System L amino acid transporter LAT1 accumulates O-(2-fluoroethyl)-L-tyrosine (FET)

Cortical Origin-Dependent Metabolic and Molecular Heterogeneity in Gliomas: Insights from 18F-FET PET

Objectives: The objective of this study is to explore the potential variations in metabolic activity across gliomas originating from distinct cortical regions, as assessed by O-(2-18F-fluoroethyl)-L-tyrosine positron emission tomography (18F-FET PET). Also, this study seeks to elucidate whether these metabolic disparities correlate with the molecular characteristics and clinical prognoses of the tumors. Specifically, this research aims to determine whether variations in 18F-FET PET uptake are indicative of underlying genetic or biochemical differences that could influence patients' outcomes. Methods: The researchers retrospectively included 107 patients diagnosed with gliomas from neocortex and mesocortex, all of whom underwent hybrid PET/MR examinations, including 18F-FET PET and diffusion weighted imaging (DWI), prior to surgery. The mean and maximum tumor-to-background ratio (TBR) and apparent diffusion coefficient (ADC) values were calculated based on whole tumor volume segmentations. Comparisons of TBR, ADC values, and survival outcomes were performed to determine statistical differences between groups. Results: Among glioblastomas (GBMs, WHO grade 4) originating from the two cortical regions, there was a significant difference in the human Telomerase Reverse Transcriptase (TERT) promoter mutation rate, while no difference was observed in O6-Methylguanine-DNA Methyltransferase (MGMT) promoter methylation status. For WHO grade 3 gliomas, significant differences were found in the TERT promoter mutation rate and the proportion of 1p/19q co-deletion between the two cortical regions, whereas no difference was noted in MGMT methylation status. For WHO grade 2 gliomas, no molecular phenotypic differences were observed between the two cortical regions. In terms of survival, only GBMs originating from the mesocortex demonstrated significantly longer survival compared to those from the neocortex, while no statistically significant differences were found in survival for the other two groups. Conclusions: Gliomas originating from different cortical regions exhibit variations in metabolic activity, molecular phenotypes, and clinical outcomes.

FET PET to differentiate between post-treatment changes and recurrence in high-grade gliomas: a single center multidisciplinary clinic controlled study

PURPOSE

The clinico-radiological dilemma in post-treatment high-grade gliomas, between disease recurrence (TR) and treatment-related changes (TRC), still persists. FET (Fluoro-ethyl-tyrosine) PET has been extensively used as problem-solving modality for cases where MR imaging is inconclusive. We incorporated a systematic imaging and clinical follow-up algorithm in a multi-disciplinary clinic (MDC) setting to analyse our cohort of FET PET in post-treatment gliomas.

METHODS

We retrospectively analyzed 171 patients of post-treatment grade III and IV glioma with equivocal findings on MRI. 185-222 MBq of 18 F-FET was injected and dedicated static imaging of brain was performed at 20 min. TBR (Tumor to background ratio) was used as semi-quantitative parameter. Cutoff of 2.5 was used for image interpretation. Imaging findings were confirmed with histopathological diagnosis, wherever available or in a multidisciplinary joint clinic based on serial imaging.

RESULTS

121 of 171 patients showed recurrent disease on FET PET, on follow up, 109 were confirmed with recurrence; 7 patients showed TRC, whereas 5 were treated with bevacizumab, with no further clinico-radiological deterioration, thus confirming TRC. 50 patients showed TRC on FET PET, on follow up on follow up, 40 were confirmed as true-negative. 10 patients who showed TBR less than 2.5 had confirmed TR on subsequent MR imaging. The overall sensitivity and specificity was 91.6 and 76.9% respectively, with a diagnostic accuracy of 87.13%.

CONCLUSION

There is potential for FET PET to be used along with MRI in the post treatment algorithm of high-grade glial tumors.

Advances in Glioblastoma Diagnosis: Integrating Genetics, Noninvasive Sampling, and Advanced Imaging

Glioblastoma is the most common primary brain tumor in adult patients, and despite standard-of-care treatment, median survival has remained less than two years. Advances in our understanding of molecular mutations have led to changes in the diagnostic criteria of glioblastoma, with the WHO classification integrating important mutations into the grading system in 2021. We sought to review the basics of the important genetic mutations associated with glioblastoma, including known mechanisms and roles in disease pathogenesis/treatment. We also examined new advances in image processing as well as less invasive and noninvasive diagnostic tools that can aid in the diagnosis and surveillance of those undergoing treatment for glioblastoma. Our review is intended to serve as an overview of the current state-of-the-art in the diagnosis and management of glioblastoma.

PET imaging of gliomas: Status quo and quo vadis?

PET imaging, particularly using amino acid tracers, has become a valuable adjunct to anatomical MRI in the clinical management of patients with glioma. Collaborative international efforts have led to the development of clinical and technical guidelines for PET imaging in gliomas. The increasing readiness of statutory health insurance agencies, especially in European countries, to reimburse amino acid PET underscores its growing importance in clinical practice. Integrating artificial intelligence and radiomics in PET imaging of patients with glioma may significantly improve tumor detection, segmentation, and response assessment. Efforts are ongoing to facilitate the clinical translation of these techniques. Considerable progress in computer technology developments (eg quantum computers) may be helpful to accelerate these efforts. Next-generation PET scanners, such as long-axial field-of-view PET/CT scanners, have improved image quality and body coverage and therefore expanded the spectrum of indications for PET imaging in Neuro-Oncology (eg PET imaging of the whole spine). Encouraging results of clinical trials in patients with glioma have prompted the development of PET tracers directing therapeutically relevant targets (eg the mutant isocitrate dehydrogenase) for novel anticancer agents in gliomas to improve response assessment. In addition, the success of theranostics for the treatment of extracranial neoplasms such as neuroendocrine tumors and prostate cancer has currently prompted efforts to translate this approach to patients with glioma. These advancements highlight the evolving role of PET imaging in Neuro-Oncology, offering insights into tumor biology and treatment response, thereby informing personalized patient care. Nevertheless, these innovations warrant further validation in the near future.

The pons as an optimal background reference region for spinal 18F-FET PET/MRI evaluation

BACKGROUND

This study aims to evaluate the effect of various background reference regions on spinal 18F-FET PET imaging, with a focus on distinguishing between spinal tumors and myelitis. To enhance diagnostic accuracy, we investigated the pons and several other spinal cord area as potential references, given the challenges in interpreting spinal PET results.

RESULTS

A retrospective analysis was conducted on 30 patients, 15 with cervical myelitis and 15 with cervical tumors, who underwent O-(2-[18F]-fluoroethyl)-L-tyrosine (FET) PET/MR imaging. The stability of uptake across four regions, including the pons, C2, C2-C7, and T1-T3, was compared. The standardized uptake value ratio (SUVR) was then evaluated using various background regions, and their effectiveness in differentiating between spinal tumors and myelitis was compared. Additionally, we correlated the SUVR values derived from these regions with the Ki-67 proliferation index in tumor patients. The study found no significant difference in SUVmax (U = 110, p = 0.93) and SUVmean (U = 89, p = 0.35) values at lesion sites between myelitis and tumor patients. The pons had the highest average uptake (p < 0.001) compared to the other three regions. However, its coefficient of variation (CV) was significantly lower than that of the C2-C7 (p < 0.0001) and T1-T3 segments (p < 0.05). The SUVRmax values, calculated using the regions of pons, C2-C7 and T1-T3, were found to significantly differentiate between tumors and myelitis (p < 0.05). However, only the pons-based SUVRmean was able to significantly distinguish between the two groups (p < 0.05). Additionally, the pons-based SUVRmax (r = 0.63, p = 0.013) and SUVRmean (r = 0.67, p = 0.007) demonstrated a significant positive correlation with the Ki-67 index.

CONCLUSIONS

This study suggests that the pons may be considered a suitable reference region for spinal 18F-FET PET imaging, which can improve the differentiation between spinal tumors and myelitis. The significant correlation between pons-based SUVR values and the Ki-67 index further highlights the potential of this approach in assessing tumor cell proliferation.

A prospective clinical study of the influence of oral protein intake on [18F]FET-PET uptake and test-retest repeatability in glioma

BACKGROUND

O-(2-[18F]fluoroethyl)-L-tyrosine positron emission tomography ([18F]FET PET) scanning is used in routine clinical management and evaluation of gliomas with a recommended 4 h prior fasting. Knowledge of test-retest variation of [18F]FET PET imaging uptake metrics and the impact of accidental protein intake can be critical for interpretation. The aim of this study was to investigate the repeatability of [18F]FET-PET metrics and to assess the impact of protein-intake prior to [18F]FET PET scanning of gliomas.

RESULTS

Test-retest variability in the non-protein group was good with absolute (and relative) upper and lower limits of agreement of + 0.15 and - 0.13 (+ 9.7% and - 9.0%) for mean tumour-to-background ratio (TBRmean), + 0.43 and - 0.28 (+ 19.6% and - 11.8%) for maximal tumour-to-background ratio (TBRmax), and + 2.14 cm3 and - 1.53 ml (+ 219.8% and - 57.3%) for biological tumour volume (BTV). Variation was lower for uptake ratios than for BTV. Protein intake was associated with a 27% increase in the total sum of plasma concentration of the L-type amino acid transporter 1 (LAT1) relevant amino acids and with decreased standardized uptake value (SUV) in both healthy appearing background brain tissue (mean SUV - 25%) and in tumour (maximal SUV - 14%). Oral intake of 24 g of protein 1 h prior to injection of tracer tended to increase variability, but the effects on derived tumour metrics TBRmean and TBRmax were only borderline significant, and changes generally within the variability observed in the group with no protein intake.

CONCLUSION

The test-retest repeatability was found to be good, and better for TBRmax and TBRmean than BTV, with the methodological limitation that tumour growth may have influenced results. Oral intake of 24 g of protein one hour before a [18F]FET PET scan decreases uptake of [18F]FET in both tumour and in healthy appearing brain, with no clinically significant difference on the most commonly used tumour metrics.

Radiosynthesis and Preclinical Evaluation of m-[18F]FET and [18F]FET-OMe as Novel [18F]FET Analogs for Brain Tumor Imaging

O-([18F]Fluoroethyl)-l-tyrosine ([18F]FET) is actively transported into the brain and cancer cells by LAT1 and possibly other amino acid transporters, which enables brain tumor imaging by positron emission tomography (PET). However, tumor delivery of this probe in the presence of competing amino acids may be limited by a relatively low affinity for LAT1. The aim of the present work was to evaluate the meta-substituted [18F]FET analog m-[18F]FET and the methyl ester [18F]FET-OMe, which were designed to improve tumor delivery by altering the physicochemical, pharmacokinetic, and/or transport properties. Both tracers could be prepared with good radiochemical yields of 41-56% within 66-90 min. Preclinical evaluation with [18F]FET as a reference tracer demonstrated reduced in vitro uptake of [18F]FET-OMe by U87 glioblastoma cells and no advantage for in vivo tumor imaging. In contrast, m-[18F]FET showed significantly improved in vitro uptake and accelerated in vivo tumor accumulation in an orthotopic glioblastoma model. As such, our work identifies m-[18F]FET as a promising alternative to [18F]FET for brain tumor imaging that deserves further evaluation with regard to its transport properties and in vivo biodistribution.

18F-FACBC and 18F-FDG PET/MRI in the evaluation of 3 patients with primary central nervous system lymphoma: a pilot study

BACKGROUND

This PET/MRI study compared contrast-enhanced MRI, 18F-FACBC-, and 18F-FDG-PET in the detection of primary central nervous system lymphomas (PCNSL) in patients before and after high-dose methotrexate chemotherapy. Three immunocompetent PCNSL patients with diffuse large B-cell lymphoma received dynamic 18F-FACBC- and 18F-FDG-PET/MRI at baseline and response assessment. Lesion detection was defined by clinical evaluation of contrast enhanced T1 MRI (ce-MRI) and visual PET tracer uptake. SUVs and tumor-to-background ratios (TBRs) (for 18F-FACBC and 18F-FDG) and time-activity curves (for 18F-FACBC) were assessed.

RESULTS

At baseline, seven ce-MRI detected lesions were also detected with 18F-FACBC with high SUVs and TBRs (SUVmax:mean, 4.73, TBRmax: mean, 9.32, SUVpeak: mean, 3.21, TBRpeak:mean: 6.30). High TBR values of 18F-FACBC detected lesions were attributed to low SUVbackground. Baseline 18F-FDG detected six lesions with high SUVs (SUVmax: mean, 13.88). In response scans, two lesions were detected with ce-MRI, while only one was detected with 18F-FACBC. The lesion not detected with 18F-FACBC was a small atypical MRI detected lesion, which may indicate no residual disease, as this patient was still in complete remission 12 months after initial diagnosis. No lesions were detected with 18F-FDG in the response scans.

CONCLUSIONS

18F-FACBC provided high tumor contrast, outperforming 18F-FDG in lesion detection at both baseline and in response assessment. 18F-FACBC may be a useful supplement to ce-MRI in PCNSL detection and response assessment, but further studies are required to validate these findings. Trial registration ClinicalTrials.gov. Registered 15th of June 2017 (Identifier: NCT03188354, https://clinicaltrials.gov/study/NCT03188354 ).

Exploring the mechanism of 18F-fluorodopa uptake in recurrent high-grade gliomas: A comprehensive histomolecular-positron emission tomography analysis

BACKGROUND

Dihydroxy-6-[18F]fluoro-L-phenylalanine (18F-FDOPA) positron emission tomography (PET) is a valuable tool for managing high-grade gliomas (HGGs), but there is a lack of literature on its relationship with glioma subtypes since the 2021 reclassification of brain tumors. There is also debate surrounding the mechanism of 18F-FDOPA uptake, particularly after chemoradiation therapy. This study aimed to investigate the correlation between 18F-FDOPA uptake and histomolecular characteristics, particularly L-amino acid transporter 1 (LAT1) expression, in recurrent gliomas, and examine their impact on survival in HGGs.

METHODS

Thirty-nine patients with recurrent HGGs (14 isocitrate dehydrogenase [IDH]-mutant, 25 IDH-wildtype) who underwent a brain 18F-FDOPA PET/computed tomography (CT) or PET/magnetic resonance imaging (MRI) followed by surgical resection of the 18F-FDOPA-avid lesion within 6 months, were retrospectively reviewed. PET results were compared with histological examination and for SCL7A5/LAT1 immunostaining. The study also examined the relationship between PET parameters, LAT1 expression, and survival outcomes.

RESULTS

Astrocytoma IDH-mutant G4 had higher 18F-FDOPA uptake than glioblastoma IDH-wildtype G4 (maximum tumor-to-normal brain ratio [TBRmax] 5 [3.4-9] vs. 3.8 [2.8-5.9], p = 0.02). IDH-mutant gliomas had higher LAT1 expression than IDH-wildtype gliomas (100 [14-273] vs. 15.5 [0-137], p < 0.05) as well as higher TBRmax (5 [2.4-9] vs. 3.8 [2.8-6], p < 0.05). In survival analysis, LAT1 score >100 was a predictor for longer progression-free survival in IDH-mutant HGGs.

CONCLUSIONS

To our knowledge, our study is the first to suggest a link between LAT1 expression and IDH mutation status. We showed that higher TBRmax was associated with higher LAT1 expression and IDH mutation status. Further studies are needed to better understand the mechanisms underlying amino acid PET tracers uptake, especially in the post-radiation and chemotherapy settings.

Results from a phase I study of 4-l-[131I]iodo-phenylalanine ([131I]IPA) with external radiation therapy in patients with recurrent glioblastoma (IPAX-1)

Background

Glioblastoma (GBM), the most common malignant brain tumor, is associated with devastating outcomes. IPAX-1 was a multicenter, open-label, single-arm phase I study to evaluate carrier-added 4-L-[131I]iodo-phenylalanine ([131I]IPA) plus external radiation therapy (XRT) in recurrent GBM.

Methods

A total of 10 adults with recurrent GBM who had received first-line debulking surgery plus radio-chemotherapy, were randomized to a single-dose regimen (1f; 131I-IPA 2 GBq before XRT); a fractionated parallel dose regimen (3f-p; 3 131I-IPA 670 MBq fractions, in parallel with second-line XRT), or a fractionated sequential dose regimen (3f-s; 3 131I-IPA 670 MBq fractions before and after XRT). Metabolic tumor responses were determined using O-(2-[18F]fluoroethyl)-l-tyrosine positron emission tomography, while single-photon emission computed tomography was used to guide [131I]IPA tumor dosimetry.

Results

All dose regimens were well tolerated. Organ-absorbed radiation doses in red marrow (0.38 Gy) and kidney (1.28 Gy) confirmed no radiation-based toxicity. Stable disease was observed in 4 of the 9 patients at 3 months post-treatment (3-month follow-up [FU], 1 patient did not reach protocol-mandated end of study), yielding a response rate of 44.4%. At the 3-month FU, 6 patients demonstrated metabolic stable disease. Median progression-free survival was 4.3 months (95% confidence interval [CI]: 3.3-4.5), while median overall survival was 13 months (95% CI: 7.1-27).

Conclusions

Single or fractionated doses of [131I]IPA plus XRT were associated with acceptable tolerability and specific tumor targeting in patients with recurrent GBM, warranting further investigation.

Diagnostic accuracy of anti-3-[18F]-FACBC PET/MRI in gliomas

PURPOSE

The primary aim was to evaluate whether anti-3-[18F]FACBC PET combined with conventional MRI correlated better with histomolecular diagnosis (reference standard) than MRI alone in glioma diagnostics. The ability of anti-3-[18F]FACBC to differentiate between molecular and histopathological entities in gliomas was also evaluated.

METHODS

In this prospective study, patients with suspected primary or recurrent gliomas were recruited from two sites in Norway and examined with PET/MRI prior to surgery. Anti-3-[18F]FACBC uptake (TBRpeak) was compared to histomolecular features in 36 patients. PET results were then added to clinical MRI readings (performed by two neuroradiologists, blinded for histomolecular results and PET data) to assess the predicted tumor characteristics with and without PET.

RESULTS

Histomolecular analyses revealed two CNS WHO grade 1, nine grade 2, eight grade 3, and 17 grade 4 gliomas. All tumors were visible on MRI FLAIR. The sensitivity of contrast-enhanced MRI and anti-3-[18F]FACBC PET was 61% (95%CI [45, 77]) and 72% (95%CI [58, 87]), respectively, in the detection of gliomas. Median TBRpeak was 7.1 (range: 1.4-19.2) for PET positive tumors. All CNS WHO grade 1 pilocytic astrocytomas/gangliogliomas, grade 3 oligodendrogliomas, and grade 4 glioblastomas/astrocytomas were PET positive, while 25% of grade 2-3 astrocytomas and 56% of grade 2-3 oligodendrogliomas were PET positive. Generally, TBRpeak increased with malignancy grade for diffuse gliomas. A significant difference in PET uptake between CNS WHO grade 2 and 4 gliomas (p < 0.001) and between grade 3 and 4 gliomas (p = 0.002) was observed. Diffuse IDH wildtype gliomas had significantly higher TBRpeak compared to IDH1/2 mutated gliomas (p < 0.001). Adding anti-3-[18F]FACBC PET to MRI improved the accuracy of predicted glioma grades, types, and IDH status, and yielded 13.9 and 16.7 percentage point improvement in the overall diagnoses for both readers, respectively.

CONCLUSION

Anti-3-[18F]FACBC PET demonstrated high uptake in the majority of gliomas, especially in IDH wildtype gliomas, and improved the accuracy of preoperatively predicted glioma diagnoses.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov ID: NCT04111588, URL: https://clinicaltrials.gov/study/NCT04111588.

Preparation of NIn-Methyl-6-[18F]fluoro- and 5-Hydroxy-7-[18F]fluorotryptophans as Candidate PET-Tracers for Pathway-Specific Visualization of Tryptophan Metabolism

Tryptophan (Trp) is an essential proteinogenic amino acid and metabolic precursor for several signaling molecules that has been implicated in many physiological and pathological processes. Since the two main branches of Trp metabolism-serotonin biosynthesis and kynurenine pathway-are differently affected by a variety of neurological and neoplastic diseases, selective visualization of these pathways is of high clinical relevance. However, while positron emission tomography (PET) with existing probes can be used for non-invasive assessment of total Trp metabolism, optimal imaging agents for pathway-specific PET imaging are still lacking. In this work, we describe the preparation of two 18F-labeled Trp derivatives, NIn-methyl-6-[18F]fluorotryptophan (NIn-Me-6-[18F]FTrp) and 5-hydroxy-7-[18F]fluorotryptophan (5-HO-7-[18F]FTrp). We also report feasible synthetic routes for the preparation of the hitherto unknown boronate radiolabeling precursors and non-radioactive reference compounds. Under optimized conditions, alcohol-enhanced Cu-mediated radiofluorination of the respective precursors afforded NIn-Me-6-[18F]FTrp and 5-HO-7-[18F]FTrp as application-ready solutions in radiochemical yields of 45 ± 7% and 29 ± 4%, respectively. As such, our work provides access to two promising candidate probes for pathway-specific visualization of Trp metabolism in amounts sufficient for their preclinical evaluation.

Prediction of Glioma Grade and IDH Status Using 18F-FET PET/CT Dynamic and Multiparametric Texture Analysis

Mutations in isocitrate dehydrogenase (IDH) represent an independent predictor of better survival in patients with gliomas. We aimed to assess grade and IDH mutation status in patients with untreated gliomas, by evaluating the respective value of 18F-FET PET/CT via dynamic and texture analyses. A total of 73 patients (male: 48, median age: 47) who underwent an 18F-FET PET/CT for initial glioma evaluation were retrospectively included. IDH status was available in 61 patients (20 patients with WHO grade 2 gliomas, 41 with grade 3-4 gliomas). Time-activity curve type and 20 parameters obtained from static analysis using LIFEx© v6.30 software were recorded. Respective performance was assessed using receiver operating characteristic curve analysis and stepwise multivariate regression analysis adjusted for patients' age and sex. The time-activity curve type and texture parameters derived from the static parameters showed satisfactory-to-good performance in predicting glioma grade and IDH status. Both time-activity curve type (stepwise OR: 101.6 (95% CI: 5.76-1791), p = 0.002) and NGLDM coarseness (stepwise OR: 2.08 × 1043 (95% CI: 2.76 × 1012-1.57 × 1074), p = 0.006) were independent predictors of glioma grade. No independent predictor of IDH status was found. Dynamic and texture analyses of 18F-FET PET/CT have limited predictive value for IDH status when adjusted for confounding factors. However, they both help predict glioma grade.

Stimuli-Responsive Multifunctional Nanomedicine for Enhanced Glioblastoma Chemotherapy Augments Multistage Blood-to-Brain Trafficking and Tumor Targeting

Minimal therapeutic advances have been achieved over the past two decades for glioblastoma (GBM), which remains an unmet clinical need. Here, hypothesis-driven stimuli-responsive nanoparticles (NPs) for docetaxel (DTX) delivery to GBM are reported, with multifunctional features that circumvent insufficient blood-brain barrier (BBB) trafficking and lack of GBM targeting-two major hurdles for anti-GBM therapies. NPs are dual-surface tailored with a i) brain-targeted acid-responsive Angiopep-2 moiety that triggers NP structural rearrangement within BBB endosomal vesicles, and ii) L-Histidine moiety that provides NP preferential accumulation into GBM cells post-BBB crossing. In tumor invasive margin patient cells, the stimuli-responsive multifunctional NPs target GBM cells, enhance cell uptake by 12-fold, and induce three times higher cytotoxicity in 2D and 3D cell models. Moreover, the in vitro BBB permeability is increased by threefold. A biodistribution in vivo trial confirms a threefold enhancement of NP accumulation into the brain. Last, the in vivo antitumor efficacy is validated in GBM orthotopic models following intratumoral and intravenous administration. Median survival and number of long-term survivors are increased by 50%. Altogether, a preclinical proof of concept supports these stimuli-responsive multifunctional NPs as an effective anti-GBM multistage chemotherapeutic strategy, with ability to respond to multiple fronts of the GBM microenvironment.

Prediction of response to lomustine-based chemotherapy in glioma patients at recurrence using MRI and FET PET

BACKGROUND

We evaluated O-(2-[18F]fluoroethyl)-l-tyrosine (FET) PET and MRI for early response assessment in recurrent glioma patients treated with lomustine-based chemotherapy.

METHODS

Thirty-six adult patients with WHO CNS grade 3 or 4 gliomas (glioblastoma, 69%) at recurrence (median number of recurrences, 1; range, 1-3) were retrospectively identified. Besides MRI, serial FET PET scans were performed at baseline and early after chemotherapy initiation (not later than two cycles). Tumor-to-brain ratios (TBR), metabolic tumor volumes (MTV), the occurrence of new distant hotspots with a mean TBR >1.6 at follow-up, and the dynamic parameter time-to-peak were derived from all FET PET scans. PET parameter thresholds were defined using ROC analyses to predict PFS of ≥6 months and OS of ≥12 months. MRI response assessment was based on RANO criteria. The predictive values of FET PET parameters and RANO criteria were subsequently evaluated using univariate and multivariate survival estimates.

RESULTS

After treatment initiation, the median follow-up time was 11 months (range, 3-71 months). Relative changes of TBR, MTV, and RANO criteria predicted a significantly longer PFS (all P ≤ .002) and OS (all P ≤ .045). At follow-up, the occurrence of new distant hotspots (n ≥ 1) predicted a worse outcome, with significantly shorter PFS (P = .005) and OS (P < .001). Time-to-peak changes did not predict a significantly longer survival. Multivariate survival analyses revealed that new distant hotspots at follow-up FET PET were most potent in predicting non-response (P < .001; HR, 8.578).

CONCLUSIONS

Data suggest that FET PET provides complementary information to RANO criteria for response evaluation of lomustine-based chemotherapy early after treatment initiation.

Glioblastoma immuno-endothelial multicellular microtissue as a 3D in vitro evaluation tool of anti-cancer nano-therapeutics

Despite being the most prevalent and lethal type of adult brain cancer, glioblastoma (GBM) remains intractable. Promising anti-GBM nanoparticle (NP) systems have been developed to improve the anti-cancer performance of difficult-to-deliver therapeutics, with particular emphasis on tumor targeting strategies. However, current disease modeling toolboxes lack close-to-native in vitro models that emulate GBM microenvironment and bioarchitecture, thus partially hindering translation due to poorly predicted clinical responses. Herein, human GBM heterotypic multicellular tumor microtissues (MCTMs) are generated through high-throughput 3D modeling of U-251 MG tumor cells, tissue differentiated macrophages isolated from peripheral monocytes, and brain microvascular primary endothelial cells. GBM MCTMs mimicked tumor spatial organization, extracellular matrix production and necrosis areas. The bioactivity of a model drug, docetaxel (DTX), and of tumor-targeted DTX-loaded polymeric NPs with a surface L-Histidine moiety (H-NPs), were assessed in the MCTMs. MCTMs cell uptake and anti-proliferative effect was 8- and 3-times higher for H-NPs, respectively, compared to the non-targeted NPs and to free DTX. H-NPs provided a decrease of MCTMs anti-inflammatory M2-macrophages, while increasing their pro-inflammatory M1 counterparts. Moreover, H-NPs showed a particular biomolecular signature through reduced secretion of an array of medium cytokines (IFN-γ, IL-1β, IL-1Ra, IL-6, IL-8, TGF-β). Overall, MCTMs provide an in vitro biomimetic model to recapitulate key cellular and structural features of GBM and improve in vivo drug response predictability, fostering future clinical translation of anti-GBM nano-therapeutic strategies.

Diagnostic Value of 18 F-FACBC PET/MRI in Brain Metastases

PURPOSE

The study aims to evaluate whether combined 18 F-FACBC PET/MRI could provide additional diagnostic information compared with MRI alone in brain metastases.

PATIENTS AND METHODS

Eighteen patients with newly diagnosed or suspected recurrence of brain metastases received dynamic 18 F-FACBC PET/MRI. Lesion detection was evaluated on PET and MRI scans in 2 groups depending on prior stereotactic radiosurgery (SRS group) or not (no-SRS group). SUVs, time-activity curves, and volumetric analyses of the lesions were performed.

RESULTS

In the no-SRS group, 29/29 brain lesions were defined as "MRI positive." With PET, 19/29 lesions were detected and had high tumor-to-background ratios (TBRs) (D max MR , ≥7 mm; SUV max , 1.2-8.4; TBR, 3.9-25.9), whereas 10/29 lesions were undetected (D max MR , ≤8 mm; SUV max , 0.3-1.2; TBR, 1.0-2.7). In the SRS group, 4/6 lesions were defined as "MRI positive," whereas 2/6 lesions were defined as "MRI negative" indicative of radiation necrosis. All 6 lesions were detected with PET (D max MR , ≥15 mm; SUV max , 1.4-4.2; TBR, 3.6-12.6). PET volumes correlated and were comparable in size with contrast-enhanced MRI volumes but were only partially congruent (mean DSC, 0.66). All time-activity curves had an early peak, followed by a plateau or a decreasing slope.

CONCLUSIONS

18 F-FACBC PET demonstrated uptake in brain metastases from cancer of different origins (lung, gastrointestinal tract, breast, thyroid, and malignant melanoma). However, 18 F-FACBC PET/MRI did not improve detection of brain metastases compared with MRI but might detect tumor tissue beyond contrast enhancement on MRI. 18 F-FACBC PET should be further evaluated in recurrent brain metastases.

Use of multimodality imaging, histology, and treatment feasibility to characterize a transgenic Rag2-null rat model of glioblastoma

Many drugs that show potential in animal models of glioblastoma (GBM) fail to translate to the clinic, contributing to a paucity of new therapeutic options. In addition, animal model development often includes histologic assessment, but multiparametric/multimodality imaging is rarely included despite increasing utilization in patient cancer management. This study developed an intracranial recurrent, drug-resistant, human-derived glioblastoma tumor in Sprague-Dawley Rag2-Rag2 ^tm1Hera knockout rat and was characterized both histologically and using multiparametric/multimodality neuroimaging. Hybrid ¹⁸F-fluoroethyltyrosine positron emission tomography and magnetic resonance imaging, including chemical exchange saturation transfer (¹⁸F-FET PET/CEST MRI), was performed for full tumor viability determination and characterization. Histological analysis demonstrated human-like GBM features of the intracranially implanted tumor, with rapid tumor cell proliferation (Ki67 positivity: 30.5 ± 7.8%) and neovascular heterogeneity (von Willebrand factor VIII:1.8 to 5.0% positivity). Early serial MRI followed by simultaneous ¹⁸F-FET PET/CEST MRI demonstrated consistent, predictable tumor growth, with exponential tumor growth most evident between days 35 and 49 post-implantation. In a second, larger cohort of rats, ¹⁸F-FET PET/CEST MRI was performed in mature tumors (day 49 post-implantation) for biomarker determination, followed by evaluation of single and combination therapy as part of the model development and validation. The mean percentage of the injected dose per mL of ¹⁸F-FET PET correlated with the mean %CEST (r = 0.67, P < 0.05), but there was also a qualitative difference in hot spot location within the tumor, indicating complementary information regarding the tumor cell demand for amino acids and tumor intracellular mobile phase protein levels. Finally, the use of this glioblastoma animal model for therapy assessment was validated by its increased overall survival after treatment with combination therapy (temozolomide and idasanutlin) (P < 0.001). Our findings hold promise for a more accurate tumor viability determination and novel therapy assessment in vivo in a recently developed, reproducible, intracranial, PDX GBM.

Can Radiomics Provide Additional Information in [18F]FET-Negative Gliomas?

Simple Summary

Amino acid positron emission tomography (PET) complements standard magnetic resonance imaging (MRI) since it directly visualizes the increased amino acid transport into tumor cells. Amino acid PET using O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) has proven to be relevant, for example, for glioma classification, identification of tumor progression or recurrence, or for the delineation of tumor extent. Nevertheless, a relevant proportion of low-grade gliomas (30%) and few high-grade gliomas (5%) were found to show no or even decreased amino acid uptake by conventional visual analysis of PET images. Advanced image analysis with the extraction of radiomic features is known to provide more detailed information on tumor characteristics than conventional analyses. Hence, this study aimed to investigate whether radiomic features derived from dynamic [¹⁸F]FET PET data differ between [¹⁸F]FET-negative glioma and healthy background and thus provide information that cannot be extracted by visual read.

Abstract

The purpose of this study was to evaluate the possibility of extracting relevant information from radiomic features even in apparently [¹⁸F]FET-negative gliomas. A total of 46 patients with a newly diagnosed, histologically verified glioma that was visually classified as [¹⁸F]FET-negative were included. Tumor volumes were defined using routine T2/FLAIR MRI data and applied to extract information from dynamic [¹⁸F]FET PET data, i.e., early and late tumor-to-background (TBR_5–15, TBR_20–40) and time-to-peak (TTP) images. Radiomic features of healthy background were calculated from the tumor volume of interest mirrored in the contralateral hemisphere. The ability to distinguish tumors from healthy tissue was assessed using the Wilcoxon test and logistic regression. A total of 5, 15, and 69% of features derived from TBR_20–40, TBR_5–15, and TTP images, respectively, were significantly different. A high number of significantly different TTP features was even found in isometabolic gliomas (after exclusion of photopenic gliomas) with visually normal [¹⁸F]FET uptake in static images. However, the differences did not reach satisfactory predictability for machine-learning-based identification of tumor tissue. In conclusion, radiomic features derived from dynamic [¹⁸F]FET PET data may extract additional information even in [¹⁸F]FET-negative gliomas, which should be investigated in larger cohorts and correlated with histological and outcome features in future studies.

LAT1, a novel pharmacological target for the treatment of glioblastoma

The L-Type Amino Acid transporter, LAT1 (SLC7A5), has a crucial role in mediating amino acid uptake into the cells, thus modulating cell growth and proliferation as well as other intracellular functions. Different studies have reported a central role of LAT1 in glioblastoma development and progression, suggesting that the modulation of its activity could be a novel therapeutic strategy. LAT1 also has an important role in the peripheral immune system, by regulating the activation status of several immune cells through modulation of the mechanistic target of rapamycin kinase. In glioblastoma (GBM), the blood-brain barrier is disrupted, which allows the recruitment of peripheral immune cells to the tumour site. These cells, together with resident microglia, contribute to cancer growth and progression. Currently, little is known about the function of LAT1 in the reprogramming of the immune component of the tumour microenvironment in the context of GBM. In this article, we review the available data on the role of LAT1 in the regulation of GBM biology, including its potential role in the tumour microenvironment, particularly in infiltrating-peripheral immune cells and resident microglial cells. In addition, we review the available data on the main pharmacological inhibitors of LAT1, aiming to evaluate their possible role as novel therapeutics for GBM.

Functional Characterization of the Solute Carrier LAT-1 (SLC7A5/SLC2A3) in Human Brain Capillary Endothelial Cells with Rapid UPLC-MS/MS Quantification of Intracellular Isotopically Labelled L-Leucine

The solute carrier L-type amino acid transporter 1 (LAT-1/SLC7A5) is a viable target for drug delivery to the central nervous system (CNS) and tumors due to its high abundance at the blood-brain barrier and in tumor tissue. LAT-1 is only localized on the cell surface as a heterodimer with CD98, which is not required for transporter function. To support future CNS drug-delivery development based on LAT-1 targeting, we established an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) assay for stable isotopically labeled leucine ([¹³C₆, ¹⁵N]-L-leucine), with a dynamic range of 0.1-1000 ng/mL that can be applied for the functional testing of LAT-1 activity when combined with specific inhibitors and, consequently, the LAT-1 inhibition capacity of new compounds. The assay was established in a 96-well format, facilitating high-throughput experiments, and, hence, can support the screening for novel inhibitors. Applicable recommendations of the US Food and Drug Administration and European Medicines Agency for bioanalytical method validation were followed to validate the assay. The assay was applied to investigate the IC₅₀ of two well-known LAT-1 inhibitors on hCMEC/D3 cells: the highly specific LAT-1 inhibitor JPH203, which was also used to demonstrate LAT-1 specific uptake, and the general system L inhibitor BCH. In addition, the [¹³C₆, ¹⁵N]-L-leucine uptake was determined on two human brain capillary endothelial cell lines (NKIM-6 and hCMEC/D3), which were characterized for their expressional differences of LAT-1 at the protein and mRNA level and the surface amount of CD98. The IC₅₀ values of the inhibitors were in concordance with previously reported values. Furthermore, the [¹³C₆, ¹⁵N]-L-leucine uptake was significantly higher in hCMEC/D3 cells compared to NKIM-6 cells, which correlated with higher expression of LAT-1 and a higher surface amount of CD98. Therefore, the UPLC-MS/MS quantification of ([¹³C₆, ¹⁵N]-L-leucine is a feasible strategy for the functional characterization of LAT-1 activity in cells or tissue.

PET Imaging in Neuro-Oncology: An Update and Overview of a Rapidly Growing Area

Simple Summary

Positron emission tomography (PET) is a functional imaging technique which plays an increasingly important role in the management of brain tumors. Owing different radiotracers, PET allows to image different metabolic aspects of the brain tumors. This review outlines currently available PET radiotracers and their respective indications in neuro-oncology. It specifically focuses on the investigation of gliomas, meningiomas, primary central nervous system lymphomas as well as brain metastases. Recent advances in the production of PET radiotracers, image analyses and translational applications to peptide radionuclide receptor therapy, which allow to treat brain tumors with radiotracers, are also discussed. The objective of this review is to provide a comprehensive overview of PET imaging’s potential in neuro-oncology as an adjunct to brain magnetic resonance imaging (MRI).

Abstract

PET plays an increasingly important role in the management of brain tumors. This review outlines currently available PET radiotracers and their respective indications. It specifically focuses on ¹⁸F-FDG, amino acid and somatostatin receptor radiotracers, for imaging gliomas, meningiomas, primary central nervous system lymphomas as well as brain metastases. Recent advances in radiopharmaceuticals, image analyses and translational applications to therapy are also discussed. The objective of this review is to provide a comprehensive overview of PET imaging’s potential in neuro-oncology as an adjunct to brain MRI for all medical professionals implicated in brain tumor diagnosis and care.

L-type amino acid transporter (LAT) 1 expression in 18F-FET-negative gliomas

Background

O-(2-[¹⁸F]-fluoroethyl)-L-tyrosine (¹⁸F-FET) is a highly sensitive PET tracer for glioma imaging, and its uptake is suggested to be driven by an overexpression of the L-type amino-acid transporter 1 (LAT1). However, 30% of low- and 5% of high-grade gliomas do not present enhanced ¹⁸F-FET uptake at primary diagnosis (“¹⁸F-FET-negative gliomas”) and the pathophysiologic basis for this phenomenon remains unclear. The aim of this study was to determine the expression of LAT1 in a homogeneous group of newly diagnosed ¹⁸F-FET-negative gliomas and to compare them to a matched group of ¹⁸F-FET-positive gliomas. Forty newly diagnosed IDH-mutant astrocytomas without 1p/19q codeletion were evaluated (n = 20 ¹⁸F-FET-negative (tumour-to-background ratio (TBR) < 1.6), n = 20 ¹⁸F-FET-positive gliomas (TBR > 1.6)). LAT1 immunohistochemistry (IHC) was performed using SLC7A5/LAT1 antibody. The percentage of LAT1-positive tumour cells (%) and the staining intensity (range 0–2) were multiplied to an overall score (H-score; range 0–200) and correlated to PET findings as well as progression-free survival (PFS).

Results

IHC staining of LAT1 expression was positive in both, ¹⁸F-FET-positive as well as ¹⁸F-FET-negative gliomas. No differences were found between the ¹⁸F-FET-negative and ¹⁸F-FET-positive group with regard to percentage of LAT1-positive tumour cells, staining intensity or H-score. Interestingly, the LAT1 expression showed a significant negative correlation with the PFS (p = 0.031), whereas no significant correlation was found for TBR_max, neither in the overall group nor in the ¹⁸F-FET-positive group only (p = 0.651 and p = 0.140).

Conclusion

Although LAT1 is reported to mediate the uptake of ¹⁸F-FET into tumour cells, the levels of LAT1 expression do not correlate with the levels of ¹⁸F-FET uptake in IDH-mutant astrocytomas. In particular, the lack of tracer uptake in ¹⁸F-FET-negative gliomas cannot be explained by a reduced LAT1 expression. A higher LAT1 expression in IDH-mutant astrocytomas seems to be associated with a short PFS. Further studies regarding mechanisms influencing the uptake of ¹⁸F-FET are necessary.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00865-9.

Evaluation of 3-l- and 3-d-[18F]Fluorophenylalanines as PET Tracers for Tumor Imaging

Simple Summary

The early detection and treatment of malignant brain tumors can significantly improve the survival time and life quality of affected patients. Whereas positron emission tomography (PET) with O-(2-[¹⁸F]fluoroethyl)tyrosine ([¹⁸F]FET) offers improved diagnostic accuracy compared to other imaging methods, there is still a need for PET tracers with better tumor-specificity. A higher protein incorporation rate, as well as a higher affinity for the amino acid transporter LAT1, could provide probes with superior image quality compared to [¹⁸F]FET. The aim of the present study was a preclinical evaluation of the two enantiomeric phenylalanine (Phe) analogues, 3-l- and 3-d-[¹⁸F]fluorophenylalanine ([¹⁸F]FPhes), as possible alternatives to [¹⁸F]FET. Based on promising in vitro evaluation results, the radiolabeled amino acids were studied in vivo in two subcutaneous and one orthotopic rodent tumor xenograft models using µPET. The results show that 3-l- and 3-d-[¹⁸F]FPhe enable high-quality visualization of tumors with certain advantages over [¹⁸F]FET, making them promising candidates for further preclinical and clinical evaluations.

Abstract

Purpose: The preclinical evaluation of 3-l- and 3-d-[¹⁸F]FPhe in comparison to [¹⁸F]FET, an established tracer for tumor imaging. Methods: In vitro studies were conducted with MCF-7, PC-3, and U87 MG human tumor cell lines. In vivo µPET studies were conducted in healthy rats with/without the inhibition of peripheral aromatic l-amino acid decarboxylase by benserazide pretreatment (n = 3 each), in mice bearing subcutaneous MCF-7 or PC-3 tumor xenografts (n = 10), and in rats bearing orthotopic U87 MG tumor xenografts (n = 14). Tracer accumulation was quantified by SUV_max, SUV_mean and tumor-to-brain ratios (TBrR). Results: The uptake of 3-l-[¹⁸F]FPhe in MCF-7 and PC-3 cells was significantly higher relative to [¹⁸F]FET. The uptake of all three tracers was significantly reduced by the suppression of amino acid transport systems L or ASC. 3-l-[¹⁸F]FPhe but not 3-d-[¹⁸F]FPhe exhibited protein incorporation. In benserazide-treated healthy rats, brain uptake after 42–120 min was significantly higher for 3-d-[¹⁸F]FPhe vs. 3-l-[¹⁸F]FPhe. [¹⁸F]FET showed significantly higher uptake into subcutaneous MCF-7 tumors (52–60 min p.i.), while early uptake into orthotopic U87 MG tumors was significantly higher for 3-l-[¹⁸F]FPhe (SUV_max: 3-l-[¹⁸F]FPhe, 107.6 ± 11.3; 3-d-[¹⁸F]FPhe, 86.0 ± 4.3; [¹⁸F]FET, 90.2 ± 7.7). Increased tumoral expression of LAT1 and ASCT2 was confirmed immunohistologically. Conclusion: Both novel tracers enable accurate tumor delineation with an imaging quality comparable to [¹⁸F]FET.

Targeting Amino Acid Metabolic Reprogramming via L-Type Amino Acid Transporter 1 (LAT1) for Endocrine-Resistant Breast Cancer

The PI3K/Akt/mTOR pathway has been well known to interact with the estrogen receptor (ER)-pathway and to be also frequently upregulated in aromatase inhibitor (AI)-resistant breast cancer patients. Intracellular levels of free amino acids, especially leucine, regulate the mammalian target of rapamycin complex 1 (mTORC1) activation. L-type amino acid transporters such as LAT1 and LAT3 are associated with the uptake of essential amino acids. LAT1 expression could mediate leucine uptake, mTORC1 signaling, and cell proliferation. Therefore, in this study, we explored amino acid metabolism, including LAT1, in breast cancer and clarified the potential roles of LAT1 in the development of therapeutic resistance and the eventual clinical outcome of the patients. We evaluated LAT1 and LAT3 expression before and after neoadjuvant hormone therapy (NAH) and examined LAT1 function and expression in estrogen deprivation-resistant (EDR) breast carcinoma cell lines. Tumors tended to be in advanced stages in the cases whose LAT1 expression was high. LAT1 expression in the EDR cell lines was upregulated. JPH203, a selective LAT1 inhibitor, demonstrated inhibitory effects on cell proliferation in EDR cells. Hormone therapy changed the tumor microenvironment and resulted in metabolic reprogramming through inducing LAT1 expression. LAT1 expression then mediated leucine uptake, enhanced mTORC1 signaling, and eventually resulted in AI resistance. Therefore, LAT1 could be the potential therapeutic target in AI-resistant breast cancer patients.

A systematic review of the utility of amino acid PET in assessing treatment response to bevacizumab in recurrent high-grade glioma

Background. Currently, bevacizumab (BEV), an antiangiogenic agent, is used as an adjunctive therapy to re-irradiation and surgery in patients with recurrent high-grade gliomas (rHGG). BEV has shown to decrease enhancement on MRI, but it is often unclear if these changes are due to tumor response to BEV or treatment-induced changes in the blood brain barrier. Preliminary studies show that amino acid PET can aid in distinguishing these changes on MRI.

Methods. The authors performed a systematic review of PubMed and Embase through July 2020 with the search terms ‘bevacizumab’ or ‘Avastin’ and ‘recurrent glioma’ and ‘PET,’ yielding 38 papers, with 14 meeting inclusion criteria.

Results. Thirteen out of fourteen studies included in this review used static PET and three studies used dynamic PET to evaluate the use of BEV in rHGG. Six studies used the amino acid tracer [18F]FET, four studies used [11C]MET, and four studies used [18F]FDOPA.

Conclusion. [18F]FET, [11C]MET, and [18F]FDOPA PET in combination with MRI have shown promising results for improving accuracy in diagnosing tumor recurrence, detecting early treatment failure, and distinguishing between tumor progression and treatment-induced changes in patients with rHGG treated with BEV.

Brain FET PET tumor-to-white mater ratio to differentiate recurrence from post-treatment changes in high-grade gliomas

BACKGROUND AND PURPOSE

Highergrade glial neoplasms undergo standard treatment with surgery, radiotherapy, and alkylating agents. There is often a clinical/neuroimaging dilemma in the post-treatment setting to differentiate disease recurrence from treatment-related changes. FET (fluoro-ethyl-tyrosine) PET has emerged as a molecular imaging modality for cases where MR imaging is inconclusive. This study aims to develop a cutoff on FET PET for differentiating true recurrence from post-treatment changes.

METHODS

We retrospectively analyzed72 patientswith post-treatment grade 3 or 4 brain gliomas. Five to six mCi of ¹⁸ F-FET was injected and static imaging of the brain was performed at 20 min. A tumor-to-white matter (T/Wm) ratio was used as semiquantitative parameter. A T/Wm cutoff of 2.5 was used for image interpretation. Imaging findings were confirmed by either histopathologic diagnosis in a multidisciplinary joint clinic or based on follow-up of clinical and neuroimaging findings.

RESULTS

Forty-one of 72 patients (57%) showed recurrent disease on FET PET. Thirty-five of them were confirmed to have tumor recurrence; six patients showed post-treatment changes. Thirty-one of 72 patients (43%) showed post-treatment changes on FET PET; 27 were confirmed as post-treatment change and four patients had tumor recurrence on subsequent MR imaging. An optimum T/Wm cutoff of 2.65 was derived based on receiver operating characteristic analysis with a sensitivity of 80% and specificity of 87.5%.

CONCLUSION

Static FET PET can be used as problem-solving imaging modality with a T/Wm cutoff of 2.65 to differentiate late recurrence from post-treatment changes in grade 3 or 4 brain gliomas with equivocal MR features.

A Linearized Fit Model for Robust Shape Parameterization of FET-PET TACs

The kinetic analysis of [Formula: see text]-FET time-activity curves (TAC) can provide valuable diagnostic information in glioma patients. The analysis is most often limited to the average TAC over a large tissue volume and is normally assessed by visual inspection or by evaluating the time-to-peak and linear slope during the late uptake phase. Here, we derived and validated a linearized model for TACs of [Formula: see text]-FET in dynamic PET scans. Emphasis was put on the robustness of the numerical parameters and how reliably automatic voxel-wise analysis of TAC kinetics was possible. The diagnostic performance of the extracted shape parameters for the discrimination between isocitrate dehydrogenase (IDH) wildtype (wt) and IDH-mutant (mut) glioma was assessed by receiver-operating characteristic in a group of 33 adult glioma patients. A high agreement between the adjusted model and measured TACs could be obtained and relative, estimated parameter uncertainties were small. The best differentiation between IDH-wt and IDH-mut gliomas was achieved with the linearized model fitted to the averaged TAC values from dynamic FET PET data in the time interval 4-50 min p.i.. When limiting the acquisition time to 20-40 min p.i., classification accuracy was only slightly lower (-3%) and was comparable to classification based on linear fits in this time interval. Voxel-wise fitting was possible within a computation time ≈ 1 min per image slice. Parameter uncertainties smaller than 80% for all fits with the linearized model were achieved. The agreement of best-fit parameters when comparing voxel-wise fits and fits of averaged TACs was very high (p < 0.001).

Long-term metabolic evolution of brain metastases with suspected radiation necrosis following stereotactic radiosurgery: longitudinal assessment by F-DOPA PET

BACKGROUND

The evolution of radiation necrosis (RN) varies depending on the combination of radionecrotic tissue and active tumor cells. In this study, we characterized the long-term metabolic evolution of RN by sequential PET/CT imaging with 3,4-dihydroxy-6-[18F]-fluoro-l-phenylalanine (F-DOPA) in patients with brain metastases following stereotactic radiosurgery (SRS).

METHODS

Thirty consecutive patients with 34 suspected radionecrotic brain metastases following SRS repeated F-DOPA PET/CT every 6 months or yearly in addition to standard MRI monitoring. Diagnoses of local progression (LP) or RN were confirmed histologically or by clinical follow-up. Semi-quantitative parameters of F-DOPA uptake were extracted at different time points, and their diagnostic performances were compared with those of corresponding contrast-enhanced MRI.

RESULTS

Ninety-nine F-DOPA PET scans were acquired over a median period of 18 (range: 12-66) months. Median follow-up from the baseline F-DOPA PET/CT was 48 (range 21-95) months. Overall, 24 (70.6%) and 10 (29.4%) lesions were classified as RN and LP, respectively. LP occurred after a median of 18 (range: 12-30) months from baseline PET. F-DOPA tumor-to-brain ratio (TBR) and relative standardized uptake value (rSUV) increased significantly over time in LP lesions, while remaining stable in RN lesions. The parameter showing the best diagnostic performance was rSUV (accuracy = 94.1% for the optimal threshold of 1.92). In contrast, variations of the longest tumor dimension measured on contrast-enhancing MRI did not distinguish between RN and LP.

CONCLUSION

F-DOPA PET has a high diagnostic accuracy for assessing the long-term evolution of brain metastases following SRS.

Voxelwise Principal Component Analysis of Dynamic [S-Methyl-11C]Methionine PET Data in Glioma Patients

Recent works have demonstrated the added value of dynamic amino acid positron emission tomography (PET) for glioma grading and genotyping, biopsy targeting, and recurrence diagnosis. However, most of these studies are based on hand-crafted qualitative or semi-quantitative features extracted from the mean time activity curve within predefined volumes. Voxelwise dynamic PET data analysis could instead provide a better insight into intra-tumor heterogeneity of gliomas. In this work, we investigate the ability of principal component analysis (PCA) to extract relevant quantitative features from a large number of motion-corrected [S-methyl-¹¹C]methionine ([¹¹C]MET) PET frames. We first demonstrate the robustness of our methodology to noise by means of numerical simulations. We then build a PCA model from dynamic [¹¹C]MET acquisitions of 20 glioma patients. In a distinct cohort of 13 glioma patients, we compare the parametric maps derived from our PCA model to these provided by the classical one-compartment pharmacokinetic model (1TCM). We show that our PCA model outperforms the 1TCM to distinguish characteristic dynamic uptake behaviors within the tumor while being less computationally expensive and not requiring arterial sampling. Such methodology could be valuable to assess the tumor aggressiveness locally with applications for treatment planning and response evaluation. This work further supports the added value of dynamic over static [¹¹C]MET PET in gliomas.

Diagnosis of Pseudoprogression Following Lomustine-Temozolomide Chemoradiation in Newly Diagnosed Glioblastoma Patients Using FET-PET

PURPOSE

The CeTeG/NOA-09 phase III trial demonstrated a significant survival benefit of lomustine-temozolomide chemoradiation in patients with newly diagnosed glioblastoma with methylated O⁶-methylguanine-DNA methyltransferase (MGMT) promoter. Following lomustine-temozolomide chemoradiation, late and prolonged pseudoprogression may occur. We here evaluated the value of amino acid PET using O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (FET) for differentiating pseudoprogression from tumor progression.

EXPERIMENTAL DESIGN

We retrospectively identified patients (i) who were treated off-study according to the CeTeG/NOA-09 protocol, (ii) had equivocal MRI findings after radiotherapy, and (iii) underwent additional FET-PET imaging for diagnostic evaluation (number of scans, 1-3). Maximum and mean tumor-to-brain ratios (TBR_max, TBR_mean) and dynamic FET uptake parameters (e.g., time-to-peak) were calculated. In patients with more than one FET-PET scan, relative changes of TBR values were evaluated, that is, an increase or decrease of >10% compared with the reference scan was considered as tumor progression or pseudoprogression. Diagnostic performances were evaluated using ROC curve analyses and Fisher exact test. Diagnoses were confirmed histologically or clinicoradiologically.

RESULTS

We identified 23 patients with 32 FET-PET scans. Within 5-25 weeks after radiotherapy (median time, 9 weeks), pseudoprogression occurred in 11 patients (48%). The parameter TBR_mean calculated from the FET-PET performed 10 ± 7 days after the equivocal MRI showed the highest accuracy (87%) to identify pseudoprogression (threshold, <1.95; P = 0.029). The integration of relative changes of TBR_mean further improved the accuracy (91%; P < 0.001). Moreover, the combination of static and dynamic parameters increased the specificity to 100% (P = 0.005).

CONCLUSIONS

The data suggest that FET-PET parameters are of significant clinical value to diagnose pseudoprogression related to lomustine-temozolomide chemoradiation.

The TSPO Ligands MGV-1 and 2-Cl-MGV-1 Differentially Inhibit the Cigarette Smoke-Induced Cytotoxicity to H1299 Lung Cancer Cells

Simple Summary

In this study, we investigated the impact of CS on various TSPO-related mitochondrial processes, and the protective ability of our novel TSPO ligands against such CS-induced cellular damages. Our results support the previously reported role of TSPO in apoptotic cell death. Moreover, the present data demonstrate the protective effect of our TSPO ligands against CS-induced cellular damage.

Abstract

TSPO is involved in cigarette smoke (CS)-induced cellular toxicity, which may result in oral and pulmonary diseases and lung cancer. H1299 lung cancer cells were exposed directly to CS. The H1299 cells were pretreated with our TSPO ligands MGV-1 and 2-Cl-MGV-1 (Ki = 825 nM for both) at a concentration of 25 µM 24 h prior to CS exposure. Cell death and apoptotic markers were measured, in addition to TSPO expression levels, ATP synthase activity, generation of reactive oxygen species (ROS), depolarization of mitochondrial membrane potential (ΔΨm), cAMP and LDH levels. Pretreatment with MGV-1 and 2-Cl-MGV-1 (25 µM), 24 h prior to CS exposure, differentially attenuated the CS-induced cellular insult as well as cell death in H1299 lung cancer cells. These protective effects included prevention of ATP synthase reversal, ROS generation, depolarization of the mitochondrial membrane and elevation in LDH. The preventive efficacy of 2-Cl-MGV-1 was superior to that achieved by MGV-1. Both ligands did not prevent the elevation in cAMP. These findings may indicate a mild protective effect of these TSPO ligands in CS-related pulmonary and keratinocyte cellular pathology.

TERT-Promoter Mutational Status in Glioblastoma - Is There an Association With Amino Acid Uptake on Dynamic 18F-FET PET?

Objective

The mutation of the ‘telomerase reverse transcriptase gene promoter’ (TERTp) has been identified as an important factor for individual prognostication and tumorigenesis and will be implemented in upcoming glioma classifications. Uptake characteristics on dynamic ¹⁸F-FET PET have been shown to serve as additional imaging biomarker for prognosis. However, data on the correlation of TERTp-mutational status and amino acid uptake on dynamic ¹⁸F-FET PET are missing. Therefore, we aimed to analyze whether static and dynamic ¹⁸F-FET PET parameters are associated with the TERTp-mutational status in de-novo IDH-wildtype glioblastoma and whether a TERTp-mutation can be predicted by dynamic ¹⁸F-FET PET.

Methods

Patients with de-novo IDH-wildtype glioblastoma, WHO grade IV, available TERTp-mutational status and dynamic ¹⁸F-FET PET scan prior to any therapy were included. Here, established clinical parameters maximal and mean tumor-to-background-ratios (TBR_max/TBR_mean), the biological-tumor-volume (BTV) and minimal-time-to-peak (TTP_min) on dynamic PET were analyzed and correlated with the TERTp-mutational status.

Results

One hundred IDH-wildtype glioblastoma patients were evaluated; 85/100 of the analyzed tumors showed a TERTp-mutation (C228T or C250T), 15/100 were classified as TERTp-wildtype. None of the static PET parameters was associated with the TERTp-mutational status (median TBR_max 3.41 vs. 3.32 (p=0.362), TBR_mean 2.09 vs. 2.02 (p=0.349) and BTV 26.1 vs. 22.4 ml (p=0.377)). Also, the dynamic PET parameter TTP_min did not differ in both groups (12.5 vs. 12.5 min, p=0.411). Within the TERTp-mutant subgroups (i.e., C228T (n=23) & C250T (n=62)), the median TBR_max (3.33 vs. 3.69, p=0.095), TBR_mean (2.08 vs. 2.09, p=0.352), BTV (25.4 vs. 30.0 ml, p=0.130) and TTP_min (12.5 vs. 12.5 min, p=0.190) were comparable, too.

Conclusion

Uptake characteristics on dynamic ¹⁸F-FET PET are not associated with the TERTp-mutational status in glioblastoma However, as both, dynamic ¹⁸F-FET PET parameters as well as the TERTp-mutation status are well-known prognostic biomarkers, future studies should investigate the complementary and independent prognostic value of both factors in order to further stratify patients into risk groups.

18F-FACBC PET/MRI in the evaluation of human brain metastases: a case report

Background

Patients with metastatic cancer to the brain have a poor prognosis. In clinical practice, MRI is used to delineate, diagnose and plan treatment of brain metastases. However, MRI alone is limited in detecting micro-metastases, delineating lesions and discriminating progression from pseudo-progression. Combined PET/MRI utilises superior soft tissue images from MRI and metabolic data from PET to evaluate tumour structure and function. The amino acid PET tracer ¹⁸F-FACBC has shown promising results in discriminating high- and low-grade gliomas, but there are currently no reports on its use on brain metastases. This is the first study to evaluate the use of ¹⁸F-FACBC on brain metastases.

Case presentation

A middle-aged female patient with brain metastases was evaluated using hybrid PET/MRI with ¹⁸F-FACBC before and after stereotactic radiotherapy, and at suspicion of recurrence. Static/dynamic PET and contrast-enhanced T1 MRI data were acquired and analysed. This case report includes the analysis of four ¹⁸F-FACBC PET/MRI examinations, investigating their utility in evaluating functional and structural metastasis properties.

Conclusion

Analysis showed high tumour-to-background ratios in brain metastases compared to other amino acid PET tracers, including high uptake in a very small cerebellar metastasis, suggesting that ¹⁸F-FACBC PET can provide early detection of otherwise overlooked metastases. Further studies to determine a threshold for ¹⁸F-FACBC brain tumour boundaries and explore its utility in clinical practice should be performed.

Advanced Imaging Techniques for Radiotherapy Planning of Gliomas

The accuracy of target delineation in radiation treatment (RT) planning of cerebral gliomas is crucial to achieve high tumor control, while minimizing treatment-related toxicity. Conventional magnetic resonance imaging (MRI), including contrast-enhanced T1-weighted and fluid-attenuated inversion recovery (FLAIR) sequences, represents the current standard imaging modality for target volume delineation of gliomas. However, conventional sequences have limited capability to discriminate treatment-related changes from viable tumors, owing to the low specificity of increased blood-brain barrier permeability and peritumoral edema. Advanced physiology-based MRI techniques, such as MR spectroscopy, diffusion MRI and perfusion MRI, have been developed for the biological characterization of gliomas and may circumvent these limitations, providing additional metabolic, structural, and hemodynamic information for treatment planning and monitoring. Radionuclide imaging techniques, such as positron emission tomography (PET) with amino acid radiopharmaceuticals, are also increasingly used in the workup of primary brain tumors, and their integration in RT planning is being evaluated in specialized centers. This review focuses on the basic principles and clinical results of advanced MRI and PET imaging techniques that have promise as a complement to RT planning of gliomas.

Inhibition of Arginase 1 Liberates Potent T Cell Immunostimulatory Activity of Human Neutrophil Granulocytes

Myeloid cell arginase-mediated arginine depletion with consecutive inhibition of T cell functions is a key component of tumor immune escape. Both, granulocytic myeloid-derived suppressor cells (G-MDSC) and conventional mature human polymorphonuclear neutrophil granulocytes (PMN) express high levels of arginase 1 and can act as suppressor cells of adaptive anti-cancer immunity. Here we demonstrate that pharmacological inhibition of PMN-derived arginase 1 not only prevents the suppression of T cell functions but rather leads to a strong hyperactivation of T cells. Human PMN were incubated in cell culture medium in the absence or presence of an arginase inhibitor. T cells from healthy donors were then activated either polyclonally or in an antigen-specific manner in the supernatants of the PMN cultures at different PMN-T cell ratios. T cell proliferation was completely suppressed in these supernatants in the absence of an arginase inhibitor. Arginase inhibition led to a strong hyperinduction of T cell proliferation, which exceeded control activation conditions up to 25-fold. The hyperinduction was correlated with higher PMN-T cell ratios and was only apparent when PMN arginase activity was blocked sufficiently. The T cell stimulatory factor was liberated very early by PMN and was present in the < 3 kDa fraction of the PMN supernatants. Increased T cell production of specific proinflammatory cytokines by PMN supernatant in the presence of arginase inhibitor was apparent. Upon arginase inhibition, downregulation of important T cell membrane activation and costimulation proteins was completely prevented or de novo induction accelerated. Antigen-specific T cell cytotoxicity against tumor cells was enhanced by PMN supernatant itself and could be further increased by PMN arginase blockade. Finally, we analyzed anergic T cells from multiple myeloma patients and noticed a complete reversal of anergy and the induction of strong proliferation upon T cell activation in PMN supernatants by arginase inhibition. In summary, we discovered a potent PMN-mediated hyperactivation of human T cells, which is apparent only when PMN arginase-mediated arginine depletion is concurrently inhibited. Our findings are clearly relevant for the analysis and prevention of human tumor immune escape in conjunction with the application of arginase inhibitors already being developed clinically.

Advanced imaging techniques for neuro-oncologic tumor diagnosis, with an emphasis on PET-MRI imaging of malignant brain tumors

PURPOSE OF REVIEW

This review will explore the latest in advanced imaging techniques, with a focus on the complementary nature of multiparametric, multimodality imaging using magnetic resonance imaging (MRI) and positron emission tomography (PET).

RECENT FINDINGS

Advanced MRI techniques including perfusion-weighted imaging (PWI), MR spectroscopy (MRS), diffusion-weighted imaging (DWI), and MR chemical exchange saturation transfer (CEST) offer significant advantages over conventional MR imaging when evaluating tumor extent, predicting grade, and assessing treatment response. PET performed in addition to advanced MRI provides complementary information regarding tumor metabolic properties, particularly when performed simultaneously. 18F-fluoroethyltyrosine (FET) PET improves the specificity of tumor diagnosis and evaluation of post-treatment changes. Incorporation of radiogenomics and machine learning methods further improve advanced imaging. The complementary nature of combining advanced imaging techniques across modalities for brain tumor imaging and incorporating technologies such as radiogenomics has the potential to reshape the landscape in neuro-oncology.

18F-Boramino acid PET/CT in healthy volunteers and glioma patients

PURPOSE

In this work, the safety, biodistribution, and radiation dosimetry of large neutral amino acid transporter type-1 (LAT-1) targeting PET tracer ¹⁸F-trifluorobborate-derived tyrosine (denoted as ¹⁸F-FBY) has been investigated. It is designed as a first-in-human study in healthy volunteers and to assay LAT-1 expression level in glioma patients.

METHODS

Six healthy volunteers (3 M, 3 F) underwent whole-body PET acquisitions at multiple time points after bolus injection of ¹⁸F-FBY. Regions of interest (ROIs) were mapped manually on major organs, and then the time-activity curves (TACs) were obtained. Dosimetry was calculated with the OLINDA/EXM software. Thirteen patients who were suspected of glioma were scanned with PET/CT at 30 min after ¹⁸F-FBY injection. Within 7 days after PET/CT, the tumor was removed surgically, and LAT-1 immunohistochemical staining for LAT-1 was performed on tumor samples and correlated with ¹⁸F-FBY PET imaging.

RESULTS

¹⁸F-FBY was well tolerated by all healthy volunteers, and no adverse symptoms were observed or reported. ¹⁸F-FBY is rapidly cleared from the blood circulation and excreted mainly through the kidneys and urinary tract. The effective dose (ED) was 0.0039 ± 0.0006 mSv/MBq. In 14 surgical confirmed gliomas (one of the patiens had two gliomas), ¹⁸F-FBY uptake increased consistently with tumor grade, with maximum standard uptake values (SUV_max) of 0.28 ± 0.14 and 2.84 ± 0.46 and tumor-to-normal contralateral activity (T/N) ratio of 2.30 ± 1.26 and 24.56 ± 6.32 in low- and high-grade tumors, respectively. In addition to the significant difference in the uptakes between low- and high-grade gliomas (P < 0.001), the immunohistochemical staining confirmed the positive correlations between the SUV_max, LAT-1 expression (r² = 0.80, P < 0.001), and Ki-67 labeling index (r² = 0.79, P < 0.001).

CONCLUSION

¹⁸F-FBY is a PET tracer with favorable dosimetry profile and pharmacokinetics. It has the potential to assay LAT-1 expression in glioma patients and may provide imaging guidance for further boron neutron capture therapy of gliomas.

TRIAL REGISTRATION

clinicaltrials.gov (NCT03980431).

The IMiD target CRBN determines HSP90 activity toward transmembrane proteins essential in multiple myeloma

The complex architecture of transmembrane proteins requires quality control (QC) of folding, membrane positioning, and trafficking as prerequisites for cellular homeostasis and intercellular communication. However, it has remained unclear whether transmembrane protein-specific QC hubs exist. Here we identify cereblon (CRBN), the target of immunomodulatory drugs (IMiDs), as a co-chaperone that specifically determines chaperone activity of HSP90 toward transmembrane proteins by means of counteracting AHA1. This function is abrogated by IMiDs, which disrupt the interaction of CRBN with HSP90. Among the multiple transmembrane protein clients of CRBN-AHA1-HSP90 revealed by cell surface proteomics, we identify the amino acid transporter LAT1/CD98hc as a determinant of IMiD activity in multiple myeloma (MM) and present an Anticalin-based CD98hc radiopharmaceutical for MM radio-theranostics. These data establish the CRBN-AHA1-HSP90 axis in the biogenesis of transmembrane proteins, link IMiD activity to tumor metabolism, and nominate CD98hc and LAT1 as attractive diagnostic and therapeutic targets in MM.

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CRBN functions as a transmembrane protein-specific co-chaperone of HSP90

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Disruption of CRBN-HSP90 interaction determines the anti-tumor activity of IMiDs

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The CD98hc/LAT1 complex is a central target of IMiDs in multiple myeloma

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CD98hc-Anticalin is a theranostic tool in multiple myeloma

Amino Acid Transporters on the Guard of Cell Genome and Epigenome

Tumorigenesis is driven by metabolic reprogramming. Oncogenic mutations and epigenetic alterations that cause metabolic rewiring may also upregulate the reactive oxygen species (ROS). Precise regulation of the intracellular ROS levels is critical for tumor cell growth and survival. High ROS production leads to the damage of vital macromolecules, such as DNA, proteins, and lipids, causing genomic instability and further tumor evolution. One of the hallmarks of cancer metabolism is deregulated amino acid uptake. In fast-growing tumors, amino acids are not only the source of energy and building intermediates but also critical regulators of redox homeostasis. Amino acid uptake regulates the intracellular glutathione (GSH) levels, endoplasmic reticulum stress, unfolded protein response signaling, mTOR-mediated antioxidant defense, and epigenetic adaptations of tumor cells to oxidative stress. This review summarizes the role of amino acid transporters as the defender of tumor antioxidant system and genome integrity and discusses them as promising therapeutic targets and tumor imaging tools.

Imaging temozolomide-induced changes in the myeloid glioma microenvironment

Rationale: The heterogeneous nature of gliomas makes the development and application of novel treatments challenging. In particular, infiltrating myeloid cells play a role in tumor progression and therapy resistance. Hence, a detailed understanding of the dynamic interplay of tumor cells and immune cells in vivo is necessary. To investigate the complex interaction between tumor progression and therapy-induced changes in the myeloid immune component of the tumor microenvironment, we used a combination of [¹⁸F]FET (amino acid metabolism) and [¹⁸F]DPA-714 (TSPO, GAMMs, tumor cells, astrocytes, endothelial cells) PET/MRI together with immune-phenotyping. The aim of the study was to monitor temozolomide (TMZ) treatment response and therapy-induced changes in the inflammatory tumor microenvironment (TME).

Methods: Eighteen NMRI^nu/nu mice orthotopically implanted with Gli36dEGFR cells underwent MRI and PET/CT scans before and after treatment with TMZ or DMSO (vehicle). Tumor-to-background (striatum) uptake ratios were calculated and areas of unique tracer uptake (FET vs. DPA) were determined using an atlas-based volumetric approach.

Results: TMZ therapy significantly modified the spatial distribution and uptake of both tracers. [¹⁸F]FET uptake was significantly reduced after therapy (-53 ± 84%) accompanied by a significant decrease of tumor volume (-17 ± 6%). In contrast, a significant increase (61 ± 33%) of [¹⁸F]DPA-714 uptake was detected by TSPO imaging in specific areas of the tumor. Immunohistochemistry (IHC) validated the reduction in tumor volumes and further revealed the presence of reactive TSPO-expressing glioma-associated microglia/macrophages (GAMMs) in the TME.

Conclusion: We confirm the efficiency of [¹⁸F]FET-PET for monitoring TMZ-treatment response and demonstrate that in vivo TSPO-PET performed with [¹⁸F]DPA-714 can be used to identify specific reactive areas of myeloid cell infiltration in the TME.

Current trends in the use of O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) in neurooncology

The diagnostic potential of PET using the amino acid analogue O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET) in brain tumor diagnostics has been proven in many studies during the last two decades and is still the subject of multiple studies every year. In addition to standard magnetic resonance imaging (MRI), positron emission tomography (PET) using [18F]FET provides important diagnostic data concerning brain tumor delineation, therapy planning, treatment monitoring, and improved differentiation between treatment-related changes and tumor recurrence. The pharmacokinetics, uptake mechanisms and metabolism have been well described in various preclinical studies. The accumulation of [18F]FET in most benign lesions and healthy brain tissue has been shown to be low, thus providing a high contrast between tumor tissue and benign tissue alterations. Based on logistic advantages of F-18 labelling and convincing clinical results, [18F]FET has widely replaced short lived amino acid tracers such as L-[11C]methyl-methionine ([11C]MET) in many centers across Western Europe. This review summarizes the basic knowledge on [18F]FET and its contribution to the care of patients with brain tumors. In particular, recent studies about specificity, possible pitfalls, and the utility of [18F]FET PET in tumor grading and prognostication regarding the revised WHO classification of brain tumors are addressed.

FET PET Radiomics for Differentiating Pseudoprogression from Early Tumor Progression in Glioma Patients Post-Chemoradiation

Simple Summary

Following chemoradiation with alkylating agents in glioma patients, structural magnetic resonance imaging (MRI) may suggest tumor progression which subsequently improves during the course of the disease without any treatment change. This phenomenon has been termed pseudoprogression. Despite advances in medical imaging, a reliable diagnosis of pseudoprogression remains a challenging task. Radiomics is a subdiscipline of artificial intelligence and allows the identification and extraction of imaging features from various routine imaging modalities. These features can be used for the generation of mathematical models to improve diagnostics in patients with brain tumors. The present study highlights the potential of radiomics obtained from amino acid positron emission tomography (PET) for the diagnosis of pseudoprogression. In 34 patients with suspicious MRI early after chemoradiation completion, our radiomics model correctly identified all patients with pseudoprogression.

Abstract

Currently, a reliable diagnostic test for differentiating pseudoprogression from early tumor progression is lacking. We explored the potential of O-(2-[¹⁸F]fluoroethyl)-L-tyrosine (FET) positron emission tomography (PET) radiomics for this clinically important task. Thirty-four patients (isocitrate dehydrogenase (IDH)-wildtype glioblastoma, 94%) with progressive magnetic resonance imaging (MRI) changes according to the Response Assessment in Neuro-Oncology (RANO) criteria within the first 12 weeks after completing temozolomide chemoradiation underwent a dynamic FET PET scan. Static and dynamic FET PET parameters were calculated. For radiomics analysis, the number of datasets was increased to 102 using data augmentation. After randomly assigning patients to a training and test dataset, 944 features were calculated on unfiltered and filtered images. The number of features for model generation was limited to four to avoid data overfitting. Eighteen patients were diagnosed with early tumor progression, and 16 patients had pseudoprogression. The FET PET radiomics model correctly diagnosed pseudoprogression in all test cohort patients (sensitivity, 100%; negative predictive value, 100%). In contrast, the diagnostic performance of the best FET PET parameter (TBR_max) was lower (sensitivity, 81%; negative predictive value, 80%). The results suggest that FET PET radiomics helps diagnose patients with pseudoprogression with a high diagnostic performance. Given the clinical significance, further studies are warranted.

Effects of Photons Irradiation on 18F-FET and 18F-DOPA Uptake by T98G Glioblastoma Cells

The differential diagnosis between brain tumors recurrence and early neuroinflammation or late radionecrosis is still an unsolved problem. The new emerging magnetic resonance imaging, computed tomography, and positron emission tomography diagnostic modalities still lack sufficient accuracy. In the last years, a great effort has been made to develop radiotracers able to detect specific altered metabolic pathways or tumor receptor markers. Our research project aims to evaluate irradiation effects on radiopharmaceutical uptake and compare the kinetic of the fluorinate tracers. T98G glioblastoma cells were irradiated at doses of 2, 10, and 20 Gy with photons, and ¹⁸F-DOPA and ¹⁸F-FET tracer uptake was evaluated. Activity and cell viability at different incubation times were measured. ¹⁸F-FET and ¹⁸F-DOPA are accumulated via the LAT-1 transporter, but ¹⁸F-DOPA is further incorporated, whereas ¹⁸F-FET is not metabolized. Therefore, time-activity curves (TACs) tend to plateau with ¹⁸F-DOPA and to a rapid washout with ¹⁸F-FET. After irradiation, ¹⁸F-DOPA TAC resembles the ¹⁸F-FET pattern. ¹⁸F-DOPA activity peak we observed at 20 min might be fictitious, because earlier time points have not been evaluated, and a higher activity peak before 20 min cannot be excluded. In addition, the activity retained in the irradiated cells remains higher in comparison to the sham ones at all time points investigated. This aspect is similar in the ¹⁸F-FET TAC but less evident. Therefore, we can hypothesize the presence of a second intracellular compartment in addition to the amino acidic pool one governed by LAT-1, which could explain the progressive accumulation of ¹⁸F-DOPA in unirradiated cells.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

The bibliography includes 159 references.

Photopenic defects on O-(2-[18F]-fluoroethyl)-L-tyrosine PET: clinical relevance in glioma patients

BACKGROUND

O-(2-[18F]-fluoroethyl)-L-tyrosine (FET) PET has a sensitivity of more than 90% to detect gliomas. In the remaining small fraction of gliomas without increased tracer uptake, some tumors even show photopenic defects whose clinical significance is unclear.

METHODS

Glioma patients with a negative FET PET scan prior to neuropathological confirmation were identified retrospectively. Gliomas were rated visually as (i) having indifferent FET uptake or (ii) photopenic, if FET uptake was below background activity. FET uptake in the area of signal hyperintensity on the T2/fluid attenuated inversion recovery-weighted MRI was evaluated by mean standardized uptake value (SUV) and mean tumor-to-brain ratio (TBR). The progression-free survival (PFS) of photopenic gliomas was compared with that of gliomas with indifferent FET uptake.

RESULTS

Of 100 FET-negative gliomas, 40 cases with photopenic defects were identified. Fifteen of these 40 cases (38%) had World Health Organization (WHO) grades III and IV gliomas. FET uptake in photopenic gliomas was significantly decreased compared with both the healthy-appearing brain tissue (SUV, 0.89 ± 0.26 vs 1.08 ± 0.23; P < 0.001) and gliomas with indifferent FET uptake (TBR, 0.82 ± 0.09 vs 0.96 ± 0.13; P < 0.001). Irrespective of the applied treatment, isocitrate dehydrogenase (IDH)-mutated WHO grade II diffuse astrocytoma patients with indifferent FET uptake (n = 25) had a significantly longer PFS than patients with IDH-mutated diffuse astrocytomas (WHO grade II) with photopenic defects (n = 11) (51 vs 24 mo; P = 0.027). The multivariate survival analysis indicated that photopenic defects predict an unfavorable PFS (P = 0.009).

CONCLUSION

Photopenic gliomas in negative FET PET scans should be managed more actively, as they seem to have a higher risk of harboring a higher-grade glioma and an unfavorable outcome.

L-Type amino acid transporter 1 as a target for drug delivery

Our growing understanding of membrane transporters and their substrate specificity has opened a new avenue in the field of targeted drug delivery. The L-type amino acid transporter 1 (LAT1) has been one of the most extensively investigated transporters for delivering drugs across biological barriers. The transporter is predominantly expressed in cerebral cortex, blood-brain barrier, blood-retina barrier, testis, placenta, bone marrow and several types of cancer. Its physiological function is to mediate Na⁺ and pH independent exchange of essential amino acids: leucine, phenylalanine, etc. Several drugs and prodrugs designed as LAT1 substrates have been developed to improve targeted delivery into the brain and cancer cells. Thus, the anti-parkinsonian drug, L-Dopa, the anti-cancer drug, melphalan and the anti-epileptic drug gabapentin, all used in clinical practice, utilize LAT1 to reach their target site. These examples provide supporting evidence for the utility of the LAT1-mediated targeted delivery of the (pro)drug. This review comprehensively summarizes recent advances in LAT1-mediated targeted drug delivery. In addition, the use of LAT1 is critically evaluated and limitations of the approach are discussed.

Boron Accumulation in Brain Tumor Cells through Boc-Protected Tryptophan as a Carrier for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary therapeutic approach. Nonradioactive boron-10 atoms accumulated in tumor cells combining with the neutron beams produce two highly energetic particles that could eradicate the cell that takes it and the neighboring cells. Small molecules that carry boron atom, e.g. 5- and 6-boronated and 2,7-diboronated tryptophans, were assessed for their boron accumulation in U87-MG, LN229, and 3T3 for BNCT. TriBoc tryptophan, TB-6-BT, shows boron-10 at 300 ppm in both types of tumor cells with a tumor to normal ratio (T/N) of 5.19-5.25 (4 h). TB-5-BT and DBA-5-BT show boron-10 at 300 ppm (2 h) in U87-MG cells. TB-5-BT exerts a T/N of >9.66 (1 h) in LN229 compared with the current clinical boronophenyl alanine with a highest T/N of 2.3 (1 h) and accumulation concentration of <50 ppm. TB-5-BT and TB-6-BT warrant further animal study.