3'-Deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT) transport in newly diagnosed glioma: correlation with nucleoside transporter expression, vascularization, and blood-brain barrier permeability

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

PURPOSE

This review aimed to summarize the available literature on the clinical application of [¹⁸F] FLT PET imaging in primary brain tumours.

METHODS

A comprehensive search strategy based on Pubmed/Medline, Scopus, Web of Science, Cochrane Library, Google Scholar, and the Embase databases was carried on using the following search string: ('3` Fluorothymidine'/exp OR 'FLT' OR '[81F]-FLT' OR '[¹⁸F] Fluorothymidine') AND ('pet'/exp OR 'pet' OR 'positron emission tomography') AND ('glioma'/exp OR 'glioma' OR 'brain tumour'/exp OR 'brain tumour'). The search was updated till March 2021 and only articles in English and studies investigating the clinical applications of [¹⁸F] FLT PET and PET/CT in primary brain tumours were considered eligible for inclusion.

RESULTS

The literature search ultimately yielded 52 studies included in the systematic review, with main results as follows: a) the uptake of [¹⁸F] FLT may guide stereotactic biopsy but does not discriminate between grade II and III glioma. b) [¹⁸F] FLT uptake and texture parameters correlate with overall survival (OS) in newly diagnosed gliomas. c) In patients with recurrent glioma, proliferative volume (PV) and tumour-to-normal brain (T/N) uptake ratio are independent predictors of survival. d) Patients demonstrating response to therapy at [¹⁸F] FLT PET scan show longer OS compared to non-responders. e) [¹⁸F] FLT PET demonstrated good performance in discriminating tumour recurrence from radionecrosis. However, controversial results exist in comparative literature examining the performance of [¹⁸F] FLT vs. other radiotracers in the assessment of recurrence.

CONCLUSION

[¹⁸F] FLT PET imaging has demonstrated potential benefits for grading, diagnostic and prognostic purposes, despite the small sample size studies due to the relatively low availability of the radiotracer.

Multiple positron emission tomography tracers for use in the classification of gliomas according to the 2016 World Health Organization criteria

Background

The molecular diagnosis of gliomas such as isocitrate dehydrogenase (IDH) status (wild-type [wt] or mutation [mut]) is especially important in the 2016 World Health Organization (WHO) classification. Positron emission tomography (PET) has afforded molecular and metabolic diagnostic imaging. The present study aimed to define the interrelationship between the 2016 WHO classification of gliomas and the integrated data from PET images using multiple tracers, including ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG), ¹¹C-methionine (¹¹C-MET), ¹⁸F-fluorothymidine (¹⁸F-FLT), and ¹⁸F-fluoromisonidazole (¹⁸F-FMISO).

Methods

This retrospective, single-center study comprised 113 patients with newly diagnosed glioma based on the 2016 WHO criteria. Patients were divided into 4 glioma subtypes (Mut, Codel, Wt, and glioblastoma multiforme [GBM]). Tumor standardized uptake value (SUV) divided by mean normal cortical SUV (tumor–normal tissue ratio [TNR]) was calculated for ¹⁸F-FDG, ¹¹C-MET, and ¹⁸F-FLT. Tumor–blood SUV ratio (TBR) was calculated for ¹⁸F-FMISO. To assess the diagnostic accuracy of PET tracers in distinguishing glioma subtypes, a comparative analysis of TNRs and TBR as well as the metabolic tumor volume (MTV) were calculated by Scheffe's multiple comparison procedure for each PET tracer following the Kruskal–Wallis test.

Results

The differences in mean ¹⁸F-FLT TNR and ¹⁸F-FMISO TBR were significant between GBM and other glioma subtypes (P < .001). Regarding the comparison between Gd-T1WI volumes and ¹⁸F-FLT MTVs or ¹⁸F-FMISO MTVs, we identified significant differences between Wt and Mut or Codel (P < .01).

Conclusion

Combined administration of 4 PET tracers might aid in the preoperative differential diagnosis of gliomas according to the 2016 WHO criteria.

Imaging-guided precision medicine in glioblastoma patients treated with immune checkpoint modulators: research trend and future directions in the field of imaging biomarkers and artificial intelligence

Immunotherapies that employ immune checkpoint modulators (ICMs) have emerged as an effective treatment for a variety of solid cancers, as well as a paradigm shift in the treatment of cancers. Despite this breakthrough, the median survival time of glioblastoma patients has remained at about 2 years. Therefore, the safety and anti-cancer efficacy of combination therapies that include ICMs are being actively investigated. Because of the distinct mechanisms of ICMs, which restore the immune system’s anti-tumor capacity, unconventional immune-related phenomena are increasingly being reported in terms of tumor response and progression, as well as adverse events. Indeed, immunotherapy response assessments for neuro-oncology (iRANO) play a central role in guiding cancer patient management and define a “wait and see strategy” for patients treated with ICMs in monotherapy with progressive disease on MRI. This article deciphers emerging research trends to ameliorate four challenges unaddressed by the iRANO criteria: (1) patient selection, (2) identification of immune-related phenomena other than pseudoprogression (i.e., hyperprogression, the abscopal effect, immune-related adverse events), (3) response assessment in combination therapies including ICM, and (4) alternatives to MRI. To this end, our article provides a structured approach for standardized selection and reporting of imaging modalities to enable the use of precision medicine by deciphering the characteristics of the tumor and its immune environment. Emerging preclinical or clinical innovations are also discussed as future directions such as immune-specific targeting and implementation of artificial intelligence algorithms.

Demonstration of Nucleoside Transporter Activity in the Nose-to-Brain Distribution of [18F]Fluorothymidine Using PET Imaging

To evaluate the role of nucleoside transporters in the nose-to-brain uptake of [¹⁸F]fluorothymidine (FLT), an equilibrative nucleoside transporter (ENT1,2) and concentrative nucleoside transporter (CNT1-3) substrate, using PET to measure local tissue concentrations. Anesthetized Sprague-Dawley rats were administered FLT by intranasal (IN) instillation or tail-vein injection (IV). NBMPR (nitrobenzylmercaptopurine riboside), an ENT1 inhibitor, was administered either IN or intraperitoneally (IP). Dynamic PET imaging was performed for up to 40 min. A CT was obtained for anatomical co-registration and attenuation correction. Time-activity curves (TACs) were generated for the olfactory bulb (OB) and remaining brain, and the area-under-the-curve (AUC) for each TAC was calculated to determine the total tissue exposure of FLT. FLT concentrations were higher in the OB than in the rest of the brain following IN administration. IP administration of NBMPR resulted in increased OB and brain FLT exposure following both IN and IV administration, suggesting that NBMPR decreases the clearance rate of FLT from the brain. When FLT and NBMPR were co-administered IN, there was a decrease in the OB AUC while an increase in the brain AUC was observed. The decrease in OB exposure was likely the result of inhibition of ENT1 uptake activity in the nose-to-brain transport pathway. FLT distribution patterns show that nucleoside transporters, including ENT1, play a key role in the distribution of transporter substrates between the nasal cavity and the brain via the OB.

Brain Tumors: An Update on Clinical PET Research in Gliomas

A previous review published in 2012 demonstrated the role of clinical PET for diagnosis and management of brain tumors using mainly FDG, amino acid tracers, and ¹⁸F-fluorothymidine. This review provides an update on clinical PET studies, most of which are motivated by prediction of prognosis and planning and monitoring of therapy in gliomas. For FDG, there has been additional evidence supporting late scanning, and combination with ¹³N ammonia has yielded some promising results. Large neutral amino acid tracers have found widespread applications mostly based on ¹⁸F-labeled compounds fluoroethyltyrosine and fluorodopa for targeting biopsies, therapy planning and monitoring, and as outcome markers in clinical trials. ¹¹C-alpha-methyltryptophan (AMT) has been proposed as an alternative to ¹¹C-methionine, and there may also be a role for cyclic amino acid tracers. ¹⁸F-fluorothymidine has shown strengths for tumor grading and as an outcome marker. Studies using ¹⁸F-fluorocholine (FCH) and ⁶⁸Ga-labeled compounds are promising but have not yet clearly defined their role. Studies on radiotherapy planning have explored the use of large neutral amino acid tracers to improve the delineation of tumor volume for irradiation and the use of hypoxia markers, in particular ¹⁸F-fluoromisonidazole. Many studies employed the combination of PET with advanced multimodal MR imaging methods, mostly demonstrating complementarity and some potential benefits of hybrid PET/MR.

A boronate-caged [¹⁸F]FLT probe for hydrogen peroxide detection using positron emission tomography

Reactive oxygen species (ROS) play important roles in the development and progression of cancer and other diseases, motivating the development of translatable technologies for biological ROS imaging. Here we report Peroxy-Caged-[¹⁸F]Fluorodeoxy thymidine-1 (PC-FLT-1), an oxidatively immolative positron emission tomography (PET) probe for H₂O₂ detection. PC-FLT-1 reacts with H₂O₂ to generate [¹⁸F]FLT, allowing its peroxide-dependent uptake and retention in proliferating cells. The relative uptake of PC-FLT-1 was evaluated using H₂O₂-treated UOK262 renal carcinoma cells and a paraquat-induced oxidative stress cell model, demonstrating ROS-dependent tracer accumulation. The data suggest that PC-FLT-1 possesses promising characteristics for translatable ROS detection and provide a general approach to PET imaging that can be expanded to the in vivo study of other biologically relevant analytes.

An intra-individual comparison of MRI, [18F]-FET and [18F]-FLT PET in patients with high-grade gliomas

Objectives

Intra-individual spatial overlap analysis of tumor volumes assessed by MRI, the amino acid PET tracer [¹⁸F]-FET and the nucleoside PET tracer [¹⁸F]-FLT in high-grade gliomas (HGG).

Methods

MRI, [¹⁸F]-FET and [¹⁸F]-FLT PET data sets were retrospectively analyzed in 23 HGG patients. Morphologic tumor volumes on MRI (post-contrast T1 (cT1) and T2 images) were calculated using a semi-automatic image segmentation method. Metabolic tumor volumes for [¹⁸F]-FET and [¹⁸F]-FLT PETs were determined by image segmentation using a threshold-based volume of interest analysis. After co-registration with MRI the morphologic and metabolic tumor volumes were compared on an intra-individual basis in order to estimate spatial overlaps using the Spearman's rank correlation coefficient and the Mann-Whitney U test.

Results

[¹⁸F]-FLT uptake was negative in tumors with no or only moderate contrast enhancement on MRI, detecting only 21 of 23 (91%) HGG. In addition, [¹⁸F]-FLT uptake was mainly restricted to cT1 tumor areas on MRI and [¹⁸F]-FLT volumes strongly correlated with cT1 volumes (r = 0.841, p<0.001). In contrast, [¹⁸F]-FET PET detected 22 of 23 (96%) HGG. [¹⁸F]-FET uptake beyond areas of cT1 was found in 61% of cases and [¹⁸F]-FET volumes showed only a moderate correlation with cT1 volumes (r = 0.573, p<0.001). Metabolic tumor volumes beyond cT1 tumor areas were significantly larger for [¹⁸F]-FET compared to [¹⁸F]-FLT tracer uptake (8.3 vs. 2.7 cm³, p<0.001).

Conclusion

In HGG [¹⁸F]-FET but not [¹⁸F]-FLT PET was able to detect metabolic active tumor tissue beyond contrast enhancing tumor on MRI. In contrast to [¹⁸F]-FET, blood-brain barrier breakdown seems to be a prerequisite for [¹⁸F]-FLT tracer uptake.