Albumin-Binding PSMA Ligands: Optimization of the Tissue Distribution Profile

Development of radioligands with an albumin-binding moiety of 4-(P-Iodophenyl) butyric acid for theranostic applications

The rapid clearance of imaging probes from blood circulation is beneficial for receptor imaging, as it minimizes non-target tissue exposure and improves tumor-to-background contrast. However, this rapid clearance can hinder radioligand therapy by limiting tumor uptake of radiolabeled compounds. An optimal blood half-life is crucial, as it enhances the uptake of radiolabeled compounds in targets, improving tumor uptake and retention of small molecule drugs, and thus therapeutic outcomes. To address this, strategies to extend blood half-life have been developed, with the addition of an albumin-binding moiety (ABM) being particularly effective. Among these, 4-(p-iodophenyl)butyric acid (IPBA) has emerged as a versatile ABM for radiopharmaceutical design. IPBA conjugation has successfully enhanced tissue distribution profiles across various cancer types. This review highlights recent progress in the design, radiosynthesis, and application of IPBA-based small molecular radioligands, providing insights for future clinical development of IPBA-based radiopharmaceuticals.

Therapeutic evaluation of [212Pb]Pb-AB001 and [177Lu]Lu-PSMA-617 in a mouse model of disseminated prostate cancer

BACKGROUND

Metastatic castration-resistant prostate cancer (mCRPC) frequently leads to bone and soft tissue metastases, leading to poor prognosis. The beta-emitting radioligand [177Lu]Lu-PSMA-617 targets the prostate-specific membrane antigen (PSMA) and may be less efficient against micrometastatic disease. The alpha-emitting radioligand [212Pb]Pb-AB001 could offer enhanced treatment by delivering high energy over a short range. This study compared the efficacy of [212Pb]Pb-AB001 and [177Lu]Lu-PSMA-617 in a mouse model of disseminated prostate cancer.

METHODS

Binding and internalisation of radioligands were evaluated in PC-3 PIP-luc cells. A mouse model was established by intracardiac injection of these cells. Treatments with 0.24‒1.0 MBq [212Pb]Pb-AB001 or 22‒66 MBq [177Lu]Lu-PSMA-617 were initiated 7 d post-cell inoculation. Metastatic burden was measured using bioluminescence imaging, and PSMA-targeted uptake was determined with [18F]F-PSMA-1007 µPET/µCT. Gamma-autoradiography evaluated [212Pb]Pb-AB001 distribution, and bone metastases were identified by radiography.

RESULTS

Both radioligands displayed comparable in vitro binding. In vivo studies revealed metastatic formation in clinically relevant organs. µPET/µCT demonstrated increased [18F]F-PSMA-1007 uptake in metastases, matching the bioluminescence imaging results. Focal [212Pb]Pb-AB001 distribution in the metastatic xenograft indicated heterogeneously distributed micrometastases in the organs. A median survival up to 47 d was achieved with [212Pb]Pb-AB001, compared to 25 d for controls and 27 d for [177Lu]Lu-PSMA-617. An activity-dependent reduction in bone metastases was observed for [177Lu]Lu-PSMA-617, while no bone lesions were detected in [212Pb]Pb-AB001-treated mice.

CONCLUSION

[212Pb]Pb-AB001 showed significant efficacy against micrometastases and advantages over [177Lu]Lu-PSMA-617 in preventing or treating early bone metastases for the investigated injected activities. This implies clinical potential for treating mCRPC, including patients at risk of early metastatic disease, but further studies including dosimetry and toxicity analyses are required with regards to activity levels.

Development of Novel Gastrin-Releasing Peptide Receptor-Targeted Radioligand with Albumin Binder to Improve Accumulation in Tumor

Gastrin-releasing peptide receptor (GRPR) is a promising target for cancer radiotheranostics combining nuclear imaging with targeted radionuclide therapy. Improving the accumulation of radioligands in tumors by introducing an albumin binder (ALB) is useful to promote the efficacy of radiotheranostics. In this study, we designed and synthesized a novel GRPR-targeted radioligand [111In]In-AMTG-DA1 containing an ALB moiety to improve tumor accumulation. [111In]In-AMTG-DA1 showed marked binding ability to albumin, high affinity for GRPR, and high-level stability in vitro. In biodistribution studies, the tumor accumulation of [111In]In-AMTG-DA1 was much higher than that of the control ligand without an ALB moiety. The introduction of ALB increased the tumor area under the curve (AUC) value of [111In]In-AMTG-DA1 by 3.5 times. In a single-photon emission computed tomography (SPECT) study, [111In]In-AMTG-DA1 visualized a GRPR-expressing tumor clearly at 24 h postinjection. Our findings suggest the favorable pharmacokinetics of [111In]In-AMTG-DA1 as a GRPR-targeted radioligand exhibiting a high-level accumulation in tumors.

Design and Preclinical Evaluation of Novel uPAR-Targeting Radiopeptides Modified with an Albumin-Binding Entity

Several studies have focused on the development and application of radiolabeled DOTA-AE105 for targeting the urokinase-type plasminogen activator receptor (uPAR), which is expressed on various cancer types. The aim of this project was to design and evaluate novel uPAR-targeting radiopeptides with improved pharmacokinetic properties in view of their therapeutic application. Five peptides (uPAR-01, uPAR-02, uPAR-03, uPAR-04, and uPAR-05) were synthesized based on the AE105 peptide backbone, a DOTA chelator, and the 4-(p-iodophenyl)butanoate moiety as an albumin binder. The peptides were obtained in 20-29 synthetic steps using solid-phase peptide synthesis with a 6-34% overall yield. In saline, the 177Lu-labeled peptides (100 MBq/nmol) were stable (>93% intact radiopeptides) in the presence of l-ascorbic acid over 24 h. The new radiopeptides were also stable (>98% intact radiopeptides) in mouse and human blood plasma, while only ∼13% of [177Lu]Lu-DOTA-AE105 was intact after a 4 h incubation period. The uPAR-binding affinities (KD values) determined with uPAR-transfected human embryonic kidney cells (HEK-uPAR) ranged from 10 to 57 nM and were, thus, similar to that of [177Lu]Lu-DOTA-AE105 (KD: 20 ± 1 nM). Compared to [177Lu]Lu-DOTA-AE105, the radiopeptides showed the anticipated increased binding affinity to plasma proteins both in mouse (31- to 104-fold) and human blood plasma (43- to 136-fold). The tissue distribution of the novel radiopeptides in nude mice bearing HEK-uPAR xenografts showed substantial activity retention in the blood (12-16% IA/g and 4.5-13% IA/g at 4 and 24 h p.i., respectively), while [177Lu]Lu-DOTA-AE105 was rapidly cleared (<0.1% IA/g at 4 h p.i.). As a result, the accumulation of the new radiopeptides in HEK-uPAR xenografts (3.6-11% and 3.1-10% IA/g at 4 and 24 h p.i., respectively) was increased in comparison to that of [177Lu]Lu-DOTA-AE105 (<1% IA/g at 4 h p.i.). Importantly, the metabolic stability of the new radiopeptides in mice was enhanced as compared to that of [177Lu]Lu-DOTA-AE105. [177Lu]Lu-uPAR-02 showed the most promising tissue distribution profile with over 10-fold higher activity retention in the HEK-uPAR xenograft than observed after injection of [177Lu]Lu-DOTA-AE105. As a result, the xenograft-to-kidney ratio of [177Lu]Lu-uPAR-02 was >3-fold higher than that of [177Lu]Lu-DOTA-AE105.

Two Targets, One Mission: Heterobivalent Metal-Based Radiopharmaceuticals for Prostate Cancer Imaging and Therapy

Prostate cancer (PCa) is a significant healthcare challenge, associated with considerable mortality and morbidity among men, particularly in developed countries. PCa mortality and morbidity are primarily related to its most advanced form, metastatic castration-resistant PCa (mCRPC), for which there is presently no cure. Therefore, novel therapeutic approaches to increase mCRPC survival are critically needed. Due to PCa tumor heterogeneity and a complex tumor microenvironment, the efficacy of single-target radiopharmaceuticals, such as the Food and Drug Administration-approved [177Lu]Lu-PSMA-617, is currently under reassessment. The design and development of PCa dual-target radiopharmaceuticals have garnered considerable attention, due to their benefits over single-target counterparts, namely increased therapeutic specificity and efficacy, as well as the ability to overcome the challenge of inconsistent tumor visualization caused by variable receptor expression across diverse lesions, thereby enabling more comprehensive imaging. Several PCa biomarkers are currently being investigated as potential targets for dual-target radiopharmaceuticals, including prostate-specific membrane antigen, gastrin-releasing peptide receptor, integrin αvβ3 receptor, fibroblast activation protein, sigma-1 receptor, as well as albumin, the radiosensitive cell nucleus, and mitochondria. This review explores recent advancements in heterobivalent metal-based radiopharmaceuticals for dual targeting in PCa, highlighting their significance in theranostic and personalized medicine.

Preclinical comparison of (radio)lanthanides using mass spectrometry and nuclear imaging techniques: biodistribution of lanthanide-based tumor-targeting agents and lanthanides in ionic form

PURPOSE

With the growing interest in exploring radiolanthanides for nuclear medicine applications, the question arises as to whether they are generally interchangeable without affecting a biomolecule's pharmacokinetic properties. The goal of this study was to investigate similarities and differences of four (radio)lanthanides simultaneously applied as complexes of biomolecules or in ionic form.

METHODS

Inductively coupled plasma mass spectrometry (ICP-MS) was employed for the simultaneous detection of four lanthanides (Ln = lutetium, terbium, gadolinium and europium) in biological samples. In vitro tumor cell uptake and in vivo biodistribution studies were performed with Ln-DOTATATE, Ln-DOTA-LM3, Ln-PSMA-617 and Ln-OxFol-1. AR42J cells, PC-3 PIP cells and KB cells expressing the somatostatin receptor, the prostate-specific membrane antigen and the folate receptor, respectively, were used in vitro as well as to obtain the respective tumor mouse models for in vivo studies. The distribution of lanthanides in ionic form was investigated in immunocompetent mice. Dual-isotope SPECT/CT imaging studies were performed with mice administered with the radiolabeled biomolecules or chloride salts of lutetium-177 and terbium-161.

RESULTS

Similar in vitro cell uptake was observed for all four lanthanide complexes of each biomolecule into the respective tumor cell lines. AR42J tumor uptake of Ln-DOTATATE and Ln-DOTA-LM3 in mice showed similar values for all lanthanide complexes (3.8‒5.1% ID/g and 4.5‒5.0% ID/g; 1 h p.i., respectively). Accumulation of Ln-PSMA-617 in PC-3 PIP tumors (24-25% ID/g; 1 h p.i.) and of Ln-OxFol-1 in KB tumors (28-31% ID/g; 24 h p.i.) were also equal for the four lanthanide complexes of each biomolecule. After injection of lanthanide chloride salts (LnCl3; Ln = natLu, natTb, natGd, natEu), the liver uptake was different for each metal (~ 12% ID/g, ~ 22% ID/g, ~ 31% ID/g and ~ 37% ID/g; 24 h p.i., respectively) which could be ascribed to the radii of the respective lanthanide ions. In the bones, accumulation was considerably higher for lutetium than for other lanthanides (25 ± 5% ID/g vs. 14‒15% ID/g; 24 h p.i.). These data were confirmed visually by 177Lu/161Tb-based dual-isotope SPECT/CT images.

CONCLUSIONS

The presented study confirmed similar properties of Ln-complexes, suggesting that lutetium-177 can be replaced by other radiolanthanides, most probably without affecting the tissue distribution profile of the resultant radiopharmaceuticals. On the other hand, the different radii of the lanthanide ions affected their uptake and resorption mechanisms in liver and bones when injected in uncomplexed form.

Synthesis and Imaging of Novel CDK19-Targeted Tracers Incorporating an Albumin-Binding Moiety

Cyclin-dependent kinase 19 (CDK19) is a potential target for the diagnosis and treatment of prostate cancer. We have previously studied a series of CDK19-targeted PET tracers, but in-depth drug optimization is needed to improve the physiochemical properties of such large and polar tracers. The albumin strategy has received widespread attention in recent years, and we synthesized 68Ga-IRM-14a and 68Ga-IRM-14b based on the strategy. After in vivo imaging studies in mice, we found that introducing albumin moiety will significantly change the physicochemical properties of existing large polarity tracers, thereby increasing tissue uptake and retention, which is beneficial for future treatment. In short, the albumin strategy will be an important strategy in the field of radiopharmaceutical optimization.

Novel Radiotheranostic Ligands Targeting Prostate-Specific Membrane Antigen Based on Dual Linker Approach

Radiotheranostics using prostate-specific membrane antigen (PSMA)-targeting radioligands offers precision medicine by performing radionuclide therapy based on results of diagnosis. Albumin binder (ALB) binds to albumin reversibly and contributes to effective radiotheranostics by enhancing tumor accumulation of PSMA-targeting radioligands. We newly developed two ALB-containing PSMA-targeting radioligands including dual functional linkers, a hydrophilic linker, d-glutamic acid, and a hydrophobic linker, 4-(aminomethyl)benzoic acid, with the opposite arrangement (PNT-DA6 and PNT-DA7). A biodistribution study of [111In]In-PNT-DA6 indicated that the introduction and arrangement of dual functional linkers contributed to improved pharmacokinetics. A single photon emission computed tomography study of [111In]In-PNT-DA6 produced a clear PSMA-expressing tumor image. Moreover, [225Ac]Ac-PNT-DA6 showed the inhibition of tumor growth in targeted radionuclide therapy in PSMA-expressing tumor-bearing mice. These results indicated that [111In]In-PNT-DA6 and [225Ac]Ac-PNT-DA6 exhibited useful characteristics as PSMA-targeting radiotheranostic ligands.

Current Status and Perspectives of Novel Radiopharmaceuticals with Heterologous Dual-targeted Functions: 2013-2023

Radiotracers provide molecular- and cellular-level information in a noninvasive manner and have become important tools for precision medicine. In particular, the successful clinical application of radioligand therapeutic (RLT) has further strengthened the role of nuclear medicine in clinical treatment. The complicated microenvironment of the lesion has rendered traditional single-targeted radiopharmaceuticals incapable of fully meeting the requirements. The design and development of dual-targeted and multitargeted radiopharmaceuticals have rapidly emerged. In recent years, significant progress has been made in the development of heterologous dual-targeted radiopharmaceuticals. This perspective aims to provide a comprehensive overview of the recent progress in these heterologous dual-targeted radiopharmaceuticals, with a special focus on the design of ligand structures, pharmacological properties, and preclinical and clinical evaluation. Furthermore, future directions are discussed from this perspective.

Structure-affinity-pharmacokinetics relationships of 111In-labeled PSMA-targeted ligands with different albumin binders

INTRODUCTION

Prostate-specific membrane antigen (PSMA) is a promising target for treating metastatic castration-resistant prostate cancer. Our previous report presented 111In- or 225Ac-labeled PSMA-NAT-DA1 (PNT-DA1) as a PSMA-targeted ligand. To improve its therapeutic efficiency, PNT-DA1 contains 4-(p-iodophenyl)butyric acid (IPBA), which is known as an albumin binder (ALB) moiety. However, few reports have examined the relationship between the chemical modification of the ALB moiety and pharmacokinetics of PSMA-targeted radioligands. To assess this relationship, we designed, synthesized, and evaluated four [111In]In-PNT-DA1 analogues with ALB moieties different from IPBA.

METHODS

The [111In]In-PNT-DA1 analogues were synthesized from their corresponding precursors through ligand substitution reaction. The stability of [111In]In-PNT-DA1 analogues in mouse plasma, their affinity for human serum albumin (HSA), their binding to mouse plasma proteins, and their affinity for PSMA were evaluated in vitro. The tissue distribution profile of the radioligands was assessed in biodistribution studies using LNCaP tumor-bearing nude mice.

RESULTS

All [111In]In-PNT-DA1 analogues were obtained at a high radiochemical yield and purity. These analogues were highly stable in mouse plasma after 24 h. The binding affinity for HSA significantly varied among the different ALB moieties. Moreover, high affinity for mouse plasma proteins was observed for all [111In]In-PNT-DA1 analogues compared with their counterparts without an ALB moiety. The affinity for PSMA was comparable for all radioligands. In the biodistribution assay, the pharmacokinetics of [111In]In-PNT-DA1 analogues varied markedly depending on the type of ALB moiety. In particular, tumor area under the curve (AUC) values were increased for radioligands with higher blood retention, while some previous studies reported that compounds with moderate blood retention exhibited the highest tumor AUC values.

CONCLUSION

The introduction of an appropriate ALB moiety into the ligand may lead to the development of more useful PSMA-targeted radioligands with higher tumor accumulation.

Effect of Linker Entities on Pharmacokinetics of 111In-Labeled Prostate-Specific Membrane Antigen-Targeting Ligands with an Albumin Binder

In the field of radiopharmaceutical development targeting cancer, an albumin binder (ALB) is commonly used to improve accumulation of radioligands in tumors because it has high binding affinity for albumin and extends the circulation time of radioligands. The further development of ALB-containing radioligands is also expected to regulate their pharmacokinetics. In this study, we newly designed and synthesized [111In]In-PNT-DA1 derivatives, prostate-specific membrane antigen (PSMA)-targeting radioligands including a functional linker (d-glutamic acid or 4-(aminomethyl)benzoic acid), and evaluated the relationships among the structure, albumin-binding affinity, and pharmacokinetics. These derivatives showed a different binding affinity for albumin by the introduction of a linker. Biodistribution studies revealed that the introduction of a linker affects the pharmacokinetics of each derivative. The biodistribution studies also suggested that moderate albumin-binding affinity enhances the tumor/kidney ratio of the derivative. SPECT imaging using [111In]In-PNT-DA3 with the highest tumor/kidney ratio among [111In]In-PNT-DA1 derivatives led to clear visualization of a PSMA-positive LNCaP tumor. The results suggest that the appropriate introduction of linker entities may be necessary to improve the pharmacokinetics of PSMA-targeting radioligands.

Effect of Ibuprofen as an Albumin Binder on Melanoma-Targeting Properties of 177Lu-Labeled Ibuprofen-Conjugated Alpha-Melanocyte-Stimulating Hormone Peptides

The purpose of this study was to examine how the introduction of ibuprofen (IBU) affected tumor-targeting and biodistribution properties of 177Lu-labeled IBU-conjugated alpha-melanocyte-stimulating hormone peptides. The IBU was used as an albumin binder and conjugated to the DOTA-Lys moiety without or with a linker to yield DOTA-Lys(IBU)-GG-Nle-CycMSHhex {1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-Lys(IBU)-Gly-Gly-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-CONH2}, DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex, DOTA-Lys(Asn-IBU)-GGNle-CycMSHhex, and DOTA-Lys(Dab-IBU)-GGNle-CycMSHhex peptides. Their melanocortin-receptor 1 (MC1R) binding affinities were determined on B16/F10 melanoma cells first. Then the biodistribution of 177Lu-labeled peptides was determined on B16/F10 melanoma-bearing C57 mice at 2 h postinjection to choose the lead peptide for further examination. The full biodistribution and melanoma imaging properties of 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex were further evaluated using B16/F10 melanoma-bearing C57 mice. DOTA-Lys(IBU)-GG-Nle-CycMSHhex, DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex, DOTA-Lys(Asn-IBU)-GGNle-CycMSHhex, and DOTA-Lys(Dab-IBU)-GGNle-CycMSHhex displayed the IC50 values of 1.41 ± 0.37, 1.52 ± 0.08, 0.03 ± 0.01, and 0.58 ± 0.06 nM on B16/F10 melanoma cells, respectively. 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex exhibited the lowest liver and kidney uptake among all four designed 177Lu peptides. Therefore, 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex was further evaluated for its full biodistribution and melanoma imaging properties. The B16/F10 melanoma uptake of 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex was 19.5 ± 3.12, 24.12 ± 3.35, 23.85 ± 2.08, and 10.80 ± 2.89% ID/g at 0.5, 2, 4, and 24 h postinjection, respectively. Moreover, 177Lu-DOTA-Lys(Asp-IBU)-GGNle-CycMSHhex could clearly visualize the B16/F10 melanoma lesions at 2 h postinjection. The conjugation of IBU with or without a linker to GGNle-CycMSHhex affected the MC1R binding affinities of the designed peptides. The charge of the linker played a key role in the liver and kidney uptake of 177Lu-Asp-IBU, 177Lu-Asn-IBU, and 177Lu-Dab-IBU. 177Lu-Asp-IBU exhibited higher tumor/liver and tumor/kidney uptake ratios than those of 177Lu-Asn-IBU and 177Lu-Dab-IBU, underscoring its potential evaluation for melanoma therapy in the future.

Comparison of novel PSMA-targeting [177Lu]Lu-P17-087 with its albumin binding derivative [177Lu]Lu-P17-088 in metastatic castration-resistant prostate cancer patients: a first-in-human study

PURPOSE

Prostate-specific membrane antigen (PSMA) is a promising target for diagnosis and radioligand therapy (RLT) of prostate cancer. Two novel PSMA-targeting radionuclide therapy agents, [177Lu]Lu-P17-087, and its albumin binder modified derivative, [177Lu]Lu-P17-088, were evaluated in metastatic castration-resistant prostate cancer (mCRPC) patients. The primary endpoint was dosimetry evaluation, the second endpoint was radiation toxicity assessment (CTCAE 5.0) and PSA response (PCWG3).

METHODS

Patients with PSMA-positive tumors were enrolled after [68Ga]Ga-PSMA-11 PET/CT scan. Five mCRPC patients received [177Lu]Lu-P17-087 and four other patients received [177Lu]Lu-P17-088 (1.2 GBq/patient). Multiple whole body planar scintigraphy was performed at 1.5, 4, 24, 48, 72, 120 and 168 h after injection and one SPECT/CT imaging was performed at 24 h post-injection for each patient. Dosimetry evaluation was compared in both patient groups.

RESULTS

Patients showed no major clinical side-effects under this low dose treatment. As expected [177Lu]Lu-P17-088 with longer blood circulation (due to its albumin binding) exhibited higher effective doses than [177Lu]Lu-P17-087 (0.151 ± 0.036 vs. 0.056 ± 0.019 mGy/MBq, P = 0.001). Similarly, red marrow received 0.119 ± 0.068 and 0.048 ± 0.020 mGy/MBq, while kidney doses were 0.119 ± 0.068 and 0.046 ± 0.022 mGy/MBq, respectively. [177Lu]Lu-P17-087 demonstrated excellent tumor uptake and faster kinetics; while [177Lu]Lu-P17-088 displayed a slower washout and higher average dose (7.75 ± 4.18 vs. 4.72 ± 2.29 mGy/MBq, P = 0.018). After administration of [177Lu]Lu-P17-087 and [177Lu]Lu-P17-088, 3/5 and 3/4 patients showed reducing PSA values, respectively.

CONCLUSION

[177Lu]Lu-P17-088 and [177Lu]Lu-P17-087 displayed different pharmacokinetics but excellent PSMA-targeting dose delivery in mCRPC patients. These two agents are promising RLT agents for personalized treatment of mCRPC. Further studies with increased dose and frequency of RLT are warranted to evaluate the potential therapeutic efficacy.

TRIAL REGISTRATION

177Lu-P17-087/177Lu-P17-088 in Patients with Metastatic Castration-resistant Prostate Cancer (NCT05603559, Registered at 25 October, 2022). URL OF REGISTRY: https://classic.

CLINICALTRIALS

gov/ct2/show/NCT05603559 .

Development and first-in-human study of PSMA-targeted PET tracers with improved pharmacokinetic properties

PURPOSE

A series of new 68Ga-labeled tracers based on [68Ga]Ga-PSMA-617 were developed to augment the tumor-to-kidney ratio and reduce the activity accumulation in bladder, ultimately minimize radiation toxicity to the urinary system.

METHODS

We introduced quinoline group, phenylalanine and decanoic acid into different tracers to enhance their lipophilicity, strategically limiting their metabolic pathway through the urinary system. Their binding affinity onto LNCaP cells was determined through in vitro saturation assays and competition binding assays. In vivo metabolic study, PET imaging and biodistribution experiment were performed in LNCaP tumor-bearing B-NSG male mice. The most promising tracer was selected for first-in-human study.

RESULTS

Four radiotracers were synthesized with radiochemical purity (RCP) > 95% and molar activity in a range of 20.0-25.5 GBq/μmol. The binding affinities (Ki) of TWS01, TWS02 to PSMA were in the low nanomolar range (< 10 nM), while TWS03 and TWS04 exhibited binding affinities with Ki > 20 nM (59.42 nM for TWS03 and 37.14 nM for TWS04). All radiotracers exhibited high stability in vivo except [68Ga]Ga-TWS03. Micro PET/CT imaging and biodistribution analysis revealed that [68Ga]Ga-TWS02 enabled clear tumor visualization in PET images at 1.5 h post-injection, with higher tumor-to-kidney ratio (T/K, 0.93) and tumor-to-muscle ratio (T/M, 107.62) compared with [68Ga]Ga-PSMA-617 (T/K: 0.39, T/M: 15.01) and [68Ga]Ga-PSMA-11 (T/K: 0.15, T/M: 24.00). In first-in-human study, [68Ga]Ga-TWS02 effectively detected PCa-associated lesions including primary and metastatic lesions, with lower accumulation in urinary system, suggesting that [68Ga]Ga-TWS02 might be applied in the detection of bladder invasion, with minimized radiation toxicity to the urinary system.

CONCLUSION

Introduction of quinoline group, phenylalanine and decanoic acid into different tracers can modulate the binding affinity and pharmacokinetics of PSMA in vivo. [68Ga]Ga-TWS02 showed high binding affinity to PSMA, excellent pharmacokinetic properties and clear imaging of PCa-associated lesions, making it a promising radiotracer for the clinical diagnosis of PCa. Moreover, TWS02 with a chelator DOTA could also label 177Lu and 225Ac, which could be used for PCa treatment without significant side effects.

TRIAL REGISTRATION

The clinical evaluation of this study was registered On October 30, 2021 at https://www.chictr.org.cn/ (No: ChiCTR2100052545).

Synthesis and preclinical evaluation of a novel probe [18F]AlF-NOTA-IPB-GPC3P for PET imaging of GPC3 positive tumor

Glypican-3 (GPC3) is markedly overexpressed in hepatocellular carcinoma (HCC) and not expressed in normal liver tissues. In this study, a novel peptide PET imaging agent ([18F]AlF-NOTA-IPB-GPC3P) was developed to target GPC3 expressed in tumors. The overall radiochemical yield of [18F]AlF-NOTA-IPB-GPC3P was 10-15 %, and its lipophilicity, expressed as the logD value at a pH of 7.4, was -1.18 ± 0.06 (n = 3). Compared to the previously reported tracer [18F]AlF-GP2633, [18F]AlF-NOTA-IPB-GPC3P exhibited higher cellular uptake (15.13 vs 5.96) and internalized rate (80.63 % vs 35.93 %) in Huh7 cells at 120 min. Micro-PET/CT and biodistribution studies further demonstrated that [18F]AlF-NOTA-IPB-GPC3P exhibited significantly increased tumor uptake and prolonged tumor residence in Huh7 tumors compared to [18F]AlF-GP2633 (4.66 ± 0.22 % ID/g vs 0.72 ± 0.09 % ID/g at 60 min, p < 0.001; 5.05 ± 0.23 % ID/g vs 0.35 ± 0.08 % ID/g at 120 min, p < 0.001, respectively). Furthermore, the tumor-to-organ ratios of [18F]AlF-NOTA-IPB-GPC3P surpassed those of [18F]AlF-GP2633. Our results support the utilization of [18F]AlF-NOTA-IPB-GPC3P as a PET imaging agent targeting the GPC3 receptor for tumor detection.

Novel CDK19-Targeted Radiotracers: A Potential Design Strategy to Improve the Pharmacokinetics and Tumor Uptake

Cyclin-dependent kinase 19 (CDK19) is overexpressed in prostate cancer, making it an attractive target for both imaging and therapy. Since little is known about the optimized approach for radioligands of nuclear proteins, linker optimization strategies were used to improve pharmacokinetics and tumor absorption, including the adjustment of the length, flexibility/rigidity, and hydrophilicity/lipophilicity of linkers. Molecular docking was conducted for virtual screening and followed by IC50 determination. Both BALB/c mice and P-16 xenografts were used for tissue distribution and PET/CT imaging. The ligand 68Ga-10c demonstrated high absorption in tumor 5 min after injection and sustains long-term imaging within 3 h. Furthermore, 68Ga-10c exhibited slow clearance within the tumor and was predominantly metabolized in both the liver and kidneys, showing the potential to alleviate metabolic pressure and enhance tissue safety. Therefore, the linker optimization strategy is well suited for CDK19 and provides a reference for the radioactive ligands of other nuclear targets.

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

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

Design and Preclinical Evaluation of a Novel Prostate-Specific Membrane Antigen Radioligand Modified with a Transthyretin Binder

Transthyretin binders have previously been used to improve the pharmacokinetic properties of small-molecule drug conjugates and could, thus, be utilized for radiopharmaceuticals as an alternative to the widely explored "albumin binder concept". In this study, a novel PSMA ligand modified with a transthyretin-binding entity (TB-01) was synthesized and labeled with lutetium-177 to obtain [177Lu]Lu-PSMA-TB-01. A high and specific uptake of [177Lu]Lu-PSMA-TB-01 was found in PSMA-positive PC-3 PIP cells (69 ± 3% after 4 h incubation), while uptake in PSMA-negative PC-3 flu cells was negligible (<1%). In vitro binding studies showed a 174-fold stronger affinity of [177Lu]Lu-PSMA-TB-01 to transthyretin than to human serum albumin. Biodistribution studies in PC-3 PIP/flu tumor-bearing mice confirmed the enhanced blood retention of [177Lu]Lu-PSMA-TB-01 (16 ± 1% IA/g at 1 h p.i.), which translated to a high tumor uptake (69 ± 13% IA/g at 4 h p.i.) with only slow wash-out over time (31 ± 8% IA/g at 96 h p.i.), while accumulation in the PC-3 flu tumor and non-targeted normal tissue was reasonably low. Further optimization of the radioligand design would be necessary to fine-tune the biodistribution and enable its use for therapeutic purposes. This study was the first of this kind and could motivate the use of the "transthyretin binder concept" for the development of future radiopharmaceuticals.

Towards the Magic Radioactive Bullet: Improving Targeted Radionuclide Therapy by Reducing the Renal Retention of Radioligands

Targeted radionuclide therapy (TRT) is an emerging field and has the potential to become a major pillar in effective cancer treatment. Several pharmaceuticals are already in routine use for treating cancer, and there is still a high potential for new compounds for this application. But, a major issue for many radiolabeled low-to-moderate-molecular-weight molecules is their clearance via the kidneys and their subsequent reuptake. High renal accumulation of radioactive compounds may lead to nephrotoxicity, and therefore, the kidneys are often the dose-limiting organs in TRT with these radioligands. Over the years, different strategies have been developed aiming for reduced kidney retention and enhanced therapeutic efficacy of radioligands. In this review, we will give an overview of the efforts and achievements of the used strategies, with focus on the therapeutic potential of low-to-moderate-molecular-weight molecules. Among the strategies discussed here is coadministration of compounds that compete for binding to the endocytic receptors in the proximal tubuli. In addition, the influence of altering the molecular design of radiolabeled ligands on pharmacokinetics is discussed, which includes changes in their physicochemical properties and implementation of cleavable linkers or albumin-binding moieties. Furthermore, we discuss the influence of chelator and radionuclide choice on reabsorption of radioligands by the kidneys.

New ionic liquids based on 5-fluorouracil: Tuning of BSA binding and cytotoxicity

In this work, we describe the synthesis, interactions with bovine serum albumin, and cytotoxicity of new ionic liquids based on 5-fluorouracil (API-ILs) with different cations (imidazolium, choline, isoquinolinium, guanidinium). The secondary and tertiary structure of BSA in solutions with different concentrations of API-ILs was monitored by the circular dichroism (CD) technique. The addition of API-ILs does not lead to structural changes in BSA. A quenching of fluorescence spectra intensity of BSA in presence of all API-ILs was observed, allowing the quantification of binding between API-ILs and BSA. The preferred localization of both ions in API-ILs differs significantly depending on the structure of the cation according to molecular docking. The aggregation of BSA in presence of API-ILs was analyzed by the dynamic light scattering (DLS) method, revealing a moderate increase in particle size. Cytotoxicity and selectivity of API-ILs on cancer and normal cell lines were estimated, showing a clear modification of the pharmaceutic activity of ionic liquid compared to 5-fluorouracil.

Dosimetric Analysis of a Phase I Study of PSMA-Targeting Radiopharmaceutical Therapy With [177Lu]Ludotadipep in Patients With Metastatic Castration-Resistant Prostate Cancer

OBJECTIVE

177Lutetium [Lu] Ludotadipep is a novel prostate-specific membrane antigen targeting therapeutic agent with an albumin motif added to increase uptake in the tumors. We assessed the biodistribution and dosimetry of [177Lu]Ludotadipep in patients with metastatic castration-resistant prostate cancer (mCRPC).

MATERIALS AND METHODS

Data from 25 patients (median age, 73 years; range, 60-90) with mCRPC from a phase I study with activity escalation design of single administration of [177Lu]Ludotadipep (1.85, 2.78, 3.70, 4.63, and 5.55 GBq) were assessed. Activity in the salivary glands, lungs, liver, kidneys, and spleen was estimated from whole-body scan and abdominal SPECT/CT images acquired at 2, 24, 48, 72, and 168 h after administration of [177Lu]Ludotadipep. Red marrow activity was calculated from blood samples obtained at 3, 10, 30, 60, and 180 min, and at 24, 48, and 72 h after administration. Organ- and tumor-based absorbed dose calculations were performed using IDAC-Dose 2.1.

RESULTS

Absorbed dose coefficient (mean ± standard deviation) of normal organs was 1.17 ± 0.81 Gy/GBq for salivary glands, 0.05 ± 0.02 Gy/GBq for lungs, 0.14 ± 0.06 Gy/GBq for liver, 0.77 ± 0.28 Gy/GBq for kidneys, 0.12 ± 0.06 Gy/GBq for spleen, and 0.07 ± 0.02 Gy/GBq for red marrow. The absorbed dose coefficient of the tumors was 10.43 ± 7.77 Gy/GBq.

CONCLUSION

[177Lu]Ludotadipep is expected to be safe at the dose of 3.7 GBq times 6 cycles planned for a phase II clinical trial with kidneys and bone marrow being the critical organs, and shows a high tumor absorbed dose.

Development of probes for radiotheranostics with albumin binding moiety to increase the therapeutic effects of astatine-211 (211At)

PURPOSE

We have developed probes for multiradionuclides radiotheranostics using RGD peptide ([67Ga]Ga-DOTA-c[RGDf(4-I)K] ([67Ga]1) and Ga-DOTA-[211At]c[RGDf(4-At)K] ([211At]2)) for clinical applications. The introduction of an albumin binding moiety (ABM), such as 4-(4-iodophenyl)-butyric acid (IPBA), that has high affinity with the blood albumin and prolongs the circulation half-life can improve the pharmacokinetics of drugs. To perform more effective targeted alpha therapy (TAT), we designed and synthesized Ga-DOTA-K([211At]APBA)-c(RGDfK) ([211At]5) with 4-(4-astatophenyl)-butyric acid (APBA), which has an astato group instead of an iodo group in IPBA. We evaluated whether APBA functions as ABM and [211At]5 is effective for TAT. In addition, we prepared 67Ga-labeled RGD peptide without ABM, [67Ga]Ga-DOTA-K-c(RGDfK) ([67Ga]3), and 125I-labeled RGD peptide with ABM, Ga-DOTA-K([125I]IPBA)-c(RGDfK) ([125I]4), to compare with [211At]5.

METHODS

Biodistribution experiments of [67Ga]3 without ABM, [125I]4 and [211At]5 with ABM were conducted in normal mice and U-87 MG tumor-bearing mice. In addition, two doses of [211At]5 (370 or 925 kBq) were administered to U-87 MG tumor-bearing mice to confirm the therapeutic effects.

RESULTS

The blood retention of [125I]4 and [211At]5 was remarkably increased compared to [67Ga]3. Also, [125I]4 and [211At]5 showed similar biodistribution and significantly greater tumor accumulation and retention compared to [67Ga]3. In addition, [211At]5 inhibited tumor growth in a dose-dependent manner.

CONCLUSION

The functionality of APBA as ABM like IPBA, and the usefulness of [211At]5 as the radionuclide therapy agent for TAT was revealed.

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.

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.

PSMA-GCK01: A Generator-Based 99mTc/188Re Theranostic Ligand for the Prostate-Specific Membrane Antigen

Prostate-specific membrane antigen (PSMA) theranostics have been introduced with 68Ga and 177Lu, the most used radionuclides. However, 188Re is a well-known generator-based therapeutic nuclide that completes a theranostic tandem with 99mTc and may offer an interesting alternative to the currently used radionuclides. In the present work, we aimed at the development of a PSMA-targeted 99mTc/188Re theranostic tandem. Methods: The ligand HYNIC-iPSMA was chosen as the lead structure. Its HYNIC chelator has limitations for 188Re labeling and was replaced by mercaptoacetyltriserine to obtain PSMA-GCK01, a precursor for stable 99mTc and 188Re labeling. 99mTc-PSMA-GCK01 was used for in vitro evaluation of the ligand and comparison with 99mTc-EDDA/HYNIC-iPSMA. Planar imaging using 99mTc-PSMA-GCK01 and organ biodistribution with 188Re-PSMA-GCK01 were performed using LNCaP tumor-bearing mice. Finally, the theranostic tandem was applied for imaging and therapy in 3 prostate cancer patients in compassionate care. Results: Efficient radiolabeling of PSMA-GCK01 with both radionuclides was demonstrated. Cell-based assays with 99mTc-PSMA-GCK01 versus 99mTc-EDDA/HYNIC-iPSMA revealed comparable uptake characteristics. Planar imaging and organ distribution revealed good tumor uptake of both 99mTc-PSMA-GCK01 and 188Re-PSMA-GCK01 at 1 and 3 h after injection, with low uptake in nontarget organs. In patients, similar distribution patterns were observed for 99mTc-PSMA-GCK01 and 188Re-PSMA-GCK01 and in comparison with 177Lu-PSMA-617. Conclusion: The ligand PSMA-GCK01 labels stably with 99mTc and 188Re, both generator-based radionuclides, and thus provides access to on-demand labeling at reasonable costs. Preclinical evaluation of the compounds revealed favorable characteristics of the PSMA-targeted theranostic tandem. This result was confirmed by successful translation into first-in-humans application.

Predicting the effect of different folate doses on [68Ga]Ga-PSMA-11 organ and tumor uptake using physiologically based pharmacokinetic modeling

BACKGROUND

Folate intake might reduce [⁶⁸Ga]Ga-PSMA-11 uptake in tissues due to a competitive binding to the PSMA receptor. For diagnostic imaging, this could impact decision making, while during radioligand therapy this could affect treatment efficacy. The relationship between folate dose, timing of dosing and tumor and organ uptake is not well established. The aim of this study was to develop a physiologically based pharmacokinetic (PBPK) model to predict the effect of folates on [⁶⁸Ga]Ga-PSMA-11 PET/CT uptake in salivary glands, kidneys and tumors.

METHODS

A PBPK model was developed for [⁶⁸Ga]Ga-PSMA-11 and folates (folic acid and its metabolite 5-MTHF), with compartments added that represent salivary glands and tumor. Reactions describing receptor binding, internalization and intracellular degradation were included. Model evaluation for [⁶⁸Ga]Ga-PSMA-11 was performed by using patient scan data from two different studies (static and dynamic), while for folates data from the literature were used for evaluation. Simulations were performed to assess the effect of different folate doses (150 µg, 400 µg, 5 mg and 10 mg) on accumulation in salivary glands, kidney and tumor, also for patients with different tumor volumes (10, 100, 500 and 1000 mL).

RESULTS

Final model evaluation showed that predictions adequately described data for both [⁶⁸Ga]Ga-PSMA-11 and folates. Predictions of a 5-MTFH dose of 150 µg and folic acid dose of 400 µg (in case of administration at the same time as [⁶⁸Ga]Ga-PSMA-11 (t = 0)) showed no clinically relevant effect on salivary glands and kidney uptake. However, the effect of a decrease in salivary glands and kidney uptake was determined to be clinically relevant for doses of 5 mg (34% decrease for salivary glands and 32% decrease for kidney) and 10 mg (36% decrease for salivary glands and 34% decrease for kidney). Predictions showed that tumor uptake was not relevantly affected by the co-administration of folate for all different folate doses (range 150 µg-10 mg). Lastly, different tumor volumes did not impact the folate effect on [⁶⁸Ga]Ga-PSMA-11 biodistribution.

CONCLUSION

Using a PBPK model approach, high doses of folate (5 and 10 mg) were predicted to show a decrease of [⁶⁸Ga]Ga-PSMA-11 salivary glands and kidney uptake, while intake by means of folate containing food or vitamin supplements showed no relevant effects. In addition, tumor uptake was not affected by folate administration in the simulated dose ranges (150 µg-10 mg). Differences in tumor volume are not expected to impact folate effects on [⁶⁸Ga]Ga-PSMA-11 organ uptake.

Development of Novel 111In/225Ac-Labeled Agent Targeting PSMA for Highly Efficient Cancer Radiotheranostics

Prostate-specific membrane antigen (PSMA) is a promising target for metastatic castration-resistant prostate cancer. We previously reported the effectiveness of PSMA-DA1 as a PSMA-targeting radiotheranostic agent containing an albumin binder moiety. To further enhance tumor uptake, we newly designed PSMA-NAT-DA1 (PNT-DA1) by the introduction of a lipophilic linker into PSMA-DA1. The PSMA affinity of [¹¹¹In]In-PNT-DA1 was increased (K_d = 8.20 nM) compared with that of [¹¹¹In]In-PSMA-DA1 (K_d = 89.4 nM). [¹¹¹In]In-PNT-DA1 showed markedly high tumor accumulation (131.6% injected dose/g at 48 h post-injection), and [¹¹¹In]In-PNT-DA1 enabled the visualization of the tumor clearly at 24 h post-injection with SPECT/CT imaging. The administration of [²²⁵Ac]Ac-PNT-DA1 (2.5 kBq) led to shrinkage of the tumor without marked toxicity, and the antitumor effects of [²²⁵Ac]Ac-PNT-DA1 were superior to those of [²²⁵Ac]Ac-PSMA-DA1 and [²²⁵Ac]Ac-PSMA-617, which is the current gold standard for PSMA-targeting ²²⁵Ac-endoradiotherapy. These results suggest that the combination of [¹¹¹In]In-PNT-DA1 and [²²⁵Ac]Ac-PNT-DA1 comprises a promising method of PSMA-targeting radiotheranostics.

Preclinical Evaluation of a PSMA-Targeting Homodimer with an Optimized Linker for Imaging of Prostate Cancer

Prostate-specific membrane antigen (PSMA) targeting radiopharmaceuticals have been successfully used for diagnosis and therapy of prostate cancer. Optimization of the available agents is desirable to improve tumor uptake and reduce side effects to non-target organs. This can be achieved, for instance, via linker modifications or multimerization approaches. In this study, we evaluated a small library of PSMA-targeting derivatives with modified linker residues, and selected the best candidate based on its binding affinity to PSMA. The lead compound was coupled to a chelator for radiolabeling, and subject to dimerization. The resulting molecules, 22 and 30, were highly PSMA specific (IC₅₀ = 1.0-1.6 nM) and stable when radiolabeled with indium-111 (>90% stable in PBS and mouse serum up to 24 h). Moreover, [¹¹¹In]In-30 presented a high uptake in PSMA expressing LS174T cells, with 92.6% internalization compared to 34.1% for PSMA-617. Biodistribution studies in LS174T mice xenograft models showed that [¹¹¹In]In-30 had a higher tumor and kidney uptake compared to [¹¹¹In]In-PSMA-617, but increasing T/K and T/M ratios at 24 h p.i. Tumors could be clearly visualized at 1 h p.i. by SPECT/CT after administration of [¹¹¹In]In-22 and [¹¹¹In]In-PSMA-617, while [¹¹¹In]In-30 showed a clear signal at later time-points (e.g., 24 h p.i.).

Characterization of Non-Specific Uptake and Retention Mechanisms of [177Lu]Lu-PSMA-617 in the Salivary Glands

The radionuclide therapy [¹⁷⁷Lu]Lu-PSMA-617 was recently FDA-approved for treatment of metastatic castration-resistant prostate cancer. Salivary gland toxicity is currently considered as the main dose-limiting side effect. However, its uptake and retention mechanisms in the salivary glands remain elusive. Therefore, our aim was to elucidate the uptake patterns of [¹⁷⁷Lu]Lu-PSMA-617 in salivary gland tissue and cells by conducting cellular binding and autoradiography experiments. Briefly, A-253 and PC3-PIP cells, and mouse kidney and pig salivary gland tissue, were incubated with 5 nM [¹⁷⁷Lu]Lu-PSMA-617 to characterize its binding. Additionally, [¹⁷⁷Lu]Lu-PSMA-617 was co-incubated with monosodium glutamate, ionotropic or metabotropic glutamate receptor antagonists. Low, non-specific binding was observed in salivary gland cells and tissues. Monosodium glutamate was able to decrease [¹⁷⁷Lu]Lu-PSMA-617 in PC3-PIP cells, mouse kidney and pig salivary gland tissue. Kynurenic acid (ionotropic antagonist) decreased the binding of [¹⁷⁷Lu]Lu-PSMA-617 to 29.2 ± 20.6% and 63.4 ± 15.4%, respectively, with similar effects observed on tissues. (RS)-MCPG (metabotropic antagonist) was able to decrease the [¹⁷⁷Lu]Lu-PSMA-617 binding on A-253 cells to 68.2 ± 16.8% and pig salivary gland tissue to 53.1 ± 36.8%. To conclude, we showed that the non-specific binding on [¹⁷⁷Lu]Lu-PSMA-617 could be reduced by monosodium glutamate, kynurenic acid and (RS)-MCPG.

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

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

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

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

Inductively Coupled Plasma Mass Spectrometry─A Valid Method for the Characterization of Metal Conjugates in View of the Development of Radiopharmaceuticals

This study addresses the question whether inductively coupled plasma mass spectrometry (ICP-MS) can be used as a method for the in vitro and in vivo characterization of non-radioactive metal conjugates to predict the properties of analogous radiopharmaceuticals. In a "proof-of-concept" study, the prostate-specific membrane antigen (PSMA)-targeting [¹⁷⁵Lu]Lu-PSMA-617 and [¹⁵⁹Tb]Tb-PSMA-617 were compared with their respective radiolabeled analogues, [¹⁷⁷Lu]Lu-PSMA-617 (PLUVICTO, Novartis) and [¹⁶¹Tb]Tb-PSMA-617. ICP-MS and conventional γ-counting of the cell samples revealed almost identical results (<6% absolute difference between the two technologies) for the in vitro uptake and internalization of the (radio)metal conjugates, irrespective of the employed methodology. In vivo, an equal uptake in PSMA-positive PC-3 PIP tumor xenografts was determined 1 h after the injection of [¹⁷⁵Lu]Lu-/[¹⁷⁷Lu]Lu-PSMA-617 (41 ± 6% ID/g and 44 ± 12% IA/g, respectively) and [¹⁵⁹Tb]Tb-/[¹⁶¹Tb]Tb-PSMA-617 (44 ± 5% ID/g and 44 ± 5% IA/g, respectively). It was further revealed that it is crucial to use the same ratios of the (radio)metal-labeled and unlabeled ligands for both methodologies to obtain equal data in organs in which receptor saturation was reached such as the kidneys (12 ± 2% ID/g vs 10 ± 1% IA/g, 1 h after injection). The data of this study demonstrate that the use of high-sensitivity ICP-MS allows reliable and predictive quantification of compounds labeled with stable metal isotopes in cell and tissue samples obtained in preclinical studies. It can, hence, be employed as a valid alternative to the state-of-the-art γ-counting methodology to detect radioactive ligands.

Improve the Biodistribution with Bulky and Lipophilic Modification Strategies on Lys-Urea-Glu-Based PSMA-Targeting Radiotracers

Since prostate-specific membrane antigen (PSMA) is upregulated in nearly all stages of prostate cancer (PCa), PSMA can be considered a viable diagnostic biomarker and treatment target in PCa. In this study, we have developed five ⁶⁸Ga-labeled PSMA-targeted tracers, ⁶⁸Ga-Flu-1, ⁶⁸Ga-Flu-2, ⁶⁸Ga-9-Ant, ⁶⁸Ga-1-Nal, and ⁶⁸Ga-1-Noi, to investigate the effect of lipophilic bulky groups on the pharmacokinetics of PSMA inhibitors compared to ⁶⁸Ga-PSMA-11 and then explore their in vitro and in vivo properties. ⁶⁸Ga-labeled PSMA inhibitors were obtained in 88.53-99.98% radiochemical purity and at the highest specific activity of up to 20 MBq/μg. These compounds revealed a highly efficient uptake and internalization into LNCaP cells and increased over time. PET imaging and biodistribution studies were performed in mice bearing PSMA expressing LNCaP prostate cancer xenografts. All tracers enabled clear visualization of tumors in PET images with excellent tumor-to-background contrast. The biodistribution studies showed that all these radioligands were excreted mainly via the renal pathway. The in vivo biodistribution of ⁶⁸Ga-Flu-1 revealed higher tumor uptake (40.11 ± 9.24 %ID/g at 2 h p.i.) compared to ⁶⁸Ga-PSMA-11 (28.10 ± 5.96 %ID/g at 2 h p.i.). Both in vitro and in vivo experiments showed that chemical modification of the lysine fragment significantly impacts tumor-targeting and pharmacokinetic properties. Great potential to serve as new PET tracers for prostate cancer has been revealed with these radiotracers─⁶⁸Ga-Flu-1 in particular.

Synthesis and preclinical evaluation of novel 99mTc-labeled PSMA ligands for radioguided surgery of prostate cancer

BACKGROUND

Radioguided surgery (RGS) has recently emerged as a valuable new tool in the management of recurrent prostate cancer (PCa). After preoperative injection of a ^99mTc-labeled prostate-specific membrane antigen (PSMA) inhibitor, radioguided intraoperative identification and resection of lesions is facilitated by means of suitable γ-probes. First clinical experiences show the feasibility of RGS and suggest superiority over conventional lymph node dissection in recurrent PCa. However, commonly used [^99mTc]Tc-PSMA-I&S exhibits slow whole-body clearance, thus hampering optimal tumor-to-background ratios (TBR) during surgery. We therefore aimed to develop novel ^99mTc-labeled, PSMA-targeted radioligands with optimized pharmacokinetic profile to increase TBR at the time of surgery.

METHODS

Three ^99mTc-labeled N4-PSMA ligands were preclinically evaluated and compared to [^99mTc]Tc-PSMA-I&S. PSMA affinity (IC₅₀) and internalization were determined on LNCaP cells. Lipophilicity was assessed by means of the distribution coefficient logD_7.4 and an ultrafiltration method was used to determine binding to human plasma proteins. Biodistribution studies and static µSPECT/CT-imaging were performed at 6 h p.i. on LNCaP tumor-bearing CB17-SCID mice.

RESULTS

The novel N4-PSMA tracers were readily labeled with [^99mTc]TcO₄^- with RCP > 95%. Comparable and high PSMA affinity was observed for all [^99mTc]Tc-N4-PSMA-ligands. The ligands showed variable binding to human plasma and medium to low lipophilicity (logD_7.4 - 2.6 to - 3.4), both consistently decreased compared to [^99mTc]Tc-PSMA-I&S. Biodistribution studies revealed comparable tumor uptake among all [^99mTc]Tc-N4-PSMA-ligands and [^99mTc]Tc-PSMA-I&S, while clearance from most organs was superior for the novel tracers. Accordingly, increased TBR were achieved. [^99mTc]Tc-N4-PSMA-12 showed higher TBR than [^99mTc]Tc-PSMA-I&S for blood and all evaluated tissue. In addition, a procedure suitable for routine clinical production of [^99mTc]Tc-N4-PSMA-12 was established. Labeling with 553 ± 187 MBq was achieved with RCP of 98.5 ± 0.6% (n = 10).

CONCLUSION

High tumor accumulation and favorable clearance from blood and non-target tissue make [^99mTc]Tc-N4-PSMA-12 an attractive tracer for RGS, possibly superior to currently established [^99mTc]Tc-PSMA-I&S. Its GMP-production according to a method presented here and first clinical investigations with this novel radioligand is highly recommended.

Pharmacological Optimization of PSMA-Based Radioligand Therapy

Prostate cancer (PCa) is the most common malignancy in men of middle and older age. The standard treatment strategy for PCa ranges from active surveillance in low-grade, localized PCa to radical prostatectomy, external beam radiation therapy, hormonal treatment and chemotherapy. Recently, the use of prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) for metastatic castration-resistant PCa has been approved. PSMA is predominantly, but not exclusively, expressed on PCa cells. Because of its high expression in PCa, PSMA is a promising target for diagnostics and therapy. To understand the currently used RLT, knowledge about pharmacokinetics (PK) and pharmacodynamics (PD) of the PSMA ligand and the PSMA protein itself is crucial. PK and PD properties of the ligand and its target determine the duration and extent of the effect. Knowledge on the concentration-time profile, the target affinity and target abundance may help to predict the effect of RLT. Increased specific binding of radioligands to PSMA on PCa cells may be associated with better treatment response, where nonspecific binding may increase the risk of toxicity in healthy organs. Optimization of the radioligand, as well as synergistic effects of concomitant agents and an improved dosing strategy, may lead to more individualized treatment and better overall survival.

[225Ac]Ac-SibuDAB for Targeted Alpha Therapy of Prostate Cancer: Preclinical Evaluation and Comparison with [225Ac]Ac-PSMA-617

In the present study, SibuDAB, an albumin-binding PSMA ligand, was investigated in combination with actinium-225 and the data were compared with those of [225Ac]Ac-PSMA-617. In vitro, [225Ac]Ac-SibuDAB and [225Ac]Ac-PSMA-617 showed similar tumor cell uptake and PSMA-binding affinities as their 177Lu-labeled counterparts. The in vitro binding to serum albumin in mouse and human blood plasma, respectively, was 2.8-fold and 1.4-fold increased for [225Ac]Ac-SibuDAB as compared to [177Lu]Lu-SibuDAB. In vivo, this characteristic was reflected by the longer retention of [225Ac]Ac-SibuDAB in the blood than previously seen for [177Lu]Lu-SibuDAB. Similar to [225Ac]Ac-PSMA-617, [225Ac]Ac-SibuDAB was well tolerated at 30 kBq per mouse. Differences in blood cell counts were observed between treated mice and untreated controls, but no major variations were observed between values obtained for [225Ac]Ac-SibuDAB and [225Ac]Ac-PSMA-617. [225Ac]Ac-SibuDAB was considerably more effective to treat PSMA-positive tumor xenografts than [225Ac]Ac-PSMA-617. Only 5 kBq per mouse were sufficient to eradicate the tumors, whereas tumor regrowth was observed for mice treated with 5 kBq [225Ac]Ac-PSMA-617 and only one out of six mice survived until the end of the study. The enhanced therapeutic efficacy of [225Ac]Ac-SibuDAB as compared to that of [225Ac]Ac-PSMA-617 and reasonable safety data qualify this novel radioligand as a candidate for targeted α-therapy of prostate cancer.

Dosimetry in Radiopharmaceutical Therapy

The application of radiopharmaceutical therapy for the treatment of certain diseases is well established, and the field is expanding. New therapeutic radiopharmaceuticals have been developed in recent years, and more are in the research pipeline. Concurrently, there is growing interest in the use of internal dosimetry as a means of personalizing, and potentially optimizing, such therapy for patients. Internal dosimetry is multifaceted, and the current state of the art is discussed in this continuing education article. Topics include the context of dosimetry, internal dosimetry methods, the advantages and disadvantages of incorporating dosimetry calculations in radiopharmaceutical therapy, a description of the workflow for implementing patient-specific dosimetry, and future prospects in the field.

Albumin-Mediated Size Exclusion Chromatography: The Apparent Molecular Weight of PSMA Radioligands as Novel Parameter to Estimate Their Blood Clearance Kinetics

A meticulously adjusted pharmacokinetic profile and especially fine-tuned blood clearance kinetics are key characteristics of therapeutic radiopharmaceuticals. We, therefore, aimed to develop a method that allowed the estimation of blood clearance kinetics in vitro. For this purpose, ¹⁷⁷Lu-labeled PSMA radioligands were subjected to a SEC column with human serum albumin (HSA) dissolved in a mobile phase. The HSA-mediated retention time of each PSMA ligand generated by this novel 'albumin-mediated size exclusion chromatography' (AMSEC) was converted to a ligand-specific apparent molecular weight (MW_app), and a normalization accounting for unspecific interactions between individual radioligands and the SEC column matrix was applied. The resulting normalized MW_app,norm. could serve to estimate the blood clearance of renally excreted radioligands by means of their influence on the highly size-selective process of glomerular filtration (GF). Based on the correlation between MW and the glomerular sieving coefficients (GSCs) of a set of plasma proteins, GSC_calc values were calculated to assess the relative differences in the expected GF/blood clearance kinetics in vivo and to select lead candidates among the evaluated radioligands. Significant differences in the MW_app,norm. and GSC_calc values, even for stereoisomers, were found, indicating that AMSEC might be a valuable and high-resolution tool for the preclinical selection of therapeutic lead compounds for clinical translation.

Synthesis and Evaluation of Two Long-Acting SSTR2 Antagonists for Radionuclide Therapy of Neuroendocrine Tumors

Somatostatin receptor subtype 2 (SSTR2) has become an essential target for radionuclide therapy of neuroendocrine tumors (NETs). JR11 was introduced as a promising antagonist peptide to target SSTR2. However, due to its rapid blood clearance, a better pharmacokinetic profile is necessary for more effective treatment. Therefore, two JR11 analogs (8a and 8b), each carrying an albumin binding domain, were designed to prolong the blood residence time of JR11. Both compounds were labeled with lutetium-177 and evaluated via in vitro assays, followed by in vivo SPECT/CT imaging and ex vivo biodistribution studies. [177Lu]Lu-8a and [177Lu]Lu-8b were obtained with high radiochemical purity (>97%) and demonstrated excellent stability in PBS and mouse serum (>95%). [177Lu]Lu-8a showed better affinity towards human albumin compared to [177Lu]Lu-8b. Further, 8a and 8b exhibited binding affinities 30- and 48-fold lower, respectively, than that of the parent peptide JR11, along with high cell uptake and low internalization rate. SPECT/CT imaging verified high tumor accumulation for [177Lu]Lu-8a and [177Lu]Lu-JR11 at 4, 24, 48, and 72 h post-injection, but no tumor uptake was observed for [177Lu]Lu-8b. Ex vivo biodistribution studies revealed high and increasing tumor uptake for [177Lu]Lu-8a. However, its extended blood circulation led to an unfavorable biodistribution profile for radionuclide therapy.

86Y-Labeled Albumin-Binding Fibroblast Activation Protein Inhibitor for Late-Time-Point Cancer Diagnosis

Fibroblast activation protein inhibitor (FAPI) is a novel quinoline-based radiopharmaceutical that has theranostic potential, yet the limited tumor retention hinders late-time diagnosis and radionuclide treatment. This study synthesized four albumin-binding FAPIs (TE-FAPI-01 to 04) and evaluated their in vitro stability, binding affinity, in vivo biodistribution, and tumor uptake with ⁶⁸Ga, ⁸⁶Y, and ¹⁷⁷Lu labeling, aiming to select the best molecule that has favorable pharmacokinetics to extend the blood circulation and tumor uptake in FAP-expressing tumors. All TE-FAPIs were stable in saline and plasma and displayed high FAP-binding affinity, with IC₅₀ values ranging from 3.96 to 34.9 nmol/L. The capabilities of TE-FAPIs to be retained in circulation were higher than that of FAPI-04, and TE-FAPI-04 displayed minimum physiological uptake in major organs compared with other molecules. TE-FAPI-03 and TE-FAPI-04 exhibited persistent tumor accumulation, with tumor radioactivity 24 h after administration of 2.84 ± 1.19%ID/g and 3.86 ± 1.15%ID/g for ¹⁷⁷Lu-TE-FAPI-03 and ¹⁷⁷Lu-TE-FAPI-04, respectively, both of which outperformed ¹⁷⁷Lu-FAPI-04 (0.34 ± 0.07%ID/g). TE-FAPI-04 was recognized as the albumin-binding FAPI with the most favorable pharmacokinetics and imaging performance. The enhanced circulation half-life and tumor uptake of TE-FAPI-04 aided the theranostics of malignant tumors and warrant further clinical investigations.

Refined Chelator Spacer Moieties Ameliorate the Pharmacokinetics of PSMA-617

Prostate-specific membrane antigen (PSMA) binding tracers are promising agents for the targeting of prostate tumors. To further optimize the clinically established radiopharmaceutical PSMA-617, novel PSMA ligands for prostate cancer endoradiotherapy were developed. A series of PSMA binding tracers that comprise a benzyl group at the chelator moiety were obtained by solid-phase synthesis. The compounds were labeled with ⁶⁸Ga or ¹⁷⁷Lu. Competitive cell-binding assays and internalization assays were performed using the cell line C4-2, a subline of the PSMA positive cell line LNCaP (human lymph node carcinoma of the prostate). Positron emission tomography (PET) imaging and biodistribution studies were conducted in a C4-2 tumor bearing BALB/c nu/nu mouse model. All ⁶⁸Ga-labeled ligands were stable in human serum over 2 h; ¹⁷⁷Lu-CA030 was stable over 72 h. The PSMA ligands revealed inhibition potencies [K_i] (equilibrium inhibition constants) between 4.8 and 33.8 nM. The percentage of internalization of the injected activity/10⁶ cells of ⁶⁸Ga-CA028, ⁶⁸Ga-CA029, and ⁶⁸Ga-CA030 was 41.2 ± 2.7, 44.3 ± 3.9, and 53.8 ± 5.4, respectively; for the comparator ⁶⁸Ga-PSMA-617, 15.5 ± 3.1 was determined. Small animal PET imaging of the compounds showed a high tumor-to-background contrast. Organ distribution studies revealed high specific uptake in the tumor, that is, approximately 34.4 ± 9.8% of injected dose per gram (%ID/g) at 1 h post injection for ⁶⁸Ga-CA028. At 1 h p.i., ⁶⁸Ga-CA028 and ⁶⁸Ga-CA030 demonstrated lower kidney uptake than ⁶⁸Ga-PSMA-617, but at later time points, kidney time–activity curves converge. In line with the preclinical data, first diagnostic PET imaging using ⁶⁸Ga-CA028 and ⁶⁸Ga-CA030 revealed high-contrast detection of bone and lymph node lesions in patients with metastatic prostate cancer. The novel PSMA ligands, in particular CA028 and CA030, are promising agents for targeting PSMA-positive tumor lesions as shown in the preclinical evaluation and in a first patient, respectively. Thus, clinical translation of ⁶⁸Ga-CA028 and ⁶⁸Ga/¹⁷⁷Lu-CA030 for diagnostics and endoradiotherapy of prostate cancer in larger cohorts of patients is warranted.

IgG-Binding Nanobody Capable of Prolonging Nanobody-Based Radiotracer Plasma Half-Life and Enhancing the Efficacy of Tumor-Targeted Radionuclide Therapy

Nanobodies have been developed rapidly as targeted probes for molecular imaging owing to their high affinity, outstanding tissue penetration, and rapid blood clearance. However, the short retention time at the tumor site limits their application in targeted radionuclide therapy. In this study, we designed a dual-targeting nanobody referred to as MIRC213-709, which can specifically bind to the HER2 receptor in tumor cell lines with high affinity (by nanobody MIRC213) and endogenous IgG in plasma to prolong the half-life by the MIRC213 C-terminal fusion nanobody, MIRC709. The nanobodies were site-specifically radiolabeled with ^99mTc and ¹⁷⁷Lu, and radiochemical purity was >95% after purification. The long blood circulation time and tumor retention property of ^99mTc/¹⁷⁷Lu-MIRC213-709 were confirmed by a blood clearance assay, single-photon emission computed tomography (SPECT), and a biodistribution study. The blood clearance assay showed that the distribution phase half-life (T_1/2α) and elimination phase half-life (T_1/2β) of ^99mTc-MIRC213-709 were 6.74- and 19.04-fold longer than those of ^99mTc-MIRC213, respectively. The SPECT/CT and biodistribution results showed that the highest uptake of ¹⁷⁷Lu-MIRC213 in the NCI-N87 model was 5.24 ± 0.95% ID/g at 6 h p.i., while the highest uptake of ¹⁷⁷Lu-MIRC213-709 in the NCI-N87 model was 30.82 ± 7.29% ID/g at 48 h p.i. Compared with ¹⁷⁷Lu-MIRC213, ¹⁷⁷Lu-MIRC213-709 had a 16.9-fold increased tumor cumulative uptake (2606 ± 195.1 vs 153.9 ± 22.37% ID/g·h). The targeted radionuclide therapy assay was performed in the NCI-N87 tumor model, and treatment monitoring ended on day 32. The post-treatment/pretreatment tumor volumes were 12.99 ± 1.66, 3.58 ± 0.96, 1.26 ± 0.17, and 1.54 ± 0.50 in the 0, 9, and 18 MBq single-dose groups and the two 9 MBq divided dose group (14 days apart), respectively. All treatment groups showed significant therapeutic effects (P < 0.0001). Thus, fusion with the IgG-binding nanobody MIRC709 provides MIRC213 derivatives with improved metabolic properties for targeted radionuclide therapy.

Tailoring the NIR-II Photoluminescence of Single Thiolated Au25 Nanoclusters by Selective Binding to Proteins

Atomically precise gold nanoclusters are a fascinating class of nanomaterials that exhibit molecule-like properties and have outstanding photoluminescence (PL). Their ultrasmall size, molecular chemistry, and biocompatibility make them extremely appealing for selective biomolecule labeling in investigations of biological mechanisms at the cellular and anatomical levels. In this work, we report a simple route to incorporate a preformed Au₂₅ nanocluster into a model bovine serum albumin (BSA) protein. A new approach combining small-angle X-ray scattering and molecular modeling provides a clear localization of a single Au₂₅ within the protein to a cysteine residue on the gold nanocluster surface. Attaching Au₂₅ to BSA strikingly modifies the PL properties with enhancement and a redshift in the second near-infrared (NIR-II) window. This study paves the way to conrol the design of selective sensitive probes in biomolecules through a ligand-based strategy to enable the optical detection of biomolecules in a cellular environment by live imaging.

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

Simple Summary

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

Abstract

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

Albumin Binder-Conjugated Fibroblast Activation Protein Inhibitor Radiopharmaceuticals for Cancer Therapy

Fibroblast activation protein (FAP) has become an attractive target for diagnosis and therapy, and a series of FAP inhibitor (FAPI)-based radiotracers has been developed and had excellent performance for diagnosis outcomes in clinical applications. Yet, their fast clearance and insufficient tumor retention have hampered their further clinical application in cancer treatment. In this study, we developed 2 albumin binder-conjugated FAPI radiotracers, TEFAPI-06 and TEFAPI-07. They were derived from FAPI-04 and were optimized by conjugating 2 types of well-studied albumin binders, 4-(p-iodophenyl) butyric acid moiety (TEFAPI-06) and truncated Evans blue moiety (TEFAPI-07), to try to overcome the above limitations at the expense of prolonging the blood circulation. Methods: TEFAPI-06 and TEFAPI-07 were synthesized and labeled with 68Ga, 86Y, and 177Lu successfully. A series of cell assays was performed to identify the binding affinity and FAP specificity in vitro. PET imaging, SPECT imaging, and biodistribution studies were performed to evaluate the pharmacokinetics in pancreatic cancer patient-derived xenograft (PDX) animal models. The cancer treatment efficacy of 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were evaluated in pancreatic cancer PDX-bearing mice. Results: The binding affinities (dissociation constants) to FAP of 68Ga-TEFAPI-06 and 68Ga-TEFAPI-07 were 10.16 ± 2.56 nM and 7.81 ± 2.28 nM, respectively, which were comparable with that of 68Ga-FAPI-04. Comparative PET imaging of HT-1080-FAP and HT-1080 tumor-bearing mice and a blocking study showed the FAP-targeting ability in vivo of these 2 tracers. Compared with 177Lu-FAPI-04, PET imaging, SPECT imaging, and biodistribution studies of TEFAPI-06 and TEFAPI-07 demonstrated their remarkably enhanced tumor accumulation and retention, respectively. Notable tumor growth inhibition by 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 were observed, whereas the control group and the group treated by 177Lu-FAPI-04 showed a slight therapeutic effect. Conclusion: Two albumin binder-conjugated FAPI radiopharmaceuticals have been developed and evaluated in vitro and in vivo. Significantly improved tumor uptake and retention were observed, compared with the original FAPI tracer. Both 177Lu-TEFAPI-06 and 177Lu-TEFAPI-07 showed remarkable growth inhibition of PDX tumors, whereas the side effects were almost negligible, demonstrating that these radiopharmaceuticals are promising for further clinical translational studies.

Preclinical investigations using [177Lu]Lu-Ibu-DAB-PSMA toward its clinical translation for radioligand therapy of prostate cancer

[177Lu]Lu-Ibu-DAB-PSMA was previously characterized with moderate albumin-binding properties enabling high tumor accumulation but reasonably low retention in the blood. The aim of this study was to investigate [177Lu]Lu-Ibu-DAB-PSMA in preclinical in vivo experiments and compare its therapeutic efficacy and potential undesired side effects with those of [177Lu]Lu-PSMA-617 and the previously developed [177Lu]Lu-PSMA-ALB-56. BALB/c nude mice without tumors were investigated on Day 10 and 28 after injection of 10 MBq radioligand. It was revealed that most plasma parameters were in the same range for all groups of mice and histopathological examinations of healthy tissue did not show any alternations in treated mice as compared to untreated controls. Based on these results, a therapy study over twelve weeks was conducted with PC-3 PIP tumor-bearing mice for comparison of the radioligands’s therapeutic efficacy up to an activity of 10 MBq (1 nmol) per mouse. In agreement with the increased mean absorbed tumor dose, [177Lu]Lu-Ibu-DAB-PSMA (~ 6.6 Gy/MBq) was more effective to inhibit tumor growth than [177Lu]Lu-PSMA-617 (~ 4.5 Gy/MBq) and only moderately less potent than [177Lu]Lu-PSMA-ALB-56 (~ 8.1 Gy/MBq). As a result, the survival of mice treated with 2 MBq of an albumin-binding radioligand was significantly increased (p < 0.05) compared to that of mice injected with [177Lu]Lu-PSMA-617 or untreated controls. The majority of mice treated with 5 MBq or 10 MBq [177Lu]Lu-Ibu-DAB-PSMA or [177Lu]Lu-PSMA-ALB-56 were still alive at study end. Hemograms of immunocompetent mice injected with 30 MBq [177Lu]Lu-Ibu-DAB-PSMA or 30 MBq [177Lu]Lu-PSMA-617 showed values in the same range as untreated controls. This was, however, not the case for mice treated with [177Lu]Lu-PSMA-ALB-56 which revealed a drop in lymphocytes and hemoglobin at Day 10 and Day 28 after injection. The data of this study demonstrated a significant therapeutic advantage of [177Lu]Lu-Ibu-DAB-PSMA over [177Lu]Lu-PSMA-617 and a more favorable safety profile as compared to that of [177Lu]Lu-PSMA-ALB-56. Based on these results, [177Lu]Lu-Ibu-DAB-PSMA may has the potential for a clinical translation.

Advances in PSMA theranostics

•

PSMA is an appealing target for theranostic because it is a transmembrane protein with a known substrate that is overexpessed on prostate cancer cells and internalizes upon ligand binding.

•

There are a number of PSMA theranostic ligands in clinical evaluation, clinical trial, or clinically approved.

•

PSMA theranostic ligands increase progression-free survival, overall survival, and pain in patients with metastatic castration resistant prostate cancer.

•

A major obstacle to PSMA-targeted radioligand therapy is off-target toxicity in salivary glands.

The validation of prostate specific membrane antigen (PSMA) as a molecular target in metastatic castration-resistant prostate cancer has stimulated the development of multiple classes of theranostic ligands that specifically target PSMA. Theranostic ligands are used to image disease or selectively deliver cytotoxic radioactivity to cells expressing PSMA according to the radioisotope conjugated to the ligand. PSMA theranostics is a rapidly advancing field that is now integrating into clinical management of prostate cancer patients. In this review we summarize published research describing the biological role(s) and activity of PSMA, highlight the most clinically advanced PSMA targeting molecules and biomacromolecules, and identify next generation PSMA ligands that aim to further improve treatment efficacy. The goal of this review is to provide a comprehensive assessment of the current state-of-play and a roadmap to achieving further advances in PSMA theranostics.

Preclinical Investigations to Explore the Difference between the Diastereomers [177Lu]Lu-SibuDAB and [177Lu]Lu-RibuDAB toward Prostate Cancer Therapy

[¹⁷⁷Lu]Lu-Ibu-DAB-PSMA, a radioligand modified with ibuprofen as the albumin binder, showed higher accumulation in PSMA-positive tumors of mice than the clinically used [¹⁷⁷Lu]Lu-PSMA-617 but lower retention in non-targeted tissues than previously developed albumin-binding PSMA radioligands. The aim of this study was to investigate whether the stereochemistry of the incorporated ibuprofen affects the radioligand's in vitro and in vivo properties and to select the more favorable radioligand for further development. For this purpose, SibuDAB and RibuDAB containing (S)- and (R)-ibuprofen, respectively, were synthesized and labeled with lutetium-177. In vitro, the two isomers had similar properties; however, [¹⁷⁷Lu]Lu-SibuDAB showed increased binding to mouse and human plasma proteins (91 ± 1 and 88 ± 2%, respectively) compared to [¹⁷⁷Lu]Lu-RibuDAB (75 ± 2 and 79 ± 2%, respectively). In vivo, [¹⁷⁷Lu]Lu-SibuDAB was metabolically more stable than [¹⁷⁷Lu]Lu-RibuDAB with ∼90 vs ∼67% intact radioligand detected in the blood at 4 h post injection (p.i.). In line with the lower albumin-binding affinity, the blood clearance of [¹⁷⁷Lu]Lu-RibuDAB in mice was considerably faster [27% of injected activity (% IA), 1 h p.i.] than for [¹⁷⁷Lu]Lu-SibuDAB (50% IA, 1 h p.i.). Time-dependent biodistribution studies performed in tumor-bearing athymic nude mice showed high PSMA-specific tumor uptake for both isomers. A twofold increased area under the curve (AUC_0→8d) of the blood retention was determined for [¹⁷⁷Lu]Lu-SibuDAB as compared to [¹⁷⁷Lu]Lu-RibuDAB, whereas the kidney AUC_0→8d value of [¹⁷⁷Lu]Lu-SibuDAB was only half as high as for [¹⁷⁷Lu]Lu-RibuDAB. As a result, a more favorable tumor-to-kidney AUC_0→8d ratio was obtained for [¹⁷⁷Lu]Lu-SibuDAB, which was also visualized on SPECT/CT images. Based on its improved kidney clearance and higher metabolic stability, [¹⁷⁷Lu]Lu-SibuDAB was selected as the more favorable radioligand. Therapy studies performed with [¹⁷⁷Lu]Lu-SibuDAB (5 MBq/mouse) demonstrated the anticipated therapeutic superiority over the current gold-standard [¹⁷⁷Lu]Lu-PSMA-617 (5 MBq/mouse). The significantly increased survival time of mice treated with [¹⁷⁷Lu]Lu-SibuDAB as compared to those injected with [¹⁷⁷Lu]Lu-PSMA-617 justifies further development of this novel radioligand toward clinical application.

An Albumin-Binding PSMA Ligand with Higher Tumor Accumulation for PET Imaging of Prostate Cancer

Prostate-specific membrane antigen (PSMA) is an ideal target for the diagnosis and treatment of prostate cancer. Due to the short half-life in blood, small molecules/peptides are rapidly cleared by the circulatory system. Prolonging the half-life of PSMA probes has been considered as an effective strategy to improve the tumor detection. Herein, we reported a ⁶⁴Cu-labeled PSMA tracer conjugating with maleimidopropionic acid (MPA), ⁶⁴Cu-PSMA-CM, which showed an excellent ability to detect PSMA-overexpressing tumors in delayed time. Cell experiments in PSMA-positive 22Rv1 cells, human serum albumin binding affinity, and micro-PET imaging studies in 22Rv1 model were performed to investigate the albumin binding capacity and PSMA specificity. Comparisons with ⁶⁴Cu-PSMA-BCH were performed to explore the influence of MPA on the biological properties. ⁶⁴Cu-PSMA-CM could be quickly prepared within 30 min. The uptake of ⁶⁴Cu-PSMA-CM in 22Rv1 cells increased over time and it could bind to HSA with a high protein binding ratio (67.8 ± 1.5%). When compared to ⁶⁴Cu-PSMA-BCH, ⁶⁴Cu-PSMA-CM demonstrated higher and prolonged accumulation in 22Rv1 tumors, contributing to high tumor-to-organ ratios. These results showed that ⁶⁴Cu-PSMA-CM was PSMA specific with a higher tumor uptake, which demonstrated that MPA is an optional strategy for improving the radioactivity concentration in PSMA-expressing tumors and for developing the ligands for PSMA radioligand therapy.