Ga-68 PSMA PET/CT in recurrent high-grade gliomas: evaluating PSMA expression in vivo

Prostate-Specific Membrane Antigen Use in Glioma Management: Past, Present, and Future

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a membrane-bound metallopeptidase highly expressed in the neovasculature of many solid tumors including gliomas. It is a particularly enticing therapeutic target due to its ability to internalize, thereby delivering radioligands or pharmaceuticals to the intracellular compartment. Targeting the neovasculature of gliomas using PSMA for diagnosis and management has been a recent area of increased study and promise. The purpose of this review is to synthesize the current state and future directions of PSMA use in the histopathologic study, imaging, and treatment of gliomas.

METHODS

PubMed and Scopus databases were used to conduct a literature review on PSMA use in gliomas in June 2023. Terms searched included "PSMA," "Prostate-Specific Membrane Antigen" OR "PSMA" OR "PSMA PET" AND "glioma" OR "high grade glioma" OR "glioblastoma" OR "GBM."

RESULTS

Ninety-four publications were screened for relevance with 61 studies, case reports, and reviews being read to provide comprehensive context for the historical, contemporary, and prospective use of PSMA in glioma management.

CONCLUSIONS

PSMA PET imaging is currently a promising and accurate radiographic tool for the diagnosis and management of gliomas. PSMA histopathology likely represents a viable tool for helping predict glioma behavior. More studies are needed to investigate the role of PSMA-targeted therapeutics in glioma management, but preliminary reports have indicated its potential usefulness in treatment.

Advancements in Image-Based Models for High-Grade Gliomas Might Be Accelerated

The first half of 2022 saw the publication of several major research advances in image-based models and artificial intelligence applications to optimize treatment strategies for high-grade gliomas, the deadliest brain tumors. We review them and discuss the barriers that delay their entry into clinical practice; particularly, the small sample size and the heterogeneity of the study designs and methodologies used. We will also write about the poor and late palliation that patients suffering from high-grade glioma can count on at the end of life, as well as the current legislative instruments, with particular reference to Italy. We suggest measures to accelerate the gradual progress in image-based models and end of life care for patients with high-grade glioma.

Potential of PSMA-targeting radioligand therapy for malignant primary and secondary brain tumours using super-selective intra-arterial administration: a single centre, open label, non-randomised prospective imaging study

BACKGROUND

The aim of this study was to provide quantitative evidence for the potential of PSMA-targeting radioligand therapy (RLT) as treatment approach for malignant brain tumours, and to explore whether tumour uptake could be enhanced by super-selective intra-arterial (ssIA)-administration.

METHODS

Ten patients (n = 5 high-grade glioma, n = 5 brain metastasis) received 1.5 MBq/kg [68Ga]Ga-PSMA-11 intravenously and, within 7 days, intra-arterially (i.e., selectively in tumour-feeding arteries), followed twice by PET-MRI at 90, 165 and 240 min post-injection. Patient safety was monitored for each procedure. Standardised uptake values (SUVs) were obtained for tumour, healthy-brain, salivary glands and liver. Tumour-to-salivary-gland (T/SG) and tumour-to-liver (T/L) uptake-ratios were calculated.

FINDINGS

No adverse events requiring study termination occurred. All patients showed uptake of [68Ga]Ga-PSMA-11 at the tumour site. Uptake was a median 15-fold higher following ssIA-administration (SUVmax median: 142.8, IQR: 102.8-245.9) compared to IV-administration (10.5, IQR:7.5-13.0). According to the bootstrap analysis, mean SUVmax after ssIA (168.8, 95% CI: 110.6-227.0) was well beyond the 95% confidence-interval of IV administration (10.5, 95% CI: 8.4-12.7). Uptake in healthy-brain was negligible, independent of administration route (SUVmean <0.1-0.1). Off-target uptake was comparable, resulting in more favourable T/SG- and T/L-ratios of 8.4 (IQR: 4.4-11.5) and 26.5 (IQR: 14.0-46.4) following ssIA, versus 0.5 (IQR: 0.4-0.7) and 1.8 (IQR: 1.0-2.7) for IV-administration.

INTERPRETATION

ssIA-administration is safe and leads to a median fifteen-fold higher radioligand uptake at the tumour site, therewith qualifying more patients for treatment and enhancing the potential of therapy. These results open new avenues for the development of effective RLT-based treatment strategies for patients with brain tumours.

FUNDING

Semmy Foundation.

Use of 18 F-PSMA PET to Distinguish Cerebral Radiation Necrosis From Tumor Recurrence

ABSTRACT

Brain metastasis in prostate adenocarcinoma is extremely rare and usually arises in the setting of widespread osseous and visceral metastases. Surgical resection and radiation therapy, including stereotactic radiosurgery, are the mainstays of treatment for brain metastasis. Radiation necrosis is a common complication of radiotherapy for brain metastasis, and distinguishing it from tumor recurrence by MRI is difficult because of overlapping findings. We present a 73-year-old man with prostate cancer with a solitary brain metastasis where PET with 18 F-piflufolostat helped detect disease recurrence in the setting of ambiguous MRI findings.

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.

Nuclear Medicine and Cancer Theragnostics: Basic Concepts

Cancer theragnostics is a novel approach that combines diagnostic imaging and radionuclide therapy. It is based on the use of a pair of radiopharmaceuticals, one optimized for positron emission tomography imaging through linkage to a proper radionuclide, and the other bearing an alpha- or beta-emitter isotope that can induce significant damage to cancer cells. In recent years, the use of theragnostics in nuclear medicine clinical practice has increased considerably, and thus investigation has focused on the identification of novel radionuclides that can bind to molecular targets that are typically dysregulated in different cancers. The major advantages of the theragnostic approach include the elimination of multi-step procedures, reduced adverse effects to normal tissues, early diagnosis, better predictive responses, and personalized patient care. This review aims to discuss emerging theragnostic molecules that have been investigated in a series of human malignancies, including gliomas, thyroid cancer, neuroendocrine tumors, cholangiocarcinoma, and prostate cancer, as well as potent and recently introduced molecular targets, like cell-surface receptors, kinases, and cell adhesion proteins. Furthermore, special reference has been made to copper radionuclides as theragnostic agents and their radiopharmaceutical applications since they present promising alternatives to the well-studied gallium-68 and lutetium-177.

Discriminating Inflammatory Radiation-Related Changes From Early Recurrence in Patients With Glioblastomas: A Preliminary Analysis of 68Ga-PSMA-11 PET/CT Compared With 18F-FDOPA PET/CT

PURPOSE OF THE REPORT

Using morphological and functional imaging to discriminate recurrence from postradiation-related modifications in patients with glioblastomas remains challenging. This pilot study aimed to assess the feasibility of using 68Ga-prostate-specific membrane antigen (PSMA) 11 PET/CT compared with 18F-FDOPA PET/CT to detect early recurrence.

METHODS

Nine patients followed up for glioblastomas who received MRI during 12 months of follow-up were referred for both 68Ga-PSMA-11 and 18F-FDOPA PET/CT. The SUVmax, lesion-to-striatum ratio, lesion-to-normal parenchyma ratio, and lesion-to-salivary gland ratio were calculated.

RESULTS

Good correlation between 18F-FDOPA and 68Ga-PSMA PET/CT findings was seen in 5 patients. In 4 patients, the findings of both examinations were consistent with recurrence but were better visualized with the PSMA PET/CT. Examinations of the fifth patient were suggestive of postradiation-related changes and were better analyzed with the PSMA PET/CT, which displayed relatively low uptake compared with DOPA PET/CT. Conversely, 4 patients showed conflicting results: recurrence was not detected on the PSMA PET/CT because of previously introduced bevacizumab treatment; in another patient, both examinations were consistent with recurrence, but there was an uptake mismatch at the suspected lesion sites, and 2 patients presented with inconsistent findings.

CONCLUSIONS

Despite a few discrepancies, this study highlights the potential role of 68Ga-PSMA-11 PET/CT for discriminating postradiation inflammation from recurrence. 68Ga-PSMA-11 PET/CT has an excellent lesion-to-background ratio, and false-positive and false-negative results could be minimized through implementing certain protocols before performing the examination. More powerful prospective studies are required to validate our results.

Contribution of nuclear medicine to the diagnosis and management of primary brain tumours

Positron emission tomography (PET) is a powerful tool that can help physicians manage primary brain tumours at diagnosis and follow-up. In this context, PET imaging is used with three main types of radiotracers: ¹⁸F-FDG, amino acid radiotracers, and ⁶⁸Ga conjugated to somatostatin receptor ligands (SSTRs). At initial diagnosis, ¹⁸F-FDG helps to characterize primary central nervous system (PCNS) lymphomas and high-grade gliomas, amino acid radiotracers are indicated for gliomas, and SSTR PET ligands are indicated for meningiomas. Such radiotracers provide information on tumour grade or type, assist in directing biopsies and help with treatment planning. During follow-up, in the presence of symptoms and/or MRI modifications, the differential diagnosis between tumour recurrence and post-therapeutic changes, in particular radiation necrosis, may be challenging, and there is strong interest in using PET to evaluate therapeutic toxicity. PET may also contribute to identifying specific complications, such as postradiation therapy encephalopathy, encephalitis associated with PCNS lymphoma, and stroke-like migraine after radiation therapy (SMART) syndrome associated with glioma recurrence and temporal epilepsy, originally illustrated in this review. This review summarizes the main contribution of PET to the diagnosis, management, and follow-up of brain tumours, specifically gliomas, meningiomas, and primary central nervous system lymphomas.

The Role of PET Imaging in the Differential Diagnosis between Radiation Necrosis and Recurrent Disease in Irradiated Adult-Type Diffuse Gliomas: A Systematic Review

Adult-type diffuse gliomas are treated with a multimodality treatment approach that includes radiotherapy both in the primary setting, and in the case of progressive or recurrent disease. Radiation necrosis represents a major complication of radiotherapy. Recurrent disease and treatment-related changes are often indistinguishable using conventional imaging methods. The present systematic review aims at assessing the diagnostic role of PET imaging using different radiopharmaceuticals in differentiating radiation necrosis and disease relapse in irradiated adult-type diffuse gliomas. We conducted a comprehensive literature search using the PubMed/MEDLINE and EMBASE databases for original research studies of interest. In total, 436 articles were assessed for eligibility. Ten original papers, published between 2014 and 2022, were selected. Four articles focused on [¹⁸F]FDG, seven on amino acid tracers ([¹⁸F]FET n = 3 and [¹¹C]MET n = 4), one on [¹¹C]CHO, and one on [⁶⁸Ga]Ga-PSMA. Visual assessment, semi-quantitative methods, and radiomics were applied for image analysis. Furthermore, 2/10 papers were comparative studies investigating different radiopharmaceuticals. The present review, the first one on the topic in light of the new 2021 CNS WHO classification, highlighted the usefulness of PET imaging in distinguishing radiation necrosis and tumour recurrence, but revealed high heterogeneity among studies.

Challenges and opportunities for advanced neuroimaging of glioblastoma

Glioblastoma is the most aggressive of glial tumours in adults. On conventional magnetic resonance (MR) imaging, these tumours are observed as irregular enhancing lesions with areas of infiltrating tumour and cortical expansion. More advanced imaging techniques including diffusion-weighted MRI, perfusion-weighted MRI, MR spectroscopy and positron emission tomography (PET) imaging have found widespread application to diagnostic challenges in the setting of first diagnosis, treatment planning and follow-up. This review aims to educate readers with regard to the strengths and weaknesses of the clinical application of these imaging techniques. For example, this review shows that the (semi)quantitative analysis of the mentioned advanced imaging tools was found useful for assessing tumour aggressiveness and tumour extent, and aids in the differentiation of tumour progression from treatment-related effects. Although these techniques may aid in the diagnostic work-up and (post-)treatment phase of glioblastoma, so far no unequivocal imaging strategy is available. Furthermore, the use and further development of artificial intelligence (AI)-based tools could greatly enhance neuroradiological practice by automating labour-intensive tasks such as tumour measurements, and by providing additional diagnostic information such as prediction of tumour genotype. Nevertheless, due to the fact that advanced imaging and AI-diagnostics is not part of response assessment criteria, there is no harmonised guidance on their use, while at the same time the lack of standardisation severely hampers the definition of uniform guidelines.

[68Ga]Ga-PSMA PET/MRI, histological PSMA expression and preliminary experience with [177Lu]Lu-PSMA therapy in relapsing high-grade glioma

Purpose

High-grade gliomas (HGG) are still associated with a dismal prognosis. Prostate specific membrane antigen (PSMA) is discussed as a theranostic target for PSMA-directed radioligand therapy ([¹⁷⁷Lu]Lu-PSMA RLT). Here, we report on the correlation of [⁶⁸Ga]Ga-PSMA uptake with histological PSMA expression and on our preliminary experience with [¹⁷⁷Lu]Lu-PSMA RLT in relapsing HGG.

Methods

Patients with relapsing HGG underwent [⁶⁸Ga]Ga-PSMA PET/MRI to evaluate eligibility for an individualized treatment approach with [¹⁷⁷Lu]Lu-PSMA. Standard uptake values (SUV) for tumor and liver and respective tumor-to-background ratios (compared to the liver) (TBR) on [⁶⁸Ga]Ga-PSMA PET/MRI were assessed. Eligibility criteria for [¹⁷⁷Lu]Lu-PSMA therapy were exhaustion of all standard treatment options available and TBR_max>1.0. In 11 samples, immunohistochemical PSMA expression was determined, quantified using the H-score and correlated with uptake on [⁶⁸Ga]Ga-PSMA PET/MRI.

Results

We included 20 patients with a median age of 53 years (IQR 42-57). The median SUV on [⁶⁸Ga]Ga-PSMA PET/MRI was 4.5 (3.7-6.2) for SUV_max and 1.4 (1.1-1.7) for SUV_mean. The respective TBR was maximum 0.6 (0.4-0.8) and mean 0.3 (0.2-0.4). High TBR_max correlated with increased endothelial PSMA expression [H-score of 65 (62.5-77.5)]. Three patients (15%) presented a TBR_max>1.0 and qualified for [¹⁷⁷Lu]Lu-PSMA RLT. No treatment related toxicity was observed.

Conclusion

Only a minority of patients with relapsing HGG qualified for [¹⁷⁷Lu]Lu-PSMA RLT. Our data demonstrates that PSMA expression in the neo-vasculature corresponds to PSMA uptake on [⁶⁸Ga]Ga-PSMA PET/MRI and might be used as a screening tool for patient selection. Future prospective studies need to focus the debate on TBR_max thresholds as inclusion criteria for PSMA RLT.

PSMA Radioligand Uptake as a Biomarker of Neoangiogenesis in Solid Tumours: Diagnostic or Theragnostic Factor?

Due to its overexpression on the surface of prostate cancer cells, prostate-specific membrane antigen (PSMA) is a relatively novel effective target for molecular imaging and radioligand therapy (RLT) in prostate cancer. Recent studies reported that PSMA is expressed in the neovasculature of various types of cancer and regulates tumour cell invasion as well as tumour angiogenesis. Several authors explored the role of diagnostic and therapeutic PSMA radioligands in various malignancies. In this narrative review, we describe the current status of the literature on PSMA radioligands' application in solid tumours other than prostate cancer to explore their potential role as diagnostic or therapeutic agents, with particular regard to the relevance of PSMA radioligand uptake as neoangiogenetic biomarker. Hence, a comprehensive review of the literature was performed to find relevant articles on the applications of PSMA radioligands in non-prostate solid tumours. Data on the general, methodological and clinical aspects of all included studies were collected. Forty full-text papers were selected for final review, 8 of which explored PSMA radioligand PET/CT performances in gliomas, 3 in salivary gland malignancies, 6 in thyroid cancer, 2 in breast cancer, 16 in renal cell carcinoma and 5 in hepatocellular carcinoma. In the included studies, PSMA radioligand PET showed promising performance in patients with non-prostate solid tumours. Further studies are needed to better define its potential role in oncological patients management, especially in those undergoing antineoangiogenic therapies, and to assess the efficacy of PSMA-RLT in this clinical context.

Diagnostic Accuracy of PET/CT or PET/MRI Using PSMA-Targeting Radiopharmaceuticals in High-Grade Gliomas: A Systematic Review and a Bivariate Meta-Analysis

Background: Several studies proposed the use of positron emission tomography (PET) with Prostate Specific Membrane Antigen (PSMA)-targeting radiopharmaceuticals in brain tumors. Our aim is to calculate the diagnostic accuracy of these methods in high-grade gliomas (HGG) with a bivariate meta-analysis. Methods: A comprehensive literature search of studies on the diagnostic accuracy of PET/CT or PET/MRI with PSMA-targeting radiopharmaceuticals in HGG was performed. Original articles evaluating these imaging methods both in the differential diagnosis between HGG and low-grade gliomas (LGG) and in the assessment of suspicious HGG recurrence were included. Pooled sensitivity, specificity, positive and negative likelihood ratios (LR+ and LR-), and diagnostic odds ratio (DOR) including 95% confidence intervals (95% CI) were calculated. Statistical heterogeneity was also assessed using the I² test. Results: The meta-analysis of six selected studies (157 patients) provided the following results about PET/CT or PET/MRI with PSMA-targeting radiopharmaceuticals in the diagnosis of HGG: sensitivity 98.2% (95% CI: 75.3–99.9%), specificity 91.2% (95% CI: 68.4–98.1%), LR+ 4.5 (95% CI: 2.2–9.3), LR− 0.07 (95% CI: 0.04–0.15), and DOR 70.1 (95% CI: 19.6–250.9). No significant statistical heterogeneity among the included studies was found (I² = 0%). Conclusions: the quantitative data provided demonstrate the high diagnostic accuracy of PET/CT or PET/MRI with PSMA-targeting radiopharmaceuticals for HGG detection. However, more studies are needed to confirm the promising role of PSMA-targeted PET in this clinical setting.

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.