A novel cytoplasmic tail MXXXL motif mediates the internalization of prostate-specific membrane antigen

PSMA antibody, humanized PSMA.CAR10.3, or Cetuximab increases prostate cancer localization of NF-κB p50-deficient immature myeloid cells (p50-IMC) and phagocytosis by their macrophage progeny

Adoptive transfer of immature myeloid cells lacking the repressive NF-κB p50 subunit (p50-IMC) slows the growth of syngeneic murine prostate cancer and other tumors. Directing p50-IMC to tumors using Fc receptor-bound antibodies (Abs) or surface chimeric antigen receptors (CARs) may increase tumor localization and subsequent phagocytosis of cancer cells by their mature myeloid progeny, potentiating anti-tumor T cell activation. PSMA and EGFR are found on aggressive human prostate cancers, and p50-IMC express receptors that bind the antibody Fc domain. p50-IMC combined with PSMA Ab, EGFR Ab (Cetuximab), or fully humanized PSMA.CAR10.3 manifest increased localization to Myc-CaP murine prostate cancer tumors expressing PSMA or EGFR. Tumor localization is further increased when myelo-depleting 5-fluorouracil precedes p50-IMC administration. Additionally, we find that PSMA Ab, EGFR Ab, or PSMA.CAR10.3 increase in vitro phagocytosis of Myc-CaP cells expressing PSMA or EGFR by p50-IMC-derived macrophages, including in M2-promoting IL-4, which is a component of the immune-suppressive tumor microenvironment. Lack of tolerance of human PSMA or EGFR by immune-competent mice and lack of expression of human PSMA protein in the prostate of AR2-Probasin-hPSMA transgenic mice precluded our ability to determine whether human-specific PSMA or EGFR antibody or PSMA.CAR10.3 increases anti-tumor efficacy of murine p50-IMC. Nevertheless, this study indicates the potential clinical utility of adding a tumor-directing antibody or CAR, including the novel, fully humanized PSMA.CAR10.3, to proinflammatory p50-IMC to optimize the activation of anti-tumor immunity in prostate cancer and other malignancies, and understanding PSMA toxicity in normal but not malignant prostate epithelium may reveal a novel therapeutic opportunity.

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.

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.

A peculiar distribution on 18F-DCFPyL-PSMA PET scan for a patient with prostate cancer and protein S deficiency

A 54-year-old male with biopsy-confirmed Gleason 4+4 prostate cancer underwent 18F-DCFPyL-PSMA PET scan to identify occult metastatic disease. This scan revealed abnormal radionuclide uptake not only in the prostate but also within the patient's vasculature. The scan was repeated after a week with a separate tracer batch, yielding the same result. Standard staging was performed using computed tomography and a Technetium-99 bone scan, revealing no metastatic disease. The patient's protein S deficiency is thought to have caused this peculiar tracer distribution. With the advent of PSMA PET for staging in prostate cancer, clinicians must be familiar with situations that may render unusual results.

Cyclohexanediamine Triazole (CHDT) Functionalization Enables Labeling of Target Molecules with Al18F/68Ga/111In

The Al18F-labeling approach offers a one-step access to radiofluorinated biomolecules by mimicking the labeling process for radiometals. Although these labeling conditions are considered to be mild compared to classic radiofluorinations, improvements of the chelating units have led to the discovery of (±)-H3RESCA, which allows Al18F-labeling already at ambient temperature. While the suitability of (±)-H3RESCA for functionalization and radiofluorination of proteins is well established, its use for small molecules or peptides is less explored. Herein, we advanced this acyclic pentadentate ligand by introducing an alkyne moiety for the late-stage functionalization of biomolecules via click chemistry. We show that in addition to Al18F-labeling, the cyclohexanediamine triazole (CHDT) moiety allows stable complexation of 68Ga and 111In. Three novel CHDT-functionalized PSMA inhibitors were synthesized and their Al18F-, 68Ga-, and 111In-labeled analogs were subjected to a detailed in vitro radiopharmacological characterization. Stability studies in vitro in human serum revealed among others a high kinetic inertness of all radiometal complexes. Furthermore, the Al18F-labeled PSMA ligands were characterized for their biodistribution in a LNCaP derived tumor xenograft mouse model by PET imaging. One radioligand, Al[18F]F-CHDT-PSMA-1, bearing a small azidoacetyl linker at the glutamate-urea-lysine motif, provided an in vivo performance comparable to that of [18F]PSMA-1007 but with even higher tumor-to-blood and tumor-to-muscle ratios at 120 min p.i. Overall, our results highlight the suitability of the novel CHDT moiety for functionalization and radiolabeling of small molecules or peptides with Al18F, 68Ga, and 111In and the triazole ring seems to entail favorable pharmacokinetic properties for molecular imaging purposes.

Prostate-Specific Membrane Antigen Biology and Pathophysiology in Prostate Carcinoma, an Update: Potential Implications for Targeted Imaging and Therapy

Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, was shown to be expressed 100-1000 fold higher in prostate adenocarcinoma as compared to normal prostate epithelium. Given the enzymatic function of PSMA with the presence of an internalization triggering motif, various Glu-urea-Lys-based inhibitors have been developed and, amongst others, radiolabeled with positron emitters for targeted positron emission tomography imaging such as 68Ga-PSMA-HBED-CC Glu-urea-Lys(Ahx) as well as with beta and alpha-emitting radioisotopes for targeted therapy, e.g., 177Lu-PSMA-617. In this paper, we review and discuss the potential implications for targeted imaging and therapy of altered PSMA-glycosylation, of PSMA-driven activation of the P13K/Akt/mTOR, of the evolution over time and the relationship with androgen signaling and changes in DNA methylation of PSMA, and of androgen deprivation therapy (ADT) in prostate carcinoma.

PSMA-targeted radiotheranostics in modern nuclear medicine: then, now, and what of the future?

In 1853, the perception of prostate cancer (PCa) as a rare ailment prevailed, was described by the eminent Londoner surgeon John Adams. Rapidly forward to 2018, the landscape dramatically altered. Currently, men face a one-in-nine lifetime risk of PCa, accentuated by improved diagnostic methods and an ageing population. With more than three million men in the United States alone grappling with this disease, the overall risk of succumbing to stands at one in 39. The intricate clinical and biological diversity of PCa poses serious challenges in terms of imaging, ongoing monitoring, and disease management. In the field of theranostics, diagnostic and therapeutic approaches that harmoniously merge targeted imaging with treatments are integrated. A pivotal player in this arena is radiotheranostics, employing radionuclides for both imaging and therapy, with prostate-specific membrane antigen (PSMA) at the forefront. Clinical milestones have been reached, including FDA- and/or EMA-approved PSMA-targeted radiodiagnostic agents, such as [18F]DCFPyL (PYLARIFY®, Lantheus Holdings), [18F]rhPSMA-7.3 (POSLUMA®, Blue Earth Diagnostics) and [68Ga]Ga-PSMA-11 (Locametz®, Novartis/ ILLUCCIX®, Telix Pharmaceuticals), as well as PSMA-targeted radiotherapeutic agents, such as [177Lu]Lu-PSMA-617 (Pluvicto®, Novartis). Concurrently, ligand-drug and immune therapies designed to target PSMA are being advanced through rigorous preclinical research and clinical trials. This review delves into the annals of PSMA-targeted radiotheranostics, exploring its historical evolution as a signature molecule in PCa management. We scrutinise its clinical ramifications, acknowledge its limitations, and peer into the avenues that need further exploration. In the crucible of scientific inquiry, we aim to illuminate the path toward a future where the enigma of PCa is deciphered and where its menace is met with precise and effective countermeasures. In the following sections, we discuss the intriguing terrain of PCa radiotheranostics through the lens of PSMA, with the fervent hope of advancing our understanding and enhancing clinical practice.

Prostate-Specific Membrane Antigen-Targeted Therapies for Prostate Cancer: Towards Improving Therapeutic Outcomes

CONTEXT

Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein overexpressed in most prostate cancers and exploited as a target for PSMA-targeted therapies. Different approaches to target PSMA-expressing cancer cells have been developed, showing promising results in clinical trials.

OBJECTIVE

To discuss the regulation of PSMA expression and the main PSMA-targeted therapeutic concepts illustrating their clinical development and rationalizing combination approaches with examples.

EVIDENCE ACQUISITION

We performed a detailed literature search using PubMed and reviewed the American Society of Clinical Oncology and European Society of Medical Oncology annual meeting abstracts up to September 2023.

EVIDENCE SYNTHESIS

We present an overarching description of the different strategies to target PSMA. The outcomes of PSMA-targeted therapies strongly rely on surface-bound PSMA expression. However, PSMA heterogeneity at different levels (interpatient and inter/intratumoral) limits the efficacy of PSMA-targeted therapies. We highlight the molecular mechanisms governing PSMA regulation, the understanding of which is crucial to designing therapeutic strategies aimed at upregulating PSMA expression. Thus far, homeobox B13 (HOXB13) and androgen receptor (AR) have emerged as critical transcription factors positively and negatively regulating PSMA expression, respectively. Furthermore, epigenetic regulation of PSMA has been also reported recently. In addition, many established therapeutic approaches harbor the potential to upregulate PSMA levels as well as potentiate DNA damage mediated by current radioligands.

CONCLUSIONS

PSMA-targeted therapies are rapidly advancing, but their efficacy is strongly limited by the heterogeneous expression of the target. A thorough comprehension of how PSMA is regulated will help improve the outcomes through increasing PSMA expression and will provide the basis for synergistic combination therapies.

PATIENT SUMMARY

Prostate-specific membrane antigen (PSMA) is overexpressed in most prostate cancers. PSMA-targeted therapies have shown promising results, but the heterogeneous expression of PSMA limits their efficacy. We propose to better elucidate the regulation of PSMA expression to increase the levels of the target and improve the therapeutic outcomes.

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.

Prostate-Specific Membrane Antigen PET/Computed Tomography: Pearls and Pitfalls

Prostate-specific membrane antigen PET (PSMA-PET) has emerged as a powerful imaging tool for prostate cancer primary staging, biochemical recurrence, and advanced disease assessment. This article offers a concise overview of the benefits and challenges associated with PSMA-PET for prostate cancer evaluation. The article highlights the advantages of PSMA-PET over conventional imaging, such as its higher sensitivity and specificity for detecting metastases, and the potential for guiding personalized treatment decisions. However, it also explores the limitations and potential pitfalls for interpretation. Overall, the article aims to provide valuable insights for clinicians and diagnostic imaging physicians in clinical practice.

Medicinal (Radio) Chemistry: Building Radiopharmaceuticals for the Future

Radiopharmaceuticals are increasingly playing a leading role in diagnosing, monitoring, and treating disease. In comparison with conventional pharmaceuticals, the development of radiopharmaceuticals does follow the principles of medicinal chemistry in the context of imaging-altered physiological processes. The design of a novel radiopharmaceutical has several steps similar to conventional drug discovery and some particularity. In the present work, we revisited the insights of medicinal chemistry in the current radiopharmaceutical development giving examples in oncology, neurology, and cardiology. In this regard, we overviewed the literature on radiopharmaceutical development to study overexpressed targets such as prostate-specific membrane antigen and fibroblast activation protein in cancer; β-amyloid plaques and tau protein in brain disorders; and angiotensin II type 1 receptor in cardiac disease. The work addresses concepts in the field of radiopharmacy with a special focus on the potential use of radiopharmaceuticals for nuclear imaging and theranostics.

The efficacy of natural bioactive compounds against prostate cancer: Molecular targets and synergistic activities

Globally, prostate cancer (PCa) is regarded as a challenging health issue, and the number of PCa patients continues to rise despite the availability of effective treatments in recent decades. The current therapy with chemotherapeutic drugs has been largely ineffective due to multidrug resistance and the conventional treatment has restricted drug accessibility to malignant tissues, necessitating a higher dosage resulting in increased cytotoxicity. Plant-derived bioactive compounds have recently attracted a great deal of attention in the field of PCa treatment due to their potent effects on several molecular targets and synergistic effects with anti-PCa drugs. This review emphasizes the molecular mechanism of phytochemicals on PCa cells, the synergistic effects of compound-drug interactions, and stem cell targeting for PCa treatment. Some potential compounds, such as curcumin, phenethyl-isothiocyanate, fisetin, baicalein, berberine, lutein, and many others, exert an anti-PCa effect via inhibiting proliferation, metastasis, cell cycle progression, and normal apoptosis pathways. In addition, multiple studies have demonstrated that the isolated natural compounds: d-limonene, paeonol, lanreotide, artesunate, and bicalutamide have potential synergistic effects. Further, a significant number of natural compounds effectively target PCa stem cells. However, further high-quality studies are needed to firmly establish the clinical efficacy of these phytochemicals against PCa.

Advances in non-radioactive PSMA-targeted small molecule-drug conjugates in the treatment of prostate cancer

Most patients with advanced prostate cancer (PCa) will develop metastatic castration-resistant prostate cancer (mCRPC) after androgen deprivation therapy, at this time the tumor enters the end stage, and the clinical treatment is very complicated, which requires rationalization of drugs to prolong the life of patients while improving their quality of life. Prostate-specific membrane antigen (PSMA) is a promising biological target for drug delivery in mCRPC due to its high level of specific expression in PCa cell membranes and low expression in normal tissues. Non-radioactive PSMA-targeted small molecule-drug conjugates (SMDCs) are gradually becoming a heat of discovery due to their good affinity and specificity; simple synthesis steps and transport management methods. Non-radioactive PSMA-targeted SMDCs under investigation can be divided into two categories: SMDCs and dual-ligand coupled drugs, among which SMDCs are the most widespread form of this type of conjugate. SMDCs have three key components: cytotoxic load, linker, and small molecule targeting ligands. SMDCs are internalized into the cell after binding to PSMA on the cell membrane and stored in endosomes and lysosomes, where they are usually enzymatically cleaved to allow precise release of cytotoxic molecules and uniform diffusion into the tumor tissue. More than a dozen non-radioactive PSMA-targeted SMDCs have been developed, many of which have shown favorable properties in both in vitro and in vivo evaluations, demonstrating more favorable results than unmodified cytotoxic drugs. Therefore, non-radioactive PSMA-targeted SMDCs have great therapeutic potential for mCRPC as a form of targeted therapy.

Current Status of Radiolabeled Monoclonal Antibodies Targeting PSMA for Imaging and Therapy

Prostate cancer (PCa) is one of the most prevalent cancer diagnoses among men in the United States and in several other developed countries. The prostate specific membrane antigen (PSMA) has been recognized as a promising molecular target in PCa, which has led to the development of specific radionuclide-based tracers for imaging and radiopharmaceuticals for PSMA targeted therapy. These compounds range from small molecule ligands to monoclonal antibodies (mAbs). Monoclonal antibodies play a crucial role in targeting cancer cell-specific antigens with a high degree of specificity while minimizing side effects to normal cells. The same mAb can often be labeled in different ways, such as with radionuclides suitable for imaging with Positron Emission Tomography (β+ positrons), Gamma Camera Scintigraphy (γ photons), or radiotherapy (β- electrons, α-emitters, or Auger electrons). Accordingly, the use of radionuclide-based PSMA-targeting compounds in molecular imaging and therapeutic applications has significantly grown in recent years. In this article, we will highlight the latest developments and prospects of radiolabeled mAbs that target PSMA for the detection and treatment of prostate cancer.

Joint EANM/SNMMI procedure guideline for the use of 177Lu-labeled PSMA-targeted radioligand-therapy (177Lu-PSMA-RLT)

Prostate-specific membrane antigen (PSMA) is expressed by the majority of clinically significant prostate adenocarcinomas, and patients with target-positive disease can easily be identified by PSMA PET imaging. Promising results with PSMA-targeted radiopharmaceutical therapy have already been obtained in early-phase studies using various combinations of targeting molecules and radiolabels. Definitive evidence of the safety and efficacy of [177Lu]Lu-PSMA-617 in combination with standard-of-care has been demonstrated in patients with metastatic castration-resistant prostate cancer, whose disease had progressed after or during at least one taxane regimen and at least one novel androgen-axis drug. Preliminary data suggest that 177Lu-PSMA-radioligand therapy (RLT) also has high potential in additional clinical situations. Hence, the radiopharmaceuticals [177Lu]Lu-PSMA-617 and [177Lu]Lu-PSMA-I&T are currently being evaluated in ongoing phase 3 trials. The purpose of this guideline is to assist nuclear medicine personnel, to select patients with highest potential to benefit from 177Lu-PSMA-RLT, to perform the procedure in accordance with current best practice, and to prepare for possible side effects and their clinical management. We also provide expert advice, to identify those clinical situations which may justify the off-label use of [177Lu]Lu-PSMA-617 or other emerging ligands on an individual patient basis.

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.

Delayed Imaging Improves Lesion Detectability in [99mTc]Tc-PSMA-I&S SPECT/CT in Recurrent Prostate Cancer

Our objective was to compare the ability to detect histopathologically confirmed lymph node metastases by early and delayed [^99mTc]Tc-PSMA-I&S SPECT/CT in early biochemically recurrent prostate cancer. Methods: We retrospectively analyzed 222 patients selected for radioguided surgery using [^99mTc]Tc-PSMA-I&S SPECT/CT at different time points after injection (≤4 h and >15 h). In total, 386 prostate-specific membrane antigen (PSMA) PET predetermined lesions were analyzed on SPECT/CT using a 4-point scale, and the results were compared between early and late imaging groups, with uni- and multivariate analyses performed including prostate-specific antigen, injected [^99mTc]Tc-PSMA-I&S activity, Gleason grade group, initial TNM stage, and, stratified by size, PSMA PET/CT-positive lymph nodes. PSMA PET/CT findings served as the standard of reference. Results: [^99mTc]Tc-PSMA-I&S SPECT/CT had a significantly higher positivity rate for detecting lesions in the late than the early imaging group (79%, n = 140/178, vs. 27%, n = 12/44 [P < 0.05] on a patient basis; 60%, n = 195/324, vs. 21%, n = 13/62 [P < 0.05] on a lesion basis). Similar positivity rates were found when lesions were stratified by size. Multivariate analysis found that SUV_max on PSMA PET/CT and the uptake time of [^99mTc]Tc-PSMA-I&S were independent predictors for lesion detectability on SPECT/CT. Conclusion: Late imaging (>15 h after injection) should be preferred when [^99mTc]Tc-PSMA-I&S SPECT/CT is used for lesion detection in early biochemical recurrence of prostate cancer. However, the performance of PSMA SPECT/CT is clearly inferior to that of PSMA PET/CT.

D2B-Functionalized Gold Nanoparticles: Promising Vehicles for Targeted Drug Delivery to Prostate Cancer

Despite the multitude of therapeutic agents available to treat prostate cancer (PC), there are still no effective and safe measures to treat the tumor. It remains a challenge to develop a simple approach to target PC with specific antibodies. In our study, D2B monoclonal antibodies against a prostate-specific membrane antigen (PSMA) were used. We investigated the functionalization of gold nanoparticles (AuNPs) with D2B to generate favorable physicochemical and biological properties that mediate specific binding to PC. For this purpose, AuNPs with a size of about 25 nm were synthesized in water using sodium citrate as a reducing and stabilizing agent and then coated with D2B. Major physicochemical properties of naked and D2B-coated AuNPs were investigated by ultraviolet-visible (UV-vis) spectroscopy, dynamic light scattering (DLS), and zeta potential measurements. The successful binding of D2B to AuNPs-citrate caused a 15 nm red shift in the UV-vis. This was assessed by DLS as an increase in zeta potential from ∼-45 to ∼-23 mV and in the size of AuNPs from ∼25 to ∼63 nm. Scanning electron microscopy confirmed the size shift of AuNPs, which was detected as an exterior organic layer of D2Bs surrounding each AuNP. Even at high exposure levels of the bioconjugates, PSMA-PC-3 cells exhibited minimal cytotoxicity. The specific and dose-dependent binding of AuNPs-D2B to PC-3-PSMA cells was validated by flow cytometry analysis. Our data provide effective drug delivery systems in PC theranostics.

The "Ins and Outs" of Prostate Specific Membrane Antigen (PSMA) as Specific Target in Prostate Cancer Therapy

Prostate-specific membrane antigen (PSMA) is expressed in epithelial cells of the prostate gland and is strongly upregulated in prostatic adenocarcinoma, with elevated expression correlating with metastasis, progression, and androgen independence. Because of its specificity, PSMA is a major target of prostate cancer therapy; however, detectable levels of PSMA are also found in other tissues, especially in salivary glands and kidney, generating bystander damage of these tissues. Antibody target therapy has been used with relative success in reducing tumor growth and prostate specific antigen (PSA) levels. However, since antibodies are highly stable in plasma, they have prolonged time in circulation and accumulate in organs with an affinity for antibodies such as bone marrow. For that reason, a second generation of PSMA targeted therapeutic agents has been developed. Small molecules and minibodies have had promising clinical trial results, but concerns about their specificity had arisen with side effects due to accumulation in salivary glands and kidneys. Herein we study the specificity of small molecules and minibodies that are currently being clinically tested. We observed a high affinity of these molecules for PSMA in prostate, kidney and salivary gland, suggesting that their effect is not prostate specific. The search for specific prostate target agents must continue so as to optimally treat patients with prostate cancer, while minimizing deleterious effects in other PSMA expressing tissues.

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.

Theranostic Small-Molecule Prodrug Conjugates for Targeted Delivery and Controlled Release of Toll-like Receptor 7 Agonists

We previously reported the design and synthesis of a small-molecule drug conjugate (SMDC) platform that demonstrated several advantages over antibody–drug conjugates (ADCs) in terms of in vivo pharmacokinetics, solid tumor penetration, definitive chemical structure, and adaptability for modular synthesis. Constructed on a tri-modal SMDC platform derived from 1,3,5-triazine (TZ) that consists of a targeting moiety (Lys-Urea-Glu) for prostate-specific membrane antigen (PSMA), here we report a novel class of chemically identical theranostic small-molecule prodrug conjugates (T-SMPDCs), [^18/19F]F-TZ(PSMA)-LEGU-TLR7, for PSMA-targeted delivery and controlled release of toll-like receptor 7 (TLR7) agonists to elicit de novo immune response for cancer immunotherapy. In vitro competitive binding assay of [¹⁹F]F-TZ(PSMA)-LEGU-TLR7 showed that the chemical modification of Lys-Urea-Glu did not compromise its binding affinity to PSMA. Receptor-mediated cell internalization upon the PSMA binding of [¹⁸F]F-TZ(PSMA)-LEGU-TLR7 showed a time-dependent increase, indicative of targeted intracellular delivery of the theranostic prodrug conjugate. The designed controlled release of gardiquimod, a TLR7 agonist, was realized by a legumain cleavable linker. We further performed an in vivo PET/CT imaging study that showed significantly higher uptake of [¹⁸F]F-TZ(PSMA)-LEGU-TLR7 in PSMA⁺ PC3-PIP tumors (1.9 ± 0.4% ID/g) than in PSMA⁻ PC3-Flu tumors (0.8 ± 0.3% ID/g) at 1 h post-injection. In addition, the conjugate showed a one-compartment kinetic profile and in vivo stability. Taken together, our proof-of-concept biological evaluation demonstrated the potential of our T-SMPDCs for cancer immunomodulatory therapies.

Factors Influencing the Therapeutic Efficacy of the PSMA Targeting Radioligand 212Pb-NG001

Simple Summary

Prostate-specific membrane antigen (PSMA) is a protein overexpressed in metastatic castration-resistant prostate cancer and a promising target for targeted radionuclide therapy. PSMA-targeted alpha therapy is of growing interest due to the high-emission energy and short range of alpha particles, resulting in a prominent cytotoxic potency. This study assesses the influence of various factors on the in vitro and in vivo therapeutic efficacy of the alpha particle generating PSMA-targeting radioligand ²¹²Pb-NG001.

Abstract

This study aimed to determine the influence of cellular PSMA expression, radioligand binding and internalization, and repeated administrations on the therapeutic effects of the PSMA-targeting radioligand ²¹²Pb-NG001. Cellular binding and internalization, cytotoxicity, biodistribution, and the therapeutic efficacy of ²¹²Pb-NG001 were investigated in two human prostate cancer cell lines with different PSMA levels: C4-2 (PSMA+) and PC-3 PIP (PSMA+++). Despite 10-fold higher PSMA expression on PC-3 PIP cells, cytotoxicity and therapeutic efficacy of the radioligand was only 1.8-fold better than for the C4-2 model, possibly explained by lower cellular internalization and less blood-rich stroma in PC-3 PIP xenografts. Mice bearing subcutaneous PC-3 PIP xenografts were treated with 0.2, 0.4, and 0.8 MBq of ²¹²Pb-NG001 that resulted in therapeutic indexes of 2.7, 3.0, and 3.5, respectively. A significant increase in treatment response was observed in mice that received repeated injections compared to the corresponding single dose (therapeutic indexes of 3.6 for 2 × 0.2 MBq and 4.4 for 2 × 0.4 MBq). The results indicate that ²¹²Pb-NG001 can induce therapeutic effects at clinically transferrable doses, both in the C4-2 model that resembles solid tumors and micrometastases with natural PSMA expression and in the PC-3 PIP model that mimics poorly vascularized metastases.

A Pilot Study of Dynamic 18F-DCFPyL PET/CT Imaging of Prostate Adenocarcinoma in High-Risk Primary Prostate Cancer Patients

PURPOSE

The primary aim of this study was to investigate the pharmacokinetics of 18F-DCFPyL, an 18F-labeled PSMA-based ligand, and to explore the utility of early time point positron emission tomography (PET) imaging extracted from PET data to distinguish malignant primary prostate from benign prostate tissue.

PROCEDURES

Ten consecutive patients with biopsy-proven high-risk prostate cancer underwent a dynamic 18F-DCFPyL PET/CT scan of the pelvis for the first 45 min post-injection (p.i.) followed by a static PET/CT at 2 h p.i. 18F-DCFPyL uptake values and kinetics were compared between benign prostate tissue and prostate cancer, including quantitative pharmacokinetic PET parameters extracted from 18F-DCFPyL time activity curves generated from dynamic data using a two-tissue compartment model and Patlak plots.

RESULTS

18F-DCFPyL uptake values were significantly higher in primary prostate tumors than those in benign prostatic hyperplasia (BPH) and normal prostate tissue at 5 min, 30 min, and 120 min p.i. (P = 0.0002), when examining both SUVmax and SUVmean values. The two-tissue compartment model found an overall influx value (Ki) of 0.063 in primary prostate cancer, demonstrating a Ki over 15-fold higher in malignant prostate tissue compared with BPH (Ki = 0.004) and normal prostate tissue (Ki = 0.005) (P = 0.0001).

CONCLUSION

High-risk primary prostate cancer is readily identified on dynamic and static, delayed, 18F-DCFPyL PET images. The tumor-to-background ratio increases over time, with optimal 18F-DCFPyL PET/CT imaging at 120 min p.i. for evaluation of prostate cancer, but not necessarily ideal for clinical application. Primary prostate cancer demonstrates different uptake kinetics in comparison to BPH and normal prostate tissue. The 15-fold difference in Ki between prostate cancer and non-cancer (BPH and normal) tissues translates to an ability to distinguish prostate cancer from normal tissue at time points as early as 5 to 10 min p.i.

Human blood biocompatibility and immunogenicity of scFvD2B PEGylated gold nanoparticles

Single chain variable D2B antibody fragments (scFvD2Bs) exhibit high affinity binding to prostate specific membrane antigens overexpressed in metastatic prostate cancer (PC). Conjugation of scFvD2B to gold nanoparticles (AuNPs) would enhance its stability and plasma half-life circulation to shuttle theranostic agents in PC. In this study, we synthesized PEGylated scFvD2B-AuNPs (AuNPs-scFvD2B-PEG) and tested their integrity, biocompatibility, and immunogenicity in freshly withdrawn human blood. Prior to blood incubation, Zeta potential measurements, UV-Vis spectroscopy, and dynamic light scattering (DLS) were used to assess the physicochemical properties of our nano-complexes in the presence or absence of PEGylation. A surface plasmon resonance band shift of 2 and 4 nm confirmed the successful coating for AuNPs-scFvD2B and AuNPs-scFvD2B-PEG, respectively. Likewise, DLS revealed a size increase of ∼3 nm for AuNPs-scFvD2B and ∼19 nm for AuNPs-scFvD2B-PEG. Zeta potential increased from -34 to -19 mV for AuNPs-scFvD2B and reached -3 mV upon PEGylation. Similar assessment measures were applied post-incubation in human blood with additional immunogenicity tests, such as hemolysis assay, neutrophil function test, and pyridine formazan extraction. Interestingly, grafting PEG chains on AuNPs-scFvD2B precluded the binding of blood plasma proteins and reduced neutrophil activation level compared with naked AuNPs-citrate counterparts. Most likely, a hydrated negative PEG cloud shielded the NPs rendering blood compatiblility with less than 10% hemolysis. In conclusion, the biocompatible AuNPs-scFvD2B-PEG presents promising characteristics for PC targeted therapy, with minimal protein adsorption affinity, low immunorecognition, and reduced hemolytic activity.

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.

A Prostate-Specific Membrane Antigen-Targeted Near-Infrared Conjugate for Identifying Pulmonary Squamous Cell Carcinoma during Resection

Pulmonary squamous cell carcinoma is the second most common lung cancer subtype and has a low 5-year survival rate at 17.6%. Complete resection with negative margins can be curative, but a high number of patients suffer early postoperative recurrence due to inadequate disease clearance at the index operation. Intraoperative molecular imaging (IMI) with tumor-targeted optical contrast agents is effective in improving resection completeness for other tumor types, but there are no IMI tracers targeted to pulmonary squamous cell carcinoma. In this report, we describe the use of a novel prostate-specific membrane antigen (PSMA)-targeted near-infrared conjugate (OTL78) to identify pulmonary squamous cell carcinoma. We identified PSMA as a viable target by examining its expression in human lung tumor specimens from a surgical cohort. Ninety-four percent of tumors expressed PSMA in either the pulmonary squamous cells or the tumor neovasculature. Using in vitro and in vivo models, we found that OTL78 reliably localized pulmonary squamous cell carcinoma in a PSMA-dependent manner. Finally, we found that IMI with OTL78 markedly improved surgeons' ability to identify residual disease after surgery in a preclinical model. Ultimately, this novel optical tracer may aid surgical resection of pulmonary squamous cell carcinoma and potentially improve long-term outcomes.

Cancer immunomodulation using bispecific aptamers

Evasion of immune destruction is a major hallmark of cancer. Recent US Food and Drug Administration (FDA) approvals of various immunomodulating therapies underline the important role that reprogramming the immune system can play in combating this disease. However, a wide range of side effects still limit the therapeutic potential of immunomodulators, suggesting a need for more precise reagents with negligible off-target and on-target/off-tumor effects. Aptamers are single-chained oligonucleotides that bind their targets with high specificity and affinity owing to their three-dimensional (3D) structures, and they are one potential way to address this need. In particular, bispecific aptamers (bsApts) have been shown to induce artificial immune synapses that promote T cell activation and subsequent tumor cell lysis in various in vitro and in vivo pre-clinical models. We discuss these advances here, along with gaps in bsApt biology at both the cellular and resident tissue levels that should be addressed to accelerate their translation into the clinic. The broad application, minimal production cost, and relative lack of immunogenicity of bsApts give them some ideal qualities for manipulating the immune system. Building upon lessons from other novel therapies, bsApts could soon provide clinicians with an immunomodulating toolbox that is not only potent and efficacious but exercises a wide therapeutic index.

Going Nuclear with Amino Acids and Proteins - Basic Biochemistry and Molecular Biology Primer for the Technologist

In recent years, there has been an influx of new tracers into the field of nuclear medicine and molecular imaging. Most of these tracers that have been FDA approved for clinical imaging exploit various mechanisms of protein biochemistry and molecular biology to bring about their actions, such as amino acid metabolism, protein folding, receptor-ligand interactions, and surface transport mechanisms. In this review, we attempt to paint a clear picture of the basic biochemistry and molecular biology of protein structure, translation, transcription, post-translational modifications, and protein targeting, in the context of the various radiopharmaceuticals currently used clinically, all in an easy-to-understand language for entry level technologists in the field. Tracer characteristics, including indications, dosage, injection-to-imaging time, and the logic behind the normal and pathophysiologic biodistribution of these newer molecular tracers, are also discussed.

A New Class of PSMA-617-Based Hybrid Molecules for Preoperative Imaging and Intraoperative Fluorescence Navigation of Prostate Cancer

The development of PSMA-targeting low-molecular-weight hybrid molecules aims at advancing preoperative imaging and accurate intraoperative fluorescence guidance for improved diagnosis and therapy of prostate cancer. In hybrid probe design, the major challenge is the introduction of a bulky dye to peptidomimetic core structures without affecting tumor-targeting properties and pharmacokinetic profiles. This study developed a novel class of PSMA-targeting hybrid molecules based on the clinically established theranostic agent PSMA-617. The fluorescent dye-bearing candidates of the strategically designed molecule library were evaluated in in vitro assays based on their PSMA-binding affinity and internalization properties to identify the most favorable hybrid molecule composition for the installation of a bulky dye. The library’s best candidate was realized with IRDye800CW providing the lead compound. Glu-urea-Lys-2-Nal-Chx-Lys(IRDye800CW)-DOTA (PSMA-927) was investigated in an in vivo proof-of-concept study, with compelling performance in organ distribution studies, PET/MRI and optical imaging, and with a strong PSMA-specific tumor uptake comparable to that of PSMA-617. This study provides valuable insights about the design of PSMA-targeting low-molecular-weight hybrid molecules, which enable further advances in the field of peptidomimetic hybrid molecule development.

PSMA-Targeting Imaging and Theranostic Agents-Current Status and Future Perspective

In the past two decades, extensive efforts have been made to develop agents targeting prostate-specific membrane antigen (PSMA) for prostate cancer imaging and therapy. To date, represented by two recent approvals of [⁶⁸Ga]Ga-PSMA-11 and [¹⁸F]F-DCFPyL by the United States Food and Drug Administration (US-FDA) for positron emission tomography (PET) imaging to identify suspected metastases or recurrence in patients with prostate cancer, PSMA-targeting imaging and theranostic agents derived from small molecule PSMA inhibitors have advanced to clinical practice and trials of prostate cancer. The focus of current development of new PSMA-targeting agents has thus shifted to the improvement of in vivo pharmacokinetics and higher specific binding affinity with the aims to further increase the detection sensitivity and specificity and minimize the toxicity to non-target tissues, particularly the kidneys. The main strategies involve systematic chemical modifications of the linkage between the targeting moiety and imaging/therapy payloads. In addition to a summary of the development history of PSMA-targeting agents, this review provides an overview of current advances and future promise of PSMA-targeted imaging and theranostics with focuses on the structural determinants of the chemical modification towards the next generation of PSMA-targeting agents.

Narrative review: prostate-specific membrane antigen-radioligand therapy in metastatic castration-resistant prostate cancer

Radioactive-labelled ligands targeting the prostate-specific membrane antigen (PSMA), a transmembrane protein overexpressed in prostate cancer (PC), have shown promising activity in treatment of metastatic castration-resistant prostate cancer (mCRPC). PSMA-617 and PSMA-I&T (imaging and therapy), both labeled to the beta-emitter lutetium-177 (Lu177), are most frequently used in clinical routine and have shown a favorable side-effect profile. Common side effects are transient xerostomia. Severe side effects, e.g., treatment-associated myelosuppression, are rare. Currently treatment with Lu177-PSMA outside clinical trials is available for compassionate use for patients who exhausted conventional therapies. Previous retro- and prospective studies reported promising results with ≥50% PSA declines observed in at least one third of patients. Retrospective data suggests worse biochemical response in patients with visceral metastases. Preliminary data from the randomized phase II (TheraP) trial showed an improved biochemical response rate of Lu177-PSMA as compared to cabazitaxel in patients progressing after docetaxel. Following these promising data, the results of the randomized, prospective phase III VISION study are eagerly anticipated. A major challenge remains resistance to radioligand therapy with Lu177-PSMA. As an alternative, a PSMA-ligand labeled to the alpha-emitter Actinium-225 (Ac-225) may be offered to patients, which shows promising activity in patients developing progression under Lu177-PSMA at the cost of higher toxicity. Mostly permanent xerostomia is a relevant side effect resulting in treatment discontinuation in up to a quarter of patients. This review summarizes the literature on activity and toxicity of PSMA-targeted radioligand therapy in mCRPC.

Prostate-Specific Membrane Antigen (PSMA) Theranostics for Treatment of Oligometastatic Prostate Cancer

Theranostics, a combination of therapy and diagnostics, is a field of personalized medicine involving the use of the same or similar radiopharmaceutical agents for the diagnosis and treatment of patients. Prostate-specific membrane antigen (PSMA) is a promising theranostic target for the treatment of prostate cancers. Diagnostic PSMA radiopharmaceuticals are currently used for staging and diagnosis of prostate cancers, and imaging can predict response to therapeutic PSMA radiopharmaceuticals. While mainly used in the setting of metastatic, castrate-resistant disease, clinical trials are investigating the use of PSMA-based therapy at earlier stages, including in hormone-sensitive or hormone-naïve prostate cancers, and in oligometastatic prostate cancers. This review explores the use of PSMA as a theranostic target and investigates the potential use of PSMA in earlier stage disease, including hormone-sensitive metastatic prostate cancer, and oligometastatic prostate cancer.

Radiomics for Identification and Prediction in Metastatic Prostate Cancer: A Review of Studies

Metastatic Prostate Cancer (mPCa) is associated with a poor patient prognosis. mPCa spreads throughout the body, often to bones, with spatial and temporal variations that make the clinical management of the disease difficult. The evolution of the disease leads to spatial heterogeneity that is extremely difficult to characterise with solid biopsies. Imaging provides the opportunity to quantify disease spread. Advanced image analytics methods, including radiomics, offer the opportunity to characterise heterogeneity beyond what can be achieved with simple assessment. Radiomics analysis has the potential to yield useful quantitative imaging biomarkers that can improve the early detection of mPCa, predict disease progression, assess response, and potentially inform the choice of treatment procedures. Traditional radiomics analysis involves modelling with hand-crafted features designed using significant domain knowledge. On the other hand, artificial intelligence techniques such as deep learning can facilitate end-to-end automated feature extraction and model generation with minimal human intervention. Radiomics models have the potential to become vital pieces in the oncology workflow, however, the current limitations of the field, such as limited reproducibility, are impeding their translation into clinical practice. This review provides an overview of the radiomics methodology, detailing critical aspects affecting the reproducibility of features, and providing examples of how artificial intelligence techniques can be incorporated into the workflow. The current landscape of publications utilising radiomics methods in the assessment and treatment of mPCa are surveyed and reviewed. Associated studies have incorporated information from multiple imaging modalities, including bone scintigraphy, CT, PET with varying tracers, multiparametric MRI together with clinical covariates, spanning the prediction of progression through to overall survival in varying cohorts. The methodological quality of each study is quantified using the radiomics quality score. Multiple deficits were identified, with the lack of prospective design and external validation highlighted as major impediments to clinical translation. These results inform some recommendations for future directions of the field.

Antibodies targeting Prostate-Specific Membrane Antigen positive prostate cancer: from diagnostic imaging to theranostics

PURPOSE OF REVIEW

Targeting Prostate-Specific Membrane Antigen (PSMA) has paved the way for personalized medicine in prostate cancer (PCa) patients. This review aims to highlight the role of PSMA targeting antibodies in PCa, for diagnostic and therapeutic purposes.

RECENT FINDINGS

PSMA Positron Emission Tomography/Computed Tomography has been a game changer in the diagnosis of PCa in the recent decade. Two anti-PSMA monoclonal antibodies have been studied in PCa: 7E11-C35 (limited use) and J591. J591 antibody was used for diagnostic purposes coupled with different radionuclides. Most importantly, it was combined to numerous therapeutic radionuclides such as Lutetium-177 (177Lu), Yttrium-90 (90Y), Indium-111 (111In), and Actinium-225 (225Ac). It was also conjugated to drugs forming antibody-drug conjugates (e.g. MLN2704 and PSMA-ADC). These compounds were tested in recent phase I/II clinical trials.

SUMMARY

PSMA targeting antibodies are very promising for further clinical investigation and continue to be a momentous research area, for both imaging and therapeutic settings. Although some clinical trials resulted in unfavorably safety profiles for some antibodies, they validated PSMA as a crucial immunoconjugate target.

Targeted delivery of cytotoxic proteins to prostate cancer via conjugation to small molecule urea-based PSMA inhibitors

Prostate cancer cells are characterized by a remarkably low proliferative rate and the production of high levels of prostate-specific proteases. Protein-based toxins are attractive candidates for prostate cancer therapy because they kill cells via proliferation-independent mechanisms. However, the non-specific cytotoxicity of these potent cytotoxins must be redirected to avoid toxicity to normal tissues. Prostate-Specific Membrane Antigen (PSMA) is membrane-bound carboxypeptidase that is highly expressed by prostate cancer cells. Potent dipeptide PSMA inhibitors have been developed that can selectively deliver and concentrate imaging agents within prostate cancer cells based on continuous PSMA internalization and endosomal cycling. On this basis, we conjugated a PSMA inhibitor to the apoptosis-inducing human protease Granzyme B and the potent Pseudomonas exotoxin protein toxin fragment, PE35. We assessed selective PSMA binding and entrance into tumor cell to induce cell death. We demonstrated these agents selectively bound to PSMA and became internalized. PSMA-targeted PE35 toxin was selectively toxic to PSMA producing cells in vitro. Intratumoral and intravenous administration of this toxin produced marked tumor killing of PSMA-producing xenografts with minimal host toxicity. These studies demonstrate that urea-based PSMA inhibitors represent a simpler, less expensive alternative to antibodies as a means to deliver cytotoxic proteins to prostate cancer cells.

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

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

PSMA: a game changer in the diagnosis and treatment of advanced prostate cancer

Although management of advanced prostate cancer is evolving, a lot of work remains to be done for patients who have exhausted all options. Molecular targeting of prostate specific membrane antigen (PSMA) is valuable not only for diagnostic but also for therapeutic reasons. PSMA is thus considered to be useful in a theranostic approach. PSMA scans are upcoming diagnostic modalities which detect metastatic lesions that are missed by conventional imaging modalities. PSMA ligand therapy is also an upcoming treatment modality that has been proven to be beneficial with minimal toxicity in patients with advanced prostate cancer that have progressed on prior therapy. In this review article, we summarize the current knowledge regarding PSMA diagnostics and PSMA ligand therapies and discuss their implication in the treatment of advanced prostate cancer.

Target-Specific Magnetic Resonance Imaging of Human Prostate Adenocarcinoma Using NaDyF4-NaGdF4 Core-Shell Nanoparticles

We illustrate the development of NaDyF₄-NaGdF₄ core-shell nanoparticles (NPs) for targeting prostate cancer cells using a preclinical 9.4 T magnetic resonance imaging (MRI) of live animals. The NPs composed of paramagnetic Dy³⁺ and Gd³⁺ (T₂- and T₁-contrast agents, respectively) demonstrate proton relaxivities of r₁ = 20.2 mM^-1 s^-1 and r₂ = 32.3 mM^-1 s^-1 at clinical 3 T and r₁ = 9.4 mM^-1 s^-1 and r₂ = 144.7 mM^-1 s^-1 at preclinical 9.4 T. The corresponding relaxivity values per NP are r₁ = 19.4 × 10⁵ mM_NP^-1 s^-1 and r₂ = 33.0 × 10⁵ mM_NP^-1 s^-1 at 3 T and r₁ = 9.0 × 10⁵ mM_NP^-1 s^-1 and r₂ = 147.0 × 10⁵ mM_NP^-1 s^-1 at 9.4 T. In vivo active targeting of human prostate tumors grown in nude mice revealed docking of anti-prostate-specific membrane antigen (PSMA) antibody-tagged NPs at tumor sites post-24 h of their intravenous injection. On the other hand, in vivo passive targeting showed preferential accumulation of NPs at tumor sites only within 2 h of their injection, ascribed to the enhanced permeation and retention effect of the tumor. A biodistribution study employing the harvested organs of mice, post-24 h injection of NPs, quantified active targeting as nearly twice as efficient as passive targeting. These outcomes provide potential opportunities for noninvasive diagnosis using NaDyF₄-NaGdF₄ core-shell NPs for target-specific MRI.

More Than Meets the Eye: Scientific Rationale behind Molecular Imaging and Therapeutic Targeting of Prostate-Specific Membrane Antigen (PSMA) in Metastatic Prostate Cancer and Beyond

Simple Summary

Prostate-specific membrane antigen (PSMA) is a transmembrane protein that is overexpressed in prostate cancer and correlates with the aggressiveness of the disease. PSMA is a promising target for imaging and therapeutics in prostate cancer patients validated in prospective trials. However, the role of PSMA in prostate cancer progression is poorly understood. In this review, we discuss the biology and scientific rationale behind the use of PSMA and other targets in the detection and theranostics of metastatic prostate cancer.

Abstract

Prostate cancer is the second most common cancer type in men globally. Although the prognosis for localized prostate cancer is good, no curative treatments are available for metastatic disease. Better diagnostic methods could help target therapies and improve the outcome. Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein that is overexpressed on malignant prostate tumor cells and correlates with the aggressiveness of the disease. PSMA is a clinically validated target for positron emission tomography (PET) imaging-based diagnostics in prostate cancer, and during recent years several therapeutics have been developed based on PSMA expression and activity. The expression of PSMA in prostate cancer can be very heterogeneous and some metastases are negative for PSMA. Determinants that dictate clinical responses to PSMA-targeting therapeutics are not well known. Moreover, it is not clear how to manipulate PSMA expression for therapeutic purposes and develop rational treatment combinations. A deeper understanding of the biology behind the use of PSMA would help the development of theranostics with radiolabeled compounds and other PSMA-based therapeutic approaches. Along with PSMA several other targets have also been evaluated or are currently under investigation in preclinical or clinical settings in prostate cancer. Here we critically elaborate the biology and scientific rationale behind the use of PSMA and other targets in the detection and therapeutic targeting of metastatic prostate cancer.

Radionuclide-based molecular imaging allows CAR-T cellular visualization and therapeutic monitoring

Chimeric antigen receptor T cell (CAR-T) therapy is a new and effective form of adoptive cell therapy that is rapidly entering the mainstream for the treatment of CD19-positive hematological cancers because of its impressive effect and durable responses. Huge challenges remain in achieving similar success in patients with solid tumors. The current methods of monitoring CAR-T, including morphological imaging (CT and MRI), blood tests, and biopsy, have limitations to assess whether CAR-T cells are homing to tumor sites and infiltrating into tumor bed, or to assess the survival, proliferation, and persistence of CAR-T cells in solid tumors associated with an immunosuppressive microenvironment. Radionuclide-based molecular imaging affords improved CAR-T cellular visualization and therapeutic monitoring through either a direct cellular radiolabeling approach or a reporter gene imaging strategy, and endogenous cell imaging is beneficial to reflect functional information and immune status of T cells. Focusing on the dynamic monitoring and precise assessment of CAR-T therapy, this review summarizes the current applications of radionuclide-based noninvasive imaging in CAR-T cells visualization and monitoring and presents current challenges and strategic choices.

Inhibitors of prostate-specific membrane antigen in the diagnosis and therapy of metastatic prostate cancer - a review of patent literature

INTRODUCTION

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II, is a potential target protein for imaging and treatment of patients with prostate cancer because of its overexpression during metastasis. Various PSMA-targeted imaging and therapeutic probes have been designed and synthesized based on the Lys-urea-Glu motif. Structural modifications have been made exclusively in the linker region, while maintaining the Lys-urea-Glu structure that interacts with S1 and S1' pockets.

AREA COVERED

This review includes WIPO-listed patents (from January 2017 to June 2020) reporting PSMA-targeted probes based on the Lys-urea-Glu or Glu-urea-Glu structure.

EXPERT OPINION

: PSMA-targeted imaging agents labeled with radionuclides such as fluorine-18, copper-64, gallium-68, and technetium-99m have been successfully translated into clinical phase for the early diagnosis of metastatic prostate cancer. Recently, PSMA-targeted therapeutic agents labeled with iodine-131, lutetium-177, astatine-211, and lead-212 have also been developed with notable progress. Most PSMA-targeted agents are based on the Lys-urea-Glu or Glu-urea-Glu structure, demonstrate strong PSMA-binding affinity in nanomolar range, and achieve diverse structural modifications in the non-pharmacophore pocket. By exploiting the S1 accessory pocket or the tunnel region of the PSMA active site, the in vivo efficacy and pharmacokinetic profiles of the PMSA-targeted agents can be effectively modulated.

Molecular imaging of orthotopic prostate cancer with nanobubble ultrasound contrast agents targeted to PSMA

Ultrasound imaging is routinely used to guide prostate biopsies, yet delineation of tumors within the prostate gland is extremely challenging, even with microbubble (MB) contrast. A more effective ultrasound protocol is needed that can effectively localize malignancies for targeted biopsy or aid in patient selection and treatment planning for organ-sparing focal therapy. This study focused on evaluating the application of a novel nanobubble ultrasound contrast agent targeted to the prostate specific membrane antigen (PSMA-targeted NBs) in ultrasound imaging of prostate cancer (PCa) in vivo using a clinically relevant orthotopic tumor model in nude mice. Our results demonstrated that PSMA-targeted NBs had increased extravasation and retention in PSMA-expressing orthotopic mouse tumors. These processes are reflected in significantly different time intensity curve (TIC) and several kinetic parameters for targeted versus non-targeted NBs or LUMASON MBs. These, may in turn, lead to improved image-based detection and diagnosis of PCa in the future.

Cytoplasmic Localization of Prostate-Specific Membrane Antigen Inhibitors May Confer Advantages for Targeted Cancer Therapies

Targeted imaging and therapy approaches based on novel prostate-specific membrane antigen (PSMA) inhibitors have fundamentally changed the treatment regimen of prostate cancer. However, the exact mechanism of PSMA inhibitor internalization has not yet been studied, and the inhibitors' subcellular fate remains elusive. Here, we investigated the intracellular distribution of peptidomimetic PSMA inhibitors and of PSMA itself by stimulated emission depletion (STED) nanoscopy, applying a novel nonstandard live cell staining protocol. Imaging analysis confirmed PSMA cluster formation at the cell surface of prostate cancer cells and clathrin-dependent endocytosis of PSMA inhibitors. Following the endosomal pathway, PSMA inhibitors accumulated in prostate cancer cells at clinically relevant time points. In contrast with PSMA itself, PSMA inhibitors were found to eventually distribute homogeneously in the cytoplasm, a molecular condition that promises benefits for treatment as cytoplasmic and in particular perinuclear enrichment of the radionuclide carriers may better facilitate the radiation-mediated damage of cancerous cells. This study is the first to reveal the subcellular fate of PSMA/PSMA inhibitor complexes at the nanoscale and aims to inspire the development of new approaches in the field of prostate cancer research, diagnostics, and therapeutics. SIGNIFICANCE: This study uses STED fluorescence microscopy to reveal the subcellular fate of PSMA/PSMA inhibitor complexes near the molecular level, providing insights of great clinical interest and suggestive of advantageous targeted therapies. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/8/2234/F1.large.jpg.

A targeted near-infrared nanoprobe for deep-tissue penetration and imaging of prostate cancer

The current challenge in fluorescence guided surgery (FGS) for prostate cancer (PCa) is in the design of imaging probes with high selectivity, clear visualization of tumour margins, and minimal toxicity. This report aims to design and develop a novel NIR-nanoprobe, and evaluate its potential in the penetration of PCa tumour tissues. The PSMA receptor-targeted quantum dot (PSMA-QD655) is a NIR, deep-tissue imaging agent, which has the potential for intraoperative navigation during surgery and improved detection specificity for PCa. The probe was designed and synthesized by conjugating functionalized amino-PEG quantum dots (QDs) through a heterobifunctional linker to a DUPA targeted polypeptide construct. The nanoprobe was evaluated in vitro in PSMA⁺ PCa cell lines for specificity and its binding affinity was determined by flow cytometric analysis. The penetration efficacy was tested further on large PCa 3D tumour spheroids (dia ∼1200 μm, thickness ∼450 μm) by deep tissue multiphoton imaging. PSMA-QD655 was found to be an efficient deep tissue intra-operative guided surgical tool with a high affinity (K_D = 15.3 nM) and penetrative capacity. The results have been demonstrated in vitro in 2D and 3D tissue models, mimicking cancer lesions in vivo. In summary, we have developed a deep-tissue imaging NIR nanoprobe targeting prostatic lesions that (i) binds to PSMA⁺ tumour with sub-nanomolar affinity and high specificity, (ii) shows an excellent safety profile in primary cell lines in vitro and (iii) shows high penetrative capacity in a 3D prostate tumour model (∼450 μm tissue depth).

[68Ga]Ga-PSMA-11 in prostate cancer: a comprehensive review

Imaging of the prostate-specific membrane antigen (PSMA) has become an important tool for managing patients with recurrent prostate cancer, and one of the most frequently employed radiopharmaceuticals is [68Ga]Ga-PSMA-11. Herein, we summarize the preclinical development and the clinical applications of [68Ga]Ga-PSMA-11 and present side-by-side comparisons with other radiopharmaceuticals or imaging modalities, in order to assist imagers and clinicians in recommending, performing, and interpreting the results of [68Ga]Ga-PSMA-11 PET scans in patients with prostate cancer.

Decreased prostate-specific membrane antigen levels in the seminal plasma of oligoasthenoteratozoospermic men

Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein with glutamate carboxypeptidase activity. However, its precise function in the prostate, prostasomes and seminal plasma with regard to male fertility remains unknown. This study was conducted to investigate the seminal plasma PSMA levels in fertile men and patients with oligoasthenoteratozoospermia (OAT) and to analyse its association with sperm parameters. Twenty fertile men and twenty patients admitted at the urology clinic of our institution with the diagnosis of OAT were included in the study. Following semen analysis, seminal plasma was isolated from semen ejaculates. PSMA concentrations in the seminal plasma were determined by ELISA. The correlations between seminal PSMA concentrations and semen parameters were statistically analysed. Seminal plasma PSMA concentration was significantly lower in OAT patients compared to fertile controls (p < .01). In fertile men, PSMA concentration was significantly correlated with the sperm concentration (r = -.481, p < .05), whereas in the patient group no statistically significant correlation was found between the sperm parameters and seminal PSMA level. This is the first study in the literature to investigate PSMA levels in the seminal plasma from infertile men. Decreased levels of seminal plasma PSMA might suggest a role for compromised prostasome function in the pathogenesis of OAT syndrome.

Functional Characterization of TMEM127 Variants Reveals Novel Insights into Its Membrane Topology and Trafficking

CONTEXT

TMEM127 is a poorly known tumor suppressor gene associated with pheochromocytomas, paragangliomas, and renal carcinomas. Our incomplete understanding of TMEM127 function has limited our ability to predict variant pathogenicity.

PURPOSE

To better understand the function of the transmembrane protein TMEM127 we undertook cellular and molecular evaluation of patient-derived germline variants.

DESIGN

Subcellular localization and steady-state levels of tumor-associated, transiently expressed TMEM127 variants were compared to the wild-type protein using immunofluorescence and immunoblot analysis, respectively, in cells genetically modified to lack endogenous TMEM127. Membrane topology and endocytic mechanisms were also assessed.

RESULTS

We identified 3 subgroups of mutations and determined that 71% of the variants studied are pathogenic or likely pathogenic through loss of membrane-binding ability, stability, and/or internalization capability. Investigation into an N-terminal cluster of missense variants uncovered a previously unrecognized transmembrane domain, indicating that TMEM127 is a 4- transmembrane, not a 3-transmembrane domain-containing protein. Additionally, a C-terminal variant with predominant plasma membrane localization revealed an atypical, extended acidic, dileucine-based motif required for TMEM127 internalization through clathrin-mediated endocytosis.

CONCLUSION

We characterized the functional deficits of several germline TMEM127 variants and identified novel structure-function features of TMEM127. These findings will assist in determining pathogenicity of TMEM127 variants and will help guide future studies investigating the cellular role of TMEM127.