Noninvasive Positron Emission Tomography Research Articles

Quantitative PET imaging and modeling of molecular blood-brain barrier permeability.

Blood-brain barrier (BBB) disruption is involved in the pathogenesis and progression of many neurological and systemic diseases. Non-invasive assessment of BBB permeability in humans has mainly been performed with dynamic contrast-enhanced magnetic resonance imaging, evaluating the BBB as a structural barrier. Here, we developed a novel non-invasive positron emission tomography (PET) method in humans to measure the BBB permeability of molecular radiotracers that cross the BBB through different transport mechanisms. Our method uses high-temporal resolution dynamic imaging and kinetic modeling to jointly estimate cerebral blood flow and tracer-specific BBB transport rate from a single dynamic PET scan and measure the molecular permeability-surface area (PS) product of the radiotracer. We show our method can resolve BBB PS across three PET radiotracers with greatly differing permeabilities, measure reductions in BBB PS of 18F-fluorodeoxyglucose (FDG) in healthy aging, and demonstrate a possible brain-body association between decreased FDG BBB PS in patients with metabolic dysfunction-associated steatotic liver inflammation. Our method opens new directions to efficiently study the molecular permeability of the human BBB in vivo using the large catalogue of available molecular PET tracers.

Translocator protein (18kDa) positron emission tomography imaging as a biomarker of neuroinflammation in epilepsy.

Increasing evidence indicate that neuroinflammation triggered by glial cells plays a significant role in epileptogenesis. To this effect, the overexpression of translocator protein 18kDa (TSPO) in activated microglia and astrocytes has been identified as an inflammatory biomarker in epilepsy. It is now possible to quantify neuroinflammation using non-invasive positron emission tomography (PET) imaging of TSPO. With the advancement of radiotracers, TSPO PET has become an innovative tool in elucidating the "neuroinflammatory machinery" of drug-resistant epilepsy. Furthermore, TSPO PET has demonstrated potential in detecting MRI-negative epileptogenic zones (EZ) and provided an innovative perspective in epileptic medical treatment. This manuscript presents a comprehensive exploration of the neuroinflammatory mechanisms of epilepsy, alongside a thorough review of TSPO PET studies conducted in clinical and preclinical settings. The primary objective is to deepen our understanding of epilepsy progression and to establish TSPO PET as an effective monitoring tool for treatment efficacy.

Abstract PO3-07-10: Advanced CD8 ImmunoPET imaging predicts radiation response in primary TNBC

Abstract Background: Tumor immunology and immunosuppression has been observed to drive changes in intratumoral response to cytotoxic and immunotherapy in triple negative breast cancer (TNBC). While radiation therapy is standard of care for TNBC, there is a lack of research into the relationship between long term treatment response and CD8 immune infiltration in the context of a radiation resistant model. Positron emission tomography (PET) imaging allows for noninvasive monitoring of the tumor microenvironment that can precede changes in tumor volume, including approaches that allow for immune imaging of CD8 T-cell trafficking. Noninvasive PET imaging of radiation response has the potential to identify windows of enhanced response to secondary therapy and can guide interventional therapy. The goal of this study is to understand how radiation can prime the tumor microenvironment to be combined with other therapeutics in TNBC through changes in CD8 immune infiltration with [89Zr]-CD8 ImmunoPET imaging. Methods: Syngeneic parental 4T1 (radiation sensitive) and a developed radiation-resistant 4T1 sub-cell line were orthotopically injected into BALB/c mice (N = 5 control, N = 9-10 radiation treated for each model). After reaching a tumor volume of ~100 mm3, mice began treatment with 2 Gy fractionated radiation daily from day 0-5. On day 6, mice were injected with ~50 µCi of [89Zr]-CD8 minibody (IAB42) and imaged 24 hours post injection. The mean standardized uptake value (SUV) was quantified and normalized to heart SUV to extract out a tumor:heart SUVratio that describe the CD8 infiltration within tumors. Following imaging, tumors were either excised immediately for immunofluorescence against CD8 or monitored for longitudinal changes in tumor volume. A non-parametric T-test was used to assess for significance between groups. Results: In radiation sensitive 4T1 tumors, [89Zr]-CD8 ImmunoPET imaging indicated a 23% increase in CD8 immune infiltration, in radiation-treated tumors compared to control tumors on day 7 (p = 0.02). These immune alterations occurred prior to any changes in tumor volume (p = 0.63). Longitudinally, radiation treated tumors exhibited significant changes in tumor volume beginning on day 20 (p = 0.02), which is sustained until study endpoint on day 41 (p < 0.01). In radiation resistant 4T1 tumors, no significant change was observed in short term CD8 immune infiltration (p = 0.43) or in longitudinal tumor volume (p = 0.31) in response to radiation therapy, when compared to untreated tumors. Conclusions: [89Zr]-CD8 ImmunoPET imaging reveals significant increases in CD8 immune infiltration that is predictive of eventual response to radiation therapy in radiation sensitive models of TNBC. [89Zr]-CD8 PET imaging of radiation therapy response has the potential to increase the efficacy of precision medicine and prime the tumor microenvironment for secondary therapeutics, thereby improving tumor kill and reducing patient toxicity. Citation Format: Patrick Song, Shannon Lynch, Chloe DeMellier, Alessandro Mascioni, Fang Jia, Anna Sorace. Advanced CD8 ImmunoPET imaging predicts radiation response in primary TNBC [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-07-10.

Synthesis and biological evaluation of a novel 68Ga-labeled GalNAc-PET probe for asialoglycoprotein receptor imaging

PurposeThe asialoglycoprotein receptor (ASGPR), expressed exclusively on mammalian hepatocyte membranes, recognizes ligands terminated with β-D-galactose (Gal) and N-acetylglucosamine (GalNAc). This study aims to develop a 68Ga-labeled monovalent GalNAc derivative that particularly interacts with ASGPR. It also aims to assess the potential utility of this derivative in visualizing ASGPR-related liver dysfunction using positron emission tomography (PET)/ computed tomography (CT) imaging. MethodsThe PET imaging probe 68Ga-NOTA-GalNAc was developed and characterized with regard to radiochemical purity, biocompatibility, biodistribution patterns, and specific ASGPR-targeting ability. Groups of normal mice, mice with acute liver injury (ALI) induced by CCl4, mice with early-stage liver fibrosis induced by CCl4, mice with nonalcoholic fatty liver disease (NAFLD), and orthotopic HepG2 hepatoma-bearing mice were imaged with 68Ga-NOTA-GalNAc PET to evaluate the potential of this technique in monitoring ASGPR-related liver dysfunction. Results68Ga-NOTA-GalNAc, a hydrophilic compound with high radiochemical purity (>99 %) and good liver targeting ability, particularly binds ASGPR on the surface of hepatocytes with moderate affinity (KD = 6.82 μM). Compared with control, ASGPR-related liver dysfunction groups, even the group of mice with ALI (P <0.0001), revealed sensitive distinctions in the liver uptake value of 68Ga-NOTA-GalNAc. The 68Ga-NOTA-GalNAc absorbed in the livers of mice with early-stage liver fibrosis and NAFLD group is significantly less than that absorbed in the livers of mice in the normal group (P <0.0001). Hepatoma can be visualized in orthotopic HepG2 hepatoma-bearing mice because of different radioactivity accumulated in the nontumor region and lesion region, corresponding to the ASGPR expression confirmed with immunohistochemical staining. Conclusion68Ga-NOTA-GalNAc has the potential to be a specific, noninvasive, and sensitive PET imaging agent for ASGPR-related liver dysfunction.

Radionuclide-Labeled Antisilencing Function 1a Inhibitory Peptides for Tumor Identification and Individualized Therapy.

Immune checkpoint blockade (ICB) therapy is promising to revolutionize cancer regimens, but the low response rate and the lack of a suitable patient stratification method have impeded universal profit to cancer patients. Noninvasive positron emission tomography (PET) imaging in the whole body, upon coupling with specific biomarkers closely related to the immune response, could provide spatiotemporal information to prescribe cancer therapy. Herein, we demonstrate that antisilencing function 1a (ASF1a) could serve as a biomarker target to delineate tumor immune microenvironments by immune PET (iPET). The iPET radiotracer (68Ga-AP1) is designed to target ASF1a in tumors and predict immune response, and the signal intensity predicts anti-PD-1 (αPD-1) therapy response in a negative correlation manner. The ICB-resistant tumors with a high level of ASF1a as revealed by iPET (ASF1aHigh-iPET) are prescribed to be treated by either the combined 177Lu-labeled AP1 and αPD-1 or the standalone α particle-emitting 225Ac-labeled AP1, both achieving enhanced therapeutic efficacy and prolonged survival time. Our study not only replenishes the iPET arsenal for immune-related response evaluation by designing a reliable biomarker and a facile radiotracer but also provides optional therapeutic strategies for ICB-resistant tumors with versatile radionuclide-labeled AP1 peptides, which is promising for real-time clinical diagnosis and individualized therapy planning simultaneously.

The diagnostic value of 68Ga-NOTA-MAL-Cys-MZHER2:342 PET/CT imaging for HER2-positive lung adenocarcinoma.

The human epidermal growth factor receptor 2 gene (HER2) has been identified as a potential therapeutic target in lung adenocarcinoma (LUAD). Non-invasive positron emission tomography (PET) imaging provides a reliable strategy for in vivo determination of HER2 expression through whole-body detection of abnormalities. The PET tracer 68Ga-NOTA-MAL-Cys-MZHER2:342 has shown promising results for HER2-positive breast and gastric cancers. This study aims to evaluate the performance of 68Ga-NOTA-MAL-Cys-MZHER2:342 in vitro and in vivo models and in clinical patients with HER2-positive LUAD. NOTA-MAL-Cys-MZHER2:342 was synthesized and labeled with 68Ga. Cell uptake, cell binding ability, and stability studies of 68Ga-NOTA-MAL-Cys-MZHER2:342 were assessed both in the Calu-3 lung cancer (LC) cell line and normal mice. In vivo assessment in tumor-bearing mice was conducted using microPET imaging and biodistribution experiments. Additionally, preliminary PET/CT imaging analysis was performed on HER2-positive LC patients. 68Ga-NOTA-MAL-Cys-MZHER2:342 was prepared with a radiochemical purity (RCP) exceeding 95%. The tracer demonstrated high cell uptake in HER2-overexpressing Calu-3 cells, with an IC50 of 158.9, an adequate 1.73 nM. Good stability was exhibited both in vitro and in vivo. MicroPET imaging of Calu-3-bearing mice revealed high tumor uptake and notable tumor-to-background ratios. Positive outcomes were also observed in two HER2-positive LUAD patients. 68Ga-NOTA-MAL-Cys-MZHER2:342 demonstrated satisfactory stability, sensitivity, and specificity. These findings suggest that 68Ga-NOTA-MAL-Cys-MZHER2:342 PET/CT imaging provides a novel tool for non-invasive visual assessment of HER2 expression in LUAD patients.

Ankylosing spondylitis PET imaging and quantifications via P2X7 receptor-targeting radioligand [18F]GSK1482160.

Ankylosing spondylitis (AS) is a chronic inflammatory disease of the axial spine; however, the quantitative detection of inflammation in AS remains a challenge in clinical settings. We aimed to investigate the feasibility of using a specific P2X7R-targeting 18F-labeled tracer [18F]GSK1482160 for positron emission tomography (PET) imaging and the quantification of AS. The radioligand [18F]GSK1482160 was obtained based on nucleophilic aliphatic substitution. Dynamic [18F]GSK1482160 and [18F]FDG micro-PET/CT imaging were performed on AS mice (n = 8) and age-matched controls (n = 8). Tracer kinetics modeling was performed using Logan's graphical arterial input function analysis to quantify the in vivo expression of P2X7R. The post-PET tissues were collected for hematoxylin-eosin(H&E), immunohistochemical (IHC), and immunofluorescence (IF) staining. [18F]GSK1482160 PET/CT imaging revealed that the specific binding in the ankle joint and sacroiliac joint (SIJ) of the AS at 8weeks group (BPNDankle-AS-8W (non-displaceable binding potential of the ankle) 3.931 ± 0.74; BPND SIJ-AS-8W (BPBD of the SIJ) 4.225 ± 0.84) were significantly higher than the controls at 8weeks group (BPNDankle-Ctr-8W 0.325 ± 0.15, BPNDSJJ-Ctr-8W 0.319 ± 0.17) respectively, and the AS at 14weeks group (BPNDankle-AS-14W 12.212 ± 2.25; BPNDSJJ-AS-14W 13.389 ± 3.60) were significantly higher than the controls at 14weeks group (BPNDankle-Ctr-14W 0.204 ± 0.16, BPNDSJJ-Ctr-14W 0.655 ± 0.35) respectively. The four groups had no significant difference in the [18F]FDG uptake of ankle and SIJ. IHC and IF staining revealed that the overexpression of P2X7R was colocalized with activated macrophages from the ankle synovium and spinal endplate in mice with AS, indicating that quantification of P2X7R may contribute to the understanding of the pathogenesis of inflammation in human AS. This study developed a novel P2X7R-targeting PET tracer [18F]GSK1482160 to detect the expression of P2X7R in AS mouse models and provided powerful non-invasive PET imaging and quantification for AS.

Automated Synthesis and Preclinical Evaluation of Optimized Integrin α6-Targeted Positron Emission Tomography Imaging of Pancreatic Cancer.

Integrin α6 has been considered a promising biomarker, is overexpressed in many tumors, and plays a vital role in tumor formation, recurrence, and metastasis. In this study, we identified a novel high-affinity integrin α6-targeted peptide named RD2 (Arg-Trp-Tyr-Asp-PEG4)2-Lys-Lys and developed a 18F-radiolabeled peptide tracer ([18F]-AlF-NOTA-RD2) and evaluated its potential application in positron emission tomography (PET) imaging of pancreatic cancer. [18F]-AlF-NOTA-RD2 was produced using GMP (Good Manufacturing Practice of Medical Products)-compliant automatic radiosynthesis on a single GE FASTLab2 cassette-type synthesis module. The stability of [18F]-AlF-NOTA-RD2 was analyzed in phosphate-buffered saline (PBS) and fetal bovine serum (FBS). The cell uptake assay of the tracer was assessed using PANC-1 cells. In addition, small-animal PET imaging and biodistribution studies of [18F]-AlF-NOTA-RD2 were performed in pancreatic cancer subcutaneous tumor-bearing mice. The PET tracer [18F]-AlF-NOTA-RD2 was obtained with a radiochemical yield of 23.7 ± 4.7%, radiochemical purity of >99%, and molar activity of 165.7 ± 59.1 GBq/μmol. [18F]-AlF-NOTA-RD2 exhibited good in vitro stability in PBS and FBS. LogP octanol water value for the tracer was -2.28 ± 0.05 (n = 3). The binding affinity of RD2 to the integrin α6 protein (Kd = 0.13 ± 3.65 μM, n = 3) was significantly higher than that of the RWY (CRWYDENAC) (Kd = 6.97 ± 1.44 μM, n = 3). Small-animal PET imaging and biodistribution also revealed that [18F]-AlF-NOTA-RD2 displayed rapid and good tumor uptake and lower liver background uptake in PANC-1 tumor-bearing mice. [18F]-AlF-NOTA-RD2 showed significant radioactivity accumulation in tumors and was successfully blocked by NOTA-RD2. Compared with [18F]-FDG, [18F]-AlF-NOTA-RD2 PET imaging and biodistribution studies in PANC-1 xenograft tumor-bearing mice confirmed a good tumor-to-muscle ratio (8.69 ± 2.03 vs 1.41 ± 0.23, respectively) at 0.5 h and (2.99 ± 3.02 vs 1.43 ± 0.17, respectively) at 1 h post injection. Autoradiography of human pancreatic cancer tumor tissues further confirmed high accumulation of [18F]-AlF-NOTA-RD2. In summary, we developed an optimized integrin α6-targeted imaging tracer and obtained high radioactivity products with a cassette-type synthesis module; moreover, the tracer exhibited good binding affinity with integrin α6 and good target specificity for PANC-1 cells in xenograft pancreatic tumor-bearing mice, demonstrating its promising application as a noninvasive PET radiotracer of integrin α6 expression in pancreatic cancer.

Feasibility of [18F]FSPG PET for Early Response Assessment to Combined Blockade of EGFR and Glutamine Metabolism in Wild-Type KRAS Colorectal Cancer.

Early response assessment is critical for personalizing cancer therapy. Emerging therapeutic regimens with encouraging results in the wild-type (WT) KRAS colorectal cancer (CRC) setting include inhibitors of epidermal growth factor receptor (EGFR) and glutaminolysis. Towards predicting clinical outcome, this preclinical study evaluated non-invasive positron emission tomography (PET) with (4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG) in treatment-sensitive and treatment-resistant WT KRAS CRC patient-derived xenografts (PDXs). Tumor-bearing mice were imaged with [18F]FSPG PET before and one week following the initiation of treatment with either EGFR-targeted monoclonal antibody (mAb) therapy, glutaminase inhibitor therapy, or the combination. Imaging was correlated with tumor volume and histology. In PDX that responded to therapy, [18F]FSPG PET was significantly decreased from baseline at 1-week post-therapy, prior to changes in tumor volume. In contrast, [18F]FSPG PET was not decreased in non-responding PDX. These data suggest that [18F]FSPG PET may serve as an early metric of response to EGFR and glutaminase inhibition in the WT KRAS CRC setting.

In Vivo Preclinical Assessment of the VEGF Targeting Potential of the Newly Synthesized [52Mn]Mn-DOTAGA-Bevacizumab Using Experimental Cervix Carcinoma Mouse Model.

Among humanized monoclonal antibodies, bevacizumab specifically binds to vascular endothelial growth factor A (VEGF-A). VEGF-A is an overexpressed biomarker in cervix carcinoma and is involved in the development and maintenance of tumor-associated neo-angiogenesis. The non-invasive positron emission tomography using radiolabeled target-specific antibodies (immuno-PET) provides the longitudinal and quantitative assessment of tumor target expression. Due to antibodies having a long-circulating time, radioactive metal ions (e.g., 52Mn) with longer half-lives are the best candidates for isotope conjugation. The aim of our preclinical study was to assess the biodistribution and tumor-targeting potential of 52Mn-labeled DOTAGA-bevacizumab. The VEGF-A targeting potential of the new immuno-PET ligand was assessed by using the VEGF-A expressing KB-3-1 (human cervix carcinoma) tumor-bearing CB17 SCID mouse model and in vivo PET/MRI imaging. Due to the high and specific accumulation found in the subcutaneously located experimental cervix carcinoma tumors, [52Mn]Mn-DOTAGA-bevacizumab is a promising PET probe for the detection of VEGF-A positive gynecological tumors, for patient selection, and monitoring the efficacy of therapies targeting angiogenesis.

Rectal delivery of 89Zr-labeled infliximab-loaded nanoparticles enables PET imaging-guided localized therapy of inflammatory bowel disease.

Inflammatory bowel diseases (IBDs) like Crohn's disease and ulcerative colitis involve chronic gastrointestinal inflammation. The pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-α) drives IBD pathogenesis. Anti-TNF-α therapies using monoclonal antibodies (mAbs) like infliximab (INF) help treat IBD but have limitations. We developed inflammation-targeting polyphenol-poloxamer nanoparticles loaded with the anti-inflammatory mAb INF (INF@PPNP) as a novel IBD therapy. Characterization showed that INF@PPNP had favorable stability and purity. Radiolabeling INF@PPNP with 89Zr enabled tracking localization with positron emission tomography (PET) imaging. Rectal administration of 89Zr-INF@PPNP led to colon delivery with remarkably reduced systemic exposure versus intravenous INF revealed by non-invasive PET imaging. 89Zr-INF@PPNP retention at inflamed foci indicated prolonged INF@PPNP action. INF@PPNP rectally achieved similar anti-inflammatory effects as intravenously injected INF, demonstrating the high therapeutic potential. Our findings support the use of nanoparticle-based rectal administration for localized drug delivery, prolonging drug activity and minimizing systemic exposure, ultimately offering an effective approach for treating IBD.

Noninvasive Quantitative PET Imaging in Humans of the Pancreatic Beta-Cell Mass Biomarkers VMAT2 and Dopamine D2/D3 Receptors In Vivo.

Noninvasive quantitative imaging of beta-cells can provide information on changes in cellular transporters, receptors, and signaling proteins that may affect function and/or loss of mass, both of which contribute to the loss of insulin secretion and glucose regulation of patients with type 1 or type 2 diabetes (T1D/T2D). We have developed and optimized the use of two positron emission tomography (PET) radioligands, [18F]FP-(+)-DTBZ and [11C](+)-PHNO, targeting beta-cell VMAT2 and dopamine (D2/D3) receptors, respectively. Here we describe our optimized methodology for the clinical use of these two tracers for quantitative PET imaging of beta-cell biomarkers in vivo. We also briefly discuss our previous results and their implications and value towards extending the use of PET radioligand beyond the original goal of quantitative imaging of beta-cell mass to the potential to provide insight into the biology of beta-cell loss of mass and/or function and to evaluate the efficacy of therapeutics to prevent or restore functional beta-cell mass.

PET Imaging of the Neurotensin Targeting Peptide NOTA-NT-20.3 Using Cobalt-55, Copper-64 and Gallium-68.

Neurotensin receptor 1 (NTSR1) is an emerging target for imaging and therapy of many types of cancer. Nuclear imaging of NTSR1 allows for noninvasive assessment of the receptor levels of NTSR1 on the primary tumor, as well as potential metastases. This work focuses on a the neurotensin peptide analogue NT-20.3 conjugated to the chelator NOTA for radiolabeling for use in noninvasive positron emission tomography (PET). NOTA-NT-20.3 was radiolabeled with gallium-68, copper-64, and cobalt-55 to determine the effect that modification of the radiometal has on imaging and potential therapeutic properties of NOTA-NT-20.3. In vitro assays investigating cell uptake and subcellular localization of the radiolabeled peptides were performed using human colorectal adenocarcinoma HT29 cells. In vivo PET/CT imaging was used to determine the distribution and clearance of the peptide in mice bearing NTSR1 expressing HT29 tumors. Cell uptake studies showed that the highest uptake was obtained with [55Co] Co-NOTA-NT-20.3 (18.70 ± 1.30%ID/mg), followed by [64Cu] Cu-NOTA-NT-20.3 (15.46 ± 0.91%ID/mg), and lastly [68Ga] Ga-NOTA-NT-20.3 (10.94 ± 0.46%ID/mg) (p &lt; 0.001). Subcellular distribution was similar across the three constructs, with the membranous fraction containing the highest amount of radioactivity. In vivo PET/CT imaging of the three constructs revealed similar distribution and tumor uptake at the 1 h imaging timepoint. Tumor uptake was receptor-specific and blockable by co-injection of non-radiolabeled NOTA-NT-20.3. SUV ratios of tumor to heart at the 24 h imaging timepoint show that [55Co] Co-NOTA-NT-20.3 (20.28 ± 3.04) outperformed [64Cu] Cu-NOTA-NT-20.3 (6.52 ± 1.97). In conclusion, our studies show that enhanced cell uptake and increasing tumor to blood ratios over time displayed the superiority of [55Co] Co-NOTA-NT-20.3 over [68Ga] Ga-NOTA-NT-20.3 and [64Cu] Cu-NOTA-NT-20.3 for the targeting of NTSR1.

Poly (ADP-ribose) polymerases as PET imaging targets for central nervous system diseases.

Poly (ADP-ribose) polymerases (PARPs) constitute of 17 members that are associated with divergent cellular processes and play a crucial role in DNA repair, chromatin organization, genome integrity, apoptosis, and inflammation. Multiple lines of evidence have shown that activated PARP1 is associated with intense DNA damage and irritating inflammatory responses, which are in turn related to etiologies of various neurological disorders. PARP1/2 as plausible therapeutic targets have attracted considerable interests, and multitudes of PARP1/2 inhibitors have emerged for treating cancer, metabolic, inflammatory, and neurological disorders. Furthermore, PARP1/2 as imaging targets have been shown to detect, delineate, and predict therapeutic responses in many diseases by locating and quantifying the expression levels of PARP1/2. PARP1/2-directed noninvasive positron emission tomography (PET) has potential in diagnosing and prognosing neurological diseases. However, quantitative PARP PET imaging in the central nervous system (CNS) has evaded us due to the challenges of developing blood-brain barrier (BBB) penetrable PARP radioligands. Here, we review PARP1/2's relevance in CNS diseases, summarize the recent progress on PARP PET and discuss the possibilities of developing novel PARP radiotracers for CNS diseases.

Positron Emission Tomography Probes for Imaging Cytotoxic Immune Cells.

Non-invasive positron emission tomography (PET) imaging of immune cells is a powerful approach for monitoring the dynamics of immune cells in response to immunotherapy. Despite the clinical success of many immunotherapeutic agents, their clinical efficacy is limited to a subgroup of patients. Conventional imaging, as well as analysis of tissue biopsies and blood samples do not reflect the complex interaction between tumour and immune cells. Consequently, PET probes are being developed to capture the dynamics of such interactions, which may improve patient stratification and treatment evaluation. The clinical efficacy of cancer immunotherapy relies on both the infiltration and function of cytotoxic immune cells at the tumour site. Thus, various immune biomarkers have been investigated as potential targets for PET imaging of immune response. Herein, we provide an overview of the most recent developments in PET imaging of immune response, including the radiosynthesis approaches employed in their development.

A novel 18F-labeled agonist for PET imaging of stimulator of interferon gene expression in tumor-bearing mice.

Stimulator of interferon genes (STING) protein plays a vital role in the immune surveillance of tumor microenvironment. Monitoring STING expression in tumors benefits the relevant STING therapy. This study aimed to develop a novel 18F-labeled agonist, dimeric amidobenzimidazole (diABZI), and firstly evaluate the feasibility of noninvasive positron emission tomography (PET) imaging of STING expression in the tumor microenvironment. An analog of the STING agonist NOTA-DABI was synthesized and labeled with 18F via Al18F-NOTA complexation (denoted as [18F]F-DABI). Physicochemical properties, STING protein-binding affinity, and specificity of [18F]F-DABI were evaluated using cell uptake and docking assays. In vivo small-animal PET imaging and biodistribution studies of [18F]F-DABI in tumor-bearing mice were performed to verify the pharmacokinetics and tumor targeting ability. The correlation between tumor uptake and STING expression was also analyzed. [18F]F-DABI was produced conveniently with high radiochemical yield (44 ± 15%), radiochemical purity (> 97%) and molar activity (15-30GBq/μmol). In vitro binding assays demonstrated that [18F]F-DABI has a favorable affinity and specificity for STING with a KD of 12.98 ± 2.07nM. In vivo studies demonstrated the specificity of [18F]F-DABI for PET imaging of STING expression with B16F10 tumor uptake of 10.93 ± 0.93%ID/g, which was significantly different from that of blocking groups (3.13 ± 0.88%ID/g, ***p < 0.0001). Furthermore, tumor uptake of [18F]F-DABI was well positively correlated with STING expression in different tumor types. Biodistribution results demonstrated that [18F]F-DABI was predominately uptaken in the liver and intestines, indicating its hepatobiliary elimination. This proof-of-concept study demonstrated a STING-binding radioligand for PET imaging, which could be used as a potential companion diagnostic tool for related STING-agonist therapies.

Scan-rescan measurement repeatability of 18F-FDG PET/MR imaging of vascular inflammation

Non-invasive positron emission tomography (PET) of vascular inflammation and atherosclerotic plaque by identifying increased uptake of 18F-fluordeoxyglucose (18F-FDG) is a powerful tool for monitoring disease activity, progression, and its response to therapy. 18F-FDG PET/computed tomography (PET/CT) of the aorta and carotid arteries has become widely used to assess changes in inflammation in clinical trials. However, the recent advent of hybrid PET/magnetic resonance (PET/MR) scanners has advantages for vascular imaging due to the reduction in radiation exposure and improved soft tissue contrast of MR compared to CT. Important for research and clinical use is an understanding of the scan-rescan repeatability of the PET measurement. While this has been studied for PET/CT, no data is currently available for vascular PET/MR imaging. In this study, we determined the scan-rescan measurement repeatability of 18F-FDG PET/MR in the aorta and carotid arteries was less than 5%, comparable to similar findings for 18F-FDG PET/CT.

PET Imaging of Active Invasive Fungal Infections with d-[5-11C]-Glutamine.

The increasing prevalence and severity of invasive fungal infections (IFIs), especially in immunocompromised populations, has amplified the need for rapid diagnosis of fungal pathogens. Radiotracers derived from d-amino acids (DAAs) show promise as bacterial-specific positron emission tomography (PET) imaging agents due to their preferential consumption by bacteria and largely nonutilization by hosts. Unlike mammals, fungi can utilize external DAAs including d-glutamine for their growth by rapidly upregulating DAA oxidases. Additionally, glutamine is essential for fungal nitrogen assimilation, survival, and virulence. We previously validated d-[5-11C]-glutamine (d-[5-11C]-Gln) as an efficient radiotracer targeting live bacterial soft-tissue infections. Here, we further expanded this investigation to evaluate its translational potential for PET imaging of IFIs in immunocompetent mouse models subcutaneously (SubQ) and intramuscularly (IM) infected with Candida albicans (C. albicans), using its l-isomer counterpart (l-[5-11C]-Gln) as a control. Comparative studies between pathogens showed significantly (p < 0.05) higher uptake in fungi (C. albicans and C. tropicalis) versus tested bacterial species for d-[5-11C]-Gln, suggesting that it could potentially serve as a more sensitive radiotracer for detection of fungal infections. Additionally, comparative PET imaging studies in immunocompetent infected mice demonstrated significantly higher infection-to-background ratios for d- versus l-[5-11C]-Gln in both SubQ (ratio = 1.97, p = 0.043) and IM (ratio = 1.97, p = 0.028) infections. Fungal infection imaging specificity was confirmed with no significant difference observed between localized inflammation sites versus untreated muscle background (heat-killed injection site/untreated muscle: ∼1.1). Taken together, this work demonstrates the translational potential of d-[5-11C]-Gln for noninvasive PET imaging of IFIs.

Image-derived Input Function in brain [18F]FDG PET data: which alternatives to the carotid siphons?

Quantification of brain [18F] fluorodeoxyglucose ([18F]FDG) positron emission tomography (PET) data requires an input function. A noninvasive alternative to gold-standard arterial sampling is the image-derived input function (IDIF), typically extracted from the internal carotid arteries (ICAs), which are however difficult to segment and subjected to spillover effects. In this work, we evaluated the feasibility of extracting the IDIF from two different vascular sites, i.e., 1) common carotids (CCA) and 2) superior sagittal sinus (SSS), other than 3) ICA in a large group of glioma patients undergoing a dynamic [18F]FDG PET acquisition on a hybrid PET/MR scanner. Comparisons are drawn between the different IDIFs in terms of peak amplitude and shape, as well as between the estimates of fractional uptake rate (Kr) obtained from the different extraction sites in terms of a) grey/white matter average absolute values, b) ratio of grey-to-white matter, and c) spatial patterns for the hemisphere contralateral to the lesion. Clinical Relevance - This work points towards new feasible IDIF extraction sites (CCA in particular) which could allow for fully noninvasive absolute PET quantification in clinical populations.

Immuno-PET imaging of PD-L1 expression in patient-derived lung cancer xenografts with [68Ga]Ga-NOTA-Nb109.

Accurate evaluation of programmed death-ligand 1 (PD-L1) expression levels in cancer patients may be useful in the identification of potential candidates for anti-programmed death-1/PD-L1 (anti-PD-1/PD-L1) immune checkpoint therapy to improve the response rate of immune checkpoint blockade therapy. This study evaluated the feasibility of the nanobody-based positron emission tomography (PET) tracer [68Ga]Ga-NOTA-Nb109 for immuno-PET imaging of PD-L1 in lung cancer patient-derived xenograft (PDX). We constructed 2 PDXs of lung adenocarcinoma (ADC) and lung squamous cell carcinoma (SCC) and used them for immuno-PET imaging. A 2-hour dynamic PET scanning was performed on the samples and the in vivo biodistribution and metabolism of [68Ga]Ga-NOTA-Nb109 were investigated using region of interest (ROI) analysis. The ex vivo biodistribution of [68Ga]Ga-NOTA-Nb109 in the 2 PDXs was investigated by static PET scanning. In addition, tumor PD-L1 expression in the 2 PDXs was evaluated by autoradiography, western blot, and immunohistochemical (IHC) analysis. Noninvasive PET imaging showed that [68Ga]Ga-NOTA-Nb109 can accurately and sensitively assess the PD-L1 expression in non-small cell lung cancer (NSCLC) PDX models. The maximum [68Ga]Ga-NOTA-Nb109 uptake by the ADC PDX LU6424 and the SCC PDX LU6437 were 3.13%±0.35% and 2.60%±0.32% injected dose per milliliter of tissue volume (ID/mL), respectively, at 20 min post injection. In vivo and ex vivo biodistribution analysis showed that [68Ga]Ga-NOTA-Nb109 was rapidly cleared through renal excretion and an enhanced signal-to-noise ratio (SNR) was achieved. Ex vivo PD-L1 expression analysis showed good agreement with in vivo PET imaging results. This study demonstrated that [68Ga]Ga-NOTA-Nb109 could be applied with PET imaging to noninvasively and accurately monitor PD-L1 expression in vivo for screening patients who may be responsive to immunotherapy and to guide the development of appropriate treatment strategies for such patients.