Synthesis and evaluation of technetium-99m- and rhenium-labeled inhibitors of the prostate-specific membrane antigen (PSMA)

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.

Pretargeted radiotherapy and synergistic treatment of metastatic, castration-resistant prostate cancer using cross-linked, PSMA-targeted lipoic acid nanoparticles

Metastatic castration-resistant prostate cancer (CRPC) is a currently incurable disease associated with high mortality. Novel therapeutic approaches for CRPC are urgently needed to improve prognosis. In this study, we developed cross-linked, PSMA-targeted lipoic acid nanoparticles (cPLANPs), which can interact with transmembrane glycoprotein to accumulate inside prostate cancer cells, where they upregulate caspase-3, downregulate anti-apoptotic B-cell lymphoma-2 (BCL-2), and thereby induce apoptosis. The trans-cyclooctene (TCO) decoration on cPLANPs acts as a bioorthogonal handle allowing pretargeted single-photon emission computed tomography and radiotherapy, which revealed significantly enhanced tumor accumulation and minimal off-target toxicity in our experiments. The developed strategy showed a strong synergistic anti-cancer effect in vivo, with a tumor inhibition rate of up to 95.6% after 14 days of treatment. Our results suggest the potential of combining bioorthogonal pretargeted radiotherapy with suitable PSMA-targeted nanoparticles for the treatment of metastatic CRPC.

Synthesis and Preclinical Evaluation of PSMA-Targeted 111In-Radioconjugates Containing a Mitochondria-Tropic Triphenylphosphonium Carrier

Nuclear DNA is the canonical target for biological damage induced by Auger electrons (AE) in the context of targeted radionuclide therapy (TRT) of cancer, but other subcellular components might also be relevant for this purpose, such as the energized mitochondria of tumor cells. Having this in mind, we have synthesized novel DOTA-based chelators carrying a prostate-specific membrane antigen (PSMA) inhibitor and a triphenyl phosphonium (TPP) group that were used to obtain dual-targeted 111In-radioconjugates ([111In]In-TPP-DOTAGA-PSMA and [111In]In-TPP-DOTAGA-G3-PSMA), aiming to promote a selective uptake of an AE-emitter radiometal (111In) by PSMA+ prostate cancer (PCa) cells and an enhanced accumulation in the mitochondria. These dual-targeted 111In-radiocomplexes are highly stable under physiological conditions and in cell culture media. The complexes showed relatively similar binding affinities toward the PSMA compared to the reference tracer [111In]In-PSMA-617, in line with their high cellular uptake and internalization in PSMA+ PCa cells. The complexes compromised cell survival in a dose-dependent manner and in the case of [111In]In-TPP-DOTAGA-G3-PSMA to a higher extent than observed for the single-targeted congener [111In]In-PSMA-617. μSPECT imaging studies in PSMA+ PCa xenografts showed that the TPP pharmacophore did not interfere with the excellent in vivo tumor uptake of the "golden standard" [111In]In-PSMA-617, although it led to a higher kidney retention. Such kidney retention does not necessarily compromise their usefulness as radiotherapeutics due to the short tissue range of the Auger/conversion electrons emitted by 111In. Overall, our results provide valuable insights into the potential use of mitochondrial targeting by PSMA-based radiocomplexes for efficient use of AE-emitting radionuclides in TRT, giving impetus to extend the studies to other AE-emitting trivalent radiometals (e.g., 161Tb or 165Er) and to further optimize the designed dual-targeting constructs.

"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.

Synthesis and clinical application of small-molecule drugs approved to treat prostatic cancer

Prostate cancer is a prevalent form of cancer that primarily affects men, with a high incidence and mortality rate. It is the second most common cancer among males, following lung cancer. Typically occurring in individuals aged 50 and above, this malignant tumor originates from abnormal cells in the prostate tissue. If left untreated, it can spread to nearby tissues, lymph nodes, and even bones. Current treatment methods include surgery, radiotherapy, and chemotherapy. However, these treatments have certain limitations and side effects. Therefore, researching and developing new small-molecule drugs to treat prostate cancer is of great significance. In recent years, many small-molecule drugs have been proven to have therapeutic effects on prostate cancer. The purpose of this review is to give a comprehensive look at the clinical uses and synthetic methods of various significant small-molecule drugs that have been approved to treat prostate cancer, to facilitate the development of more powerful and innovative drugs for the effective control of prostate cancer.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

A cell-impermeable luminogenic probe for near-infrared imaging of prostate-specific membrane antigen in prostate cancer microenvironments

Prostate-specific membrane antigen (PSMA) imaging probes are a promising tool for the diagnosis and image-guided surgery of prostate cancer (PCa). However, PSMA-specific luminescence probes for PCa detection and heterogeneity studies with high imaging contrast are lacking. Here, we report the first near-infrared (NIR) iridium(III) complex for the wash-free and specific imaging of PSMA in PCa cells and spheroids. The conjugation of a PSMA inhibitor, Lys-urea-Glu, to an iridium(III) complex synergizes the PSMA-specific affinity and biocompatibility of the inhibitor with the desirable photophysical properties of the iridium(III) complex, including NIR emission (670 nm), high photostability and a large Stokes shift. The cellular impermeability of the probe along with its strong binding affinity to PSMA enhances its specificity for PSMA, enabling the washing-free luminescent imaging of membrane PSMA with lower cytotoxicity. The probe was successfully applied for selectively visualizing PSMA-expressing cells and for the imaging of PSMA in a multicellular PCa model with good imaging penetration, indicating its potential use in complicated and heterogeneous tumor microenvironments. Furthermore, the probe showed good imaging performance in the PCa-bearing tumor mice via targeting PSMA in vivo. This work provides a novel strategy for the development of highly sensitive and specific NIR probes for PSMA in biological systems in vitro, which is of great significance for the precise diagnosis of PCa and for elucidating PCa heterogeneity.

Preclinical Development in Radiopharmaceutical Therapy for Prostate Cancer

Prostate cancer is a leading cause of cancer death in men worldwide. Among the various treatment options, radiopharmaceutical therapy has shown notable success in metastatic, castration-resistant disease. Radiopharmaceutical therapy is a systemic approach that delivers cytotoxic radiation doses precisely to the malignant tumors and/or tumor microenvironment. Therapeutic radiopharmaceuticals are composed of a therapeutic radionuclide and a high-affinity, tumor-targeting carrier molecule. Therapeutic radionuclides used in preclinical prostate cancer studies are primarily α-, β--, or Auger-electron-emitting radiometals or radiohalogens. Monoclonal antibodies, antibody-derived fragments, peptides, and small molecules are frequently used as tumor-targeting molecules. Over the years, several important membrane-associated proteases and receptors have been identified, validated, and subsequently used for preclinical radiotherapeutic development for prostate cancer. Prostate-specific membrane antigen (PSMA) is the most well-studied prostate cancer-associated protease in preclinical literature. PSMA-targeting radiotherapeutic agents are being investigated using high-affinity antibody- and small-molecule-based agents for safety and efficacy. Early generations of such agents were developed simply by replacing radionuclides of the imaging agents with therapeutic ones. Later, extensive structure-activity relationship studies were conducted to address the safety and efficacy issues obtained from initial patient data. Recent regulatory approval of the 177Lu-labeled low-molecular-weight agent, 177Lu-PSMA-617, is a significant accomplishment. Current preclinical experiments are focused on the structural modification of 177Lu-PSMA-617 and relevant investigational agents to increase tumor targeting and reduce off-target binding and toxicity in healthy organs. While lutetium-177 (177Lu) remains the most widely used radionuclide, radiolabeled analogs with iodine-131 (128I), yttrium-90 (89Y), copper-67 (67Cu), and terbium-161 (161Tb) have been evaluated as potential alternatives in recent years. In addition, agents carrying the α-particle-emitting radiohalogen, astatine-211 (211At), or radiometals, actinium-225 (225Ac), lead-212 (212Pb), radium-223 (223Ra), and thorium-227 (227Th), have been increasingly investigated in preclinical research. Besides PSMA-based radiotherapeutics, other prominent prostate cancer-related proteases, for example, human kallikrein peptidases (HK2 and HK3), have been explored using monoclonal-antibody-(mAb)-based targeting platforms. Several promising mAbs targeting receptors overexpressed on the different stages of prostate cancer have also been developed for radiopharmaceutical therapy, for example, Delta-like ligand 3 (DLL-3), CD46, and CUB domain-containing protein 1 (CDCP1). Progress is also being made using peptide-based targeting platforms for the gastrin-releasing peptide receptor (GRPR), a well-established membrane-associated receptor expressed in localized and metastatic prostate cancers. Furthermore, mechanism-driven combination therapies appear to be a burgeoning area in the context of preclinical prostate cancer radiotherapeutics. Here, we review the current developments related to the preclinical radiopharmaceutical therapy of prostate cancer. These are summarized in two major topics: (1) therapeutic radionuclides and (2) tumor-targeting approaches using monoclonal antibodies, small molecules, and peptides.

Aptamer-functionalized quantum dots as theranostic nanotools against cancer and bacterial infections: A comprehensive overview of recent trends

Aptamers (Apts) are synthetic nucleic acid ligands that can be engineered to target various molecules, including amino acids, proteins, and pharmaceuticals. Through a series of adsorption, recovery, and amplification steps, Apts are extracted from combinatorial libraries of synthesized nucleic acids. Using aptasensors in bioanalysis and biomedicine can be improved by combining them with nanomaterials. Moreover, Apt-associated nanomaterials, including liposomes, polymeric, dendrimers, carbon nanomaterials, silica, nanorods, magnetic NPs, and quantum dots (QDs), have been widely used as promising nanotools in biomedicine. Following surface modifications and conjugation with appropriate functional groups, these nanomaterials can be successfully used in aptasensing. Advanced biological assays can use Apts immobilized on QD surfaces through physical interaction and chemical bonding. Accordingly, modern QD aptasensing platforms rely on interactions between QDs, Apts, and targets to detect them. QD-Apt conjugates can be used to directly detect prostate, ovarian, colorectal, and lung cancers or simultaneously detect biomarkers associated with these malignancies. Tenascin-C, mucin 1, prostate-specific antigen, prostate-specific membrane antigen, nucleolin, growth factors, and exosomes are among the cancer biomarkers that can be sensitively detected using such bioconjugates. Furthermore, Apt-conjugated QDs have shown great potential for controlling bacterial infections such as Bacillus thuringiensis, Pseudomonas aeruginosa, Escherichia coli, Acinetobacter baumannii, Campylobacter jejuni, Staphylococcus aureus, and Salmonella typhimurium. This comprehensive review discusses recent advancements in the design of QD-Apt bioconjugates and their applications in cancer and bacterial theranostics.

Preclinical Evaluation of a New Series of Albumin-Binding 177Lu-Labeled PSMA-Based Low-Molecular-Weight Radiotherapeutics

Prostate-specific membrane antigen (PSMA)-based low-molecular-weight agents using beta(β)-particle-emitting radiopharmaceuticals is a new treatment paradigm for patients with metastatic castration-resistant prostate cancer. Although results have been encouraging, there is a need to improve the tumor residence time of current PSMA-based radiotherapeutics. Albumin-binding moieties have been used strategically to enhance the tumor uptake and retention of existing PSMA-based investigational agents. Previously, we developed a series of PSMA-based, β-particle-emitting, low-molecular-weight compounds. From this series, 177Lu-L1 was selected as the lead agent because of its reduced off-target radiotoxicity in preclinical studies. The ligand L1 contains a PSMA-targeting Lys-Glu urea moiety with an N-bromobenzyl substituent in the ε-amino group of Lys. Here, we structurally modified 177Lu-L1 to improve tumor targeting using two known albumin-binding moieties, 4-(p-iodophenyl) butyric acid moiety (IPBA) and ibuprofen (IBU), and evaluated the effects of linker length and composition. Six structurally related PSMA-targeting ligands (Alb-L1-Alb-L6) were synthesized based on the structure of 177Lu-L1. The ligands were assessed for in vitro binding affinity and were radiolabeled with 177Lu following standard protocols. All 177Lu-labeled analogs were studied in cell uptake and selected cell efficacy studies. In vivo pharmacokinetics were investigated by conducting tissue biodistribution studies for 177Lu-Alb-L2-177Lu-Alb-L6 (2 h, 24 h, 72 h, and 192 h) in male NSG mice bearing human PSMA+ PC3 PIP and PSMA- PC3 flu xenografts. Preliminary therapeutic ratios of the agents were estimated from the area under the curve (AUC0-192h) of the tumors, blood, and kidney uptake values. Compounds were obtained in >98% radiochemical yields and >99% purity. PSMA inhibition constants (Kis) of the ligands were in the ≤10 nM range. The long-linker-based agents, 177Lu-Alb-L4 and 177Lu-Alb-L5, displayed significantly higher tumor uptake and retention (p < 0.001) than the short-linker-bearing 177Lu-Alb-L2 and 177Lu-Alb-L3 and a long polyethylene glycol (PEG) linker-bearing agent, 177Lu-Alb-L6. The area under the curve (AUC0-192h) of the PSMA+ PC3 PIP tumor uptake of 177Lu-Alb-L4 and 177Lu-Alb-L5 were >4-fold higher than 177Lu-Alb-L2, 177Lu-Alb-L3, and 177Lu-Alb-L6, respectively. Also, the PSMA+ PIP tumor uptake (AUC0-192h) of 177Lu-Alb-L2 and 177Lu-Alb-L3 was ~1.5-fold higher than 177Lu-Alb-L6. However, the lowest blood AUC0-192h and kidney AUC0-192h were associated with 177Lu-Alb-L6 from the series. Consequently, 177Lu-Alb-L6 displayed the highest ratios of AUC(tumor)-to-AUC(blood) and AUC(tumor)-to-AUC(kidney) values from the series. Among the other agents, 177Lu-Alb-L4 demonstrated a nearly similar ratio of AUC(tumor)-to-AUC(blood) as 177Lu-Alb-L6. The tumor-to-blood ratio was the dose-limiting therapeutic ratio for all of the compounds. Conclusions: 177Lu-Alb-L4 and 177Lu-Alb-L6 showed high tumor uptake in PSMA+ tumors and tumor-to-blood ratios. The data suggest that linker length and composition can be modulated to generate an optimized therapeutic agent.

Evaluation of PSMA target diagnostic PET tracers for therapeutic monitoring of [177Lu]ludotadipep of prostate cancer: Screening of PSMA target efficiency and biodistribution using [18F]DCFPyL and [68Ga]PSMA-11

We have compared the similarity of the in vivo distribution of the prostate-specific membrane antigen (PSMA)-targeted positron emission tomography (PET) imaging agents [¹⁸F]DCFPyL, [⁶⁸Ga]galdotadipep, and [⁶⁸Ga]PSMA-11. This study is designed for a further selection of a PSMA-targeted PET imaging agent for the therapeutic evaluation of [¹⁷⁷Lu]ludotadipep, our previously developed prostate-specific membrane antigen (PSMA)-targeted prostate cancer therapeutic radiopharmaceutical. In vitro cell uptake was performed to evaluate the affinity to PSMA using PSMA + PC3-PIP, and PSMA- PC3-flu was used for the study. MicroPET/CT 60 min dynamic imaging and biodistribution were performed at 1, 2, and 4 h after injection. Autoradiography and immunohistochemistry were performed to evaluate the PSMA + tumor target efficiency. In the microPET/CT image, [⁶⁸Ga]PSMA-11 showed the highest uptake in the kidney among all three compounds. [¹⁸F]DCFPyL and [⁶⁸Ga]PSMA-11 showed similar patterns of in vivo biodistribution and high tumor targeting efficiency, similar to those of[⁶⁸Ga]galdotadipep. All three agents showed high uptake in tumor tissue on autoradiography, and PSMA expression was confirmed by immunohistochemistry. Thus, [¹⁸F]DCFPyL or [⁶⁸Ga]PSMA-11 can be used as a PET imaging agent to monitor [¹⁷⁷Lu]ludotadipep therapy in prostate cancer patients.

Necessity of Pelvic Lymph Node Irradiation in Patients with Recurrent Prostate Cancer after Radical Prostatectomy in the PSMA PET/CT Era: A Narrative Review

The main prostate cancer (PCa) treatments include surgery or radiotherapy (with or without ADT). However, none of the suggested treatments eliminates the risk of lymph node metastases. Conventional imaging methods, including MRI and CT scanning, are not sensitive enough for the diagnosis of lymph node metastases; however, the novel imaging method, PSMA PET/CT scanning, has provided valuable information about the pelvic LN involvement in patients with recurrent PCa (RPCa) after radical prostatectomy. The high sensitivity and negative predictive value enable accurate N staging in PCa patients. In this narrative review, we summarize the evidence on the treatment and extent of radiation in prostate-only or whole-pelvis radiation in patients with positive and negative LN involvement on PSMA PET/CT scans.

[99mTc]Tc-PSMA-T4-Novel SPECT Tracer for Metastatic PCa: From Bench to Clinic

Despite significant advances in nuclear medicine for diagnosing and treating prostate cancer (PCa), research into new ligands with increasingly better biological properties is still ongoing. Prostate-specific membrane antigen (PSMA) ligands show great potential as radioisotope carriers for the diagnosis and therapy of patients with metastatic PCa. PSMA is expressed in most types of prostate cancer, and its expression is increased in poorly differentiated, metastatic, and hormone-refractory cancers; therefore, it may be a valuable target for the development of radiopharmaceuticals and radioligands, such as urea PSMA inhibitors, for the precise diagnosis, staging, and treatment of prostate cancer. Four developed PSMA-HYNIC inhibitors for technetium-99m labeling and subsequent diagnosis were subjected to preclinical in vitro and in vivo studies to evaluate and compare their diagnostic properties. Among the studied compounds, the PSMA-T4 (Glu-CO-Lys-L-Trp-4-Amc-HYNIC) inhibitor showed the best biological properties for the diagnosis of PCa metastases. [99mTc]Tc-PSMA-T4 also showed effectiveness in single-photon emission computed tomography (SPECT) studies in humans, and soon, its usefulness will be extensively evaluated in phase 2/3 clinical trials.

Chemical synthesis of fluorinated and iodinated 17β-HSD3 inhibitors and evaluation for imaging prostate cancer tumors and tissue biodistribution

Prostate cancer is the most common cancer among men and the development of new therapeutic agents is needed for its treatment and/or diagnosis. 17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3) is involved in the production of androgens, which stimulates the proliferation of prostate cancer cells. Piperazinomethyl-androsterone sulfonamide derivatives were developed as 17β-HSD3 inhibitors and the concentration of a representative sulfonamide derivative (compound 1) was found to accumulate in prostate tumor tissues relatively to plasma in a mouse xenograft experiment. This finding gives us the opportunity to specifically target the prostate cancer tumors through the development of a radiolabelled version of compound 1 toward targeted molecular radiotherapy or radioimaging diagnosis. The chemical synthesis of fluorinated and iodinated analogs of compound 1 was achieved, leading to a series of compounds with similar levels of inhibition as the initial candidate. From 17β-HSD3 inhibition activity, molecular modeling and mouse plasma-concentration studies, the most promising compound of this series was selected, its ¹⁸F-radiolabelled version (¹⁸F-3) synthesized, and imaging/biodistribution studies engaged. When injected in mice, however, ¹⁸F-3 uptake in the target tissues (LNCaP[17β-HSD3] tumors and testicles) was not sufficient to allow their visualization by positron emission tomography. Plasma concentration values of compounds 3-8 administered orally, however, showed that the para-iodo compound 7 is the most metabolically stable and could therefore be an interesting alternative for radiolabelling and radiotreatment.

Prostate-Specific Membrane Antigen Expression and Response to DNA Damaging Agents in Prostate Cancer

PURPOSE

Prostate-specific membrane antigen (PSMA) targeting therapies such as Lutetium-177 (177Lu)-PSMA-617 are affecting outcomes from metastatic castration-resistant prostate cancer (mCRPC). However, a significant subset of patients have prostate cancer cells lacking PSMA expression, raising concerns about treatment resistance attributable at least in part to heterogeneous PSMA expression. We have previously demonstrated an association between high PSMA expression and DNA damage repair defects in mCRPC biopsies and therefore hypothesized that DNA damage upregulates PSMA expression.

EXPERIMENTAL DESIGN

To test this relationship between PSMA and DNA damage we conducted a screen of 147 anticancer agents (NCI/NIH FDA-approved anticancer "Oncology Set") and treated tumor cells with repeated ionizing irradiation.

RESULTS

The topoisomerase-2 inhibitors, daunorubicin and mitoxantrone, were identified from the screen to upregulate PSMA protein expression in castration-resistant LNCaP95 cells; this result was validated in vitro in LNCaP, LNCaP95, and 22Rv1 cell lines and in vivo using an mCRPC patient-derived xenograft model CP286 identified to have heterogeneous PSMA expression. As double-strand DNA break induction by topoisomerase-2 inhibitors upregulated PSMA, we next studied the impact of ionizing radiation on PSMA expression; this also upregulated PSMA protein expression in a dose-dependent fashion.

CONCLUSIONS

The results presented herein are the first, to our knowledge, to demonstrate that PSMA is upregulated in response to double-strand DNA damage by anticancer treatment. These data support the study of rational combinations that maximize the antitumor activity of PSMA-targeted therapeutic strategies by upregulating PSMA.

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.

A Review on the Current State and Future Perspectives of [99mTc]Tc-Housed PSMA-i in Prostate Cancer

Recently, prostate-specific membrane antigen (PSMA) has gained momentum in tumor nuclear molecular imaging as an excellent target for both the diagnosis and therapy of prostate cancer. Since 2008, after years of preclinical research efforts, a plentitude of radiolabeled compounds mainly based on low molecular weight PSMA inhibitors (PSMA-i) have been described for imaging and theranostic applications, and some of them have been transferred to the clinic. Most of these compounds include radiometals (e.g., ⁶⁸Ga, ⁶⁴Cu, ¹⁷⁷Lu) for positron emission tomography (PET) imaging or endoradiotherapy. Nowadays, although the development of new PET tracers has caused a significant drop in single-photon emission tomography (SPECT) research programs and the development of new technetium-99m (^99mTc) tracers is rare, this radionuclide remains the best atom for SPECT imaging owing to its ideal physical decay properties, convenient availability, and rich and versatile coordination chemistry. Indeed, ^99mTc still plays a relevant role in diagnostic nuclear medicine, as the number of clinical examinations based on ^99mTc outscores that of PET agents and ^99mTc-PSMA SPECT/CT may be a cost-effective alternative for ⁶⁸Ga-PSMA PET/CT. This review aims to give an overview of the specific features of the developed [^99mTc]Tc-tagged PSMA agents with particular attention to [^99mTc]Tc-PSMA-i. The chemical and pharmacological properties of the latter will be compared and discussed, highlighting the pros and cons with respect to [⁶⁸Ga]Ga-PSMA11.

New Halogen-Containing Drugs Approved by FDA in 2021: An Overview on Their Syntheses and Pharmaceutical Use

This review describes the recent Food and Drug Administration (FDA)-approved drugs (in the year 2021) containing at least one halogen atom (covalently bound). The structures proposed throughout this work are grouped according to their therapeutical use. Their synthesis is presented as well. The number of halogenated molecules that are reaching the market is regularly preserved, and 14 of the 50 molecules approved by the FDA in the last year contain halogens. This underlines the emergent role of halogens and, in particular, of fluorine and chlorine in the preparation of drugs for the treatment of several diseases such as viral infections, several types of cancer, cardiovascular disease, multiple sclerosis, migraine and inflammatory diseases such as vasculitis.

Strain-Promoted Azide-Alkyne Cycloaddition-Based PSMA-Targeting Ligands for Multimodal Intraoperative Tumor Detection of Prostate Cancer

Strain-promoted azide-alkyne cycloaddition (SPAAC) is a straightforward and multipurpose conjugation strategy. The use of SPAAC to link different functional elements to prostate-specific membrane antigen (PSMA) ligands would facilitate the development of a modular platform for PSMA-targeted imaging and therapy of prostate cancer (PCa). As a first proof of concept for the SPAAC chemistry platform, we synthesized and characterized four dual-labeled PSMA ligands for intraoperative radiodetection and fluorescence imaging of PCa. Ligands were synthesized using solid-phase chemistry and contained a chelator for 111In or 99mTc labeling. The fluorophore IRDye800CW was conjugated using SPAAC chemistry or conventional N-hydroxysuccinimide (NHS)-ester coupling. Log D values were measured and PSMA specificity of these ligands was determined in LS174T-PSMA cells. Tumor targeting was evaluated in BALB/c nude mice with subcutaneous LS174T-PSMA and LS174T wild-type tumors using μSPECT/CT imaging, fluorescence imaging, and biodistribution studies. SPAAC chemistry increased the lipophilicity of the ligands (log D range: -2.4 to -4.4). In vivo, SPAAC chemistry ligands showed high and specific accumulation in s.c. LS174T-PSMA tumors up to 24 h after injection, enabling clear visualization using μSPECT/CT and fluorescence imaging. Overall, no significant differences between the SPAAC chemistry ligands and their NHS-based counterparts were found (2 h p.i., p > 0.05), while 111In-labeled ligands outperformed the 99mTc ligands. Here, we demonstrate that our newly developed SPAAC-based PSMA ligands show high PSMA-specific tumor targeting. The use of click chemistry in PSMA ligand development opens up the opportunity for fast, efficient, and versatile conjugations of multiple imaging moieties and/or drugs.

Initial Experience of Clinical Use of [99mTc]Tc-PSMA-T4 in Patients with Prostate Cancer. A Pilot Study

Numerous different molecules of prostate-specific membrane antigen (PSMA) ligands are used to detect prostate cancer (PCa); most approaches utilize gallium PET and a few reports describe the role of SPECT/CT. [^99mTc]Tc-PSMA-T4 is a new radiopharmaceutical designed for the diagnosis of patients with PCa. We conducted a single site, prospective, preliminary case series study that included 31 patients with PCa; all had undergone clinical, biochemical or imaging examination and exhibited clear or suspicious active disease or clinical/biochemical recurrence of PCa. Whole-body (WB) SPECT/CT after i.v. administration of [^99mTc]Tc-PSMA-T4 was utilized; acquisition images were obtained at three time points. The clinical value of the images was assessed in regard to the evaluation of tumor extent in patients with confirmed PC that qualified for initial therapy and the evaluation of tumor recurrence; both provided encouraging results. The late acquisition of WB-SPECT resulted in better lesions delineation. The results of the analysis of the sensitivity/specificity were: 92%/100% in cases of primary cancer, 83%/100% in terms of pelvic lymph nodes disease, 100%/95% in other lymph nodes and soft tissue involvement, respectively, and bone mets were both 100%. An oncotropic SPECT [^99mTc]Tc-PSMA-T4 can help in selecting a rational therapeutic strategy for a patient with an initial diagnosis of PCa by assessing the extent of cancer and also after complex radical or palliative therapy in case of biochemical recurrence for re-staging.

Molecular Imaging in Primary Staging of Prostate Cancer Patients: Current Aspects and Future Trends

Accurate primary staging is the cornerstone in all malignancies. Different morphological imaging modalities are employed in the evaluation of prostate cancer (PCa). Regardless of all developments in imaging, invasive histopathologic evaluation is still the standard method for the detection and staging of the primary PCa. Magnetic resonance imaging (MRI) and computed tomography (CT) play crucial roles; however, functional imaging provides additional valuable information, and it is gaining ever-growing acceptance in the management of PCa. Targeted imaging with different radiotracers has remarkably evolved in the past two decades. [111In]In-capromab pendetide scintigraphy was a new approach in the management of PCa. Afterwards, positron emission tomography (PET) tracers such as [11C/18F]choline and [11C]acetate were developed. Nevertheless, none found a role in the primary staging. By introduction of the highly sensitive small molecule prostate-specific membrane antigen (PSMA) PET/CT, as well as recent developments in MRI and hybrid PET/MRI systems, non-invasive staging of PCa is being contemplated. Several studies investigated the role of these sophisticated modalities in the primary staging of PCa, showing promising results. Here, we recapitulate the role of targeted functional imaging. We briefly mention the most popular radiotracers, their diagnostic accuracy in the primary staging of PCa, and impact on patient management.

Magnetic Resonance Imaging of PSMA-Positive Prostate Cancer by a Targeted and Activatable Gd(III) MR Contrast Agent

Prostate-specific membrane antigen (PSMA) is a transmembrane protein that is highly expressed in aggressive prostate cancer (PCa) and has been extensively studied as a PCa diagnostic imaging biomarker. Multiple imaging modalities have exploited PSMA as a biomarker including magnetic resonance (MR), Optical, and PET imaging. Of all the imaging MR imaging provides the most detailed information, concurrently providing anatomical, functional, and potentially molecular information. However, the lower sensitivity of MR requires development of molecular MR contrast agents that provides high signal-to-noise ratios. Herein, we report the first targeted and activatable Gd(III)-based MR contrast agents prostate cancer probe 1 and 2 (PCP-1 and -2). We successfully used PCP-2 to differentiate between PSMA+ and PSMA- prostate cancer cells with both in vitro fluorescence imaging and in vivo MR imaging. The in vivo MR imaging results were further supported by ex vivo fluorescence imaging studies, showcasing the unique bimodal feature of PCP-2. Furthermore, PCP-2 highlights a unique molecular MR probe design strategy that improved the sensitivity of traditional biomarker-targeted MR imaging, addressing a critical unmet need in molecular MR imaging field. This work represents the first example of a targeted and activatable MR contrast agent that can be systemically administered in vivo to highlight PSMA+ prostate tumors, paving the way for the clinical translation of MR PSMA imaging.

18F DCFPyL PET Acquisition, Interpretation and Reporting: Suggestions Post Food and Drug Administration Approval

¹⁸F-DCFPyL was recently approved by the FDA for evaluation prior to definitive therapy and for biochemical recurrence. Here we focus on the key data that justify the clinical use of ¹⁸F-DCFPyL, as well as those aspects of protocol implementation and image interpretation that are important to the nuclear medicine physicians and radiologists who will interpret ¹⁸F-DCFPyL PET/CT and PET/MR scans.

The utility of radiolabeled PSMA ligands for tumor imaging

Prostate-specific membrane antigen (PSMA) is a glycosylated type-II transmembrane protein expressed in prostatic tissue and significantly overexpressed in several prostate cancer cells. Despite its name, PSMA has also been reported to be overexpressed in endothelial cells of benign and malignant non-prostate disease. So its clinical use was extended to detection, staging, and therapy of various tumor types. Recently small molecules targeting PSMA have been developed as imaging probes for diagnosis of several malignancies. Preliminary studies are emerging improved diagnostic sensitivity and specificity of PSMA imaging, leading to a change in patient management. In this review, we evaluated the first preclinical and clinical studies on PSMA ligands resulting future perspectives radiolabeled PSMA in staging and molecular characterization, based on histopathologic examinations of PSMA expression.

Evolution of Peptide-Based Prostate-Specific Membrane Antigen (PSMA) Inhibitors: An Approach to Novel Prostate Cancer Therapeutics

BACKGROUND

Prostate cancer is one of the most common cancers worldwide, with approximately 1.1 million cases diagnosed annually. The rapid development of molecular imaging has facilitated greater structural understanding, which can help formulate novel combinations of therapeutic regimens and more accurate diagnosis, avoiding unnecessary prostate biopsies. This accumulated knowledge also provides a greater understanding of the aggressive stages of the disease and tumor recurrence. Recently, much progress has been made on developing peptidomimetic-based inhibitors as promising candidates to effectively bind to the prostate- specific membrane antigen (PSMA), which is expressed by prostate cancer cells.

OBJECTIVE

In this review, recent advances covering small-molecule and peptide-based PSMA inhibitors will be extensively reviewed, providing a base for the rational design of future PSMA inhibitors.

METHOD

Herein, the literature on selected PSMA inhibitors that have been developed from 1996 to 2020 were reviewed, emphasizing recent synthetic advances and chemical strategies whilst highlighting therapeutic potential and drawbacks of each inhibitor.

RESULTS

Synthesized inhibitors presented in this review demonstrate the clinical application of certain PSMA inhibitors, exhibited in vitro and in vivo.

CONCLUSION

This review highlights the clinical potential of PSMA inhibitors, analyzing the advantages and setbacks of the chemical synthetic methodologies utilized, setting precedence for the discovery of novel PSMA inhibitors for future clinical applications.

[18F]-JK-PSMA-7 and [18F]-FDG tumour PET uptake in treated xenograft human prostate cancer model in mice

PURPOSE

This preclinical study aims to evaluate the extent to which a change in prostate-specific membrane antigen (PSMA) expression of castration-resistant prostate cancer (CRPC) following standard treatment is reflected in [18F]JK-PSMA-7 PET/CT.

METHODS

Castrated mice supplemented with testosterone implant were xenografted with human LNCaP CRPC. After appropriate tumour growth, androgen deprivation therapy (ADT) was carried out by the removal of the implant followed by a single injection of docetaxel (400 μg/20-g mouse) 2 weeks later. [18F]JK-PSMA-7 PET/CT were performed before ADT, then before and at days 12, 26, 47 and 69 after docetaxel administration. The [18F]JK-PSMA-7 PET data were compared to corresponding unspecific metabolic [18F]FDG PET/CT and ex vivo quantification of PSMA expression estimated by flow cytometry on repeated tumour biopsies.

RESULTS

ADT alone had no early effect on LNCaP tumours that pursued their progression. Until day 12 post-docetaxel, the [18F]JK-PSMA7 uptake was significantly higher than that of [18F]FDG, indicating the persistence of PSMA expression at those time points. From day 26 onwards when the tumours were rapidly expanding, both [18F]JK-PSMA7 and [18F]FDG uptake continuously decreased although the decrease in [18F]JK-PSMA uptake was markedly faster. The fraction of PSMA-positive cells in tumour biopsies decreased similarly over time to reach a non-specific level after the same time period.

CONCLUSION

Applying PSMA-based imaging for therapy monitoring in patients with CRPC should be considered with caution since a reduction in [18F]JK-PSMA-7 PET uptake after successive ADT and chemotherapy may be related to downregulation of PSMA expression in dedifferentiated and rapidly proliferating tumour cells.

Visualizing Tumors in Real Time: A Highly Sensitive PSMA Probe for NIR-II Imaging and Intraoperative Tumor Resection

Owing to the complex anatomical structure, precise resection of a tumor while maintaining adjacent tissue is a challenge in radical prostatectomy for prostate cancer (PCa). Optical imaging in near-infrared window II (NIR-II) is a promising technology for intraoperative guidance, whereas there is no available probe for PCa yet. In this article, a novel probe (PSMA-1092) bearing two prostate-specific membrane antigen (PSMA) binding motifs was developed, displaying excellent optical properties (λ_max = 1092 nm) and ultrahigh affinity (K_i = 80 pM) toward PSMA. The tumor was visualized with high resolution (tissue-to-normal tissue ratio = 7.62 ± 1.05) and clear margin by NIR-II imaging using PSMA-1092 in a mouse model. During the tumor resection, residual tumors missed by visible inspection were detected by the real-time imaging. Overall, PSMA-1092 displayed excellent performance in delineating the tumor margin and detecting residual tumors, demonstrating promising potential for precise PCa tumor resection in clinical practice.

Comparison of the diagnostic utility of 99mTc-PSMA scintigraphy versus 68Ga-PSMA-11 PET/CT in the detection of metastatic prostate cancer and dosimetry analysis: a gamma-camera-based alternate prostate-specific membrane antigen imaging modality

OBJECTIVE

The present study was performed for head-to-head comparison between 68Ga-prostate-specific membrane antigen (PSMA) PET/computed tomography (CT) and 99mTc-PSMA whole-body and regional single-photon emission computed tomography (SPECT)/CT for the detection of prostate cancer metastases.

METHODS

Ten patients with metastatic prostate cancer underwent 99mTc-PSMA whole-body scan after intravenous injection of 230-330 MBq 99mTc-PSMA. Anterior and posterior whole-body images were acquired at 10 min, 2, 4 and/or 5/6 h post-injection. Additional SPECT/CT images were acquired for the involved sites, where planar images did not clearly identify the metastatic sites. All patients also underwent whole-body 68Ga-PSMA PET/CT and the results between the two techniques were compared for the detection of the metastatic lesions. Dosimetry analysis of the 99mTc-PSMA studies was performed using the MIRD-OLINDA approach.

RESULTS

68Ga-PSMA PET/CT detected lesions in all 10 patients, whereas 99mTc-PSMA imaging detected lesions in 9/10 patients. 68Ga-PSMA PET/CT imaging identified a total of 112 PSMA avid metastatic lesions compared to 57 (51%) lesions on 99mTc-PSMA imaging. Eighteen out of 57 lesions were detected only on delayed 99mTc-PSMA imaging at 4 h and/or 6 h. The regional 99mTc-PSMA SPECT detected 51/83 (61.0%) lesions seen on 68Ga-PSMA PET/CT. The dosimetry results demonstrated that 99mTc-PSMA provided organs' radiation absorbed/effective doses comparable with 99mTc-PSMA imaging.

CONCLUSION

Whole-body 99mTc-PSMA combined with regional SPECT/CT could be a potential alternative to 68Ga-PSMA PET for the detection of the advanced stage metastatic prostate cancer and for response evaluation to PSMA-based targeted therapies.

Artificial intelligence in molecular imaging

AI has, to varying degrees, affected all aspects of molecular imaging, from image acquisition to diagnosis. During the last decade, the advent of deep learning in particular has transformed medical image analysis. Although the majority of recent advances have resulted from neural-network models applied to image segmentation, a broad range of techniques has shown promise for image reconstruction, image synthesis, differential-diagnosis generation, and treatment guidance. Applications of AI for drug design indicate the way forward for using AI to facilitate molecular-probe design, which is still in its early stages. Deep-learning models have demonstrated increased efficiency and image quality for PET reconstruction from sinogram data. Generative adversarial networks (GANs), which are paired neural networks that are jointly trained to generate and classify images, have found applications in modality transformation, artifact reduction, and synthetic-PET-image generation. Some AI applications, based either partly or completely on neural-network approaches, have demonstrated superior differential-diagnosis generation relative to radiologists. However, AI models have a history of brittleness, and physicians and patients may not trust AI applications that cannot explain their reasoning. To date, the majority of molecular-imaging applications of AI have been confined to research projects, and are only beginning to find their ways into routine clinical workflows via commercialization and, in some cases, integration into scanner hardware. Evaluation of actual clinical products will yield more realistic assessments of AI’s utility in molecular imaging.

Development of Heterobivalent Theranostic Probes Having High Affinity/Selectivity for the GRPR/PSMA

In this study, we describe the development of heterobivalent [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] radiotracers that display very high selectivity/specificity for gastrin-releasing peptide receptor (GRPR)-/prostate-specific membrane antigen (PSMA)-expressing cells. These studies include metallation, purification, characterization, and in vitro and in vivo evaluation of the new small-molecule-/peptide-based radiopharmaceuticals having utility for imaging and potentially therapy. Competitive displacement binding assays using PC-3 cells and LNCaP cell membranes showed high binding affinity for the GRPR or the PSMA. Biodistribution studies showed favorable excretion pharmacokinetics with high tumor uptake in PC-3 or PC-3 prostatic inhibin peptide (PIP) tumor-bearing mice. For example, tumor accumulation at the 1 h time point ranged from (4.74 ± 0.90) to (7.51 ± 2.61)%ID/g. Micro-single-photon emission computed tomography (microSPECT) molecular imaging investigations showed very high uptake in tumors with minimal accumulation of tracers in the surrounding collateral tissues in xenografted mice at 4 h postintravenous injection. In conclusion, [DUPA-6-Ahx-([¹¹¹In]In-DO3A)-8-Aoc-BBN ANT] and [DUPA-6-Ahx-([¹⁷⁷Lu]Lu-DO3A)-8-Aoc-BBN ANT] tracers displayed favorable pharmacokinetic and excretion profiles with high uptake and retention in tumors.

Photosensitizer-based multimodal PSMA-targeting ligands for intraoperative detection of prostate cancer

Incomplete resection of prostate cancer (PCa) occurs in 15%-50% of PCa patients. Disease recurrence negatively impacts oncological outcome. The use of radio-, fluorescent-, or photosensitizer-labeled ligands to target the prostate-specific membrane antigen (PSMA) has become a well-established method for the detection and treatment of PCa. Methods: Here, we developed and characterized multimodal [¹¹¹In]In-DOTA(GA)-IRDye700DX-PSMA ligands, varying in their molecular composition, for use in intraoperative radiodetection, fluorescence imaging and targeted photodynamic therapy of PCa lesions. PSMA-specificity of these ligands was determined in xenograft tumor models and on fresh human PCa biopsies. Results: Ligand structure optimization showed that addition of the photosensitizer (IRDye700DX) and additional negative charges significantly increased ligand uptake in PSMA-expressing tumors. Moreover, an ex vivo incubation study on human tumor biopsies confirmed the PSMA-specificity of these ligands on human samples, bridging the gap to the clinical situation. Conclusion: We developed a novel PSMA-targeting ligand, optimized for multimodal image-guided PCa surgery combined with targeted photodynamic therapy.

Evaluation of radioiodinated protein conjugates and their potential metabolites containing lysine-urea-glutamate (LuG), PEG and closo-decaborate(2-) as models for targeting astatine-211 to metastatic prostate cancer

INTRODUCTION

The use of lysine-urea-glutamate (LuG) for targeting the PSMA antigen on prostate cancer (PCa) is a promising method for delivering the alpha particle-emitting radionuclide astatine-211 (211At) to metastatic PCa. High kidney localization has been a problem with radiolabeled LuG derivatives, but has been adequately addressed in radiometal-labeled DOTA-LuG derivatives by linker optimization. Herein, we report an investigation of an alternate approach to diminishing the kidney concentrations of radiolabeled LuG-containing compounds.

METHODS

Our approach involves PEGylated LuG moieties and closo-decaborate (2-) moieties conjugated to streptavidin (SAv) or human serum albumin (HSA). After preparing the LuG conjugates, SAv and HSA conjugates were succinylated to decrease their kidney localization and radioiodinated for evaluation in athymic mice bearing C4-2B osseous PCa tumor xenografts.

RESULTS

Covalently attaching LuG to succinylated SAv and HSA significantly reduced kidney localization, but unfortunately succinylation resulted in decreased tumor concentrations. In contrast, a potential metabolite [131I]16b, an unconjugated LuG derivative containing a dPEG4® linker, provided tumor concentrations of ~15% ID/g at 4 h pi. A second unconjugated LuG derivative with a similar structure, but containing a dPEG12® linker, [131I]16a had tumor concentrations of ~4%ID/g at 4 h pi. Those results suggest that long PEG linkers also affect tumor localization in a negative manner.

CONCLUSION

Conjugation of PEGylated LuG derivatives to proteins can be an effective approach to diminishing kidney localization of radiolabeled LuG reagents, but the protein, linker and the method of linkage need to be further studied. Additionally, modification of the unconjugated 16b to decrease kidney localization may provide PCa targeting agents for use with radiohalogens, including 211At. Advances in knowledge and implications for patient care: This study is the first to evaluate PEGylated LuG and closo-decaborate (2-) moieties conjugated to proteins as potential methods for diminishing the kidney concentrations of radiolabeled LuG-containing compounds.

The emerging role of cell surface receptor and protein binding radiopharmaceuticals in cancer diagnostics and therapy

Targeting specific cell membrane markers for both diagnostic imaging and radionuclide therapy is a rapidly evolving field in cancer research. Some of these applications have now found a role in routine clinical practice and have been shown to have a significant impact on patient management. Several molecular targets are being investigated in ongoing clinical trials and show promise for future implementation. Advancements in molecular biology have facilitated the identification of new cancer-specific targets for radiopharmaceutical development.

Preparation and Biological Evaluation of [99mTc]Tc-CNGU as a PSMA-Targeted Radiotracer for the Imaging of Prostate Cancer

Prostate-specific membrane antigen (PSMA) is a well-established biological target that is overexpressed on the surface of prostate cancer lesions. Radionuclide-labeled small-molecule PSMA inhibitors have been shown to be promising PSMA-specific agents for the diagnosis and therapy of prostate cancer. In this study, a glutamate-urea-based PSMA-targeted ligand containing an isonitrile (CNGU) was synthesized and labeled with ^99mTc to prepare [^99mTc]Tc-CNGU with a high radiochemical purity (RCP). The CNGU ligand showed a high affinity toward PSMA (K_i value is 8.79 nM) in LNCaP cells. The [^99mTc]Tc-CNGU exhibited a good stability in vitro and hydrophilicity (log P = −1.97 ± 0.03). In biodistribution studies, BALB/c nude mice bearing LNCaP xenografts showed that the complex had a high tumor uptake with 4.86 ± 1.19% ID/g, which decreased to 1.74 ± 0.90% ID/g after a pre-injection of the selective PSMA inhibitor ZJ-43, suggesting that it was a PSMA-specific agent. Micro-SPECT imaging demonstrated that the [^99mTc]Tc-CNGU had a tumor uptake and that the uptake was reduced in the image after blocking with ZJ-43, further confirming its PSMA specificity. All of the results in this work indicated that [^99mTc]Tc-CNGU is a promising PSMA-specific tracer for the imaging of prostate cancer.

Toward the Discovery and Development of PSMA Targeted Inhibitors for Nuclear Medicine Applications

BACKGROUND

The rising incidence rate of prostate cancer (PCa) has promoted the development of new diagnostic and therapeutic radiopharmaceuticals during the last decades. Promising improvements have been achieved in clinical practice using prostate specific membrane antigen (PSMA) labeled agents, including specific antibodies and small molecular weight inhibitors. Focusing on molecular docking studies, this review aims to highlight the progress in the design of PSMA targeted agents for a potential use in nuclear medicine.

RESULTS

Although the first development of radiopharmaceuticals able to specifically recognize PSMA was exclusively oriented to macromolecule protein structure such as radiolabeled monoclonal antibodies and derivatives, the isolation of the crystal structure of PSMA served as the trigger for the synthesis and the further evaluation of a variety of low molecular weight inhibitors. Among the nuclear imaging probes and radiotherapeutics that have been developed and tested till today, labeled Glutamate-ureido inhibitors are the most prevalent PSMA-targeting agents for nuclear medicine applications.

CONCLUSION

PSMA represents for researchers the most attractive target for the detection and treatment of patients affected by PCa using nuclear medicine modalities. [99mTc]MIP-1404 is considered the tracer of choice for SPECT imaging and [68Ga]PSMA-11 is the leading diagnostic for PET imaging by general consensus. [18F]DCFPyL and [18F]PSMA-1007 are clearly the emerging PET PSMA candidates for their great potential for a widespread commercial distribution. After paving the way with new imaging tools, academic and industrial R&Ds are now focusing on the development of PSMA inhibitors labeled with alpha or beta minus emitters for a theragnostic application.

Comparison of Bone Uptake in Bone Scan and Ga-68 PSMA PET/CT Images in Patients with Prostate Cancer

OBJECTIVE

The aim of this study was to compare images from Tc-99m MDP bone scan (BS) and Ga-68 PSMA PET/CT of patients with prostate cancer in terms of bone metastases.

METHODS

Overall, 34 patients exhibited a mean age of 66 ± 9.5 (50-88) years, mean PSA of 51 ± 159ng/ml (0-912), and mean Gleason score of 8 (6-9). BS and Ga-68 PSMA PET/CT were applied to 34 patients within 30 days, and the results were evaluated, retrospectively. In both tests, radiopharmaceutical uptake in bones were compared.

RESULTS

In 7 patients (20.5%), uptake was not significant on BS and Ga-68 PSMA PET / CT images, which is related to metastasis. In 14 (41%) patients, bone metastases were observed in both examinations. However, more metastatic lesions were observed in the Ga-68 PSMA PET/CT of 3 patients and in the bone scintigraphy of 2 patients. PSMA expression was not observed on Ga-68 PSMA PET / CT in 13 (38%) patients with increased activity in bone scintigraphy. Two (6%) of these patients were thought to be metastatic, 2 (6%) were suspicious for metastasis, and 9 (26%) had no metastasis. When a lesion-based evaluation was performed, a total of 480 activities were evaluated: increased activity uptake was found in 305 BS, and 427 PSMA expression activity was detected. Furthermore, 435 of these activities were evaluated as metastatic.

CONCLUSION

Ga-68 PSMA PET/CT provides an additional contribution to the BS evaluation of activity areas because of the presence of PSMA expression and anatomical lesions. In 6% of the patients, activity on BS and metastatic appearance in CT images were observed and the presence of lesions in the absence of PSMA was determined. This suggests that bone metastases without PSMA expression may also be present.

Engineered Fragments of the PSMA-Specific 5D3 Antibody and Their Functional Characterization

Prostate-Specific Membrane Antigen (PSMA) is an established biomarker for the imaging and experimental therapy of prostate cancer (PCa), as it is strongly upregulated in high-grade primary, androgen-independent, and metastatic lesions. Here, we report on the development and functional characterization of recombinant single-chain Fv (scFv) and Fab fragments derived from the 5D3 PSMA-specific monoclonal antibody (mAb). These fragments were engineered, heterologously expressed in insect S2 cells, and purified to homogeneity with yields up to 20 mg/L. In vitro assays including ELISA, immunofluorescence and flow cytometry, revealed that the fragments retain the nanomolar affinity and single target specificity of the parent 5D3 antibody. Importantly, using a murine xenograft model of PCa, we verified the suitability of fluorescently labeled fragments for in vivo imaging of PSMA-positive tumors and compared their pharmacokinetics and tissue distribution to the parent mAb. Collectively, our data provide an experimental basis for the further development of 5D3 recombinant fragments for future clinical use.

Bivalent Inhibitors of Prostate-Specific Membrane Antigen Conjugated to Desferrioxamine B Squaramide Labeled with Zirconium-89 or Gallium-68 for Diagnostic Imaging of Prostate Cancer

Prostate-specific membrane antigen (PSMA) is a carboxypeptidase that is overexpressed in prostate cancer and is an excellent candidate for targeted diagnostic imaging and therapy. Lysine-ureido-glutamate inhibitors of PSMA radiolabeled with positron-emitting radionuclides can be used for diagnostic imaging with positron emission tomography (PET). A squaramide ester derivative of desferrioxamine B (H₃DFOSq) was used to prepare four new agents with either one or two lysine-ureido-glutamate pharmacophores. The H₃DFOSq ligand can be used to form stable complexes with either of the positron-emitting radionuclides gallium-68 (t_1/2 = 68 min) or zirconium-89 (t_1/2 = 3.3 days). The complexes were evaluated in PSMA-positive xenograft mouse models. Bivalent inhibitors, where two pharmacophores are tethered to a single DFOSq ligand, have better tumor uptake than their monovalent analogues. The ligands presented here, which can be labeled with either gallium-68 or zirconium-89, have the potential to increase the number of clinical sites that can perform diagnostic PET imaging.

Nanoparticles for Targeting of Prostate Cancer

Prostate cancer (PCa) is the leading cause of death by cancer in men. Because of the drastic decline in the survival rate of PCa patients with advanced/metastatic disease, early diagnosis of disease and therapy without toxic side effects is crucial. Chemotherapy is widely used to control the progression of PCa at the later stages; however, it is associated with off-target toxicities and severe adverse effects due to the lack of specificity. Delivery of therapeutic or diagnostic agents by using targeted nanoparticles is a promising strategy to enhance accuracy and sensitivity of diagnosis of PCa and to increase efficacy and specificity of therapeutic agents. Numerous efforts have been made in past decades to create nanoparticles with different architectural bases for specific delivery payloads to prostate tumors. Major PCa associated cell membrane protein markers identified as targets for such purposes include folate receptor, sigma receptors, transferrin receptor, gastrin-releasing peptide receptor, urokinase plasminogen activator receptor, and prostate specific membrane antigen. Among these markers, prostate specific membrane antigen has emerged as an extremely specific and sensitive targetable marker for designing targeted nanoparticle-based delivery systems for PCa. In this article, we review contemporary advances in design, specificity, and efficacy of nanoparticles functionalized against PCa. Whenever feasible, both diagnostic as well as therapeutic applications are discussed.

Imaging and Characterization of Macrophage Distribution in Mouse Models of Human Prostate Cancer

PURPOSE

Prostate carcinoma consists of tumor epithelium and malignant stroma. Until recently, diagnostic and therapeutic efforts have focused exclusively on targeting characteristics of the tumor epithelium, ignoring opportunities to target inflammatory infiltrate and extracellular matrix components. Prostate tumors are rich in tumor-associated macrophages (TAMs), which can be either of the cytotoxic M1 or protumorigenic M2 phenotype. We have quantified the proportion of each in seven common human prostate tumor lines grown subcutaneously in athymic nude mice and have imaged macrophage densities in vivo in xenografts derived from these lines.

PROCEDURES

A panel of seven human prostate cancer xenografts was generated in intact male athymic nude mice reflecting variable expression of the androgen receptor (AR) and prostate-specific membrane antigen (PSMA). Mice were imaged ex vivo using near-infrared fluorescence (NIRF) imaging for PSMA expression and total macrophage densities to enable direct comparison between the two. Tumors were harvested for sectioning and additional staining to delineate M1 and M2 phenotype along with vascular density.

RESULTS

Macrophage polarization analysis of sections revealed that all xenografts were > 94% M2 phenotype, and the few M1-polarized macrophages present were confined to the periphery. Xenografts displaying the fastest growth were associated with the highest densities of macrophages while the slowest growing tumors were characterized by focal, tumor-infiltrating macrophage densities. Xenograft sections displayed a strong positive spatial relationship between macrophages, vasculature, and PSMA expression.

CONCLUSIONS

Prostate TAM disposition can be imaged ex vivo and is associated with growth characteristics of a variety of tumor subtypes regardless of PSMA or AR expression.

Design, synthesis, radiolabeling and biological evaluation of new urea-based peptides targeting prostate specific membrane antigen

Early diagnosis of Prostate cancer (PCa) plays a vital role in successful treatment increasing the survival rate of patients. Prostate Specific Membrane Antigen (PSMA) is over-expressed in almost all types of PCa. The goal of present study is to introduce new ^99mTc-labeled peptides as a PSMA inhibitor for specific detection of PCa at early stages. Based on published PSMA-targeting compounds, a set of peptides bearing the well-known Glu-Urea-Lys pharmacophore and new non-urea containing pharmacophore were designed and assessed by in silico docking studies. The selected peptides were synthesized and radiolabeled with ^99mTc. The in-vitro tests (log P, stability in normal saline and fresh human plasma, and affinity toward PSMA-positive LNCaP cell line) and in-vivo characterizations of radiopeptides (biodistribution and Single Photon Emission Computed Tomography-Computed Tomography (SPECT-CT) imaging in normal and tumour-bearing mice) were performed. The peptides 1-3 containing Glu-Urea-Lys and Glu-GABA-Asp as pharmacophores were efficiently interacted with crystal structure of PSMA and showed the highest binding energies range from -8 to -11.2 kcal/mol. Regarding the saturation binding test, ^99mTc-labeled peptide 1 had the highest binding affinity (K_d = 13.58 nM) to PSMA-positive cells. SPECT-CT imaging and biodistribution studies showed high kidneys and tumour uptake 1 h post-injection of radiopeptide 1 and 2 (%ID/g tumour = 3.62 ± 0.78 and 1.8 ± 0.32, respectively). ^99mTc-peptide 1 (Glu-urea-Lys-Gly-Ala-Asp-Naphthylalanine-HYNIC-^99mTc) exhibited the highest binding affinity, high radiochemical purity, the most stability and high specific accumulation in prostate tumour lesions. ^99mTc-peptide 1 being of comparable efficacy and pharmacokinetic properties with the well-known PET tracer (⁶⁸Ga-PSMA-11) seems to be applied as a promising SPECT imaging agent to early diagnose of PCa and consequently increase survival rate of patients.

Novel β- and γ-Amino Acid-Derived Inhibitors of Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) is an excellent biomarker for the early diagnosis of prostate cancer progression and metastasis. The most promising PSMA-targeted agents in the clinical phase are based on the Lys-urea-Glu motif, in which Lys and Glu are α-(l)-amino acids. In this study, we aimed to determine the effect of β- and γ-amino acids in the S1 pocket on the binding affinity for PSMA. We synthesized and evaluated the β- and γ-amino acid analogues with (S)- or (R)-configuration with keeping α-(l)-Glu as the S1'-binding pharmacophore. The structure-activity relationship studies identified that compound 13c, a β-amino acid analogue with (R)-configuration, exhibited the most potent PSMA inhibitory activity with an IC₅₀ value of 3.97 nM. The X-ray crystal structure of PSMA in complex with 13c provided a mechanistic basis for the stereochemical preference of PSMA, which can guide the development of future PSMA inhibitors.

Image-Guided Surgery: Are We Getting the Most Out of Small-Molecule Prostate-Specific-Membrane-Antigen-Targeted Tracers?

Expressed on virtually all prostate cancers and their metastases, the transmembrane protein prostate-specific membrane antigen (PSMA) provides a valuable target for the imaging of prostate cancer. Not only does PSMA provide a target for noninvasive diagnostic imaging, e.g., PSMA-positron emission tomography (PSMA-PET), it can also be used to guide surgical resections of PSMA-positive lesions. The latter characteristic has led to the development of a plethora of PSMA-targeted tracers, i.e., radiolabeled, fluorescent, or hybrid. With image-guided surgery applications in mind, this review discusses these compounds based on clinical need. Here, the focus is on the chemical aspects (e.g., imaging label, spacer moiety, and targeting vector) and their impact on in vitro and in vivo tracer characteristics (e.g., affinity, tumor uptake, and clearance pattern).

Auger radiopharmaceutical therapy targeting prostate-specific membrane antigen in a micrometastatic model of prostate cancer

Auger radiopharmaceutical therapy is a promising strategy for micrometastatic disease given high linear energy transfer and short range in tissues, potentially limiting normal tissue toxicities. We previously demonstrated anti-tumor efficacy of a small-molecule Auger electron emitter targeting the prostate-specific membrane antigen (PSMA), 2-[3-[1-carboxy-5-(4-[¹²⁵I]iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid), or ¹²⁵I-DCIBzL, in a mouse xenograft model. Here, we investigated the therapeutic efficacy, long-term toxicity, and biodistribution of ¹²⁵I-DCIBzL in a micrometastatic model of prostate cancer (PC).

Methods: To test the therapeutic efficacy of ¹²⁵I-DCIBzL in micrometastatic PC, we used a murine model of human metastatic PC in which PSMA+ PC3-ML cells expressing firefly luciferase were injected intravenously in NSG mice to form micrometastatic deposits. One week later, 0, 0.37, 1.85, 3.7, 18.5, 37, or 111 MBq of ¹²⁵I-DCIBzL was administered (intravenously). Metastatic tumor burden was assessed using bioluminescence imaging (BLI). Long-term toxicity was evaluated via serial weights and urinalysis of non-tumor-bearing mice over a 12-month period, as well as final necropsy.

Results: In the micrometastatic PC model, activities of 18.5 MBq ¹²⁵I-DCIBzL and above significantly delayed development of detectable metastatic disease by BLI and prolonged survival in mice. Gross metastases were detectable in control mice and those treated with 0.37-3.7 MBq ¹²⁵I-DCIBzL at a median of 2 weeks post-treatment, versus 4 weeks for those treated with 18.5-111 MBq ¹²⁵I-DCIBzL (P<0.0001 by log-rank test). Similarly, treatment with ≥18.5 MBq ¹²⁵I-DCIBzL yielded a median survival of 11 weeks, compared with 6 weeks for control mice (P<0.0001). At 12 months, there was no appreciable toxicity via weight, urinalysis, or necropsy evaluation in mice treated with any activity of ¹²⁵I-DCIBzL, which represents markedly less toxicity than the analogous PSMA-targeted α-particle emitter. Macro-to-microscale dosimetry modeling demonstrated lower absorbed dose in renal cell nuclei versus tumor cell nuclei due to lower levels of drug uptake and cellular internalization in combination with the short range of Auger emissions.

Conclusion: PSMA-targeted radiopharmaceutical therapy with the Auger emitter ¹²⁵I-DCIBzL significantly delayed development of detectable metastatic disease and improved survival in a micrometastatic model of PC, with no long-term toxicities noted at 12 months, suggesting a favorable therapeutic ratio for treatment of micrometastatic PC.

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

Synthesis and Evaluation of 99mTc-Tricabonyl Labeled Isonitrile Conjugates for Prostate-Specific Membrane Antigen (PSMA) Image

Prostate-specific membrane antigen (PSMA) is a biomarker expressed on the surface of prostate cancer (PCa). In an effort to improve the detection and treatment of PCa, small urea-based PSMA inhibitors have been studied extensively. In the present study, we aimed to develop 99mTc-tricabonyl labeled urea-based PSMA conjugates containing isonitrile (CN-R)-coordinating ligands ([99mTc]Tc-15 and [99mTc]Tc-16). Both the PSMA conjugates were obtained at high radiochemical efficiency (≥98.5%). High in vitro binding affinity was observed for [99mTc]Tc-15 and [99mTc]Tc-16 (Kd = 5.5 and 0.2 nM, respectively) in PSMA-expressing 22Rv1 cells. Tumor xenografts were conducted using 22Rv1 cells and rapid accumulation of [99mTc]Tc-16 (1.87 ± 0.11% ID/g) was observed at 1 h post-injection, which subsequently increased to (2.83 ± 0.26% ID/g) at 4 h post-injection. However, [99mTc]Tc-15 showed moderate tumor uptake (1.48 ± 0.18% ID/g), which decreased at 4 h post-injection (0.81 ± 0.09% ID/g). [99mTc]Tc-16 was excreted from non-targeted tissues with high tumor-to-blood (17:1) and tumor-to-muscle ratio (41:1) at 4 h post-injection at approximately 4 times higher levels than [99mTc]Tc-15. Uptakes of [99mTc]Tc-15 and [99mTc]Tc-16 to PSMA-expressing tumor and tissues were significantly blocked by co-injection of 2-(Phosphonomethyl)-pentandioic acid (2-PMPA), suggesting that their uptakes are mediated by PSMA specifically. Whole-body single photon emission computed tomography imaging of [99mTc]Tc-16 verified the ex vivo biodistribution results and demonstrated clear visualization of tumors and tissues expressing PSMA compared to [99mTc]Tc-15. In conclusion, using [99mTc]Tc-16 rather than [99mTc]Tc-15 may be the preferable because of its relatively high tumor uptake and retention.