Synthesis and preclinical evaluation of [11C-carbonyl]PF-04457845 for neuroimaging of fatty acid amide hydrolase

IntroductionFatty acid amide hydrolase (FAAH) has a significant role in regulating endocannabinoid signaling in the central nervous system. As such, FAAH inhibitors are being actively sought for pain, addiction, and other indications. This has led to the recent pursuit of positron emission tomography (PET) radiotracers targeting FAAH. We report herein the preparation and preclinical evaluation of [¹¹C-carbonyl]PF-04457845, an isotopologue of the potent irreversible FAAH inhibitor.MethodsPF-04457845 was radiolabeled at the carbonyl position via automated [¹¹C]CO₂-fixation. Ex vivo brain biodistribution of [¹¹C-carbonyl]PF-04457845 was carried out in conscious rats. Specificity was determined by pre-administration of PF-04457845 or URB597 prior to [¹¹C-carbonyl]PF-04457845. In a separate experiment, rats injected with the title radiotracer had whole brains excised, homogenized and extracted to examine irreversible binding to brain parenchyma.ResultsThe title compound was prepared in 5 ± 1% (n = 4) isolated radiochemical yield based on starting [¹¹C]CO₂ (decay uncorrected) within 25 min from end-of-bombardment in > 98% radiochemical purity and a specific activity of 73.5 ± 8.2 GBq/μmol at end-of-synthesis. Uptake of [¹¹C-carbonyl]PF-04457845 into the rat brain was high (range of 1.2–4.4 SUV), heterogeneous, and in accordance with reported FAAH distribution. Saturable binding was demonstrated by a dose-dependent reduction in brain radioactivity uptake following pre-treatment with PF-04457845. Pre-treatment with the prototypical FAAH inhibitor, URB597, reduced the brain radiotracer uptake in all regions by 71–81%, demonstrating specificity for FAAH. The binding of [¹¹C-carbonyl]PF-04457845 to FAAH at 40 min post injection was irreversible as 98% of the radioactivity in the brain could not be extracted.Conclusions[¹¹C-carbonyl]PF-04457845 was rapidly synthesized via an automated radiosynthesis. Ex vivo biodistribution studies in conscious rodents demonstrate that [11C PF-04457845 is a promising candidate radiotracer for imaging FAAH in the brain with PET. These results coupled with the known pharmacology and toxicology of PF-04457845 should facilitate clinical translation of this radiotracer.     

Radiosynthesis and ex vivo evaluation of [11C-carbonyl]carbamate- and urea-based monoacylglycerol lipase inhibitors

IntroductionMonoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) are the two primary enzymes that regulate the tone of endocannabinoid signaling. Although new PET radiotracers have been discovered for imaging FAAH in vivo, no such radiotracer exists for imaging MAGL. Here we report the radiosynthesis of five candidate MAGL radiotracers and their ex vivo evaluations in mice and rats.MethodsCandidate carbamate and urea MAGL inhibitors were radiolabeled at the carbonyl position by [¹¹C]CO₂ fixation. Radiotracers were administered (tail-vein injection) to rodents and brain uptake of radioactivity measured at early and late time points ex vivo. Specificity of uptake was explored by pretreatment with unlabeled inhibitors (2 mg/kg, ip) 30 min prior to radiotracer administration.ResultsAll five candidate MAGL radiotracers were prepared in high specific activity (> 65 GBq/μmol) and radiochemical purity (> 98%). Moderate brain uptake (0.2–0.8 SUV) was observed for each candidate while pretreatment did not reduce uptake for four of the five tested. For two candidates ([¹¹C]12 and [¹¹C]14), high retention of radioactivity was observed in the blood (ca. 10 and 4 SUV at 40 min) which was blocked by pretreatment with unlabeled inhibitors. The most promising candidate, [¹¹C]18, demonstrated moderate brain uptake (ca. 0.8 SUV) which showed circa 50% blockade by pretreatment with unlabeled 18.ConclusionOne putative and four reported potent and selective MAGL inhibitors have been radiolabeled via [¹¹C]CO₂ fixation as radiotracers for this enzyme. Despite the promising in vitro pharmacological profile, none of the five candidate radiotracers exhibited in vivo behavior suitable for PET neuroimaging.     

Radiosynthesis of N-[11C]-methyl-hydroxyfasudil as a new potential PET radiotracer for rho-kinases (ROCKs)

N-[¹¹C]-methyl-hydroxyfasudil was synthesized as a new potential radiotracer for rho-kinases (ROCKs) via a two-step one-pot radiosynthesis. The first step was the methylation of the precursor N-Boc-hydroxyfasudil-sodium salt/benzo-15-crown-5 complex with [¹¹C]methyl iodide. The second step involved deprotection of the tert-butoxycarbonyl protecting group. The radiochemical and chemical purities of N-[¹¹C]-methyl-hydroxyfasudil were >95% and specific radioactivity was 1565–2565 mCi/μmol at the end of the synthesis.     

[11 C]Rhodamine-123: Synthesis and biodistribution in rodents

IntroductionRhodamine-123 is a known substrate for the efflux transporter, P-glycoprotein (P-gp). We wished to assess whether rhodamine-123 might serve as a useful substrate for developing probes for imaging efflux transporters in vivo with positron emission tomography (PET). For this purpose, we aimed to label rhodamine-123 with carbon-11 (t_1/2 = 20.4 min) and to study its biodistribution in rodents.Methods[¹¹ C]Rhodamine-123 was prepared by treating rhodamine-110 (desmethyl-rhodamine-123) with [¹¹ C]methyl iodide. The biodistribution of this radiotracer was studied with PET in wild-type mice and rats, in efflux transporter knockout mice, in wild-type rats pretreated with DCPQ (an inhibitor of P-gp) or with cimetidine (an inhibitor of organic cation transporters; OCT), and in P-gp knockout mice pretreated with cimetidine. Unchanged radiotracer in forebrain, plasma and peripheral tissues was also measured ex vivo at 30 min after radiotracer administration to wild-type and efflux transporter knockout rodents.Results[¹¹ C]Rhodamine-123 was obtained in 4.4% decay-corrected radiochemical yield from cyclotron-produced [¹¹ C]carbon dioxide. After intravenous administration of [¹¹ C]rhodamine-123 to wild-type rodents, PET and ex vivo measurements showed radioactivity uptake was very low in brain, but relatively high in some other organs such as heart, and especially liver and kidney. Inhibition of P-gp increased uptake in brain, heart, kidney and liver, but only by up to twofold. Secretion of radioactivity from kidney was markedly reduced by OCT knockout or pretreatment with cimetidine.Conclusions[¹¹ C]Rhodamine-123 was unpromising as a PET probe for P-gp function and appears to be a strong substrate of OCT in kidney. Cimetidine appears effective for blocking OCT in kidney in vivo.     

Rat Imaging and In Vivo Stability Studies using [11C]-Dimethyl-Diphenyl Ammonium,a Candidate Agent for PET-Myocardial Perfusion Imaging

BackgroundPET myocardial perfusion imaging (MPI) holds several advantages over SPECT for diagnosing coronary artery disease. The short half-lives of prevailing PET-MPI agents hamper wider clinical application of PET in nuclear cardiology; prompting the development of novel PET-MPI agents. We have previously reported on the potential of radiolabeled ammonium salts, and particularly on that of [¹¹C]dimethyl-diphenyl-ammonium ([¹¹C]DMDPA), for cardiac PET imaging. This study was designed to improve the radiosynthesis and increase the yield of [¹¹C]DMDPA, characterize more meticulously the kinetics of radioactivity distribution after its injection via micro-PET/CT studies, and further explore its potential for PET-MPI.MethodsThe radiosynthetic procedure of [¹¹C]DMDPA was improved with respect to the previously reported one. The kinetics of radioactivity distribution following injection of [¹¹C]DMDPA were investigated in juvenile and young adult male SD rats using microPET/CT, and compared to those of [¹³N]NH₃. Furthermore, the metabolic fate of [¹¹C]DMDPA in vivo was examined after its injection into rats.ResultsFollowing a radiosynthesis time of 25–27 min, 11.9 ± 1.1 GBq of [¹¹C]DMDPA was obtained, with a 43.7% ± 4.3% radiochemical yield (n = 7). Time activity curves calculated after administration of [¹¹C]DMDPA indicated rapid, high and sustained radioactivity uptake in hearts of both juvenile and young adult rats, having a two-fold higher cardiac radioactivity uptake compared to [¹³N]NH₃. Accordingly, at all time points after injection to both juvenile and young adult rats, image quality of the left ventricle was higher with [¹¹C]DMDPA compared to [¹³N]NH₃. In vivo stability studies of [¹¹C]DMDPA indicate that no radioactive metabolites could be detected in plasma, liver and urine samples of rats up to 20 min after injection, suggesting that [¹¹C]DMDPA is metabolically stable in vivo.ConclusionsThis study further illustrates that [¹¹C]DMDPA holds, at least in part, essential qualities required from a PET-MPI probe. Owing to the improved radiosynthetic procedure reported herein, [¹¹C]DMDPA can be produced in sufficient amounts for clinical use.     

Radiolabeling of the cannabinoid receptor agonist AZD1940 with carbon-11 and PET microdosing in non-human primate

IntroductionN-(2-tert-butyl-1-((4,4-difluorocyclohexyl)methyl)-1 H-benzo[d]imidazol-5-yl)ethanesulfonamide (AZD1940) is a candidate drug for treatment of neuropathic pain. As part of the preclinical evaluation of AZD1940, a microdosing study with positron emission tomography (PET) was conducted to assess brain exposure.MethodsAZD1940 was radiolabeled with carbon-11 in the benzimidazole moiety. The radioactive precursor, lithium [¹¹C]pivalate was obtained via ¹¹C-carboxylation of tert-butyl lithium. The target compound, [¹¹C]AZD1940, was in turn obtained by the microwave assisted reaction between lithium [¹¹C]pivalate and the o-phenylene diamine analog of AZD1940 (N-(3-amino-4-((4,4-difluorocyclohexyl)methylamino)phenyl)ethanesulfonamide) in neat phosphorous oxychloride. A brain PET measurement was performed in cynomolgus monkey.ResultsThe overall radiochemical yield of final formulated radiochemically pure (> 99%) [¹¹C]AZD1940 was 0.4% (uncorrected for decay) and the specific radioactivity was 13 GBq/μmol at time of administration (58 min after end of bombardment). After intravenous injection to cynomolgus monkey, the maximum concentration of radioactivity detected in the brain region of interest was 0.7% of the total injected radioactivity. The regional distribution of radioactivity within brain was homogenous.ConclusionsAZD1940 was radiolabelled with carbon-11 and its brain exposure, assessed using PET, was relatively low in comparison to peripheral organ exposure.     

Synthesis and evaluation of 11C-labeled naphthalene derivative as a novel non-peptidergic probe for the β-secretase (BACE1) imaging in Alzheimer's disease brain

IntroductionAs a first trial for in vivo imaging of β-secretase (BACE1) in Alzheimer's disease brain, we applied a novel non-peptidergic small molecule which has high affinity to the enzyme, naphthalene-1-carboxylic acid (3′-chloro-4′-fluoro-4-piperazin-1-yl-biphenyl-3-yl)amide (NCFB) into positron emission tomography (PET) probe. In the current study, N-¹¹C-methylated compound of NCFB, [¹¹C]Me-NCFB was synthesized and evaluated for the visualization of BACE1 in brain.MethodsBACE1 inhibitory constant was measured by FRET assay. [¹¹C]Me-NCFB was synthesized from NCFB with [¹¹C]methyl triflate. To evaluate properties of [¹¹C]Me-NCFB, log P value, stability in mouse plasma and brain uptake index were measured. The biodistribution in 6-week-old ddY mice was also studied.ResultsBACE1 inhibitory constant showed an affinity of Me-NCFB to the enzyme (IC₅₀ = 2.3 ± 0.80 μM). [¹¹C]Me-NCFB was synthesized in a 3.0% ± 0.55% decay-corrected radiochemical yield. [¹¹C]Me-NCFB showed high lipophilicity, high stability in mouse plasma and blood–brain barrier (BBB) permeability. Injected to 6-week-old ddY mice, [¹¹C]Me-NCFB penetrated BBB and was retained in the brain (0.79% ± 0.22% ID/g at 2 min and 0.75% ± 0.08% ID/g at 60 min after injection, respectively), moreover, rapid blood clearance was observed.Conclusion[¹¹C]Me-NCFB could have a potential as a PET probe for the imaging of BACE1 in the brain.     

Radiosynthesis and in vivo evaluation of fluorinated huprine derivates as PET radiotracers of acetylcholinesterase

IntroductionDeveloping positron emission tomography (PET) radiotracers for non-invasive study of the cholinergic system is crucial to the understanding of neurodegenerative diseases. Although several acetylcholinesterase (AChE) PET tracers radiolabeled with carbon-11 exist, no fluorinated radiotracer is currently used in clinical imaging studies. The purpose of the present study is to describe the first fluorinated PET radiotracer for this brain enzyme.MethodsThree structural analogs of huprine, a specific AChE inhibitor presenting high affinity towards AChE in vitro, were synthesized and labeled with fluorine-18 via a mesylate/fluoro-nucleophilic aliphatic substitution: ([¹⁸ F]-FHUa, [¹⁸ F]-FHUb and [¹⁸ F]-FHUc). Initial biological evaluation included in vitro autoradiography in rat with competition with an AChE inhibitor at different concentrations, and microPET-scan on anesthetized rats. In vivo PET studies in anesthetized cat focused on [¹⁸ F]-FHUa.Results and ConclusionsAlthough radiosynthesis of these huprine analogs was straightforward, they showed poor brain penetration potential, partially reversed after pharmacological inhibition of P-glycoprotein. These results indicated that current huprine analogs are not suitable for PET mapping of brain AChE receptors, but require physicochemical modulation in order to increase brain penetration.     

Preclinical and the first clinical studies on [11C]ITMM for mapping metabotropic glutamate receptor subtype 1 by positron emission tomography

IntroductionPreclinical studies and first positron emission tomography (PET) imaging studies were performed using N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-[¹¹C]methoxy-N-methylbenzamide ([¹¹C]ITMM) to map metabotropic glutamate receptor type 1 (mGluR1) in the human brain.Methods[¹¹C]ITMM was synthesized by O-methylation of the desmethyl precursor with [¹¹C]methyl triflate in the presence of NaOH at room temperature. In vitro selectivity and brain distributions of [¹¹C]ITMM in mice were characterized. Radiation absorbed-dose by [¹¹C]ITMM in humans was calculated from mouse distribution data. Acute toxicity of ITMM at 4.72 mg/kg body weight (> 74,000-fold clinical equivalent dose of [¹¹C]ITMM) was evaluated. Mutagenicity of ITMM was studied by the Ames test. Clinical PET imaging of mGluR1 with [¹¹C] ITMM was performed in a healthy volunteer.ResultsITMM had low activity for a 28-standard receptor binding profile. Regional brain uptake of [¹¹C]ITMM in mice was heterogeneous and consistent with known mGluR1 distributions. The radiation absorbed-dose by [¹¹C]ITMM in humans was sufficiently low for clinical use, and no acute toxicity or mutagenicity of ITMM occurred. A 90-min dynamic PET scan with [¹¹C]ITMM in a healthy volunteer showed a gradual increase of radioactivity in the cerebellum. Total distribution volume of [¹¹C]ITMM was highest in the cerebellum, followed by thalamus, cerebral cortex, and striatum; regional differences in brain radioactivity corresponded to the mGluR1 distribution in the brain. Peripherally, [¹¹C]ITMM was stable in humans: 60% of the plasma radioactivity remained in the unchanged form for 60 min.Conclusions[¹¹C] ITMM is a suitable radioligand for imaging mGluR1 in the human brain providing acceptable dosimetry and pharmacological safety at the dose required for PET.     

Synthesis and evaluation of new radioligands [11C]A-833834 and [11C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors

Introductionα7-nicotinic acetylcholine receptor (α7-nAChR) is one of the major neuronal nAChR subtypes. α7-nAChR is involved in variety of neuronal processes and disorders including schizophrenia and Alzheimer's disease. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be discovered.MethodsHigh binding affinity α7-nAChR ligands A-833834 and A-752274 were radiolabeled with ¹¹C. Baseline and blockade biodistribution studies in the mouse brain of [¹¹C]A-833834 (5-(6-(5-[¹¹C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole) and [¹¹C]A-752274 (2-(6-[¹¹C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one) were performed. [¹¹C]A-752274 was evaluated in a baseline baboon PET study.Results[¹¹C]A-833834 and [¹¹C]A-752274 were synthesized by radiomethylation of corresponding des-methyl precursors. The radioligands were prepared with radiochemical yield of 12%–32%, high specific radioactivity (330–403 GBq/μmol) and radiochemical purity > 95%. Dissection studies with [¹¹C]A-833834 demonstrated low specific α7-nAChR binding in the mouse brain. [¹¹C]A-752274 specifically (~ 50%) labeled α7-nAChR in the mouse thalamus. However, [¹¹CA-752274 exhibited low brain uptake in baboon (%SUV < 100).ConclusionTwo novel α7-nAChR ligands radioligands were synthesized and studied in animals. Specific binding of [¹¹C]A-833834 in the mouse brain is low due to the insufficient binding affinity of the radioligand. The very high binding affinity [¹¹C]A-752274 exhibited good specific binding in the α7-nAChR-rich mouse brain regions. The low uptake of [¹¹C]A-752274 in the baboon brain is due to its high hydrophilicity, rapid metabolism or other properties. Future development of α7-nAChR PET radioligands will be based on compounds with high binding affinities and good blood–brain barrier permeability.     

Radiosynthesis,biodistribution and imaging of [11C]YM155, a novel survivin suppressant,in a human prostate tumor-xenograft mouse model

IntroductionSepantronium bromide (YM155) is an antitumor drug in development and is a first-in-class chemical entity, which is a survivin suppressant. We developed a radiosynthesis of [¹¹C]YM155 to non-invasively evaluate its tissue and tumor distribution in mice bearing human prostate tumor xenografts.MethodsMethods utilizing [¹¹C]acetyl chloride and [¹¹C]methyl triflate, both accessible with automated radiosynthesis boxes, were evaluated. The O-methylation of ethanolamine-alkolate with [¹¹C]methyl triflate proved to be the key development toward a rapid and efficient process. The whole-body distribution of [¹¹C]YM155 in PC-3 xenografted mice was examined using a planar positron imaging system (PPIS).ResultsSufficient quantities of radiopharmaceutical grade [¹¹C]YM155 were produced for our PET imaging and distribution studies. The decay corrected (EOB) radiochemical yield was 16–22%, within a synthesis time of 47 min. The radiochemical purity was higher than 99%, and the specific activity was 29–60 GBq/μmol (EOS). High uptake levels of radioactivity (%ID/g, mean ± SE) were observed in tumor (0.0613 ± 0.0056), kidneys (0.0513 ± 0.0092), liver (0.0368 ± 0.0043) and cecum (0.0623 ± 0.0070). The highest tumor uptake was observed at an early time point (from 10 min after) following injection. Tumor-to-blood and tumor-to-muscle uptake ratios of [¹¹C]YM155, at 40 min after injection, were 26.5 (± 2.9) and 25.6 (± 3.6), respectively.ConclusionA rapid method for producing a radiopharmaceutical grade [¹¹C]YM155 was developed. An in vivo distribution study using PPIS showed high uptake of [¹¹C]YM155 in tumor tissue. Our methodology may facilitate the evaluation and prediction of response to YM155, when given as an anti-cancer agent.     

Synthesis and evaluation of [11C]PyrATP-1, a novel radiotracer for PET imaging of glycogen synthase kinase-3β (GSK-3β)

IntroductionThe dysfunction of glycogen synthase kinase-3β (GSK-3β) has been implicated in a number of diseases, including Alzheimer’s disease. The ability to non-invasively quantify GSK-3β activity in vivo is therefore of critical importance, and this work is focused upon development of inhibitors of GSK-3β radiolabeled with carbon-11 to examine quantification of the enzyme using positron emission tomography (PET) imaging.Methods¹¹C PyrATP-1 was prepared from the corresponding desmethyl-piperazine precursor in an automated synthesis module. In vivo rodent and primate imaging studies were conducted on a Concorde MicroPET P4 scanner to evaluate imaging properties and in vitro autoradiography studies with rat brain samples were carried out to examine specific binding.Results2035 ± 518 MBq (55 ± 14 mCi) of [¹¹C]PyrATP-1 was obtained (1%–2% non-corrected radiochemical yield at end-of-synthesis based upon [¹¹C]CO₂) with high chemical (> 95%) and radiochemical (> 99%) purities, and good specific activities (143 ± 52 GBq/μmol (3874 ± 1424 Ci/mmol)), n = 5. In vivo microPET imaging studies revealed poor brain uptake in rodents and non-human primates. Pretreatment of rodents with cyclosporin A resulted in moderately increased brain uptake suggesting Pgp transporter involvement. Autoradiography demonstrated high levels of specific binding in areas of the rodent brain known to be rich in GSK-3β.Conclusion¹¹C PyrATP-1 is readily synthesized using standard carbon-11 radiochemistry. However the poor brain uptake in rodents and non-human primates indicates that the radiotracer is not suitable for the purposes of quantifying GSK-3β in neurological and psychiatric disorders.     

A new 18F-labelled derivative of the MMP inhibitor CGS 27023A for PET: Radiosynthesis and initial small-animal PET studies

The CGS 27023A derivative (R)-2-(N-((6-fluoropyridin-3-yl)methyl)-4-methoxyphenyl-sulphonamido)-N-hydroxy-3-methylbutanamide 1a was identified as a very potent matrix metalloproteinase inhibitor. Here, we describe a one-step radiosynthesis of the target compound [¹⁸F]1a. The syntheses of [¹⁸F]1a resulted in a radiochemical yield of 12.1±5.9% (decay-corrected), a radiochemical purity of 98.8±0.6%, and a specific activity of 39±27 GBq/μmol at the end of synthesis within 160±18 min from the end of radionuclide production (n=5). Initial small-animal PET studies in wild-type mice (C57/BL6) showed no unfavourable tissue accumulation of [¹⁸F]1a.     

The synthesis and in vivo evaluation of [(18)F]PF-9811: a novel PET ligand for imaging brain fatty acid amide hydrolase (FAAH)

IntroductionFatty acid amide hydrolase (FAAH) is responsible for the enzymatic degradation of the fatty acid amide family of signaling lipids, including the endogenous cannabinoid (endocannabinoid) anandamide. The involvement of the endocannabinoid system in pain and other nervous system disorders has made FAAH an attractive target for drug development. Companion molecular imaging probes are needed, however, to assess FAAH inhibition in the nervous system in vivo. We report here the synthesis and in vivo evaluation of [¹⁸F]PF-9811, a novel PET ligand for non-invasive imaging of FAAH in the brain.MethodsThe potency and selectivity of unlabeled PF-9811 were determined by activity-based protein profiling (ABPP) both in vitro and in vivo. [¹⁸F]PF-9811 was synthesized in a 3-step, one-pot reaction sequence, followed by HPLC purification. Biological evaluation was performed by biodistribution and dynamic PET imaging studies in male rats. The specificity of [¹⁸F]PF-9811 uptake was evaluated by pre-administration of PF-04457845, a potent and selective FAAH inhibitor, 1 h prior to radiotracer injection.ResultsBiodistribution studies show good uptake (SUV ~ 0.8 at 90 min) of [¹⁸F]PF-9811 in rat brain, with significant reduction of the radiotracer in all brain regions (37%–73% at 90 min) in blocking experiments. Dynamic PET imaging experiments in rat confirmed the heterogeneous uptake of [¹⁸F]PF-9811 in brain regions with high FAAH enzymatic activity, as well as statistically significant reductions in signal following pre-administration of the blocking compound PF-04457845.Conclusions[¹⁸F]PF-9811 is a promising PET imaging agent for FAAH. Biodistribution and PET imaging experiments show that the tracer has good uptake in brain, regional heterogeneity, and specific binding as determined by blocking experiments with the highly potent and selective FAAH inhibitor, PF-04457845.     

Synthesis of carbon-11-labeled tariquidar derivatives as new PET agents for imaging of breast cancer resistance protein (ABCG2)

Carbon-11-labeled tariquidar derivatives were first designed and synthesized as new PET agents for imaging of breast cancer resistance protein. The target tracers were prepared by O-[¹¹C]methylation of their corresponding acid precursors using [¹¹C]CH3OTf under basic conditions and isolated by a simplified solid-phase extraction (SPE) method in 50–60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

GMP-compliant automated synthesis of [18F]AV-45 (Florbetapir F 18) for imaging β-amyloid plaques in human brain

We report herein the Good Manufacturing Practice (GMP)-compliant automated synthesis of ¹⁸F-labeled styrylpyridine, AV-45 (Florbetapir), a novel tracer for positron emission tomography (PET) imaging of β-amyloid (Aβ) plaques in the brain of Alzheimer’s disease patients. [¹⁸F]AV-45 was prepared in 105 min using a tosylate precursor with Sumitomo modules for radiosynthesis under GMP-compliant conditions. The overall yield was 25.4±7.7% with a final radiochemical purity of 95.3±2.2% (n=19). The specific activity of [¹⁸F]AV-45 reached as high as 470±135 TBq/mmol (n=19). The present studies show that [¹⁸F]AV-45 can be manufactured under GMP-compliant conditions and could be widely available for routine clinical use.     

Synthesis and biological evaluation of N-(2-[18F]Fluoropropionyl)-L-methionine for tumor imaging

IntroductionN-position radiolabeled amino acids, such as N-(2-[¹⁸F]fluoropropionyl)-L-methionine ([¹⁸F]FPMET) as a derivative of L-methionine (MET), can potentially serve as a PET tracer for tumor imaging. In the current study, radiosynthesis and biological evaluation of [¹⁸F]FPMET as a new PET tumor agent are performed.Methods[¹⁸F]FPMET was synthesized by reacting 4-nitrophenyl 2-[¹⁸F]fluoropropionate ([¹⁸F]NFP) with MET. In vitro competitive inhibition and protein incorporation experiments were performed with Hepa1-6 hepatoma cell lines. The biodistribution of [¹⁸F]FPMET was determined in S180 fibrosarcoma-bearing mice. PET/CT studies of [¹⁸F]FPMET were conducted in S180 fibrosarcoma-bearing mice, A549 lung adenocarcinoma-bearing nude mice, and PC-3 prostate cancer-bearing nude mice.Results[¹⁸F]FPMET was synthesized in 72% ± 4% uncorrected radiochemical yield (n = 10) from [¹⁸F]NFP. In vitro experiments showed that [¹⁸F]FPMET was primarily transported through Na⁺-dependent system A, system ASC, and system B^0,+, and was not incorporated into protein. Biodistribution and PET/CT imaging studies indicated that [¹⁸F]FPMET could delineate S180 fibrosarcoma, A549 lung adenocarcinoma, and PC-3 prostate cancer.ConclusionAn efficient synthesis of N-position [¹⁸F]labeled amino acids with a classic [¹⁸F]NFP prosthetic group is developed. The results support that [¹⁸F]FPMET seems to be a potential tracer for tumor imaging with PET.     

Synthesis of carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives as new selective PET σ1 receptor probes

Carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives were first designed and synthesized as new selective PET σ₁ receptor probes. The target tracers were prepared by O-[¹¹C]methylation of their corresponding phenolic hydroxyl precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified SPE method in 40–50% radiochemical yields based on [¹¹C]CO₂ and decay corrected to EOB. The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at EOS was 111–185 GBq/μmol.     

Synthesis of carbon-11-labeled tricyclic necroptosis inhibitors as new potential PET agents for imaging of tumor necrosis factor α (TNF-α)

Carbon-11-labeled tricyclic necroptosis inhibitors were first designed and synthesized as new potential PET agents for imaging of tumor necrosis factor α (TNF-α). The target tracers were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified SPE method in 50–60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

Reinvestigation of the synthesis and evaluation of [N-methyl-11C]vorozole,a radiotracer targeting cytochrome P450 aromatase

IntroductionWe reinvestigated the synthesis of [N-methyl-¹¹C]vorozole, a radiotracer for aromatase, and discovered the presence of an N-methyl isomer which was not removed in the original purification method. Herein we report the preparation and positron emission tomography (PET) studies of pure [N-methyl-¹¹C]vorozole.MethodsNorvorozole was alkylated with [¹¹C]methyl iodide as previously described and also with unlabeled methyl iodide. A high-performance liquid chromatography (HPLC) method was developed to separate the regioisomers. Nuclear magnetic resonance (NMR) spectroscopy (¹³C and 2D-nuclear Overhauser effect spectroscopy NMR) was used to identify and assign structures to the N-methylated products. Pure [N-methyl-¹¹C]vorozole and the contaminating isomer were compared by PET imaging in the baboon.ResultsMethylation of norvorozole resulted in a mixture of isomers (1:1:1 ratio) based on new HPLC analysis using a pentafluorophenylpropyl bonded silica column, in which vorozole coeluted one of its isomers under the original HPLC conditions. Baseline separation of the three labeled isomers was achieved. The N-3 isomer was the contaminant of vorozole, thus correcting the original assignment of isomers. PET studies of pure [N-methyl-¹¹C]vorozole with and without the contaminating N-3 isomer revealed that only [N-methyl-¹¹C]vorozole binds to aromatase. [N-methyl-¹¹C]Vorozole accumulated in all brain regions with highest accumulation in the aromatase-rich amygdala and preoptic area. Accumulation was blocked with vorozole and letrozole consistent with reports of some level of aromatase in many brain regions.ConclusionsThe discovery of a contaminating labeled isomer and the development of a method for isolating pure [N-methyl-¹¹C]vorozole combine to provide a new scientific tool for PET studies of the biology of aromatase and for drug research and development.