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

E-Urea-K-Based PSMA Imaging Served as an Alternative in Assessing Tumor Neovascularization via Targeting CD31

To reveal the natural correlation between prostate-specific membrane antigen (PSMA) imaging and tumor neovascularization in prostate cancer and further explore E-urea-K-based PSMA-targeted (EK-PSMA) imaging as a potential indicator of tumor neovascularization, the 22Rv1 mouse models were established and underwent 99mTc-HYNIC-ALUG SPECT/CT. Pearson correlation analysis was applied to assess the relationship between tumor tracer uptake and tumor characteristics, including size, glucose metabolism, and cell phenotypes (e.g., Ki-67, VEGF, CD31, and PSMA). Then, molecular docking further identified the key factors of EK-PSMA imaging, specifically related to tumor neovascularization. Finally, animal models with positive and negative PSMA expression (22Rv1, LNCaP, U87, SAOS-2, A549, and ACHN) were subjected to antibody-targeted blockade to verify the role of these key factors in EK-PSMA imaging. The Pearson's r values of tracer uptake correlated with CD31 and tumor size were 0.82 and 0.99, respectively (P < 0.05), and the correlations of tracer uptake with SUVmax, SUVmean, Ki-67, VEGF, and PSMA expressions were 0.47, 0.20, 0.69, -0.65, and 0.20, respectively (all P > 0.05). Molecular docking confirmed the affinity of E-urea-K to PSMA (two sites, binding scores, -5.4 kcal/mol and -6.0 kcal/mol) and CD31 (one site, binding score, -5.1 kcal/mol). The blockade of the CD31 antibody partially reduced the 99mTc-HYNIC-ALUG uptake in five other types of tumors (paired t test, P = 0.0478). The Pearson's r value of CD31 staining and tracer uptake prior to the antibody blockade was 0.84 (P < 0.05). Additionally, when removing the PSMA-positive models (22Rv1 and LNCaP), the Pearson's r value of CD31 staining and tracer uptake prior to the antibody blockade was 0.99 (P < 0.05). Thus, CD31 was found to be a mutual target of EK-PSMA imaging; therefore, EK-PSMA imaging provides a viable assessment option for tumor neovascularization, especially for PSMA-negative tumors.

Evaluating the Potential of PSMA Targeting in CNS Tumors: Insights from Large-Scale Transcriptome Profiling

BACKGROUND/OBJECTIVES

Prostate-specific membrane antigen (PSMA) is a well-established target in prostate cancer therapy that has shown potential as a theranostic target across non-central nervous system (CNS) and CNS tumor types. We aimed to investigate the pan-tissue expression pattern of the PSMA-encoding gene FOLH1 to assess whether transcriptome profiling can inform tumor diagnostic and theranostic probes.

METHODS

We assessed FOLH1 expression from the Open Pediatric Cancer Project (OpenPedCan, n = 2132 specimens), the Cancer Genome Atlas (TCGA, n = 10,411 specimens), and the Genotype Tissue Expression Project (GTEx, n = 17,382 specimens) in relation to published reports of PSMA radionuclide uptake in various tumors.

RESULTS

When comparing FOLH1 expression across tumor versus normal tissues, we found that non-CNS tumors exhibiting elevated expression of at least two-fold (FDR < 0.05) were reported to have significant PSMA radionuclide uptake in contrast to tumors with less than a two-fold elevation or with lower expression of FOLH1 relative to normal tissues. Notably, CNS tumors universally exhibited lower expression of FOLH1 relative to normal brain tissue, but we observed considerable variation in the expression of blood-tumor barrier (BTB) components associated with reports of BTB integrity and uptake of PSMA radiotracers.

CONCLUSIONS

Large-scale transcriptomics data may help guide the application of PSMA-based radionuclide therapies in non-CNS tumors, but care should be taken to account for BTB effects in CNS tumors when assessing the potential for radionuclide success. This study demonstrates that FOLH1 showed a lack of tumor-specific expression for both adult and pediatric CNS tumors when compared to normal brain tissue, suggesting that PSMA is not a desirable target in brain tumors.

Precision radiotherapy with molecular-profiling of CNS tumours

Diagnoses of CNS malignancies in the primary and metastatic setting have significantly advanced in the last decade with the advent of molecular pathology. Using a combination of immunohistochemistry, next-generation sequencing, and methylation profiling integrated with traditional histopathology, patient prognosis and disease characteristics can be understood to a much greater extent. This has recently manifested in predicting response to targeted drug therapies that are redefining management practices of CNS tumours. Radiotherapy, along with surgery, still remains an integral part of treating the majority of CNS tumours. However, the rapid advances in CNS molecular diagnostics have not yet been effectively translated into improving CNS radiotherapy. We explore several promising strategies under development to integrate molecular oncology into radiotherapy, and explore future directions that can serve to use molecular diagnostics to personalize radiotherapy. Evolving the management of CNS tumours with molecular profiling will be integral to supporting the future of precision radiotherapy.

Do we need dosimetry for the optimization of theranostics in CNS tumors?

Radiopharmaceutical theranostic treatments have grown exponentially worldwide, and internal dosimetry has attracted attention and resources. Despite some similarities with chemotherapy, radiopharmaceutical treatments are essentially radiotherapy treatments, as the release of radiation into tissues is the determinant of the observed clinical effects. Therefore, absorbed dose calculations are key to explaining dose-effect correlations and individualizing radiopharmaceutical treatments. The present article introduces the basic principles of internal dosimetry and provides an overview of available loco-regional and systemic radiopharmaceutical treatments for central nervous system (CNS) tumors. The specific characteristics of dosimetry as applied to these treatments are highlighted, along with their limitations and most relevant results. Dosimetry is performed with higher precision and better reproducibility than in the past, and dosimetric data should be systematically collected, as treatment planning and verification may help exploit the full potential of theranostic of CNS tumors.

Intra-arterial administration of PSMA-targeted radiopharmaceuticals for brain tumors: is the era of interventional theranostics next?

In recent years, prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, has emerged as a promising biomarker for theranostics, integrating diagnosis and therapy. PSMA's overexpression in various tumors, including brain metastases and high-grade gliomas, suggests its potential in neuro-oncology. Pruis et al. conducted a proof-of-concept study comparing intra-arterial (IA) and intravenous (IV) administration of 68Ga-PSMA-11 in brain tumor patients, aiming to enhance radioligand therapy (RLT) outcomes. Ten patients underwent IV and super-selective IA (ssIA) tracer administration, showing higher tumor uptake and more favorable biodistribution after ssIA administration on positron emission tomography (PET). Dosimetry modeling on the basis of PET data resulted in median absorbed radiation doses per tumor per cycle notably higher with ssIA with respect to IV administration, indicating its potential for RLT optimization. Challenges persist, notably in penetrating intact blood-brain barriers and targeting tumor cells effectively. To overcome these limitations, novel approaches like convection-enhanced delivery and focused ultrasound warrant exploration. Safety concerns, though minimal in this study, underscore the need for larger trials and AI-assisted procedures. PSMA's role in neuro-oncological theranostics is promising, but future research must address specificity and compare it with emerging targets.

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.

Prostate-Specific Membrane Antigen Radioligand Therapy in Non-Prostate Cancers: Where Do We Stand?

INTRODUCTION

The term theragnostic refers to the combination of a predictive imaging biomarker with a therapeutic agent. The promising application of prostate-specific membrane antigen (PSMA)-based radiopharmaceuticals in the imaging and treatment of prostate cancer (PCa) patients opens the way to investigate a possible role of PSMA-based radiopharmaceuticals in cancers beyond the prostate. Therefore, the aim of this review was to evaluate the role of 177Lu-PSMA radioligand therapy (RLT) in malignancies other than prostate cancer by evaluating preclinical, clinical studies, and ongoing clinical trials.

METHODS

An extensive literature search was performed in three different databases using different combinations of the following terms: "Lu-PSMA", "177Lu-PSMA", "preclinical", "mouse", "salivary gland cancer", "breast cancer", "glioblastoma", "solid tumour", "renal cell carcinoma", "HCC", "thyroid", "salivary", "radioligand therapy", and "lutetium-177". The search had no beginning date limit and was updated to April 2024. Only articles written in English were included in this review.

RESULTS

A total of four preclinical studies were selected (breast cancer model n = 3/4). PSMA-RLT significantly reduced cell viability and had anti-angiogenic effects, especially under hypoxic conditions, which increase PSMA binding and uptake. Considering the clinical studies (n = 8), the complexity of evaluating PSMA-RLT in cancers other than prostate cancer was clearly revealed, since in most of the presented cases a sufficient tumour radiation dose was not achieved. However, encouraging results can be found in some types of diseases, such as thyroid cancer. Some clinical trials are still ongoing, and results from prospective larger cohorts of patients are awaited.

CONCLUSIONS

The need for larger patient cohorts and more RLT cycles administered underscores the need for further comprehensive studies. Given the very preliminary results of both preclinical and clinical studies, ongoing clinical trials in the near future may provide stronger evidence of both the safety and therapeutic efficacy of PSMA-RLT in malignancies other than prostate cancer.

The Utility of Prostate-Specific Membrane Antigen-11 PET in Detection and Management of Central Nervous System Neoplasms

ABSTRACT

We present a case series of 5 patients diagnosed with schwannoma and 1 patient diagnosed with astrocytoma who underwent PSMA PET imaging for tumor detection. We retrospectively analyzed the records of 4 male and 2 female patients (mean age, 53.2 ± 13.2) who underwent PSMA PET imaging between March and September 2023. PET interpretation showed increased Ga-PSMA-11 accumulation in all patients with a mean SUV max of 3.11 ± 1.8. This series underscores PSMA PET's potential for CNS neoplasm detection.

Automated radiosynthesis and preclinical evaluation of two new PSMA-617 derivatives radiolabelled via [18F]AlF2+ method

BACKGROUND

In the last decade the development of new PSMA-ligand based radiopharmaceuticals for the imaging and therapy of prostate cancer has been a highly active and important area of research. The most promising derivative in terms of interaction with the antigen and clinical properties has been found to be "PSMA-617", and its lutetium-177 radiolabelled version has recently been approved by EU and USA regulatory agencies for therapeutic purposes. For the above reasons, the development of new derivatives of PSMA-617 radiolabelled with fluorine-18 may still be of great interest. This paper proposes the comparison of two different PSMA-617 derivatives functionalized with NODA and RESCA chelators, respectively, radiolabelled via [18F]AlF2+ complexation.

RESULTS

The organic synthesis of two PSMA-617 derivatives and their radiolabelling via [18F]AlF2+ complexation resulted to proceed efficiently and successfully. Moreover, stability in solution and in plasma has been evaluated. The whole radiosynthesis procedure has been fully automated, and the final products have been obtained with radiochemical yield and purity potentially suitable for clinical studies. The biodistribution of the two derivatives was performed both in prostate cancer and glioma tumour models. Compared with the reference [18F]F-PSMA-1007 and [18F]F-PSMA-617-RESCA, [18F]F-PSMA-617-NODA derivative showed a higher uptake in both tumors, faster clearance in non-target organs, and lower uptake in salivary glands.

CONCLUSION

PSMA-617 NODA and RESCA derivatives were radiolabelled successfully via [18F]AlF2+ chelation, the former being more stable in solution and human plasma. Moreover, preclinical biodistribution studies showed that [18F]F-PSMA-617-NODA might be of potential interest for clinical applications.

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.

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.

PSMA-targeted therapy for non-prostate cancers

Radioligand therapy (RLT) agents are demonstrating a crucial role in the clinical approach to aggressive malignancies such as metastatic castrate-resistant prostate cancer (m-CRPC). With the recent FDA approval of prostate-specific membrane antigen (PSMA)-targeted RLT for m-CRPC, the field has broadened its gaze to explore other cancers that express PSMA in the tumor parenchyma or tumor neovasculature. In this review article, we discuss current progress in the clinical use of PSMA RLTs in non-prostate cancers such salivary gland cancers, renal cell carcinoma, high grade glioma, and soft tissue sarcoma. We highlight early reports in small case series and clinical trials indicating promise for PSMA-targeted RLT and highlighting the importance of identifying patient cohorts who may most benefit from these interventions. Further study is indicated in non-prostate cancers investigating PSMA RLT dosimetry, PSMA PET/CT imaging as a biomarker, and assessing PSMA RLT safety and efficacy in these cancers.

Impact of [177Lu]Lu-PSMA-617 Radioligand Therapy on Reference Organ Uptake Assessed by [68Ga]Ga-PSMA-11-PET/CT

This study aims to assess the change in uptake to reference organs, including the liver, parotid and salivary glands after radioligand therapy (RLT) with [177Lu]Lu-PSMA-617 in relation to pretreatment imaging metrics. Eighty-five patients with mCRPC underwent [68Ga]Ga-PSMA-11 PET/CT imaging prior to (pre RLT PET) and after (post RLT PET) a median of 3 (IQR 2-6) RLT cycles with [177Lu]Lu-PSMA-617. PSMA-positive tumor burden was stratified into 4 groups based on modified PROMISE criteria (oligofocal, multifocal, disseminated, diffuse). Uptake (SUVmean, SUVmax) in liver tissue, parotid and submandibular glands was measured. A control group was established with 54 patients who had received two separate PET acquisitions following the same protocol (PET1, PET2) within 12 months for localized or oligofocal prostate cancer without RLT in the interim. Baseline uptake values (SUVmean, SUVmax) in parotid (10.8 ± 3.2, 16.8 ± 5.4) and submandibular glands (11.3 ± 2.8, 18.1 ± 4.7) are 2-fold compared to liver uptake (4.9 ± 1.4, 7.7 ± 2.0), with no significant change between PET 1 and PET 2 in the control group. In the RLT group, increasing tumor burden class is significantly associated with decreasing uptake in the liver (p = 0.013), parotid (p < 0.001) and submandibular glands (p < 0.001); this tumor sink effect by respective tumor burden is widely maintained after RLT (p = 0.011, p < 0.001, p < 0.001). RLT has a significant impact on salivary gland uptake with decreasing values per patient in all groups of disease burden change (up to -30.4% in submandibular glands, p < 0.001), while liver tissue shows rising values in patients with declining tumor burden throughout RLT (+18.6%, p = 0.020). Uptake in liver tissue and salivary glands on [68Ga]Ga-PSMA-11 PET/CT imaging is inversely related to tumor burden prior to and following RLT with [177Lu]Lu-PSMA-617. Per patient, salivary gland uptake is further reduced throughout RLT independently from tumor burden, while changes in liver uptake remain burden-dependent. Liver and salivary gland uptake-derived metrics and segmentation thresholds may thus be of limited value when used as reference for response assessment to RLT.

[177Lu]Lu-PSMA Therapy as an Individual Treatment Approach for Patients with High-Grade Glioma: Dosimetry Results and Critical Statement

The theranostic use of prostate-specific membrane antigen (PSMA) appears to be promising in patients with high-grade glioma. This study investigated [¹⁷⁷Lu]Lu-PSMA therapy as an individual treatment approach with a focus on intratherapeutic dosimetry. Methods: Three patients were treated with a median of 6.03 GBq (interquartile range [IQR], 5.74-6.10) of [¹⁷⁷Lu]Lu-PSMA. Intratherapeutic dosimetry was performed using a hybrid scenario with planar whole-body scintigraphy at 2, 24, and 48 h after treatment injection and SPECT/CT at 48 h after injection. Additive whole-body scintigraphy at 8 d after injection was performed on 1 patient. Results: The median doses were 0.56 Gy (IQR, 0.36-1.25 Gy) to tumor, 0.27 Gy (IQR, 0.16-0.57 Gy) to risk organs, 2.13 Gy (IQR, 1.55-2.89 Gy) to kidneys, and 0.76 Gy (IQR, 0.70-1.20 Gy) to salivary glands. Whole-body exposure was 0.11 Gy (IQR, 0.06-0.18 Gy). Conclusion: Because the intratherapeutic tumor dose is lower than that used in external radiation oncology, the effectiveness of treatment is questionable.