Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Automated radiosynthesis of no-carrier-added 4-[18F]fluoroiodobenzene: a versatile building block in 18F radiochemistry

4-[18F]Fluoroiodobenzene ([18F]FIB) is a versatile building block in 18F radiochemistry used in various transition metal-mediated C-C and C-N cross-coupling reactions and [18F]fluoroarylation reactions. Various synthesis routes have been described for the preparation of [18F]FIB. However, to date, no automated synthesis of [18F]FIB has been reported to allow access to larger amounts of [18F]FIB in high radiochemical and chemical purity. Herein, we describe an automated synthesis of no-carrier-added [18F]FIB on a GE TRACERlab™ FX automated synthesis unit starting from commercially available(4-iodophenyl)diphenylsulfonium triflate as the labelling precursor. [18F]FIB was prepared in high radiochemical yields of 89 ± 10% (decay-corrected, n = 7) within 60 min, including HPLC purification. The radiochemical purity exceeded 95%, and specific activity was greater than 40 GBq/μmol. Typically, from an experiment, 6.4 GBq of [18F]FIB could be obtained starting from 10.4 GBq of [18F]fluoride.

Electrochemical nucleophilic synthesis of di-tert-butyl-(4-[18F]fluoro-1,2-phenylene)-dicarbonate

An electrochemical method with the ability to conduct (18)F-fluorination of aromatic molecules through direct nucleophilic fluorination of cationic intermediates is presented in this paper. The reaction was performed on a remote-controlled automatic platform. Nucleophilic electrochemical fluorination of tert-butyloxycarbonyl (Boc) protected catechol, an intermediate model molecule for the positron emission tomography (PET) probe (3,4-dihydroxy-6-[(18)F]fluoro-L-phenylalanine), was performed. Fluorination was achieved under potentiostatic anodic oxidation in acetonitrile containing Et3N·3HF and other supporting electrolytes. Radiofluorination efficiency was influenced by a number of variables, including the concentration of the precursor, concentration of Et3N·3HF, type of supporting electrolyte, temperature and time, as well as applied potentials. Radio-fluorination efficiency of 10.4±0.6% (n=4) and specific activity of up to 43GBq/mmol was obtained after 1h electrolysis of 0.1M of 4-tert-butyl-diboc-catechol in the acetonitrile solution of Et3N·3HF (0.033M) and NBu4PF6 (0.05M). Density functional theory (DFT) was employed to explain the tert-butyl functional group facilitation of electrochemical oxidation and subsequent fluorination.

18F labeling of arenes

Molecular imaging has witnessed an upsurge in growth, with positron emission tomography leading the way. This trend has encouraged numerous synthetic chemists to enter the field of (18) F-radiochemistry and provide generic solutions to address the well-recognized challenges of late-stage fluorination. This Minireview focuses on recent developments in the (18)F-labeling of aromatic substrates.

Fluorine-18 labeling methods: Features and possibilities of basic reactions

Many experimental and established tracers make fluorine- 18 the most widely used radionuclide in positron emission tomography with an increasing demand for new or simpler 18F-labeling procedures. After a brief summary of the advantages of the nuclide and its major production routes, the basic features of the principal radiofluorination methods are described. These comprise direct electrophilic and nucleophilic processes, or in case of more complex molecules, the labeling of synthons and prosthetic groups for indirect built-up syntheses. While addressing the progress of no-carrier-added 18F-labeling procedures, the following chapters on more specific topics in this book are introduced. Emphasis is given to radiofluorination of arenes--especially with iodonium leaving groups. Examples of radiopharmaceutical syntheses are mentioned in order to illustrate strategic concepts of labeling with fluorine-18.

Synthesis, characterization, and first successful monkey imaging studies of metabotropic glutamate receptor subtype 5 (mGluR5) PET radiotracers

Three metabotropic glutamate receptor subtype 5 (mGluR5) PET tracers have been labeled with either carbon-11 or fluorine-18 and their in vitro and in vivo behavior in rhesus monkey has been characterized. Each of these tracers share the common features of high affinity for mGluR5 (0.08-0.23 nM vs. rat mGluR5) and moderate lipophilicity (log P 2.8-3.4). Compound 1b was synthesized using a Suzuki or Stille coupling reaction with [11C]MeI. Compounds 2b and 3b were synthesized by a SNAr reaction using a 3-chlorobenzonitrile precursor. Autoradiographic studies in rhesus monkey brain slices using 2b and 3b showed specific binding in cortex, caudate, putamen, amygdala, hippocampus, most thalamic nuclei, and lower binding in the cerebellum. PET imaging studies in monkey showed that all three tracers readily enter the brain and provide an mGluR5-specific signal in all gray matter regions, including the cerebellum. The specific signal observed in the cerebellum was confirmed by the autoradiographic studies and saturation binding experiments that showed tracer binding in the cerebellum of rhesus monkeys. In vitro metabolism studies using the unlabeled compounds showed that 1a, 2a, and 3a are metabolized slower by human liver microsomes than by monkey liver microsomes. In vivo metabolism studies showed 3b to be long-lived in rhesus plasma with only one other more polar metabolite observed.

Synthesis of N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine as a serotonin transporter imaging agent

Two 403U76 analogs, N,N-dimethyl-2-(2-nitro-4-bromophenylthio)benzylamine (4) and N,N-dimethyl-2-(2,4-dinitrophenylthio)benzylamine (8) were prepared in multi-steps synthesis as precursors for the synthesis of a new serotonin transporter imaging agent, N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (12a). Reaction of 2,5-dibromonitrobenzene (1) with 2-thio-N,N-dimethylbenzamide gave N N-dimethyl-2-(2-nitro-4-bromophenylthio)benzamide (3). N,N-Dimethyl-2-(2,4-dinitrophenylthio)benzamide (6) was synthesized similarly from the reaction of 2-bromo-1,5-dinitrobenzene (2) with 2-thio-N,N-dimethylbenzamide. Reduction of 3 and 6 with BH(3)/THF gave benzylamines 4 and 8 along with their amine boranes 5 and 7. Nucleophilic substitution of 4 or 8 with K[18F]/Kryptofix 2.2.2 in DMSO at 120 degrees C followed by reduction with NaBH(4)- Cu(OAc)(2) in EtOH at 78 degrees C and purification with HPLC gave compound 12a in approximately 5-10% yield in a synthesis time of 150min from EOB. A preliminary biodistribution study in rats showed that the uptake of compound 12a in rat brain was high (approximately 1%/g) and the ratio of the uptake of compound 12a in hypothalamus (a serotonin transporter-rich area) and cerebellum (a serotonin-transporter-devoid area) was 6/1 at 1h post-injection. These results suggest that compound 12a may be a potential new serotonin transporter PET imaging agent.

Single-step synthesis of [18F]haloperidol from the chloro-precursor and its applications in PET imaging of a cat's brain

We have established a convenient synthesis process for the synthesis of[18F]haloperidol using a single-step 18F-for-Cl exchange reaction and a new elution system for the preparative high performance liquid chromatography (HPLC) using C18 bonded vinylalcohol copolymer gel (ODP) and a basic eluent. We successfully applied the product to cat-PET study and got clear images of the striatum, showing the usefulness of this synthesis.

Studies with differentially labeled [11C]cocaine, [11C]norcocaine, [11C]benzoylecgonine, and [11C]- and 4'-[18F]fluorococaine to probe the extent to which [11C]cocaine metabolites contribute to PET images of the baboon brain

The psychostimulant drug of abuse, cocaine (benzoylecgonine methyl ester), is rapidly metabolized by cleavage of its two ester groups, to give benzoylecgonine (BE) and ecgonine methyl ester, and by N-demethylation, to give N-norcocaine (NC). The recent use of [N-methyl-11CH3]cocaine to image brain cocaine binding sites with positron emission tomography (PET) raises the question of whether PET images partially reflect the distribution and kinetics of labeled cocaine metabolites. We prepared [O-methyl-11CH3]cocaine by methylation of the sodium salt of BE with [11C]CH3I, and showed that PET baboon brain scans, as well as regional brain kinetics and plasma time-activity curves corrected for the presence of labeled metabolites, are nearly identical to those seen with [N-methyl-11CH3]cocaine. This strongly suggests that 11C metabolites do not significantly affect PET images, because the metabolite pattern is different for the two labeled forms of cocaine. In particular, nearly half the 11C in blood plasma at 30 min was [11C]CO2 when [N-methyl-11CH3]cocaine was administered, whereas [11C]CO2 was not formed from [O-methyl-11CH3]cocaine. Only a trace of [11C]NC was detected in plasma after [O-methyl-11CH3]cocaine administration. Nearly identical brain PET data were also obtained when 4'-[N-methyl-11CH3]fluorococaine and 4'-[18F]fluorococaine (prepared by nucleophilic aromatic substitution from [18F]fluoride- and 4'-nitrococaine) were compared with [N-methyl-11CH3]cocaine. In vitro assays with rat brain membranes showed that cocaine and 4'-fluorococaine were equipotent at the dopamine reuptake site, but that 4'-fluorococaine was about 100 times more potent at the 5-hydroxytryptamine reuptake site.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and preliminary evaluation of para- and meta-[18F]fluorobenzylguanidine

meta-[18F]Fluorobenzylguanidine ([18F]MFBG) and para-[18F]fluorobenzylguanidine ([18F]PFBG) were synthesized in three steps beginning with a fluoro for nitro exchange reaction on 3- and 4-nitrobenzonitrile, respectively. Overall radiochemical yields were 10-15% for [18F]MFBG and 50-55% for [18F]PFBG in a total synthesis time of 60 min. However, impurities interfered with the binding of the product to target cells. A new route was adopted for the synthesis of [18F]PFBG using 4-nitrilophenyl trimethylammonium trifluoromethanesulfonate (Q.S.) as the starting material. In addition to shortening the overall synthesis time by 10 min, this precursor also eliminated problems associated with the presence of small amounts of starting material in the preparation. In vitro binding of [18F]PFBG prepared by the Q.S. method to SK-N-SH, human neuroblastoma cells was 26.5 +/- 1.1%, compared to 16.9 +/- 1.6% when the nitro precursor was used. Selective uptake of both 18F-labeled isomers in the heart and adrenal was seen in mice. At 4 h, adrenal and heart uptake of [18F]PFBG prepared using Q.S. was 20.3 +/- 4.8 and 5.9 +/- 0.8% ID/g respectively, compared to 23.8 +/- 5.0 and 10.5 +/- 1.7% ID/g for [18F]MFBG. Based on the 5-fold higher radiochemical yields obtained with [18F]PFBG, this isomer would appear to be the more practical choice; however, in vitro and in vivo results suggest that [18F]MFBG exhibits greater similarities to MIBG.

Synthesis of (+/-)-[18F]BMY 14802, its enantiomers and their anatomical distributions in rodents

A potential antipsychotic drug, BMY 14802 was labeled with 18F and its distribution in rodents was studied. No-carrier-added (NCA) (+/-)-[18F]BMY 14802 (5) was synthesized by two methods in 5-10% radiochemical yield in a synthesis time of 130-140 min from EOB with a specific activity of 0.5-1.5 Ci/microM. (+)- and (-)-[18F]BMY 14802 was synthesized by the chiral reduction of alpha-(4-[18F]fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-b utanone (4) with chiral reducing agent, (+)- and (-)-beta-chlorodiisopinocampheylborane [(+)- and (-)-DIP chloride] in 6-10% radiochemical yield in a synthesis time of 150 min from EOB. Animal studies in mouse and in rat revealed that the distribution of 5 in each tissue was high at 5 min, the radioactivity then declined rapidly in all tissues studied except in the liver and in the small intestine. The radioactivity in the femur did not increase with time indicating in vivo defluorination may not occur. The uptakes of (+/-)-[18F]BMY 14802 and its enantiomers, (+)- and (-)-[18F]BMY 14802 in rat cerebellum, brain stem, hippocampus and spinal cord were similar and were significantly reduced by prior treatment of rat with haldol. This suggests that (+/-)-[18F]BMY 14802 and its enantiomers bind to sigma-receptors in a similar fashion.

Production of fluorine-18 labeled (3-N-methyl)benperidol for PET investigation of cerebral dopaminergic receptor binding

The multi-millicurie synthesis of the D-2 receptor ligand [18F](3-N-methyl)benperidol (NMB; 1-[3-(4'- [18F]fluorobenzoyl)propyl]-4-(2-keto-3-methyl-1- benzimidazolinyl)piperidine) is described. [18F]NMB was produced via a 3-step reaction sequence with an overall radiochemical yield of 5-10% and a specific activity greater than 3000 Ci/mmol within 100 min. In vitro binding assays indicated that NMB has high affinity for D-2 receptors in primate brain (K1 = 3.6 nM), with a receptor specificity exceeding that of spiperone. The technique described here permits the routine production of 10-20 mCi of this promising radiopharmaceutical for PET study of D-2 receptor binding in vivo.

[18F]fluorination/decarbonylation: new route to aryl [18F]fluorides

A new route to aryl [18F]fluorides without electron withdrawing ring substituents has been developed. [18F]Fluorobenzaldehydes, prepared from no-carrier-added (NCA) [18F]fluoride using nucleophilic aromatic substitution of fluoro or nitro groups, were decarbonylated using palladium on charcoal (Pd-C). By this approach 2-methoxy-4-nitrobenzaldehyde was converted to NCA 3-[18F]fluorophenol (25-30%, EOB) and 4-fluoro-2-methoxy-5-methylbenzaldehyde to carrier-added (CA) 3-[18F]fluoro-4-methylphenol (30-40%, EOB). Overall synthesis time was about 2 h. Since the 4-fluoro-2-methoxy-5-methylbenzaldehyde was in turn prepared by methylation and regiospecific formylation of 3-fluoro-4-methylphenol, the overall process represents use of a removable activating group for nucleophilic aromatic substitution with [18F]fluoride for preparation of CA and NCA aryl [18F]fluorides.

Antibodies with high affinity for spiroperidol--II. Cross reactivity with iodobenzamide and domperidone

Three radioligands, 3H-spiroperidol (3H-SPD), 3H-domperidone (3H-DOMP) and 125I-iodobenzamide (125I-IBZM), were used to investigate the antibody response to two haptens, aminospiroperidol (NH2SPD) and N-aminophenethylspiroperidol (NAPS). Although structurally different, these three radioligands each bind with high affinity to the D2 dopamine receptor. Antibodies with high affinity for 3H-SPD were elicited in rabbits following immunization with the hapten NH2SPD covalently linked to keyhole limpet hemocyanin (KLH). In addition, antibodies in the rabbit anti-NH2SPD antisera bound 125I-IBZM or 3H-DOMP. Rabbit anti-NH2SPD antibodies that bound 125I-IBZM or 3H-DOMP were found to have higher affinity for IBZM or DOMP, respectively, than for SPD. The binding properties of the anti-NH2SPD antibodies that bound 3H-SPD, 125I-IBZM and 3H-DOMP were characterized using a panel of competitive inhibitors and each radioligand appeared to bind to a distinct subpopulation of anti-NH2SPD antibodies. BALB/c mice were immunized with NH2SPD-KLH or NAPS-KLH. A population of antibodies that bound 3H-SPD and a population of antibodies that bound 3H-DOMP were detected. The population of antibodies that bound 3H-DOMP was found to be heteroclitic for DOMP, since DOMP was a more effective competitive inhibitor than SPD. Binding sites for 125I-IBZM were not detected in either the anti-NH2SPD or the anti-NAPS BALB/c antisera. However, two anti-NAPS monoclonal antibodies, N6-24 and N6-29, that bind 3H-SPD with high affinity (Kd = 10(-9) M), were also found to bind IBZM (Ki = 2 x 10(-7) M) and DOMP (Ki = 2 x 10(-6) M). Although anti-NH2SPD and anti-NAPS antibodies were identified that appeared to bind 3H-SPD, 3H-DOMP or 125I-IBZM with high affinity, none of the populations of polyclonal antibodies or monoclonal antibodies bound all three ligands with high affinity.

The radiosynthesis of [18F]PK 14105 as an alternative radioligand for peripheral type benzodiazepine binding sites

A method has been developed for labelling PK 14105 [N-methyl-N-(1-methyl-propyl)-1(2-fluoro-5-nitrophenyl)isoquinoline-3- carboxamide], a ligand that has high affinity and selectivity for peripheral type benzodiazepine binding sites (PBBS), with NCA fluorine-18 (t1/2 = 109.8 min, beta + = 96.9%). The method involves treating the 2-chloro-analogue with cyclotron-produced NCA [18F]fluoride in dimethyl sulphoxide, with rubidium carbonate as base, at 140 degrees C for 20 min. Purification is achieved by separation on a reverse phase Sep-Pak followed by PHLC on a silica gel column, to give chemically and radiochemically pure product with a specific activity of ca 7.4 GBq/mumol (200 mCi/mumol), decay-corrected to the end of radionuclide production (EOB). The radiosynthesis requires 210 min. giving a radiochemical yield of 10-20%, decay-corrected to EOB. [18F]PK 14105 was found to bind avidly to sites associated with kainic acid-induced unilateral lesions of rat striata. Such binding was blocked by pre-dosing the rat with PK 11195, so providing evidence for specific binding to PBBS. These results suggest that [18F]PK 14105 has potential for studying phenomena associated with PBBS in man by PET.

Syntheses and D2 receptor affinities of derivatives of spiperone containing aliphatic halogens

The development of a high affinity dopamine receptor ligand labeled with the positron emitting radionuclide, 18F (t 1/2 = 110 min), is of considerable interest for imaging and quantification of dopamine receptors in vivo. Derivatives of spiperone, a dopamine antagonist, labeled with 18F have been prepared, but the syntheses either proceed with inefficient fluoride utilization or involve several synthetic steps subsequent to 18F incorporation. To date, only the short-lived radioisotope of carbon, 11C (t 1/2 = 20.4 min), has been efficiently incorporated in the final synthetic step of 3-N-[11C]methyl-spiperone. 3-N-Fluoroethyl, 3-N-chloroethyl, and 3-N-bromoethyl spiperone derivatives are prepared by alkylation of spiperone with the appropriate 2-tosyloxy ethyl halide. In addition, alpha-fluorospiperone, containing fluorine alpha to the butyrophenone carbonyl, has been prepared. The 3-N-haloethyl spiperones display high affinity for dopamine receptor in vitro. Incorporation of [18F]fluoride during the final synthetic step yields a high affinity, 18F-labeled dopamine receptor-binding ligand.