Tumour status prediction by means of carbon-ion beam irradiation: comparison of washout rates between in-beam PET and DCE-MRI in rats

Objective.Tumour response to radiation therapy appears as changes in tumour vascular condition. There are several methods for analysing tumour blood circulatory changes one of which is dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), but there is no method that can observe the tumour vascular condition and physiological changes at the site of radiation therapy. Positron emission tomography (PET) has been applied for treatment verification in charged particle therapy, which is based on the detection of positron emitters produced through nuclear fragmentation reactions in a patient's body. However, the produced positron emitters are washed out biologically depending on the tumour vascular condition. This means that measuring the biological washout rate may allow evaluation of the tumour radiation response, in a similar manner to DCE-MRI. Therefore, this study compared the washout rates in rats between in-beam PET during12C ion beam irradiation and DCE-MRI.Approach.Different vascular conditions of the tumour model were prepared for six nude rats. The tumour of each nude rat was irradiated by a12C ion beam with simultaneous in-beam PET measurement. In 10-12 h, the DCE-MRI experiment was performed for the same six nude rats. The biological washout rate of the produced positron emitters (k2,1st) and the MRI contrast agent (k2a) were derived using the single tissue compartment model.Main results.A linear correlation was observed betweenk2,1standk2a, and they were inversely related to fractional necrotic volume.Significance.This is the first animal study which confirmed the biological washout rate of in-beam PET correlates closely with tumour vascular condition measured with the MRI contrast agent administrated intravenously.

Small-animal PET study for noninvasive quantification of transmembrane AMPA receptor regulatory protein -8 (TARP -8) in the brain

Transmembrane AMPA receptor regulatory protein γ-8 (TARP γ-8) mediates various AMPA receptor functions. Recently, [¹¹C]TARP-2105 was developed as a PET ligand for TARP γ-8 imaging. We performed a full kinetic analysis of [¹¹C]TARP-2105 using PET with [¹¹C]TARP-2105 for the first time. The distribution volume (V_T), which is a macro parameter consisting of the K₁-k₄ rate constants in the two-tissue compartment model analysis, exhibited the following rank order: hippocampus (1.4 ± 0.3) > amygdala (1.0 ± 0.2) > frontal cortex (0.9 ± 0.2) > striatum (0.8 ± 0.2) ≫ cerebellum (0.5 ± 0.1) ≈ thalamus (0.5 ± 0.1) > pons (0.4 ± 0.1 mL/cm³). These heterogenous V_T values corresponded with the order of biological distribution of TARP γ-8 in the brain. To validate the reference tissue model, the binding potential (BP_ND) of [¹¹C]TARP-2105 for TARP γ-8 was estimated using general methods (SRTM, MRTM0, Logan reference model, and ratio method). These BP_NDs based on reference models indicated excellent correlation (R² > 0.9) to the indirect BP_NDs based on 2TCM with moderate reproducibility (%variability ≈ 10). PET with [¹¹C]TARP-2105 enabled noninvasive BP_ND estimation and visual mapping of TARP γ-8 in the living rat brain.

Submillimeter resolution positron emission tomography for high-sensitivity mouse brain imaging

Positron emission tomography (PET) is a powerful molecular imaging technique that can provide functional information of living objects. However, the spatial resolution of PET imaging has been limited to around 1 mm which makes it difficult to visualize mouse brain functions in detail. Here we report an ultrahigh resolution small animal PET scanner we developed that can provide a resolution approaching 0.6 mm to visualize the mouse brain functions with unprecedented detail. Methods: The ultrahigh resolution small animal PET scanner had 52.5 mm inner diameter and 51.5 mm axial coverage. The PET scanner consisted of 4 rings each of which had 16 DOI detectors. Each DOI detector consisted of a 3-layer staggered LYSO crystal array with a pitch of 1 mm and 4×4 SiPM array. The physical performance was evaluated in accordance with the NEMA NU4 protocol. The spatial resolution was evaluated with various resolution phantoms. In vivo glucose metabolism imaging of mouse brain was performed. Results: The peak absolute sensitivity was 2.84% with an energy window of 400-600 keV. The 0.55 mm rod structure of a resolution phantom was resolved using the iterative algorithm. The in vivo mouse brain imaging with ¹⁸F-FDG showed clear identification of cortex, thalamus, and hypothalamus which were barely distinguishable in a commercial preclinical PET scanner that we used for imaging comparison. Conclusion: The developed ultrahigh resolution small animal PET scanner is a promising molecular imaging tool for neuroscience research using rodent models.

Performance evaluation of VRAIN: a brain-dedicated PET with a hemispherical detector arrangement

Objective.For PET imaging systems, a smaller detector ring enables less intrinsic spatial resolution loss due to the photon non-collinearity effect as well as better balance between production cost and sensitivity, and a hemispherical detector arrangement is more appropriate for brain imaging than a conventional cylindrical arrangement. Therefore, we have developed a brain-dedicated PET system with a hemispherical detector arrangement, which has been commercialized in Japan under the product name of VRAIN^TM. In this study, we evaluated imaging performance of VRAIN.Approach.The VRAIN used 54 detectors to form the main hemispherical unit and an additional half-ring behind the neck. Each detector was composed of a 12 × 12 array of lutetium fine silicate crystals (4.1 × 4.1 × 10 mm³) and a 12 × 12 array of silicon photomultipliers (4 × 4 mm²active area) with the one-to-one coupling. We evaluated the physical performance of VRAIN according to the NEMA NU 2-2018 standards. Some measurements were modified so as to fit the hemispherical geometry. In addition, we performed¹⁸F-FDG imaging in a healthy volunteer.Main results.In the phantom study, the VRAIN showed high resolution for separating 2.2 mm rods, 229 ps TOF resolution and 19% scatter fraction. With the TOF gain for a 20 cm diameter object (an assumed head diameter), the peak noise-equivalent count rate was 144 kcps at 9.8 kBq ml^-1and the sensitivity was 25 kcps MBq^-1. Overall, the VRAIN provided excellent image quality in phantom and human studies. In the human FDG images, small brain nuclei and gray matter structures were clearly visualized with high contrast and low noise.Significance.We demonstrated the excellent imaging performance of VRAIN, which supported the advantages of the hemispherical detector arrangement.

11C-Labeled Radiotracer for Noninvasive and Quantitative Assessment of the Thiocyanate Efflux System in the Brain

Thiocyanate (SCN^-) alters the potency of certain agonists for the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, and dysfunctions in AMPA receptor signaling are considered to underlie a number of neurological diseases. While humans may be exposed to SCN^- from the environment, including food sources, a carrier-mediated system transports SCN^- from the brain into the blood and is an important regulator of SCN^- distribution in the central nervous system. The assessment of this SCN^- efflux system in the brain would thus be useful for understanding the mechanisms underlying the neurotoxicity of SCN^- and for elucidating the relationship between the efflux system and brain diseases. However, the currently available technique for studying SCN^- efflux is severely limited by its invasiveness. Here, we describe the development of a SCN^- protracer, 9-pentyl-6-[¹¹C]thiocyanatopurine ([¹¹C]1), to overcome this limitation. [¹¹C]1 was synthesized by the reaction of the iodo-precursor and [¹¹C]SCN^- or the reaction of the disulfide precursor with [¹¹C]NH₄CN. The protracer [¹¹C]1 entered the brain after intravenous injection into mice and was rapidly metabolized to [¹¹C]SCN^-, which was then eliminated from the brain. The efflux of [¹¹C]SCN^- was dose-dependently inhibited by perchlorate, a monovalent anion, and the highest dose caused an 82% reduction in the efflux rate. Our findings demonstrate that [¹¹C]1 can be used for the noninvasive and quantitative assessment of the SCN^- efflux system in the brain.

Measurement of biological washout rates depending on tumor vascular status in 15O in-beam rat-PET

Objective. The biological washout of positron emitters should be modeled and corrected in order to achieve quantitative dose range verification in charged particle therapy based on positron emission tomography (PET). This biological washout effect is affected by physiological environmental conditions such as blood perfusion and metabolism, but the correlation to tumour pathology has not been studied yet. Approach. The aim of this study was to investigate the dependence of the biological washout rate on tumour vascular status in rat irradiation. Two types of tumour vascularity conditions, perfused and hypoxic, were modelled with nude rats. The rats were irradiated by a radioactive 15O ion beam and time activity curves were acquired by dynamic in-beam PET measurement. Tumour tissue sections were obtained to observe the histology as well. The biological washout rate was derived using a single-compartment model with two decay components (medium decay, k 2m and slow decay, k 2s ). Main results. All k 2m values in the vascular perfused tumour tissue were higher than the values of the normal tissue. All k 2m values in the hypoxic tumour tissue were much lower than the values of the vascular perfused tumour tissue and slightly lower than the values of the normal tissue. Significance. The dependency of the biological washout on the tumour vasculature conditions was experimentally shown.

Synthesis of [11C]carbonyl-labeled cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate ([11C-carbonyl]PK68) as a potential PET tracer for receptor-interacting protein 1 kinase

Background

Receptor-interacting protein 1 kinase (RIPK1) is a key enzyme in the regulation of cellular necroptosis. Recently, cyclohexyl (5-(2-acetamidobenzo[d]thiazol-6-yl)-2-methylpyridin-3-yl)carbamate (PK68, 5) has been developed as a potent inhibitor of RIPK1. Herein, we synthesized [¹¹C]carbonyl-labeled PK68 ([¹¹C-carbonyl]PK68, [¹¹C]PK68) as a potential PET tracer for imaging RIPK1 and evaluated its brain uptake in vivo.

Results

We synthesized [¹¹C]PK68 by reacting amine precursor 14 with [¹¹C]acetyl chloride. At the end of synthesis, we obtained [¹¹C]PK68 of 1200–1790 MBq with a radiochemical yield of 9.1 ± 5.9% (n = 10, decay-corrected to the end of irradiation) and radiochemical purity of > 99%, and a molar activity of 37–99 GBq/μmol starting from 18–33 GBq of [¹¹C]CO₂. The fully automated synthesis took 30 min from the end of irradiation. In a small-animal PET study, [¹¹C]PK68 was rapidly distributed in the liver and kidneys of healthy mice after injection, and subsequently cleared from their bodies via hepatobiliary excretion and the intestinal reuptake pathway. Although there was no obvious specific binding of RIPK1 in the PET study, [¹¹C]PK68 demonstrated relatively high stability in vivo and provided useful structural information further candidate development.

Conclusions

In the present study, we successfully radiosynthesized [¹¹C]PK68 as a potential PET tracer and evaluated its brain uptake. We are planning to optimize the chemical structure of [¹¹C]PK68 and conduct further PET studies on it using pathological models.

Upregulation of Striatal Metabotropic Glutamate Receptor Subtype 1 (mGluR1) in Rats with Excessive Glutamate Release Induced by N-Acetylcysteine

The aim of this study is to investigate the changes in expression of metabotropic glutamate (Glu) receptor subtype 1 (mGluR1), a key molecule involved in neuroexcitetoxicity, during excessive Glu release in the brain by PET imaging. An animal model of excessive Glu release in the brain was produced by intraperitoneally implanting an Alzet osmotic pump containing N-acetylcysteine (NAC), an activator of the cysteine/Glu antiporter, into the abdomen of rats. Basal Glu concentration in the brain was measured by microdialysis, which showed that basal Glu concentration in NAC-treated rats (0.31 µM) was higher than that in saline-treated rats (0.17 µM) at day 7 after the implantation of the osmotic pump. Similarly, PET studies with [¹¹C]ITDM, a useful radioligand for mGluR1 imaging exhibited that the striatal binding potential (BP_ND) of [¹¹C]ITDM for mGluR1 in PET assessments was increased in NAC-treated animals at day 7 after implantation (2.30) compared with before implantation (1.92). The dynamic changes in striatal BP_ND during the experimental period were highly correlated with basal Glu concentration. In conclusion, density of mGluR1 is rapidly upregulated by increases in basal Glu concentration, suggesting that mGluR1 might to be a potential biomarker of abnormal conditions in the brain.

Neuroprotective effects of minocycline and KML29, a potent inhibitor of monoacylglycerol lipase, in an experimental stroke model: a small-animal positron emission tomography study

Hypoxia caused by ischemia induces acidosis and neuroexcitotoxicity, resulting in neuronal death in the central nervous system (CNS). Monoacylglycerol lipase (MAGL) is a modulator of 2-arachidonoylglycerol (2-AG), which is involved in retrograde inhibition of glutamate release in the endocannabinoid system. In the present study, we used positron emission tomography (PET) to monitor MAGL-positive neurons and neuroinflammation in the brains of ischemic rats. Additionally, we performed PET imaging to evaluate the neuroprotective effects of an MAGL inhibitor in an ischemic injury model.

Methods: Ischemic-injury rat models were induced by intraluminal right middle cerebral artery occlusion (MCAO). PET studies of the brains of the ischemic rats were performed at several experimental time points (pre-occlusion, days 2, 4, and 7 after the MCAO surgery) using [¹¹C]SAR127303 for MAGL and [¹⁸F]FEBMP for 18 kDa translocator protein (TSPO, a hall-mark of neuroinflammation). Medication using minocycline (a well-known neuroprotective agent) or KML29 (a potent MAGL inhibitor) was given immediately after the MCAO surgery and then daily over the subsequent three days.

Results: PET imaging of the ischemic rats using [¹¹C]SAR127303 showed an acute decline of radioactive accumulation in the ipsilateral side at two days after MCAO surgery (ratio of the area under the curve between the ipsilateral and contralateral sides: 0.49 ± 0.04 in the cortex and 0.73 ± 0.02 in the striatum). PET imaging with [¹⁸F]FEBMP, however, showed a moderate increase in accumulation of radioactivity in the ipsilateral hemisphere on day 2 (1.36 ± 0.11), and further increases on day 4 (1.72 ± 0.15) and day 7 (1.99 ± 0.06). Treatment with minocycline or KML29 eased the decline in radioactive accumulation of [¹¹C]SAR127303 for MAGL (minocycline-treated group: 0.82 ± 0.06 in the cortex and 0.81 ± 0.05 in the striatum; KML29-treated group: 0.72 ± 0.07 in the cortex and 0.88 ± 0.04 in the striatum) and increased uptake of [¹⁸F]FEBMP for TSPO (minocycline-treated group: 1.52 ± 0.21 in the cortex and 1.56 ± 0.11 in the striatum; KML29-treated group: 1.63 ± 0.09 in the cortex and 1.50 ± 0.17 in the striatum). In MCAO rats, minocycline treatment showed a neuroprotective effect in the sensorimotor cortex suffering from severe hypoxic injury, whereas KML29 treatment saved neurons in the striatum, including bundles of myelinated axons.

Conclusions: PET imaging allowed visualization of the different neuroprotective effects of minocycline and KML29, and indicated that combination pharmacotherapy using these drugs may be an effective therapy in acute ischemia.

Design, Synthesis, and Evaluation of 11C-Labeled 3-Acetyl-Indole Derivatives as a Novel Positron Emission Tomography Imaging Agent for Diacylglycerol Kinase Gamma (DGKγ) in Brain

Diacylglycerol kinase gamma (DGKγ) is a subtype of DGK enzyme, which catalyzes ATP-dependent conversion of diacylglycerol to phosphatidic acid. DGKγ, localized in the brain, plays an important role in the central nervous system. However, its function has not been widely investigated. Positron emission tomography (PET) imaging of DGKγ validates target engagement of therapeutic DGKγ inhibitors and investigates DGKγ levels under normal and disease conditions. In this study, we designed and synthesized a series of 3-acetyl indole derivatives as candidates for PET imaging agents for DGKγ. Among the synthesized compounds, 2-((3-acetyl-1-(6-methoxypyridin-3-yl)-2-methyl-1H-indol-5-yl)oxy)-N-methylacetamide (9) exhibited potent inhibitory activity (IC50 = 30 nM) against DGKγ and desirable physicochemical properties allowing efficient blood-brain barrier penetration and low levels of undesirable nonspecific binding. The radiolabeling of 9 followed by PET imaging of wild-type and DGKγ-deficient mice and rats indicated that [11C]9 ([11C]T-278) specifically binds to DGKγ and yields a high signal-to-noise ratio for DGKγ in rodent brains.

Off-tumor IDO1 target engagements determine the cancer-immune set point and predict the immunotherapeutic efficacy

Background

Indoleamine-2,3-dioxygenase 1 (IDO1) has been intensively pursued as a therapeutic target to reverse the immunosuppressive cancer-immune milieu and promote tumor elimination. However, recent failures of phase III clinical trials with IDO1 inhibitors involved in cancer immunotherapies highlight the urgent need to develop appropriate methods for tracking IDO1 when the cancer-immune milieu is therapeutically modified.

Methods

We utilized a small-molecule radiotracer, ¹¹C-l-1MTrp, to quantitatively and longitudinally visualize whole-body IDO1 dynamics. Specifically, we first assessed ¹¹C-l-1MTrp in mice-bearing contralateral human tumors with distinct IDO1 expression patterns. Then, we applied ¹¹C-l-1MTrp to longitudinally monitor whole-body IDO1 variations in immunocompetent melanoma-bearing mice treated with 1-methyl-l-tryptophan plus either chemotherapeutic drugs or antibodies targeting programmedcell death 1 and cytotoxic T-lymphocyte-associated protein 4.

Results

¹¹C-l-1MTrp positron emission tomography (PET) imaging accurately delineated IDO1 expression in xenograft mouse models. Moreover, we were able to visualize dynamic IDO1 regulation in the mesenteric lymph nodes (MLNs), an off-tumor IDO1 target, where the percentage uptake of ¹¹C-l-1MTrp accurately annotated the therapeutic efficacy of multiple combination immunotherapies in preclinical models. Remarkably, ¹¹C-l-1MTrp signal intensity in the MLNs was inversely related to the specific growth rates of treated tumors, suggesting that IDO1 expression in the MLNs can serve as a new biomarker of the cancer-immune set point.

Conclusions

PET imaging of IDO1 with ¹¹C-l-1MTrp is a robust method to assess the therapeutic efficacy of multiple combinatorial immunotherapies, improving our understanding of the merit and challenges of IDO1 regimens. Further validation of this animal data in humans is ongoing. We envision that our results will provide a potential precision medicine paradigm for noninvasive visualizing each patient’s individual response in combinatorial cancer immunotherapy, and tailoring optimal personalized combination strategies.

Radiotheranostic Agent 64Cu-cyclam-RAFT-c(-RGDfK-)4 for Management of Peritoneal Metastasis in Ovarian Cancer

PURPOSE

Ovarian cancer peritoneal metastases (OCPMs) are a pathophysiologically heterogeneous group of tumors that are rarely curable. α_Vβ₃ integrin (α_Vβ₃) is overexpressed on tumoral neovessels and frequently on ovarian cancer cells. Here, using two clinically relevant α_Vβ₃-positive OCPM mouse models, we studied the theranostic potential of an α_Vβ₃-specific radiopeptide, ⁶⁴Cu-cyclam-RAFT-c(-RGDfK-)₄ (⁶⁴Cu-RaftRGD), and its intra- and intertumoral distribution in relation to the tumor microenvironment.

EXPERIMENTAL DESIGN

α_Vβ₃-expressing peritoneal and subcutaneous models of ovarian carcinoma (IGR-OV1 and NIH:OVCAR-3) were established in nude mice. ⁶⁴Cu-RaftRGD was administered either intravenously or intraperitoneally. We performed intratumoral distribution (ITD) studies, PET/CT imaging and quantification, biodistribution assay and radiation dosimetry, and therapeutic efficacy and toxicity studies.

RESULTS

Intraperitoneal administration was an efficient route for targeting ⁶⁴Cu-RaftRGD to OCPMs with excellent tumor penetration. Using the fluorescence surrogate, Cy5.5-RaftRGD, in our unique high-resolution multifluorescence analysis, we found that the ITD of ⁶⁴Cu-RaftRGD was spatially distinct from, but complementary to, that of hypoxia. ⁶⁴Cu-RaftRGD-based PET enabled clear visualization of multiple OCPM deposits and ascites and biodistribution analysis demonstrated an inverse correlation between tumor uptake and tumor size (1.2-17.2 mm). ⁶⁴Cu-RaftRGD at a radiotherapeutic dose (148 MBq/0.357 nmol) showed antitumor activities by inhibiting tumor cell proliferation and inducing apoptosis, with negligible toxicity.

CONCLUSIONS

Collectively, these results demonstrate the all-in-one potential of ⁶⁴Cu-RaftRGD for imaging guided radiotherapy of OCPM by targeting both tumoral neovessels and cancerous cells. On the basis of the ITD finding, we propose that pairing α_Vβ₃- and hypoxia-targeted radiotherapies could improve therapeutic efficacy by overcoming the heterogeneity of ITD encountered with single-agent treatments.

3D Compton image reconstruction method for whole gamma imaging

Compton imaging or Compton camera imaging has been studied well, but its advantages in nuclear medicine and molecular imaging have not been demonstrated yet. Therefore, the aim of this work was to compare Compton imaging with positron emission tomography (PET) by using the same imaging platform of whole gamma imaging (WGI). WGI is a concept that combines PET with Compton imaging by inserting a scatterer ring into a PET ring. This concept utilizes diverse types of gamma rays for 3D tomographic imaging. In this paper, we remodeled our previous WGI prototype for small animal imaging, and we developed an image reconstruction method based on a list-mode ordered subset expectation maximization algorithm incorporating detector response function modeling, random correction and normalization (sensitivity correction) for either PET and Compton imaging. To the best of our knowledge, this is the world's first realization of a full-ring Compton imaging system. We selected ⁸⁹Zr as an imaging target because a ⁸⁹Zr nuclide emits a 909 keV single-gamma ray as well as a positron, and we can directly compare Compton imaging of 909 keV photons with PET, a well-established modality. We measured a cylindrical phantom and a small rod phantom filled with ⁸⁹Zr solutions of 10.3 MBq and 10.2 MBq activity, respectively, for 1 h each. The uniform radioactivity distribution of the cylindrical phantom was reconstructed with normalization in both PET and Compton imaging. Coefficients of variation for region-of-interest values were 4.2% for Compton imaging and 3.3% for PET; the difference might be explained by the difference in the detected count number. The small rod phantom experiment showed that the WGI Compton imaging had spatial resolution better than 3.0 mm at the peripheral region although the center region had lower resolution. PET resolved 2.2 mm rods clearly at any location. We measured a mouse for 1 h, 1 d after injection of 9.8 MBq ⁸⁹Zr oxalate. The ⁸⁹Zr assimilated in the mouse bony structures was clearly depicted, and Compton imaging results agreed well with PET images, especially for the region inside the scatterer ring. In conclusion, we demonstrated the performance of WGI using the developed Compton image reconstruction method. We realized Compton imaging with a quality approaching that of PET, which is supporting a future expectation that Compton imaging outperforms PET.

Identification and Development of a New Positron Emission Tomography Ligand 4-(2-Fluoro-4-[11C]methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide for Imaging Metabotropic Glutamate Receptor Subtype 2 (mGlu2)

Metabotropic glutamate receptor 2 (mGlu₂) is a known target for treating several central nervous system (CNS) disorders. To develop a viable positron emission tomography (PET) ligand for mGlu₂, we identified new candidates 5a-i that are potent negative allosteric modulators (NAMs) of mGlu₂. Among these candidates, 4-(2-fluoro-4-methoxyphenyl)-5-((1-methyl-1H-pyrazol-3-yl)methoxy)picolinamide (5i, also named as [¹¹C]MG2-1812) exhibited high potency, high subtype selectivity, and favorable lipophilicity. Compound 5i was labeled with positron-emitting carbon-11 (¹¹C) to obtain [¹¹C]5i in high radiochemical yield and high molar activity by O-[¹¹C]methylation of the phenol precursor 12 with [¹¹C]CH₃I. In vitro autoradiography with [¹¹C]5i showed heterogeneous radioactive accumulation in the brain tissue sections, ranked in the order: cortex > striatum > hippocampus > cerebellum ≫ thalamus > pons. PET study of [¹¹C]5i indicated in vivo specific binding of mGlu₂ in the rat brain. Based on the [¹¹C]5i scaffold, further optimization for new candidates is underway to identify a more suitable ligand for imaging mGlu₂.

Development of an In Vivo Method to Estimate Effective Drug Doses and Quantify Fatty Acid Amide Hydrolase in Rodent Brain using Positron Emission Tomography Tracer [11C]DFMC

Fatty acid amide hydrolase (FAAH) is a key enzyme in the endocannabinoid system. N-(3,4-Dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[¹¹C]methylphenyl)thiazol-2-yl]-1-carboxamide ([¹¹C]DFMC) was developed as an irreversible-type positron emission tomography (PET) tracer for FAAH. Here, we attempted to noninvasively estimate rate constant k₃ (rate of transfer to the specifically-bound compartment) as a direct index for FAAH in the rat brain. First, the two-tissue compartment model analysis including three parameters [K₁-k₃, two-tissue compartment model for the irreversible-type radiotracer (2TCMi)] in PET study with [¹¹C]DFMC was conducted, which provided 0.21 ± 0.04 ml·cm^-3·min^-1 of the net uptake value (K_i), an indirect index for FAAH, in the FAAH-richest region (the cingulate cortex). Subsequently, to noninvasively estimate K_i value, the reference model analysis (Patlak graphical analysis reference model) was tried using a time-activity curve of the spinal cord. In that result, the noninvasive K_i value (K^REF) was concisely estimated with high correlation (r > 0.95) to K_i values based on 2TCMi. Using estimated K^REF value, we tried to obtain calculated-k₃ based on previously defined equations. The calculated k₃ was successfully estimated with high correlation (r = 0.95) to direct k₃ in 2TCMi. Finally, the dose relationship study using calculated k₃ demonstrated that in vivo ED₅₀ value of [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate, a major inhibitor of FAAH, was 66.4 µg/kg in rat brain. In conclusion, we proposed the calculated k₃ as an alternative index corresponding to regional FAAH concentrations and suggested that PET with [¹¹C]DFMC enables occupancy study for new pharmaceuticals targeting FAAH. SIGNIFICANCE STATEMENT: In the present study, we proposed calculated k₃ as an alternative index corresponding with fatty acid amide hydrolase concentration. By using calculated k₃, in vivo ED₅₀ of [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate was successfully estimated to be 66.4 µg/kg for rats. Thus, we demonstrated the pharmacological utility of positron emission tomography with N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[¹¹C]methylphenyl)thiazol-2-yl]-1-carboxamide.

Mechanisms of glutathione-conjugate efflux from the brain into blood: Involvement of multiple transporters in the course

Accumulation of detrimental glutathione-conjugated metabolites in the brain potentially causes neurological disorders, and must therefore be exported from the brain. However, in vivo mechanisms of glutathione-conjugates efflux from the brain remain unknown. We investigated the involvement of transporters in glutathione-conjugates efflux using 6-bromo-7-[¹¹C]methylpurine ([¹¹C]1), which enters the brain and is converted into its glutathione conjugate, S-(7-[¹¹C]methylpurin-6-yl)glutathione ([¹¹C]2). In mice of control and knockout of P-glycoprotein/breast cancer resistance protein and multidrug resistance-associated protein 2 ([Mrp2]^-/-), [¹¹C]2 formed in the brain was rapidly cleared, with no significant difference in efflux rate. In contrast, [¹¹C]2 formed in the brain of Mrp1^-/- mice was slowly cleared, whereas [¹¹C]2 microinjected into the brain of control and Mrp1^-/- mice was 75% cleared within 60 min, with no significant difference in efflux rate. These suggest that Mrp1 contributes to [¹¹C]2 efflux across cell membranes, but not BBB. Efflux rate of [¹¹C]2 formed in the brain was significantly lower in Mrp4^-/- and organic anion transporter 3 (Oat3)^-/- mice compared with control mice. In conclusion, Mrp1, Oat3, and Mrp4 mediate [¹¹C]2 efflux from the brain. Mrp1 may contribute to [¹¹C]2 efflux from brain parenchymal cells, while extracellular [¹¹C]2 is likely cleared across the BBB, partly by Oat3 and Mrp4.

Modified NEMA NU-2 performance evaluation methods for a brain-dedicated PET system with a hemispherical detector arrangement

Brain PET imaging has important roles in neurology, neuro-oncology and molecular imaging research. We have developed a helmet-type PET prototype and have shown that the proposed hemispherical geometry had high potential for realizing high-sensitivity and low-cost brain imaging. However, there is no standard performance evaluation method for helmet-type PET, which would be a bottleneck to its commercialization. Therefore, we investigated appropriate performance evaluation methods for a helmet-type PET based on the NEMA NU 2-2018 standards. For those measurement methods that are not applicable to the helmet-type PET, we changed them while keeping the basic concept of the original NEMA standards. We measured spatial resolution, sensitivity, scatter fraction, count rate characteristics, accuracy of corrections for count losses and randoms, and image quality. We partially changed the measurement methods by making brain-size phantoms and by optimizing the length or the position of radioactive sources. The spatial resolution was 2.8 mm at 1-cm offset position by the filtered back-projection method. Sensitivities measured by the NEMA original setup and the proposed setup were 13.4 and 57.1 kcps/MBq. The respective values measured with our developed brain-size scatter phantom and with the conventional whole-body-size scatter phantom were: scatter fractions of 35% and 35%; peak NECRs of 25.1 kcps at 3.2 kBq/ml and 19.8 kcps at 2.6 kBq/ml ; and maximum absolute biases of 5.5% and 16.0%. The image quality was evaluated with the developed brain-size phantom, and good image quality was obtained. The helmet-type PET prototype showed high-sensitivity even with the small number of 54 detectors. The spatial resolution was better than 4.0 mm over the field-of-view. In conclusion, we proposed the performance evaluation methods for a brain-dedicated PET system with a hemispherical geometry. The proposed method could facilitate evaluation of performance characteristics of brain-dedicated PET scanners and optimization of its scanning and reconstruction parameters.

Performance evaluation of a whole-body prototype PET scanner with four-layer DOI detectors

Parallax error caused by the detector crystal thickness degrades spatial resolution at the peripheral regions of the field-of-view (FOV) of a scanner. To resolve this issue, depth-of-interaction (DOI) measurement is a promising solution to improve the spatial resolution and its uniformity over the entire FOV. Even though DOI detectors have been used in dedicated systems with a small ring diameter such as for the human brain, breast and small animals, the use of DOI detectors for a large bore whole-body PET system has not been demonstrated yet. We have developed a four-layered DOI detector, and its potential for a brain dedicated system has been proven in our previous development. In the present work, we investigated the use of the four-layer DOI detector for a large bore PET system by developing the world's first whole-body prototype. We evaluated its performance characteristics in accordance with the NEMA NU 2 standard. Furthermore, the impact of incorporating DOI information was evaluated with the NEMA NU 4 image quality phantom. Point source images were reconstructed with a filtered back projection (FBP), and an average spatial resolution of 5.2  ±  0.7 mm was obtained. For the FBP image, the four-layer DOI information improved the radial spatial resolution by 48% at the 20 cm offset position. The peak noise-equivalent count rate (NECR) was 22.9 kcps at 7.4 kBq ml^-1 and the scatter fraction was 44%. The system sensitivity was 5.9 kcps MBq^-1. For the NEMA NU 2 image quality phantom, the 10 mm sphere was clearly visualized without any artifacts. For the NEMA NU 4 image quality phantom, we measured the phantom at 0, 10 and 20 cm offset positions. As a result, we found the image with four-layer DOI could visualize the 2 mm-diameter hot cylinder although it could not be recognized on the image without DOI. The average improvements in the recovery coefficients for the five hot rods (1-5 mm) were 0.3%, 4.4% and 26.3% at the 0, 10 and 20 cm offset positions, respectively (except for the 1 mm-diameter rod at the 20 cm offset position). Although several practical issues (such as adding end-shields) remain to be addressed before the scanner is ready for clinical use, we showed that the four-layer DOI technology provided higher and more uniform spatial resolution over the FOV and improved contrast for small uptake regions located at the peripheral FOV, which could improve detectability of small and distal lesions such as nodal metastases, especially in obese patients.

Radiosynthesis and in vivo evaluation of 11C-labeled BMS-193885 and its desmethyl analog as PET tracers for neuropeptide Y1 receptors

BACKGROUND

Neuropeptide Y (NPY) has been implicated in a wide variety of physiological processes, including feeding, learning, memory, emotion, cardiovascular homeostasis, hormone secretion, and circadian rhythms. NPY Yl receptor (NPY Y1-R) is the most widely studied NPY receptor, and is involved in many of these processes. BMS-193885 (1) was previously developed as a potent and selective NPY Y1-R antagonist, which has good systemic bioavailability and brain penetration. To evaluate the characteristics of 1 in vivo, we developed ¹¹C-labeled BMS-193885 ([¹¹C]1) and its desmethyl analog ([¹¹C]2) for potential use as two new positron emission tomography (PET) tracers.

RESULTS

[¹¹C]1 was synthesized from [¹¹C]methyl iodide using 2. [¹¹C]2 was synthesized from [¹¹C]phosgene using its aniline and amine derivatives. The mean ± S.D. decay-corrected radiochemical yields of [¹¹C]1 and [¹¹C]2 from ¹¹CO₂ at the end of radionuclide production were 23 ± 3.2% (n = 6) and 24 ± 1.5% (n = 4), respectively. In biodistribution on mice, radioactivity levels for both tracers were relatively high in the kidney, small intestine, and liver at 60 min post-injection. The radioactivity levels in the kidney, lung, and spleen of mice at 30 min post-injection with [¹¹C]1 were significantly reduced by pretreatment with 1 (10 mg/kg), and levels of [¹¹C]1 in the brain of mice were significantly increased by pretreatment with the P-glycoprotein and breast cancer resistance protein inhibitor elacridar (10 mg/kg). In metabolite analysis using mouse plasma, [¹¹C]1 and [¹¹C]2 were rapidly metabolized within 30 min post-injection, and [¹¹C]1 was mainly metabolized into unlabeled 2 and radiolabeled components.

CONCLUSION

[¹¹C]1 and [¹¹C]2 were successfully synthesized with sufficient amount of radioactivity and high quality for use in vivo. Our study of [¹¹C]1 and its desmethyl analog [¹¹C]2 was useful in that it helped to elucidate the in vivo characteristics of 1.

Development of 2-(2-(3-(4-([18F]Fluoromethoxy- d2)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([¹⁸F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]MNI-659, [¹⁸F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [¹⁸F]5 can accumulate in the brain. In this study, using [¹⁸F]5 as a lead compound, we designed four new ¹⁸F-labeled ligands ([¹⁸F]6-9) to find one more suitable than [¹⁸F]5. Of these, 2-(2-(3-(4-([¹⁸F]fluoromethoxy- d₂)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([¹⁸F]9) exhibited high in vitro binding affinity ( K_i = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BP_ND) of [¹⁸F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [¹⁸F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamination with radiolabeled metabolites in the brains of rats given [¹⁸F]9 than in those given [¹⁸F]5. In conclusion, [¹⁸F]9 is a useful PET ligand for PDE10A imaging in brain.

Integrated treatment using intraperitoneal radioimmunotherapy and positron emission tomography-guided surgery with 64Cu-labeled cetuximab to treat early- and late-phase peritoneal dissemination in human gastrointestinal cancer xenografts

Peritoneal dissemination is a common cause of death from gastrointestinal cancers and is difficult to treat using current therapeutic options, particularly late-phase disease. Here, we investigated the feasibility of integrated therapy using ⁶⁴Cu-intraperitoneal radioimmunotherapy (ipRIT), alone or in combination with positron emission tomography (PET)-guided surgery using a theranostic agent (⁶⁴Cu-labeled anti-epidermal growth factor receptor antibody cetuximab) to treat early- and late-phase peritoneal dissemination in mouse models. In this study, we utilized the OpenPET system, which has open space for conducting surgery while monitoring objects at high resolution in real time, as a novel approach to make PET-guided surgery feasible. ⁶⁴Cu-ipRIT with cetuximab inhibited tumor growth and prolonged survival with little toxicity in mice with early-phase peritoneal dissemination of small lesions. For late-phase peritoneal dissemination, a combination of ⁶⁴Cu-ipRIT for down-staging and subsequent OpenPET-guided surgery for resecting large tumor masses effectively prolonged survival. OpenPET clearly detected tumors (≥3 mm in size) behind other organs in the peritoneal cavity and was useful for confirming the presence or absence of residual tumors during an operation. These findings suggest that integrated ⁶⁴Cu therapy can serve as a novel treatment strategy for peritoneal dissemination.

First demonstration of in vivo mapping for regional brain monoacylglycerol lipase using PET with [11C]SAR127303

Monoacylglycerol lipase (MAGL) is a main regulator of the endocannabinoid system within the central nervous system (CNS). Recently, [¹¹C]SAR127303 was developed as a promising radioligand for MAGL imaging. In this study, we aimed to quantify regional MAGL concentrations in the rat brain using positron emission tomography (PET) with [¹¹C]SAR127303. An irreversible two-tissue compartment model (2-TCMi, k₄ = 0) analysis was conducted to estimate quantitative parameters (k₃, K_i^2-TCMi, and λk₃). These parameters were successfully obtained with high identifiability (<10 %COV) for the following regions ranked in order from highest to lowest: cingulate cortex > striatum > hippocampus > thalamus > cerebellum > hypothalamus ≈ pons. In vitro autoradiographs using [¹¹C]SAR127303 showed a heterogeneous distribution of radioactivity, as seen in the PET images. The K_i^2-TCMi and λk₃ values correlated relatively highly with in vitro binding (r > 0.4, P < 0.005). The K_i^2-TCMi values showed high correlation and low underestimation (<10%) compared with the slope of a Patlak plot analysis with linear regression (K_i^Patlak). In conclusion, we successfully estimated regional net uptake value of [¹¹C]SAR127303 reflecting MAGL concentrations in rat brain regions for the first time.

Kinetics and metabolism of apocynin in the mouse brain assessed with positron-emission tomography

BACKGROUND

Apocynin is a constituent of the medicinal herb Picrorhiza kurroa. It is an inhibitor of nicotinamide adenine dinucleotide phosphate oxidase. This compound shows potential anti-inflammatory and antioxidant effects and has been tested as a neuroprotectant in many animal models of brain disease. In such studies, understanding the brain kinetics of apocynin would be important for interpreting its in vivo efficacy; however, little has been reported on the kinetics of apocynin in the brain.

PURPOSE

The purpose of this study is to investigate the kinetics and metabolism of apocynin in the brain of mice.

STUDY DESIGN

The kinetics and metabolism of apocynin were examined using [¹¹C]apocynin and positron-emission tomography (PET).

METHODS

In vivo PET scanning was performed in mice for 20min after intraperitoneal administration of an apocynin solution containing [¹¹C]apocynin. Metabolites in the brain were analyzed using high-performance liquid chromatography. The doses of apocynin used ranged from <1.5 µg/kg (tracer dose) to 100mg/kg.

RESULTS

Brain radioactivity during the period of 0 to 20min after administration was negligible at the tracer dose and extremely low at the dose of 10mg/kg. Moderate radioactivity was observed in the brain a few minutes after administration at the doses of 25 and 50mg/kg and rapidly decreased thereafter. At a dose of 100mg/kg, [¹¹C]apocynin resulted in a high uptake of radioactivity followed by a gradual washout. In contrast to the brain, a clear dose-dependent increase in radioactivity was not observed in the blood. The fraction of the unchanged form in the brain decreased with time, and the degree of the reduction depended on apocynin doses: apocynin was rapidly metabolized in the brain at lower doses, whereas it was slowly decomposed at higher doses. On the basis of these data, the maximum apocynin concentrations in the brain were calculated to be 10 µM (10mg/kg), 49 µM (25mg/kg), 150 µM (50mg/kg), and 380 µM (100mg/kg). A metabolite observed in the brain was found to be apocynin glucuronide but not diapocynin, an active metabolite.

CONCLUSION

These results would be useful for an evaluation of the potential efficacy of apocynin as a neuroprotective agent.

A useful PET probe [11C]BU99008 with ultra-high specific radioactivity for small animal PET imaging of I2-imidazoline receptors in the hypothalamus

INTRODUCTION

A positron emission tomography (PET) probe with ultra-high specific radioactivity (SA) enables measuring high receptor specific binding in brain regions by avoiding mass effect of the PET probe itself. It has been reported that PET probe with ultra-high SA can detect small change caused by endogenous or exogenous ligand. Recently, Kealey et al. developed [¹¹C]BU99008, a more potent PET probe for I₂-imidazoline receptors (I₂Rs) imaging, with a conventional SA (mean 76GBq/μmol) showed higher specific binding in the brain. Here, to detect small change of specific binding for I₂Rs caused by endogenous or exogenous ligand in an extremely small region, such as hypothalamus in the brain, we synthesized and evaluated [¹¹C]BU99008 with ultra-high SA as a useful PET probe for small-animal PET imaging of I₂Rs.

METHODS

[¹¹C]BU99008 was prepared by [¹¹C]methylation of N-desmethyl precursor with [¹¹C]methyl iodide. Biodistribution, metabolite analysis, and brain PET studies were conducted in rats.

RESULTS

[¹¹C]BU99008 with ultra-high SA in the range of 5400-16,600GBq/μmol were successfully synthesized (n=7), and had appropriate radioactivity for in vivo study. In the biodistribution study, the mean radioactivity levels in all investigated tissues except for the kidney did not show significant difference between [¹¹C]BU99008 with ultra-high SA and that with conventional SA. In the metabolite analysis, the percentage of unchanged [¹¹C]BU99008 at 30min after the injection of probes with ultra-high and conventional SA was similar in rat brain and plasma. In the PET study of rats' brain, radioactivity level (AUC_{30-60 min}) in the hypothalamus of rats injected with [¹¹C]BU99008 with ultra-high SA (64 [SUV ∙ min]) was significantly higher than that observed for that with conventional SA (50 [SUV ∙ min]). The specific binding of [¹¹C]BU99008 with ultra-high SA (86% of total binding) for I₂R was higher than that of conventional SA (76% of total binding).

CONCLUSION

A PET study using [¹¹C]BU99008 with ultra-high SA would thus contribute to the detection of small changes in or small regions with I₂R expression and hence may be useful in elucidating new functions of I₂R.

Development of a simultaneous optical/PET imaging system for awake mice

Simultaneous measurements of multiple physiological parameters are essential for the study of brain disease mechanisms and the development of suitable therapies to treat them. In this study, we developed a measurement system for simultaneous optical imaging and PET for awake mice. The key elements of this system are the OpenPET, optical imaging and fixation apparatus for an awake mouse. The OpenPET is our original open-type PET geometry, which can be used in combination with another device because of the easily accessible open space of the former. A small prototype of the axial shift single-ring OpenPET was used. The objective lens for optical imaging with a mounted charge-coupled device camera was placed inside the open space of the AS-SROP. Our original fixation apparatus to hold an awake mouse was also applied. As a first application of this system, simultaneous measurements of cerebral blood flow (CBF) by laser speckle imaging (LSI) and [(11)C]raclopride-PET were performed under control and 5% CO2 inhalation (hypercapnia) conditions. Our system successfully obtained the CBF and [(11)C]raclopride radioactivity concentration simultaneously. Accumulation of [(11)C]raclopride was observed in the striatum where the density of dopamine D2 receptors is high. LSI measurements could be stably performed for more than 60 minutes. Increased CBF induced by hypercapnia was observed while CBF under the control condition was stable. We concluded that our imaging system should be useful for investigating the mechanisms of brain diseases in awake animal models.

Long-term effects of cerebral hypoperfusion on neural density and function using misery perfusion animal model

We investigated the chronic effects of cerebral hypoperfusion on neuronal density and functional hyperemia using our misery perfusion mouse model under unilateral common carotid artery occlusion (UCCAO). Neuronal density evaluated 28 days after UCCAO using [(11)C]flumazenil-PET and histology indicated no neurologic deficit in the hippocampus and neocortex. CBF response to sensory stimulation was assessed using laser-Doppler flowmetry. Percentage changes in CBF response of the ipsilateral hemisphere to UCCAO were 18.4 ± 3.0%, 6.9 ± 2.8%, 6.8 ± 2.3% and 4.9 ± 2.4% before, and 7, 14 and 28 days after UCCAO, respectively. Statistical significance was found at 7, 14 and 28 days after UCCAO (P < 0.01). Contrary to our previous finding (Tajima et al. 2014) showing recovered CBF response to hypercapnia on 28 days after UCCAO using the same model, functional hyperemia was sustained and became worse 28 days after UCCAO.

Regorafenib as a potential adjuvant chemotherapy agent in disseminated small colon cancer: Drug selection outcome of a novel screening system using nanoimprinting 3-dimensional culture with HCT116-RFP cells

Colon cancer is one of the leading causes of cancer death worldwide. Adjuvant chemotherapy following primary surgical treatment is suggested to be beneficial in eradicating invisible disseminated small tumors in colon cancer; however, an effective drug remains to be developed. Recently, we reported a novel drug screening system using a nanoimprinting 3-dimensional (3D) culture that creates multicellular spheroids, which simulate in vivo conditions and, thereby, predict effective drugs in vivo. This study aimed to perform drug selection using our recently developed 3D culture system in a human colon cancer HCT116 cell line stably expressing red fluorescent protein (HCT116-RFP), to determine the most effective agent in a selection of clinically used antitumor agents for colon cancer. In addition, we confirmed the efficacy of the selected drug regorafenib, in vivo using a mouse model of disseminated small tumors. HCT116-RFP cells were cultured using a nanoimprinting 3D culture and in vitro drug selection was performed with 8 clinically used drugs [bevacizumab, capecitabine, cetuximab, 5-fluorouracil (5-FU), irinotecan, oxaliplatin, panitumumab and regorafenib]. An in vivo study was performed in mice bearing HCT116-RFP intraperitoneally disseminated small tumors using 3'-[18F]-fluoro-3'-deoxythymidine-positron emission tomography and fluorescence microscopy imaging to evaluate the therapeutic effects. Regorafenib was determined to be the most effective drug in the 3D culture, and significantly inhibited tumor growth in vivo, compared to the untreated control and 5-FU-treated group. The drug 5-FU is commonly used in colon cancer treatment and was used as a reference. Our results demonstrate that regorafenib is a potentially efficacious adjuvant chemotherapeutic agent for the treatment of disseminated small colon cancer and, therefore, warrants further preclinical and clinical studies.

Development of a small single-ring OpenPET prototype with a novel transformable architecture

The single-ring OpenPET (SROP), for which the detector arrangement has a cylinder shape cut by two parallel planes at a slant angle to form an open space, is our original proposal for in-beam PET. In this study, we developed a small prototype of an axial-shift type SROP (AS-SROP) with a novel transformable architecture for a proof-of-concept. In the AS-SROP, detectors originally forming a cylindrical PET are axially shifted little by little. We designed the small AS-SROP prototype for 4-layer depth-of-interaction detectors arranged in a ring diameter of 250 mm. The prototype had two modes: open and closed. The open mode formed the SROP with the open space of 139 mm and the closed mode formed a conventional cylindrical PET. The detectors were simultaneously moved by a rotation handle allowing them to be transformed between the two modes. We evaluated the basic performance of the developed prototype and carried out in-beam imaging tests in the HIMAC using (11)C radioactive beam irradiation. As a result, we found the open mode enabled in-beam PET imaging at a slight cost of imaging performance; the spatial resolution and sensitivity were 2.6 mm and 5.1% for the open mode and 2.1 mm and 7.3% for the closed mode. We concluded that the AS-SROP can minimize the decrease of resolution and sensitivity, for example, by transforming into the closed mode immediately after the irradiation while maintaining the open space only for the in-beam PET measurement.

A ¹¹C-labeled 1,4-dihydroquinoline derivative as a potential PET tracer for imaging of redox status in mouse brain

A disturbance in redox balance has been implicated in the pathogenesis of a number of diseases. This study sought to examine the feasibility of imaging brain redox status using a (11)C-labeled dihydroquinoline derivative ([(11)C]DHQ1) for positron emission tomography (PET). The lipophilic PET tracer [(11)C]DHQ1 was rapidly oxidized to its hydrophilic form in mouse brain homogenate. The redox modulators diphenyleneiodonium and apocynin significantly reduced the initial velocity of [(11)C]DHQ1 oxidation, and apocynin also caused concentration-dependent inhibition of the initial velocity. Moreover, [(11)C]DHQ1 readily entered the brain by diffusion after administration and underwent oxidation into the hydrophilic cationic form, which then slowly decreased. By contrast, apocynin treatment inhibited the in vivo oxidation of [(11)C]DHQ1 to the hydrophilic cationic form, leading to a rapid decrease of radioactivity in the brain. Thus, the difference in the [(11)C]DHQ1 kinetics reflects the alteration in redox status caused by apocynin. In conclusion, [(11)C]DHQ1 is a potential PET tracer for imaging of redox status in the living brain.

Utility of Translocator Protein (18 kDa) as a Molecular Imaging Biomarker to Monitor the Progression of Liver Fibrosis

Hepatic fibrosis is the wound healing response to chronic hepatic injury caused by various factors. In this study, we aimed to evaluate the utility of translocator protein (18 kDa) (TSPO) as a molecular imaging biomarker for monitoring the progression of hepatic fibrosis to cirrhosis. Model rats were induced by carbon tetrachloride (CCl₄), and liver fibrosis was assessed. Positron emission tomography (PET) with N-benzyl-N-methyl-2-[7,8-dihydro-7-(2-[¹⁸F]fluoroethyl)-8-oxo-2-phenyl-9H-purin-9-yl]-acetamide ([¹⁸F]FEDAC), a radioprobe specific for TSPO, was used for noninvasive visualisation in vivo. PET scanning, immunohistochemical staining, ex vivo autoradiography, and quantitative reverse-transcription polymerase chain reaction were performed to elucidate the relationships among radioactivity uptake, TSPO levels, and cellular sources enriching TSPO expression in damaged livers. PET showed that uptake of radioactivity in livers increased significantly after 2, 4, 6, and 8 weeks of CCl₄ treatment. Immunohistochemistry demonstrated that TSPO was mainly expressed in macrophages and hepatic stellate cells (HSCs). TSPO-expressing macrophages and HSCs increased with the progression of liver fibrosis. Interestingly, the distribution of radioactivity from [¹⁸F]FEDAC was well correlated with TSPO expression, and TSPO mRNA levels increased with the severity of liver damage. TSPO was a useful molecular imaging biomarker and could be used to track the progression of hepatic fibrosis to cirrhosis with PET.

Development of 1-N-(11)C-Methyl-L- and -D-Tryptophan for pharmacokinetic imaging of the immune checkpoint inhibitor 1-Methyl-Tryptophan

1-Methyl-tryptophan (1MTrp) is known as a specific inhibitor targeting the immune- checkpoint protein indoleamine-2,3-dioxygenase, in two stereoisomers of levorotary (l) and dextrorotary (d). A long-standing debate exists in immunology and oncology: which stereoisomer has the potential of antitumor immunotherapy. Herein, we developed two novel radioprobes, 1-N-¹¹C-methyl-l- and -d-tryptophan (¹¹C-l-1MTrp and ¹¹C-d-1MTrp), without modifying the chemical structures of the two isomers, and investigated their utility for pharmacokinetic imaging of the whole body. ¹¹C-l-1MTrp and ¹¹C-d-1MTrp were synthesized rapidly with radiochemical yields of 47 ± 6.3% (decay-corrected, based on ¹¹C-CO₂), a radiochemical purity of >98%, specific activity of 47–130 GBq/μmol, and high enantiomeric purity. PET/CT imaging in rats revealed that for ¹¹C-l-1MTrp, the highest distribution of radioactivity was observed in the pancreas, while for ¹¹C-D-1MTrp, it was observed in the kidney. Ex vivo biodistribution confirmed the PET/CT results, indicating the differences in pharmacokinetics between the two isomers. Both ¹¹C-l-1MTrp and ¹¹C-d-1MTrp are therefore useful PET probes for delineating the distribution and action of the checkpoint inhibitor 1MTrp in vivo. This study represents the first step toward using whole-body and real-time insight to disentangle the antitumor potential of the two stereoisomers of 1MTrp, and it can facilitate the development of 1MTrp immunotherapy.

[(11)C]Raclopride binding in the striatum of minimally restrained and free-walking awake mice in a positron emission tomography study

Anesthesia and restraint stress have profound impacts on brain functions, including neural activity and cerebrovascular function, possibly influencing functional and neurochemical positron emission tomography (PET) imaging data. For circumventing this effect, we developed an experimental system enabling PET imaging of free-walking awake mice with minimal restraints by fixing the head to a holder. The applicability of this system was investigated by performing PET imaging of D2 dopamine receptors with [(11)C]raclopride under the following three different conditions: (1) free-walking awake state; (2) 1.5% isoflurane anesthesia; and (3) whole-body restraint without anesthesia. [(11)C]raclopride binding potential (BP(ND)) values under isoflurane anesthesia and restrained awake state were significantly lower than under free-walking awake state (P < 0.01). Heart rates in restrained awake mice were significantly higher than those in free-walking awake mice (P < 0.01), suggesting that free-walking awake state minimized restraint stress during the PET scan. [(11)C] raclopride-PET with methamphetamine (METH) injection was also performed in awake and anesthetized mice. METH-induced reduction of [(11)C]raclopride BP(ND) in anesthetized mice showed a trend to be less than that in free-walking awake mice, implying that pharmacological modulation of dopaminergic transmissions could be sensitively captured by PET imaging of free-walking awake mice. We concluded that our system is of utility as an in vivo assaying platform for studies of brain functions and neurotransmission elements in small animals, such as those modeling neuropsychiatric disorders.

Improved Visualization and Specific Binding for Metabotropic Glutamate Receptor Subtype 1 (mGluR1) Using [11C]ITMM with Ultra-High Specific Activity in Small-Animal PET

Metabotropic glutamate receptor subtype 1 (mGluR1) is a crucial target in the development of new medications to treat central nervous system (CNS) disorders. Recently, we developed N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-[¹¹C]methoxy-N-methyl-benzamide ([¹¹C]ITMM) as a useful positron emission tomography (PET) probe for mGluR1 in clinical studies. Here, we aimed to improve visualization and threshold of specific binding for mGluR1 using [¹¹C]ITMM with ultra-high specific activity (SA) of > 3,500 GBq/μmol in rat brains. A two-tissue compartment model indicated large differences between the two SAs in the constants k₃ and k₄, representing binding ability for mGluR1, while constants K₁ and k₂ showed no differences. The total distribution volume (V_T) values of conventional and ultra-high SA were 9.1 and 11.2 in the thalamus, 7.7 and 9.7 in the striatum, and 6.4 and 8.5 mL/cm³ in the substantia nigra, respectively. The specific binding of [¹¹C]ITMM with ultra-high SA was significantly higher than the conventional SA, especially in the basal ganglia. Parametric PET images scaled with V_T of the ultra-high SA clearly identified regional differences in the rat brain. In conclusion, PET studies using [¹¹C]ITMM with ultra-high SA could sufficiently improve visualization and specific binding for mGluR1, which could help further understanding for mGluR1 functions in CNS disorders.

Preclinical assessment of early tumor response after irradiation by positron emission tomography with 2-amino-[3-¹¹C]isobutyric acid

The positron emission tomography (PET) probe, 2-amino-[3-¹¹C]isobutyric acid ([3-¹¹C]AIB), is reported to accumulate less in inflammatory lesions than 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) and has the potential for evaluation of the efficacy of radiotherapy. To determine whether [3-¹¹C]AIB is useful to monitor early metabolic change in tumors after radiotherapy, we evaluated the temporal change in [3-¹¹C]AIB tumor uptake, tumor volume, histological features and expression of amino acid transporters early after radiotherapy in a mouse tumor model. PET with [3-¹¹C]AIB was conducted in mice bearing a subcutaneous tumor (SY, derived from small cell lung cancer) in two schedules: schedule 1, before (day -1) and after (days 1 and 3) 15 Gy of radiation and schedule 2, days -1, 1 and 5. [3-¹¹C]AIB tumor uptake tended to increase on day 1 after irradiation and decreased thereafter. Tumor uptake was not correlated with tumor volume in schedule 1. Although tumor uptake was correlated with tumor volume in schedule 2, this correlation was lost when the day 5 data of greatly reduced tumor volumes were excluded. In a separate group of tumor-bearing mice, excised tumor sections were stained with terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) or anti-Ki-67 antibody. There was no correlation between tumor uptake and percentages of TUNEL- or Ki-67-positive cells. Expression of amino acid transporters, SLC38A1, SLC38A2 and SLC38A4, was determined by real-time RT-PCR. SLC38A1 and SLC38A2 were expressed in SY tumors, and a significant correlation was observed between [3-¹¹C]AIB tumor uptake and SLC38A1 expression. In conclusion, early change in [3-¹¹C]AIB tumor uptake after irradiation reflected the temporal change in amino acid transporter expression, while it was independent of change in tumor volume, apoptosis and cell proliferation. PET with [3-¹¹C]AIB has the potential for use in non-invasive evaluation of early metabolic change after irradiation before morphological change of tumors.

OAT3-mediated extrusion of the 99mTc-ECD metabolite in the mouse brain

After administration of the (99m)Tc complex with N,N'-1,2-ethylenediylbis-L-cysteine diethyl ester ((99m)Tc-ECD), a brain perfusion imaging agent, the radioactive metabolite is trapped in primate brain, but not in mouse and rat. Here, we investigate the involvement of metabolite extrusion by organic anion transporter 3 (OAT3), which is highly expressed at the blood-brain barrier in mice, in this species difference. The efflux rate of radioactivity in the cerebrum of Oat3(-/-) mice at later phase was 20% of that of control mice. Thus, organic anion transporters in mouse brain would be involved in the low brain retention of radioactivity after (99m)Tc-ECD administration.

Molecular imaging of ectopic metabotropic glutamate 1 receptor in melanoma with a positron emission tomography radioprobe (18) F-FITM

Oncoimaging using positron emission tomography (PET) with a specific radioprobe would facilitate individualized cancer management. Evidence indicates that ectopically expressed metabotropic glutamate 1 (mGlu1) receptor independently induces melanocyte carcinogenesis, and it is therefore becoming an important target for personalized diagnosis and treatment strategies for melanomas. Here, we report the development of an oncoprotein-based PET imaging platform in melanomas for noninvasive visualization and quantification of mGlu1 with a novel mGlu1-specific radioprobe, 4-(18)F-fluoro-N-[4-[6-(isopropyl amino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methylbenzamide ((18)F-FITM). (18)F-FITM shows excellent pharmacokinetics, namely the dense and specific accumulation in mGlu1-positive melanomas versus mGlu1-negative hepatoma and normal tissues. Furthermore, the accumulation levels of radioactivity corresponded to the extent of tumor and to levels of mGlu1 protein expression in melanomas and melanoma metastasis. The (18)F-FITM PET imaging platform, as a noninvasive personalized diagnostic tool, is expected to open a new avenue for defining individualized therapeutic strategies, clinical trials, patient management and understanding mGlu1-triggered oncologic events in melanomas.

Visualization of acute liver damage induced by cycloheximide in rats using PET with [(18)F]FEDAC, a radiotracer for translocator protein (18 kDa)

Liver damage induced by drug toxicity is an important concern for both medical doctors and patients. The aim of this study was to noninvasively visualize acute liver damage using positron emission tomography (PET) with N-benzyl-N-methyl-2-[7,8-dihydro-7-(2-[¹⁸F]fluoroethyl)-8-oxo-2-phenyl-9H-purin-9-yl]acetamide ([¹⁸F]FEDAC), a radiotracer specific for translocator protein (18 kDa, TSPO) as a biomarker for inflammation, and to determine cellular sources enriching TSPO expression in the liver. A mild acute liver damage model was prepared by a single intraperitoneal injection of cycloheximide (CHX) into rats. Treatment with CHX induced apoptosis and necrotic changes in hepatocytes with slight neutrophil infiltration. The uptake of radioactivity in the rat livers was measured with PET after injection of [¹⁸F]FEDAC. The uptake of [¹⁸F]FEDAC increased in livers damaged from treatment with CHX compared to the controls. Presence of TSPO was examined in the liver tissue using quantitative reverse transcriptase-polymerase chain reaction and immunohistochemical assays. mRNA expression of TSPO was elevated in the damaged livers compared to the controls, and the level was correlated with the [¹⁸F]FEDAC uptake and severity of damage. TSPO expression in the damaged liver sections was mainly found in macrophages (Kupffer cells) and neutrophils, but not in hepatocytes. The elevation of TSPO mRNA expression was derived from the increase of the number of macrophages with TSPO and neutrophils with TSPO in damaged livers. From this study we considered that PET imaging with [¹⁸F]FEDAC represented the mild liver damage through the enhanced TSPO signal in inflammatory cells. We conclude that this method may be a useful tool for diagnosis in early stage of acute liver damage.

Novel system using microliter order sample volume for measuring arterial radioactivity concentrations in whole blood and plasma for mouse PET dynamic study

This study aimed to develop a new system, named CD-Well, for mouse PET dynamic study. CD-Well allows the determination of time-activity curves (TACs) for arterial whole blood and plasma using 2-3 µL of blood per sample; the minute sample size is ideal for studies in small animals. The system has the following merits: (1) measures volume and radioactivity of whole blood and plasma separately; (2) allows measurements at 10 s intervals to capture initial rapid changes in the TAC; and (3) is compact and easy to handle, minimizes blood loss from sampling, and delay and dispersion of the TAC. CD-Well has 36 U-shaped channels. A drop of blood is sampled into the opening of the channel and stored there. After serial sampling is completed, CD-Well is centrifuged and scanned using a flatbed scanner to define the regions of plasma and blood cells. The length measured is converted to volume because the channels have a precise and uniform cross section. Then, CD-Well is exposed to an imaging plate to measure radioactivity. Finally, radioactivity concentrations are computed. We evaluated the performance of CD-Well in in vitro measurement and in vivo (18)F-fluorodeoxyglucose and [(11)C]2-carbomethoxy-3β-(4-fluorophenyl) tropane studies. In in vitro evaluation, per cent differences (mean±SE) from manual measurement were 4.4±3.6% for whole blood and 4.0±3.5% for plasma across the typical range of radioactivity measured in mouse dynamic study. In in vivo studies, reasonable TACs were obtained. The peaks were captured well, and the time courses coincided well with the TAC derived from PET imaging of the heart chamber. The total blood loss was less than 200 µL, which had no physiological effect on the mice. CD-Well demonstrates satisfactory performance, and is useful for mouse PET dynamic study.