Deep learning-based whole-body PSMA PET/CT attenuation correction utilizing Pix-2-Pix GAN.

PURPOSE: Sequential PET/CT studies oncology patients can undergo during their treatment follow-up course is limited by radiation dosage. We propose an artificial intelligence (AI) tool to produce attenuation-corrected PET (AC-PET) images from non-attenuation-corrected PET (NAC-PET) images to reduce need for low-dose CT scans. METHODS: A deep learning algorithm based on 2D Pix-2-Pix generative adversarial network (GAN) architecture was developed from paired AC-PET and NAC-PET images. 18F-DCFPyL PSMA PET-CT studies from 302 prostate cancer patients, split into training, validation, and testing cohorts (n = 183, 60, 59, respectively). Models were trained with two normalization strategies: Standard Uptake Value (SUV)-based and SUV-Nyul-based. Scan-level performance was evaluated by normalized mean square error (NMSE), mean absolute error (MAE), structural similarity index (SSIM), and peak signal-to-noise ratio (PSNR). Lesion-level analysis was performed in regions-of-interest prospectively from nuclear medicine physicians. SUV metrics were evaluated using intraclass correlation coefficient (ICC), repeatability coefficient (RC), and linear mixed-effects modeling. RESULTS: Median NMSE, MAE, SSIM, and PSNR were 13.26%, 3.59%, 0.891, and 26.82, respectively, in the independent test cohort. ICC for SUVmax and SUVmean were 0.88 and 0.89, which indicated a high correlation between original and AI-generated quantitative imaging markers. Lesion location, density (Hounsfield units), and lesion uptake were all shown to impact relative error in generated SUV metrics (all p < 0.05). CONCLUSION: The Pix-2-Pix GAN model for generating AC-PET demonstrates SUV metrics that highly correlate with original images. AI-generated PET images show clinical potential for reducing the need for CT scans for attenuation correction while preserving quantitative markers and image quality.

Exploring the role of combined external beam radiotherapy and targeted radioligand therapy with [177Lu]Lu-PSMA-617 for prostate cancer - from bench to bedside.

Management of prostate cancer (PC) might be improved by combining external beam radiotherapy (EBRT) and prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) with lutetium-177 (177Lu)-labeled PSMA inhibitors. We hypothesized a higher efficacy of the combination due to augmentation of the radiation dose to the tumor and interactions of EBRT with PSMA expression potentially increasing radiopharmaceutical uptake. Therefore, this study analyzed the influence of radiation on PSMA expression levels in vitro. The results were translated to evaluate the efficacy of the combination of photon EBRT and [177Lu]Lu-PSMA-617 in a murine PC xenograft model. Finally, a clinical case report on a combined elective field EBRT with RLT dose escalation illustrates a proof-of-concept. Methods: PSMA gene and protein expression were assessed in human PSMA-overexpressing LNCaP cells after irradiation using reverse transcription quantitative polymerase chain reaction (RT-qPCR), flow cytometry and On-Cell Western assays. In the in vivo therapy study, LNCaP tumor-bearing BALB/c nu/nu mice were irradiated once with 2 Gy X-ray EBRT and injected with 40 MBq [177Lu]Lu-PSMA-617 after 4 h or received single or no treatment (n = 10 each). Tumor-absorbed doses by [177Lu]Lu-PSMA-617 were calculated according to the Medical Internal Radiation Dosimetry (MIRD) formalism after deriving time-activity curves using a gamma probe. An exemplified patient case is demonstrated where fractionated EBRT (54 Gy to prostate; 45 Gy to pelvic lymphatics) and three cycles of [177Lu]Lu-PSMA-617 (3.4-6.0 GBq per cycle) were sequentially combined under concurrent androgen deprivation for treating locally advanced PC. Results: At 4 h following irradiation with 2-8 Gy, LNCaP cells displayed a PSMA protein upregulation by around 18% relative to non-irradiated cells, and a stronger upregulation on mRNA level (up to 2.6-fold). This effect was reversed by 24 h when PSMA protein levels were downregulated by up to 22%. Mice treated with the combination therapy showed significantly improved outcomes regarding tumor control and median survival (p < 0.0001) as compared to single or no treatment. Relative to monotherapy with PSMA-RLT or EBRT, the tumor doubling time was prolonged 1.7- or 2.7-fold and the median survival was extended by 24% or 60% with the combination, respectively. Additionally, tumors treated with EBRT exhibited a 14% higher uptake of the radiopharmaceutical as evident from the calculated tumor-absorbed dose, albeit with high variability in the data. Concerning the patient case, the tri-modality treatment was well tolerated and the patient responded with a long-lasting complete biochemical remission for five years following end of PSMA-RLT. The patient then developed a biochemical relapse with oligo-recurrent disease on follow-up imaging. Conclusion: The present preclinical and clinical data demonstrate that the combination of EBRT with dose escalation by PSMA-RLT improves tumor control and potentially prolongs survival. This may pave the way for further clinical investigations of this approach to explore the curative potential of the combination therapy.

Salvage Radiotherapy PSMA PET/CT-guided in Men With PSA Recurrence.

BACKGROUND/AIM: To evaluate the clinical outcome in men with recurrent prostate cancer (PCa) treated by salvage radiotherapy (sRT) prostate-specific membrane antigen positron emission tomography/computed tomography (PSMA PET/CT)-guided. PATIENTS AND METHODS: From January 2021 to January 2023, 33 patients who previously underwent definitive/systemic therapy were submitted to sRT PSMA PET/CT-guided for PCa recurrence: 16 (48.5%) on the prostate bed (PB), 12 (36.4%) on the lymph node (LN) and five (15.1%) on the bone. The median PSA value was 3.3 ng/ml (range=0.3-15.5 ng/ml): 0.2-0.5 ng/ml (18.2% cases), 0.51-1 ng/ml (39.4% cases) and >1 ng/ml (42.4% cases). Median 18F PSMA PET/CT standardized uptake value (SUVmax) was evaluated on PB, vs. LN vs. bones PCa recurrences and was equal to 12.5 vs. 19.0 vs. 30.1, respectively. RESULTS: Overall, at a median follow up of 12 months, 23/33 patients (69.7%) had local control without distant progression (PSA and SUVmax evaluation): 14/16 (87.5%) vs. 7/12 (58.3%) vs. 2/5 (40%) underwent sRT on the PB vs. LN vs. bone metastases, respectively. CONCLUSION: PSMA PET/CT allows to perform sRT early in men with PCa recurrence and low PSA values obtaining a complete clinical response in approximately 70% of the cases one year from treatment.

Limited prognostic role of routine serum markers (AP, CEA, LDH and NSE) in oligorecurrent prostate cancer patients undergoing PSMA-radioguided surgery.

INTRODUCTION: We evaluated the prognostic role of pre-salvage prostate-specific membrane antigen-radioguided surgery (PSMA-RGS) serum levels of alkaline phosphatase (AP), carcinoembryonic antigen (CEA), lactate dehydrogenase (LDH), and neuron-specific enolase (NSE). MATERIALS AND METHODS: Patients who consecutively underwent PSMA-RGS for prostate cancer (PCa) oligorecurrence between January 2019 and January 2022 were selected. Biomarkers were assessed one day before surgery. Cox regression and logistic regression models tested the relationship between biochemical recurrence-free survival (BFS), 6- and 12-month biochemical recurrence (BCR), and several independent variables, including biomarkers. RESULTS: 153 consecutive patients were analyzed. In the univariable Cox regression analysis, none of the biomarkers achieved predictor status (AP: hazard ratio [HR] = 1.03, 95% CI 0.99, 1.01; p = 0.19; CEA: HR = 1.73, 95% CI 0.94, 1.21; p = 0.34; LDH: HR = 1.01, 95% CI 1.00, 1.01; p = 0.05; NSE: HR = 1.02, 95% CI 0.98, 1.06; p = 0.39). The only independent predictor of BFS was the number of positive lesions on PSMA PET (HR = 1.17, 95% CI 1.02, 1.30; p = 0.03). The number of positive lesions was confirmed as independent predictor for BCR within 6 and 12 months (BCR < 6 months: odds ratio [OR] = 1.1, 95% CI 1.0, 1.3; p = 0.04; BCR < 12 months: OR = 1.1, 95% CI 1.0, 1.3; p = 0.04). CONCLUSION: The assessment of AP, CEA, LDH, and NSE before salvage PSMA-RGS showed no prognostic impact. Further studies are needed to identify possible predictors that will optimize patient selection for salvage PSMA-RGS.

Precise prostate cancer diagnosis using fluorescent nanoprobes for detecting PSA and PSMA in serum.

Novel Au-Se bond-based nanoprobes were designed for concurrent detection of PSA and PSMA in serum samples, aiming to enhance the early diagnosis of prostate cancer. These probes demonstrate robust stability, specificity and accuracy, underscoring their potential as non-invasive tools for diagnosis.

Heterogeneous PSMA ligand uptake inside parotid glands.

The purpose of this investigation is to quantify the spatial heterogeneity of prostate-specific membrane antigen (PSMA) positron emission tomography (PET) uptake within parotid glands. We aim to quantify patterns in well-defined regions to facilitate further investigations. Furthermore, we investigate whether uptake is correlated with computed tomography (CT) texture features. METHODS: Parotid glands from [18F]DCFPyL PSMA PET/CT images of 30 prostate cancer patients were analyzed. Uptake patterns were assessed with various segmentation schemes. Spearman's rank correlation coefficient was calculated between PSMA PET uptake and feature values of a Grey Level Run Length Matrix using a long and short run length emphasis (GLRLML and GLRLMS) in subregions of the parotid gland. RESULTS: PSMA PET uptake was significantly higher (p < 0.001) in lateral/posterior regions of the glands than anterior/medial regions. Maximum uptake was found in the lateral half of parotid glands in 50 out of 60 glands. The difference in SUVmean between parotid halves is greatest when parotids are divided by a plane separating the anterior/medial and posterior/lateral halves symmetrically (out of 120 bisections tested). PSMA PET uptake was significantly correlated with CT GLRLML (p < 0.001), and anti-correlated with CT GLRLMS (p < 0.001). CONCLUSION: Uptake of PSMA PET is heterogeneous within parotid glands, with uptake biased towards lateral/posterior regions. Uptake within parotid glands was strongly correlated with CT texture feature maps.

Genomic Correlates of Prostate-Specific Membrane Antigen Expression and Response to 177Lu-PSMA-617: A Retrospective Multicenter Cohort Study.

PURPOSE: While 177Lu-PSMA-617 (LuPSMA) is an effective therapy for many patients with metastatic castration-resistant prostate cancer (mCRPC), biomarkers associated with outcomes are not well defined. We hypothesized that prostate cancer mutational profile may associate with clinical activity of LuPSMA. We devised a study to evaluate associations between mCRPC mutational profile with LuPSMA clinical outcomes. METHODS: This was a multicenter retrospective analysis of patients with mCRPC with next-generation sequencing (NGS) who received LuPSMA. PSA50 response (ie, ≥50% decline in prostate-specific antigen [PSA]) rate, PSA progression free survival (PSA PFS), and overall survival (OS) were compared between genetically defined subgroups. RESULTS: One hundred twenty-six patients with NGS results who received at least one cycle of LuPSMA were identified. The median age was 73 (IQR, 68-78) years, 124 (98.4%) received ≥1 prior androgen receptor-signaling inhibitor, and 121 (96%) received ≥1 taxane-based chemotherapy regimen. Fifty-eight (46%) patients with a DNA damage repair gene mutation (DNA damage response group) and 59 (46.8%) with a mutation in TP53, RB1, or PTEN tumor suppressor genes (TSG group) were identified. After adjusting for relevant confounders, the presence of ≥1 TSG mutation was associated with shorter PSA PFS (hazard ratio [HR], 1.93 [95% CI, 1.05 to 3.54]; P = .034) and OS (HR, 2.65 [95% CI, 1.15 to 6.11]; P = .023). There was improved OS favoring the DNA damage response group (HR, 0.37 [95% CI, 0.14 to 0.97]; P = .044) on multivariable analysis. Univariate analysis of patients with ATM mutations had significantly higher rates of PSA50 response, PSA PFS, and OS. CONCLUSION: Outcomes on LuPSMA varied on the basis of mutational profile. Prospective studies to define the clinical activity of LuPSMA in predefined genomic subgroups are justified.

Prostate-Specific Membrane Antigen-Avid Neurofibroma Mimicking Cutaneous Metastasis in Metastatic Castration-Resistant Prostate Cancer on 18 F-PSMA-1007 PET/CT.

ABSTRACT: The occurrence of cutaneous metastases in prostate cancer is exceedingly rare. Many benign lesions and nonprostatic cancers can express the prostate-specific membrane antigen (PSMA). They can potentially mimic metastasis of prostate cancer and lead to misinterpretation of PSMA PET/CT findings. Additionally, it has significant management and prognostic implications. We present a rare case of an 88-year-old man with metastatic castration-resistant prostate cancer who showed a PSMA-expressing subcutaneous nodule in the scalp on 18 F-PSMA-1007 PET/CT, raising the suspicion of cutaneous metastasis. However, its biopsy revealed a neurofibroma, altering the disease prognosis and management.

Head-to-head comparison of prostate-specific membrane antigen positron emission tomography/computed tomography and multiparametric magnetic resonance imaging in the detection of biochemical recurrence of prostate cancer: a systematic review and meta-analysis.

AIM: Our main goal of this meta-analytical analysis was to evaluate the diagnostic effectiveness of prostate-specific membrane antigen (PSMA) positron emission tomography (PET)/computed tomography (CT) against multiparametric magnetic resonance imaging (mpMRI) in the context of identifying biochemical recurrence in patients with prostate cancer (PCa). MATERIALS AND METHODS: A thorough search covering articles published until March 2023 was carried out across major databases such as PubMed, Embase, and Web of Science. Studies examining the direct comparison of PSMA PET/CT and mpMRI in patients with PCa suffering biochemical recurrence were included in the inclusion criteria. Using the renowned Quality Assessment of Diagnostic Performance Studies-2 technique, each study's methodological rigor was assessed. RESULTS: We analyzed data from six eligible studies involving 290 patients in total. The combined data showed that for PSMA PET/CT and mpMRI, respectively, the pooled overall detection rates for recurrent PCa after definitive treatment were 0.69 (95% confidence interval [CI]: 0.45-0.89) and 0.70 (95% CI: 0.44-0.91). The detection rates for local recurrence were specifically 0.52 (95% CI: 0.39-0.65) and 0.62 (95% CI: 0.31-0.89), while they were 0.50 (95% CI: 0.26-0.74) and 0.32 (95% CI: 0.18-0.48) for lymph node metastasis. Notably, there was no discernible difference between the two imaging modalities in terms of the overall detection rate (P = 0.95). The detection rates for local recurrence and lymph node metastasis did not differ statistically significantly (P = 0.55, 0.23). CONCLUSION: The performance of PSMA PET/CT and mpMRI in identifying biochemical recurrence in PCa appears to be comparable. However, the meta-analysis' findings came from research with modest sample sizes. In this context, more extensive research should be conducted in the future.

Impact of Circulating Tumor Cell-Expressed Prostate-Specific Membrane Antigen and Prostate-Specific Antigen Transcripts in Different Stages of Prostate Cancer.

PURPOSE: Prostate-specific membrane antigen (PSMA)-based images, which visually quantify PSMA expression, are used to determine prostate cancer micrometastases. This study evaluated whether a circulating tumor cell (CTC)-based transcript platform, including PSMA mRNA, could help identify potential prognostic markers in prostate cancer. EXPERIMENTAL DESIGN: We prospectively enrolled 21 healthy individuals and 247 patients with prostate cancer [localized prostate cancer (LPCa), n = 94; metastatic hormone-sensitive prostate cancer (mHSPC), n = 44; and metastatic castration-resistant prostate cancer (mCRPC), n = 109]. The mRNA expression of six transcripts [PSMA, prostate-specific antigen (PSA), AR, AR-V7, EpCAM, and KRT 19] from CTCs was measured, and their relationship with biochemical recurrence (BCR) in LPCa and mCRPC progression-free survival (PFS) rate in mHSPC was assessed. PSA-PFS and radiological-PFS were also calculated to identify potential biomarkers for predicting androgen receptor signaling inhibitor (ARSI) and taxane-based chemotherapy resistance in mCRPC. RESULTS: CTC detection rates were 75.5%, 95.3%, and 98.0% for LPCa, mHSPC, and mCRPC, respectively. In LPCa, PSMA [hazard ratio (HR), 3.35; P = 0.028) and PSA mRNA (HR, 1.42; P = 0.047] expressions were associated with BCR. Patients with mHSPC with high PSMA (HR, 4.26; P = 0.020) and PSA mRNA (HR, 3.52; P = 0.042) expressions showed significantly worse mCRPC-PFS rates than those with low expression. Increased PSA and PSMA mRNA expressions were significantly associated with shorter PSA-PFS and radiological PFS in mCPRC, indicating an association with drug resistance. CONCLUSIONS: PSMA and PSA mRNA expressions are associated with BCR in LPCa. In advanced prostate cancer, PSMA and PSA mRNA can also predict rapid progression from mHSPC to mCRPC and ARSI or taxane-based chemotherapy resistance.

Implementation of PSMA PET/CT and alignment of ordering to SNMMI appropriate use criteria in a large network system.

INTRODUCTION: The Society of Nuclear Medicine and Molecular Imaging (SNMMI) provides appropriate use criteria (AUC) for prostate-specific membrane antigen positron emission tomography/computed tomography (PSMA PET/CT) which include guidance on imaging in newly diagnosed prostate cancer and in patients with biochemically recurrent (BCR) disease. This study aims to examine trends in PSMA implementation and the prevalence and outcomes of scans ordered in scenarios deemed rarely appropriate or not meeting SNMMI AUC. METHODS: We retrospectively identified patients who were diagnosed with presumptive National Comprehensive Cancer Network unfavorable intermediate, high, or very high risk prostate cancer, patients who underwent staging for BCR, and all patients staged with PSMA between July 2021 and March 2023. Positivity was validated by adherence to a predetermined reference standard. RESULTS: The frequency of PSMA use increased in initial staging from 24% to 80% and work-up of BCR from 91% to 99% over our study period. In addition, 5% (17/340) of PSMA scans ordered for initial staging did not meet AUC and 3% (15/557) of posttreatment scans were deemed rarely appropriate. Initial staging orders not meeting SNMMI AUC resulted in no positivity (0/17), while rarely appropriate posttreatment scans were falsely positive in 75% (3/4) of cases. Urologists (53%, 17/32) comprised the largest ordering specialty in rarely appropriate use. CONCLUSION: The frequency of PSMA use rose across the study period. A significant minority of patients received PSMA PET/CT in rarely appropriate scenarios yielding no positivity in initial staging and significant false positivity post-therapy. Further education of providers and electronic medical record-based interventions could help limit the rarely appropriate use of PET imaging.

A PSMA-targeted doxorubicin small-molecule drug conjugate.

We developed a model small-molecule drug conjugate (SMDC) that employed doxorubicin as a representative chemotherapeutic targeted to the cell membrane biomarker PSMA (prostate-specific membrane antigen) expressed on prostate cancer cells. The strategy capitalized on the clatherin-mediated internalization of PSMA to facilitate the selective uptake and release of doxorubicin in the target cells. The SMDC was prepared and assessed for binding kinetics, plasma stability, cell toxicity, and specificity towards PSMA expressing prostate cancer cell lines. We observed high affinity of the SMDC for PSMA (IC50 5 nM) with irreversible binding, as well as specific effectiveness against PSMA(+) cells. These findings validated the strategy for a small molecule-based approach in targeted cancer therapy.

PSMA-targeted dendrimer as an efficient anticancer drug delivery vehicle for prostate cancer.

Prostate cancer (PCa) is the second leading cause of cancer-related deaths among men in the United States. Although early-stage treatments exhibit promising 5-year survival rates, the treatment options for advanced stage disease are constrained, with short survival benefits due to the challenges associated with effective and selective drug delivery to PCa cells. Even though targeting Prostate Specific Membrane Antigen (PSMA) has been extensively explored and is clinically employed for imaging and radio-ligand therapy, the clinical success of PSMA-based approaches for targeted delivery of chemotherapies remains elusive. In this study, we combine a generation 4 hydroxy polyamidoamine dendrimer (PD) with irreversible PSMA ligand (CTT1298) to develop a PSMA-targeted nanoplatform (PD-CTT1298) for selective intracellular delivery of potent chemotherapeutics to PCa. PD-CTT1298-Cy5 exhibits a PSMA IC50 in the nanomolar range and demonstrates selective uptake in PSMA (+) PCa cells via PSMA mediated internalization. When systemically administered in a prostate tumor xenograft mouse model, PD-CTT1298-Cy5 selectively targets PSMA (+) tumors with significantly less accumulation in PSMA (-) tumors or upon blocking of the PSMA receptors. Moreover, the dendrimer clears rapidly from the off-target organs limiting systemic side-effects. Further, the conjugation of an anti-cancer agent, cabozantinib to the PSMA-targeted dendrimer translates to a significantly enhanced anti-proliferative activity in vitro compared to the free drug. These findings highlight the potential of PD-CTT1298 nanoplatform as a versatile approach for selective delivery of high payloads of potent chemotherapeutics to PCa, where dose related systemic side-effects are a major concern.

Lutetium-177 Labelled Anti-PSMA Monoclonal Antibody (Lu-TLX591) Therapy for Metastatic Prostate Cancer: Treatment Toxicity and Outcomes.

INTRODUCTION: Whilst prostate cancer is the fourth most common cancer globally, effective therapies for patients with advanced disease are lacking. In recent years, interest in using theranostic agents to treat castrate-resistant prostate cancer (CRPC) and metastatic prostate cancer has emerged. Lu-TLX591 monoclonal antibody is a potential agent of significance; however, to date, reports on its toxicity and efficacy have been limited to small clinical trials in heavily pretreated patients. This retrospective study describes the real-world toxicity and efficacy profile of Lu-TLX591. METHODS: Eighteen patients received Lu-TLX591 at two private oncology centres in Australia. Patients were eligible if they had CRPC or metastatic prostate cancer and prostate-specific membrane antigen (PSMA)-avid disease confirmed by PSMA-positron emission tomography (PET). Patients received two cycles of Lu-TLX591 monoclonal antibody (177 Lu-DOTA-rosopatamab) each dosed from 1.01-2.85 GBq, 14 days apart. Patient side effects, blood test results and radiology reports were recorded on the patient's electronic medical record (eMR). RESULTS: Prominent side effects included fatigue (55.6%), anorexia (16.7%), nausea (11.1%), and transfusion reactions (11.1%). All-grade haematological toxicities included lymphopenia (61.1%), anaemia (22.2%), leukopenia (27.8%), neutropenia (27.8%), and thrombocytopenia (27.8%). Grade 4 toxicity included lymphopenia (6.7%) and thrombocytopenia (6.7%). Patients' prostate-specific antigen (PSA) responses were as follows; ≥ 30% PSA decline (27.8%), ≥ 50% PSA decline (11.4%) and any PSA decline (38.9%). Follow-up radiology revealed 54.5% stable disease, 45.4% disease progression and 9.1% disease regression. CONCLUSION: Lu-TLX591 was safely administered at acceptable toxicity and its efficacy reflects previous clinical trials. Larger studies are required and are underway (NCT04786847; NCT05146973; NCT04876651) to determine Lu-TLX591 effectiveness amongst different prostate cancer populations and compare its efficacy against peptide-based radiopharmaceutical agents.

An Unexpected Seminal Vesicle Pitfall in PSMA PET.

ABSTRACT: A 76-year-old man undergoing hormone therapy for prostate cancer was referred for 68 Ga-PSMA-11-PET (PSMA PET) due to persistently detectable PSA level. No PSMA-positive tumor lesions were detected, so a delayed phase imaging was performed, which revealed focal PSMA uptake in the right seminal vesicle together with contrast accumulation on excretory phase contrast-enhanced CT. These findings were finally determined to be secondary to urinary reflux as a consequence of a prostatic enucleation he had undergone 5 months earlier following an episode of acute urinary retention.

PSMA-targeted SMART molecules outfitted with SN38.

Herein, we report the modular synthesis and evaluation of a prostate-specific membrane antigen (PSMA) targeted small molecule drug conjugate (SMDC) carrying the chemotherapeutic agent, SN38. Due to the fluorogenic properties of SN38, payload release kinetics from the platform was observed in buffers representing the pH conditions of systemic circulation and cellular internalization. It was found that this platform is stable with minimal payload release at physiological pH with most rapid payload release observed at pH values representing the endosome complex. We confirmed selective payload release and chemotherapeutic efficacy for PSMA(+) prostate cancer cells over PSMA(-) cells. These results demonstrate that chemotherapeutic agents with limited solubility can be conjugated to a water-soluble targeting and linker platform without attenuating efficacy.

Parallel Metabolomics and Lipidomics of a PSMA/GCPII Deficient Mouse Model Reveal Alteration of NAAG Levels and Brain Lipid Composition.

Glutamate carboxypeptidase II (GCPII, also known as PSMA or FOLH1) is responsible for the cleavage of N-acetyl-aspartyl-glutamate (NAAG) to N-acetyl-aspartate and glutamate in the central nervous system and facilitates the intestinal absorption of folate by processing dietary folyl-poly-γ-glutamate in the small intestine. The physiological function of GCPII in other organs like kidneys is still not known. GCPII inhibitors are neuroprotective in various conditions (e.g., ischemic brain injury) in vivo; however, their utilization as potential drug candidates has not been investigated in regard to not yet known GCPII activities. To explore the GCPII role and possible side effects of GCPII inhibitors, we performed parallel metabolomic and lipidomic analysis of the cerebrospinal fluid (CSF), urine, plasma, and brain tissue of mice with varying degrees of GCPII deficiency (fully deficient in Folh1, -/-; one allele deficient in Folh1, +/-; and wild type, +/+). Multivariate analysis of metabolites showed no significant differences between wild-type and GCPII-deficient mice (except for NAAG), although changes were observed between the sex and age. NAAG levels were statistically significantly increased in the CSF, urine, and plasma of GCPII-deficient mice. However, no difference in NAAG concentrations was found in the whole brain lysate likely because GCPII, as an extracellular enzyme, can affect only extracellular and not intracellular NAAG concentrations. Regarding the lipidome, the most pronounced genotype-linked changes were found in the brain tissue. In brains of GCPII-deficient mice, we observed statistically significant enrichment in phosphatidylcholine-based lipids and reduction of sphingolipids and phosphatidylethanolamine plasmalogens. We hypothesize that the alteration of the NAA-NAAG axis by absent GCPII activity affected myelin composition. In summary, the absence of GCPII and thus similarly its inhibition do not have detrimental effects on metabolism, with just minor changes in the brain lipidome.

Development and Preclinical Evaluation of [211At]PSAt-3-Ga: An Inhibitor for Targeted α-Therapy of Prostate Cancer.

The application of prostate-specific membrane antigen (PSMA)-targeted α-therapy is a promising alternative to ß--particle-based treatments. 211At is among the potential α-emitters that are favorable for this concept. Herein, 211At-based PSMA radiopharmaceuticals were designed, developed, and evaluated. Methods: To identify a 211At-labeled lead, a surrogate strategy was applied. Because astatine does not exist as a stable nuclide, it is commonly replaced with iodine to mimic the pharmacokinetic behavior of the corresponding 211At-labeled compounds. To facilitate the process of structural design, iodine-based candidates were radiolabeled with the PET radionuclide 68Ga to study their preliminary in vitro and in vivo properties before the desired 211At-labeled lead compound was formed. The most promising candidate from this evaluation was chosen to be 211At-labeled and tested in biodistribution studies. Results: All 68Ga-labeled surrogates displayed affinities in the nanomolar range and specific internalization in PSMA-positive LNCaP cells. PET imaging of these compounds identified [68Ga]PSGa-3 as the lead compound. Subsequently, [211At]PSAt-3-Ga was synthesized in a radiochemical yield of 35% and showed tumor uptake of 19 ± 8 percentage injected dose per gram of tissue (%ID/g) at 1 h after injection and 7.6 ± 2.9 %ID/g after 24 h. Uptake in off-target tissues such as the thyroid (2.0 ± 1.1 %ID/g), spleen (3.0 ± 0.6 %ID/g), or stomach (2.0 ± 0.4 %ID/g) was low, indicating low in vivo deastatination of [211At]PSAt-3-Ga. Conclusion: The reported findings support the use of iodine-based and 68Ga-labeled variants as a convenient strategy for developing astatinated compounds and confirm [211At]PSAt-3 as a promising radiopharmaceutical for targeted α-therapy.