Impurity profiling of N,N'-ethylenebis-l-cysteine diethyl ester (Bicisate).

A HPLC-UV-CAD method with a HILIC column for impurity profiling of the 99mTc chelating agent bicisate has been developed and evaluated. Bicisate and its impurities were separated by means of isocratic elution on a zwitterionic stationary phase using a mixture of 7.5mmol/L trifluoroacetic acid and acetonitrile (47.5:52.5 V/V) as the mobile phase. Five different bicisate batches of a manufacturer were tested using the method. In addition LC-MS experiments were conducted in order to identify the impurities. The predominant impurities found were the oxidation product (disulfide), the monoester of ethylene dicysteine and an unknown compound with an m/z of 293 in ESI positive mode. A new degradation product of bicisate, bicisate lactam, was identified during sample solution stability assessment.

Pretargeted PET Imaging Using a Bioorthogonal 18F-Labeled trans-Cyclooctene in an Ovarian Carcinoma Model.

In cancer research, pretargeted positron emission tomography (PET) imaging has emerged as an effective two-step approach that combines the excellent target affinity and selectivity of antibodies with the advantages of using short-lived radionuclides such as fluorine-18. One possible approach is based on the bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction between tetrazines and trans-cyclooctene (TCO) derivatives. Here, we report the first successful use of an 18F-labeled small TCO compound, [18F]1 recently developed in our laboratory, to perform pretargeted immuno-PET imaging. The study was performed in an ovarian carcinoma mouse model, using a trastuzumab-tetrazine conjugate.

Al18F-Labeling Of Heat-Sensitive Biomolecules for Positron Emission Tomography Imaging.

Positron emission tomography (PET) using radiolabeled biomolecules is a translational molecular imaging technology that is increasingly used in support of drug development. Current methods for radiolabeling biomolecules with fluorine-18 are laborious and require multistep procedures with moderate labeling yields. The Al18F-labeling strategy involves chelation in aqueous medium of aluminum mono[18F]fluoride ({Al18F}2+) by a suitable chelator conjugated to a biomolecule. However, the need for elevated temperatures (100-120 °C) required for the chelation reaction limits its widespread use. Therefore, we designed a new restrained complexing agent (RESCA) for application of the AlF strategy at room temperature. Methods. The new chelator RESCA was conjugated to three relevant biologicals and the constructs were labeled with {Al18F}2+ to evaluate the generic applicability of the one-step Al18F-RESCA-method. Results. We successfully labeled human serum albumin with excellent radiochemical yields in less than 30 minutes and confirmed in vivo stability of the Al18F-labeled protein in rats. In addition, we efficiently labeled nanobodies targeting the Kupffer cell marker CRIg, and performed µPET studies in healthy and CRIg deficient mice to demonstrate that the proposed radiolabeling method does not affect the functional integrity of the protein. Finally, an affibody targeting HER2 (PEP04314) was labeled site-specifically, and the distribution profile of (±)-[18F]AlF(RESCA)-PEP04314 in a rhesus monkey was compared with that of [18F]AlF(NOTA)-PEP04314 using whole-body PET/CT. Conclusion. This generic radiolabeling method has the potential to be a kit-based fluorine-18 labeling strategy, and could have a large impact on PET radiochemical space, potentially enabling the development of many new fluorine-18 labeled protein-based radiotracers.

Synthesis and preclinical evaluation of [11C]MA-PB-1 for in vivo imaging of brain monoacylglycerol lipase (MAGL).

MAGL is a potential therapeutic target for oncological and psychiatric diseases. Our objective was to develop a PET tracer for in vivo quantification of MAGL. We report [11C]MA-PB-1 as an irreversible MAGL inhibitor PET tracer. The in vitro inhibitory activity, ex vivo distribution, brain kinetics and specificity of [11C]MA-PB-1 binding were studied. Ex vivo biodistribution and microPET showed good brain uptake which could be blocked by pretreatment with both MA-PB-1 and a structurally non-related MAGL inhibitor MJN110. These initial results suggest that [11C]MA-PB-1 is a suitable tracer for in vivo imaging of MAGL.

Micro-flow photosynthesis of new dienophiles for inverse-electron-demand Diels-Alder reactions. Potential applications for pretargeted in vivo PET imaging.

Pretargeted PET imaging has emerged as an effective two-step in vivo approach that combines the superior affinity and selectivity of antibodies with the rapid pharmacokinetics and favorable dosimetry of smaller molecules radiolabeled with short-lived radionuclides. This approach can be based on the bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction between tetrazines and trans-cyclooctene (TCO) derivatives. We aimed to develop new [18F]TCO-dienophiles with high reactivity for IEDDA reactions, and favorable in vivo stability and pharmacokinetics. New dienophiles were synthesized using an innovative micro-flow photochemistry process, and their reaction kinetics with a tetrazine were determined. In vivo stability and biodistribution of the most promising 18F-radiolabeled-TCO-derivative ([18F]3) was investigated, and its potential for in vivo pretargeted PET imaging was assessed in tumor-bearing mice. We demonstrated that [18F]3 is a suitable dienophile for IEDDA reactions and for pretargeting applications.

Preclinical Evaluation of 18F-JNJ64349311, a Novel PET Tracer for Tau Imaging.

In this study, we have synthesized and evaluated 18F-JNJ64349311, a tracer with high affinity for aggregated tau (inhibition constant value, 8 nM) and high (≥500×) in vitro selectivity for tau over ß-amyloid, in comparison with the benchmark compound 18F-AV1451 (18F-T807) in mice, rats, and a rhesus monkey. Methods: In vitro binding characteristics were determined for Alzheimer's disease, progressive supranuclear palsy, and corticobasal degeneration patient brain tissue slices using autoradiography studies. Ex vivo biodistribution studies were performed in mice. Radiometabolites were quantified in the brain and plasma of mice and in the plasma of a rhesus monkey using high-performance liquid chromatography. Dynamic small-animal PET studies were performed in rats and a rhesus monkey to evaluate tracer pharmacokinetics in the brain. Results: Mouse biodistribution studies showed moderate initial brain uptake and rapid brain washout. Radiometabolite analyses after injection of 18F-JNJ64349311 in mice showed the presence of a polar radiometabolite in plasma, but not in the brain. Semiquantitative autoradiography studies on postmortem tissue sections of human Alzheimer's disease brains showed highly displaceable binding to tau-rich regions. No specific binding was, however, found on human progressive supranuclear palsy and corticobasal degeneration brain slices. Small-animal PET scans of Wistar rats revealed moderate initial brain uptake (SUV, ∼1.5 at 1 min after injection) and rapid brain washout. Gradual bone uptake was, however, also observed. Blocking and displacement did not affect brain time-activity curves, suggesting no off-target specific binding of the tracer in the healthy rat brain. A small-animal PET scan of a rhesus monkey revealed moderate initial brain uptake (SUV, 1.9 at 1 min after injection) with a rapid washout. In the monkey, no bone uptake was detected during the 120-min scan. Conclusion: This biologic evaluation suggests that 18F-JNJ64349311 is a promising tau PET tracer candidate, with a favorable pharmacokinetic profile, as compared with 18F-AV1451.

Striatal phosphodiesterase 10A availability is altered secondary to chronic changes in dopamine neurotransmission.

BACKGROUND: Phosphodiesterase 10A (PDE10A) is an important regulator of nigrostriatal dopamine (DA) neurotransmission. However, little is known on the effect of alterations in DA neurotransmission on PDE10A availability. Here, we used [18F]JNJ42259152 PET to measure changes in PDE10A availability, secondary to pharmacological alterations in DA release and to investigate whether these are D1- or D2-receptor driven. RESULTS: Acute treatment of rats using D-amphetamine (5 mg, s.c. and 1 mg/kg i.v.) did not result in a significant change in PDE10A BPND compared to baseline conditions. 5-day D-amphetamine treatment (5 mg/kg, s.c.) increased striatal PDE10A BPND compared to the baseline (+24 %, p = 0.03). Treatment with the selective D2 antagonist SCH23390 (1 mg/kg) and D-amphetamine decreased PDE10A binding (-22 %, p = 0.03). Treatment with only SCH23390 further decreased PDE10A binding (-26 %, p = 0.03). No significant alterations in PDE10A mRNA levels were observed. CONCLUSIONS: Repeated D-amphetamine treatment significantly increased PDE10A binding, which is not observed upon selective D1 receptor blocking. This study suggests a potential pharmacological interaction between PDE10A enzymes and drugs modifying DA neurotransmission. Therefore, PDE10A binding in patients with neuropsychiatric disorders might be modulated by chronic DA-related treatment.

What We Observe In Vivo Is Not Always What We See In Vitro: Development and Validation of 11C-JNJ-42491293, A Novel Radioligand for mGluR2.

Positive allosteric modulators (PAM) of metabotropic glutamate receptor 2 (mGluR2) are a potential therapy for anxiety, schizophrenia, and addiction. Aside from pathophysiologic imaging studies, an mGluR2 PET tracer would enable confirmation of sufficient central target engagement and assist dose selection for proof-of-concept studies of PAM compounds. 11C-JNJ-42491293, a novel high-affinity radioligand (human 50% inhibitory concentration = 9.6 nM) for the PAM site of mGluR2, was evaluated as a selective mGluR2 PAM PET tracer. METHODS: In vitro and ex vivo autoradiography binding experiments in Wistar and in mGluR2 knockout and wildtype rats as well as in vivo biodistribution and brain PET imaging studies in wildtype and mGluR2 knockout rats in a primate and in humans were performed. RESULTS: In vitro binding studies and in vivo imaging studies in Wistar rats showed moderate brain uptake, with a distribution pattern fully consistent with the reported intracerebral distribution of mGluR2. Given these promising findings, biodistribution, dosimetry, and brain kinetic modeling of 11C-JNJ-42491293 were determined in humans. Because of an unexpected high myocardial retention, additional 11C-JNJ-42491293 imaging studies were performed in recently available mGluR2 knockout and wildtype rats and in a monkey using a structurally distinct mGluR2 PAM ligand with affinity for the same site, demonstrating off-target binding in vivo that could not have been anticipated from previous in vitro experiments. To date, the target of this non-mGluR2 tracer binding remains unknown. CONCLUSION: On the basis of in vivo selectivity issues suggested by human distribution and demonstrated in knockout rat models, 11C-JNJ-42491293 was considered unsuitable as a specific PET ligand for in vivo imaging of mGluR2. These results emphasize the importance of elaborated in vitro/in vivo comparative studies and, when available, validation with knockout animal models or structurally distinct ligands with affinity for the same site, in radiotracer development.

Carbon-11 and Fluorine-18 Radiolabeled Pyridopyrazinone Derivatives for Positron Emission Tomography (PET) Imaging of Phosphodiesterase-5 (PDE5).

The cyclic guanosine monophosphate (cGMP) specific phosphodiesterase type 5 (PDE5) plays an important role in various pathologies including pulmonary arterial hypertension and cardiomyopathy. PDE5 represents an important therapeutic and/or prognostic target, but noninvasive assessment of PDE5 expression is lacking. The purpose of this study was to develop and evaluate pyridopyrazinone derivatives labeled with carbon-11 or fluorine-18 as PDE5-specific PET tracers. In biodistribution studies, highest PDE5-specific retention was observed for [11C]-12 and [18F]-17 in the lungs of wild-type mice and in the myocardium of transgenic mice with cardiomyocyte-specific PDE5 overexpression at 30 min postinjection. In vivo dynamic microPET images in rats revealed that both tracers crossed the blood-brain barrier but brain retention was not PDE5-specific. Both [11C]-12 and [18F]-17 showed specific binding to PDE5 in myocardium of transgenic mice; however [18F]-17 showed significantly higher PDE5-specific inhibitable binding than [11C]-12.

Synthesis, Biodistribution and In vitro Evaluation of Brain Permeable High Affinity Type 2 Cannabinoid Receptor Agonists [11C]MA2 and [18F]MA3.

The type 2 cannabinoid receptor (CB2) is a member of the endocannabinoid system and is known for its important role in (neuro)inflammation. A PET-imaging agent that allows in vivo visualization of CB2 expression may thus allow quantification of neuroinflammation. In this paper, we report the synthesis, radiosynthesis, biodistribution and in vitro evaluation of a carbon-11 ([11C]MA2) and a fluorine-18 ([18F]MA3) labeled analog of a highly potent N-arylamide oxadiazole CB2 agonist (EC50 = 0.015 nM). MA2 and MA3 behaved as potent CB2 agonist (EC50: 3 nM and 0.1 nM, respectively) and their in vitro binding affinity for hCB2 was found to be 87 nM and 0.8 nM, respectively. Also MA3 (substituted with a fluoro ethyl group) was found to have higher binding affinity and EC50 values when compared to the originally reported trifluoromethyl analog 12. [11C]MA2 and [18F]MA3 were successfully synthesized with good radiochemical yield, high radiochemical purity and high specific activity. In mice, both tracers were efficiently cleared from blood and all major organs by the hepatobiliary pathway and importantly these compounds showed high brain uptake. In conclusion, [11C]MA2 and [18F]MA3 are shown to be high potent CB2 agonists with good brain uptake, these favorable characteristics makes them potential PET probes for in vivo imaging of brain CB2 receptors. However, in view of its higher affinity and selectivity, further detailed evaluation of MA3 as a PET tracer for CB2 is warranted.

[18F]JNJ42259152 binding to phosphodiesterase 10A, a key regulator of medium spiny neuron excitability, is altered in the presence of cyclic AMP.

Phosphodiesterase 10A (PDE10A) is a key regulator of medium spiny neuron excitability. Therefore, it plays an important role in the regulation of motor, reward, and cognitive processes. Despite the interest in PDE10A as a drug and positron emission tomography (PET) imaging target, little is known about the regulation of PDE10A enzymatic activity. This study aimed to further investigate the role of cAMP in the regulation of PDE10A activity and PDE10A PET imaging. Using [18 F]JNJ42259152 as radioligand, we investigated alterations in PDE10A binding secondary to changes in cAMP levels. An in vitro striatum homogenate binding assay was developed to determine KD and Bmax of [18 F]JNJ42259152. Homogenate binding was assessed after addition of increasing concentrations of exogenous cAMP (1, 10, and 100 µM). Rats were treated using JNJ49137530 and rolipram to induce in vivo alterations of cAMP. The effect of the induced cAMP alterations on PDE10A binding was assessed by comparing [18 F]JNJ42259152 microPET studies after treatment to microPET studies acquired at baseline conditions prior to treatment. In vitro binding affinity of [18 F]JNJ42259152 was higher in the presence of cAMP compared to baseline conditions (KD  = 3.17 ± 0.91 nM with 10 µM cAMP vs. KD  = 6.62 ± 0.7 nM at baseline). Inhibition of PDE4 using rolipram significantly increased [18 F]JNJ42259152 binding (BPND  = 2.61 ± 0.50 vs. 1.91 ± 0.36 at baseline). Administration of the PDE2 inhibitor JNJ49137530 significantly increased PDE10A binding potential (BPND  = 2.74 ± 0.22 vs. 2.05 ± 0.16 at baseline). Our data indicate an important role for cAMP in the regulation of PDE10A activity. Additionally, our data show a profound interaction between several PDEs in striatum.

Preclinical Evaluation of a P2X7 Receptor-Selective Radiotracer: PET Studies in a Rat Model with Local Overexpression of the Human P2X7 Receptor and in Nonhuman Primates.

UNLABELLED: The P2X7 receptor (P2X7R) orchestrates neuroinflammation, and this is the basis for an increased interest in the development of antagonists inhibiting P2X7R function in the brain. This study provides the preclinical evaluation of (11)C-JNJ-54173717, a PET tracer for P2X7R in both rats and nonhuman primates. METHODS: (11)C-JNJ-54173717 is a high-affinity radiotracer for the human P2X7R (hP2X7R). Biodistribution and radiometabolite studies were performed. Viral vectors encoding either enhanced green fluorescent protein-hP2X7R or 3flag-hP2X7R were engineered and validated in cell culture. hP2X7R was regionally overexpressed in the rat striatum after stereotactic injection of viral vectors. Dynamic small-animal PET studies were performed in vector-injected rats and in healthy monkeys using (11)C-JNJ-54173717. RESULTS: The affinity of JNJ-54173717 was 1.6 ± 0.1 nM in a rat cortex P2X7R membrane binding assay. In a functional assay at the recombinant human and rat P2X7R orthologs, the half maximal inhibitory concentration (IC50) of JNJ-54173717 was 4.2 ± 0.01 nM and 7.6 ± 0.01 nM, respectively. The rat biodistribution study showed that (11)C-JNJ-54173717 crossed the blood-brain barrier and was cleared from plasma mainly via the hepatobiliary pathway. A polar radiometabolite was found in rat plasma. No radiometabolites were detected in rat brain. Dynamic small-animal PET showed binding of (11)C-JNJ-54173717 in the striatum expressing hP2X7R, with rapid washout from the noninjected control striatum and other brain regions. Likewise, (11)C-JNJ-54173717 PET signal was blocked by a chemically distinct P2X7R ligand, indicating specific binding to P2X7R in the monkey brain. CONCLUSION: JNJ-54173717 is a high-affinity P2X7R antagonist. An animal rat model stably expressing hP2X7R was developed and validated, identifying favorable characteristics for (11)C-JNJ-54173717 as a PET radioligand for in vivo visualization of hP2X7R. (11)C-JNJ-54173717 selectively visualized P2X7R in the monkey brain, and this radioligand will be further evaluated in a clinical setting to study P2X7R expression levels in neurodegenerative disorders.

Dynamic 68Ga-DOTATOC PET/CT and static image in NET patients. Correlation of parameters during PRRT.

PURPOSE: To investigate the relationship between the dynamic parameters (Ki) and static image-derived parameters of 68Ga-DOTATOC-PET, to determine which static parameter best reflects underlying somatostatin-receptor-expression (SSR) levels on neuroendocrine tumours (NETs). PATIENTS, METHODS: 20 patients with metastasized NETs underwent a dynamic and static 68Ga-DOTATOC-PET before PRRT and at 7 and 40 weeks after the first administration of 90Y-DOTATOC (in total 4 cycles were planned); 175 lesions were defined and analyzed on the dynamic as well as static scans. Quantitative analysis was performed using the software PMOD. One to five target lesions per patient were chosen and delineated manually on the baseline dynamic scan and further, on the corresponding static 68Ga-DOTATOC-PET and the dynamic and static 68Ga-DOTATOC-PET at the other time-points; SUVmax and SUVmean of the lesions was assessed on the other six scans. The input function was retrieved from the abdominal aorta on the images. Further on, Ki was calculated using the Patlak-Plot. At last, 5 reference regions for normalization of SUVtumour were delineated on the static scans resulting in 5 ratios (SUVratio). RESULTS: SUVmax and SUVmean of the tumoural lesions on the dynamic 68Ga-DOTATOC-PET had a very strong correlation with the corresponding parameters in the static scan (R²: 0.94 and 0.95 respectively). SUVmax, SUVmean and Ki of the lesions showed a good linear correlation; the SUVratios correlated poorly with Ki. A significantly better correlation was noticed between Ki and SUVtumour(max and mean) (p < 0.0001). CONCLUSIONS: As the dynamic parameter Ki correlates best with the absolute SUVtumour, SUVtumour best reflects underlying SSR-levels in NETs.

Comparison of New Tau PET-Tracer Candidates With [18F]T808 and [18F]T807.

Early clinical results of two tau tracers, [(18)F]T808 and [(18)F]T807, have recently been reported. In the present study, the biodistribution, radiometabolite quantification, and competition-binding studies were performed in order to acquire comparative preclinical data as well as to establish the value of T808 and T807 as benchmark compounds for assessment of binding affinities of eight new/other tau tracers. Biodistribution studies in mice showed high brain uptake and fast washout.In vivoradiometabolite analysis using high-performance liquid chromatography showed the presence of polar radiometabolites in plasma and brain. No specific binding of [(18)F]T808 was found in transgenic mice expressing mutant human P301L tau. In semiquantitative autoradiography studies on human Alzheimer disease slices, we observed more than 50% tau selective blocking of [(18)F]T808 in the presence of 1 µmol/L of the novel ligands. This study provides a straightforward comparison of the binding affinity and selectivity for tau of the reported radiolabeled tracers BF-158, BF-170, THK5105, lansoprazole, astemizole, and novel tau positron emission tomography ligands against T807 and T808. Therefore, these data are helpful to identify structural requirements for selective interaction with tau and to compare the performance of new highly selective and specific radiolabeled tau tracers.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT.

Clinical trials aiming to develop disease-altering drugs for Alzheimer's disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

New Chelators for Low Temperature Al(18)F-Labeling of Biomolecules.

The Al(18)F labeling method is a relatively new approach that allows radiofluorination of biomolecules such as peptides and proteins in a one-step procedure and in aqueous solution. However, the chelation of the {Al(18)F}(2+) core with the macrocyclic chelators NOTA or NODA requires heating to 100-120 °C. Therefore, we have developed new polydentate ligands for the complexation of {Al(18)F}(2+) with good radiochemical yields at a temperature of 40 °C. The stability of the new Al(18)F-complexes was tested in phosphate buffered saline (PBS) at pH 7.4 and in rat serum. The stability of the Al(18)F-L3 complex was found to be comparable to that of the previously reported Al(18)F-NODA complex up to 60 min in rat serum. Moreover, the biodistribution of Al(18)F-L3 in healthy mice showed the absence of in vivo defluorination since no significant bone uptake was observed, whereas the major fraction of activity at 60 min p.i. was observed in liver and intestines, indicating hepatobiliary clearance of the radiolabeled ligand. The acyclic chelator H3L3 proved to be a good lead candidate for labeling of heat-sensitive biomolecules with fluorine-18. In order to obtain a better understanding of the different factors influencing the formation and stability of the complex, we carried out more in-depth experiments with ligand H3L3. As a proof of concept, we successfully conjugated the new AlF-chelator with the urea-based PSMA inhibitor Glu-NH-CO-NH-Lys to form Glu-NH-CO-NH-Lys(Ahx)L3, and a biodistribution study in healthy mice was performed with the Al(18)F-labeled construct. This new class of AlF-chelators may have a great impact on PET radiochemical space as it will stimulate the rapid development of new fluorine-18 labeled peptides and other heat-sensitive biomolecules.

Quantification of TSPO overexpression in a rat model of local neuroinflammation induced by intracerebral injection of LPS by the use of [(18)F]DPA-714 PET.

PURPOSE: [(18)F]DPA-714 is a radiotracer with high affinity for TSPO. We have characterized the kinetics of [(18)F]DPA-714 in rat brain and evaluated its ability to quantify TSPO expression with PET using a neuroinflammation model induced by unilateral intracerebral injection of lipopolysaccharide (LPS). METHODS: Dynamic small-animal PET scans with [(18)F]DPA-714 were performed in Wistar rats on a FOCUS-220 system for up to 3 h. Both plasma and perfused brain homogenates were analysed using HPLC to quantify radiometabolites. Full kinetic modelling of [(18)F]DPA-714 brain uptake was performed using a metabolite-corrected arterial plasma input function. Binding potential (BPND) calculated as the distribution volume ratio minus one (DVR-1) between affected and healthy brain tissue was used as the outcome measure and evaluated against reference tissue models. RESULTS: The percentage of intact [(18)F]DPA-714 in arterial plasma samples was 92 ± 4 % at 10 min, 75 ± 8 % at 40 min and 52 ± 6 % at 180 min. The radiometabolite fraction in brain was negligible (<3 % at 30 min). Among the models investigated, the reversible two-tissue (2T) compartment model best described [(18)F]DPA-714 brain kinetics. BPND values obtained with a simplified and a multilinear reference tissue model (SRTM, MRTM) using the contralateral striatum as the reference region correlated well (Spearman's r = 0.96, p ≤ 0.003) with 2T BPND values calculated as DVR-1, and showed comparable bias (bias range 17.94 %, 20.32 %). Analysis of stability over time suggested that the acquisition time should be at least 90 min for SRTM and MRTM. CONCLUSION: Quantification of [(18)F]DPA-714 binding to TSPO with full kinetic modelling is feasible using a 2T model. SRTM and MRTM can be suggested as reasonable substitutes with the contralateral striatum as the reference region and a scan duration of at least 90 min. However, selection of the reference region depends on the disease model used.

PET imaging of TSPO in a rat model of local neuroinflammation induced by intracerebral injection of lipopolysaccharide.

OBJECTIVE: The goal of this study was to measure functional and structural aspects of local neuroinflammation induced by intracerebral injection of lipopolysaccharide (LPS) in rats using TSPO microPET imaging with [(18)F]DPA-714, magnetic resonance imaging (MRI), in vitro autoradiography and immunohistochemistry (IHC) in order to characterize a small animal model for screening of new PET tracers targeting neuroinflammation. METHODS: Rats were injected stereotactically with LPS (50 µg) in the right striatum and with saline in the left striatum. [(18)F]DPA-714 microPET, MRI, in vitro autoradiography and IHC studies were performed at different time points after LPS injection for 1 month. RESULTS: Analysis of the microPET data demonstrated high uptake of the tracer in the LPS injected site with an affected-to-non-affected side-binding potential ratio (BPright-to-left) of 3.0 at 3 days after LPS injection. This BP ratio decreased gradually over time to 0.9 at 30 days after LPS injection. In vitro autoradiography ([(18)F]DPA-714) and IHC (CD68, GFAP and TSPO) confirmed local neuroinflammation in this model. Dynamic contrast enhanced (DCE) MRI demonstrated BBB breakdown near the LPS injection site at day 1, which gradually resolved over time and was absent at 1 month after LPS injection. CONCLUSION: The LPS model is useful for first screening of newly developed tracers because of the easy design and the robust, unilateral inflammatory reaction allowing the use of the contralateral region as control. Additionally, this model can be used to test and follow up the benefits of anti-inflammatory therapies by non-invasive imaging.

Retention of [(18)F]fluoride on reversed phase HPLC columns.

As [(18)F]fluoride is a starting reagent in the radiosynthesis of most fluorine-18 labeled positron emission tomography (PET) tracers, its chromatographic behavior on reversed phase (RP) HPLC columns is important for the purification performance and accuracy of RP HPLC quality control methods. We have investigated the chromatographic behavior and recovery of [(18)F]fluoride as a function of the type and brand of RP HPLC column, the pH and the composition of the mobile phase. Elution and elution profile of [(18)F]fluoride from six RP-HPLC columns (Waters XBridge C18 3 mm × 100 mm 3.5 µm; Grace Platinum EPS C18 4.6 mm × 100 mm, 3 µm; Waters XTerra C18 4.6 mm × 250 mm, 5 µm; Phenomenex C18 4.6 mm × 150 mm, 5 µm; Hamilton PRP-1 column 4.1 mm × 150 mm, 5 µm; Merck KGaA Chromolith Performance C18 3 mm × 100 mm) eluted with mobile phase composed of phosphate or acetate buffers (pH 2, 3, 4, 5, 7.3 and 9) and acetonitrile or ethanol as organic modifier were characterized. The elution profile was determined by on-line radioactivity measurement in the column eluate and recovery was calculated by comparison of radioactivity eluted with the HPLC column present or absent in the chromatographic flow path. Interestingly, [(18)F]fluoride recovery increased with increasing pH. At pH 3 all packed silica-based columns showed significant retention of fluorine-18, whereas almost no retention was observed on a polymeric PRP-1 column. However at pH 5, [(18)F]fluoride recovery was above 90% for each tested column. In addition, small differences were observed when changing the composition of the mobile phase. We therefore recommend to use a mobile phase with pH > 5 for silica based C18 columns for both quality control and semi-preparative HPLC of fluorine-18 labeled PET radiopharmaceuticals. If required a lower pH can be used in combination with a polymer based HPLC column.

Sodium cholate, a solubilizing agent for the necrosis avid radioiodinated hypericin in rabbits with acute myocardial infarction.

OBJECTIVES: We determined whether sodium cholate (NaCh) could act as a solubilizing agent for the necrosis avid iodine-123-labeled hypericin ((123)I-Hyp) and investigated biodistribution and targetability of this formulation in rabbits with acute myocardial infarction (AMI). MATERIALS AND METHODS: Solubility of radioiodinated hypericin/hypericin (Hyp) in NaCh solutions was evaluated by microscopy. Hyp with (123)I-sodium iodide was performed using hydrogen peroxide as oxidant in 0.06 M NaCh. Radiochemical yield determination and purification were conducted using high performance liquid chromatography. (123)I-Hyp was solubilized in 0.06 M NaCh containing 1.9 × 10(-4 )M Hyp. The formulation was macroscopically inspected and intravenously injected to five rabbits with AMI. At 24 h, biodistribution was evaluated by tissue gamma counting (TGC) and necrosis targetability was assessed by TGC, autoradiography, fluorescence examination and histology. RESULTS: Microscopically NaCh at 0.06 M shows the best properties for solubilizing the radioiodinated Hyp/Hyp. (123)I-Hyp in 0.06 M NaCh was achieved in 85% with radiochemical purity of 99% after purification. NaCh-dissolved (123)I-Hyp/Hyp shows no particles. By TGC, animals exhibited higher (p = 0.003) radioactivity accumulation in AMI (0.8 ± 0.2% ID/g) than in normal myocardium (0.05 ± 0.02% ID/g), as confirmed by autoradiography, fluorescence measurement and histology. Among organs, the highest uptake of radioactivity was found in liver (15.7 ± 0.6% ID), large (9.7 ± 1.0% ID) and small (5.9 ± 0.6% ID) intestines. CONCLUSION: Necrosis avidity of NaCh-dissolved (123)I-Hyp/Hyp and its hepatobiliary excretion were demonstrated. The suitability of NaCh as solubilizing agent of (123)I-Hyp for hotspot imaging of AMI was proved.