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

LAT1 expression in colorectal cancer cells is unresponsive to HIF-1/2α accumulation under experimental hypoxia

L-type amino acid transporter 1 (LAT1) is upregulated in various cancer types and contributes to disease progression. Previous studies have demonstrated or suggested that hypoxia-inducible factors (HIFs), the key transcription factors in hypoxic responses, control the expression of LAT1 gene in several types of cancer cells. However, this regulatory relationship has not been investigated yet in colorectal cancer (CRC), one of the cancer types in which the increased LAT1 expression holds prognostic significance. In this study, we found that neither LAT1 mRNA nor protein is induced under hypoxic condition (1% O2) in CRC HT-29 cells in vitro, regardless of the prominent HIF-1/2α accumulation and HIFs-dependent upregulation of glucose transporter 1. The hypoxic treatment generally did not increase either the mRNA or protein expression of LAT1 in eight CRC cell lines tested, in contrast to the pronounced upregulation by amino acid restriction. Interestingly, knockdown of von Hippel-Lindau ubiquitin ligase to inhibit the proteasomal degradation of HIFs caused an accumulation of HIF-2α and increased the LAT1 expression in certain CRC cell lines. This study contributes to delineating the molecular mechanisms responsible for the pathological expression of LAT1 in CRC cells, emphasizing the ambiguity of HIFs-dependent transcriptional upregulation of LAT1 across cancer cells.

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.

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.

PET Imaging and Protein Expression of Prostate-Specific Membrane Antigen in Glioblastoma: A Multicenter Inventory Study

Upregulation of prostate-specific membrane antigen (PSMA) in neovasculature has been described in glioblastoma multiforme (GBM), whereas vasculature in nonaffected brain shows hardly any expression of PSMA. It is unclear whether PSMA-targeting tracer uptake on PET is based on PSMA-specific binding to neovasculature or aspecific uptake in tumor. Here, we quantified uptake of various PSMA-targeting tracers in GBM and correlated this with PSMA expression in tumor biopsy samples from the same patients. Methods: Fourteen patients diagnosed with de novo (n = 8) or recurrent (n = 6) GBM underwent a preoperative PET scan after injection of 1.5 MBq/kg [68Ga]Ga-PSMA-11 (n = 7), 200 MBq of [18F]DCFpyl (n = 3), or 200 MBq of [18F]PSMA-1007 (n = 4). Uptake in tumor and tumor-to-background ratios, with contralateral nonaffected brain as background, were determined. In a subset of patients, PSMA expression levels from different regions in the tumor tissue samples (n = 40), determined using immunohistochemistry (n = 35) or RNA sequencing (n = 13), were correlated with tracer uptake on PET. Results: Moderate to high (SUVmax, 1.3-20.0) heterogeneous uptake was found in all tumors irrespective of the tracer type used. Uptake in nonaffected brain was low, resulting in high tumor-to-background ratios (6.1-359.0) calculated by dividing SUVmax of tumor by SUVmax of background. Immunohistochemistry showed variable PSMA expression on endothelial cells of tumor microvasculature, as well as on dispersed individual cells (of unknown origin), and granular staining of the neuropil. No correlation was found between in vivo uptake and PSMA expression levels (for immunohistochemistry, r = -0.173, P = 0.320; for RNA, r = -0.033, P = 0.915). Conclusion: Our results indicate the potential use of various PSMA-targeting tracers in GBM. However, we found no correlation between PSMA expression levels on immunohistochemistry and uptake intensity on PET. Whether this may be explained by methodologic reasons, such as the inability to measure functionally active PSMA with immunohistochemistry, tracer pharmacokinetics, or the contribution of a disturbed blood-brain barrier to tracer retention, should still be investigated.

Translocator protein (18kDA) (TSPO) marks mesenchymal glioblastoma cell populations characterized by elevated numbers of tumor-associated macrophages

TSPO is a promising novel tracer target for positron-emission tomography (PET) imaging of brain tumors. However, due to the heterogeneity of cell populations that contribute to the TSPO-PET signal, imaging interpretation may be challenging. We therefore evaluated TSPO enrichment/expression in connection with its underlying histopathological and molecular features in gliomas. We analyzed TSPO expression and its regulatory mechanisms in large in silico datasets and by performing direct bisulfite sequencing of the TSPO promotor. In glioblastoma tissue samples of our TSPO-PET imaging study cohort, we dissected the association of TSPO tracer enrichment and protein labeling with the expression of cell lineage markers by immunohistochemistry and fluorescence multiplex stains. Furthermore, we identified relevant TSPO-associated signaling pathways by RNA sequencing.We found that TSPO expression is associated with prognostically unfavorable glioma phenotypes and that TSPO promotor hypermethylation is linked to IDH mutation. Careful histological analysis revealed that TSPO immunohistochemistry correlates with the TSPO-PET signal and that TSPO is expressed by diverse cell populations. While tumor core areas are the major contributor to the overall TSPO signal, TSPO signals in the tumor rim are mainly driven by CD68-positive microglia/macrophages. Molecularly, high TSPO expression marks prognostically unfavorable glioblastoma cell subpopulations characterized by an enrichment of mesenchymal gene sets and higher amounts of tumor-associated macrophages.In conclusion, our study improves the understanding of TSPO as an imaging marker in gliomas by unveiling IDH-dependent differences in TSPO expression/regulation, regional heterogeneity of the TSPO PET signal and functional implications of TSPO in terms of tumor immune cell interactions.

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.

Recapitulating the Key Advances in the Diagnosis and Prognosis of High-Grade Gliomas: Second Half of 2021 Update

High-grade gliomas (World Health Organization grades III and IV) are the most frequent and fatal brain tumors, with median overall survivals of 24–72 and 14–16 months, respectively. We reviewed the progress in the diagnosis and prognosis of high-grade gliomas published in the second half of 2021. A literature search was performed in PubMed using the general terms “radio* and gliom*” and a time limit from 1 July 2021 to 31 December 2021. Important advances were provided in both imaging and non-imaging diagnoses of these hard-to-treat cancers. Our prognostic capacity also increased during the second half of 2021. This review article demonstrates slow, but steady improvements, both scientifically and technically, which express an increased chance that patients with high-grade gliomas may be correctly diagnosed without invasive procedures. The prognosis of those patients strictly depends on the final results of that complex diagnostic process, with widely varying survival rates.

Recent Advances in 18F-Labeled Amino Acids Synthesis and Application

Radiolabeled amino acids are an important class of agents for positron emission tomography imaging that target amino acid transporters in many tumor types. Traditional ¹⁸F-labeled amino acid synthesis strategies are always based on nucleophilic aromatic substitution reactions with multistep radiosynthesis and low radiochemical yields. In recent years, new ¹⁸F-labeling methodologies such as metal-catalyzed radiofluorination and heteroatom (B, P, S, Si, etc.)-¹⁸F bond formation are being effectively used to synthesize radiopharmaceuticals. This review focuses on recent advances in the synthesis, radiolabeling, and application of a series of ¹⁸F-labeled amino acid analogs using new ¹⁸F-labeling strategies.

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.

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.