Advances in PET imaging of brain tumors: a referring physician's perspective

Recent advancements in brain tumor targeting using magnetic nanoparticles

Transport of drugs through the blood-brain barrier to the brain and the toxic effects of drugs on the healthy cells can limit the effectiveness of chemotherapeutic agents. In recent years, magnetic nanoparticles (MNPs) have received much attention as targeted therapeutic and diagnostic systems due to their simplicity, ease of preparation and ability to tailor their properties such as their composition, size, surface morphology, etc. for biomedical applications. MNPs are utilized in drug delivery, radio therapeutics, hyperthermia treatment, gene therapy, biotherapeutics and diagnostic imaging. The present review will address the challenges in brain tumor targeting and discuss the application and recent developments in brain tumor targeting using MNPs.

Glutamine Metabolism in Brain Tumors

Altered metabolism is a hallmark of cancer cells. Tumor cells rewire their metabolism to support their uncontrolled proliferation by taking up nutrients from the microenvironment. The amino acid glutamine is a key nutrient that fuels biosynthetic processes including ATP generation, redox homeostasis, nucleotide, protein, and lipid synthesis. Glutamine as a precursor for the neurotransmitter glutamate, and plays a critical role in the normal functioning of the brain. Brain tumors that grow in this glutamine/glutamate rich microenvironment can make synaptic connections with glutamatergic neurons and reprogram glutamine metabolism to enable their growth. In this review, we examine the functions of glutamate/glutamine in the brain and how brain tumor cells reprogram glutamine metabolism. Altered glutamine metabolism can be leveraged to develop non-invasive imaging strategies and we review these imaging modalities. Finally, we examine if targeting glutamine metabolism could serve as a therapeutic strategy in brain tumors.

Amide proton transfer imaging seems to provide higher diagnostic performance in post-treatment high-grade gliomas than methionine positron emission tomography

OBJECTIVES

To compare the diagnostic performance of amide proton transfer (APT) imaging and 11-C methionine positron emission tomography (MET-PET) for in vivo molecular imaging of protein metabolism in post-treatment gliomas.

MATERIALS AND METHODS

This study included 43 patients (12 low and 31 high grade) with post-treatment gliomas who underwent both APT and MET-PET imaging within 3 weeks. APT-weighted voxel values and semi-quantitative tumour-to-normal ratios (TNR) were obtained from tumour portions. The voxel-wise relationships between TNR and APT were assessed. The diagnostic performance for recurrence of high-grade gliomas was calculated, using the area under the receiver operating characteristic curve (AUC) with maximum (TNR_max and APT_max) and 90% histogram values (TNR90 and APT90).

RESULTS

A moderate positive correlation between TNR and APT was found in low-grade recurrences (r = 0.47, p < 0.001), but not in high-grade ones (r = -0.24, p < 0.001). For distinguishing recurrence in post-treatment high-grade gliomas, APT_max (AUC, 0.88) and APT90 (AUC, 0.78-0.83) had a similar to better diagnostic performance than TNR_max (AUC, 0.71, p = 0.08) or TNR90 (AUC, 0.53-0.59, p = 0.01-0.05).

CONCLUSIONS

In post-treatment high-grade gliomas, APT provides different regional information to MET-PET and provides higher diagnostic performance. This difference needs to be considered when using APT or MET-PET as a surrogate marker for tumour protein metabolism.

KEY POINTS

• APT and TNR values in low-grade recurrence showed a moderate voxel-wise correlation. • APT and TNR demonstrated regional differences in post-treatment high-grade gliomas. • APT90 showed better diagnostic performance than TNR90 in high-grade recurrence.

Metabolic Reprogramming in Brain Tumors

Next-generation sequencing has substantially enhanced our understanding of the genetics of primary brain tumors by uncovering several novel driver genetic alterations. How many of these genetic modifications contribute to the pathogenesis of brain tumors is not well understood. An exciting paradigm emerging in cancer biology is that oncogenes actively reprogram cellular metabolism to enable tumors to survive and proliferate. We discuss how some of these genetic alterations in brain tumors rewire metabolism. Furthermore, metabolic alterations directly impact epigenetics well beyond classical mechanisms of tumor pathogenesis. Metabolic reprogramming in brain tumors is also influenced by the tumor microenvironment contributing to drug resistance and tumor recurrence. Altered cancer metabolism can be leveraged to noninvasively image brain tumors, which facilitates improved diagnosis and the evaluation of treatment effectiveness. Many of these aspects of altered metabolism provide novel therapeutic opportunities to effectively treat primary brain tumors.

Glutamine-based PET imaging facilitates enhanced metabolic evaluation of gliomas in vivo

Glucose and glutamine are the two principal nutrients that cancer cells use to proliferate and survive. Many cancers show altered glucose metabolism, which constitutes the basis for in vivo positron emission tomography (PET) imaging with (18)F-fluorodeoxyglucose ((18)F-FDG). However, (18)F-FDG is ineffective in evaluating gliomas because of high background uptake in the brain. Glutamine metabolism is also altered in many cancers, and we demonstrate that PET imaging in vivo with the glutamine analog 4-(18)F-(2S,4R)-fluoroglutamine ((18)F-FGln) shows high uptake in gliomas but low background brain uptake, facilitating clear tumor delineation. Chemo/radiation therapy reduced (18)F-FGln tumor avidity, corresponding with decreased tumor burden. (18)F-FGln uptake was not observed in animals with a permeable blood-brain barrier or neuroinflammation. We translated these findings to human subjects, where (18)F-FGln showed high tumor/background ratios with minimal uptake in the surrounding brain in human glioma patients with progressive disease. These data suggest that (18)F-FGln is avidly taken up by gliomas, can be used to assess metabolic nutrient uptake in gliomas in vivo, and may serve as a valuable tool in the clinical management of gliomas.

The role of imaging in the management of progressive glioblastoma : a systematic review and evidence-based clinical practice guideline

QUESTION

Which imaging techniques most accurately differentiate true tumor progression from pseudo-progression or treatment related changes in patients with previously diagnosed glioblastoma?

TARGET POPULATION

These recommendations apply to adults with previously diagnosed glioblastoma who are suspected of experiencing progression of the neoplastic process.

RECOMMENDATIONS LEVEL II

Magnetic resonance imaging with and without gadolinium enhancement is recommended as the imaging surveillance method to detect the progression of previously diagnosed glioblastoma.

LEVEL II

Magnetic resonance spectroscopy is recommended as a diagnostic method to differentiate true tumor progression from treatment-related imaging changes or pseudo-progression in patients with suspected progressive glioblastoma.

LEVEL III

The routine use of positron emission tomography to identify progression of glioblastoma is not recommended.

LEVEL III

Single-photon emission computed tomography imaging is recommended as a diagnostic method to differentiate true tumor progression from treatment-related imaging changes or pseudo-progression in patients with suspected progressive glioblastoma.

Accuracy of distinguishing between dysembryoplastic neuroepithelial tumors and other epileptogenic brain neoplasms with [¹¹C]methionine PET

BACKGROUND

Dysembryoplastic neuroepithelial tumors (DNTs) represent a prevalent cause of epileptogenic brain tumors, the natural evolution of which is much more benign than that of most gliomas. Previous studies have suggested that [(11)C]methionine positron emission tomography (MET-PET) could help to distinguish DNTs from other epileptogenic brain tumors, and hence optimize the management of patients. Here, we reassessed the diagnostic accuracy of MET-PET for the differentiation between DNT and other epileptogenic brain neoplasms in a larger population.

METHODS

We conducted a retrospective study of 77 patients with focal epilepsy related to a nonrapidly progressing brain tumor on MRI who underwent MET-PET, including 52 with a definite histopathology. MET-PET data were assessed by a structured visual analysis that distinguished normal, moderately abnormal, and markedly abnormal tumor methionine uptake and by semiquantitative ratio measurements.

RESULTS

Pathology showed 21 DNTs (40%), 10 gangliogliomas (19%), 19 low-grade gliomas (37%), and 2 high-grade gliomas (4%). MET-PET visual findings significantly differed among the various tumor types (P < .001), as confirmed by semiquantitative analyses (P < .001 for all calculated ratios), regardless of gadolinium enhancement on MRI. All gliomas and gangliogliomas were associated with moderately or markedly increased tumor methionine uptake, whereas 9/21 DNTs had normal methionine uptake. Receiver operating characteristics analysis of the semiquantitative ratios showed an optimal cutoff threshold that distinguished DNTs from other tumor types with 90% specificity and 89% sensitivity.

CONCLUSIONS

Normal MET-PET findings in patients with an epileptogenic nonrapidly progressing brain tumor are highly suggestive of DNT, whereas a markedly increased tumor methionine uptake makes this diagnosis unlikely.

Multifunctional nanoparticles for brain tumor imaging and therapy

Brain tumors are a diverse group of neoplasms that often carry a poor prognosis for patients. Despite tremendous efforts to develop diagnostic tools and therapeutic avenues, the treatment of brain tumors remains a formidable challenge in the field of neuro-oncology. Physiological barriers including the blood-brain barrier result in insufficient accumulation of therapeutic agents at the site of a tumor, preventing adequate destruction of malignant cells. Furthermore, there is a need for improvements in brain tumor imaging to allow for better characterization and delineation of tumors, visualization of malignant tissue during surgery, and tracking of response to chemotherapy and radiotherapy. Multifunctional nanoparticles offer the potential to improve upon many of these issues and may lead to breakthroughs in brain tumor management. In this review, we discuss the diagnostic and therapeutic applications of nanoparticles for brain tumors with an emphasis on innovative approaches in tumor targeting, tumor imaging, and therapeutic agent delivery. Clinically feasible nanoparticle administration strategies for brain tumor patients are also examined. Furthermore, we address the barriers towards clinical implementation of multifunctional nanoparticles in the context of brain tumor management.

Synthesis and evaluation of ¹⁸F labeled FET prodrugs for tumor imaging

INTRODUCTION

O-(2-[(18)F]fluoroethyl)-L-tyrosine (FET, [(18)F]1) is a useful amino-acid-based imaging agent for brain tumors. This paper reports the synthesis and evaluation of three FET prodrugs, O-(2-[(18)F]fluoroethyl)-L-tyrosyl-L-glycine (FET-Gly, [(18)F]2), O-(2-[(18)F]fluoroethyl)-L-tyrosyl-L-alanine (FET-Ala, [(18)F]3) and N-acetyl O-(2-[(18)F]fluoroethyl)-L-tyrosine (AcFET, [(18)F]4), which could be readily hydrolyzed to FET in vivo for tumor imaging. We investigated their metabolism in the blood and imaging properties in comparison to FET ([(18)F]1).

METHODS

Three new [(18)F]FET derivatives, 2-4, were prepared from their corresponding tosylate-precursors through nucleophilic fluorination and subsequent deprotection reactions. In vitro uptake studies were carried out in 9L glioma cancer cell lines. In vitro and in vivo hydrolysis studies were conducted to evaluate the hydrolysis of FET prodrugs in blood and in Fisher 344 rats. Biodistribution and PET imaging studies were then performed in rats bearing 9L tumors.

RESULTS

New FET prodrugs were prepared with 3-28% decay corrected radiochemical yields, good enantiomeric purity (>95%) and high radiochemical purity (>95%). FET-Gly ([(18)F]2), FET-Ala ([(18)F]3), and AcFET ([(18)F]4) exhibited negligible uptake in comparison to the high uptake of FET ([(18)F]1) in 9L cells. Metabolism studies of FET-Gly ([(18)F]2), FET-Ala ([(18)F]3), and AcFET ([(18)F]4) in rat and human blood showed that FET-Ala ([(18)F]3) was hydrolyzed to FET ([(18)F]1) faster than FET-Gly ([(18)F]2) or AcFET ([(18)F]4). Most of the FET-Ala (79%) was converted to FET ([(18)F]1) within 5min in blood in vivo. Biodistribution studies demonstrated that FET-Ala ([(18)F]3) displayed the highest tumor uptake. The tumor-to-background ratios of FET-Ala ([(18)F]3) and FET ([(18)F]1) were comparable and appeared to be better than those of FET-Gly ([(18)F]2) and AcFET ([(18)F]4). PET imaging studies showed that both FET ([(18)F]1) and FET-Ala ([(18)F]3) could visualize tumors effectively, and that they share similar imaging characteristics.

CONCLUSIONS

FET-Ala ([(18)F]3) demonstrated promising properties as a prodrug of FET ([(18)F]1), which could be used in PET imaging of tumor amino acid metabolism.

Comparison of 3'-deoxy-3'-[18F]fluorothymidine PET and O-(2-[18F]fluoroethyl)-L-tyrosine PET in patients with newly diagnosed glioma

PURPOSE

The purpose of this prospective study was to clarify the value of FLT PET and FET PET for the noninvasive grading and prognosis of newly diagnosed gliomas.

MATERIALS AND METHODS

Twenty patients with newly diagnosed gliomas were investigated with FLT and FET PET before surgery. FLT and FET uptakes were assessed by the maximum standardized uptake (SUVmax) of tumor, and the ratio to uptake in the normal brain parenchyma (TNR). All tumors were graded by WHO system.

RESULTS

FLT PET detected all 17 high-grade gliomas (HGG) and did not detect all 3 low-grade gliomas (LGG). FET PET detected all 20 HGG and LGG regardless of grading. The average FLT SUVmax in HGG and LGG was 1.51 ± 0.72 and 0.30 ± 0.07, and the average FLT TNR in HGG and LGG was 5.52 ± 3.09 and 1.12 ± 0.14, respectively. The differences of FLT SUVmax and TNR between HGG and LGG were statistically significant (p=0.0069, p=0.0070). The average FET SUVmax in HGG and LGG was 2.68 ± 0.86 and 1.36 ± 0.15, and the average FET TNR in HGG and LGG was 2.31 ± 0.73 and 1.27 ± 0.12, respectively. The differences of FET SUVmax and TNR between HGG and LGG were statistically significant (p=0.0129, p=0.0095).

CONCLUSIONS

FET PET has higher sensitivity in detection of gliomas rather than FLT PET, but it seems that FLT PET is better than FET PET for noninvasive grading and predicting prognosis of newly diagnosed gliomas, considering high contrast of FLT and overlap of FET uptakes between HGG and LGG.

Potential role for magnetoencephalography in distinguishing low- and high-grade gliomas: a preliminary study with histopathological confirmation

Gliomas are the most common form of tumor in the CNS and are exceptionally heterogeneous. Accurately characterizing gliomas, in terms of grade and type, is essential for predicting the rate of tumor progression. Histopathological grading and analysis based on biopsied tissue remains the gold standard, but non- and semi-invasive neuroimaging also plays a key role. Neuroimaging has been used to guide and optimize biopsies for several decades, but more recently molecular imaging and variants of MRI have shown promise in independently predicting glioma grade. Here we evaluated whether magnetoencephalographic (MEG) measurements of population-level physiology within the glioma space were predictive of the inherent grade of the tissue, based on definitive histopathological analyses. High-density MEG data were recorded from 11 patients who were undergoing functional mapping in preparation for resective surgery. The primary results indicated that glioma grade was positively correlated with the local amplitude of activity within the glioma space in the theta (4-7 Hz), alpha (8-14 Hz), and beta bands (14-30 Hz). Additionally, activity within the glioma was significantly elevated relative to the nonaffected homologue area in the same frequency bands. These results indicate that pathological levels of synchronization exist within the tumor space and that MEG may be a viable tool for noninvasively differentiating gliomas by their grade. Although these results should be considered preliminary and are only correlative in nature, these data suggest that MEG can potentially detect neurophysiological signatures or markers that predict the inherent grade of a glial tumor.