Automated preparation of hypoxic cell marker [18F]FRP-170 by on-column hydrolysis.

An automated synthesis for the preparation of the novel hypoxic cell marker, [(18)F]FRP-170 3, [(18)F]1-[2-fluoro-1-(hydroxymethyl)ethoxy]methyl-2- nitroimidazole, was developed using an on-column basic-hydrolysis step. The (18)F-labeled protected intermediate 2 was retained on a Sep-Pak Plus C18 cartridge and, in the same cartridge at room temperature, hydrolyzed by NaOH for deacetylation to give [(18)F]FRP-170. The elution method from the cartridge was optimized for direct injection of the crude product into an HPLC column. Thus, [(18)F]FRP-170 was prepared in 20-30% decay-corrected radiochemical yield within 60 min.     

Synthesis of [18F]fluoromethyl iodide, a synthetic precursor for fluoromethylation of radiopharmaceuticals.

[18F]fluoroiodomethane was labeled via nucleophilic substitution of diiodomethane with [18F]fluoride, and labeling conditions were optimized. The optimized labeling yield was 40 +/- 8% (decay-corrected). The synthesis and purification of [18F]fluoroiodomethane took 15 min. The reactions of [18F]fluoroiodomethane with amine, carboxylic acid, thiol and phenoxide groups produced fluoromethylated derivatives with various yields (12-95%). The results indicated that [18F]fluoroiodomethane is a valuable synthetic precursor for the introduction of an [18F]fluoromethyl group into radiopharmaceuticals.     

Synthesis and preclinical evaluation of the choline transport tracer deshydroxy-[18F]fluorocholine ([18F]dOC)

11C-labeled choline ([11C]CHO) and 18F-fluorinated choline analogues have been demonstrated to be valuable tracers for in vivo imaging of neoplasms by means of positron emission tomography (PET). The objective of the present study was to evaluate whether deshydroxy-[18F]fluorocholine, ([18F]dOC), a non-metabolizable [18F]fluorinated choline analogue, can serve as a surrogate for cholines that are able to be phosphorylated and thus allow PET-imaging solely by addressing the choline transport system. The specificity of uptake of [18F]dOC was compared with that of [11C]choline ([11C]CHO) in cultured rat pancreatic carcinoma and PC-3 human prostate cancer cells in vitro. In addition, biodistribution of [18F]dOC and [11C]CHO was compared in AR42J- and PC-3 tumor bearing mice. The in vitro studies revealed that membrane transport of both compounds can be inhibited in a concentration dependent manner by similar concentrations of cold choline (IC50 [18F]dOC= 11 microM; IC50 [11C]CHO=13 microM. In vitro studies with PC-3 and AR42J cells revealed that the internalized fraction of [18F]dOC after 5 min incubation time is comparable to that of [11C]CHO, whereas the uptake of [11C]CHO was superior after 20 min incubation time. As for [11C]CHO, kidney and liver were also the primary sites of uptake for [18F]dOC in vivo. Biodistribution data after simultaneous injection of both tracers into AR42J tumor bearing mice revealed slightly higher tumor uptake for [18F]dOC at 10 min post-injection, whereas [11C]CHO uptake was higher at later time points. In conclusion, [18F]dOC is taken up into AR42J rat pancreatic carcinoma and PC-3 human prostate cancer cells by a choline specific transport system. Similar transport rates of [18F]dOC and [11C]CHO result in comparable cellular uptake levels at early time points. In contrast to [18F]dOC, which is transported but not intracellularly trapped, the choline kinase substrate [11C]CHO is transported into tumor cells and retained. Thus, the signal obtained by imaging early after injection is mainly reflecting transport, whereas a valid quantification of choline kinase activity needs imaging at later time points. Further studies have to clarify whether quantification of the transport capacity or the choline kinase activity result in a better pathophysiological correlate and thus is the more useful process for tumor characterization.     

Fully automated [18F]fluorocholine synthesis in the TracerLab MX FDG Coincidence synthesizer

INTRODUCTION: We developed a new fully automated method for the radiosynthesis of [18F]fluorocholine by modifying the commercial 2-[18F]fluoro-2-d-deoxy-glucose ([18F]FDG) synthesizer module (GE TracerLab MX, formerly Coincidence). METHODS: [18F]Flurocholine was synthesized by (18)F-fluoroalkylation of N,N-dimethylaminoethanol using [18F]fluorobromomethane as fluoromethylating agent. [18F]Fluorobromomethane was produced by reaction of dibromomethane with [18F]fluoride, assisted by Kryptofix 2.2.2. RESULTS: After purification on solid-phase extraction cartridges, the [18F]fluorocholine was obtained in 15-25% radiochemical yields (decay not corrected), with more than 99% radiochemical purity. Specific activity was more than 37 GBq/micromol. Synthesis time was less than 35 min. CONCLUSION: This new automated synthesis technique provides high and reproducible yields that could be dedicated for routine use with the same [18F]FDG disposable cassette system.     

PET/CT in patients with hepatocellular carcinoma using [18F]fluorocholine: preliminary comparison with [18F]FDG PET/CT

Purpose The diagnostic accuracy of [¹⁸F]fluorodeoxyglucose (FDG) PET is insufficient to characterise hepatocellular carcinoma (HCC) in liver masses and to diagnose all cases of recurrent HCC. HCC has been reported to take up [¹¹C]acetate, but routine use of this tracer is difficult. Choline is another tracer of lipid metabolism, present in large amounts in HCC. In a proof-of-concept study, we evaluated [¹⁸F]fluorocholine (FCH) uptake by HCC and compared FCH PET/CT with FDG PET/CT.Methods Twelve patients with newly diagnosed (n=8) or recurrent HCC (n=4) were prospectively enrolled. HCC was assessed by histology in eight cases and by American Association for the Study of Liver Diseases (AASLD) criteria in four cases. All patients underwent whole-body PET/CT 10 min after injection of 4 MBq/kg FCH. Within 1 week, 9 of the 12 patients also underwent whole-body FDG PET/CT 1 h after injection of 5 MBq/kg FDG.Results The per-patient analysis showed a detection rate of 12/12 using FCH PET/CT for both newly diagnosed and recurrent HCC. The median signal to noise ratio was 1.5±0.38. There was a trend towards a higher FCH SUV_max in well-differentiated HCC (15.6±7.9 vs 11.9±0.9, NS). Of the nine patients who underwent FCH and FDG PET/CT, all nine were positive with FCH whereas only five were positive with FDG.Conclusion FCH provides a high detection rate for HCC, making it potentially useful in the initial evaluation of HCC or in the detection of recurrent disease. The favourable result of this proof-of-concept study opens the way to a phase III prospective study.     

2-[18F]Fluoroethanol and 3-[18F]fluoropropanol: facile preparation,biodistribution in mice,and their application as nucleophiles in the synthesis of [18F]fluoroalkyl aryl ester and ether PET tracers

Introduction2-[¹⁸F]Fluoroethoxy and 3-[¹⁸F]fluoropropoxy groups are common moieties in the structures of radiotracers used with positron emission tomography. The objectives of this study were (1) to develop an efficient one-step method for the preparation of 2-[¹⁸F]fluoroethanol (2-[¹⁸F]FEtOH) and 3-[¹⁸F]fluoropropanol (3-[¹⁸F]FPrOH); (2) to demonstrate the feasibility of using 2-[¹⁸F]FEtOH as a nucleophile for the synthesis of 2-[¹⁸F]fluoroethyl aryl esters and ethers; and (3) to determine the biodistribution profiles of 2-[¹⁸F]FEtOH and 3-[¹⁸F]FPrOH in mice.Methods2-[¹⁸F]FEtOH and 3-[¹⁸F]FPrOH were prepared by reacting n-Bu₄N[¹⁸F]F with ethylene carbonate and 1,3-dioxan-2-one, respectively, in diethylene glycol at 165 °C and purified by distillation. 2-[¹⁸F]fluoroethyl 4-fluorobenzoate and 1-(2-[¹⁸F]fluoroethoxy)-4-nitrobenzene were prepared by coupling 2-[¹⁸F]FEtOH with 4-fluorobenzoyl chloride and 1-fluoro-4-nitrobenzene, respectively. Biodistribution and PET/CT imaging studies of 2-[¹⁸F]FEtOH and 3-[¹⁸F]FPrOH were performed in normal female Balb/C mice.ResultsThe preparation of 2-[¹⁸F]FEtOH and 3-[¹⁸F]FPrOH took 60 min, and their decay-corrected yields were 88.6 ± 2.0% (n = 9) and 65.6 ± 10.2% (n = 5), respectively. The decay-corrected yields for the preparation of 2-[¹⁸F]fluoroethyl 4-fluorobenzoate and 1-(2-[¹⁸F]fluoroethoxy)-4-nitrobenzene were 36.1 ± 5.4% (n = 3) and 27.7 ± 10.7% (n = 3), respectively. Imaging/biodistribution studies in mice using 2-[¹⁸F]FEtOH showed high initial radioactivity accumulation in all major organs followed by very slow clearance. On the contrary, by using 3-[¹⁸F]FPrOH, radioactivity accumulated in all major organs was cleared rapidly, but massive in vivo defluorination (31.3 ± 9.57%ID/g in bone at 1 h post-injection) was observed.ConclusionsUsing 2-[¹⁸F]FEtOH/3-[¹⁸F]FPrOH as a nucleophile is a competitive new strategy for the synthesis of 2-[¹⁸F]fluoroethyl/3-[¹⁸F]fluoropropyl aryl esters and ethers. Our biodistribution data emphasize the importance of in vivo stability of PET tracers containing a 2-[¹⁸F]fluoroethyl or 3-[¹⁸F]fluoropropyl group due to high background and high bone uptake resulting from 2-[¹⁸F]FEtOH and 3-[¹⁸F]FPrOH, respectively. This is especially important for their aryl ester derivatives which are prone to in vivo hydrolysis.     

Synthesis of [18F]2-deoxy-2-fluoro-D-glucose from highly reactive [18F]tetraethylammonium fluoride prepared by hydrolysis of [18F]fluorotrimethylsilane

18F-Labeled fluorotrimethylsilane was prepared by nucleophilic substitution of chlorotrimethylsilane with reactor produced [18F]fluoride. Hydrolysis of fluorotrimethylsilane by aqueous tetraethylammonium hydroxide followed by removal of water with a mechanical pump gave a powerful source of no carrier added nucleophilic 18F. Reaction of this purified 18F preparation with 4,6-benzylidene-1-beta-O-methyl D-mannopyranoside-2.3-cyclic sulfate was complete in 2 min at 80 degrees C and gave two labeled products with similar retention times on reverse phase HPLC. Allowing for decay and handling losses during deprotection, the maximum yield of [18F]2-deoxy-2-fluoro-D-glucose from no-carrier added tetraethylammonium fluoride was 50%. Incorporation of 18F into organic products was 30% complete in 10 min at room temperature. An identical time-course was observed for reaction of 3-O-triflyl-1,2-5,6-diisopropylidene-D-allofuranose, the starting material for 3-deoxy-3-fluoro-D-glucose. Reaction of tetraethylammonium fluoride with chlorotrimethylsilane was more rapid and much more tolerant of water than the fluorosugar reactions. Chlorotrimethylsilane can be used to recover unreacted 18F from reaction mixtures.     

N-[18F]fluoroacetyl-D-glucosamine: a potential agent for cancer diagnosis

Positron labeled substrates such as sugars, amino acids, and nucleosides have been investigated for the in-vivo evaluation of biochemical processes in cancerous tissue. Hexosamines are obligatory structural components of many biologically important macromolecules, including membrane glycoproteins and mucopolysaccharide. We evaluated a new synthesized pharmaceutical, N-[18F]fluoroacetyl-D-glucosamine (18F-FAG), which is a structural analog of N-acetyl-D-glucosamine. C3H/HeMsNRS mice bearing spontaneous hepatomas were used for the tissue distribution study. At 60 min after injection, high uptakes were found in tumor (5.16, mean value of %dose/g), liver (3.71), and kidney (3.27). The tumor uptake of 18F-FAG showed the highest value in all tissue. In the PET study, VX-2 carcinoma of the rabbit was clearly visualized. Our preliminary results suggest that 18F-FAG has potential as a new agent for tumor imaging.     

Synthesis of 2'-deoxy-2'-[18F]fluoro-beta-D-arabinofuranosyl nucleosides, [18F]FAU, [18F]FMAU, [18F]FBAU and [18F]FIAU,as potential PET agents for imaging cellular proliferation. Synthesis of [18F]labelled FAU,FMAU, FBAU,FIAU

An efficient and reliable synthesis of 2'-deoxy-2'-[(18)F]fluoro-beta-D-arabinofuranosyl nucleosides is presented. Overall decay-corrected radiochemical yields of 35-45% of 4 analogs, FAU, FMAU, FBAU and FIAU are routinely obtained in >98% radiochemical purity and with specific activities of greater than 3 Ci/micromol (110 MBq/micromol) in a synthesis time of approximately 3 hours. When approximately 220 mCi (8.15 GBq) of starting [(18)F]fluoride is used, 25 -30 mCi (0.93 -1.11 GBq) of product (enough to image two patients sequentially) is typically obtained.     

Development of microwave-based automated nucleophilic [18F]fluorination system and its application to the production of [18F]flumazenil

IntroductionThis study presents the development of an automated radiosynthesis system integrating a microwave reactor and its subsequent application in the synthesis of [¹⁸F]flumazenil, a potentially useful compound in the evaluation of central benzodiazepine receptor density.MethodsPreparation of dry [K/K₂₂₂]⁺¹⁸F^? complex and radiofluorination of the nitro-flumazenil precursor were achieved using the developed microwave-based radiosynthesis system. The crude product was prepurified in a C18 cartridge followed by reversed-phase preparative high-performance liquid chromatography. The isolated [¹⁸F]flumazenil was evaporated in vacuo and reconstituted in an ethanol-free solution.ResultsOptimum incorporation of ¹⁸F^? in the nitro-precursor was achieved in 5 min time utilizing 2 mg of precursor in N,N-dimethylformamide reacted at 160°C which gave an incorporation yield of 40±5%. The radiochemical yield obtained at the end of synthesis was 26±4% (EOB) with a radiochemical purity of >99% and a total synthesis time of about 55–60 min. The produced [¹⁸F]flumazenil was observed to be stable for at least 8 h.ConclusionThe developed [¹⁸F]flumazenil radiosynthesis system offers shorter reaction time, simplicity in operation and applicability for use in routine clinical practice.     

Biodistribution,pharmacokinetics and PET Imaging of [18F]FMISO, [18F]FDG and [18F]FAc in a sarcoma- and inflammation-bearing mouse model

2-Deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG), [¹⁸F]fluoroacetate ([¹⁸F]FAc) and [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO) were all considered to be positron emission tomography (PET) probes for tumor diagnosis, though based on different rationale of tissue uptake. This study compared the biodistribution, pharmacokinetics and imaging of these three tracers in a sarcoma- and inflammation-bearing mouse model.MethodsC3H mice were inoculated with 2×10⁵ KHT sarcoma cells in the right thigh on Day 0. Turpentine oil (0.1 ml) was injected in the left thigh on Day 11 to induce inflammatory lesion. Biodistribution, pharmacokinetics and microPET imaging of [¹⁸F]FMISO, [¹⁸F]FDG and [¹⁸F]FAc were performed on Day 14 after tumor inoculation.ResultsThe inflammatory lesions were clearly visualized by [¹⁸F]FDG/microPET and autoradiography at 3 days after turpentine oil injection. The tumor-to-muscle and inflammatory lesion-to-muscle ratios derived from microPET imaging were 6.79 and 1.48 for [¹⁸F]FMISO, 8.12 and 4.69 for [¹⁸F]FDG and 3.72 and 3.19 for [¹⁸F]FAc at 4 h post injection, respectively. Among these, the tumor-to-inflammation ratio was the highest (4.57) for [¹⁸F]FMISO compared with that of [¹⁸F]FDG (1.73) and [¹⁸F]FAc (1.17), whereas [¹⁸F]FAc has the highest bioavailability (area under concentration of radiotracer vs. time curve, 116.2 h×percentage of injected dose per gram of tissue).ConclusionsMicroPET images and biodistribution studies showed that the accumulation of [¹⁸F]FMISO in the tumor is significantly higher than that in inflammatory lesion at 4 h post injection. [¹⁸F]FDG and [¹⁸F]FAc delineated both tumor and inflammatory lesions. Our results demonstrated the potential of [¹⁸F]FMISO/PET in distinguishing tumor from inflammatory lesion.     

[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.     

Comparison of [18F]FDOPA, [18F]FMT and [18F]FECNT for imaging dopaminergic neurotransmission in mice

INTRODUCTION: The clinically established positron emission tomography (PET) tracers 6-[(18)F]-fluoro-l-DOPA ([(18)F]FDOPA), 6-[(18)F]-fluoro-l-m-tyrosine ([(18)F]FMT) and 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-[(18)F]-fluoroethyl)-nortropane ([(18)F]FECNT) serve as markers of presynaptic integrity of dopaminergic nerve terminals in humans. This study describes our efforts to adopt the methodology of human Parkinson's disease (PD) PET studies to mice. METHODS: The PET imaging characteristics of [(18)F]FDOPA, [(18)F]FMT and [(18)F]FECNT were analyzed in healthy C57BL/6 mice using the dedicated small-animal PET tomograph quad-HIDAC. Furthermore, [(18)F]FECNT was tested in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. RESULTS: [(18)F]FDOPA and [(18)F]FMT failed to clearly visualize the mouse striatum, whereas PET experiments using [(18)F]FECNT proved that the employed methodology is capable of delineating the striatum in mice with exquisite resolution. Moreover, [(18)F]FECNT PET imaging of healthy and MPTP-lesioned mice demonstrated that the detection and quantification of striatal degeneration in lesioned mice can be accomplished. CONCLUSIONS: This study shows the feasibility of using [(18)F]FECNT PET to analyze noninvasively the striatal degeneration in the MPTP mouse model of PD. This methodology can be therefore considered as a viable complement to established in vivo microdialysis and postmortem techniques.     

Synthesis of a [18F]labeled chelidonine derivative as a possible antitumor agent.

[(+/-)-[5bRS-(5b alpha, 6beta, 12b alpha)]-5b,6,7,12b,13,14-hexahydro-13-methyl[1,3]benzodioxolo[5,6-c]-1,3-dioxolo[4,5-1] phenanthridin-6-yl]-4-[18F]fluorobenzoate (6b) an aromatic ester of chelidonine was prepared as a possible PET radiotracer. Compound (6b) was prepared in no-carrier-added (n.c.a) form from the 4-N,N,N-trimethylanilinium triflate derivative (5) in one step. The best results were obtained using Kryptofix(2.2.2)/[18F] and DMSO as the solvent at 90 degrees C. Column chromatography afforded the desired compound in overall radiochemical yield of 65-70% (EOS) with a specific activity of about 3000 Ci/mmole and radiochemical purity greater than 95% in 15 min.     

Radiofluorination with reactor-produced Cesium [18F]fluoride: No-carrier-added [18F]2-fluoronicotine and [18F]6-fluoronicotine

We have synthesised no-carrier-added [¹⁸F]2-fluoronicotine by the halogen-exchange reaction of reactor-produced Cs¹⁸F upon 2-bromonicotine in refluxing DMSO. No special drying of the Cs¹⁸F was required. [¹⁸F]2-Fluoronicotine was isolated by reversed-phase HPLC and production of 220 MBq (6 mCi) ready for injection represented a decay-corrected radiochemical yield of 23% and required 3.5 h from end-of-bombardment. Similarly, [¹⁸F]6-fluoronicotine was prepared from 6-bromonicotine in amounts of 80 MBq (2.2 mCi) or 8% yield. These quantities are sufficient for studies in humans.     

An improved preparation of [18F]FPBM: A potential serotonin transporter (SERT) imaging agent

IntroductionIn vivo positron emission tomography (PET) imaging of the serotonin transporter (SERT) is a valuable tool in drug development and in monitoring brain diseases with altered serotonergic function. We have developed a two-step labeling reaction for the preparation of the high serotonin affinity ligand [¹⁸F]FPBM ([¹⁸F]2-(2′-((dimethylamino)methyl)-4′-(3-fluoropropoxy)phenylthio)benzenamine, 1).MethodTo improve and automate the radiolabeling of [¹⁸F]FPBM, 1, an intermediate, [¹⁸F]3-fluoropropyltosylate, [¹⁸F]4, was prepared first, and then it was reacted with the phenol precursor (4-(2-aminophenylthio)-3-((dimethylamino)methyl)phenol, 3) to afford [¹⁸F]FPBM, 1. To optimize the labeling, this O-alkylation reaction was evaluated under different temperatures, using different bases and varying amounts of precursor 3. The desired product was obtained after a solid phase extraction (SPE) purification.ResultsThis two-step radiolabeling reaction successfully produced the desired [¹⁸F]FPBM, 1, with an excellent radiochemical purity (> 95%, n = 8). Radiochemical yields were between 31% and 39% (decay corrected, total time of labeling: 70 min, n = 8). The SPE purification cannot completely remove pseudo-carriers in the final dose of [¹⁸F]FPBM, 1. The concentrations of major pseudo-carriers were measured by UV-HPLC (476–676, 68–95 and 50–71 μg for precursor 3, O-hydroxypropyl and O-allyloxy derivatives, 5 and 6, respectively). To investigate the potential inhibition of SERT binding of these pseudo-carriers, we performed in vitro competition experiments evaluated by autoradiography. Known amounts of ‘standard’ FPBM, 1, of the pseudo-carriers, 5 and 6, were added to the HPLC-purified [¹⁸F]1 dose. The inhibition of ‘standard’ FPBM, 1, binding to the SERT binding sites, using monkey brain sections, were measured (EC₅₀ = 13, 46, 7.1 and 8.3 nM, respectively for 1, precursor 3, O-hydroxypropyl and O-allyloxy derivative of 3).ConclusionAn improved radiolabeling method by a SPE purification for preparation of [¹⁸F]FPBM, 1, was developed. The results suggest that it is feasible to use this labeling method to prepare [¹⁸F]FPBM, 1, without affecting in vivo SERT binding.     

Investigations to the synthesis of n.c.a. [18F]FClO3 as electrophilic fluorinating agent.

An approach to synthesize the electrophilic fluorinating agent no-carrier-added (n.c.a.) [18F]perchloryl fluoride ([18F]FClO3) in superacidic media in the presence of KClO4 or anhydrous perchloric acid starting from [18F]fluoride was demonstrated in this study. However, the radiochemical yields were low (1-6%) and poorly reproducible. Fluorosulphonic acid proved to be an essential intermediate as revealed by non-radioactive experiments. A key problem in the preparation of [18F]FClO3 is the assumed kinetic inhibition due to the unfavourable stoichiometric ratio of the ClO4 moiety to [18F]HSO3F.     

1-[18F]fluoro-2-propanol p-toluenesulfonate: a synthon for the preparation of N-([18F]fluoroisopropyl)amines

The new radiochemical synthon 1-[¹⁸F]fluoro-2-propanol p-toluenesulfonate 2′a is prepared with a radiochemical yield of 45% [corrected for decay to beginning of synthesis (BOS), synthesis time 40 min]. This compound is used to prepare the [¹⁸F]fluoroisopropyl-alkylated derivatives of benzylamine and norephedrine with a yield of 7 and 2% respectively (BOS, synthesis time 90 min). This alkylation reaction with 2′a gives a good perspective for the preparation of [¹⁸F]fluoro-labelled analogues of β₁-adrenergic receptor binding ligands for PET.