40-60% ##STR00020## administering to the subject the .sup.18F-labeled fludarabine of the formula (B) or one of its salts; and detecting said .sup.18F-labeled fludarabine.

2. The method of claim 1, wherein said .sup.18F-labeled fludarabine is a PET probe.

3. The method of claim 1, wherein the step (ii) consists of reacting said protected 2-[.sup.18F] fluoropurine derivative and said partially unprotected 2-[.sup.18F] fluropurine derivative with an alcohol, followed by aqueous ammonia and then by heating at a temperature between 50° C. to 90° C. for between 5 min to 45 min.

4. The method of claim 1, wherein said protected 2-[.sup.18F] fluoropurine derivative and optionally the partially unprotected 2-[.sup.18F] fluoropurine derivative are obtained with a yield of 70-100%.

5. A method for performing PET imaging studies on a subject comprising: synthesizing a .sup.18F-labeled fludarabine corresponding to the formula (B) from a protected 2-nitropurine derivative having the formula (1c) ##STR00021## in which R and R′ are identical or different protecting groups; R.sub.2 is hydrogen, an optionally substituted alkyl group, an optionally substituted aryl group, an optionally substituted furanose group, or an optionally substituted pyranose group; R.sub.3 is hydrogen, an optionally substituted alkyl group, an optionally substituted aryl group, a halogen, a —OR.sub.8 group or a —SR.sub.8 group, with R.sub.8 being hydrogen, an optionally substituted alkyl group or an optionally substituted aryl group, wherein said synthesizing includes the following successive steps of (i) reacting the protected 2-nitropurine derivative having the formula (1c) with a source of [.sup.18F] labeled fluoride ions in a solvent of CH.sub.3CN at a temperature between 55° C. and 60° C. for a duration of between 1 min and 30 min to obtain a protected 2-[.sup.18F] fluoropurine derivative and optionally a partially unprotected 2-[.sup.18F] fluoropurine derivative, and (ii) unprotecting said protected 2-[.sup.18F] fluoropurine derivative and said partially unprotective 2-[.sup.18F] fluoropurine derivative to obtain said .sup.18F-labeled fludarabine corresponding to the formula (B) with a yield of 40-60% ##STR00022## administering to the subject the .sup.18F-labeled fludarabine of the formula (B) or one of its salts; and detecting said .sup.18F-labeled fludarabine, wherein said PET imaging studies are used for in vivo mapping of malignant hematopoietic cells.

6. The method of claim 5, wherein the .sup.18F-labeled fludarabine is a PET probe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows the radio-thin-layer chromatographies (or “RTLC”) obtained, at t=5 min, from a fluorination reaction mixture using as a source of cations K.sub.2CO.sub.1;

(2) FIG. 1B shows the radio-thin-layer chromatographies (or “RTLC”) obtained, at t=5 min, from a fluorination reaction mixture using as a source of cations K.sub.2SO.sub.4;

(3) FIG. 2 shows the evolution as a function of the duration of the reaction mixture composition during the fluorination step using K.sub.2SO.sub.4 as a source of cations;

(4) FIG. 3 shows the evolution as a function of the duration of the reaction mixture composition during the fluorination step using K.sub.2CO.sub.3 as a source of cations; and

(5) FIG. 4 shows the comparison of the percentages of the partially deprotected product in the reactional environment as a function of the time and the nature of the potassium salt used as a source of cations (K.sub.2SO.sub.4 or K.sub.2CO.sub.3).

DETAILED EXPLANATION OF THE PARTICULAR EMBODIMENTS

Material and Methods

(6) In general, all the chemical products and solvents are of ACS quality (Analytical grade chemical solvent) or of HPLC (High-Performance Liquid Chromatography) quality and are used without any other purification, unless otherwise specified.

(7) Dichloromethane (CH.sub.2Cl.sub.2) has been distilled over P.sub.2O.sub.5. Pyridine was dried and distilled on KOH. Acetonitrile, DMF, THF, Dioxane are purified on resin on a Millipore apparatus and filtered with a 0.22 μm filter (Millipak). The fludarabine (2F-ARA-A) was obtained from Sigma.

(8) HPLC analyses were carried out with an HPLC pump (model: L-6200 Intelligent Pump, Merck), a U.V. detector Merck L-4250 (λ=254 nm) in series with a flux β detector by Novelec. HPLC chromatograms were recorded by a dual-channel interface/module (Varian star 800) connected to a PC provided with the Galaxy software (Varian).

(9) Thin layer chromatographies (TLC) were made on silica plates (gel 60 F.sub.254) and visualized using an U.V. lamp (λ=254 nm) or by immersion in an appropriate coloration agent (KMnO.sub.4, Vanillin, Iodine or PMA) followed by gentle heating. The radioactive compounds were localized on the TLC using an imager (Packard Instant Imager) connected to a PC.

(10) Flash chromatographies were made on columns of silica gel (SiO.sub.2 40-63 μm, Merck).

(11) NMR (Nuclear Magnetic Resonance): NMR .sup.1H spectra were recorded using a Bruker apparatus at 250 or 400 MHz (DPX 250 or DRX 400). Chemical shifts δ are indicated in ppm using the TMS as reference. Coupling constants J are given in Hertz (Hz). The multiplicities are indicated using the following abbreviations: s=singulet, d=doublet, t=triplet, q=quartet, m=multiplet, bs=broad singulet.

(12) .sup.13C {.sup.1H} NMP spectra were recorded using a Broker apparatus at 62.9 or 100.6 MHz. Chemical shifts δ are indicated in ppm using deuterated solvent as reference. Coupling constants J are given in Hertz (Hz).

(13) .sup.19F RMN spectra were recorded using a Brüker apparatus (Advanced DRX 400), (376 MHz). Chemical shifts δ are indicated in ppm using the CFCl.sub.3 as external reference.

(14) The fluoride ion [.sup.18F]F.sup.− in water was obtained from a cyclotron (IBA, Cyclone 18/9 RF) using a proton beam on 95% [.sup.18O] enriched water (Cambridge Isotope Laboratories Inc.). The fluoride ion [.sup.18F]F.sup.− in water was purified on a QMA ion-exchange resin (ABX, Advanced biochemical compounds) eluted with 500 μl of aqueous K.sub.2CO.sub.3 (1 mg/ml to 5 mg/ml).

(15) Water was evaporated by azeotropic distillation with acetonitrile at 110° C. using a nitrogen stream using stirring/heating module (Pierce).

(16) Radioactivity measurements were carried out using a Capintec CRC-15 dose calibrator.

Example 1

Preparation of [.SUP.18.F]Fluoroadenosine

(17) ##STR00014##

Diagram 4

(18) Chemistry

(19) Pentabenzoyled adenosine and the 2-nitro-pentabenzoyladenosine 1 were prepared according to the methods previously described (Article of Braendvang et al., 2006, Synthesis, 18, 2993 and International Application WO 2005/056571) and purified by silica gel chromatography.

(20) Protected 2-fluoro-adenosine 2 and 3 were obtained according to the modified protocol (1.1 equivalent of Bu.sub.4NF in the CH.sub.3CN instead of an excess of reactant in DMF) enabling their isolation in high yields.

(21) ##STR00015##

Diagram 5

(22) Tetrabutylammonium fluoride (TBAF) (1.3 eq, 600 μL, 0.6 mmol, 1 M in THF) has been added drop wise for 1 min to a suspension of 2-nitro-adenosine 1 (379 mg, 0.45 mmol) in dry acetonitrile (15 mL) at 0° C. The mixture was stirred for 20 min and the resulting solution was vacuum evaporated without heating. The product has been purified by flash chromatography (CH.sub.2Cl.sub.2-acetone). The compounds 2 and 3 have been isolated with a yield of 58% (211 mg) and 35% (130 mg), respectively.

(23) A solution of the protected compound 2 (185 mg, 0.23 mmol) in MeOH (40 mL) was saturated by a stream of ammoniac gaz for 15 min at 0° C. The resulting mixture was stirred for 14 hours at room temperature, then evaporated under reduced pressure and purified by flash silica gel chromatography (AcOEt-MeOH 83:17) to give 51 mg (77%) of 2-fluoroadenosine 4.

2-fluoro-6-N,N-dibenzoyl-9-(2′,3′,5′-tri-O-benzoyl-β-D-ribofuranosyl)-9H-purine 2

(24) .sup.1H NMR (400 MHz, CDCl.sub.3): δ=4.82 (m, 3H, H.sub.4′ H.sub.5′); 6.17 (t, 1H, H.sub.3′, J=5.4 Hz); 6.22 (t, 1H, H.sub.2′, J=5.8 Hz); 6.46 (d, 1H, H.sub.1′, J=5.3 Hz); 7.28-7.52 (m, 15H, Ar); 7.85 (dd, 4H, J=1.2 and 8.4 Hz; H.sub.Ar); 7.95 (dd, 2H, J=1.2 and 8.4 Hz, H.sub.Ar); 8.02 (dd, 2H, J=1.2 et 8.4 Hz, H.sub.Ar); 8.12 (dd, 2H, J=1.2 et 8.40, H.sub.Ar); 8.20 (s, 1H, H.sub.8).

(25) .sup.13C NMR (100.6 MHz, CDCl.sub.3): δ=63.9 (C.sub.5′); 71.7 (C.sub.4′); 74.4 (C.sub.3′); 81.4 (C.sub.2′); 87.0 (C.sub.1′); 126.4 (d, J.sub.CF=5.0 Hz, C.sub.5); 128.7; 128.9; 129.0; 129.1; 129.3; 129.5; 129.9; 130.1; 130.2; 130.3; 133.7; 133.9; 134.0; 134.2; 134.3; 143.7 (d, J.sub.F=3.8 Hz, C.sub.8); 154.3 (d, J.sub.CF=16.9 Hz, C.sub.6); 154.9 (d, J.sub.CF=17.0 Hz, C.sub.4); 158.3 (d, J.sub.CF=218.9 Hz, C.sub.2); 165.5 (C═O); 165.7 (C═O); 166.5 (C═O); 172.1 (2×C═O).

(26) .sup.19F NMR (376.5 MHz, CDCl.sub.3): δ=−49.1

2-fluoro-6-N-benzoyl-9-(2′,3′,5′-tri-O-benzoyl-β-D-ribofuranosyl)-9H-purine 3

(27) (two rotamers: ratio 80/20).

(28) .sup.1H NMR (400 MHz, CDCl.sub.3): δ=4.74 (m, 3.6H, H.sub.4′ H.sub.5′); 6.12 (m, 2.4H, H.sub.3. H.sub.2′); 6.39 (d, 0.8H, H.sub.1′, J=5.5 Hz); 6.51 (d, 0.2H, H.sub.1′, J=5.5 Hz); 7.28-7.51 (m, 15H, H.sub.Ar); 7.83-8.01 (m, 10H, H.sub.Ar); 8.08 (s, 0.8H, H.sub.8); 8.39 (s, 0.2H, H.sub.8); 8.94 (s, 0.8H, NH); 9.14 (s, 0.2H, NH).

(29) .sup.19F NMR (376.5 MHz, CDCl.sub.3): δ=−47.6

2-fluoro-9-(β-D-ribofuranosyl)-9H-purine 4

(30) .sup.1H NMR (400 MHz, DMSO-D.sub.6): δ=3.60 (m, 2H, H.sub.5′); 3.94 (m, 1H, H.sub.4′); 4.13 (m, 1H, H.sub.3′); 4.52 (m, 1H, H.sub.2′); 5.07 (t, 1H, OH-5′, J=5.6 Hz); 5.20 (d, 1H, OH-3′, J=4.7 Hz); 5.47 (d, 1H, OH-2′, J=5.9 Hz); 5.79 (d, 1H, H.sub.1′, J=5.9 Hz); 7.87 (bs, 2H, NH.sub.2); 8.35 (s, 1H, H.sub.8).

(31) .sup.19F NMR (376.5 MHz, DMSO-D.sub.6): δ=−52.5

(32) Radiochemistry

(33) Method A

(34) The ion fluoride [.sup.18F]F.sup.− in water was adsorbed on a QMA (ABX) ion-exchange resin and eluted with aqueous (500 μL, 1 mg/mL) K.sub.2CO.sub.3 and Kryptofix (K.sub.222, 15−25 mg) in acetonitrile (500 μL). Water was evaporated under a nitrogen stream at 110° C. by azeotropic distillation using acetonitrile (3×1 mL). The precursor 1 (5-5.5 mg in 500-800 μL of acetonitrile) was added to the dry [.sup.18F]KF complex. Heating at 55-60° C. for 5 to 10 min (preferably 8 min) provided the expected [.sup.18F] fluoroadenosine 2 in a radiochemical yield of more than 90% (thin-layer chromatography SiO.sub.2 eluted with CH.sub.2Cl.sub.2/acetone 95/5, Packard instant imager). The solution was diluted with 500 μL of ethyl acetate (AcOEt) and adsorbed on a silica cartridge Sep-Pak (WATERS). Elution with 3-3.5 mL of AcOEt followed by 5 mL of dry air with the evaporation of the solvent has made it possible to provide the protected [.sup.18F]2-fluoroadenosine 2 with a overall yield of 86% (decay corrected) from [.sup.18F]KF.

(35) Deprotection was carried out in a conventional manner by the addition of 500 μL of methanol followed by 500 μL of aqueous ammonia (29% in water) and heating at 65-70° C. for 20 min. After cooling, acetic acid (0.7 mL, 40% in water or 0.3 ml of pure acetic acid) was added and the clear solution was purified by a semi-preparative HPLC chromatography on a μBondapak column (water/ethanol 97/3 5 mL/min) to obtain the [.sup.18F]-fluoroadenosine 4 with a overall yield of 50% (decay corrected).

(36) Method B

(37) The B method is an alternative to selectively obtain the mono-hydrolyzed fluoroadenosine [.sup.18F] 3. Subsequent deprotection can be carried out using more drastic conditions (80-85° C.) to obtain the [.sup.18F]-fluoroadenosine 4 and this strategy improves the deprotection step (90%).

(38) 2-phenyl-ethylamine (1 mg) in 100 μL of acetonitrile was added to the crude [.sup.18F] fluoroadenosine 2 in acetonitrile (obtained according to method A) and the solution was heated for 10 min at 60° C. A TLC and HPLC analysis indicated a quantitative formation of the intermediate marked/labelled mono-deprotected that is the compound [.sup.18F] 3. The solution was diluted with 500 μL of AcOEt and adsorbed on a silica Sep-Pak cartridge (WATERS). Elution with 3.5 mL of AcOEt followed by 5 mL of dry air and the evaporation of the solvent makes it possible to obtain the compound [.sup.18F] 3 with a 60% non-optimized yield (decay corrected).

(39) Final deprotection was carried out by adding 500 μL of methanol followed by 400 μL of ammonia (29% in water) and heating at 80-85° C. for 20 min. After cooling, acetic acid (0.7 mL, 40% in the water) was added and the clear solution was purified by a semi-preparative HPLC chromatography on a μBondapak column (water/ethanol 97/3, 5 mL/min) to produce the marked/labelled 2-fluoroadenosine [.sup.18F] 4.

Example 2

Preparation of [.SUP.18.F]Fludarabine

(40) ##STR00016##

Diagram 6: Synthesis of the [.SUP.18.F]Fludarabine

(41) Chemistry

(42) Synthesis of the precursor 7 can be achieved through protection and nitration of the 9(β-D-arabinofuranosyl)-9H-purine according to the protocols described for its ribose analogue (Article of Braendvang et al., 2006, Synthesis, 18, 2993 and international application WO 2005/056571).

(43) Alternatively, compound 7 can also be obtained from the adenosine or the guanosine (diagram 6) (Article of Gimisis et al., 1998, Tetrahedron, 54, 573 and international application WO 9412514).

(44) ##STR00017##

Diagram 7: Synthesis of Precursor 7

(45) The reactants and yields of the different steps for the synthesis of the precursor 7 (diagram 7) are: i: TBDMSCl, DABCO, AgNO.sub.3, THF, 53%. ii: Tf.sub.2O, DMAP, DIPEA, Py. 67%. iii: PhCOOK, DMSO, 96%. iv: Bu.sub.4NF, THF, 81%. v: PhCOCl, Py, 95%. vi: TBAN, TFAA, CH.sub.2Cl.sub.2, 45%.

Synthesis of the 6-N,N-dibenzoyl-9-(2′,3′,5′-tri-O-benzoyl-β-D-arabinofuranosyl)-9H-purine 6

(46) Benzoyl chloride (750 μL, 6.49 mmol) was added to a solution of the compound 5 (181 mg, 0.82 mmol) in the anhydrous pyridine (7 mL) and the mixture was refluxed for 6 hours. After cooling and extraction (CH.sub.2Cl.sub.2), the organic phase was washed with saturated aqueous NaHCO.sub.3 (10 mL) and brine (2×10 mL), and dried over Na.sub.2SO.sub.4. Filtration and evaporation under reduced pressure yielded the crude product as a pale yellow solid, which was purified by flash chromatography (CH.sub.2Cl.sub.2: acetone, 95:5) to yield 606 mg (96%) of 6.

(47) .sup.1H NMR (250 MHz, CDCl.sub.3): δ=4.73 (m, 3H, H.sub.4′ and H.sub.5′); 5.91 (m, 2H, H.sub.3′ and H.sub.2′); 6.48 (d, 1H, H.sub.1′, J=4.2 Hz); 7.20-7.39 (m, 15H, H.sub.Ar); 7.68-7.72 (m, 5H, H.sub.Ar); 7.96-8.00 (m, 5H, H.sub.Ar); 8.38 (s, 1H, H.sub.2); 8.58 (s, 1H, H.sub.8).

6-N, N-dibenzoyl-9-(2′,3′,5′-tri-O-benzoyl-β-D-arabinofuranosyl)-2-nitro-9H-purine 7

(48) A nitrated mixture was prepared by adding 2,2,2-trifluoroacetic anhydride (160 μL, 1.15 mmol) for 2 min to a solution of tetrabutylammonium nitrate (351 mg, 1.15 mmol) in dry methylene chloride (15 mL) at 0° C. After 45 min, the solution was added to 6 (606 mg, 0.77 mmol) in dry methylene chloride (15 mL) at 0° C. After 14 hours at room temperature (and protected from light), the reaction mixture was poured into a cold mixture of water (30 mL), saturated aqueous NaHCO.sub.3 (20 mL) and CH.sub.2Cl.sub.2-Et.sub.2O (1:2, 20 mL). The aqueous layer was extracted from CH.sub.2Cl.sub.2-Et.sub.2O (1:2, 2×20 mL). The combined organic extracts was washed with brine (2×20 mL), dried (Na.sub.2SO.sub.4), and vacuum evaporated (without heating above 40° C.). The product was purified by flash chromatography (CH.sub.2Cl.sub.2-acetone 95:5) to produce 201 mg (45%) of 7.

(49) .sup.1H NMR (250 MHz, CDCl.sub.3): δ=4.79 (m, 3H, H.sub.4′ H.sub.5′); 5.86 (m, 2H, H.sub.3′ H.sub.2′); 6.83 (d, 1H, H.sub.1′, J=5.1 Hz); 7.21-7.47 (m, 15H, H.sub.Ar); 7.66-7.73 (m, 6H, H.sub.Ar); 7.95 (dd, 2H, J=1.4 and 7.8 Hz, H.sub.Ar); 8.05 (dd, 2H, J=1.4 and 7.8 Hz, H.sub.Ar); 8.52 (s, 1H, H.sub.8).

(50) .sup.13C NMR (62.9 MHz, CDCl.sub.3): δ=63.5 (C.sub.5′); 76.3 (C.sub.2′); 77.1 (C.sub.3′); 81.6 (C.sub.4′); 84.7 (C.sub.1′); 127.8 (C.sub.5); 128.6; 128.9; 129.1; 129.2; 129.3; 129.5; 129.6; 129.8; 130.1; 130.2; 130.5; 133.7; 133.8; 133.9; 134.5; 134.6; 147.7 (C.sub.8); 153.0 (C.sub.6); 153.1 (C.sub.2); 154.0 (C.sub.4); 165.1 (C═O); 165.8 (C═O); 166.6 (C═O); 171.8 (2×C═O).

(51) Radiochemistry

(52) [18F]-FLUDARABINE:

(53) Using 4.8 to 5.5 mg de 6-N,N-dibenzoyl-9-(2′,3′,5′-tri-O-benzoyl-β-D-arabinofuranosyl)-2-nitro-9H-purine 7 and the methods (A or B) previously described, the [.sup.18F]-fludarabine has been obtained with an overall radiochemical yield of 63% (decay corrected) in about 85 min after the semi-preparative HPLC purification (Water/Ethanol 97/3 5 mL/min).

Example 3

Evolution of the Fluorination Reaction Mixture as a Function of the Potassium Salt Used as Source of Cations

(54) Fluorination reaction is carried out in DMSO at 140° C. in presence of potassium salt (K.sub.2CO.sub.3, 0.5-1 mg or K.sub.2SO.sub.4 3-5 mg) and kryptofix K2.2.2 (20 mg) using 5 mg of precursor (2-nitro purine). Reaction products obtained present the following structure:

(55) ##STR00018##

(56) Analysis of the reaction mixture is made by radio-TLC (SiO.sub.2, eluted CH.sub.2Cl.sub.2/Acetone 95/5) from a 15-25 μL sampling taken every 5 min.

(57) Radioactive products are identified by co-elution with non-radioactive references (product 1 Rf=0.49; product 2 Rf=0.75; cf Drawing 1).

(58) FIG. 2 shows that in K.sub.2SO.sub.4 presence, the product 1 (partially deprotected) is majoritarily formed during the fluorination reaction. The composition of the mixture evolves as a function of time and product 1 becomes largely majoritary after 15 min to the detriment of product 2 (progressive deprotection of product 2 in these reaction conditions).

(59) When the fluorination reaction is carried out in K.sub.2CO.sub.3 presence, product 2 (entirely protected) is majoritarily formed and its proportion remains stable over time (61 to 70%), as shown in FIG. 3.

(60) FIG. 4 makes it possible to observe the influence of potassium salt nature (K.sub.2SO.sub.4 versus K.sub.2CO.sub.3) on the formation of product 1. Using K.sub.2SO.sub.4 makes it possible to majoritarily obtain product 1 and after 20 min of reaction, product 1 represents 95% (ratio 1/2=81/4) of the fluorination products.

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