Radiation Dosimetry of a Novel Adenosine A(2A) Receptor Radioligand [C-11]Preladenant Based on PET/CT Imaging and Ex Vivo Biodistribution in Rats

[C-11]Preladenant was developed as a novel adenosine A(2A) receptor PET radioligand. The aim of this study was to determine the radiation dosimetry of [C-11]preladenant and to investigate whether dosimetry estimation based on organ harvesting can be replaced by positron emission tomography (PET)/x-ray computed tomography (CT) imaging in rats. Male Wistar rats (n = 35) were i.v. injected with [C-11]preladenant. The tracer biodistribution was determined by organ harvesting at 1, 5, 15, 30, 60, and 90 min post injection. Hollow organs including the stomach, intestines, and urinary bladder were harvested with contents. In 10 rats, a 90-min dynamic PET/CT scan of the torso was acquired. Twenty volumes of interest (VOIs) were manually drawn on the PET image using the CT image of the same animal as anatomical reference. The dynamic time-activity curves were used to calculate organ residence times (RTs). Human radiation dosimetry estimates, derived from rat data, were calculated with OLINDA/EXM 1.1. PET-imaging and organ-harvesting estimated comparable organ RTs, with differences of 6-27 %, except for the lungs, pancreas, and urinary bladder, with differences of 48, 53, and 60, respectively. The critical organ was the small intestine with a dose of 25 mu Sv/MBq. The effective doses (EDs) calculated from imaging-based and organ-harvesting-derived data were 5.5 and 5.6 mu Sv/MBq, respectively, using the International Commission on Radiological Protection 60 tissue weighting factors. The ED of [C-11]preladenant (2 mSv for a 370-MBq injected dose) is comparable with other C-11-labeled PET tracers. Estimation of the radiation dosimetry of [C-11]preladenant by PET/CT imaging in rats is feasible and gives comparable results to organ harvesting, provided that small VOIs are used and the content of hollow organs is taken into account. Dosimetry by PET imaging can strongly reduce the number of laboratory animals required

Preclinical Evaluation and Quantification of F-18-Fluoroethyl and F-18-Fluoropropyl Analogs of SCH442416 as Radioligands for PET Imaging of the Adenosine A(2A) Receptor in Rat Brain

The cerebral adenosine A(2A) receptor is an attractive therapeutic target for neuropsychiatric disorders. F-18-fluoroethyl and F-18-fluoropropyl analogs of F-18-labeled pyrazolo[4,3-e]-1,2,4-triazolo [1,5-c] pyrimidine (SCH442416) (F-18-FESCH and F-18-FPSCH, respectively) were developed as A(2A) receptor-specific PET ligands. Our aim was to determine an appropriate compartmental model for tracer kinetics, evaluate a reference tissue approach, and select the most suitable PET ligand. Methods: A 90-min dynamic PET scan with arterial blood sampling and metabolite analysis was acquired for 22 healthy male Wistar rats starting at the time of F-18-FESCH (n = 12) and F-18-FPSCH (n = 10) injection. For each tracer, half the animals were vehicle-treated whereas the other half were pretreated with the A(2A) receptor-selective antagonist KW-6002, inducing full blocking. Regional tissue total volume of distribution (V-T) was estimated by 1- and 2-tissue-compartment modeling (1TCM and 2TCM, respectively) and Logan graphical analysis. Midbrain, cerebellum, and hippocampus were evaluated as the reference region by comparing baseline V-T with V-T under full blocking conditions and comparing striatal nondisplaceable binding potential (BPND) using a simplified reference tissue model (SRTM) with distribution volume ratio minus 1 (DVR - 1) for 60- and 90-min scans. Results: On the basis of the Akaike information criterion, 1TCM and 2TCM were the most appropriate models for F-18-FPSCH (baseline striatal VT, 3.7 6 1.1) and F-18-FESCH (baseline striatal V-T, 5.0 6 2.0), respectively. Baseline striatal V-T did not significantly differ between tracers. After pretreatment, striatal V-T was reduced significantly, with no significant decrease in hippocampus, midbrain, or cerebellum V-T. Baseline striatal SRTM BPND did not differ significantly from DVR - 1 except for F-18-FPSCH when using a 60-min scan and midbrain as the reference region, whereas Bland-Altman analysis found a smaller bias for F-18-FESCH and a 60-min scan. After pretreatment, striatal SRTM BPND did not significantly differ from zero except for F-18-FPSCH when using hippocampus as the reference region. Striatal SRTM BPND using midbrain or cerebellum as the reference region was significantly lower for F-18-FPSCH (range, 1.41-2.62) than for F-18-FESCH (range, 1.64-3.36). Conclusion: Dynamic PET imaging under baseline and blocking conditions determined F-18-FESCH to be the most suitable PET ligand for quantifying A(2A) receptor expression in the rat brain. Accurate quantification is achieved by a 60-min dynamic PET scan and the use of either cerebellum or midbrain as the reference region

Use of 11C-MPDX and PET to study adenosine A1 receptor occupancy by nonradioactive agonists and antagonists

BACKGROUND: Adenosine A1 receptors (A1Rs) in human and rodent brains can be visualized with the radioligand 8-dicyclopropylmethyl-1-(11)C-methyl-3-propylxanthine ((11)C-MPDX) and PET. Here we investigated whether A1R occupancy by nonradioactive agonists and antagonists can be assessed with this technique. METHODS: Small-animal PET scans with arterial blood sampling were obtained for 4 groups of isoflurane-anesthetized Wistar rats: controls (n = 7); pretreated with a centrally active A1R agonist, N(6)-cyclopentyladenosine (CPA; 0.25 mg/kg intraperitoneally; dissociation constant, 0.48 nM; n = 7); pretreated with a moderate dose of caffeine (antagonist for A1Rs and adenosine A2A receptors; 4 mg/kg intraperitoneally; dissociation constant, 11 μM; n = 6); and pretreated with a high dose of caffeine (40 mg/kg intraperitoneally; n = 6). RESULTS: The administration of CPA resulted in a strong reduction (>50%) in the heart rate, and caffeine administration resulted in a small increase (10%-15%). A caffeine dose of 4 mg/kg (n = 6) resulted in 65.9% A1R occupancy, and a dose of 40 mg/kg (n = 6) resulted in 98.5% occupancy (calculated from a modified Lassen plot). However, the administration of CPA resulted in an increase in (11)C-MPDX binding in the brain. CONCLUSION: Small-animal PET with (11)C-MPDX can be used to assess antagonist but not agonist binding at A1Rs. Changes in tracer uptake after the administration of CPA resembled previously reported changes induced by treatment of rats with ethanol and an adenosine kinase inhibitor (ABT702). Thus, the administration of an exogenous agonist or increasing the level of an endogenous agonist have similar effects. Agonists and antagonists may bind to different sites on the A1R protein having allosteric interactions

Use of 11C-MPDX and PET to Study Adenosine A1 Receptor Occupancy by Nonradioactive Agonists and Antagonists

Adenosine A1 receptors (A1Rs) in human and rodent brains can be visualized with the radioligand 8-dicyclopropylmethyl-1-11C-methyl-3-propylxanthine (11C-MPDX) and PET. Here we investigated whether A1R occupancy by nonradioactive agonists and antagonists can be assessed with this technique. Methods: Small-animal PET scans with arterial blood sampling were obtained for 4 groups of isoflurane-anesthetized Wistar rats: controls (n = 7); pretreated with a centrally active A1R agonist, N6-cyclopentyladenosine (CPA; 0.25 mg/kg intraperitoneally; dissociation constant, 0.48 nM; n = 7); pretreated with a moderate dose of caffeine (antagonist for A1Rs and adenosine A2A receptors; 4 mg/kg intraperitoneally; dissociation constant, 11 µM; n = 6); and pretreated with a high dose of caffeine (40 mg/kg intraperitoneally; n = 6). Results: The administration of CPA resulted in a strong reduction (>50%) in the heart rate, and caffeine administration resulted in a small increase (10%–15%). A caffeine dose of 4 mg/kg (n = 6) resulted in 65.9% A1R occupancy, and a dose of 40 mg/kg (n = 6) resulted in 98.5% occupancy (calculated from a modified Lassen plot). However, the administration of CPA resulted in an increase in 11C-MPDX binding in the brain. Conclusion: Small-animal PET with 11C-MPDX can be used to assess antagonist but not agonist binding at A1Rs. Changes in tracer uptake after the administration of CPA resembled previously reported changes induced by treatment of rats with ethanol and an adenosine kinase inhibitor (ABT702). Thus, the administration of an exogenous agonist or increasing the level of an endogenous agonist have similar effects. Agonists and antagonists may bind to different sites on the A1R protein having allosteric interactions.

An improved synthesis of n-(4-[18 f]fluorobenzoyl)-interleukin-2 for the preclinical pet imaging of tumour-infiltrating t-cells in ct26 and mc38 colon cancer models

10.3390/molecules26061728Molecules266172

In vivo evaluation of [C-11]preladenant positron emission tomography for quantification of adenosine A(2A) receptors in the rat brain

[(11)C]Preladenant was developed as a novel adenosine A2A receptor positron emission tomography radioligand. The present study aims to evaluate the suitability of [(11)C]preladenant positron emission tomography for the quantification of striatal A2A receptor density and the assessment of striatal A2A receptor occupancy by KW-6002. Sixty- or ninety-minute dynamic positron emission tomography imaging was performed on rats. Tracer kinetics was quantified by the two-tissue compartment model, Logan graphical analysis and several reference tissue-based models. Test-retest reproducibility was assessed by repeated imaging on two consecutive days. Two-tissue compartment model and Logan plot estimated comparable distribution volume (VT) values of ∼10 in the A2A receptor-rich striatum and substantially lower values in all extra-striatal regions (∼1.5-2.5). The simplified reference tissue model with midbrain or occipital cortex as the reference region proved to be the best non-invasive model for quantification of A2A receptor, showing a striatal binding potential (BPND) value of ∼5.5, and a test-retest variability of ∼5.5%. The brain metabolite analysis showed that at 60-min post injection, 17% of the radioactivity in the brain was due to radioactive metabolites. The ED50 of KW-6002 in rat striatum for i.p. injection was 0.044-0.062 mg/kg. The study demonstrates that [(11)C]preladenant is a suitable tracer to quantify striatal A2A receptor density and assess A2A receptor occupancy by A2A receptor-targeting molecules

Granzyme B PET Imaging in Response to In Situ Vaccine Therapy Combined with αPD1 in a Murine Colon Cancer Model

Immune checkpoint inhibitors (ICIs) block checkpoint receptors that tumours use for immune evasion, allowing immune cells to target and destroy cancer cells. Despite rapid advancements in immunotherapy, durable response rates to ICIs remains low. To address this, combination clinical trials are underway assessing whether adjuvants can enhance responsiveness by increasing tumour immunogenicity. CpG-oligodeoxynucleotides (CpG-ODN) are synthetic DNA fragments containing an unmethylated cysteine-guanosine motif that stimulate the innate and adaptive immune systems by engaging Toll-like receptor 9 (TLR9) present on the plasmacytoid dendritic cells (pDCs) and B cells. Here, we have assessed the ability of AlF-mNOTA-GZP, a peptide tracer targeting granzyme B, to serve as a PET imaging biomarker in response to CpG-ODN 1585 in situ vaccine therapy delivered intratumourally (IT) or intraperitoneally (IP) either as monotherapy or in combination with αPD1. [18F]AlF-mNOTA-GZP was able to differentiate treatment responders from non-responders based on tumour uptake. Furthermore, [18F]AlF-mNOTA-GZP showed positive associations with changes in tumour-associated lymphocytes expressing GZB, namely GZB+ CD8+ T cells, and decreases in suppressive F4/80+ cells. [18F]AlF-mNOTA-GZP tumour uptake was mediated by GZB expressing CD8+ cells and successfully stratifies therapy responders from non-responders, potentially acting as a non-invasive biomarker for ICIs and combination therapy evaluation in a clinical setting