Induction of PSMA and Internalization of an Anti-PSMA mAb in the Vascular Compartment

All that glitters is not gold: high uptake on PSMA PET in non-prostate cancers does not mean that treatment with [177Lu]Lu-PSMA-radioligand will be successful

BACKGROUND

The main objective is to discuss why treatment of non-prostate cancers with [177Lu]Lu-PSMA-radioligand achieved only low tumor dose in most published cases, despite high uptake on PSMA PET. We use a patient with renal cell carcinoma as an illustrative example. Furthermore, we discuss how the problem with early washout and low tumor dose might be overcome by using a radionuclide with shorter half-life, matching the target binding residence time.

CASE PRESENTATION

[68Ga]Ga-PSMA-11 PET/CT of a 56-year old man with metastatic renal cell carcinoma showed high lesion uptake. One dose of 6.9 GBq [177Lu]Lu-PSMA-I&T was administrated. Post-therapy dosimetry was performed with SPECT/CT and whole-body planar imaging after 5, 24 and 48 h. Doses to target lesions were only 0.2-0.5 Gy. No treatment effect was achieved.

CONCLUSION

Rapid tumor washout of [177Lu]Lu-PSMA-I&T and low tumor dose despite high uptake of [68Ga]Ga-PSMA-11 are most likely caused by localization of PSMA-receptors on neovasculature rather than on the tumor cells, and unlike in prostate cancer cells, the PSMA-RL / PSMA-receptor complex is not internalized. To overcome the problem with early washout, the use of a radionuclide with shorter half-life matching the target binding residence time will be needed.

Prostate Specific Membrane Antigen Expression in a Syngeneic Breast Cancer Mouse Model

PURPOSE

Prostate specific membrane antigen (PSMA) has been studied in human breast cancer (BCa) biopsies, however, lack of data on PSMA expression in mouse models impedes development of PSMA-targeted therapies, particularly in improving breast conserving surgery (BCS) margins. This study aimed to validate and characterize the expression of PSMA in murine BCa models, demonstrating that PSMA can be utilized to improve therapies and imaging techniques.

METHODS

Murine triple negative breast cancer 4T1 cells, and human cell lines, MDA-MB-231, MDA-MB-468, implanted into the mammary fat pads of BALB/c mice, were imaged by our PSMA targeted theranostic agent, PSMA-1-Pc413, and tumor to background ratios (TBR) were calculated to validate selective uptake. Immunohistochemistry was used to correlate PSMA expression in relation to CD31, an endothelial cell biomarker highlighting neovasculature. PSMA expression was also quantified by Reverse Transcriptase Polymerase Chain Reaction (RT-PCR).

RESULTS

Accumulation of PSMA-1-Pc413 was observed in 4T1 primary tumors and associated metastases. Average TBR of 4T1 tumors were calculated to be greater than 1.5-ratio at which tumor tissues can be distinguished from normal structures-at peak accumulation with the signal intensity in 4T1 tumors comparable to that in high PSMA expressing PC3-pip tumors. Extraction of 4T1 tumors and lung metastases followed by RT-PCR analysis and PSMA-CD31 co-staining shows that PSMA is consistently localized on tumor neovasculature with no expression in tumor cells and surrounding normal tissues.

CONCLUSION

The selective uptake of PSMA-1-Pc413 in these cancer tissues as well as the characterization and validation of PSMA expression on neovasculature in this syngeneic 4T1 model emphasizes their potential for advancements in targeted therapies and imaging techniques for BCa. PSMA holds great promise as an oncogenic target for BCa and its associated metastases.

Evaluation of Prostate-Specific Membrane Antigen (PSMA) Immunohistochemical Expression in Early-Stage Breast Cancer Subtypes

Breast cancer, known for its diverse subtypes, ranks as one of the leading causes of cancer-related deaths. Prostate-specific membrane antigen (PSMA), primarily associated with prostate cancer, has also been identified in breast cancer, though its role remains unclear. This study aimed to evaluate PSMA expression across different subtypes of early-stage breast cancer and investigate its correlation with clinicopathological factors. This retrospective study included 98 breast cancer cases. PSMA expression was examined in both tumor cells and tumor-associated blood vessels. The analysis revealed PSMA expression in tumor-associated blood vessels in 88 cases and in tumor cells in 75 cases. Ki67 expression correlated positively with PSMA expression in blood vessels (p < 0.0001, RSpearman 0.42) and tumor cells (p = 0.010, RSpearman 0.26). The estrogen and progesterone receptor expression correlated negatively with PSMA levels in blood vessels (p = 0.0053, R Spearman -0.26 and p = 0.00026, R Spearman -0.347, respectively). Human epidermal growth factor receptor 2 (HER2) status did not significantly impact PSMA expression. We did not detect any statistically significant differences between breast cancer subtypes. These findings provide evidence for a heterogenous PSMA expression in breast cancer tissue and suggest its correlation with tumor aggressiveness. Despite the limited sample size, the study provides valuable insights into the potential of PSMA as a prognostic, diagnostic, and therapeutic target in the management of breast cancer.

The clinical signification and application value of [68Ga]Ga-PSMA imaging in thyroid malignancy

PURPOSE

Approximately 5% of differentiated thyroid cancers lose the ability to uptake iodine, leading to limited treatment options and poor prognosis due to invasion and distant metastasis. PSMA imaging probes have been proposed as a potential diagnostic and therapeutic tool for iodine-refractory thyroid cancer. However, there are limited reports and significant heterogeneity in patient selection, warranting further exploration of the application value of PSMA in thyroid cancer.

METHODS

We performed Western Blot, PCR, and [68Ga]Ga-PSMA uptake experiments on cell lines and conducted in vivo small animal imaging. Clinical and radiological results of included differentiated thyroid cancer patients were collected. (Trial registration number: 2021-669, Trial registration date: December 30, 2021).

RESULTS

PSMA expression levels were significantly higher in poorly differentiated thyroid cancer (7.86 ± 1.90 vs. 1.00 ± 0, P < 0.01; 7.86 ± 1.90 vs. 0.03 ± 0.02, P < 0.01), but [68Ga]Ga-PSMA imaging correlated with tumor burden, such as 18F-FDG (8.08 ± 7.74 and 5.67 ± 4.23, P = 0.01) and Tg levels (307.1 ± 183.4 vs. 118.0 ± 116.1, P = 0.002).

CONCLUSION

Our results showed that PSMA expression increased with the decrease of thyroid cancer differentiation. However, the level of [68Ga]Ga-PSMA uptake in thyroid cancer patients was not significantly associated with the degree of thyroid cancer differentiation, but also with the metabolism and burden of tumors such as 2-[18F]FDG and Tg levels. These findings provide additional clinical significance and application value for PSMA in thyroid cancer.

The prostate-specific membrane antigen holds potential as a vascular target for endogenous radiotherapy with [177Lu]Lu-PSMA-I&T for triple-negative breast cancer

INTRODUCTION

Overexpression of prostate-specific membrane antigen (PSMA) on the vasculature of triple-negative breast cancer (TNBC) presents a promising avenue for targeted endogenous radiotherapy with [177Lu]Lu-PSMA-I&T. This study aimed to assess and compare the therapeutic efficacy of a single dose with a fractionated dose of [177Lu]Lu-PSMA-I&T in an orthotopic model of TNBC.

METHODS

Rj:NMRI-Foxn1nu/nu mice were used as recipients of MDA-MB-231 xenografts. The single dose group was treated with 1 × 60 ± 5 MBq dose of [177Lu]Lu-PSMA-I&T, while the fractionated dose group received 4 × a 15 ± 2 MBq dose of [177Lu]Lu-PSMA-I&T at 7 day intervals. The control group received 0.9% NaCl. Tumor progression was monitored using [18F]FDG-PET/CT. Ex vivo analysis encompassed immunostaining, TUNEL staining, H&E staining, microautoradiography, and autoradiography.

RESULTS

Tumor volumes were significantly smaller in the single dose (p < 0.001) and fractionated dose (p < 0.001) groups. Tumor growth inhibition rates were 38% (single dose) and 30% (fractionated dose). Median survival was notably prolonged in the treated groups compared to the control groups (31d, 28d and 19d for single dose, fractionated dose and control, respectively). [177Lu]Lu-PSMA-I&T decreased the size of viable tumor areas. We further demonstrated, that [177Lu]Lu-PSMA-I&T binds specifically to the tumor-associated vasculature.

CONCLUSION

This study highlights the potential of [177Lu]Lu-PSMA-I&T for endogenous radiotherapy of TNBC.

PSMA-Expression Is Highly Associated with Histological Subtypes of Renal Cell Carcinoma: Potential Implications for Theranostic Approaches

In renal cell carcinoma (RCC), accurate imaging methods are required for treatment planning and response assessment to therapy. In addition, there is an urgent need for new therapeutic options, especially in metastatic RCC. One way to combine diagnostics and therapy in a so-called theranostic approach is the use of radioligands directed against surface antigens. For instance, radioligands against prostate-specific membrane antigen (PSMA) have already been successfully used for diagnosis and radionuclide therapy of metastatic prostate cancer. Recent studies have demonstrated that PSMA is expressed not only in prostate cancer but also in the neovasculature of several solid tumors, which has raised hopes to use PSMA-guided theranostic approaches in other tumor entities, too. However, data on PSMA expression in different histopathological subtypes of RCC are sparse. Because a better understanding of PSMA expression in RCC is critical to assess which patients would benefit most from theranostic approaches using PSMA-targeted ligands, we investigated the expression pattern of PSMA in different subtypes of RCC on protein level. Immunohistochemical staining for PSMA was performed on formalin-fixed, paraffin-embedded archival material of major different histological subtypes of RCC (clear cell RCC (ccRCC)), papillary RCC (pRCC) and chromophobe RCC (cpRCC). The extent and intensity of PSMA staining were scored semi-quantitatively and correlated with the histological RCC subtypes. Group comparisons were calculated with the Kruskal-Wallis test. In all cases, immunoreactivity was detected only in the tumor-associated vessels and not in tumor cells. Staining intensity was the strongest in ccRCC, followed by cpRCC and pRCC. ccRCC showed the most diffuse staining pattern, followed by cpRCC and pRCC. Our results provide a rationale for PSMA-targeted theranostic approaches in ccRCC and cpRCC.

The current status of prostate cancer treatment and PSMA theranostics

In the treatment of cancer, understanding the disease status, or accurate staging, is extremely important, and various imaging techniques are used. Computed tomography (CT), magnetic resonance imaging, and scintigrams are commonly used for solid tumors, and advances in these technologies have improved the accuracy of diagnosis. In the clinical practice of prostate cancer, CT and bone scans have been considered especially important for detecting metastases. Nowadays, CT and bone scans are called conventional methods because positron emission tomography (PET), especially prostate-specific membrane antigen (PSMA)/PET, is extremely sensitive in detecting metastases. Advances in functional imaging, such as PET, are advancing the diagnosis of cancer by allowing information to be added to the morphological diagnosis. Furthermore, PSMA is known to be upregulated depending on the malignancy of the prostate cancer grade and resistance to therapy. Therefore, it is often highly expressed in castration-resistant prostate cancer (CRPC) with poor prognosis, and its therapeutic application has been attempted for around two decades. PSMA theranostics refers to a type of cancer treatment that combines both diagnosis and therapy using a PSMA. The theranostic approach uses a radioactive substance attached to a molecule that targets PSMA protein on cancer cells. This molecule is injected into the patient's bloodstream and can be used for both imaging the cancer cells with a PET scan (PSMA PET imaging) and delivering radiation directly to the cancer cells (PSMA-targeted radioligand therapy), with the aim of minimizing damage to healthy tissue. Recently, in an international phase III trial, the impact of ¹⁷⁷Lu-PSMA-617 therapy was studied in patients with advanced PSMA-positive metastatic CRPC who had previously been treated with specific inhibitors and regimens. The trial revealed that ¹⁷⁷Lu-PSMA-617 significantly extended both progression-free survival and overall survival compared to standard care alone. Although there was a higher incidence of grade 3 or above adverse events with ¹⁷⁷Lu-PSMA-617, it did not negatively impact the patients' quality of life. PSMA theranostics is currently being studied and used primarily for the treatment of prostate cancer, but it has the potential to be applied to other types of cancers as well.

The Potential of PSMA as a Vascular Target in TNBC

Recent studies proving prostate-specific membrane antigen (PSMA) expression on triple-negative breast cancer (TNBC) cells and adjacent endothelial cells suggest PSMA as a promising target for therapy of until now not-targetable cancer entities. In this study, PSMA and its isoform expression were analyzed in different TNBC cells, breast cancer stem cells (BCSCs), and tumor-associated endothelial cells. PSMA expression was detected in 91% of the investigated TNBC cell lines. The PSMA splice isoforms were predominantly found in the BCSCs. Tumor-conditioned media from two TNBC cell lines, BT-20 (high full-length PSMA expression, PSMAΔ18 expression) and Hs578T (low full-length PSMA expression, no isoform expression), showed significant pro-angiogenic effect with induction of tube formation in endothelial cells. All TNBC cell lines induced PSMA expression in human umbilical vein endothelial cells (HUVEC). Significant uptake of radiolabeled ligand [⁶⁸Ga]Ga-PSMA was detected in BCSC1 (4.2%), corresponding to the high PSMA expression. Moreover, hypoxic conditions increased the uptake of radiolabeled ligand [¹⁷⁷Lu]Lu-PSMA in MDA-MB-231 (0.4% vs. 3.4%, under hypoxia and normoxia, respectively) and MCF-10A (0.3% vs. 3.0%, under normoxia and hypoxia, respectively) significantly (p < 0.001). [¹⁷⁷Lu]Lu-PSMA-induced apoptosis rates were highest in BT-20 and MDA-MB-231 associated endothelial cells. Together, these findings demonstrate the potential of PSMA-targeted therapy in TNBC.

Therapeutic potential of targeting mirnas to prostate cancer tumors: using psma as an active target

Prostate cancer (PC) is a commonly diagnosed malignancy in men and is associated with high mortality rates. Current treatments for PC include surgery, chemotherapy, and radiation therapy. However, recent advances in targeted delivery systems have yielded promising new approaches to PC treatment. As PC epithelial cells express high levels of prostate-specific membrane antigen (PSMA) on the cell surface, new drug conjugates focused on PSMA targeting have been developed. microRNAs (miRNAs) are small noncoding RNAs that regulate posttranscriptional gene expression in cells and show excellent possibilities for use in developing new therapeutics for PC. PSMA-targeted therapies based on a miRNA payload and that selectively target PC cells enhances therapeutic efficacy without eliciting damage to normal surrounding tissue. This review discusses the rationale for utilizing miRNAs to target PSMA, revealing their potential in therapeutic approaches to PC treatment. Different delivery systems for miRNAs and challenges to miRNA therapy are also explored.

Current role of prostate-specific membrane antigen-based imaging and radioligand therapy in castration-resistant prostate cancer

Castration-resistant prostate cancer (CRPC) is a therapy-resistant and lethal form of prostate cancer as well as a therapeutic challenge. Prostate-specific membrane antigen (PSMA) has been proved as a promising molecular target for optimizing the theranostics for CRPC patients. When combined with PSMA radiotracers, novel molecular imaging techniques such as positron emission tomography (PET) can provide more accurate and expedient identification of metastases when compared with conventional imaging techniques. Based on the PSMA-based PET scans, the accurate visualization of local and disseminative lesions may help in metastasis-directed therapy. Moreover, the combination of ⁶⁸Ga-labeled PSMA-based PET imaging and radiotherapy using PSMA radioligand therapy (RLT) becomes a novel treatment option for CRPC patients. The existing studies have demonstrated this therapeutic strategy as an effective and well-tolerated therapy among CRPC patients. PSMA-based PET imaging can accurately detect CRPC lesions and describe their molecular features with quantitative parameters, which can be used to select the best choice of treatments, monitor the response, and predict the outcome of RLT. This review discussed the current and potential role of PSMA‐based imaging and RLT in the diagnosis, treatment, and prediction of prognosis of CRPC.

Advances in PSMA theranostics

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PSMA is an appealing target for theranostic because it is a transmembrane protein with a known substrate that is overexpessed on prostate cancer cells and internalizes upon ligand binding.

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There are a number of PSMA theranostic ligands in clinical evaluation, clinical trial, or clinically approved.

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PSMA theranostic ligands increase progression-free survival, overall survival, and pain in patients with metastatic castration resistant prostate cancer.

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A major obstacle to PSMA-targeted radioligand therapy is off-target toxicity in salivary glands.

The validation of prostate specific membrane antigen (PSMA) as a molecular target in metastatic castration-resistant prostate cancer has stimulated the development of multiple classes of theranostic ligands that specifically target PSMA. Theranostic ligands are used to image disease or selectively deliver cytotoxic radioactivity to cells expressing PSMA according to the radioisotope conjugated to the ligand. PSMA theranostics is a rapidly advancing field that is now integrating into clinical management of prostate cancer patients. In this review we summarize published research describing the biological role(s) and activity of PSMA, highlight the most clinically advanced PSMA targeting molecules and biomacromolecules, and identify next generation PSMA ligands that aim to further improve treatment efficacy. The goal of this review is to provide a comprehensive assessment of the current state-of-play and a roadmap to achieving further advances in PSMA theranostics.

Imaging of Tumor-Associated Vascular Prostate-Specific Membrane Antigen in Woodchuck Model of Hepatocellular Carcinoma

BACKGROUND AND AIMS

Radiolabeled short peptide ligands targeting prostate-specific membrane antigen (PSMA) were developed initially for imaging and treatment of prostate cancers. While many nonprostate solid tumors including hepatocellular carcinoma (HCC) express little PSMA, their neovasculature expresses a high level of PSMA, which is avid for Gallium-68-labeled PSMA-targeting radio-ligand (⁶⁸Ga-PSMA-11) for positron emission tomography (PET). However, the lack of a spontaneous animal model of tumor-associated vascular PSMA overexpression has hindered the development and assessment of PSMA-targeting radioligands for imaging and therapy of the nonprostatic cancers. We identified detectable indigenous PSMA expression on tumor neovascular endothelia in a naturally occurring woodchuck model of HCC.

METHODS

Molecular docking was performed with 3 bait PSMA ligands and compared between human and woodchuck PSMA. Initially, PET images were acquired dynamically after intravenously injecting 37 MBq (1.0 mCi) of ⁶⁸Ga-PSMA-11 into woodchuck models of HCC. Subsequently, 10-minute static PET scans were conducted for other animals 1-hour after injection due to HCC and liver background uptake stabilization at 30-45 minutes after injection. Liver tissue samples were harvested after imaging, fresh-frozen for quantitative reverse transcription polymerase chain reaction and western blot for validation, or fixed for histology for correlation.

RESULTS

Our preclinical studies confirmed the initial clinical findings of ⁶⁸Ga-PSMA-11 uptake in HCC. The agents (ligands and antibodies) developed against human PSMA were found to be reactive against the woodchuck PSMA.

CONCLUSION

This animal model offers a unique opportunity for investigating the biogenesis of tumor-associated vascular PSMA, its functional role(s), and potentials for future treatment strategies targeting tumor vascular PSMA using already developed PSMA-targeting agents.

Angiogenesis as Therapeutic Target in Metastatic Prostate Cancer - Narrowing the Gap Between Bench and Bedside

Angiogenesis in metastatic castration-resistant prostate cancer (mCRPC) has been extensively investigated as a promising druggable biological process. Nonetheless, targeting angiogenesis has failed to impact overall survival (OS) in patients with mCRPC despite promising preclinical and early clinical data. This discrepancy prompted a literature review highlighting the tumor heterogeneity and biological context of Prostate Cancer (PCa). Narrowing the gap between the bench and bedside appears critical for developing novel therapeutic strategies. Searching clinicaltrials.gov for studies examining angiogenesis inhibition in patients with PCa resulted in n=20 trials with specific angiogenesis inhibitors currently recruiting (as of September 2021). Moreover, several other compounds with known anti-angiogenic properties - such as Metformin or Curcumin - are currently investigated. In general, angiogenesis-targeting strategies in PCa include biomarker-guided treatment stratification - as well as combinatorial approaches. Beyond established angiogenesis inhibitors, PCa therapies aiming at PSMA (Prostate Specific Membrane Antigen) hold the promise to have a substantial anti-angiogenic effect - due to PSMA´s abundant expression in tumor vasculature.

Expression of Prostate Specific Membrane Antigen (PSMA) in Breast Cancer

Background Prostate specific membrane antigen (PSMA) is a promising protein for breast cancer patients. It has not only been detected in prostate cancer but is also expressed by tumor cells and the endothelial cells of tumor vessels in breast cancer patients. PSMA plays a role in tumor progression and tumor angiogenesis. For this reason, a number of diagnostic and therapeutic methods to target PSMA have been developed. Method This paper provides a general structured overview of PSMA and its oncogenic potential, with a special focus on its role in breast cancer. This narrative review is based on a selective literature search carried out in PubMed and the library of Freiburg University Clinical Center. The following key words were used for the search: "PSMA", "PSMA and breast cancer", "PSMA PET/CT", "PSMA tumor progression". Relevant articles were explicitly read through, processed, and summarized. Conclusion PSMA could be a new diagnostic and therapeutic alternative, particularly for triple-negative breast cancer. It appears to be a potential predictive and prognostic marker.

PSMA-positive membranes secreted from prostate cancer cells have potency to transform vascular endothelial cells into an angiogenic state

BACKGROUND

Prostate-specific membrane antigen (PSMA) is highly expressed in poorly differentiated, metastatic, and castration-resistant prostate cancers. Recently, 68Ga-PSMA positron emission tomography/computed tomography has been successfully developed as an effective diagnostic tool for prostate cancer. However, the pathophysiological functions of PSMA in prostate tumors remain unclear.

METHODS

We examined the protein expression of PSMA in tumor endothelial cells in human prostate tumors by immunohistochemistry. Prostate cancer tissues were resected by robotic surgery in 2019 at Ehime University from patients with prostate cancer. In vitro, we prepared conditioned medium (CM) derived from a PSMA-positive human prostate cancer cell line, LNCaP, cultured on collagen I gels. We then examined PSMA expression in human umbilical vascular endothelial cells (HUVECs) cultured with the CM. We assessed angiogenic activities by treatment of HUVECs with LNCaP-derived CM using a tube formation assay that mimics angiogenesis.

RESULTS

Immunohistochemistry of PSMA and CD31, a marker of endothelial cells, and PSMA-expressing tumor endothelial cells were observed in 4 of 33 prostate cancer patients (12.1%). We also found that the 10,000g pellet fraction of the LNCaP-derived CM containing PSMA-positive membranes, such as microvesicles transformed HUVECs "PSMA-negative" into "PSMA-positive." Furthermore, treatment of HUVECs with the 10,000g pellet fraction of the LNCaP-derived CM significantly promoted tube formation, mimicking angiogenesis in a PSMA-dependent manner.

CONCLUSIONS

Our findings revealed the existence of PSMA-positive tumor endothelial cells in human prostate tumors, which enhances tumor angiogenesis in prostate cancer tissues.

PSMA PET Imaging in Glioblastoma: A Preclinical Evaluation and Theranostic Outlook

BACKGROUND

Prostate specific membrane antigen (PSMA) PET imaging has recently gained attention in glioblastoma (GBM) patients as a potential theranostic target for PSMA radioligand therapy. However, PSMA PET has not yet been established in a murine GBM model. Our goal was to investigate the potential of PSMA PET imaging in the syngeneic GL261 GBM model and to give an outlook regarding the potential of PMSA radioligand therapy in this model.

METHODS

We performed an 18F-PSMA-1007 PET study in the orthotopic GL261 model (n=14 GBM, n=7 sham-operated mice) with imaging at day 4, 8, 11, 15, 18 and 22 post implantation. Time-activity-curves (TAC) were extracted from dynamic PET scans (0-120 min p. i.) in a subset of mice (n=4 GBM, n=3 sham-operated mice) to identify the optimal time frame for image analysis, and standardized-uptake-values (SUV) as well as tumor-to-background ratios (TBR) using contralateral normal brain as background were calculated in all mice. Additionally, computed tomography (CT), ex vivo and in vitro 18F-PSMA-1007 autoradiographies (ARG) were performed.

RESULTS

TAC analysis of GBM mice revealed a plateau of TBR values after 40 min p. i. Therefore, a 30 min time frame between 40-70 min p. i. was chosen for PET quantification. At day 15 and later, GBM mice showed a discernible PSMA PET signal on the inoculation site, with highest TBRmean in GBM mice at day 18 (7.3 ± 1.3 vs. 1.6 ± 0.3 in shams; p=0.024). Ex vivo ARG confirmed high tracer signal in GBM compared to healthy background (TBRmean 26.9 ± 10.5 vs. 1.6 ± 0.7 in shams at day 18/22 post implantation; p=0.002). However, absolute uptake values in the GL261 tumor remained low (e.g., SUVmean 0.21 ± 0.04 g/ml at day 18) resulting in low ratios compared to dose-relevant organs (e.g., mean tumor-to-kidney ratio 1.5E-2 ± 0.5E-2).

CONCLUSIONS

Although 18F-PSMA-1007 PET imaging of GL261 tumor-bearing mice is feasible and resulted in high TBRs, absolute tumoral uptake values remained low and hint to limited applicability of the GL261 model for PSMA-directed therapy studies. Further investigations are warranted to identify suitable models for preclinical evaluation of PSMA-targeted theranostic approaches in GBM.

Folate hydrolase-1 (FOLH1) is a novel target for antibody-based brachytherapy in Merkel cell carcinoma

Backgrounds

Folate Hydrolase‐1 (FOLH1; PSMA) is a type II transmembrane protein, luminally expressed by solid tumour neo‐vasculature. Monoclonal antibody (mAb), J591, is a vehicle for mAb‐based brachytherapy in FOLH1+ cancers. Brachytherapy is a form of radiotherapy that involves placing a radioactive material a short distance from the target tissue (e.g., on the skin or internally); brachytherapy is commonly accomplished with the use of catheters, needles, metal seeds and antibody or small peptide conjugates. Herein, FOLH1 expression in primary (p) and metastatic (m) Merkel cell carcinoma (MCC) is characterized to determine its targeting potential for J591‐brachytherapy.

Materials & Methods

Paraffin sections from pMCC and mMCC were evaluated by immunohistochemistry for FOLH1. Monte Carlo simulation was performed using the physical properties of conjugated radioisotope lutetium‐177. Kaplan–Meier survival curves were calculated based on patient outcome data and FOLH1 expression.

Results

Eighty‐one MCC tumours were evaluated. 67% (54/81) of all cases, 77% (24/31) pMCC and 60% (30/50) mMCC tumours were FOLH1+. Monte Carlo simulation showed highly localized ionizing tracks of electrons emitted from the targeted neo‐vessel. 42% (34/81) of patients with FOLH1+/− MCC had available survival data for analysis. No significant differences in our limited data set were detected based on FOLH1 status (p = 0.4718; p = 0.6470), staining intensity score (p = 0.6966; p = 0.9841) or by grouping staining intensity scores (− and + vs. ++, +++, +++) (p = 0.8022; p = 0.8496) for MCC‐specific survival or recurrence free survival, respectively.

Conclusions

We report the first evidence of prevalent FOLH1 expression within MCC‐associated neo‐vessels, in 60‐77% of patients in a large MCC cohort. Given this data, and the need for alternatives to immune therapies it is appropriate to explore the safety and efficacy of FOLH1‐targeted brachytherapy for MCC.

What's already known about this topic?

We report the first evidence of prevalent folate hydrolase‐1 (FOLH1; also known as prostate‐specific membrane antigen) expression within MCC‐associated neovessels.

What does this study add?

Herein, FOLH1 expression in Merkel cell carcinoma neovasculature is validated, and the therapeutic mechanism of specific, systemic targeting of disseminated disease with antibody‐based brachytherapy, is defined.

Comparison of Quantification of Target-Specific Accumulation of [18F]F-siPSMA-14 in the HET-CAM Model and in Mice Using PET/MRI

Simple Summary

Animal studies are essential for the development of new radiopharmaceuticals to determine specific accumulation and biodistribution. Alternative models, such as the HET-CAM model, offer the possibility of reducing animal experiments in accordance with the 3Rs principles. Accurate quantification of tumor accumulation of a PSMA-specific ligand in the HET-CAM model and comparison with corresponding animal experiments was performed using the imaging modalities PET and MRI. It was demonstrated that the HET-CAM model leads to comparable results and is suitable as an alternative to animal experiments for the initial assessment of target-specific binding of novel radiopharmaceuticals. However, as evaluation of biodistribution in ovo is still limited, further animal experiments with promising compounds are mandatory.

Abstract

Assessment of biodistribution and specific tumor accumulation is essential for the development of new radiopharmaceuticals and requires animal experiments. The HET-CAM (hens-egg test—chorioallantoic membrane) model can be used in combination with the non-invasive imaging modalities PET and MRI for pre-selection during radiopharmaceutical development to reduce the number of animal experiments required. Critical to the acceptance of this model is the demonstration of the quantifiability and reproducibility of these data compared to the standard animal model. Tumor accumulation and biodistribution of the PSMA-specific radiotracer [¹⁸F]F-siPSMA-14 was analyzed in the chick embryo and in an immunodeficient mouse model. Evaluation was based on MRI and PET data in both models. γ-counter measurements and histopathological analyses complemented these data. PSMA-specific accumulation of [¹⁸F]F-siPSMA-14 was successfully demonstrated in the HET-CAM model, similar to the results obtained by mouse model studies. The combination of MR and PET imaging allowed precise quantification of peptide accumulation, initial assessment of biodistribution, and accurate determination of tumor volume. Thus, the use of the HET-CAM model is suitable for the pre-selection of new radiopharmaceuticals and potentially reduces animal testing in line with the 3Rs principles of animal welfare.

Evolving Castration Resistance and Prostate Specific Membrane Antigen Expression: Implications for Patient Management

Metastatic castration-resistant prostate cancer (mCRPC) remains an incurable disease, despite multiple novel treatment options. The role of prostate-specific membrane antigen (PSMA) in the process of mCRPC development has long been underestimated. During the last years, a new understanding of the underlying molecular mechanisms of rising PSMA expression and its association with disease progression has emerged. Accurate understanding of these complex interactions is indispensable for a precise diagnostic process and ultimately successful treatment of advanced prostate cancer. The combination of different novel therapeutics such as androgen deprivation agents, 177LU-PSMA radioligand therapy and PARP inhibitors promises a new kind of efficacy. In this review, we summarize the current knowledge about the most relevant molecular mechanisms around PSMA in mCRPC development and how they can be implemented in mCRPC management.

Prostate-Specific Membrane Antigen (PSMA) Promotes Angiogenesis of Glioblastoma Through Interacting With ITGB4 and Regulating NF-κB Signaling Pathway

Background

Glioblastoma multiforme (GBM) is the most common primary malignant tumor in the central nervous system (CNS), causing the extremely poor prognosis. Combining the role of angiogenesis in tumor progression and the role of prostate-specific membrane antigen (PSMA) in angiogenesis, this study aims to explore the functions of PSMA in GBM.

Methods

Clinical GBM specimens were collected from 60 patients who accepted surgical treatment in Fudan University Shanghai Cancer Center between January 2018 and June 2019. Immunohistochemical staining was used to detect PSMA and CD31 expression in GBM tissues. Prognostic significance of PSMA was evaluated by bioinformatics. Human umbilical vein endothelial cells (HUVECs) transfected with PSMA overexpression plasmids or cultured with conditioned medium collected based on GBM cells, were used for CCK8, Transwell and tube formation assays. High-throughput sequencing and immunoprecipitation were used to explore the underlying mechanism. Furthermore, the in vivo experiment had been also conducted.

Results

We demonstrated that PSMA was abundantly expressed in endothelium of vessels of GBM tissues but not in vessels of normal tissues, which was significantly correlated with poor prognosis. Overexpression of PSMA could promotes proliferation, invasion and tube formation ability of human umbilical vein endothelial cells (HUVECs). Moreover, U87 or U251 conditioned medium could upregulated PSMA expression and induce similar effects on phenotypes of HUVECs, all of which could be partially attenuated by 2-PMPA treatment. The mechanistic study revealed that PSMA might promote angiogenesis of GBM through interacting with Integrin β4 (ITGB4) and activating NF-κB signaling pathway. The in vivo growth of GBM could be alleviated by the treatment of 2-PMPA.

Conclusion

This study identified PSMA as a critical regulator in angiogenesis and progression of GBM, which might be a promising therapeutic target for GBM treatment.

Nanobodies Targeting Prostate-Specific Membrane Antigen for the Imaging and Therapy of Prostate Cancer

The repertoire of methods for the detection and chemotherapeutic treatment of prostate cancer (PCa) is currently limited. Prostate-specific membrane antigen (PSMA) is overexpressed in PCa tumors and can be exploited for both imaging and drug delivery. We developed and characterized four nanobodies that present tight and specific binding and internalization into PSMA⁺ cells and that accumulate specifically in PSMA⁺ tumors. We then conjugated one of these nanobodies to the cytotoxic drug doxorubicin, and we show that the conjugate internalizes specifically into PSMA⁺ cells, where the drug is released and induces cytotoxic activity. In vivo studies show that the extent of tumor growth inhibition is similar when mice are treated with commercial doxorubicin and with a 42-fold lower amount of the nanobody-conjugated doxorubicin, attesting to the efficacy of the conjugated drug. These data highlight nanobodies as promising agents for the imaging of PCa tumors and for the targeted delivery of chemotherapeutic drugs.

Prostate-specific Membrane Antigen Based Antibody-drug Conjugates for Metastatic Castration-resistance Prostate Cancer

Cancer cells can be selectively targeted by identifying and developing antibodies to specific antigens present on the cancer cell surface. Cytotoxic agents can be conjugated to these antibodies that bind to these cell surface antigens in order to significantly increase the therapeutic index of whichever cytotoxic agent is utilized. This approach of conjugating the cytotoxic drugs to antibodies to target specific surface antigens enhances the anti-tumor activity of antibodies and improves the tumor-to-normal tissue selectivity of chemotherapy. Critical parameters in the development of these antibody-drug conjugates include: 1) selection of most appropriate antigen, 2) the ability of an antibody to be internalized after binding to the antigen, 3) cytotoxic drug potency and 4) stability of the antibody-drug conjugate. For prostate cancer, prostate-specific membrane antigen (PSMA, also known as folate hydrolase-1) is the most validated theragnostic target to date. PSMA is overexpressed on the prostate cancer cell surface, which makes it an even better target for selective drug delivery through conjugated antibodies. Here, we review the PSMA-based antibody-drug conjugates for metastatic castration-resistance prostate cancer (mCRPC).

Targeting of prostate-specific membrane antigen for radio-ligand therapy of triple-negative breast cancer

Background

Triple-negative breast cancer has extremely high risk of relapse due to the lack of targeted therapies, intra- and inter-tumoral heterogeneity, and the inherent and acquired resistance to therapies. In this study, we evaluate the potential of prostate-specific membrane antigen (PSMA) as target for radio-ligand therapy (RLT).

Methods

Tube formation was investigated after incubation of endothelial HUVEC cells in tumor-conditioned media and monitored after staining using microscopy. A binding study with ⁶⁸Ga-labeled PSMA-addressing ligand was used to indicate targeting potential of PSMA on tumor-conditioned HUVEC cells. For mimicking of the therapeutic application, tube formation potential and vitality of tumor-conditioned HUVEC cells were assessed following an incubation with radiolabeled PSMA-addressing ligand [¹⁷⁷Lu]-PSMA-617. For in vivo experiments, NUDE mice were xenografted with triple-negative breast cancer cells MDA-MB231 or estrogen receptor expressing breast cancer cells MCF-7. Biodistribution and binding behavior of [⁶⁸Ga]-PSMA-11 was investigated in both tumor models at 30 min post injection using μPET. PSMA- and CD31-specific staining was conducted to visualize PSMA expression and neovascularization in tumor tissue ex vivo.

Results

The triple-negative breast cancer cells MDA-MB231 showed a high pro-angiogenetic potential on tube formation of endothelial HUVEC cells. The induced endothelial expression of PSMA was efficiently addressed by radiolabeled PSMA-specific ligands. ¹⁷⁷Lu-labeled PSMA-617 strongly impaired the vitality and angiogenic potential of HUVEC cells. In vivo, as visualized by μPET, radiolabeled PSMA-ligand accumulated specifically in the triple-negative breast cancer xenograft MDA-MB231 (T/B ratio of 43.3 ± 0.9), while no [⁶⁸Ga]-PSMA-11 was detected in the estrogen-sensitive MCF-7 xenograft (T/B ratio of 1.1 ± 0.1). An ex vivo immunofluorescence analysis confirmed the localization of PSMA on MDA-MB231 xenograft-associated endothelial cells and also on TNBC cells.

Conclusions

Here we demonstrate PSMA as promising target for two-compartment endogenous radio-ligand therapy of triple-negative breast cancer.

Prostate specific membrane antigen (PSMA) expression in vena cava tumour thrombi of clear cell renal cell carcinoma suggests a role for PSMA-driven tumour neoangiogenesis

Background

Clear cell renal cell carcinoma (ccRCC) is a malignant renal neoplasm with a peculiar propensity to propagate as a contiguous tumor extension via the renal vein and inferior vena cava, occasionally reaching the right atrium. This intravascular tumor extension, often referred to as a tumor thrombus, represents the active growing front of the cancer. Prostate specific membrane antigen (PSMA), a glycoprotein that is extensively used in prostate cancer diagnostics, is a useful vascular marker for a variety of solid tumors. It is expressed in renal carcinomas. The aim of the current investigation was to analyse and compare the expression of PSMA at the growing front of the vena cava tumor extension with that found in the primary renal lesion.

Methods

Immunohistochemical (IHC) analysis of PSMA and CD34 was performed on archived paraffin embedded vena cava tumour thrombus tissue and matching renal tumours. These specimens were collected from radical nephrectomies of 10 patients with vena cava invasive (pT3b) ccRCC in a large tertiary hospital in Australia. Quantitative and qualitative morphometric analysis of PSMA IHC expression was performed with Aperio ImageScope morphometry using intensity and positive pixel counts of CD34 and PSMA from the IVC tumour slides and the corresponding renal tumour mass.

Results

PSMA and CD34 immunostaining were noted in the neovasculature of IVC tumour and renal tumour tissue. There was a higher PSMA/CD34 positive pixel count ratio noted in IVC tumour tissue when compared to renal tumour tissue. PSMA showed consistently increased expression in vena cava tumour, in comparison with the renal tumour mass.

Conclusions

Intravascular venous tumour extension expresses PSMA more intensely compared to intrarenal tumour tissue neovasculature. Our data suggest a possible mechanism for PSMA in neoangiogenesis and local progression of ccRCC and therefore its usefulness as a biomarker of neoangiogenesis for future diagnostic and therapeutic advancements.

PSMA expression in the Hi-Myc model; extended utility of a representative model of prostate adenocarcinoma for biological insight and as a drug discovery tool

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II (GCPII), is highly overexpressed in primary and metastatic prostate cancer (PCa). This has led to the development of radiopharmaceuticals for targeted imaging and therapy under current clinical evaluation. Despite this progress, the exact biological role of the protein in prostate cancer development and progression has not been fully elucidated. This is in part because the human PSMA and mouse PSMA (mPSMA) have different patterns of anatomical expression which confound study in the most widely utilized model organisms. Most notably, mPSMA is not expressed in the healthy murine prostate. Here, we reveal that mPSMA is highly upregulated in the prostate adenocarcinoma of the spontaneous Hi-Myc mouse model, a highly accurate and well characterized mouse model of prostate cancer development. Antibody detection and molecular imaging tools are used to confirm that mPSMA is expressed from early prostatic intraepithelial neoplasia (PIN) through adenocarcinoma.

Prostate-specific membrane antigen in breast cancer: a comprehensive evaluation of expression and a case report of radionuclide therapy

PURPOSE

Prostate-specific membrane antigen (PSMA), a protein product of the folate hydrolase 1 (FOLH1) gene, is gaining increasing acceptance as a target for positron emission tomography/computer tomography (PET/CT) imaging in patients with several cancer types, including breast cancer. So far, PSMA expression in breast cancer endothelia has not been sufficiently characterized.

METHODS

This study comprised 315 cases of invasive carcinoma of no special type (NST) and lobular breast cancer (median follow-up time 9.0 years). PSMA expression on tumor endothelia was detected by immunohistochemistry. Further, vascular mRNA expression of the FOLH1 gene (PSMA) was investigated in a cohort of patients with invasive breast cancer provided by The Cancer Genome Atlas (TCGA).

RESULTS

Sixty percent of breast cancer cases exhibited PSMA-positive endothelia with higher expression rates in tumors of higher grade, NST subtype with Her2-positivity, and lack of hormone receptors. These findings were confirmed on mRNA expression levels. The highest PSMA rates were observed in triple-negative carcinomas (4.5 × higher than in other tumors). Further, a case of a patient with metastatic breast cancer showing PSMA expression in PET/CT imaging and undergoing PSMA radionuclide therapy is discussed in detail.

CONCLUSIONS

This study provides a rationale for the further development of PSMA-targeted imaging in breast cancer, especially in triple-negative tumors.

Prostate-specific membrane antigen (PSMA) expression in breast cancer and its metastases

The present study was undertaken to investigate the expression of prostate-specific membrane antigen (PSMA) in normal breast tissues, in cancerous breast tissues and in distant metastases from patients with breast cancer. Immunohistochemical analysis was performed to determine PSMA expression and angiogenic activity using anti-PSMA mAb and anti-CD31 mAb respectively. Immunofluorescence staining was applied to confirm the exact co-localization of PSMA and CD31. We observed different patterns of PSMA expression between normal and cancerous tissues. Normal breast tissues showed PSMA expression only in normal glandular cells. However, primary breast tumors and distant metastases showed PSMA expression in tumor cells and in tumor-associated neovasculature. PSMA score group status in primary breast tumors was significantly associated with histologic type and tumor grade (p = 0.026 and p = 0.004 respectively). Distant metastases showed higher PSMA expression in tumor-associated neovasculature comparing with primary tumors. Moreover, brain tumor-associated neovasculture had significantly higher expression of PSMA comparing with bone tumor-associated neovasculture. The localized binding of PSMA mAb to the neovasculature endothelium was confirmed with the double Immunofluorescence staining. ⁶⁸Ga-PSAM imaging of a patient with metastatic breast cancer showed strong tracer uptake in all known skeletal metastases. To the best of our knowledge, this study is the second one that has assessed PSMA expression in a large number of breast cancer patients. Our findings showed that PSMA is particularly expressed in tumor-associated neovasculature of breast tumors and its distant metastases, thus enhancing the evidence on the potential usefulness of PSMA as a therapeutic vascular target.

Development of prostate specific membrane antigen targeted ultrasound microbubbles using bioorthogonal chemistry

Prostate specific membrane antigen (PSMA) targeted microbubbles (MBs) were developed using bioorthogonal chemistry. Streptavidin-labeled MBs were treated with a biotinylated tetrazine (MB_Tz) and targeted to PSMA expressing cells using trans-cyclooctene (TCO)-functionalized anti-PSMA antibodies (TCO-anti-PSMA). The extent of MB binding to PSMA positive cells for two different targeting strategies was determined using an in vitro flow chamber. The initial approach involved pretargeting, where TCO-anti-PSMA was first incubated with PSMA expressing cells and followed by MB_Tz, which subsequently showed a 2.8 fold increase in the number of bound MBs compared to experiments performed in the absence of TCO-anti-PSMA. Using direct targeting, where TCO-anti-PSMA was linked to MB_Tz prior to initiation of the assay, a 5-fold increase in binding compared to controls was observed. The direct targeting approach was subsequently evaluated in vivo using a human xenograft tumor model and two different PSMA-targeting antibodies. The US signal enhancements observed were 1.6- and 5.9-fold greater than that for non-targeted MBs. The lead construct was also evaluated in a head-to-head study using mice bearing both PSMA positive or negative tumors in separate limbs. The human PSMA expressing tumors exhibited a 2-fold higher US signal compared to those tumors deficient in human PSMA. The results demonstrate both the feasibility of preparing PSMA-targeted MBs and the benefits of using bioorthogonal chemistry to create targeted US probes.