FLT Uptake Research Articles

IMMU-19. PRECLINICAL THERAPEUTIC ACTIVITY AND PET IMAGING OF STING AGONIST 8803 IN GLIOBLASTOMA

Abstract STING (stimulator of interferon genes) activation triggers interferon (IFN) release within the tumor microenvironment from myeloid cells and other cell types, inducing proinflammatory anti-tumor immune responses. The small molecule STING agonist, 8803, elicits long-term survival in 56% (QPP4; p=0.0003) to 100% (QPP8; p<0.0001) of mice with orthotopically implanted immune checkpoint-resistant glioblastoma models after 2-3 doses. In humanized mice, in which glioblastoma-cell STING is epigenetically silenced, 8803 therapeutic activity is maintained. The combination therapy of 8803 with anti-PD-1 relatively further enhances survival in immune checkpoint-resistant models. Ex vivo flow cytometry and proteomic sequential multiplex profiling during the therapeutic window demonstrate global immunological reprogramming, specific tumor-antigen immune responses, increased tumor immune effector trafficking, and release of IFN. It has been shown that IFN signaling augments intracellular nucleotide metabolism and increases [18F]-FLT uptake in pancreatic cancer. We evaluated the longitudinal [18F]-FLT uptake in CT-2A-bearing mice using MRI/PET/CT at days 0, 2, and 4 after intratumoral 8803 delivery. Brain images were acquired before and after [18F]-FLT tracer and 8803 administrations. The CT was used for PET attenuation/correction, and the MRI was used for tumor segmentation. Regional analysis of PET-derived [18F]-FLT SUV maps used advanced image processing approaches as a function of time to enhance the voxel-by-voxel interpretation of the acquired images. [18F]-FLT uptake increased from baseline in all mice treated with 8803 compared to control mice with a peak at 48-96 hours. Concordant sequential multiplex immunofluorescence demonstrated that the STING expression was prominent along the tumor vasculature, myeloid, and tumor cells. p-IRF-3 expression, reflective of STING pathway activation, showed a diffuse pattern after treatment. Based on the preclinical data, PET imaging at 48-96 hours post-treatment represents the appropriate imaging modality and timepoint to maximize the ability to detect STING-dependent changes for a therapeutic strategy that reprograms the glioblastoma microenvironment and improves survival across multiple preclinical models of glioblastoma.

Dose-Response Relationships Between Radiation Exposure, Bone Marrow Function as Measured by 18F-Fluorothymidine Positron Emission Tomography, and Lymphocyte Counts During Chemoradiation for Non-Small Cell Lung Cancer

Purpose18F-Fluorothymidine(FLT)-PET enables sensitive imaging of bone marrow (BM) proliferation. Sequential FLT-PET/CT scans before and during chemoradiation (CRT) for non-small cell lung cancer (NSCLC) were repurposed to investigate dose-response effects of radiation on BM proliferation. Methods and materialsTwenty-six NSCLC patients underwent platinum-based CRT to 60Gy in 30 fractions with FLT-PET/CT scans at baseline, week 2 (20Gy) and week 4 (40Gy). FLT uptake in BM was isolated using Medical Image Merge software. Week 2 and week 4 FLT-PET BM scans were fused with contemporaneous radiation isodose distributions. Relationships between radiation dose and FLT BM uptake (SUVmax and visual parameters) were analyzed using generalized-linear and restricted cubic-spline models. Percentage volumes of total BM without appreciable FLT uptake (“ablated”) on week 2 and week 4 FLT-PET scans were calculated by comparisons with baseline scans. ResultsThoracic FLT uptake was ablated in BM regions exposed to cumulative radiation doses ≥3Gy by week 2. In all cases BM FLT SUVmax declined rapidly as radiation dose increased. BM proliferation significantly decreased by more than 95% after ≥3-4Gy at 2 weeks and ≥4-5Gy at 4 weeks. The ablated BM volume increased from week 2 to week 4 as BM in the penumbra accumulated radiation dose. Median percentage of total BM ablated was 13.1% (range 5.6-20.3) at 2 weeks and 15.7% (range 9.2 - 24.1) at 4 weeks. Mean lymphocyte counts fell from baseline of 2.01×109/L to 0.77 week 2, and 0.60 week 4. Lymphocyte decline strongly correlated with percentage of total BM ablated by week 4 (y=-46 -1.64 x, R2adj = 0.34, p=0.001). ConclusionsBM ablation associated with low dose-radiation exposure during CRT correlated significantly with lower week 4 lymphocyte counts. BM is a potential organ-at-risk and reducing the BM volume exposed to ≥3Gy may help preserve lymphocytes essential for effective adjuvant immunotherapy.

The relationship between gadolinium enhancement and [18 F]fluorothymidine uptake in brain lesions with the use of hybrid PET/MRI

BackgroundTo evaluate and compare the diagnostic power of [18F]FLT-PET with ceMRI in patients with brain tumours or other focal lesions.Methods121 patients with suspected brain tumour or those after brain tumour surgery were enroled in this retrospective study (61 females, 60 males, mean age 37.3 years, range 1–80 years). All patients underwent [18F]FLT-PET/MRI with gadolinium contrast agent application. In 118 of these patients, a final diagnosis was made, verified by histopathology or by follow-up. Agreement between ceMRI and [18F]FLT-PET of the whole study group was established. Further, sensitivity and specificity of ceMRI and [18F]FLT-PET were calculated for differentiation of high-grade vs. low-grade tumours, high-grade vs. low-grade tumours together with non-tumour lesions and for differentiation of high-grade tumours from all other verified lesions.Results[18F]FLT-PET and ceMRI findings were concordant in 119 cases (98%). On closer analysis of a subset of 64 patients with verified gliomas, the sensitivity and specificity of both PET and ceMRI were identical (90% and 84%, respectively) for differentiating low-grade from high-grade tumours, if the contrast enhancement and [18F]FLT uptake were considered as hallmarks of high-grade tumour. For differentiation of high-grade tumours from low-grade tumours and lesions of nontumorous aetiology (e.g., inflammatory lesions or post-therapeutic changes) in a subgroup of 93 patients by visual evaluation, the sensitivity of both PET and ceMRI was 90%, whereas the specificity of PET was slightly higher (61%) compared to ceMRI (57%). By receiver operating characteristic analysis, the sensitivity and specificity were 82% and 74%, respectively, when the threshold of SUVmax in the tumour was set to 0.9 g/ml.ConclusionWe demonstrated a generally very high correlation of [18F]FLT accumulation with contrast enhancement visible on ceMRI and a comparable diagnostic yield in both modalities for differentiating high-grade tumours from low-grade tumours and lesions of other aetiology.

Emerging Role of [18F]FLT PET/CT in Lymphoid Malignancies: A Review of Clinical Results.

Fluorine-18 fluorodeoxyglucose ([18F]FDG) is nowadays the leading positron emission tomography (PET) tracer for routine clinical work-ups in hematological malignancies; however, it is limited by false positive findings. Notably, false positives can occur in inflammatory and infective cases or in necrotic tumors that are infiltrated by macrophages and other inflammatory cells. In this context, 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) has been shown to be a promising imaging biomarker of hematological malignant cell proliferation. In this review, a total of 15 papers were reviewed to collect literature data regarding the clinical application of [18F]FLT PET/CT in hematological malignancies. This imaging modality seems to be a suitable tool for noninvasive assessment of tumor grading, also showing a correlation with Ki-67 immunostaining. Moreover, [18F]FLT PET/CT demonstrated high sensitivity in detecting aggressive lymphoma lesions, especially when applying a standardized uptake value (SUV) cutoff of 3. At baseline, the potential of [18F]FLT imaging as a predictive tool is demonstrated by the low tracer uptake in patients with a complete response. However, its use is limited in evaluating bone diseases due to its high physiological uptake in bone marrow. Interim [18F]FLT PET/CT (iFLT) has the potential to identify high-risk patients with greater precision than [18F]FDG PET/CT, optimizing risk-adapted therapy strategies. Moreover, [18F]FLT uptake showed a greater ability to differentiate tumor from inflammation compared to [18F]FDG, allowing the reduction of false-positive findings and making the first one a more selective tracer. Finally, FLT emerges as a superior independent predictor of PFS and OS compared to FDG and ensures a reliable early response assessment with greater accuracy and predictive value.

Exploring the applicability of a lesion segmentation method on [18F]fluorothymidine PET/CT images in diffuse large B-cell lymphoma

Background and purposeThe determination of the total metabolic tumour volume based on [18F]fluorothymidine ([18F]FLT) PET/CT images in diffuse large B-cell lymphoma has a potential clinical value for detecting early relapse in this type of heterogeneous lymphoproliferative tumours. Tumour segmentation is a key step in this process. For this purpose, our objective was to determine a segmentation threshold of [18F]FLT PET/CT images, based on a reference tissue uptake, on a cohort of patients with diffuse large B-cell lymphoma (DLBCL) that have been scanned at different stages of the treatment.MethodsWe enrolled 23 adult patients with DLBCL confirmed in II-IV stages without nervous system compromise. All patients were scanned using [18F]FLT PET/CT at the time of diagnosis (baseline PET), interim PET (iPET), and at the end of treatment (fPET). The administered activity was 1.8–2.6 MBq/kg body weight, performed 60–70 min after injection and without use of contrast-enhanced CT. First, we assessed the [18F]FLT uptake stability in liver and bone marrow along the patient follow-up. For the lesion segmentation, three threshold values were assessed.ResultsBoth, liver, and bone marrow can be indistinctly taken as reference tissue. The SUV threshold for a voxel to be considered as belonging to a lesion is expressed in terms of a percentage relative to the patient’s uptake in the reference tissue. Found thresholds were: for liver, 62%, 33%, 27%; and for bone marrow, 35%, 21% and 22%, for baseline, iPET and fPET stages, respectively. The relative threshold throughout the treatment has a decreasing tendency along the stages.ConclusionBased on the results obtained with [18F]FLT PET/CT during staging and follow-up in patients with DLBCL, reference values were obtained for each stage referring to liver and bone marrow uptake that could be used in clinical practice oncology.

Use of [18F]FLT/PET for assessing the tumor evolution and monitoring the antitumor activity of rosmarinic acid in a mouse 4T1 breast tumor model

The use of the tracer 18F-fluoro-3'-deoxy-3'-L-fluorothymidine ([18F]FLT) in positron emission tomography (PET) has been shown to be an effective tool for assessing tumor aggressiveness and early response to therapy. In this study, we investigated the applicability of [18F]FLT/PET to study the antitumor and anti–lung metastatic effects of rosmarinic acid (RA) in highly invasive breast cancer. 4T1 mammary carcinoma cells were injected into the flank of female Balb/c mice. The animals were treated daily with RA until day 21 after the inoculation of tumor cells. [18F]FLT/PET imaging was used to evaluate the response of primary tumors and lung metastases to RA treatment. PET Images showed a decreased [18F]FLT uptake in the lungs of mice after RA treatment. The antitumor effect of RA appears to be related to the inhibition of cell migration, cell proliferation, and blood vessel formation in the primary tumor. Furthermore, the inflammatory response was modulated by RA, which reduced the accumulation of mast cells and neutrophils in the primary tumor and of macrophages in the lungs. In conclusion, [18F]FLT/PET demonstrates the antitumor and antimetastatic effects of RA in a 4T1 breast tumor model. Furthermore, the findings suggest that RA modulates tumor angiogenesis and inflammation, resulting in antitumor and antimetastatic effects in a 4T1 breast carcinoma model.

Combination Therapy with Trastuzumab and Niraparib: Quantifying Early Proliferative Alterations in HER2+ Breast Cancer Models.

HER2-targeted treatments have improved survival rates in HER2+ breast cancer patients, yet poor responsiveness remains a major clinical obstacle. Recently, HER2+ breast cancer cells, both resistant and responsive to HER2-targeted therapies, have demonstrated sensitivity to poly-(ADP-ribose) polymerase (PARP) inhibition, independent of DNA repair deficiencies. This study seeks to describe biological factors that precede cell viability changes in response to the combination of trastuzumab and PARP inhibition. Treatment response was evaluated in HER2+ and HER2- breast cancer cells. Further, we evaluated the utility of 3'-Deoxy-3'-[18F]-fluorothymidine positron emission tomography ([18F]FLT-PET) imaging for early response assessment in a HER2+ patient derived xenograft (PDX) model of breast cancer. In vitro, we observed decreased cell viability. In vivo, we observed decreased inhibition in tumor growth in combination therapies, compared to vehicle and monotherapy-treated cohorts. Early assessment of cellular proliferation corresponds to endpoint cell viability. Standard summary statistics of [18F]FLT uptake from PET were insensitive to early proliferative changes. Meanwhile, histogram analysis of [18F]FLT uptake indicated the potential translatability of imaging proliferation biomarkers. This study highlights the potential of combined trastuzumab and PARP inhibition in HER2+ breast cancer, while demonstrating a need for optimization of [18F]FLT-PET quantification in heterogeneous models of HER2+ breast cancer.

Visualization research on ENT1/NIS dual-function gene therapy to reverse drug resistance mediated by MUC1 in GEM-resistant pancreatic cancer

PurposeTo use bifunctional target genes to increase the intracellular transport of gemcitabine (GEM) to reverse chemotherapy resistance and to simultaneously use reporter gene imaging to localize therapeutic genes. The therapeutic effect was evaluated by [18F]FLT PET/CT to visualize the effect of gene therapy. MethodsA viral gene vector containing the pancreatic cancer-targeting promoter MUC1 for specific transcription of equilibrative nucleoside transporter 1 (ENT1) and NIS (nuclide transport channel) was employed. [125I]NaI uptake tests and [131I]NaI SPECT imaging were performed to verify the function of NIS and the target function of MUC1. The correlation between [18F]FLT uptake and GEM resistance were assessed, and the influence ENT1 and thymidine kinase 1 (TK1) expression on [18F]FLT micro-PET/CT was measured, which provides a theoretical basis for the use of [18F]FLT micro-PET/CT to evaluate the efficacy of gene therapy. ResultsFirst, functions of gene therapy were confirmed: ENT1 reversed the drug resistance of GEM-resistant pancreatic cancer cells by increasing GEM intracellular transport; MUC1 drove NIS target gene expression in pancreatic cancer; and therapeutic genes could be localized using [131I]NaI SPECT reporter gene imaging. Second, the [18F]FLT uptake ratio was affected by drug resistance and GEM treatment. The mechanism underlying this effect was related to ENT1 and TK1. Increased expression of ENT1 inhibited the expression of TK1 after GEM chemotherapy to reduce the uptake of [18F]FLT. Finally, micro-PET/CT indicated that the SUVmax of [18F]FLT could predict survival time. SUVmax exhibited an increasing trend in resistant pancreatic cancer but a trend of inhibition after upregulation of ENT1, which was more significant after GEM treatment. ConclusionsBifunctional targeted genes can localize therapeutic genes through reporter gene imaging, reverse the drug resistance of GEM-resistant pancreatic cancer and be visually evaluated through [18F]FLT micro-PET/CT.

3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) Positron Emission Tomography as an In Vivo Biomarker of inhibition of CDK 4/6-Rb pathway by Palbociclib in a patient derived bladder tumor

BackgroundSeveral new generation CDK4/6 inhibitors have been developed and approved for breast cancer therapy in combination with endocrine therapeutics. Application of these inhibitors either alone or in combination in other solid tumors has been proposed, but no imaging biomarkers of response have been reported in non-breast cancer animal models. The purpose of this study was to evaluate 3'-[18F]fluoro-3'-deoxythymidine ([18F]FLT) Positron Emission Tomography (PET) as in vivo biomarker of response to palbociclib in a non-breast cancer model.MethodsTwenty-four NSG mice bearing patient derived xenografts (PDX) of a well-characterized bladder tumor were randomized into 4 treatment groups: vehicle (n = 6); palbociclib (n = 6); temozolomide (n = 6); and palbociclib plus temozolomide (n = 6) and treated with two cycles of therapy or vehicle. Tumor uptake of [18F]FLT was determined by micro-PET/CT at baseline, 3 days, and 9 days post initiation of therapy. Following the second cycle of therapy, the mice were maintained until their tumors reached a size requiring humane termination.Results[18F]FLT uptake decreased significantly in the palbociclib and combination arms (p = 0.0423 and 0.0106 respectively at day 3 and 0.0012 and 0.0031 at day 9) with stable tumor volume. In the temozolomide arm [18F]FLT uptake increased with day 9 uptake significantly different than baseline (p = 0.0418) and progressive tumor growth was observed during the treatment phase. All groups exhibited progressive disease after day 22, 10 days following cessation of therapy.ConclusionSignificant decreases in [18F]FLT uptake as early as three days post initiation of therapy with palbociclib, alone or in combination with temozolomide, in this bladder cancer model correlates with an absence of tumor growth during therapy that persists until day 18 for the palbociclib group and day 22 for the combination group (6 days and 10 days) following cessation of therapy. These results support early modulation of [18F]FLT as an in vivo biomarker predictive of palbociclib therapy response in a non-breast cancer model.

PD1 blockade alters cell-cycle distribution and affects 3'-deoxy-3'-[18F]fluorothymidine uptake in a mouse CT26 tumor model.

We previously reported that alterations of the tumor microenvironment (TME) by programmed death receptor-1 (PD1) blockade affected tumor glucose metabolism and tumor 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) uptake. In cancer cells, high glycolysis allows cells to sustain rapid proliferation since glycolysis is closely related to the proliferation of cancer cells. Therefore, imaging of cellular proliferation may provide more detail of TME alterations. In this study, we investigated how TME alterations by PD1 blockade affects the uptake of 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT), which is a 18F-radiolabeled thymidine derivative and is taken up by proliferating cells. Mice inoculated with murine colon carcinoma CT26 cells were intraperitoneally administered an anti-PD1 antibody on Day 0, when the tumor volume exceeded 50 mm3, and Day 5. [18F]FLT-PET imaging was performed pre-treatment (Day 0) and post treatment (Day 7). Tumor infiltrating lymphocytes (TILs) were identified by flow cytometry. [18F]FLT accumulation and localization in tumor tissue was evaluated by autoradiography and immunohistochemistry. The cell-cycle distribution of tumors and CT26 cells exposed to cytokines (interleukin-2, interferon [INF]-γ, and tumor necrosis factor [TNF]-α) was analyzed by flow cytometry. PD1 blockade increased CD8+ and CD4+ T cells in tumor tissue and significantly suppressed tumor proliferation; however, tumor [18F]FLT uptake remained unchanged. Autoradiography and immunohistochemistry showed that [18F]FLT was mainly taken up by cancer cells, but not TILs. Flow cytometric analysis demonstrated that the population of cells in G2/M phase increased after PD1 blockade. Moreover, INF-γ and TNF-α significantly increased cells in G2/M phase in vitro. PD1 blockade-induced alteration of the TME increased CT26 tumor cells in the G2/M phase, which have high thymidine kinase 1 activity. Therefore, [18F]FLT is taken up by tumor cells even if tumor proliferation is suppressed. This observation may be useful for evaluating the response to immunotherapy.

18F] Fluorothymidine Positron Emission Tomography Imaging in Primary Brain Tumours: A Systematic Review.

This review aimed to summarize the available literature on the clinical application of [18F] FLT PET imaging in primary brain tumours. A comprehensive search strategy based on Pubmed/Medline, Scopus, Web of Science, Cochrane Library, Google Scholar, and the Embase databases was carried on using the following search string: ('3` Fluorothymidine'/exp OR 'FLT' OR '[81F]-FLT' OR '[18F] Fluorothymidine') AND ('pet'/exp OR 'pet' OR 'positron emission tomography') AND ('glioma'/exp OR 'glioma' OR 'brain tumour'/exp OR 'brain tumour'). The search was updated till March 2021 and only articles in English and studies investigating the clinical applications of [18F] FLT PET and PET/CT in primary brain tumours were considered eligible for inclusion. The literature search ultimately yielded 52 studies included in the systematic review, with main results as follows: a) the uptake of [18F] FLT may guide stereotactic biopsy but does not discriminate between grade II and III glioma. b) [18F] FLT uptake and texture parameters correlate with overall survival (OS) in newly diagnosed gliomas. c) In patients with recurrent glioma, proliferative volume (PV) and tumour-to-normal brain (T/N) uptake ratio are independent predictors of survival. d) Patients demonstrating response to therapy at [18F] FLT PET scan show longer OS compared to non-responders. e) [18F] FLT PET demonstrated good performance in discriminating tumour recurrence from radionecrosis. However, controversial results exist in comparative literature examining the performance of [18F] FLT vs. other radiotracers in the assessment of recurrence. [18F] FLT PET imaging has demonstrated potential benefits for grading, diagnostic and prognostic purposes, despite the small sample size studies due to the relatively low availability of the radiotracer.

Quantification of uptake in pelvis F-18 FLT PET-CT images using a 3D localization and segmentation CNN.

The purpose of this work was to develop and validate a deep convolutional neural network (CNN) approach for the automated pelvis segmentation in computed tomography (CT) scans to enable the quantification of active pelvic bone marrow by means of Fluorothymidine F-18 (FLT) tracer uptake measurement in positron emission tomography (PET) scans. This quantification is a critical step in calculating bone marrow dose for radiopharmaceutical therapy clinical applications as well as external beam radiationdoses. An approach for the combined localization and segmentation of the pelvis in CT volumes of varying sizes, ranging from full-body to pelvis CT scans, was developed that utilizes a novel CNN architecture in combination with a random sampling strategy. The method was validated on 34 planning CT scans and 106 full-body FLT PET-CT scans using a cross-validation strategy. Specifically, two different training and CNN application options were studied, quantitatively assessed, and statisticallycompared. The proposed method was able to successfully locate and segment the pelvis in all test cases. On all data sets, an average Dice coefficient of 0.9396 0.0182 or better was achieved. The relative tracer uptake measurement error ranged between 0.065% and 0.204%. The proposed approach is time-efficient and shows a reduction in runtime of up to 95% compared to a standard U-Net-based approach without a localizationcomponent. The proposed method enables the efficient calculation of FLT uptake in the pelvis. Thus, it represents a valuable tool to facilitate bone marrow preserving adaptive radiation therapy and radiopharmaceutical dose calculation. Furthermore, the method can be adapted to process other bone structures as well asorgans.

FLT-PET/CT in Non-Small Cell Lung Cancer Treated With SBRT- A Pilot Study

<h3>Purpose/Objective(s)</h3> To prospectively assess the value of FLT-PET in differentiating tumor recurrence from radiation-induced lung fibrosis. The primary objective was to compare FLT uptake in three cohort of patients: 1) before SBRT, 2) stable lung fibrosis after SBRT, and 3) suspicious/proven local recurrence. The secondary objective was to optimize FLT PET imaging sequences for lung cancer patients and compare FLT uptake in 4D (respiratory sorted) versus free-breathing FLT-PET scans. <h3>Materials/Methods</h3> Early-stage (T1-2N0M0 or T3N0M0 limited to the chest wall as per 7^th edition American Joint committee on cancer staging manual) presumed or biopsy-proven non-small cell lung carcinoma, aged more than 18 years and planned for SBRT at the institution or followed up after SBRT with radiographic findings of fibrosis or recurrence were considered eligible for the study. Patients were assigned to one of three study cohorts. All patients underwent imaging with FLT PET CT- prior to SBRT in cohort 1 and at fibrosis or recurrence in cohort 2 and 3. The planned sample size was 20 patients in each cohort, 60 patients in total. The FLT PET SUV variables like SUV max, SUV mean, SUV2Dpeak, SUV3Dpeak, SUV 50 and SUV95 were compared between the three cohorts using Kruskal Wallis test. The correlation of 4D phased matched and 3D PET SUV variables was performed using spearman correlation coefficient. Statistical significance was set at p<0.05. <h3>Results</h3> Forty one patients were accrued in the study (20 in cohort 1, 16 in cohort 2 and 5 in cohort3). The median age of the entire cohort was 77 years (range 46-89). Majority were diagnosed with stage I lung cancer (86%) and 48Gy in 4 fractions was the most commonly used dose fractionation (59%). A 4D FLT PET was performed in 35 patients. There was no significant difference in the FLT uptake SUV variables between 4D (respiratory sorted) versus free-breathing scans (r=0.8-1). Using the 3D FLT PET information, SUV peak, SUV mean, SUV max were significantly lower in fibrosis cohort compared to recurrence and pretreatment cohorts (Table 1). The delayed FLT PET values SUV 50 and 95 values in recurrence cohort were significantly similar to pretreatment cohort and significantly different compared to fibrosis. <h3>Conclusion</h3> FLT PET may be helpful in differentiating SBRT related fibrosis from recurrence. This study was limited by the small sample size in recurrence cohort and larger validation studies are needed to ascertain utility of FLT PET to confirm the findings.

Assessing Bone Marrow Activity with [18F]FLT PET in Patients with Essential Thrombocythemia and Prefibrotic Myelofibrosis: A Proof of Concept.

Objectives: This study aims to assess the value of FLT-PET as a non-invasive tool to differentiate between patients with ET and Pre-PMF. This study is a pilot study to have a proof of concept only. Methods: This is a prospective, interventional study where a total of 12 patients were included. Each patient underwent FLT PET imaging as well as bone marrow examination (gold standard). In addition, semi-quantitative (SUVmax and SUVmean) measurements of FLT uptake in the liver, spleen, and Lspine, SUVmean, as well as the Total Lesion Glycolysis (TLG) of the Lspine were performed. Results from the two patient cohorts were compared using = Kruskal-Wallis statistical test. A P-value of <.05 is considered to be statistically significant. Results: The differences in FLT SUVmax and SUVmean measurements in the three organs (liver, spleen, and LSpine) between the ET and Pre-PMF patients were not statistically significant (P > .05). In contrast, TLG measurements in the LSpine were statistically different (P = .013), and therefore, compared to gold standard bone marrow results, TLG can separate ET and Pre-PMF patients. Conclusion: This study is a proof of concept about the potential to discriminate between ET and pre-PMF patients in a non-invasive way. TLG of the LSpine in FLT PET images is a potential quantitative parameter to distinguish between ET and pre-PMF patients.

Assessing Bone Marrow Activity in Patients with Essential Thrombocythemia and Prefibrotic Myelofibrosis: Results of a Pilot Study of [18F]FLT PET

▪BackgroundAmong several groups of clinicians and hematopathologists a conflict of opinion has been repeatedly expressed concerning the validity of bone marrow (BM) features characterizing myeloproliferative neoplasms (MPNs). In this regard, controversy is mainly focused on the distinction between essential thrombocythemia (ET) and pre-fibrotic/early primary myelofibrosis (pre-PMF) Although other groups confirmed the characteristic BM features and emphasized the clinical impact to discriminate both MPN subtypes the existence of pre-PMF has been questioned, including clinical usefulness and particularly reproducibility of the corresponding diagnostic guidelines. In this context, it has been criticized that the MPN classification proposed by the World Health Organization (WHO), updated in 2008 and revised in 2016,was focused on BM morphology as the gold standard of diagnosis.The current standard for follow-up of these patients is based on pathological markers (peripheral blood counts and/ bone marrow histomorphology) and molecular markers. Bone marrow examination is the gold standard method to assess the disease's extent; it offers detailed information about cellularity, the morphology of each lineage, the degree of fibrosis, and the transformation and dysplastic features. However, many patients are reluctant to go for this invasive technique which precludes precise disease activity assessment at the desirable frequencies. A non-invasive technique that can offer reliable prognostic and predictive information about the disease is lacking.The objective of this study is to explore the diagnostic value of FLT-PET in malignant hematopoiesis of Pre-PMF and ET. The potential to use FLT-PET metrics to differentiate between Pre-PMF and ET is assessedMethodsA total of 13 patients (mean age of 43.23 ± 14.42 years, 7 males and 6 females) with Essential Thrombocythemia (ET) and/or Prefibrotic myelofibrosis were included in this study. One male subject was excluded due to an inconclusive diagnosis. The study was approved by the institutional review board. Written informed consents were obtained from all subjects. Each subject underwent FLT PET imaging as well as bone marrow examination (gold standard) and all were tested for JAK2v617F , CALR and MPL . Semi-quantitative (SUVmax and SUVmean) measurements of FLT uptake in the liver, spleen and Lumbar spine, SUVmean, as well as the Total Lesion Glycolysis (TLG) of the Lspine were performed. Results from the two patient cohorts were compared using = Kruskal-Wallis statistical test. A P-value of <0.05 is considered to be statistically significant.Discussion:Pre‐PMF and ET exhibited different features of bone marrow; however, this is not always easy to judge objectively, making pathologists' distinction often suboptimal. And in another scenario, bone marrow which is mandated for diagnosis, cannot be obtained due to technical issues or patient-related factors. In the 2016 revised classification,pre-PMF was recognized as a separate entity, distinct from ET. Thrombosis and hemorrhage represent two of the main causes of morbidity and mortality in patients with ET. Incidence of arterial and venous thrombosis prior to diagnosis revealed no significant differences (23% /20 and 9/8%) in WHO-defined ET compared with pre-PMF; thrombotic complications were also similar during the follow-thrombosis is not significantly different, whereas bleeding is more frequent in pre-PMF.From clinical prespective it is important to differentiate between the two categories.ResultsThe differences in FLT SUVmax and SUVmean measurements in the three organs (liver, spleen, and LSpine) between the ET and Pre-PMF patients were not statistically significant (P>0.05). In contrast, TLG measurements in the LSpine were statistically different (P=0.013), and therefore, compared to gold standard bone marrow results, TLG can separate ET and Pre-PMF patients.ConclusionTLG of the Lumbar Spine in FLT PET images is a potential quantitative parameter to discriminate between ET and PRE-PMF patients [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Cell proliferation detected using [18F]FLT PET/CT as an early marker of abdominal aortic aneurysm

BackgroundAbdominal aortic aneurysm (AAA) is a focal aortic dilatation progressing towards rupture. Non-invasive AAA-associated cell proliferation biomarkers are not yet established. We investigated the feasibility of the cell proliferation radiotracer, fluorine-18-fluorothymidine ([18F]FLT) with positron emission tomography/computed tomography (PET/CT) in a progressive pre-clinical AAA model (angiotensin II, AngII infusion). Methods and ResultsFourteen-week-old apolipoprotein E-knockout (ApoE−/−) mice received saline or AngII via osmotic mini-pumps for 14 (n = 7 and 5, respectively) or 28 (n = 3 and 4, respectively) days and underwent 90-minute dynamic [18F]FLT PET/CT. Organs were harvested from independent cohorts for gamma counting, ultrasound scanning, and western blotting. [18F]FLT uptake was significantly greater in 14- (n = 5) and 28-day (n = 3) AAA than in saline control aortae (n = 5) (P < 0.001), which reduced between days 14 and 28. Whole-organ gamma counting confirmed greater [18F]FLT uptake in 14-day AAA (n = 9) compared to saline-infused aortae (n = 4) (P < 0.05), correlating positively with aortic volume (r = 0.71, P < 0.01). Fourteen-day AAA tissue showed increased expression of thymidine kinase-1, equilibrative nucleoside transporter (ENT)-1, ENT-2, concentrative nucleoside transporter (CNT)-1, and CNT-3 than 28-day AAA and saline control tissues (n = 3 each) (all P < 0.001). Conclusions[18F]FLT uptake is increased during the active growth phase of the AAA model compared to saline control mice and late-stage AAA.

STING-driven interferon signaling triggers metabolic alterations in pancreas cancer cells visualized by [18F]FLT PET imaging

Type I interferons (IFNs) are critical effectors of emerging cancer immunotherapies designed to activate pattern recognition receptors (PRRs). A challenge in the clinical translation of these agents is the lack of noninvasive pharmacodynamic biomarkers that indicate increased intratumoral IFN signaling following PRR activation. Positron emission tomography (PET) imaging enables the visualization of tissue metabolic activity, but whether IFN signaling-induced alterations in tumor cell metabolism can be detected using PET has not been investigated. We found that IFN signaling augments pancreatic ductal adenocarcinoma (PDAC) cell nucleotide metabolism via transcriptional induction of metabolism-associated genes including thymidine phosphorylase (TYMP). TYMP catalyzes the first step in the catabolism of thymidine, which competitively inhibits intratumoral accumulation of the nucleoside analog PET probe 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT). Accordingly, IFN treatment up-regulates cancer cell [18F]FLT uptake in the presence of thymidine, and this effect is dependent upon TYMP expression. In vivo, genetic activation of stimulator of interferon genes (STING), a PRR highly expressed in PDAC, enhances the [18F]FLT avidity of xenograft tumors. Additionally, small molecule STING agonists trigger IFN signaling-dependent TYMP expression in PDAC cells and increase tumor [18F]FLT uptake in vivo following systemic treatment. These findings indicate that [18F]FLT accumulation in tumors is sensitive to IFN signaling and that [18F]FLT PET may serve as a pharmacodynamic biomarker for STING agonist-based therapies in PDAC and possibly other malignancies characterized by elevated STING expression.

18F]Fluorothymidine Uptake in the Porcine Pancreatic Elastase-Induced Model of Abdominal Aortic Aneurysm.

The porcine pancreatic elastase (PPE) model is a common preclinical model of abdominal aortic aneurysms (AAA). Some notable characteristics of this model include the low aortic rupture rate, non-progressive disease course, and infra-renal AAA formation. Enhanced [18F]fluorothymidine ([18F]FLT) uptake on positron emission tomography/computed tomography (PET/CT) has previously been reported in the angiotensin II-induced murine model of AAA. Here, we report our preliminary findings of investigating [18F]FLT uptake in the PPE murine model of AAA. [18F]FLT uptake was found to be substantially increased in the abdominal areas recovering from the surgery, whilst it was not found to be significantly increased within the PPE-induced AAA, as confirmed using in vivo PET/CT and ex vivo whole-organ gamma counting (PPE, n = 7; controls, n = 3). This finding suggests that the [18F]FLT may not be an appropriate radiotracer for this specific AAA model, and further studies with larger sample sizes are warranted to elucidate the pathobiology contributing to the reduced uptake of [18F]FLT in this model.

Comparison of the [18F]-FDG and [18F]-FLT PET Tracers in the Evaluation of the Preclinical Proton Therapy Response in Hepatocellular Carcinoma.

The main objective of the present study was to compare the 2-deoxy-2-[18F]fluoro-D-glucose ([18F]-FDG) and 3'-[18F]fluoro-3'-deoxythymidine ([18F]-FLT) PET imaging biomarkers for the longitudinal follow-up of small animal proton therapy studies in the context of hepatocellular carcinoma (HCC). SK-HEP-1 cells were injected into NMRI nude mice to mimic human HCC. The behavior of [18F]-FDG and [18F]-FLT tumor uptake was evaluated after proton therapy procedures. The proton single-fraction doses were 5, 10, and 20 Gy, with a dose rate of 10 Gy/min. The experimental protocol consisted of 8 groups of 10 mice, each group experiencing a particular dose/radiotracer condition. A reference PET exam was performed on each mouse the day before the irradiation procedure, followed by PET exams every 3 days up to 16 days after irradiation. [18F]-FDG uptake showed a linear dose-dependent increase in the first days after treatment (37%, p < 0.05), while [18F]-FLT uptake decreased in a dose-dependent manner (e.g., 21% for 5 Gy compared to 10 Gy, p = 1.1e-2). At the later time point, [18F]-FDG normalized activity showed an 85% decrease (p < 0.01) for both 10 and 20 Gy doses and no variation for 5 Gy. Conversely, a significant 61% (p = 0.002) increase was observed for [18F]-FLT normalized activity at 5 Gy and no variation for higher doses. We showed that the use of the [18F]-FDG and [18F]-FLT radiolabeled molecules can provide useful and complementary information for longitudinal follow-up of small animal proton therapy studies in the context of HCC. [18F]-FDG PET imaging enables a treatment monitoring several days/weeks postirradiation. On the other hand, [18F]-FLT could represent a good candidate to monitor the treatment few days postirradiation, in the context of hypo-fractioned and close irradiation planning. This opens new perspectives in terms of treatment efficacy verification depending on the irradiation scheme.

IMG-02. USEFUL DIAGNOSIS OF PEDIATRIC CYSTIC BRAIN TUMORS USING MULTIPLE POSITRON EMISSION TOMOGRAPHY STUDIES

OBJECTIVEPediatric brain tumors are primarily diagnosed using MRI or CT examination; however, determining the correct diagnosis using only morphological MRI can sometimes be challenging. Positron emission tomography (PET) uses radiotracers for metabolic and molecular imaging. We examined the accumulation of multiple PET (FDG, MET, FLT, and FMISO) studies for diagnosing pediatric cystic brain tumors.METHODSWe performed PET scans for eight pediatric patients (five pilocytic astrocytoma, one pleomorphic xanthoastrocytoma, one diffuse astrocytoma with IDH1 mutation, one ganglioglioma) from April 2010 to December 2019. The resulting studies were compared by measuring the tumor-to-normal lesion (T/N) ratio of FDG, MET, and FLT and the tumor-to-blood value (T/B) ratio of FMISO between each pediatric cystic brain tumor.RESULTSAll pediatric brain tumors showed tumor uptake of FDG, MET, and FLT. We could not examine FMISO PET for one diffuse astrocytoma with IDH1 mutation. The T/N ratios of FDG, MET, and FLT and the T/B ratio of FMISO were 1.07, 2.76, 4.6, and 1.12 for pilocytic astrocytoma; 0.65, 4.6, 7.67, and 1.38 for pleomorphic xanthoastrocytoma; 0.61, 2.14, and 3.82 for diffuse astrocytoma with IDH1 mutation; and 0.79, 1.78, 5, and 1.49 for ganglioglioma, respectively. The T/N ratios of MET and FLT for pleomorphic xanthoastrocytoma were high, but the Ki-67 labeling index was 1%. In the ganglioglioma, the T/N ratio of FLT was high, but the T/N ratio of MET was low.CONCLUSIONSpecialized multiple PET accumulation patterns for tumors are useful for discriminating each tumor.