Radiation dosimetry and biodistribution of the translocator protein radiotracer [11C]DAA1106 determined with PET/CT in healthy human volunteers

IntroductionWhen microglia become activated (an integral part of neuroinflammation), cellular morphology changes and expression of translocator protein (TSPO) 18 kDa is increased. Over the past several years, [¹¹C]DAA1106 has emerged as a reliable radiotracer for labeling TSPO with high affinity during positron emission tomography (PET) scanning. While [¹¹C]DAA1106 PET scanning has been used in several research studies, a radiation dosimetry study of this radiotracer in humans has not yet been published.MethodsTwelve healthy participants underwent full body dynamic [¹¹C]DAA1106 PET scanning, with 8 sequential whole body scans (approximately 12 bed positions each), following a single injection. Regions of interest were drawn manually, and time activity curves (TACs) were obtained for 15 organs. OLINDA/EXM 1.1 was used to compute radiation absorbed doses to the target organs, as well as effective dose (ED) and effective dose equivalent (EDE).ResultsThe ED and EDE were 4.06 ± 0.58 μSv/MBq and 5.89 ± 0.83 μSv/MBq, respectively. The highest absorbed doses were to the heart wall, kidney, liver, pancreas, and spleen. TACs revealed that peak dose rates are during the first scan (at 6 min) for all organs other than the urinary bladder wall, which had its peak dose rate during the fourth scan (at 30 min).ConclusionsThe recently developed radiotracer [¹¹C]DAA1106 has its EDE and target-organ absorbed dose such that, for a single administration, its radiation dosimetry is well within the U.S. FDA guidelines for basic research studies in adults. This dose level implies that the dosimetry for multiple [¹¹C]DAA1106 scans within a given year also falls within FDA guidelines, and this favorable property makes this radiotracer suitable for examining microglial activation repeatedly over time, which may in the future be useful for longitudinal tracking of disease progression and monitoring of therapy response in conditions marked by neuroinflammation (e.g., head trauma and multiple sclerosis).     

Whole-body biodistribution and dosimetry estimates of a novel radiotracer for imaging of serotonin 4 receptors in brain: [18F]MNI-698

IntroductionA new radiotracer for imaging the serotonin 4 receptors (5-HT₄) in brain, [¹⁸F]MNI-698, was recently developed by our group. Evaluation in nonhuman primates indicates the novel radiotracer holds promise as an imaging agent of 5-HT₄ in brain. This paper aims to describe the whole-body biodistribution and dosimetry estimates of [¹⁸F]MNI-698.MethodsWhole-body positron emission tomography (PET) images were acquired over 240 minutes after intravenous bolus injection of [¹⁸F]MNI-698 in adult rhesus monkeys. Different models were investigated for quantification of radiation absorbed and effective doses using OLINDA/EXM 1.0 software.ResultsThe radiotracer main elimination route was found to be urinary and the critical organ was the urinary bladder. Modeling of the urinary bladder voiding interval had a considerable effect on the estimated effective dose. Normalization of rhesus monkeys’ organs and whole-body masses to human equivalent reduced the calculated dosimetry values. The effective dose ranged between 0.017 and 0.027 mSv/MBq.ConclusionThe dosimetry estimates, obtained when normalizing organ and whole-body weights and applying the urinary bladder model, indicate that the radiation doses from [¹⁸F]MNI-698 comply with limits and guidelines recommended by key regulatory authorities that govern the translation of radiotracers to human clinical trials. The timing of urinary bladder emptying should be considered when designing future clinical protocols with [¹⁸F]MNI-698, in order to minimize the subject absorbed doses.     

Synthesis,metabolite analysis,and in vivo evaluation of [11C]irinotecan as a novel positron emission tomography (PET) probe

IntroductionIrinotecan is a semisynthetic derivative of camptothecin that exerts potent antitumor activity by inhibiting topoisomerase I. Despite much research into the complex pharmacokinetic profile and pharmacodynamic effects of irinotecan, unpredictable and severe side effects are still commonly observed. In this study, we synthesized [¹¹C]irinotecan as a positron emission tomography (PET) probe, performed the metabolite analysis, and evaluated the biodistribution and kinetics of [¹¹C]irinotecan using small animal PET.Methods[¹¹C]Irinotecan was synthesized by two routes using [¹¹C]phosgene and [¹¹C]carbon dioxide fixation. Metabolites in the plasma of mice following injection of [¹¹C] irinotecan were investigated using a combination of column-switching high-performance liquid chromatography (HPLC) and on-line solid-phase extraction (SPE). Whole-body PET studies were conducted in wild-type mice and P-glycoprotein and breast cancer resistance protein (Pgp/Bcrp) knockout mice.Results[¹¹C]Irinotecan was successfully synthesized by the two abovementioned routes. Decay-corrected radiochemical yields based on [¹¹C]carbon dioxide using [¹¹C]phosgene and [¹¹C]carbon dioxide fixation were 8.8 ± 2.0% (n = 8) and 16.9 ± 2.9 % (n = 5), respectively. Metabolite analysis of the plasma of mice following injection of [¹¹C]irinotecan was successfully performed using the column-switching HPLC and on-line SPE combination resulting in greater than 87 % recovery of radioactivity from HPLC. In the PET study in mice, the radioactivity levels in the brain, liver, and small intestine were slightly increased by inhibition of the Pgp/Bcrp function for more than 30 min after [¹¹C]irinotecan injection. This result demonstrated that in vivo behavior of [¹¹C] irinotecan and radioactive metabolites are influenced by the Pgp/Bcrp function.ConclusionPET studies using [¹¹C]irinotecan combined with metabolite analysis may be a useful tool for evaluating irinotecan pharmacokinetics and toxicity.     

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.     

Longitudinal observation of [11C]4DST uptake in turpentine-induced inflammatory tissue

IntroductionLongitudinal changes of 4′-[methyl-¹¹C]thiothymidine ([¹¹C]4DST) uptake were evaluated in turpentine-induced inflammation.MethodsTurpentine (0.1 ml) was injected intramuscularly into the right hind leg of male Wistar rats. Longitudinal [¹¹C]4DST uptake was evaluated by the tissue dissection method at 1, 2, 4, 7, and 14 days after turpentine injection (n = 5). The tumor selectivity index was calculated using the previously published biodistribution data in C6 glioma-bearing rats. Dynamic PET scan was performed on day 4 when maximum [¹¹C]4DST uptake was observed during the longitudinal study. Histopathological analysis and Ki-67 immunostaining were also performed.ResultsThe uptake of [¹¹C]4DST in inflammatory tissue was significantly increased on days 2–4 after turpentine injection, and then decreased. On day 14, tracer uptake returned to the day 1 level. The maximum SUV of inflamed muscle was 0.6 and was 3 times higher than that of the contralateral healthy muscle on days 2–4 after turpentine injection. However, tumor selectivity index remains very high (> 10) because of the low inflammation uptake. A dynamic PET scan showed that the radioactivity in inflammatory tissues peaked at 5 min after [¹¹C]4DST injection, and then washed out until 20 min. At intervals > 20 min, radioactivity levels were constant and double that of healthy muscle. The changes in Ki-67 index were paralleled with those of [¹¹C]4DST uptake, indicating cell proliferation-dependent uptake of [¹¹C]4DST in inflammatory tissues.ConclusionIn our animal model, low but significant levels of [¹¹C]4DST uptake were observed in subacute inflammation.     

Evaluation of [18F]Nifene biodistribution and dosimetry based on whole-body PET imaging of mice

Introduction[¹⁸F]Nifene is a novel radiotracer specific to the nicotinic acetylcholine α4β2 receptor class. In preparation for using this tracer in humans we have performed whole-body PET studies in mice to evaluate the in vivo biodistribution and dosimetry of [¹⁸F]Nifene.MethodsSeven BALB/c mice (3 males, 4 females) received IV tail injections of [¹⁸F]Nifene and were scanned for 2 h in an Inveon dedicated PET scanner. Each animal also received a high resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest (VOI) on the following organs: brain, large intestine, small intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, uterus and urinary bladder. All organ time activity curves had the decay correction reversed and were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource (RADAR) animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms.ResultsThe highest mouse residence times were found in urinary bladder, liver, bone, small intestine and kidneys. The largest doses in mice were found in urinary bladder and kidneys for both females and males. The elimination of radiotracer was primarily via kidney and urinary bladder with the urinary bladder being the limiting organ. The projected human effective doses were 1.51E-02 mSv/MBq for the adult male phantom and 1.65E-02 mSv/MBq for the adult female model phantom.ConclusionThis study indicates that the whole-body mouse imaging can be used as a preclinical tool for initial estimation of the absorbed doses of [¹⁸F]Nifene in humans.     

Human whole-body biodistribution and dosimetry of a new PET tracer, [11C]ketoprofen methyl ester,for imagings of neuroinflammation

IntroductionNeuroinflammatory processes play an important role in the pathogenesis of Alzheimer's disease and other brain disorders, and nonsteroidal anti-inflammatory drugs (NSAIDs) are considered therapeutic candidates. As a biomarker of neuroinflammatory processes, ¹¹C-labeled ketoprofen methyl ester ([¹¹C]KTP-Me) was designed to allow cerebral penetration of ketoprofen (KTP), an active form of a selective cyclooxygenase-1 inhibitor that acts as an NSAID. Rat neuroinflammation models indicate that [¹¹C]KTP-Me enters the brain and is retained in inflammatory lesions, accumulating in activated microglia. [¹¹C]KTP-Me is washed out from normal tissues, leading to the present first-in-human exploratory study.Methods[¹¹C]KTP-Me was synthesized by rapid C-[¹¹C]methylation of [¹¹C]CH₃I and the corresponding arylacetate precursor, purified with high-performance liquid chromatography, and prepared as an injectable solution including PEG400, providing radiochemical purity of > 99% and specific activity of > 25 GBq/μmol at injection. Six young healthy male humans were injected with [¹¹C]KTP-Me and scanned with PET camera to determine the early-phase brain time course followed by three whole-body scans starting 8, 20, and 40 min post-injection, together with sequential blood sampling and labeled metabolite analysis.ResultsNo adverse effects were observed during PET scanning after [¹¹C]KTP-Me injection. [¹¹C]KTP-Me was rapidly metabolized to ¹¹C-labeled ketoprofen ([¹¹C]KTP) within 2–3 min and was gradually cleared from blood. The radioactivity entered the brain with an average peak cortical SUV of 1.5 at 2 min. The cortical activity was gradually washed out. Whole-body images indicated that the urinary bladder was the major excretory pathway. The organ with the highest radiation dose was the urinary bladder (average dose of 41μGy/MBq, respectively). The mean effective dose was 4.7 μSv/MBq, which was comparable to other ¹¹C-labeled radiopharmaceuticals.Conclusion[¹¹C]KTP-Me demonstrated a favorable dosimetry, biodistribution, and safety profile. [¹¹C]KTP-Me entered the human brain, and the radioactivity was washed out from cerebral tissue. These data warrant further exploratory studies on patients with neuroinflammation.     

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.     

Equivalence of arterial and venous blood for [11C]CO2-metabolite analysis following intravenous administration of 1-[11C]acetate and 1-[11C]palmitate

PurposeSampling of arterial blood for metabolite correction is often required to define a true radiotracer input function in quantitative modeling of PET data. However, arterial puncture for blood sampling is often undesirable. To establish whether venous blood could substitute for arterial blood in metabolite analysis for quantitative PET studies with 1-[¹¹C]acetate and 1-[¹¹C]palmitate, we compared the results of [¹¹C]CO₂-metabolite analyses performed on simultaneously collected arterial and venous blood samples.MethodsPaired arterial and venous blood samples were drawn from anesthetized pigs at 1, 3, 6, 8, 10, 15, 20, 25 and 30 min after i.v. administration of 1-[¹¹C]acetate and 1-[¹¹C]palmitate. Blood radioactivity present as [¹¹C]CO₂ was determined employing a validated 10-min gas-purge method. Briefly, total blood ¹¹C radioactivity was counted in base-treated [¹¹C]-blood samples, and non-[¹¹C]CO₂ radioactivity was counted after the [¹¹C]-blood was acidified using 6 N HCl and bubbled with air for 10 min to quantitatively remove [¹¹C]CO₂.ResultsAn excellent correlation was found between concurrent arterial and venous [¹¹C]CO₂ levels. For the [¹¹C]acetate study, the regression equation derived to estimate the venous [¹¹C]CO₂ from the arterial values was: y = 0.994x + 0.004 (r² = 0.97), and for the [¹¹C]palmitate: y = 0.964x ? 0.001 (r² = 0.9). Over the 1–30 min period, the fraction of total blood ¹¹C present as [¹¹C]CO₂ rose from 4% to 64% for acetate, and 0% to 24% for palmitate. The rate of [¹¹C]CO₂ appearance in venous blood appears similar for the pig model and humans following i.v. [¹¹C]-acetate administration.ConclusionVenous blood [¹¹C]CO₂ values appear suitable as substitutes for arterial blood samples in [¹¹C]CO₂ metabolite analysis after administration of [¹¹C]acetate or [¹¹C]palmitateAdvances in Knowledge and Implications for Patient CareQuantitative PET studies employing 1-[¹¹C]acetate and 1-[¹¹C]palmitate can employ venous blood samples for metabolite correction of an image-derived tracer arterial input function, thereby avoiding the risks of direct arterial blood sampling.     

Enhanced radiosyntheses of [11C]raclopride and [11C]DASB using ethanolic loop chemistry

IntroductionTo improve the synthesis and quality control of carbon-11 labeled radiopharmaceuticals, we report the fully automated loop syntheses of [¹¹C]raclopride and [¹¹C]DASB using ethanol as the only organic solvent for synthesis module cleaning, carbon-11 methylation, HPLC purification, and reformulation.MethodsEthanolic loop chemistry is fully automated using a GE TRACERLab FX_C-Pro synthesis module, and is readily adaptable to any other carbon-11 synthesis apparatus. Precursors (1 mg) were dissolved in ethanol (100 μL) and loaded into the HPLC loop. [¹¹C]MeOTf was passed through the HPLC loop and then the labeled products were purified by semi-preparative HPLC and reformulated into ethanolic saline.ResultsBoth [¹¹C]raclopride (3.7% RCY; > 95% RCP; SA = 20831 Ci/mmol; n = 64) and [¹¹C]DASB, both with (3.0% RCY; > 95% RCP; SA = 15152 Ci/mmol; n = 9) and without (3.0% RCY; > 95% RCP; SA = 10931 Ci/mmol; n = 3) sodium ascorbate, have been successfully prepared using the described methodology. Doses are suitable for human use and the described methods are now employed for routine clinical production of both radiopharmaceuticals at the University of Michigan.ConclusionsEthanolic loop chemistry is a powerful technique for preparing [¹¹C]raclopride and [¹¹C]DASB, and we are in the process of adapting it for other carbon-11 radiopharmaceuticals prepared in our laboratories ([¹¹C]PMP, [¹¹C]PBR28 etc.).     

Ex Vivo and In Vivo Evaluation of the Norepinephrine Transporter Ligand [(11)C]MRB for Brown Adipose Tissue Imaging

IntroductionIt has been suggested that brown adipose tissue (BAT) in humans may play a role in energy balance and obesity. We conducted ex vivo and in vivo evaluation using [¹¹C]MRB, a highly selective NET (norepinephrine transporter) ligand for BAT imaging at room temperature, which is not achievable with [¹⁸F]FDG.MethodsPET images of male Sprague–Dawley rats with [¹⁸F]FDG and [¹¹C]MRB were compared. Relative [¹⁸F]FDG or [¹¹C]MRB retention at 20, 40 and 60 min post-injection was quantified on awake rats after exposing to cold (4 °C for 4 h) or remaining at room temperature. Rats pretreated with unlabeled MRB or nisoxetine 30 min before [¹¹C]MRB injection were also assessed. The [¹¹C]MRB metabolite profile in BAT was evaluated.ResultsPET imaging demonstrated intense [¹¹C]MRB uptake (SUV of 2.9 to 3.3) in the interscapular BAT of both room temperature and cold-exposed rats and this uptake was significantly diminished by pretreatment with unlabeled MRB; in contrast, [¹⁸F]FDG in BAT was only detected in rats treated with cold. Ex vivo results were concordant with the imaging findings; i.e. the uptake of [¹¹C]MRB in BAT was 3 times higher than that of [¹⁸F]FDG at room temperature (P = 0.009), and the significant cold-stimulated uptake in BAT with [¹⁸F]FDG (10-fold, P = 0.001) was not observed with [¹¹C]MRB (P = 0.082). HPLC analysis revealed 94%–99% of total radioactivity in BAT represented unchanged [¹¹C]MRB.ConclusionsOur study demonstrates that BAT could be specifically labeled with [¹¹C]MRB at room temperature and under cold conditions, supporting a NET-PET strategy for imaging BAT in humans under basal conditions.     

Novel fluorine-18 PET radiotracers based on flumazenil for GABAA imaging in the brain

IntroductionTwo 7-fluoroimidazobenzodiazepines (AH114726 and GEH120348), analogs of flumazenil, were labeled with fluorine-18 and evaluated as alternative radioligands for in vivo imaging of the GABA_A/benzodiazepine receptor by comparing them to [¹¹C]flumazenil in rhesus monkey.MethodsRadiotracers were prepared from the corresponding nitro-precursors in an automated synthesis module, and primate imaging studies were conducted on a Concorde MicroPET P4 scanner. The brain was imaged for 60 (12 × 5 min frames) or 90 min (18 × 5 min frames), and data was reconstructed using the 3D MAP algorithm. Specificity of [¹⁸F]AH114726 and [¹⁸F]GEH120348 was confirmed by displacement studies using unlabeled flumazenil.Results[¹⁸F]GEH120348 and [¹⁸F]AH114726 were obtained in 13–24% yields (end of synthesis) with high chemical (> 95%) and radiochemical (> 99%) purities, and high specific activities (2061 ± 985 Ci/mmol). The in vivo pharmacokinetics of [¹⁸F]AH114726 and [¹⁸F]GEH120348 were determined in a non-human primate and directly compared with [¹¹C]flumazenil. Both fluorine-18 radioligands showed time-dependent regional brain distributions that correlated with the distribution of [¹¹C]flumazenil and the known concentrations of GABA_A/benzodiazepine receptors in the monkey brain. [¹⁸F]AH114726 exhibited maximal brain uptake and tissue time-radioactivity curves that were most similar to [¹¹C]flumazenil. In contrast, [¹⁸F]GEH120348 showed higher initial brain uptake but very different pharmacokinetics with continued accumulation of radioactivity into the cortical regions of high GABA/benzodiazepine receptor concentrations and very little clearance from the regions of low receptor densities. Rapid washout of both radiotracers occurred upon treatment with unlabeled flumazenil.ConclusionThe ease of the radiochemical synthesis, together with in vivo brain pharmacokinetics most similar to [¹¹C]flumazenil, support that [¹⁸F]AH114726 is a suitable option for imaging the GABA_A receptor.     

Evaluation of PET radioligands for in vivo visualization of phosphodiesterase 5 (PDE5)

IntroductionThe cyclic guanosine monophosphate (cGMP) specific phosphodiesterase type 5 (PDE5) is considered to play an important role in various etiologies such as pulmonary arterial hypertension (PAH) and chronic heart failure. This PDE5 modulation represents an important prognostic and/or therapeutic target; however, there is currently no method to non-invasively evaluate the PDE5 expression levels in vivo.MethodsRadiolabeled tracers were prepared by N-alkylation of the corresponding precursors with [¹¹C]methyl trifluoromethanesulfonate ([¹¹C]CH₃OTf) or 2-[¹⁸ F]fluoroethyl trifluoromethanesulfonate ([¹⁸ F]FEtOTf). Biodistribution of radiolabeled tracers was studied in NMRI mice and their specific binding to PDE5 was investigated by comparing their lung retention as the enzyme is abundantly expressed in this organ.ResultsThe overall radiochemical yields ranged between 24% and 60% for labeled radiotracers with radiochemical purity of > 99%. The highest retention in the lungs at 30 min post injection was observed for vardenafil derivatives [¹¹C]-7 and [¹⁸ F]-11 and the retention of the ethoxyethyl pyrazolopyrimidine derivative [¹¹C]-37 was moderate. The other investigated compounds [¹¹C]-8, [¹¹C]-14, [¹¹C]-21 and [¹¹C]-33 showed lower retention in lungs in agreement with their lower in-vitro affinity for PDE5.ConclusionAmong the different radiolabeled PDE5 inhibitors evaluated in this study, the vardenafil derivatives [¹¹C]-7 and [¹⁸ F]-11 are found to be promising tracers for in vivo visualization of PDE5.     

Palladium mediated 11C-cyanation and characterization in the non-human primate brain of the novel mGluR5 radioligand [11C]AZD9272

IntroductionThe aims of the present positron emission tomography (PET) study were to set up a system for ¹¹C-cyanation labeling of the selective mGluR5-antagonist [¹¹C]AZD9272 and to perform the first in vivo characterization of [¹¹C]AZD9272 binding in cynomolgus monkeys.Methods[¹¹C]AZD9272 was labeled using palladium mediated ¹¹C-cyanation. Altogether seven PET measurements were performed in three cynomolgus monkeys including baseline and co-injection experiments with unlabelled AZD9272 (0.04 and 0.4 mg/kg). Radiometabolites in plasma were measured using HPLC.Results[¹¹C]AZD9272 was prepared in over 50% incorporation yield from hydrogen [¹¹C]cyanide in a total synthesis time of 45–50 min. The radiochemical purity of the radioligand in its final formulation was high (> 99%) and the mean specific radioactivity was 47 GBq/ μmol (1278 Ci/mmol, n = 7) calculated at end of bombardment (EOB). In the baseline measurements 10% of the total injected radioactivity was present in monkey brain at five minutes after i.v. injection. The radioactivity concentration was high in the caudate, cingulate gyrus and thalamus whereas it was moderate in the temporal cortex and lower for the cerebellum. After co-injection with cold AZD9272 the binding of [¹¹C]AZD9272 was reduced in a dose-dependent fashion. Analysis of radiometabolites showed relatively slow metabolism and resulted only in hydrophilic radiometabolites.ConclusionA fast and efficient method was developed to label AZD9272 with ¹¹C. PET-examination in Cynomolgus monkeys showed that [¹¹C]AZD9272 entered the brain to a high extent, that binding was saturable and that the regional radioactivity pattern was in accordance with the known distribution of mGluR5. The results support further examination of [¹¹C]AZD9272 binding in human subjects.     

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.     

Pre-clinical evaluation of [68Ga]Ga-DO3A-VS-Cys40-Exendin-4 for imaging of insulinoma

IntroductionInsulinoma is the most common form of pancreatic endocrine tumors responsible for hyperinsulinism in adults. These tumors overexpress glucagon like peptide-1 (GLP-1) receptor, and biologically stable GLP-1 analogs have therefore been proposed as potential imaging agents. Here, we evaluate the potential of a positron emission tomography (PET) tracer, [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin-4, for imaging and quantification of GLP-1 receptors (GLP-1R) in insulinoma.Methods[⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin-4 was evaluated for binding to GLP-1R by in vitro autoradiography binding studies in INS-1 tumor from xenografts. In vivo biodistribution was investigated in healthy control mice, INS-1 xenografted and PANC1 xenografted immunodeficient mice at two different doses of peptide: 2.5 μg/kg (baseline) and 100 μg/kg (block). In vivo imaging of [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin-4 in xenografted mice was evaluated by small animal PET/CT using a direct comparison with the clinically established insulinoma marker [¹¹C]5-hydroxy-tryptophan ([¹¹C]5-HTP).ResultsGLP-1 receptor density could be quantified in INS-1 tumor biopsies. [⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin-4 showed significant uptake (p ≤ 0.05) in GLP1-R positive tissues such as INS-1 tumor, lungs and pancreas upon comparison between baseline and blocking studies. In vivo imaging showed concordant results with higher tumor-to-muscle ratio in INS-1 xenografted mice compared with [¹¹C]5-HTP.Conclusion[⁶⁸Ga]Ga-DO3A-VS-Cys⁴⁰-Exendin-4 has high affinity and specificity for GLP-1R expressed on insulinoma in vitro and in vivo.     

Synthesis and preclinical evaluation of the radiolabeled P-glycoprotein inhibitor [11C]MC113

ObjectivesWith the aim to develop a PET tracer to visualize P-glycoprotein (Pgp) expression levels in different organs, the Pgp inhibitor MC113 was labeled with ¹¹C and evaluated using small-animal PET.Methods[¹¹C]MC113 was synthesized by reaction of O-desmethyl MC113 with [¹¹C]methyl triflate. Small-animal PET was performed with [¹¹C]MC113 in FVB wild-type and Mdr1a/b^(-/-) mice (n = 3 per group) and in a mouse model of high (EMT6Ar1.0) and low (EMT6) Pgp expressing tumor grafts (n = 5). In the tumor model, PET scans were performed before and after administration of the reference Pgp inhibitor tariquidar (15 mg/kg).ResultsBrain uptake of [¹¹C]MC113, expressed as area under the time-activity curve from time 0 to 60 min (AUC_0-60), was moderately but not significantly increased in Mdr1a/b^(-/-) compared with wild-type mice (mean ± SD AUC_0-60, Mdr1a/b^(-/-): 88 ± 7 min, wild-type: 62 ± 6 min, P = 0.100, Mann Whitney test). In the tumor model, AUC_0-60 values were not significantly different between EMT6Ar1.0 and EMT6 tumors. Neither in brain nor in tumors was activity concentration significantly changed in response to tariquidar administration. Half-maximum effect concentrations (IC₅₀) for inhibition of Pgp-mediated rhodamine 123 efflux from CCRFvcr1000 cells were 375 ± 60 nM for MC113 versus 8.5 ± 2.5 nM for tariquidar.Conclusion[¹¹C]MC113 showed higher brain uptake in mice than previously described Pgp PET tracers, suggesting that [¹¹C]MC113 was only to a low extent effluxed by Pgp. However, [¹¹C]MC113 was found unsuitable to visualize Pgp expression levels presumably due to insufficiently high Pgp binding affinity of MC113 in relation to Pgp densities in brain and tumors.     

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.     

Dosimetry of 64Cu-DOTA-AE105, a PET tracer for uPAR imaging

⁶⁴Cu-DOTA-AE105 is a novel positron emission tomography (PET) tracer specific to the human urokinase-type plasminogen activator receptor (uPAR). In preparation of using this tracer in humans, as a new promising method to distinguish between indolent and aggressive cancers, we have performed PET studies in mice to evaluate the in vivo biodistribution and estimate human dosimetry of ⁶⁴Cu-DOTA-AE105.MethodsFive mice received iv tail injection of ⁶⁴Cu-DOTA-AE105 and were PET/CT scanned 1, 4.5 and 22 h post injection. Volume-of-interest (VOI) were manually drawn on the following organs: heart, lung, liver, kidney, spleen, intestine, muscle, bone and bladder. The activity concentrations in the mentioned organs [%ID/g] were used for the dosimetry calculation. The %ID/g of each organ at 1, 4.5 and 22 h was scaled to human value based on a difference between organ and body weights. The scaled values were then exported to OLINDA software for computation of the human absorbed doses. The residence times as well as effective dose equivalent for male and female could be obtained for each organ. To validate this approach, of human projection using mouse data, five mice received iv tail injection of another ⁶⁴Cu-DOTA peptide-based tracer, ⁶⁴Cu-DOTA-TATE, and underwent same procedure as just described. The human dosimetry estimates were then compared with observed human dosimetry estimate recently found in a first-in-man study using ⁶⁴Cu-DOTA-TATE.ResultsHuman estimates of ⁶⁴Cu-DOTA-AE105 revealed the heart wall to receive the highest dose (0.0918 mSv/MBq) followed by the liver (0.0815 mSv/MBq), All other organs/tissue were estimated to receive doses in the range of 0.02–0.04 mSv/MBq. The mean effective whole-body dose of ⁶⁴Cu-DOTA-AE105 was estimated to be 0.0317 mSv/MBq. Relatively good correlation between human predicted and observed dosimetry estimates for ⁶⁴Cu-DOTA-TATE was found. Importantly, the effective whole body dose was predicted with very high precision (predicted value: 0.0252 mSv/Mbq, Observed value: 0.0315 mSv/MBq) thus validating our approach for human dosimetry estimation.ConclusionFavorable dosimetry estimates together with previously reported uPAR PET data fully support human testing of ⁶⁴Cu-DOTA-AE105.     

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.