The homodimer of prostate-specific membrane antigen is a functional target for cancer therapy

Targeted and Synergistic Codelivery of Chemotherapeutic and Nucleic Acid Drugs by Liposome-Coated MPDA Nanoparticles for Advanced Prostate Cancer Treatment

Docetaxel (DTX)-based chemotherapy is the primary therapeutic approach for advanced prostate cancer (PCa) when endocrine therapy proves ineffective. Traditional chemotherapy exhibits poor specificity and induces severe side effects, such as immunosuppression, neurotoxicity, and hypersensitivity. In this study, we aimed to develop a new targeted nanodrug delivery system to accurately identify PCa cells and deliver drugs. We prepared mesoporous polydopamine (MPDA) nanoparticles using a one-pot method. After loading DTX onto MPDA, siRNA was attached to the surface, which was coated with polyethylene glycol lipids film (PEG-Lips); together, this formed MDS@L. The aptamer A10-3.2 was coupled to the surface of PEG-Lips to obtain MDS@LA, which was characterized using different techniques, including transmission electron microscopy and Fourier transform infrared spectroscopy. MDS@LA exhibited excellent stability, acid-responsive release, and photothermal properties, enhancing its antitumor effects. Both in vitro and in vivo experiments revealed that MDS@LA precisely targeted PCa cells and effectively delivered DTX and siRNA, leading to significant inhibition of PCa cell growth and proliferation. This versatile nanoplatform offers a promising, precise, and efficient therapeutic approach for advanced PCa, addressing the limitations of conventional chemotherapy.

Balancing enthalpy and entropy in inhibitor binding to the prostate-specific membrane antigen (PSMA)

Understanding the molecular mechanism of inhibitor binding to prostate-specific membrane antigen (PSMA) is of fundamental importance for designing targeted drugs for prostate cancer. Here we designed a series of PSMA-targeting inhibitors with distinct molecular structures, which were synthesized and characterized using both experimental and computational approaches. Microsecond molecular dynamics simulations revealed the structural and thermodynamic details of PSMA-inhibitor interactions. Our findings emphasize the pivotal role of the inhibitor's P1 region in modulating binding affinity and selectivity and shed light on the binding-induced conformational shifts of two key loops (the entrance lid and the interface loop). Binding energy calculations demonstrate the enthalpy-entropy balance in the thermodynamic driving force of different inhibitors. The binding of inhibitors in monomeric form is entropy-driven, in which the solvation entropy from the binding-induced water restraints plays a key role, while the binding of inhibitors in dimeric form is enthalpy-driven, due to the promiscuous PSMA-inhibitor interactions. These insights into the molecular driving force of protein-ligand binding offer valuable guidance for rational drug design.

Prostate-specific membrane antigen as target for vasculature-directed therapeutic strategies in solid tumors

Prostate-specific membrane antigen (PSMA) is one of the few biomarkers which has been successfully translated to the clinic as theranostic biomarker for patients with prostate cancer. In the context of prostate cancer, PSMA is overexpressed on the cell membrane of tumor cells, making it a viable target for interventions with urea-based small molecule inhibitors or antibodies conjugated to radioactive isotopes. Interestingly, in several non-prostatic cancers, expression of PSMA appears to be associated with the tumor neovasculature. This offers novel therapeutic opportunities for treatments targeting the vasculature in non-prostatic cancers. In this review, we discuss PSMA and its potential as target for vasculature-directed therapeutic approaches, including radioligand therapy, fusion protein vaccination and CAR T-cell therapy.

Biological determinants of PSMA expression, regulation and heterogeneity in prostate cancer

Prostate-specific membrane antigen (PSMA) is an important cell-surface imaging biomarker and therapeutic target in prostate cancer. The PSMA-targeted theranostic 177Lu-PSMA-617 was approved in 2022 for men with PSMA-PET-positive metastatic castration-resistant prostate cancer. However, not all patients respond to PSMA-radioligand therapy, in part owing to the heterogeneity of PSMA expression in the tumour. The PSMA regulatory network is composed of a PSMA transcription complex, an upstream enhancer that loops to the FOLH1 (PSMA) gene promoter, intergenic enhancers and differentially methylated regions. Our understanding of the PSMA regulatory network and the mechanisms underlying PSMA suppression is evolving. Clinically, molecular imaging provides a unique window into PSMA dynamics that occur on therapy and with disease progression, although challenges arise owing to the limited resolution of PET. PSMA regulation and heterogeneity - including intertumoural and inter-patient heterogeneity, temporal changes, lineage dynamics and the tumour microenvironment - affect PSMA theranostics. PSMA response and resistance to radioligand therapy are mediated by a number of potential mechanisms, and complementary biomarkers beyond PSMA are under development. Understanding the biological determinants of cell surface target regulation and heterogeneity can inform precision medicine approaches to PSMA theranostics as well as other emerging therapies.

Prognostic value of baseline alkaline phosphatase of 177Lu-PSMA radioligand therapy in metastatic castration-resistant prostate cancer: A systematic review and meta-analysis

BACKGROUND

Current studies have shown inconsistent results regarding the impact of baseline alkaline phosphatase (ALP) levels on the prognosis of metastatic castration-resistant prostate cancer (mCRPC) patients who undergo 177Lu-prostate-specific membrane antigen (PSMA) radioligand therapy (PRLT). Therefore, a comprehensive meta-analysis is needed to clarify the implications.

METHODS

This study was carried out in full compliance with the PRISMA protocol 2020, and a comprehensive search was conducted through PubMed, Web of Science, and Embase for published literature up to April 1st, 2024. Random-effects models were used to assess the correlation between baseline ALP and overall survival (OS) or progression-free survival (PFS) of mCRPC patients treated with 177Lu-PRLT, with a significance level set at α = 0.05.

RESULTS

A total of 12 articles were included in this study. The pooled effect estimates for baseline ALP and OS were 1.134 (95% CI: 1.035-1.245), I2 = 78.7%, P < 0.05. Regarding baseline ALP and PFS, the pooled effect estimate was found to be 2.14 (95% CI: 1.232-3.718), I2 = 93.3%, P < 0.05. Subgroup analysis revealed minimal heterogeneity among articles using a cut-off value ≥220U/L when examining the association between baseline ALP and OS; whereas for baseline ALP and PFS, there was also minimal heterogeneity observed among articles that adjusted for confounders.

CONCLUSION

This meta-analysis demonstrates a significant association between elevated baseline ALP levels in mCRPC patients prior to 177Lu-PRLT treatment and inferior OS and PFS. Timely monitoring of baseline ALP levels can provide valuable insights for clinical decision-making and patient counseling.

Theranostics Nuclear Medicine in Prostate Cancer

Theranostic Nuclear Medicine is based on the idea of combining the same molecule (or drug) with different radioisotopes for both diagnosis and treatment, a concept that emerged in the early 1940s with the use of radioactive iodine for thyroid diseases. Theranostic Nuclear Medicine has since expanded to diseases of higher incidence, such as prostate cancer, with several imaging methods used to assess the extent of the disease and the corresponding radiopharmaceuticals used for treatment. For example, by detecting osteoblastic metastases by bone scintigraphy, corresponding radiopharmaceuticals with therapeutic properties can be administered to eliminate or reduce pain associated with metastases and/or determine overall survival gain. The purpose of this review is to discuss the role of Theranostic Nuclear Medicine in prostate cancer, addressing the main diagnostic imaging studies with their corresponding treatments in the Theranostic model.

Preclinical Efficacy of a PSMA-Targeted Actinium-225 Conjugate (225Ac-Macropa-Pelgifatamab): A Targeted Alpha Therapy for Prostate Cancer

PURPOSE

Initially, prostate cancer responds to hormone therapy, but eventually resistance develops. Beta emitter-based prostate-specific membrane antigen (PSMA)-targeted radionuclide therapy is approved for the treatment of metastatic castration-resistant prostate cancer. Here we introduce a targeted alpha therapy (TAT) consisting of the PSMA antibody pelgifatamab covalently linked to a macropa chelator and labeled with actinium-225 and compare its efficacy and tolerability with other TATs.

EXPERIMENTAL DESIGN

The in vitro characteristics and in vivo biodistribution, antitumor efficacy, and tolerability of 225Ac-macropa-pelgifatamab (225Ac-pelgi) and other TATs were investigated in cell line- and patient-derived prostate cancer xenograft models. The antitumor efficacy of 225Ac-pelgi was also investigated in combination with the androgen receptor inhibitor darolutamide.

RESULTS

Actinium-225-labeling of 225Ac-pelgi was efficient already at room temperature. Potent in vitro cytotoxicity was seen in PSMA-expressing (LNCaP, MDA-PCa-2b, and C4-2) but not in PSMA-negative (PC-3 and DU-145) cell lines. High tumor accumulation was seen for both 225Ac-pelgi and 225Ac-DOTA-pelgi in the MDA-PCa-2b xenograft model. In the C4-2 xenograft model, 225Ac-pelgi showed enhanced antitumor efficacy with a T/Cvolume (treatment/control) ratio of 0.10 compared with 225Ac-DOTA-pelgi, 225Ac-DOTA-J591, and 227Th-HOPO-pelgifatamab (227Th-pelgi; all at 300 kBq/kg) with T/Cvolume ratios of 0.37, 0.39, and 0.33, respectively. 225Ac-pelgi was less myelosuppressive than 227Th-pelgi. 225Ac-pelgi showed dose-dependent treatment efficacy in the patient-derived KuCaP-1 model and strong combination potential with darolutamide in both cell line- (22Rv1) and patient-derived (ST1273) xenograft models.

CONCLUSIONS

These results provide a strong rationale to investigate 225Ac-pelgi in patients with prostate cancer. A clinical phase I study has been initiated (NCT06052306).

DNA-encoded chemical libraries enable the discovery of potent PSMA-ligands with substantially reduced affinity towards the GCPIII anti-target

Prostate-specific membrane antigen (PSMA) is a tumor-associated protein that has been successfully targeted with small organic ligands and monoclonal antibodies. Pluvicto™ is a PSMA-targeted radioligand therapeutic (RLT) recently approved by the FDA for the treatment of metastatic castration-resistant prostate cancer (2022 FDA marketing authorization). Although a large Phase III clinical trial (VISION trial) demonstrated clinical benefits in patients treated with Pluvicto™, the therapeutic window of the drug is narrowed by its undesired accumulation in healthy organs. Glutamate carboxypeptidase III (GCPIII), an enzyme sharing 70% identity with PSMA, may be responsible for the off-target accumulation of PSMA-RLTs in salivary glands and kidneys. In this work, we designed and synthesized affinity and selectivity maturation DNA-encoded chemical libraries (ASM-DELs) comprising 18'284'658 compounds that were screened in parallel against PSMA and GCPIII with the aim to identify potent and selective PSMA ligands for tumor-targeting applications. Compound A70-B104 was isolated as the most potent and selective ligand (KD of 900 pM for PSMA, KD of 40 nM for GCPIII). 177Lu-A70-B104-DOTA, a radiolabeled derivative of compound A70-B104, presented selective accumulation in PSMA-positive cancer lesions (i.e., 7.4% ID g-1, 2 hour time point) after systemic administration in tumor-bearing mice. The results of autoradiography experiments showed that 177Lu-A70-B104-DOTA selectively binds to PSMA-positive cancer tissues, while negligible binding on human salivary glands was observed.

Recent advances in the development of 225Ac- and 211At-labeled radioligands for radiotheranostics

Radiotheranostics utilizes a set of radioligands incorporating diagnostic or therapeutic radionuclides to achieve both diagnosis and therapy. Imaging probes using diagnostic radionuclides have been used for systemic cancer imaging. Integration of therapeutic radionuclides into the imaging probes serves as potent agents for radionuclide therapy. Among them, targeted alpha therapy (TAT) is a promising next-generation cancer therapy. The α-particles emitted by the radioligands used in TAT result in a high linear energy transfer over a short range, inducing substantial damage to nearby cells surrounding the binding site. Therefore, the key to successful cancer treatment with minimal side effects by TAT depends on the selective delivery of radioligands to their targets. Recently, TAT agents targeting biomolecules highly expressed in various cancer cells, such as sodium/iodide symporter, norepinephrine transporter, somatostatin receptor, αvβ3 integrin, prostate-specific membrane antigen, fibroblast-activation protein, and human epidermal growth factor receptor 2 have been developed and have made remarkable progress toward clinical application. In this review, we focus on two radionuclides, 225Ac and 211At, which are expected to have a wide range of applications in TAT. We also introduce recent fundamental and clinical studies of radiopharmaceuticals labeled with these radionuclides.

Control of the antitumour activity and specificity of CAR T cells via organic adapters covalently tethering the CAR to tumour cells

On-target off-tumour toxicity limits the anticancer applicability of chimaeric antigen receptor (CAR) T cells. Here we show that the tumour-targeting specificity and activity of T cells with a CAR consisting of an antibody with a lysine residue that catalytically forms a reversible covalent bond with a 1,3-diketone hapten can be regulated by the concentration of a small-molecule adapter. This adapter selectively binds to the hapten and to a chosen tumour antigen via a small-molecule binder identified via a DNA-encoded library. The adapter therefore controls the formation of a covalent bond between the catalytic antibody and the hapten, as well as the tethering of the CAR T cells to the tumour cells, and hence the cytotoxicity and specificity of the cytotoxic T cells, as we show in vitro and in mice with prostate cancer xenografts. Such small-molecule switches of T-cell cytotoxicity and specificity via an antigen-independent 'universal' CAR may enhance the control and safety profile of CAR-based cellular immunotherapies.

Structure-activity relationship study of mesyl and busyl phosphoramidate antisense oligonucleotides for unaided and PSMA-mediated uptake into prostate cancer cells

Phosphorothioate (PS) group is a key component of a majority of FDA approved oligonucleotide drugs that increase stability to nucleases whilst maintaining interactions with many proteins, including RNase H in the case of antisense oligonucleotides (ASOs). At the same time, uniform PS modification increases nonspecific protein binding that can trigger toxicity and pro-inflammatory effects, so discovery and characterization of alternative phosphate mimics for RNA therapeutics is an actual task. Here we evaluated the effects of the introduction of several N-alkane sulfonyl phosphoramidate groups such as mesyl (methanesulfonyl) or busyl (1-butanesulfonyl) phosphoramidates into gapmer ASOs on the efficiency and pattern of RNase H cleavage, cellular uptake in vitro, and intracellular localization. Using Malat1 lncRNA as a target, we have identified patterns of mesyl or busyl modifications in the ASOs for optimal knockdown in vitro. Combination of the PSMA ligand-mediated delivery with optimized mesyl and busyl ASOs resulted in the efficient target depletion in the prostate cancer cells. Our study demonstrated that other N-alkanesulfonyl phosphoramidate groups apart from a known mesyl phosphoramidate can serve as an essential component of mixed backbone gapmer ASOs to reduce drawbacks of uniformly PS-modified gapmers, and deserve further investigation in RNA therapeutics.

Construction of truncated PSMA as a PET reporter gene for CAR T cell trafficking

In solid tumors, there are multiple barriers for a chimeric antigen receptor (CAR) T cell to surmount in order to reach the tumor site. For better understanding whether CAR T cells effectively infiltrate into tumor site, and simultaneously, whether there are off-target effects, real-time monitoring technologies need to be established. Cell-based positron emission tomography reporter genes have been developed to monitor engineered cells in living subjects. In this study, we reported the construction of a novel reporter gene truncated prostate-specific membrane antigen (ΔPSMA) pending for monitoring CAR T cells using 68Ga-PSMA-617 and a method for tracking the distribution of CAR T cells in vivo was developed. Data were provided to demonstrate that ΔPSMA was predominantly localized on the plasma membrane and could take up 68Ga-PSMA-617 in vitro in a time-dependent manner. And the expression of ΔPSMA did not affect CAR expression and cytolytic capacity of CAR T cells. CAR-ΔPSMA T cell xenografts in nude mice were clearly imaged by positron emission tomography 60 min after injection of 68Ga-PSMA-617. PSMA paired with 68Ga-PSMA-617 was capable of identifying approximately 1 × 104 engineered CAR T cells. The ability to image small numbers of CAR T cells in vivo would be helpful to accelerate the translation of cell-based therapies into the clinic, and it may reinforce our understanding of treatment success, failure, and toxicity.

"One Method to Label Them All": A Single Fully Automated Protocol for GMP-Compliant 68Ga Radiolabeling of PSMA-11, Transposable to PSMA-I&T and PSMA-617

BACKGROUND

Prostate-specific membrane antigen (PSMA) is an ideal target for molecular imaging and targeted radionuclide therapy in prostate cancer. Consequently, various PSMA ligands were developed. Some of these molecules are functionalized with a chelator that can host radiometals, such as 68Ga for PET imaging. The 68Ga radiolabeling step benefits from process automation, making it more robust and reducing radiation exposure.

OBJECTIVE

To design a single automated radiolabeling protocol for the GMP-compliant preparation of [68Ga]Ga-PSMA-11, transposable to the production of [68Ga]Ga-PSMA-617 and [68Ga]Ga-PSMA-I&T.

METHODS

A GAIA® synthesis module and a GALLIAD® generator were used. Radio-TLC and radio-HPLC methods were validated for radiochemical purity (RCP) determination. Three [68Ga]Ga-PSMA-11 validation batches were produced and thoroughly tested for appearance and pH, radionuclide identity and purity, RCP, stability, residual solvent and sterility. Minimal modifications were made to the reagents and disposables for optimal application to other PSMA ligands.

RESULTS

[68Ga]Ga-PSMA-11 for clinical application was produced in 27 min. The 3 validation batches met the quality criteria expected by the European Pharmacopoeia to allow routine production. For optimal transposition to PSMA-617, the solid phase extraction cartridge was changed to improve purification of the radiolabeled product. For application to PSMA-I&T, the buffer solution initially used was replaced by HEPES 2.7 M to achieve good radiochemical yields. Residual HEPES content was checked in the final product and was below the Ph. Eur. threshold.

CONCLUSION

A single automated radiolabeling method on the GAIA® module was developed and implemented for 68Ga radiolabeling of 3 PSMA ligands, with slight adjustments for each molecule.

177Lu-labeled PSMA targeting therapeutic with optimized linker for treatment of disseminated prostate cancer; evaluation of biodistribution and dosimetry

Introduction

Prostate specific membrane antigen (PSMA), highly expressed in metastatic castration-resistant prostate cancer (mCRPC), is an established therapeutic target. Theranostic PSMA-targeting agents are widely used in patient management and has shown improved outcomes for mCRPC patients. Earlier, we optimized a urea-based probe for radionuclide visualization of PSMA-expression in vivo using computer modeling. With the purpose to develop a targeting agent equally suitable for radionuclide imaging and therapy, the agent containing DOTA chelator was designed (BQ7876). The aim of the study was to test the hypothesis that 177Lu-labeled BQ7876 possesses target binding and biodistribution properties potentially enabling its use for radiotherapy.

Methods

BQ7876 was synthesized and labeled with Lu-177. Specificity and affinity of [177Lu]Lu-BQ7876 to PSMA-expressing PC3-pip cells was evaluated and its processing after binding to cells was studied. Animal studies in mice were performed to assess its biodistribution in vivo, target specificity and dosimetry. [177Lu]Lu-PSMA-617 was simultaneously evaluated for comparison.

Results

BQ7876 was labeled with Lu-177 with radiochemical yield >99%. Its binding to PSMA was specific in vitro and in vivo when tested in antigen saturation conditions as well as in PSMA-negative PC-3 tumors. The binding of [177Lu]Lu-BQ7876 to living cells was characterized by rapid association, while the dissociation included a rapid and a slow phase with affinities KD1 = 3.8 nM and KD2 = 25 nM. The half-maximal inhibitory concentration for natLu-BQ7876 was 59 nM that is equal to 61 nM for natLu-PSMA-617. Cellular processing of [177Lu]Lu-BQ7876 was accompanied by slow internalization. [177Lu]Lu-BQ7876 was cleared from blood and normal tissues rapidly. Initial elevated uptake in kidneys decreased rapidly, and by 3 h post injection, the renal uptake (13 ± 3%ID/g) did not differ significantly from tumor uptake (9 ± 3%ID/g). Tumor uptake was stable between 1 and 3 h followed by a slow decline. The highest absorbed dose was in kidneys, followed by organs and tissues in abdomen.

Discussion

Biodistribution studies in mice demonstrated that targeting properties of [177Lu]Lu-BQ7876 are not inferior to properties of [177Lu]Lu-PSMA-617, but do not offer any decisive advantages.

Evaluation of a photodynamic therapy agent using a canine prostate cancer model

BACKGROUND

Male dogs can develop spontaneous prostate cancer, which is similar physiologically to human disease. Recently, Tweedle and coworkers have developed an orthotopic canine prostate model allowing implanted tumors and therapeutic agents to be tested in a more translational large animal model. We used the canine model to evaluate prostate-specific membrane antigen (PSMA)-targeted gold nanoparticles as a theranostic approach for fluorescence (FL) imaging and photodynamic therapy (PDT) of early stage prostate cancer.

METHODS

Dogs (four in total) were immunosuppressed with a cyclosporine-based immunosuppressant regimen and their prostate glands were injected with Ace-1-hPSMA cells using transabdominal ultrasound (US) guidance. Intraprostatic tumors grew in 4-5 weeks and were monitored by ultrasound (US). When tumors reached an appropriate size, dogs were injected intravenously (iv) with PSMA-targeted nano agents (AuNPs-Pc158) and underwent surgery 24 h later to expose the prostate tumors for FL imaging and PDT. Ex vivo FL imaging and histopathological studies were performed to confirm PDT efficacy.

RESULTS

All dogs had tumor growth in the prostate gland as revealed by US. Twenty-four hours after injection of PSMA-targeted nano agents (AuNPs-Pc158), the tumors were imaged using a Curadel FL imaging device. While normal prostate tissue had minimal fluorescent signal, the prostate tumors had significantly increased FL. PDT was activated by irradiating specific fluorescent tumor areas with laser light (672 nm). PDT bleached the FL signal, while fluorescent signals from the other unexposed tumor tissues were unaffected. Histological analysis of tumors and adjacent prostate revealed that PDT damaged the irradiated areas to a depth of 1-2 mms with the presence of necrosis, hemorrhage, secondary inflammation, and occasional focal thrombosis. The nonirradiated areas showed no visible damages by PDT.

CONCLUSION

We have successfully established a PSMA-expressing canine orthotopic prostate tumor model and used the model to evaluate the PSMA-targeted nano agents (AuNPs-Pc158) in the application of FL imaging and PDT. It was demonstrated that the nano agents allowed visualization of the cancer cells and enabled their destruction when they were irradiated with a specific wavelength of light.

Application of 18F-PSMA-1007 PET/MR Imaging in Early Biochemical Recurrence of Prostate Cancer: Results of a Prospective Study of 60 Patients with Very Low PSA Levels ≤ 0.5 ng/mL

The use of 18F-PSMA-1007 and the role of PET/MR in the diagnosis of prostate cancer are not conclusively confirmed. There are reports indicating the potential pros and cons of using 18F-PSMA-1007 as well as the PET/MR technique in prostate cancer recurrence, but they are not yet included in the EAU guidelines. The aim of the study was to assess the effectiveness of 18F-PSMA-1007 PET/MR in detecting BCR lesions at very low PSA levels <0.5 ng/mL.

METHODS

Sixty patients with BCR after radical prostatectomy (RP) with PSA ranged 0.1-0.5 ng/mL were enrolled in a prospective study. All patients underwent simultaneous whole-body and pelvic 18F-PSMA-1007 PET/MR. The obtained results were verified by 12-month follow-up.

RESULTS

Fifty-three lesions were detected in 45 patients with 75% detection rate. The mean PSA value was 0.31 ng/mL. Of all PSMA-positive foci, 91% were localized in the pelvis, and only 9% of lesions were located in the extrapelvic region. Local recurrences were detected in 29%, PSMA-positive lymph nodes were detected in 64% of patients and bone metastases lesions were detected in 7% of patients.

CONCLUSIONS

18F-PSMA-1007 PET/MR seems to be an excellent diagnostic tool in patients with early BCR with very low PSA levels, especially with dt PSA < 6 months. The synergistic effect of combining 18F-PSMA-1007 and whole-body PET/MR with precise multiparametric assessment of pelvic lesions is of particular benefit in early BCR.

Innovation in Radionuclide Therapy for the Treatment of Prostate Cancers: Radiochemical Perspective and Recent Therapeutic Practices

Prostate cancer represents the second cause of death by cancer in males in western countries. While early-stage diseases are accessible to surgery and/or external radiotherapy, advanced metastatic prostate cancers are primarily treated with androgen deprivation therapy, to which new generation androgen receptor antagonists or taxane-based chemotherapies are added in the case of tumor relapse. Nevertheless, patients become invariably resistant to castration with a median survival that rarely exceeds 3 years. This fostered the search for alternative strategies, independent of the androgen receptor signaling pathway. In this line, radionuclide therapies may represent an interesting option as they could target either the microenvironment of sclerotic bone metastases with the use of radiopharmaceuticals containing samarium-153, strontium-89 or radium-223 or tumor cells expressing the prostate-specific membrane antigen (PSMA), a protein found at the surface of prostate cancer cells. This review gives highlights the chemical properties of radioligands targeting prostate cancer cells and recapitulates the clinical trials evaluating the efficacy of radionuclide therapies, alone or in combination with other approved treatments, in patients with castration-resistant prostate tumors. It discusses some of the encouraging results obtained, especially the benefit on overall survival that was reported with [¹⁷⁷Lu]-PSMA-617. It also addresses the specific requirements for the use of this particular class of drugs, both in terms of medical staff coordination and adapted infrastructures for efficient radioprotection.

Lutetium-177 Prostate-Specific Membrane Antigen-617 Treatment in Metastatic Castration-Resistant Prostate Adenocarcinoma: Results of Single-Center Experience

OBJECTIVE

Lutetium-177 prostate-specific membrane antigen-617 is a novel alternative therapeutic option in metastatic castration-resistant prostate cancer, especially useful for patients who do not respond to standard therapy methods. The aim of this study was to define the efficacy and safety profile of lutetium-177 prostate- specific membrane antigen-617 treatment in a group of patients with metastatic castration-resistant prostate cancer.

MATERIALS AND METHODS

Study group included 34 men with metastatic castration-resistant prostate cancer (median, 69.6 ± 7.7 years) who were treated with lutetium-177 prostate-specific membrane antigen-617 therapy (22/34; 4 courses, 12/34; 2 courses). Patients were evaluated by physical examination, Eastern coop- erative oncology group performance status, gallium-68 prostate-specific membrane antigen positron emis- sion tomography/computed tomography, brief pain inventory-short form questionnaire, biochemical tests, and complete blood counts. Treatment response and adverse effects were examined by brief pain inventory scores, SUVmax values, biochemical tests, and complete blood counts. Independent variables were analyzed statistically (significance; P < .05).

RESULTS

The Eastern cooperative oncology group performance was grade 0 in 5/34 (14.7%), grade 1 in 25/34 (73.5%), and grade 2 in 4/34 (11.8%) patients. Distribution of patient numbers according to brief pain inven- tory scores (score: <1, scores: 1-4, and scores: 5-10) was 2, 10 and 22 at the beginning, 6, 16 and 12 after the second course, and 10, 10 and 2 after the fourth course of treatment, respectively. Serum prostate-specific antigen decreased in 15 of 22 patients (68%) (P < .05). Before and after the treatment, we found a substan- tial decrease in SUVmax values (22.3 vs. 11.8, P < .001) and brief pain inventory scores (score ≥ 5; 22/34 pts vs. 0/22 pts). The counts of white blood cells (P < .05), hemoglobin (P < .05), and thrombocytes (P = .001) were all significantly lower at the conclusion of the therapy. The most important adverse events were severe leukopenia (1/34 pts; 2.29 × 103/μL) and thrombocytopenia (3/34 pts; 32 000, 36 000, 32 000 106/L). Q1 Conclusion: We found that lutetium-177 prostate-specific membrane antigen-617 therapy is a promising treatment method for metastatic castration-resistant prostate cancer patients who are unresponsive to conventional therapy, according to our biochemical, positron emission tomography/computed tomography, and pain score outcomes.

Imageological/Structural Study regarding the Improved Pharmacokinetics by 68Ga-Labeled PEGylated PSMA Multimer in Prostate Cancer

PMSA (prostate-specific membrane antigen) is currently the most significant target for diagnosing and treating PCa (prostate cancer). Herein, we reported a series ⁶⁸Ga/¹⁷⁷Lu-labeled multimer PSMA tracer conjugating with PEG chain, including [⁶⁸Ga]Ga-DOTA-(1P-PEG₄), [⁶⁸Ga]Ga-DOTA-(2P-PEG₀), [⁶⁸Ga]Ga-DOTA-(2P-PEG₄), and [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-DOTA-(2P-PEG₄)₂, which showed an advantage of a multivalent effect and PEGylation to achieve higher tumor accumulation and faster kidney clearance. To figure out how structural optimizations based on a PSMA multimer and PEGylation influence the probe's tumor-targeting ability, biodistribution, and metabolism, we examined PSMA molecular probes' affinities to PC-3 PIP (PSMA-highly-expressed PC-3 cell line), and conducted pharmacokinetics analysis, biodistribution detection, small animal PET/CT, and SPECT/CT imaging. The results showed that PEG₄ and PSMA dimer optimizations enhanced the probes' tumor-targeting ability in PC-3 PIP tumor-bearing mice models. Compared with the PSMA monomer, the PEGylated PSMA dimer reduced the elimination half-life in the blood and increased uptake in the tumor, and the biodistribution results were consistent with PET/CT imaging results. [⁶⁸Ga]Ga-DOTA-(2P-PEG₄)₂ exhibited higher tumor-to-organ ratios. When labeled by lutetium-177, relatively high accumulation of DOTA-(2P-PEG₄)₂ was still detected in PC-3 PIP tumor-bearing mice models after 48 h, indicating its prolonged tumor retention time. Given the superiority in imaging, simple synthetic processes, and structural stability, DOTA-(2P-PEG₄)₂ is expected to be a promising tumor-targeting diagnostic molecular probe in future clinical practice.

Synthesis and radiolabelling of PSMA-targeted derivatives containing GYK/MVK cleavable linkers

Targeted radionuclide therapy (TRT) is a promising strategy to treat different types of cancer. TRT relies on a targeting vector used to deliver a therapeutic radionuclide specifically to the tumour site. Several low molecular weight ligands targeting the prostate-specific membrane antigen (PSMA) have been synthesized, but their pharmacokinetic properties still need to be optimized. Hereby, we describe the synthesis of new conjugates, featuring the cleavable linkers Gly-Tyr-Lys (GYK) and Met-Val-Lys (MVK), to reduce the dose delivered to the kidneys. Compounds were synthesized by solid-phase peptide synthesis (SPPS) and obtained in greater than 95% chemical purity. Radiolabelling was performed with both In-111 and Lu-177 to validate potential use of the compounds as both imaging and therapeutic agents. Radiochemical purity greater than 80% was obtained for both nuclides, but significant radiolysis was observed for the methionine-containing analogue. The results obtained thus far with the GYK-PSMA conjugate could warrant further biological investigations.

Targeted Radiation and Immune Therapies-Advances and Opportunities for the Treatment of Prostate Cancer

Prostate cancer is the most diagnosed malignancy in men in the United States and the second leading cause of cancer-related death. For localized disease, radiation therapy is a standard treatment that is often curative. For metastatic disease, radiation therapy has been primarily used for palliation, however, several newer systemic radiation therapies have been demonstrated to significantly improve patient outcomes and improve survival. In particular, several targeted radionuclide therapies have been approved for the treatment of advanced-stage cancer, including strontium-89, samarium-153, and radium-223 for bone-metastatic disease, and lutetium-177-labeled PSMA-617 for patients with prostate-specific membrane antigen (PSMA)-expressing metastatic castration-resistant prostate cancer (mCRPC). Contrarily, immune-based treatments have generally demonstrated little activity in advanced prostate cancer, with the exception of the autologous cellular vaccine, sipuleucel-T. This has been attributed to the presence of an immune-suppressive prostate cancer microenvironment. The ability of radiation therapy to not only eradicate tumor cells but also potentially other immune-regulatory cells within the tumor immune microenvironment suggests that targeted radionuclide therapies may be well poised to combine with immune-targeted therapies to eliminate prostate cancer metastases more effectively. This review provides an overview of the recent advances of targeted radiation agents currently approved for prostate cancer, and those being investigated in combination with immunotherapy, and discusses the challenges as well as the opportunities in this field.

Synthesis and evaluation of a multifunctional probe with a high affinity for prostate-specific membrane antigen (PSMA) and bone

Prostate cancer frequently metastasizes to the bone. Because patients with bone metastases suffer from skeletal-related events, the diagnosis and treatment of bone metastases in the early stage are important. In this study, to improve the sensitivity of detecting bone metastases in patients with prostate cancer, we designed, synthesized, and evaluated a multifunctional radiotracer, [⁶⁷Ga]Ga-D₁₁-PSMA-617 ([⁶⁷Ga]3), with an undeca-aspartic acid as a bone-seeking moiety between [⁶⁷Ga]Ga-DOTA and a prostate-specific membrane antigen (PSMA) ligand based on the lysine-urea-glutamate motif. [⁶⁷Ga]3 showed a high affinity for hydroxyapatite and high uptake in PSMA-positive LNCaP cells. Moreover, in biodistribution experiments using tumor-bearing mice, [⁶⁷Ga]3 exhibited high accumulation in the bone and PSMA-positive tumor although the accumulation of [⁶⁷Ga]3 in the PSMA-positive tumor was lower than that of [⁶⁷Ga]Ga-PSMA-617. This study provides valuable information for developing radiotheranostic probes combining multiple carriers with different mechanisms.

A Supramolecular Self-Assembling Nanoagent by Inducing Intracellular Aggregation of PSMA for Prostate Cancer Molecularly Targeted Theranostics

Prostate cancer (PCa) with prostate-specific membrane antigen (PSMA)-specific high expression is well suited for molecularly targeted theranostics. PSMA expression correlates with the malignancy of PCa, and its dimeric form can promote tumor progression by exerting enzymatic activity to activate downstream signal transduction. However, almost no studies have shown that arresting the procancer signaling of the PSMA receptors themselves can cause tumor cell death. Meanwhile, supramolecular self-assembling peptides are widely used to design anticancer agents due to their unique and excellent properties. Here, a PSMA-targeting supramolecular self-assembling nanotheranostic agent, DBT-2FFGACUPA, which actively targets PSMA receptors on PCa cell membranes and induces them to enter the cell and form large aggregates, is developed. This process not only selectively images PSMA-positive tumor cells but also suppresses the downstream procancer signals of PSMA, causing tumor cell death. This work provides an alternative approach and an advanced agent for molecularly targeted theranostics options in PCa that can induce tumor cell death without relying on any reported anticancer drugs.

PSMA PET Imaging and Therapy in Adenoid Cystic Carcinoma and Other Salivary Gland Cancers: A Systematic Review

Simple Summary

Adenoid cystic carcinoma (ACC) and other salivary gland cancers (SGC) are rare conditions with limited treatment options when they recur or spread to other parts of the body. There is increasing interest in the interaction of radioactive labeled proteins 68Gallium- Prostate Specific Membrane Antigen (68Ga-PSMA) with their corresponding receptors on tumor cells (PSMA receptor) which can be detected on scans. This innovation has created diagnostic and therapeutic progress in management of metastatic prostate cancer. These interactions are also found in SGCs though studies are currently limited. Our systematic review aims to collate available published scientific information on this technology to better inform its potential use, pitfalls and its future directions as a diagnostic and therapeutic option in SGCs. We concluded that the 68Ga-PSMA scans can be useful in detecting ACC and SGC not detected on standard radioimaging and that small studies have shown the therapeutic potential of this innovation in advanced or metastatic ACC and SGC.

Abstract

Adenoid cystic carcinoma (ACC) and other salivary gland cancers (SGCs) are rare tumors where application of prostate specific membrane antigen (PSMA) positron emission tomography (PET) and PSMA radioligand therapy have yet to be studied extensively. This review explores the role of PSMA PET imaging and therapy as a theranostic tool for ACC and other SGCs based on current literature. A comprehensive literature search on PubMed and Embase was performed. All relevant studies containing information on PSMA PET imaging in ACC and SGC were included. Ten studies (one prospective, three retrospective, five case reports and one review paper) were included. For ACC, the mean maximum standardized uptake value (SUVmax) for local recurrence and distant metastases ranged from 2.41 to 13.8 and 2.04 to 14.9, respectively. In SGC, the meanSUVmax ranged from 1.2–12.50. Most studies observed PSMA expression positivity on immunohistochemistry (IHC) when there was PSMA PET uptake. PSMA PET was able to detect lesions not detected on standard imaging. Despite the small number of studies and wide intra-patient and inter-tumor variation of PSMA uptake in ACC and SGC, 68Gallium (68Ga)-PSMA PET has promising prospects as a diagnostic and radioligand therapeutic option. Further studies to answer the various theranostics considerations are required to guide its use in the real-world setting.

Prostate-specific membrane antigen-targeted theranostics: past, present, and future approaches

Although prostate cancer is the type of cancer most commonly survived by men in the United States, it remains the second most common cause of death from cancer, largely owing to metastatic disease. Patients with metastatic castration-resistant prostate cancer (mCRPC) whose disease has progressed on standard-of-care therapies have few options and a poor prognosis. Prostate-specific membrane antigen (PSMA) is a type II integral membrane protein that is commonly expressed in prostate cancer. Expression is limited on extra-prostatic tissues other than the salivary glands, lacrimal glands, duodenal epithelium, Kupffer cells, and renal tubules. PSMA-directed theranostics has emerged to exploit the specificity of PSMA for prostate cancer cells and has demonstrated promising results in the clinic. Radionuclides linked to PSMA inhibitors/binders have resulted in US Food and Drug Administration (FDA) approval of 2 radiodiagnostics for PSMA-directed positron emission tomography/computed tomography. In addition, these radionuclides have led to the development of lutetium Lu 177PSMA-617 therapy, which is currently under priority FDA review. Multiple novel PSMA-targeted modalities have been developed and are currently under clinical investigation, including ligand-drug and cellular immune therapies. In this review, we discuss the development of PSMA-directed theranostics, along with its clinical implications, limitations, and future directions.

Selective delivery of clinically approved tubulin binding agents through covalent conjugation to an active targeting moiety

Abstract:

The efficacy and tolerability of tubulin binding agents are hampered by their low specificity for cancer cells, like most clinically used anticancer agents. To improve specificity, tubulin binding agents have been covalently conjugated to agents which target cancer cells to give actively targeted drug conjugates. These conjugates are designed to increase uptake of the drug by cancer cells, while having limited uptake by normal cells thereby improving efficacy and tolerability. Approaches used include attachment to small molecules, polysaccharides, peptides, proteins and antibodies that exploit the overexpression of receptors for these substances. Antibody targeted strategies have been the most successful to date with six such examples having gained clinical approval. Many other conjugate types, especially those targeting the folate receptor, have shown promising efficacy and toxicity profiles in pre-clinical models and in early-stage clinical studies. Presented herein is a discussion of the success or otherwise of the recent strategies used to form these actively targeted conjugates.

Development of targeted nanoparticles loaded with antiviral drugs for SARS-CoV-2 inhibition

Recently, a novel coronavirus, known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has raised global concerns, being the etiological agent of the current pandemic infectious coronavirus disease 2019 (COVID-19). Specific prophylactic treatments like vaccines, have been authorized for use by regulatory bodies in multiple countries, however there is an urgent need to identify new, safe, and targeted therapeutics as post-exposure therapy for COVID-19. Among a plethora of potential pharmacological targets, the angiotensin-converting enzyme 2 (ACE2) membrane receptor, which plays a crucial role in viral entry, is representing an attractive intervention opportunity for SARS-CoV-2 antiviral discovery process. In this scenario, we envisioned that binding to ACE2 by multivalent attachment of ligands to nanocarriers incorporating antiviral therapeutics, it would increase receptor avidity and impart specificity to these nanovectors for host cells, particularly in the pulmonary tract, which is the primary entry route for SARS-CoV-2. Herein, we report the design and development of novel polymeric nanoparticles (NP), densely grafted with various ligands to selectively bind to ACE2, as innovative nanovectors for targeted drug delivery. We first evaluated the impact of these biocompatible targeted NP (TNP) on ligand binding toward ACE2 and measured their competition ability vs a model of spike protein (Lipo-S1). Next, we tested the effectiveness of the most performing nanoprotopype, TNP-1, loaded with a model anti-SARS-CoV-2 drug such as remdesivir (RDV), on antiviral activity against SARS-CoV-2 infected Vero E6 cells. The RDV-TNP-1 exhibited a significantly improved antiviral effect compared to RDV at the same concentration. Interestingly, unloaded TNP (TNP-1E) also exhibited a basal antiviral activity, potentially due to a direct competitive mechanism with viral particles for the ACE2 binding site. We also measured the anti-exopeptidase activity of TNP-1E against ACE2 protein. Collectively, these insights warrant in-depth preclinical development for our nanoprototypes, for example as potential inhalable drug carriers, with the perspective of a clinical translation.

Glycoforms of human prostate-specific membrane antigen (PSMA) in human cells and prostate tissue

INTRODUCTION

N-glycosylation is a ubiquitous and variable posttranslational modification that regulates physiological functions of secretory and membrane-associated proteins and the dysregulation of glycosylation pathways is often associated with cancer growth and metastasis. Prostate-specific membrane antigen (PSMA) is an established biomarker for prostate cancer imaging and therapy.

METHODS

Mass spectrometry was used to analyze the distribution of the site-specific glycoforms of PSMA in insect, human embryonic kidney, and prostate cancer cells, and in prostate tissue upon immunoaffinity enrichment.

RESULTS

While recombinant PSMA expressed in insect cells was decorated mainly by paucimannose and high mannose glycans, complex, hybrid, and high mannose glycans were detected in samples from human cells and tissue. We noted an interesting spatial distribution of the glycoforms on the PSMA surface-high mannose glycans were the dominant glycoforms at the N459, N476, and N638 sequons facing the plasma membrane, while the N121, N195, and N336 sites, located at the exposed apical PSMA domain, carried primarily complex glycans. The presence of high mannose glycoforms at the former sequons likely results from the limited access of enzymes of the glycosynthetic pathway required for the synthesis of the complex structures. In line with the limited accessibility of membrane-proximal sites, no glycosylation was observed at the N51 site positioned closest to the membrane.

CONCLUSIONS

Our study presents initial descriptive analysis of the glycoforms of PSMA observed in cell lines and in prostate tissue. It will hopefully stimulate further research into PSMA glycoforms in the context of tumor staging, noninvasive detection of prostate tumors, and the impact of glycoforms on physicochemical and enzymatic characteristics of PSMA in a tissue-specific manner.

Emerging methods for prostate cancer imaging: evaluating cancer structure and metabolic alterations more clearly

Imaging plays a fundamental role in all aspects of the cancer management pathway. However, conventional imaging techniques are largely reliant on morphological and size descriptors that have well-known limitations, particularly when considering targeted-therapy response monitoring. Thus, new imaging methods have been developed to characterise cancer and are now routinely implemented, such as diffusion-weighted imaging, dynamic contrast enhancement, positron emission technology (PET) and magnetic resonance spectroscopy. However, despite the improvement these techniques have enabled, limitations still remain. Novel imaging methods are now emerging, intent on further interrogating cancers. These techniques are at different stages of maturity along the biomarker pathway and aim to further evaluate the cancer microstructure (vascular, extracellular and restricted diffusion for cytometry in tumours) magnetic resonance imaging (MRI), luminal water fraction imaging] as well as the metabolic alterations associated with cancers (novel PET tracers, hyperpolarised MRI). Finally, the use of machine learning has shown powerful potential applications. By using prostate cancer as an exemplar, this Review aims to showcase these potentially potent imaging techniques and what stage we are at in their application to conventional clinical practice.

UpFrontPSMA: a randomized phase 2 study of sequential 177 Lu-PSMA-617 and docetaxel vs docetaxel in metastatic hormone-naïve prostate cancer (clinical trial protocol)

OBJECTIVE

To assess the activity and safety of sequential lutetium-177 (¹⁷⁷ Lu)-PSMA-617 and docetaxel vs docetaxel on a background of androgen deprivation therapy (ADT) in men with de novo metastatic hormone-naïve prostate cancer (mHNPC).

PATIENTS AND METHODS

UpFrontPSMA (NCT04343885) is an open-label, randomized, multicentre, phase 2 trial, recruiting 140 patients at 12 Australian centres. Key eligibility criteria include: prostate cancer with a histological diagnosis within 12 weeks of screening commencement; prostate-specific antigen (PSA) >10 ng/mL at diagnosis; ≤4 weeks on ADT; evidence of metastatic disease on computed tomography (CT) and/or bone scan; high-volume prostate-specific membrane antigen (PSMA)-avid disease with a maximum standardized uptake value >15; and absence of extensive discordant fluorodeoxyglcuose (FDG)-positive, PSMA-negative disease. ⁶⁸ Ga-PSMA-11 and ¹⁸ F-FDG positron-emission tomography (PET)/CT undergo central review to determine eligibility. Patients are randomized 1:1 to experimental treatment, Arm A (¹⁷⁷ Lu-PSMA-617 7.5GBq q6w × 2 cycles followed by docetaxel 75 mg/m² q3w × 6 cycles), or standard-of-care treatment, Arm B (docetaxel 75 mg/m² q3w × 6 cycles). All patients receive continuous ADT. Patients are stratified based on disease volume on conventional imaging and duration of ADT at time of registration. The primary endpoint is the proportion of patients with undetectable PSA (≤0.2 ng/L) at 12 months after study treatment commencement. Secondary endpoints include safety, time to castration resistance, overall survival, PSA and radiographic progression-free survival, objective tumour response rate, early PSMA PET response, health-related quality of life, and frequency and severity of adverse events. Enrolment commenced in April 2020.

RESULTS AND CONCLUSIONS

The results of this trial will generate data on the activity and safety of ¹⁷⁷ Lu-PSMA-617 in men with de novo mHNPC in a randomized phase 2 design.

PSMA Expression in Glioblastoma as a Basis for Theranostic Approaches: A Retrospective, Correlational Panel Study Including Immunohistochemistry, Clinical Parameters and PET Imaging

Aim

The aim of the current study was to enlighten the evolution of prostate-specific membrane antigen (PSMA) expression in glioblastoma between initial diagnosis and recurrence in order to provide preliminary insight for further clinical investigations into innovative PSMA-directed treatment concepts in neuro-oncology.

Methods

Patients who underwent resection for de-novo glioblastoma (GBM) and had a re-resection in case of a recurrent tumor following radiochemotherapy and subsequent chemotherapy were included (n = 16). Histological and immunohistochemical stainings were performed at initial diagnosis and at recurrence (n = 96 tissue specimens). Levels of PSMA expression both in endothelial and non-endothelial cells as well as vascular density (CD34) were quantified via immunohistochemistry and changes between initial diagnosis and recurrence were determined. Immunohistochemical findings were correlated with survival and established clinical parameters.

Results

PSMA expression was found to be present in all GBM tissue samples at initial diagnosis as well as in all but one case of recurrent tumor samples. The level of PSMA expression in glioblastoma varied inter-individually both in endothelial and non-endothelial cells. Likewise, the temporal evolution of PSMA expression highly varied in between patients. The level of vascular PSMA expression at recurrence and its change between initial diagnosis and recurrence was associated with post recurrence survival time: Patients with high vascular PSMA expression at recurrence as well as patients with increasing PSMA expression throughout the disease course survived shorter than patients with low vascular PSMA expression or decreasing vascular PSMA expression. There was no significant correlation of PSMA expression with MGMT promoter methylation status or Ki-67 labelling index.

Conclusion

PSMA is expressed in glioblastoma both at initial diagnosis and at recurrence. High vascular PSMA expression at recurrence seems to be a negative prognostic marker. Thus, PSMA expression in GBM might present a promising target for theranostic approaches in recurrent glioblastoma. Especially PSMA PET imaging and PSMA-directed radioligand therapy warrant further studies in brain tumor patients.

Prostate-specific membrane antigen (PSMA)-targeted photodynamic therapy enhances the delivery of PSMA-targeted magnetic nanoparticles to PSMA-expressing prostate tumors

Enhanced vascular permeability in tumors plays an essential role in nanoparticle delivery. Prostate-specific membrane antigen (PSMA) is overexpressed on the epithelium of aggressive prostate cancers (PCs). Here, we evaluated the feasibility of increasing the delivery of PSMA-targeted magnetic nanoparticles (MNPs) to tumors by enhancing vascular permeability in PSMA(+) PC tumors with PSMA-targeted photodynamic therapy (PDT).

Method: PSMA(+) PC3 PIP tumor-bearing mice were given a low-molecular-weight PSMA-targeted photosensitizer and treated with fluorescence image-guided PDT, 4 h after. The mice were then given a PSMA-targeted MNP immediately after PDT and monitored with fluorescence imaging and T₂-weighted magnetic resonance imaging (T₂-W MRI) 18 h, 42 h, and 66 h after MNP administration. Untreated PSMA(+) PC3 PIP tumor-bearing mice were used as negative controls.

Results: An 8-fold increase in the delivery of the PSMA-targeted MNPs was detected using T₂-W MRI in the pretreated tumors 42 h after PDT, compared to untreated tumors. Additionally, T₂-W MRIs revealed enhanced peripheral intra-tumoral delivery of the PSMA-targeted MNPs. That finding is in keeping with two-photon microscopy, which revealed higher vascular densities at the tumor periphery.

Conclusion: These results suggest that PSMA-targeted PDT enhances the delivery of PSMA-targeted MNPs to PSMA(+) tumors by enhancing the vascular permeability of the tumors.

Dimeric and Multimeric DNA Aptamers for Highly Effective Protein Recognition

Multivalent interactions frequently occur in biological systems and typically provide higher binding affinity and selectivity in target recognition than when only monovalent interactions are operative. Thus, taking inspiration by nature, bivalent or multivalent nucleic acid aptamers recognizing a specific biological target have been extensively studied in the last decades. Indeed, oligonucleotide-based aptamers are suitable building blocks for the development of highly efficient multivalent systems since they can be easily modified and assembled exploiting proper connecting linkers of different nature. Thus, substantial research efforts have been put in the construction of dimeric/multimeric versions of effective aptamers with various degrees of success in target binding affinity or therapeutic activity enhancement. The present review summarizes recent advances in the design and development of dimeric and multimeric DNA-based aptamers, including those forming G-quadruplex (G4) structures, recognizing different key proteins in relevant pathological processes. Most of the designed constructs have shown improved performance in terms of binding affinity or therapeutic activity as anti-inflammatory, antiviral, anticoagulant, and anticancer agents and their number is certainly bound to grow in the next future.

Application of an inhibitor-based probe to reveal the distribution of membrane PSMA in dSTORM imaging

Relying on an inhibitor-based probe, we reveal the clustered distribution of membrane PSMA by dSTORM imaging and uncover its potential interaction with folate receptor. This inhibitor-based strategy realizes more accurate labeling than antibody labeling, which would make it a powerful tool in the field of dSTORM imaging.

Voxel-based comparison of [68Ga]Ga-RM2-PET/CT and [68Ga]Ga-PSMA-11-PET/CT with histopathology for diagnosis of primary prostate cancer

BACKGROUND

Focal therapies or focally escalated therapies of primary prostate cancer are becoming more and more important. This increases the need to identify the exact extension of the intraprostatic tumor and possible dominant intraprostatic lesions by imaging techniques. While the prostate-specific membrane antigen (PSMA) is already a well-established target for imaging of prostate cancer cells, the gastrin-releasing peptide receptor (GRPR) seems to provide interesting additional information. Histopathology was used to examine the extent to which the single and combined image information of PET scans targeting GRPR and PSMA might lead to better tumor delineation.

METHODS

Eight patients with histologically proven primary prostate cancer underwent two positron emission tomography with computer tomography scans, [68Ga]Ga-RM2-PET/CT (RM2-PET) and [68Ga]Ga-PSMA-11-PET/CT (PSMA-PET), prior to radical prostatectomy. RM2-PET data were correlated voxel-wise to a voxel-based model of the histopathologic tumor volume information. The results were compared to, correlated to, and combined with the correlation of PSMA-PET data analyzed analogously.

RESULTS

In 4/8 patients, RM2-PET showed a higher signal in histologically proven tumor regions compared to PSMA. There were also tumor regions where PSMA-PET showed a higher signal than GRPR in 4/8 patients. A voxel-wise correlation of RM2-PET against histopathology yielded similar results compared to the correlation of PSMA-PET against histopathology, while PSMA-PET is the slightly better performing imaging technique. The combined information of both tracers yielded the best overall result, although this effect was not statistically significant compared to RM2-PET alone.

CONCLUSIONS

Qualitative and quantitative findings in this preliminary study with 8 patients indicate that RM2-PET and PSMA-PET partially show not only the same, but also distinct regions of prostate cancer. Patients with pPCa might profit from information given by tracers targeting GRPR and PSMA simultaneously, in terms of a better delineation of the gross tumor volume.

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.

Multi-Modal PET and MR Imaging in the Hen's Egg Test-Chorioallantoic Membrane (HET-CAM) Model for Initial in Vivo Testing of Target-Specific Radioligands

The validation of novel target-specific radioligands requires animal experiments mostly using mice with xenografts. A pre-selection based on a simpler in vivo model would allow to reduce the number of animal experiments, in accordance with the 3Rs principles (reduction, replacement, refinement). In this respect, the chick embryo or hen’s egg test–chorioallantoic membrane (HET-CAM) model is of special interest, as it is not considered an animal until day 17. Thus, we evaluated the feasibility of quantitative analysis of target-specific radiotracer accumulation in xenografts using the HET-CAM model and combined positron emission tomography (PET) and magnetic resonance imaging (MRI). For proof-of-principle we used established prostate-specific membrane antigen (PSMA)-positive and PSMA-negative prostate cancer xenografts and the clinically widely used PSMA-specific PET-tracer [⁶⁸Ga]Ga-PSMA-11. Tracer accumulation was quantified by PET and tumor volumes measured with MRI (n = 42). Moreover, gamma-counter analysis of radiotracer accumulation was done ex-vivo. A three- to five-fold higher ligand accumulation in the PSMA-positive tumors compared to the PSMA-negative tumors was demonstrated. This proof-of-principle study shows the general feasibility of the HET-CAM xenograft model for target-specific imaging with PET and MRI. The ultimate value for characterization of novel target-specific radioligands now has to be validated in comparison to mouse xenograft experiments.

Membrane Protein Dimerization in Cell-Derived Lipid Membranes Measured by FRET with MC Simulations

Many membrane proteins are thought to function as dimers or higher oligomers, but measuring membrane protein oligomerization in lipid membranes is particularly challenging. Förster resonance energy transfer (FRET) and fluorescence cross-correlation spectroscopy are noninvasive, optical methods of choice that have been applied to the analysis of dimerization of single-spanning membrane proteins. However, the effects inherent to such two-dimensional systems, such as the excluded volume of polytopic transmembrane proteins, proximity FRET, and rotational diffusion of fluorophore dipoles, complicate interpretation of FRET data and have not been typically accounted for. Here, using FRET and fluorescence cross-correlation spectroscopy, we introduce a method to measure surface protein density and to estimate the apparent Förster radius, and we use Monte Carlo simulations of the FRET data to account for the proximity FRET effect occurring in confined two-dimensional environments. We then use FRET to analyze the dimerization of human rhomboid protease RHBDL2 in giant plasma membrane vesicles. We find no evidence for stable oligomers of RHBDL2 in giant plasma membrane vesicles of human cells even at concentrations that highly exceed endogenous expression levels. This indicates that the rhomboid transmembrane core is intrinsically monomeric. Our findings will find use in the application of FRET and fluorescence correlation spectroscopy for the analysis of oligomerization of transmembrane proteins in cell-derived lipid membranes.

Preclinical Efficacy of a PSMA-Targeted Thorium-227 Conjugate (PSMA-TTC), a Targeted Alpha Therapy for Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) is an attractive target for radionuclide therapy of metastatic castration-resistant prostate cancer (mCRPC). PSMA-targeted alpha therapy (TAT) has shown early signs of activity in patients with prostate cancer refractory to beta radiation. We describe a novel, antibody-based TAT, the PSMA-targeted thorium-227 conjugate PSMA-TTC (BAY 2315497) consisting of the alpha-particle emitter thorium-227 complexed by a 3,2-HOPO chelator covalently linked to a fully human PSMA-targeting antibody.

EXPERIMENTAL DESIGN

PSMA-TTC was characterized for affinity, mode of action, and cytotoxic activity in vitro. Biodistribution, pharmacokinetics, and antitumor efficacy were investigated in vivo using cell line and patient-derived xenograft (PDX) models of prostate cancer.

RESULTS

PSMA-TTC was selectively internalized into PSMA-positive cells and potently induced DNA damage, cell-cycle arrest, and apoptosis in vitro. Decrease in cell viability was observed dependent on the cellular PSMA expression levels. In vivo, PSMA-TTC showed strong antitumor efficacy with T/C values of 0.01 to 0.31 after a single injection at 300 to 500 kBq/kg in subcutaneous cell line and PDX models, including models resistant to standard-of-care drugs such as enzalutamide. Furthermore, inhibition of both cancer and cancer-induced abnormal bone growth was observed in a model mimicking prostate cancer metastasized to bone. Specific tumor uptake and efficacy were demonstrated using various PSMA-TTC doses and dosing schedules. Induction of DNA double-strand breaks was identified as a key mode of action for PSMA-TTC both in vitro and in vivo.

CONCLUSIONS

The strong preclinical antitumor activity of PSMA-TTC supports its clinical evaluation, and a phase I trial is ongoing in mCRPC patients (NCT03724747).

TheraP: a randomized phase 2 trial of 177 Lu-PSMA-617 theranostic treatment vs cabazitaxel in progressive metastatic castration-resistant prostate cancer (Clinical Trial Protocol ANZUP 1603)

OBJECTIVE

To assess the activity and safety of cabazitaxel chemotherapy vs that of treatment with ¹⁷⁷ Lu-PSMA-617, a novel radiolabelled small molecule that binds with high affinity to prostate-specific membrane antigen (PSMA), in men with metastatic castration-resistant prostate cancer (mCRPC) who have received prior docetaxel treatment.

PATIENTS AND METHODS

The TheraP trial (ANZUP 1603) is an open-label, randomized, stratified, two-arm multicentre phase 2 trial comparing the activity and safety of cabazitaxel chemotherapy vs ¹⁷⁷ Lu-PSMA-617 therapy in the treatment of men with mCRPC. Key eligibility criteria include prior docetaxel chemotherapy, rising prostate-specific antigen (PSA) level, sufficient PSMA avidity, as defined by centrally reviewed ⁶⁸ Ga-PSMA-11 and fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) with no discordant FDG-avid PSMA-negative sites of disease. Patients in the control group receive standard treatment with cabazitaxel (20 mg/m² ) i.v. every 3 weeks with prednisolone 10 mg daily orally, for a maximum of 10 cycles. Patients in the experimental group receive ¹⁷⁷ Lu-PSMA-617 (8.5 GBq decreasing by 0.5 GBq per cycle) i.v. every 6 weeks, for up to a maximum of six cycles. In the event of an exceptional response as defined on centrally reviewed post-therapy single-photon emission CT imaging, treatment will be suspended but can recommence on progression. The trial aims to include 200 patients who will be centrally randomized to one of the two treatment groups, in a 1:1 ratio. The primary endpoint is PSA response. Secondary endpoints are overall survival, progression-free survival (PFS), radiographic PFS, PSA PFS, objective tumour response, pain response, pain PFS, health-related quality of life, and frequency and severity of adverse events. The treatment and outcomes of patients excluded on the basis of low PSMA avidity or discordant FDG-avid disease on screening ⁶⁸ Ga-PSMA-11 and Fluorine-18 (¹⁸ F)-FDG-PET/CT scan will also be assessed. Enrolment in the study commenced on 29 January 2018.

RESULTS AND CONCLUSIONS

¹⁷⁷ Lu-PSMA-617 offers a potential additional life-prolonging treatment option for men with mCRPC. The results of this trial will determine, for the first time in a randomized design, the activity and safety of ¹⁷⁷ Lu-PSMA-617, as compared with cabazitaxel chemotherapy in men with progressive mCRPC.

Does lymph node localization affect prostate-specific membrane antigen uptake?

OBJECTIVE

To investigate the relationship between lymph node (LN) size and localization and prostate-specific membrane antigen (PSMA) uptake in patients with prostate cancer.

PATIENTS AND METHODS

Between February 2015 and February 2017, a total of 178 patients diagnosed with prostate adenocarcinoma by gallium-68 PSMA PET/computed tomography were evaluated. Seventy-eight patients with LN PSMA uptake were included in the study.The short-axis diameter and maximum and mean standardized uptake values (SUV) were measured from axial images of LNs with PSMA uptake. LN conglomerates were considered a single lesion. Patients were divided into two groups according to the presence (group 1) or absence (group 2) of bone uptake in addition to the LN uptake; subgroups were constituted according to the short-axis diameter and PSMA-positive LN region for each group. LNs were classified according to their localization as pelvic, abdominal, or cervical/thoracic.

RESULTS

In both groups, the SUV were significantly higher in LNs with short-axis diameters more than 8 mm than those with diameters up to 8 mm (P < 0.001 and < 0.001, respectively). Also, both groups showed a moderate correlation between the LN size and SUV.When the analysis was carried out according to the localization, but after excluding the LN conglomerates, the correlation between the pelvic and abdominal LNs was higher in the second group.

CONCLUSION

The highest significant positive correlation between LN size and SUV was in patients with pelvic and abdominal LN metastases without bone involvement according to the gallium-68 PSMA PET/computed tomography images in our study.

[PSA increase after definitive treatment]

Following definitive treatment with curative intent a subset of patients with prostate cancer experience biochemical recurrence. In these patients clinical parameters are mostly used to decide if a local or systemic disease recurrence is present. While salvage radiation treatment is advocated for local recurrence after radical prostatectomy, no standard recommendations exist in cases of local recurrence after primary radiation therapy although salvage prostatectomy may be considered. Imaging procedures have traditionally not routinely been recommended for the onset of prostate-specific antigen (PSA) relapse; however, prostate-specific membrane antigen (PSMA) positron emission tomography (PET) computed tomography (CT) exhibits high detection rates even at low PSA values. Thus, the current German guidelines state that PSMA PET/CT can be considered if this could result in a decisive change in further treatment management. Currently, a positive influence on oncological long-term outcome, however, has not yet been proven.

A Novel Fully Human Antibody targeting Extracellular Domain of PSMA Inhibits Tumor Growth in Prostate Cancer

Prostate cancer is the most commonly diagnosed malignancy in men and the second leading cause of cancer-related death. It is of vital importance to develop new strategies for prostate cancer therapy. PSMA (prostate-specific membrane antigen) is specifically expressed in prostate cancer and the neovasculature of certain cancer types, thus is considered to be an ideal target for cancer therapy. In our previous study, we have obtained a PSMA-specific single-chain variable fragment (scFv), named gy1, from a large yeast display naïve human scFv library. In this study, we reconstructed the PSMA scFv into a fully human antibody (named PSMAb) and evaluated its characterization both in vitro and in vivo We showed that PSMAb can specifically bind with and internalize into PSMA⁺ cells. The binding affinity of PSMAb is measured to be at nanomolar level, and PSMAb has very good thermostability. In vivo study showed that near IR dye-labeled PSMAb can specifically localize at PSMA⁺ tumors, and the application of PSMAb in vivo significantly inhibited the growth of PSMA⁺ tumors, but not PSMA^- tumors. At the studied doses, no obvious toxicity was observed when applied in vivo, as shown by the relative normal liver and kidney function and normal structure of important organs, shown by hematoxylin and eosin staining. In addition, PSMAb may inhibit tumor growth through antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity mechanisms. Our results indicated that the novel fully human antibody, PSMAb, deserve further study for PSMA-targeted diagnosis and therapy for prostate cancer and other cancer types with vascular PSMA expression.