Prostate-specific membrane antigen and fibroblast activation protein distribution in prostate cancer: preliminary data on immunohistochemistry and PET imaging

Introduction Fibroblast activation protein (FAP) has been recently presented as new imaging target for malignant diseases and offers high contrast to surrounding normal tissue. FAP tracer uptake has been reported in various tumor entities. The aim of this study was to compare FAP and Prostate-specific membrane antigen (PSMA) expression in primary prostate cancer employing histological analyses and PET imaging in two small patient collectives. Methods Two independent small patient collectives were included in this study. For cohort A, data of 5 prostate cancer patients and 3 patients with benign prostate hyperplasia were included. Patients with prostate cancer were initially referred for PSMA PET staging. Radical prostatectomy was performed in all patients and prostate specimen of patients and biopsies of healthy controls were available for further evaluation. Histological workup included HE and immunohistochemistry using PSMA Ab, FAP Ab. Cohort B consists of 6 Patients with diagnosed mCRPC and available PSMA as well as FAP PET. Results Patients with proven prostate cancer infiltration exhibited strong positivity for PSMA in both primary tumors and lymph node metastases while stainings for FAP were found positive in some cases, but not all (2/5). Controls with BPH presented moderate PSMA staining and in one case also with a positive FAP staining (1/3). PET imaging with FAP seemed to result in more precise results in case of low PSMA expression than PSMA-PET. Conclusions While PSMA staining intensity is a valid indicator of prostate cancer in both primary tumor and lymph node metastases, the expression of FAP seems to be heterogeneous but not necessarily linked to cancer-associated fibroblasts. It is also present in inflammation-associated myofibroblasts. Therefore, its ultimate role in prostate cancer diagnosis remains a subject of discussion.

Impact of the molar activity and PSMA expression level on [18F]AlF-PSMA-11 uptake in prostate cancer

This two-part preclinical study aims to evaluate prostate specific membrane antigen (PSMA) as a valuable target for expression-based imaging applications and to determine changes in target binding in function of varying apparent molar activities (MAapp) of [18F]AlF-PSMA-11. For the evaluation of PSMA expression levels, male NOD/SCID mice bearing prostate cancer (PCa) xenografts of C4-2 (PSMA+++), 22Rv1 (PSMA+) and PC-3 (PSMA−) were administered [18F]AlF-PSMA-11 with a medium MAapp (20.24 ± 3.22 MBq/nmol). SUVmean and SUVmax values were respectively 3.22 and 3.17 times higher for the high versus low PSMA expressing tumors (p < 0.0001). To evaluate the effect of varying MAapp, C4-2 and 22Rv1 xenograft bearing mice underwent additional [18F]AlF-PSMA-11 imaging with a high (211.2 ± 38.9 MBq/nmol) and/or low MAapp (1.92 ± 0.27 MBq/nmol). SUV values showed a significantly increasing trend with higher MAapp. Significant changes were found for SUVmean and SUVmax between the high versus low MAapp and medium versus low MAapp (both p < 0.05), but not between the high versus medium MAapp (p = 0.055 and 0.25, respectively). The effect of varying MAapp was more pronounced in low expressing tumors and PSMA expressing tissues (e.g. salivary glands and kidneys). Overall, administration of a high MAapp increases the detection of low expression tumors while also increasing uptake in PSMA expressing tissues, possibly leading to false positive findings. In radioligand therapy, a medium MAapp could reduce radiation exposure to dose-limiting organs with only limited effect on radionuclide accumulation in the tumor.

Prostate-Specific Membrane Antigen in Anaplastic and Poorly Differentiated Thyroid Cancer—A New Diagnostic and Therapeutic Target?

Several studies have demonstrated an expression of the prostate-specific membrane antigen (PSMA) in the cancer-related neovasculature of thyroid malignancies. Due to the poor prognosis and limited therapeutic options for patients with anaplastic (ATC) and poorly differentiated (PDTC) thyroid carcinoma, the aim of our study was to investigate the theranostic approach of PSMA expression in these patients. The PSMA uptake on Gallium-68 (68Ga)-PSMA-positron emission tomography/computed tomography (PET/CT) and glucose uptake on F-18-Fluordeoxyglucose (18F-FDG)-PET/CTs were analysed in two ATC and six PDTC patients. The PSMA expression in corresponding patients’ tissue samples was detected by immunohistochemistry. In addition, various tissue sections from 22 ATC and six PDTC patients were examined concerning PSMA expression. 68Ga-PSMA-PET/CT showed heterogeneous PSMA expression among patients and lesions. Six of the eight analyzed patients (two ATC, four PDTC) showed increased glucose metabolism without increased PSMA uptake after PET/CT. In one patient (PDTC), 18F-FDG-PET/CT tracer uptake was positive and 68Ga-PSMA-PET/CT showed heterogeneous results. Another patient (PDTC) evidenced only PSMA-positive lesions and received two cycles of Lutetium-177 (177Lu)-PSMA therapy, which kept his disease stable for seven months. There was a correlation between immunohistochemical PSMA expression and uptake on 68Ga-PMSA-PET/CT in three of the examined patients. Twenty-seven of the analyzed 39 ATC and 13 of the analyzed 22 PDTC tissue sections showed a strong PSMA expression. Considering the rarity of PDTC and ATC, which is the reason for the small patient population we studied, the findings of this study confirm the high diagnostic sensitivity and superiority of 18F-FDG-PET/CT in comparison to 68Ga-PSMA-PET/CT in the diagnosis of ATC and PDTC. However, it can be suggested that 68Ga-PMSA-PET/CT can be considered as a beneficial adjunct to the well-established 18F-FDG-PET/CT for a few individual selected patients with ATC and PDTC to detect lesions not discovered by 18F-FDG-PET/CT and to determine patients’ eligibility for a radioligand therapy. Radiolabelled PSMA-ligands may, in the future, represent a theranostic approach with only minor side effects for a few individual selected patients with ATC and PDTC who need alternative treatment options in case of progression when established therapies are no longer effective. However, due to the small sample size of our collective, larger studies are needed to allow for a final evaluation on the significance of PSMA-targeted diagnostic and therapy for ATC and PDTC.

Hot needles can confirm accurate lesion sampling intraoperatively using [18F]PSMA-1007 PET/CT-guided biopsy in patients with suspected prostate cancer

Purpose Prostate-specific membrane antigen (PSMA)-targeted PET is increasingly used for staging prostate cancer (PCa) with high accuracy to detect significant PCa (sigPCa). [68 Ga]PSMA-11 PET/MRI-guided biopsy showed promising results but also persisting limitation of sampling error, due to impaired image fusion. We aimed to assess the possibility of intraoperative quantification of [18F]PSMA-1007 PET/CT uptake in core biopsies as an instant confirmation for accurate lesion sampling. Methods In this IRB-approved, prospective, proof-of-concept study, we included five consecutive patients with suspected PCa. All underwent [18F]PSMA-1007 PET/CT scans followed by immediate PET/CT-guided and saturation template biopsy (3.1 ± 0.3 h after PET). The activity in biopsy cores was measured as counts per minute (cpm) in a gamma spectrometer. Pearson’s test was used to correlate counts with histopathology (WHO/ISUP), tumor length, and membranous PSMA expression on immunohistochemistry (IHC). Results In 43 of 113 needles, PCa was present. The mean cpm was overall significantly higher in needles with PCa (263 ± 396 cpm) compared to needles without PCa (73 ± 44 cpm, p < 0.001). In one patient with moderate PSMA uptake (SUVmax 8.7), 13 out of 24 needles had increased counts (100–200 cpm) but only signs of inflammation and PSMA expression in benign glands on IHC. Excluding this case, ROC analysis resulted in an AUC of 0.81, with an optimal cut-off to confirm PCa at 75 cpm (sens/spec of 65.1%/87%). In all 4 patients with PCa, the first or second PSMA PET-guided needle was positive for sigPCa with high counts (156–2079 cpm). Conclusions [18F]PSMA-1007 uptake in PCa can be used to confirm accurate lesion sampling of the dominant tumor intraoperatively. This technique could improve confidence in imaging-based biopsy guidance and reduce the need for saturation biopsy. Trial registration number NCT03187990, 15/06/2017.

TAMI-15. PROSTATE SPECIFIC MEMBRANE ANTIGEN EXPRESSION IN GLIOBLASTOMA TUMOR AND ENDOTHELIAL CELLS

INTRODUCTION Prostate specific membrane antigen (PSMA) has emerged in recent years as a potential target for PET imaging of CNS tumors. Originally studied for imaging in prostate cancer, it was incidentally found to have selective uptake in some CNS tumors. Early studies have shown promise for PSMA tracer use in glioblastoma (GBM), but more research is needed to characterize the PSMA expression profile in GBM. METHODS Formalin-fixed paraffin-embedded tissues of 26 GBMs were immunostained with PSMA antibody. Stained sections were scored, using 5 images per tumor, in total 130 observations. Sections were assessed for extent and intensity of PSMA expression, in tumor cells and neovascular endothelial cells. Extent of expression: 0% (none), 1-9% (very low), 10-39% (low), 40-69% (moderate), &gt; 70% (high). Intensity of expression: no staining (none); barely perceptible staining (low); readily apparent at low magnification (moderate); and maximum-intensity staining (high). RESULTS Extent of PSMA expression in tumor cells was mostly very low (76 out of 130, 58%), but intensity was often moderate (42, 32%), low (41, 32%), or high (39, 30%). The most prevalent combination was very low extent with low or moderate intensity (both 30, 23%). Endothelial cells were absent in 61 (47%) sections. Extent of expression in endothelial cells was mostly high (28, 41%), and intensity was often moderate (25, 36%) or high (20, 29%). The most prevalent combination was high extent with moderate (14, 20%) or high intensity (13, 19%). CONCLUSION PSMA expression was variable and was seen in both vascular endothelial cells and tumor cells. The mechanism of PSMA expression in GBM may not be straightforward, but rather may vary by cell type and tumor mutation status. Further tissue studies to understand the specific mechanism are needed to establish the potential use of PSMA PET as a neuroimaging modality in GBM.

Deep Membrane Proteome Profiling Reveals Overexpression of Prostate-Specific Membrane Antigen (PSMA) in High-Risk Human Paraganglioma and Pheochromocytoma, Suggesting New Theranostic Opportunity

Pheochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine tumors arising from chromaffin cells of adrenal medulla or sympathetic or parasympathetic paraganglia, respectively. To identify new therapeutic targets, we performed a detailed membrane-focused proteomic analysis of five human paraganglioma (PGL) samples. Using the Pitchfork strategy, which combines specific enrichments of glycopeptides, hydrophobic transmembrane segments, and non-glycosylated extra-membrane peptides, we identified over 1800 integral membrane proteins (IMPs). We found 45 “tumor enriched” proteins, i.e., proteins identified in all five PGLs but not found in control chromaffin tissue. Among them, 18 IMPs were predicted to be localized on the cell surface, a preferred drug targeting site, including prostate-specific membrane antigen (PSMA), a well-established target for nuclear imaging and therapy of advanced prostate cancer. Using specific antibodies, we verified PSMA expression in 22 well-characterized human PPGL samples. Compared to control chromaffin tissue, PSMA was markedly overexpressed in high-risk PPGLs belonging to the established Cluster 1, which is characterized by worse clinical outcomes, pseudohypoxia, multiplicity, recurrence, and metastasis, specifically including SDHB, VHL, and EPAS1 mutations. Using immunohistochemistry, we localized PSMA expression to tumor vasculature. Our study provides the first direct evidence of PSMA overexpression in PPGLs which could translate to therapeutic and diagnostic applications of anti-PSMA radio-conjugates in high-risk PPGLs.

Impact of the Specific Activity and PSMA Expression Level on 18F-AlF-PSMA-11 Uptake in Prostate Cancer

This two-part preclinical study aims to evaluate prostate specific membrane antigen (PSMA) as a valuable target for expression-based imaging applications and to determine changes in target binding in function of varying specific activities (SA) of 18F-AlF-PSMA-11. For the evaluation of PSMA expression levels, male NOD/SCID mice bearing prostate cancer (PCa) xenografts of C4-2 (PSMA+++), 22Rv1 (PSMA+) and PC-3 (PSMA-) were administered 18F-AlF-PSMA-11 with a medium SA (20.26 ± 3.25 MBq/µg). SUVmean and SUVmax values were respectively 3.22 and 3.17 times higher for the high vs low PSMA expressing tumors (p < 0.0001). To evaluate the effect of varying SA, C4-2 and 22Rv1 xenograft bearing mice underwent additional 18F-AlF-PSMA-11 imaging with a high (213.3 ± 39.3 MBq/µg) and/or low SA (1.94 ± 0.27 MBq/µg). SUV values showed a significantly increasing trend with higher SA. Significant changes were found for SUVmean and SUVmax between the high vs low SA and medium vs low SA (both p < 0.05), but not between the high vs medium SA (p = 0.055 and 0.25, respectively). The effect of varying SA was more pronounced in low expressing tumors and PSMA expressing tissues (e.g. salivary glands and kidneys). Overall, administration of a high SA increases the detection of low expression tumors while also increasing uptake in PSMA expressing tissues, possibly leading to false positive findings. In radioligand therapy, a medium SA could reduce radiation exposure to dose-limiting organs with only limited effect on radionuclide accumulation in the tumor.

Non-Prostate Cancer Tumours: Incidence on 18f-Dcfpyl Psma Pet/Ct and Psma Expression Characteristics in 1445 Patients.

PurposeProstate cancer (PCa) imaging has been revolutionized by Positron emission tomography (PET) tracers targeted to prostate specific membrane antigen (PSMA). Identification and characterization of non-PCa tumors has become an increasing clinical dilemma with growing use. The primary aim of this retrospective multicenter analysis was to determine atypical PSMA expression in PCa and expression in non-PCa tumors to aid providing clinically relevant reports and guiding multidisciplinary discussion.MethodsRetrospective multicenter study examining 1445 consecutive 18F-DCFPyL PSMA PET/CT between 2016-2020. Referrals were from public and private, secondary and tertiary referral centres serving New Zealand and Melbourne. Repeat studies were excluded. Lesions atypical for PCa were categorized into four groups: 1. Atypical PCa metastases 2. Non-PCa tumors 3. Benign and 4. Indeterminate.Results67 patients had lesions atypical for PCa metastases; 11.9% atypical prostate cancer metastases, 25.4% non-PCa tumors, 40.3% indeterminate, 10.4% benign. With the exception of Renal Cell Carcinoma (RCC), the non-PCa tumors, indeterminate and benign lesions demonstrated low PSMA expression. Most atypical PCa metastases demonstrated significant expression. Limitations included retrospective design and lack of histopathological correlation/follow up in some.Conclusions: Non-PCa tumor detection is low. Lesions demonstrating significant PSMA expression were almost exclusively PCa metastases. Differentiation of atypical PCa metastases from second primary malignancies is vital to avoid unnecessary investigation, delayed therapy and additional costs.