Prostate-specific Membrane Antigen Inhibitor Research Articles

Synthesis, chemical and biochemical characterization of Lu2O3-iPSMA nanoparticles activated by neutron irradiation.

Among the nanomaterials, rare sesquioxides (lanthanide oxides such as Lu2O3) are of interest due to their adequate thermal conductivity, excellent chemical stability, and high light output. The prostate-specific membrane antigen (PSMA) is an integral multifunctional protein overexpressed in various types of cancer cells. The radiolabeled PSMA inhibitor peptides (iPSMA) have demonstrated their usefulness as specific probes in the treatment and detection of a wide variety of neoplasms, mainly due to their high in vivo recognition by the PSMA protein. The objective of this research was to synthesize Lu2O3-iPSMA nanoparticles (NPs) and characterize their physicochemical properties before and after neutron activation, as well as to assess their biodistribution profile and in vitro potential to target cells overexpressing PSMA. The Lu2O3 NPs were synthesized by the precipitation-calcination method and conjugated to the iPSMA peptide using DOTA (1,4,7,10-tetraazocyclodecane-N,N',N″,N‴-tetraacetic acid) as a linking agent. Results of the physicochemical characterization by FT-IR and UV-Vis spectroscopies, SEM, TEM, DLS, HRTEM, SAED, DSC-TGA, and X-ray diffraction indicated the formation of Lu2O3-iPSMA NPs (diameter of 29.98±9.07nm), which were not affected in their physicochemical properties after neutron activation. 177Lu2O3-iPSMA NPs showed high affinity (Kd=5.7±1.9nM) for the PSMA protein, evaluated by the saturation assay on HepG2 hepatocellular carcinoma cells (PSMA-positive). The biodistribution profile of the nanosystem in healthy mice showed the main uptake in the liver. After irradiation, radioactive Lu2O3-iPSMA NPs exhibited radioluminescent properties, making the in vivo acquisition of their biodistribution, via optical imaging, possible. The results obtained from this research validate the execution of additional preclinical studies with the objective of evaluating the potential of the 177Lu2O3-iPSMA NPs for the targeted radiotherapy and in vivo imaging of tumors overexpressing the PSMA protein.

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 (t1/2 = 68 min) or zirconium-89 (t1/2 = 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.

Heterodimeric Radiotracer Targeting PSMA and GRPR for Imaging of Prostate Cancer-Optimization of the Affinity towards PSMA by Linker Modification in Murine Model.

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [111In]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [111In]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.

Combined Application of Albumin-Binding [177Lu]Lu-PSMA-ALB-56 and Fast-Cleared PSMA Inhibitors: Optimization of the Pharmacokinetics.

The strategy of using radioligands for targeting the prostate-specific membrane antigen (PSMA) revealed to be promising for the treatment of metastatic castration-resistant prostate cancer (mCRPC). Recently developed albumin-binding PSMA radioligands showed a remarkably increased tumor uptake because of the enhanced blood circulation, but higher accumulation of activity was also observed in off-target organs and tissues. The aim of this study was to investigate the option of using fast-cleared, small-molecular-weight PSMA inhibitors (PSMA-11, 2-PMPA, and ZJ-43) to reduce the kidney uptake of [177Lu]Lu-PSMA-ALB-56, a previously developed albumin-binding PSMA radioligand. Dual-isotope SPECT/CT imaging was performed with tumor-bearing mice coinjected with [177Lu]Lu-PSMA-ALB-56 and a 2.5-fold molar excess of [67Ga]Ga-PSMA-11. At early timepoints after injection, the high renal uptake of [67Ga]Ga-PSMA-11 reduced the accumulation of [177Lu]Lu-PSMA-ALB-56 in the kidneys substantially, whereas the tumor uptake of [177Lu]Lu-PSMA-ALB-56 was only slightly affected. These findings were confirmed in biodistribution studies, which revealed reduced uptake of [177Lu]Lu-PSMA-ALB-56 in the kidneys due to coadministered unlabeled PSMA-11 (9.1 ± 0.8% IA/g vs 46 ± 11% IA/g; 1 h p.i.). The tumor uptake of [177Lu]Lu-PSMA-ALB-56 was almost the same at 1 h p.i., irrespective of whether or not PSMA-11 was coinjected (24 ± 6% IA/g vs 27 ± 7% IA/g). The application of [177Lu]Lu-PSMA-ALB-56 with 2-PMPA or ZJ-43, respectively, showed similar results in biodistribution studies. Among all three tested PSMA inhibitors, 2-PMPA, applied at a 2.5-fold molar excess relative to [177Lu]Lu-PSMA-ALB-56, was most effective to improve the tumor-to-kidney ratios over the first hours after injection of [177Lu]Lu-PSMA-ALB-56. The concept of using a PSMA inhibitor together with [177Lu]Lu-PSMA-ALB-56 appears promising in view of a clinical translation of this and possibly other long-circulating PSMA radioligands.

Recent progress of positron radionuclide labeled small molecule inhibitors of prostate specific membrane antigen for PET/CT imaging of prostate cancer

Prostate cancer is the most common tumor of the urinary system, and its mortality rate is second only to lung cancer. With the specific and high expression on the surface of prostate cancer cells, prostate-specific membrane antigen (PSMA) has been an ideal theranostic target of prostate cancer with great clinical significance and research value. Positron emission tomography/computed tomography (PET/CT), a new modality of molecular imaging combining functional metabolic information and anatomical structure, provides high diagnostic performance for cancer detection. This paper mainly reviewed recent progress of PSMA inhibitors labeled by positron-emitting radionuclides for early diagnosis, preoperative staging, response assessment, restaging and metastasis detection of prostate cancer.

Preclinical Evaluation of a High-Affinity Sarcophagine-Containing PSMA Ligand for 64Cu/67Cu-Based Theranostics in Prostate Cancer.

The application of small molecules targeting prostate-specific membrane antigen (PSMA) has emerged as a highly promising clinical strategy for visualization and treatment of prostate cancer. Ligands that integrate the ability to both quantify the distribution of radioactivity and treat disease through the use of a matched pair of radionuclides have particular value in clinical and regulatory settings. In this study, we describe the development and preclinical evaluation of RPS-085, a ligand that binds PSMA and serum albumin and exploits the 64/67Cu radionuclide pair for prostate cancer theranostics. RPS-085 was synthesized by conjugation of a PSMA-targeting moiety, an Nε-(2-(4-iodophenyl)acetyl)lysine albumin binding group, and a bifunctionalized MeCOSar chelator. The IC50 of the metal-free RPS-085 was determined in a competitive binding assay. The affinity for human serum albumin of the radiolabeled compound was determined by high-performance affinity chromatography. Radiolabeling was performed in NH4OAc buffer at 25 °C. The stability of the radiolabeled compounds was assessed in vitro and in vivo. The biodistribution of [64/67Cu]Cu-RPS-085 was determined following intravenous administration to male BALB/c mice bearing LNCaP tumor xenografts. The radiochemical yields of [64/67Cu]Cu-RPS-085 were nearly quantitative after 20 min. The metal-free complex is a potent inhibitor of PSMA (IC50 = 29 ± 2 nM), and the radiolabeled compound has moderate affinity for human serum albumin (Kd = 9.9 ± 1.7 μM). Accumulation of the tracer in mice was primarily evident in tumor and kidneys. Activity in all other tissues, including blood, was negligible, and the radiolabeled compounds demonstrated high stability in vitro and in vivo. Tumor activity reached a maximum at 4 h post injection (p.i.) and cleared gradually over a period of 96 h. By contrast, activity in the kidney cleared rapidly from 4 to 24 h p.i. As a consequence, by 24 h p.i., the tumor-to-kidney ratio exceeds 2, and the predicted dose to tumors is significantly greater than the dose to kidneys. [64Cu]Cu-RPS-085 combines rapid tissue distribution and clearance with prolonged retention in LNCaP tumor xenografts. The pharmacokinetics should enable radioligand therapy using [67Cu]Cu-RPS-085. By virtue of its rapid kidney clearance, the therapeutic index of [67Cu]Cu-RPS-085 likely compares favorably to its parent structure, [177Lu]Lu-RPS-063, a highly avid PSMA-targeting compound. On this basis, [64/67Cu]Cu-RPS-085 show great promise as PSMA-targeting theranostic ligands for prostate cancer imaging and therapy.

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 -11.2kcal/mol. Regarding the saturation binding test, 99mTc-labeled peptide 1 had the highest binding affinity (Kd=13.58nM) to PSMA-positive cells. SPECT-CT imaging and biodistribution studies showed high kidneys and tumour uptake 1h 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.

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.

Evaluation of Radiation dosimetry of 99mTc-HYNIC-PSMA and imaging in prostate cancer

This study aims to evaluate the radiation dosimetry of a new technetium-99m‒labelled small-molecule inhibitor of prostate-specific membrane antigen (HYNIC-Glu-Urea-A, 99mTc-HYNIC-PSMA) and its feasibility as a tumor-imaging agent in prostate cancer (PCa) patients. A total of 15 PCa patients were enrolled in this study. For the dosimetry study, 5 PCa patients received whole-body planar scans at 0.5 h, 1 h, 2 h, 4 h and 8 h after 99mTc-HYNIC-PSMA injection. The Dosimetry Toolkit (GE, Milwaukee) was used to process the data and segment the organs in the SPECT/CT images, which were then projected onto planar images. The organ-specific absorbed doses, total-body absorbed doses and 99mTc-HYNIC-PSMA effective doses of patients were calculated using OLINDA/EXM 1.1 software. Whole-body SPECT/CT images were also acquired from additional 10 prostate patients to investigate the feasibility of 99mTc-HYNIC-PSMA for imaging tumors by calculating the ratio of tumor-to-background tracer uptake at 2 h after 740 MBq administration. The total-body absorbed dose was 1.54E-03 ± 2.43E-04 mGy/MBq, and the effective dose was 3.72E-03 ± 4.5E-04 mSv/MBq. Compared to published studies of other similar PSMA tracers and 99mTc-targeted conventional tracers, the absorbed doses of 99mTc-HYNIC-PSMA in all organs showed that it could be used safely in the human body. In addition, 99mTc-HYNIC-PSMA showed high tracer uptake (with a tumor-to-background ratio of 9.42 ± 2.62) in the malignant lesions of PCa patients, making it a promising radiopharmaceutical imaging method for site-specific management of PCa.

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.

Prospective comparison of 68Ga-RM2 PET/CT and 68Ga-PSMA-617 PET/CT for initial staging of prostate cancer.

21 Background: PSMA ( Prostate Specific Membrane Antigen) and GRP-R ( Gastrin Releasing Peptide Receptor) are overexpressed on prostatic cancer cells and can be targeted with a PSMA inhibitor such as 68Ga-PSMA-617 (Liu 2018) or a GRP-R antagonist such as 68Ga-RM2 (Touijer 2019). We aimed to compare 68Ga-PSMA-617 PET/CT and 68Ga-RM2 PET/CT at initial staging of prostate cancer. Methods: We prospectively enrolled 10 treatment-naïve patients with biopsy-confirmed prostate cancer. Fifteen lesions were found (two with Gleason score (GS) 6; seven GS 7(3+4), two GS 7(4+3) and four GS ≥ 8). 68Ga-PSMA-617 PET/CT and 68Ga-RM2 PET/CT were randomly performed before prostatectomy with imaging at 1 and 2 hours after injection (2MBq/kg). Manual (MS) and automatic (AS) segmentation were performed on each PET imaging and then compared to histology (standard-of- truth). Prostatectomy samples were analyzed by an experimented pathologist who identified and staged each lesion according to GS. Each prostatectomy slice available was scanned for comparison between PET and histologic segmentation. Uptake intensity (SUVmax) and signal-to-noise ratio (SNR) relative to non-lesional prostate gland parenchyma were analyzed. Per-lesion analysis was performed on final GS (prostatectomy). Results: Using AS, 68Ga-RM2 PET/CT showed better sensitivity (Se) than 68Ga-PSMA-617 PET/CT: (0.85 vs 0.75) and better specificity (Sp) (0.89 vs 0.60). MS found similar Se (0.94 vs 0.93) but better Sp (0.89 vs 0.80) with 68Ga-RM2 PET/CT. SNR of 68Ga-RM2 PET/CT was high whatever the GS was; while 68Ga-PSMA-617 PET/CT exhibited higher SNR in high GS (p = 0.044). Finally, in low GS, SNR of 68Ga-RM2 PET/CT was significantly higher than 68Ga-PSMA-617 PET/CT (p = 0.004). Conclusions: GRP-R-targeting and PSMA-targeting have been previously compared in patients with biochemically recurrence (Minamimoto 2016) but not at primary staging. Our initial results, in this indication, showed great diagnostic performance of 68Ga-RM2 PET/CT compared to 68Ga-PSMA-617 PET/CT to characterize low Gleason score tumors. 68Ga-PSMA-617 PET/CT is promising for initial staging of high Gleason score tumors. These results need to be confirmed in a larger study.

Development of 18F-Fluoroglycosylated PSMA-Ligands with Improved Renal Clearance Behavior

The prostate-specific membrane antigen (PSMA) is a type II transmembrane glycoprotein that is highly expressed in the malignant human prostate epithelium. Therefore, PSMA has emerged as a very attractive target for developing radiopharmaceuticals for the diagnosis, e.g., by positron emission tomography (PET) imaging, and radiotherapy of prostate cancer. The aim of this study was to develop 18F-labeled PSMA ligands bearing different 18F-glycosyl moieties to study the effect on the in vivo clearance behavior of radiotracers in addition to their tumor binding ability. Therefore, we applied click chemistry-based 18F-fluoroglcosylation using 2-deoxy-2-[18F]fluoroglucosyl azide or 6-deoxy-6-[18F]fluoroglucosyl azide as prosthetic groups for the radiosynthesis of the 18F-fluoroglycosylated glutamate-urea-lysine-based PSMA inhibitors 2-[18F]FGlc-PSMA ([18F]7) and 6-[18F]FGlc-PSMA ([18F]8). The PSMA inhibitory potencies were determined by competitive radioligand binding assays using 99mTc-MIP-1404 and PSMA-expressing PC-3 PIP cells, revealing moderate PSMA inhibitory potencies for [18F]7 (IC50 = 234 nM) and [18F]8 (IC50 = 59 nM). Biodistribution and small-animal PET studies were performed using PSMA-positive PC-3 PIP and PSMA-negative PC-3 tumor-bearing nude mice. PSMA inhibitors [18F]7 and [18F]8 were obtained in high radioactivity yields of 19-22% (nondecay-corrected, referred to [18F]fluoride) and with molar activities of 71-136 GBq/μmol. In the biodistribution studies, the uptake levels of [18F]7 and [18F]8 in PC-3 PIP tumors were 13 ± 3%ID/g and 6 ± 5%ID/g at 60 min p.i., respectively. PSMA-negative PC-3 tumors and all other tissues had negligible low uptake values. Interestingly, [18F]7 had high uptake in the kidneys, with remarkable retention from 30 to 60 min p.i. (74 to 72%ID/g). In contrast, [18F]8 revealed a low uptake of 7.5%ID/g in the kidneys at 30 min p.i. and was rapidly cleared through the kidney (0.9%ID/g at 120 min p.i.). In direct comparison to a 68Ga-PSMA-11 PET scan of the same mouse, [18F]7 and [18F]8 showed 2- to 3-fold higher uptake values in PC-3 PIP tumors. Both radiotracers were solely cleared via the kidneys and not via the hepatobiliary pathway. The regional kidney distribution pattern of the tracers in the kidneys revealed that 68Ga-PSMA-11 and 2-[18F]FGlc-PSMA([18F]7) mainly accumulated in the cortex of the kidneys, whereas 6-[18F]FGlc-PSMA([18F]8) showed a 10-fold lower kidney uptake with accumulation in the inner medulla or pelvis of the kidneys. Overall, the developed 6-fluoroglucosyl derivative [18F]8, with its considerably low kidney uptake and fast clearance, demonstrated high uptake in PSMA-positive tumors in vivo. This candidate could, therefore, be valuable for translation into the clinic.

Novel multifunctional 18F-labelled PET tracer with prostate-specific membrane antigen-targeting and hypoxia-sensitive moieties

Prostate cancer is one of the most frequently found cancers in men worldwide. Prostate-specific membrane antigen (PSMA) is typically highly expressed in prostate cancer, and the Glu-Urea-Lys (GUL) structure has recently received considerable attention as a key unit of PSMA-targeting agents. Additionally, one of the common characteristics of many solid tumors, such as prostate cancer, is hypoxia. In this study, novel multifunctional PSMA inhibitors containing a PSMA-targeting moiety either with or without a hypoxia-sensitive moiety (18F-PEG3-ADIBOT-2NI-GUL and 18F-PEG3-ADIBOT-GUL, respectively; ADIBOT: azadibenzocyclooctatriazole, 2NI: 2-nitroimidazole) were designed and synthesized, and their feasibility as PET tracers for prostate cancer imaging studies was examined. The compounds labelled with 18F via the copper-free click reaction were stable in human serum and showed nanomolar binding affinities in in vitro PSMA binding assays. Micro-PET and biodistribution studies indicate that both 18F-labelled inhibitors successfully accumulated in prostate cancer regions, and 18F-PEG3-ADIBOT-2NI-GUL showed a 2-fold higher tumor-to-total non-target organ ratio than that of 18F-PEG3-ADIBOT-GUL, suggesting that the synergistic effects of the PSMA-targeting GUL moiety and the hypoxia-sensitive 2-nitroimidazole moiety can increase tumor uptake of the novel PET tracers in prostate cancer. These findings suggest that this novel multifunctional PET tracer with an 18F-labelled PSMA inhibitor and a 2-nitroimidazole moiety is a potent candidate to provide better diagnosis of prostate cancer via PET imaging studies.

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.

Design and synthesis of a novel BODIPY-labeled PSMA inhibitor

Prostate-specific membrane antigen (PSMA) is a zinc-bound metalloprotease which is highly expressed in metastatic prostate cancer. It has been considered an excellent target protein for prostate cancer imaging and targeted therapy because it is a membrane protein and its active site is located in the extracellular region. We successfully synthesized and evaluated a novel PSMA ligand conjugated with BODIPY650/665. Compound 1 showed strong PSMA-inhibitory activity and selective uptake into PSMA-expressing tumors. Compound 1 has the potential to be utilized as a near infrared (NIR) optical imaging probe targeting PSMA-expressing cancers.

Looking for Drugs in All the Wrong Places: Use of GCPII Inhibitors Outside the Brain.

In tribute to our friend and colleague Michael Robinson, we review his involvement in the identification, characterization and localization of the metallopeptidase glutamate carboxypeptidase II (GCPII), originally called NAALADase. While Mike was characterizing NAALADase in the brain, the protein was independently identified by other laboratories in human prostate where it was termed prostate specific membrane antigen (PSMA) and in the intestines where it was named Folate Hydrolase 1 (FOLH1). It was almost a decade to establish that NAALADase, PSMA, and FOLH1 are encoded by the same gene. The enzyme has emerged as a therapeutic target outside of the brain, with the most notable progress made in the treatment of prostate cancer and inflammatory bowel disease (IBD). PSMA-PET imaging with high affinity ligands is proving useful for the clinical diagnosis and staging of prostate cancer. A molecular radiotherapy based on similar ligands is in trials for metastatic castration-resistant prostate cancer. New PSMA inhibitor prodrugs that preferentially block kidney and salivary gland versus prostate tumor enzyme may improve the clinical safety of this radiotherapy. The wide clinical use of PSMA-PET imaging in prostate cancer has coincidentally led to clinical documentation of GCPII upregulation in a wide variety of tumors and inflammatory diseases, likely associated with angiogenesis. In IBD, expression of the FOLH1 gene that codes for GCPII is strongly upregulated, as is the enzymatic activity in diseased patient biopsies. In animal models of IBD, GCPII inhibitors show substantial efficacy, suggesting potential theranostic use of GCPII ligands for IBD.

Synthesis and Initial Biological Evaluation of Boron-Containing Prostate-Specific Membrane Antigen Ligands for Treatment of Prostate Cancer Using Boron Neutron Capture Therapy.

Boron neutron capture therapy (BNCT) is a therapeutic modality which has been used for the treatment of cancers, including brain and head and neck tumors. For effective treatment via BNCT, efficient and selective delivery of a high boron dose to cancer cells is needed. Prostate-specific membrane antigen (PSMA) is a target for prostate cancer imaging and drug delivery. In this study, we conjugated boronic acid or carborane functional groups to a well-established PSMA inhibitor scaffold to deliver boron to prostate cancer cells and prostate tumor xenograft models. Eight boron-containing PSMA inhibitors were synthesized. All of these compounds showed a strong binding affinity to PSMA in a competition radioligand binding assay (IC50 from 555.7 to 20.3 nM). Three selected compounds 1a, 1d, and 1f were administered to mice, and their in vivo blocking of 68Ga-PSMA-11 uptake was demonstrated through a positron emission tomography (PET) imaging and biodistribution experiment. Biodistribution analysis demonstrated boron uptake of 4-7 μg/g in 22Rv1 prostate xenograft tumors and similar tumor/muscle ratios compared to the ratio for the most commonly used BNCT compound, 4-borono-l-phenylalanine (BPA). Taken together, these data suggest a potential role for PSMA targeted BNCT agents in prostate cancer therapy following suitable optimization.

Preclinical Dosimetry, Imaging, and Targeted Radionuclide Therapy Studies of Lu-177-Labeled Albumin-Binding, PSMA-Targeted CTT1403.

Prostate-specific membrane antigen (PSMA) continues to be the hallmark biomarker for prostate cancer as it is expressed on nearly all prostatic tumors. In addition, increased PSMA expression correlates with castration resistance and progression to the metastatic stage of the disease. Recently, we combined both an albumin-binding motif and an irreversible PSMA inhibitor to develop the novel PSMA-targeted radiotherapeutic agent, CTT1403. This molecule was novel in the field of PSMA-targeted agents as its key motifs resulted in extended blood circulation time and tumor uptake, rapid and extensive internalization into PSMA+ cells, and promising therapeutic efficacy. The objective of this study was to perform IND-enabling translational studies on CTT1403 in rodent models. A dose optimization study was performed in PC3-PIP (PSMA+) tumor-bearing mice. Treatment groups were randomly selected to receive one to three 14-MBq injections of CTT1403. Control groups included (1) saline, (2) non-radioactive [175Lu]CTT1403, or (3) two injections of 14MBq CTT1751, a Lu-177-labeled non-targeted albumin-binding moiety. Tumor growth was monitored up to 120days. Small-animal single photon emission tomography/X-ray computed tomography imaging was performed with CTT1403 and CTT1751 in PC3-PIP tumor-bearing mice to visualize infiltration of the Lu-177-labeled agent into the tumor. In preparation for a first-in-human study, human absorbed doses were estimated based on rat biodistribution out to 5weeks to determine a safe CTT1403 therapy dose in humans. Two to 3 injections of 14MBq CTT1403 yielded significant tumor growth inhibition and increased survival compared with all control groups and mice receiving 1 injection of 14MBq CTT1403. Five of 12 mice receiving 2 or 3 injections of CTT1403 survived to the 120-day post-treatment study endpoint. Dosimetry identified the kidneys as the dose-limiting organ, with an equivalent dose of 5.18mSv/MBq, resulting in a planned maximum dose of 4.4GBq for phase 1 clinical trials. The preclinical efficacy and dosimetry of CTT1403 suggest that this agent has significant potential to be safe and effective in humans.

Conjugates of Prostate-Specific Membrane Antigen Ligands with Antitumor Drugs

Conjugates for targeted delivery that are based on low-molecular-mass prostate specific membrane antigen (PSMA) inhibitors are widely used and developed because of the deficiencies of existing methods for treating and diagnosing prostate cancer. The major classes of low-molecular-mass PSMA inhibitors, drug classes, and their mechanisms of action are discussed. Therapeutic conjugates for targeted delivery that are based on them are analyzed. Structural features of the linker that affect the biological activity and selectivity are identified.

Radiopharmaceuticals targeting PSMA for imaging and/or therapy

Targeting the Prostate Specific Membrane Antigen (PSMA) is becoming increasingly more important for the management of prostate cancer patients at various stages. This review article describes selected radiolabelled PSMA inhibitors with optimized radiopharmaceutical properties for imaging and/or therapy.