177Lu-Labeled Albumin-Binder-Conjugated PSMA-Targeting Agents with Extremely High Tumor Uptake and Enhanced Tumor-to-Kidney Absorbed Dose Ratio

Salivary toxicity from PSMA-targeted radiopharmaceuticals: What we have learned and where we are going

Clinical trials of prostate-specific membrane antigen (PSMA) targeted radiopharmaceuticals have shown encouraging results. Some agents, like lutetium-177 [177Lu]Lu-PSMA-617 ([177Lu]Lu-PSMA-617), are already approved for late line treatment of metastatic castration-resistant prostate cancer (mCRPC). Projections are for continued growth of this treatment modality; [177Lu]Lu-PSMA-617 is being studied both in earlier stages of disease and in combination with other anti-cancer therapies. Further, the drug development pipeline is deep with variations of PSMA-targeting radionuclides, including higher energy alpha particles conjugated to PSMA-honing vectors. It is safe to assume that an increasing number of patients will be exposed to PSMA-targeted radiopharmaceuticals during the course of their cancer treatment. In this setting, it is important to better understand and mitigate the most commonly encountered toxicities. One particularly vexing side effect is xerostomia. In this review, we discuss the scope of the problem, inventories to better characterize and monitor this troublesome side effect, and approaches to preserve salivary function and effectively palliate symptoms. This article aims to serve as a useful reference for prescribers of PSMA-targeted radiopharmaceuticals, while also commenting on areas of missing data and opportunities for future research.

Development and evaluation of albumin binder-conjugated heterodimeric radiopharmaceuticals targeting integrin αvβ3 and CD13 for cancer therapy

PURPOSE

The advancement of heterodimeric tracers, renowned for their high sensitivity, marks a significant trend in the development of radiotracers for cancer diagnosis. Our prior work on [68Ga]Ga-HX01, a heterodimeric tracer targeting CD13 and integrin αvβ3, led to its approval for phase I clinical trials by the China National Medical Production Administration (NMPA). However, its fast clearance and limited tumor retention pose challenges for broader clinical application in cancer treatment. This study aims to develop a new radiopharmaceutical with increased tumor uptake and prolonged retention, rendering it a potential therapeutic candidate.

METHODS

New albumin binder-conjugated compounds were synthesized based on the structure of HX01. In vitro and in vivo evaluation of these new compounds were performed after labelling with 68Ga. Small-animal PET/CT imaging were conducted at different time points at 0.5-6 h post injection (p.i.) using BxPC-3 xenograft mice models. The one with the best imaging performance was further radiolabeled with 177Lu for small-animal SPECT/CT and ex vivo biodistribution investigation.

RESULTS

We have synthesized novel albumin binder-conjugated compounds, building upon the structure of HX01. When radiolabeled with 68Ga, all compounds demonstrated improved pharmacokinetics (PK). Small-animal PET/CT studies revealed that these new albumin binder-conjugated compounds, particularly [68Ga]Ga-L6, exhibited significantly enhanced tumor accumulation and retention compared with [68Ga]Ga-L0 without an albumin binder. [68Ga]Ga-L6 outperformed [68Ga]Ga-L7, a compound developed using a previously reported albumin binder. Furthermore, [177Lu]Lu-L6 demonstrated rapid clearance from normal tissues, high tumor uptake, and prolonged retention in small-animal SPECT/CT and biodistribution studies, positioning it as an ideal candidate for radiotherapeutic applications.

CONCLUSION

A new integrin αvβ3 and CD13 targeting compound was screened out. This compound bears a novel albumin binder and exhibits increased tumor uptake and prolonged tumor retention in BxPC-3 tumors and low background in normal organs, making it a perfect candidate for radiotherapy when radiolabeled with 177Lu.

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

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

Synthesis and evaluation of novel trifunctional chelating agents for pretargeting approach using albumin binder to improve tumor accumulation

INTRODUCTION

The pretargeting approach consists of in vivo ligation between pre-injected antibodies and low-molecular-weight radiolabeled effectors. The advantage of the pretargeting approach is to improve a tumor-to-background ratio, but the disadvantage is to compromise tumor accumulation. In this study, we applied albumin binder (ALB) to the pretargeting approach to overcome low tumor accumulation.

METHODS

We synthesized two novel trifunctional effectors containing an ALB moiety, a chelator, and a different tetrazine and two corresponding effectors without an ALB moiety. Albumin-binding assays and stability assays were performed using 111In-labeled effectors. Measurements of reaction rate constant were conducted using 111In-labeled effectors and anti-HER2 antibody trastuzumab modified by trans-cyclooctene, which drives the click reaction with tetrazine. Biodistribution studies using HER2-expressing tumor-bearing mice were performed with or without the pretargeting approach.

RESULTS

In albumin-binding assays, ALB-containing effectors exhibited a marked binding to albumin. Two ALB-containing effectors showed the difference in the reactivity and the slight difference in the stability. In biodistribution studies without the pretargeting approach, two ALB-containing effectors showed different pharmacokinetics in blood retention. With the pretargeting approach, the tumor accumulation was improved by the introduction of ALB and the highest tumor accumulation was observed in using the ALB-containing effector with higher blood retention.

CONCLUSION

These results suggest that the application of ALB to the pretargeting approach is effective to improve tumor accumulation, and the structure of tetrazine influences the utility of ALB-containing effectors.

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 .

Optimizing the pharmacokinetics of an 211At-labeled RGD peptide with an albumin-binding moiety via the administration of an albumin-binding inhibitor

PURPOSE

A probe for targeted alpha therapy (TAT) using the RGD peptide (Ga-DOTA-K([211At]APBA)-c(RGDfK) ([211At]1)) with albumin-binding moiety (ABM) was recently developed. [211At]1 highly accumulated in tumors and significantly inhibited tumor growth in U-87 MG tumor-bearing mice. However, high [211At]1 retention in blood may cause critical adverse events, such as hematotoxicity. Therefore, we attempted to accelerate the blood clearance of [211At]1 by competitively inhibiting the binding of [211At]1 to albumin to modulate the pharmacokinetics of the former.

METHODS

To evaluate the effects of albumin-binding inhibitors in normal mice, sodium 4-(4-iodophenyl)butanoate at 2, 5, or 10 molar equivalents of blood albumin was administered at 1-h postinjection of [211At]1. The biodistribution of [211At]1, SPECT/CT imaging of [67Ga]Ga-DOTA-K(IPBA)-c(RGDfK) ([67Ga]2), and the therapeutic effects of [211At]1 were compared with or without IPBA administration in U-87 MG tumor-bearing mice.

RESULTS

Blood radioactivity of [211At]1 was decreased in a dose-dependent manner with IPBA in normal mice. In U-87 MG tumor-bearing mice, the blood radioactivity and accumulation in nontarget tissues of [211At]1 were decreased by IPBA. Meanwhile, tumor [211At]1 accumulation was not changed at 3-h postinjection of IPBA. In SPECT/CT imaging of [67Ga]2, IPBA administration dramatically decreased radioactivity in nontarget tissues, and only tumor tissue was visualized. In therapeutic experiments, [211At]1 with IPBA injected-group significantly inhibited tumor growth compared to the control group.

CONCLUSION

IPBA administration (as an albumin-binding inhibitor) could modulate the pharmacokinetics and enhance the therapeutic effects of [211At]1.

Theranostics - a sure cure for cancer after 100 years?

Cancer has remained a formidable challenge in medicine and has claimed an enormous number of lives worldwide. Theranostics, combining diagnostic methods with personalized therapeutic approaches, shows huge potential to advance the battle against cancer. This review aims to provide an overview of theranostics in oncology: exploring its history, current advances, challenges, and prospects. We present the fundamental evolution of theranostics from radiotherapeutics, cellular therapeutics, and nanotherapeutics, showcasing critical milestones in the last decade. From the early concept of targeted drug delivery to the emergence of personalized medicine, theranostics has benefited from advances in imaging technologies, molecular biology, and nanomedicine. Furthermore, we emphasize pertinent illustrations showcasing that revolutionary strategies in cancer management enhance diagnostic accuracy and provide targeted therapies customized for individual patients, thereby facilitating the implementation of personalized medicine. Finally, we describe future perspectives on current challenges, emerging topics, and advances in the field.

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.

Development, preclinical evaluation and preliminary dosimetry profiling of SB03178, a first-of-its-kind benzo[h]quinoline-based fibroblast activation protein-α-targeted radiotheranostic for cancer imaging and therapy

Fibroblast activation protein-α (FAP) is a marker of cancer-associated fibroblasts (CAFs) that constitute a significant portion of most carcinomas. Since it plays a critical role in tumor growth and metastasis, its timely detection to identify tumor lesions in early developmental stages using targeted radiopharmaceuticals has gained significant impetus. In the present work, two novel FAP-targeted precursors SB03178 and SB04033 comprising of an atypical benzo[h]quinoline construct were synthesized and either chelated to diagnostic radionuclide gallium-68 or therapeutic radionuclide lutetium-177, with ≥90% radiochemical purities and 22-76% decay-corrected radiochemical yields. natGa-labeled complexes displayed dose-dependent FAP inhibition, with binding potency of natGa-SB03178 being ∼17 times higher than natGa-SB04033. To evaluate their pharmacokinetic profiles, PET imaging and ex vivo biodistribution analyses were executed in FAP-overexpressing HEK293T:hFAP tumor-bearing mice. While both tracers displayed clear tumor visualization that was primarily FAP-arbitrated, with negligible uptake in most peripheral tissues, [68Ga]Ga-SB03178 demonstrated higher tumor uptake and superior tumor-to-background contrast ratios than [68Ga]Ga-SB04033. 177Lu-labeled SB03178 was subjected to tumor retention studies, mouse dosimetry profiling and mouse-to-human dose extrapolations also using the HEK293T:hFAP tumor model. [177Lu]Lu-SB03178 exhibited a combination of high and sustained tumor uptake, with excellent tumor-to-critical organ uptake ratios resulting in a high radiation absorbed dose to the tumor and a low estimated whole-body dose to humans. Our preliminary findings are considerably encouraging to support clinical development of [68Ga]Ga-/[177Lu]Lu-SB03178 theranostic pair for use in a vast majority of FAP-overexpressing neoplasms, particularly carcinomas.

Preclinical comparison of [177Lu]Lu-rhPSMA-10.1 and [177Lu]Lu-rhPSMA-10.2 for endoradiotherapy of prostate cancer: biodistribution and dosimetry studies

BACKGROUND

Radiohybrid PSMA-targeted ligands (rhPSMA) have been introduced as a novel platform for theranostic applications. Among a variety of rhPSMA-ligands developed for radioligand therapy, two stereoisomers [177Lu]Lu-rhPSMA-10.1 and -10.2 have been synthesized and initially characterized in preclinical experiments with the aim to provide an optimized binding profile to human serum albumin, a reduction of charge, and thus accelerated kidney excretion, and unaffected or even improved tumor uptake. As both isomers showed similar in vitro characteristics and tumor uptake at 24 h post injection in tumor bearing mice and in order to identify the isomer with the most favorable pharmacokinetics for radioligand therapy, we carried out in-depth biodistribution and dosimetry studies in tumor-bearing and healthy mice.

RESULTS

rhPSMA-10.1 and -10.2 were radiolabeled with lutetium-177 according to the established procedures of other DOTA-based PSMA ligands and displayed a high and comparable stability in all buffers and human serum (> 97%, 24 h). Biodistribution studies revealed fast clearance from the blood pool (0.3-0.6%ID/g at 1 h) and other background tissues within 48 h. Distinctive differences were found in the kidneys, where [177Lu]Lu-rhPSMA-10.1 displayed lower initial uptake and faster excretion kinetics compared to [177Lu]Lu-rhPSMA-10.2 expressed by a 1.5-fold and ninefold lower uptake value at 1 h and 24 h in healthy animals, respectively. Tumor uptake was comparable and in the range of 8.6-11.6%ID/g for both isomers over 24 h and was maintained up to 168 h at a level of 2.2 ± 0.8 and 4.1 ± 1.4%ID/g for [177Lu]Lu-rhPSMA-10.1 and [177Lu]Lu-rhPSMA-10.2, respectively.

CONCLUSION

Our preclinical data on biodistribution and dosimetry indicate a more favorable profile of [177Lu]Lu-rhPSMA-10.1 compared to [177Lu]Lu-rhPSMA-10.2 for PSMA-targeted radioligand therapy. [177Lu]Lu-rhPSMA-10.1 shows fast kidney clearance kinetics resulting in excellent tumor-to-organ ratios over a therapy relevant time course. Meanwhile, [177Lu]Lu-rhPSMA-10.1 is currently being investigated in clinical phase I/II studies in patients with mCRPC (NCT05413850), in patients with high-risk localized PC (NCT06066437, Nautilus Trial) and after external beam radiotherapy (NCT06105918).

177Lu-Labeled Bivalent Ligands of Prostate-Specific Membrane Antigen for Endoradiotherapy of Prostate Cancer

Recently, we developed a bivalent prostate-specific membrane antigen (PSMA) radioligand ([18F]AlF-Bi-PSMA), which showed higher tumor uptake and retention in PSMA-positive mouse models than the clinically used radioligands, [68Ga]Ga-PSMA-11 and [18F]PSMA-1007. Here, we developed two 177Lu-labeled bivalent PSMA ligands with (DOTA-Alb-Bi-PSMA) or without an albumin-binding motif (DOTA-Bi-PSMA) to enhance radiotherapeutic efficacy with minimal toxicity. The results demonstrated that both 177Lu-labeled bivalent radioligands showed good stability, high binding affinity, and PSMA-targeting specificity in vitro. Compared with [177Lu]Lu-PSMA-617, both [177Lu]Lu-Bi-PSMA and [177Lu]Lu-Alb-Bi-PSMA showed a higher area under the curve (AUC) of tumor accumulation and superior therapeutic efficacy. However, [177Lu]Lu-Alb-Bi-PSMA exhibited a dose-dependent increase in acute damage to kidneys. In terms of the radionuclide therapy efficacy and side effects, [177Lu]Lu-Bi-PSMA exhibited well-balanced action with high tumor-to-organs AUC ratios, resulting in remarkable therapeutic efficacy and negligible side effects. These promising results warrant further investigations to achieve the clinical translation of [177Lu]Lu-Bi-PSMA.

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.

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.

Lys-urea-Aad, Lys-urea-Cmc and Lys-urea-Cms as potential pharmacophores for the design of PSMA-targeted radioligands to reduce off-target uptake in kidneys and salivary glands

High kidney and salivary gland uptake is a common feature of prostate-specific membrane antigen (PSMA)-targeted radioligands derived from the lysine-urea-glutamic acid (Lys-urea-Glu) pharmacophore. In this study we investigated if radioligands derived from lysine-urea-2-aminoadipic acid (Lys-urea-Aad), lysine-urea-S-carboxylmethylcysteine (Lys-urea-Cmc) and lysine-urea-O-carboxylmethylserine (Lys-urea-Cms) pharmacophores with/without an albumin binder could retain good PSMA-targeting capability but with minimized kidney and salivary gland uptake. Methods: HTK03177 and HTK03187 were obtained by replacing Aad in the previously reported Lys-urea-Aad-derived HTK03149 with Cmc and Cms, respectively. HTK03170, HTK04048 and HTK04028 were derived from HTK03149, HTK03177 and HTK03187, respectively, with the conjugation of an albumin-binding moiety, 4-(p-methoxyphenyl)butyric acid. In vitro competition binding assays were conducted using PSMA-expressing LNCaP prostate cancer cells and [18F]DCFPyL as the radioligand. Imaging and biodistribution studies of 68Ga-labeled HTK03177 and HTK03187, and 177Lu-labeled HTK03170, HTK04048 and HTK04028 were performed in LNCaP tumor-bearing mice. Radioligand therapy study of [177Lu]Lu-HTK03170 was carried out in LNCaP tumor-bearing mice and [177Lu]Lu-PSMA-617 was used for comparison. Results: The calculated Ki(PSMA) values of Ga-HTK03177, Ga-HTK03187, Lu-HTK03170, Lu-HTK04048 and Lu-HTK04028 were 5.0±2.4, 10.6±2.0, 1.6±0.4, 1.4±1.0 and 13.9±3.2 nM, respectively. PET Imaging and biodistribution studies at 1 h post-injection showed that both [68Ga]Ga-HTK03177 and [68Ga]Ga-HTK03187 had high uptake in LNCaP tumor xenografts (24.7±6.85 and 21.1±3.62 %ID/g, respectively) but minimal uptake in normal organs/tissues including kidneys (7.76±1.00 and 2.83±0.45 %ID/g, respectively) and salivary glands (0.22±0.02 and 0.16±0.02 %ID/g, respectively). SPECT imaging and biodistribution studies showed that the LNCaP tumor uptake of 177Lu-labeled HTK03170, HTK04048 and HTK04028 peaked at 4-24 h post-injection at ~43-65 %ID/g and was relatively sustained over time. Their peaked average uptake in kidneys (≤ 17.4 %ID/g) and salivary glands (≤ 2.92 %ID/g) was lower and continuously reduced over time. Radioligand therapy study showed that compared with [177Lu]Lu-PSMA-617 (37 MBq), a quarter dose of [177Lu]Lu-HTK03170 (9.3 MBq) led to a better median survival (63 vs 90 days). Conclusions: Our data demonstrate that that Lys-urea-Aad, Lys-urea-Cmc and Lys-urea-Cms are promising pharmacophores for the design of PSMA-targeted radioligands especially for radiotherapeutic applications to minimize toxicity to kidneys and salivary glands.

Synthesis and Evaluation of 99mTc-Labeled PSMA-Targeted Tracers Based on the Lys-Urea-Aad Pharmacophore for Detecting Prostate Cancer with Single Photon Emission Computed Tomography

Prostate-specific membrane antigen (PSMA) is a well-validated prostate cancer marker but reported PSMA-targeted tracers derived from the Lys-urea-Glu pharmacophore including the clinically validated [^99mTc]Tc-EDDA/HYNIC-iPSMA have high off-target uptake in kidneys, spleen, and salivary glands. In this study, we synthesized and evaluated three novel ^99mTc-labeled PSMA-targeted tracers and investigated if the tracers derived from the Lys-urea-Aad pharmacophore could have minimized uptake in off-target organs/tissues. In vitro competition binding assays showed that compared with HYNIC-iPSMA, the three novel ligands had slightly weaker PSMA binding affinity (average K_i = 3.11 vs. 8.96-11.6 nM). Imaging and ex vivo biodistribution studies in LNCaP tumor-bearing mice showed that [^99mTc]Tc-EDDA/HYNIC-iPSMA and the three novel tracers successfully visualized LNCaP tumor xenografts in SPECT images and were excreted mainly via the renal pathway. The average tumor uptake at 1 h post-injection varied from 5.40 to 18.8%ID/g, and the tracers derived from the Lys-urea-Aad pharmacophore had much lower uptake in the spleen and salivary glands. Compared with the clinical tracer [^99mTc]Tc-EDDA/HYNIC-iPSMA, the Lys-urea-Aad-derived [^99mTc]Tc-EDDA-KL01127 had lower background uptake and superior tumor-to-background contrast ratios and is therefore promising for clinical translation to detect prostate cancer lesions with SPECT.

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.

Novel 68Ga-Labeled Pyridine-Based Fibroblast Activation Protein-Targeted Tracers with High Tumor-to-Background Contrast

Compared to quinoline-based fibroblast activation protein (FAP)-targeted radiotracers, pyridine-based FAP-targeted tracers are expected to have faster pharmacokinetics due to their smaller molecular size and higher hydrophilicity, which we hypothesize would improve the tumor-to-background image contrast. We aim to develop 68Ga-labeled pyridine-based FAP-targeted tracers for cancer imaging with positron emission tomography (PET), and compare their imaging potential with the clinically validated [68Ga]Ga-FAPI-04. Two DOTA-conjugated pyridine-based AV02053 and AV02070 were synthesized through multi-step organic synthesis. IC50(FAP) values of Ga-AV02053 and Ga-AV02070 were determined by an enzymatic assay to be 187 ± 52.0 and 17.1 ± 4.60 nM, respectively. PET imaging and biodistribution studies were conducted in HEK293T:hFAP tumor-bearing mice at 1 h post-injection. The HEK293T:hFAP tumor xenografts were clearly visualized with good contrast on PET images by [68Ga]Ga-AV02053 and [68Ga]Ga-AV02070, and both tracers were excreted mainly through the renal pathway. The tumor uptake values of [68Ga]Ga-AV02070 (7.93 ± 1.88%ID/g) and [68Ga]Ga-AV02053 (5.6 ± 1.12%ID/g) were lower than that of previously reported [68Ga]Ga-FAPI-04 (12.5 ± 2.00%ID/g). However, both [68Ga]Ga-AV02070 and [68Ga]Ga-AV02053 showed higher tumor-to-background (blood, muscle, and bone) uptake ratios than [68Ga]Ga-FAPI-04. Our data suggests that pyridine-based pharmacophores are promising for the design of FAP-targeted tracers. Future optimization on the selection of a linker will be explored to increase tumor uptake while maintaining or even further improving the high tumor-to-background contrast.

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.

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 Three Novel 68Ga-Labeled Bispecific PSMA/FAP-Targeting Tracers for Prostate Cancer Imaging

Tumor heterogeneity limits the efficacy and reliability of monospecific radiopharmaceuticals in prostate cancer diagnosis and therapy. To overcome this limitation and improve lesion detection sensitivity, we developed and evaluated three bispecific radiotracers that can target both prostate-specific membrane antigen (PSMA) and fibroblast activation protein (FAP), which are the two key proteins overexpressed in prostate cancer. Three FAP-targeting ligands with various linker lengths were synthesized through multistep organic synthesis, and then connected to the PSMA-targeting motif. IC₅₀(PSMA) and IC₅₀(FAP) values of Ga-complexed bispecific ligands, Ga-AV01017, Ga-AV01030, and Ga-AV01038 were 25.2-71.6 and 1.25-2.74 nM, respectively. The uptake values in PSMA-expressing LNCaP tumor xenografts were 4.38 ± 0.55, 5.17 ± 0.51, and 4.25 ± 0.86 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were lower than the monospecific PSMA-targeting tracer [⁶⁸Ga]Ga-HTK03041 (23.1 ± 6.11 %ID/g). The uptake values in FAP-expressing HEK293T:hFAP tumor xenografts were 2.99 ± 0.37, 3.69 ± 0.81, 3.64 ± 0.83 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were also lower than the monospecific FAP-targeting tracer, [⁶⁸Ga]Ga-FAPI-04 (12.5 ± 2.00 %ID/g). We observed that the bispecific tracers had prolonged blood retention, in which tracers with a longer linker tend to have a higher blood uptake and lower tumor uptake. Further investigations are needed to optimize the linker selection to generate promising bispecific PSMA/FAP-targeting tracers for prostate cancer imaging.

Prostate specific membrane antigen binding radiopharmaceuticals: Current data and new concepts

Prostate specific membrane antigen (PSMA) represents a validated target for prostate cancer therapeutics. The phase III VISION study with ¹⁷⁷lutetium (¹⁷⁷Lu)-PSMA-617 represented a pivotal step forward and the FDA has now approved this agent in advanced metastatic castrate-resistant prostate cancer (mCRPC). A number of other PSMA targeted radiopharmaceuticals are now under development. Some of these agents are targeted to PSMA via monoclonal antibodies such as J591 and TLX591. Others are targeted to PSMA via small molecules such as PSMA-617, PSMA I&T, MIP-1095, etc. In addition to the use of various ligands, multiple isotopes are now in clinical trials. Beta emitters in development include ¹⁷⁷Lu, ¹³¹iodide (¹³¹I), and ⁶⁷copper (⁶⁷Cu). Targeted alpha emitters potentially include ²²⁵actinium (²²⁵Ac), ²²⁷thorium (²²⁷Th), and ²¹²lead (²¹²Pb). Phase III trials are underway with both ¹⁷⁷Lu-PSMA-617 and ¹⁷⁷Lu-PSMA I&T in mCRPC. Single dose phase I trials are complete with ²²⁵Ac-J591 but additional data are need to launch a phase III. Data are promising with ²²⁵Ac-PSMA-617 but concerns remain over salivary and renal toxicity. Tandem therapies are also considered combining both beta and alpha-targeted therapy. Taken together the field of PSMA targeted radiopharmaceuticals is rapidly developing. The targeted alpha therapies are particularly promising and several developmental paths forward are being considered in the near future.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

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.

Visceral fat area and albumin based nutrition-related prognostic index model could better stratify the prognosis of diffuse large B-cell lymphoma in rituximab era

Background

Diffuse large B-cell lymphoma (DLBCL) is a heterogeneous disease and the existing prognosis systems based on clinical variables are difficult to stratify patients accurately. Nutritional indices play a meaningful role in prognosis of solid tumors, whereas the effect on DLBCL is still equivocal. This retrospective study aimed to develop a novel model based on nutritional indices and other clinical variables to accurately differentiate the prognosis of DLBCL.

Methods

A total of 129 patients pathologically diagnosed with DLBCL in Affiliated Hospital of Xuzhou Medical University from 2014 to 2018 were retrospectively recruited. The total fat area (TFA), visceral fat area (VFA) and subcutaneous fat area (SFA) at the third lumbar vertebra level spine were obtained by computed tomography (CT) to assess the effect of nutritional status on the prognosis of DLBCL. Principal component analysis was used to reduce the dimension of nutritional indices, and continuous variables were evaluated according to X-Tile and Restricted cubic spline. Univariable and multivariable Cox proportional hazard analyses were performed on potential variables. Kaplan-Meier method was utilized to evaluate survival probabilities and the differences between groups were assessed by log-rank test.

Results

X-Tile analysis divided VFA and albumin into two and three groups when applying 114.7 cm² of VFA, 38.3 and 42.4 g/L of albumin as the optimal cut-off points, respectively. The final scoring model of nutrition-related prognostic index (NPI) comprised four independent prognostic variables. The C-index of the final model was 0.823 [95% CI (0.749~0.897)] by bootstrap resampling. Finally, a maximum score of 6 points was obtained. Compared with IPI, NCCN-IPI and GELTAMO-IPI, NPI showed better accuracy in discerning the prognostic risk of patients.

Conclusion

VFA and albumin were associated with the prognosis of DLBCL, and the NPI model based on nutritional indices could better stratify the prognosis of DLBCL.

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.

Palmitic Acid-Conjugated Radiopharmaceutical for Integrin αvβ3-Targeted Radionuclide Therapy

Peptide receptor radionuclide therapy (PRRT) is an emerging approach for patients with unresectable or metastatic tumors. Our previously optimized RGD peptide (3PRGD2) has excellent targeting specificity for a variety of integrin αvβ3/αvβ5-positive tumors and has been labeled with the therapeutic radionuclide [177Lu]LuCl3 for targeted radiotherapy of tumors. However, the rapid clearance of [177Lu]Lu-DOTA-3PRGD2 (177Lu-3PRGD2) in vivo requires two doses of 111 MBq/3 mCi to achieve effective tumor suppression, limiting its further clinical application. Albumin binders have been attached to drugs to facilitate binding to albumin in vivo to prolong the drug half-life in plasma and obtain long-term effects. In this study, we modified 3PRGD2 with albumin-binding palmitic acid (Palm-3PRGD2) and then radiolabeled Palm-3PRGD2 with 177Lu. [177Lu]Lu-DOTA-Palm-3PRGD2 (177Lu-Palm-3PRGD2) retained a specific binding affinity for integrin αvβ3/αvβ5, with an IC50 value of 5.13 ± 1.16 nM. Compared with 177Lu-3PRGD2, the 177Lu-Palm-3PRGD2 circulation time in blood was more than 6 times longer (slow half-life: 73.42 min versus 11.81 min), and the tumor uptake increased more than fivefold (21.34 ± 4.65 %IA/g and 4.11 ± 0.70 %IA/g at 12 h post-injection). Thus, the significant increase in tumor uptake and tumor retention resulted in enhanced efficacy of targeted radiotherapy, and tumor growth was completely inhibited by a single and relatively lowdose of 18.5 MBq/0.5 mCi. Thus, 177Lu-Palm-3PRGD2 shows great potential for clinical application.

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

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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.

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

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

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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.

A Comparison of Evans Blue and 4-(p-Iodophenyl)butyryl Albumin Binding Moieties on an Integrin αvβ6 Binding Peptide

Serum albumin binding moieties (ABMs) such as the Evans blue (EB) dye fragment and the 4-(p-iodophenyl)butyryl (IP) have been used to improve the pharmacokinetic profile of many radiopharmaceuticals. The goal of this work was to directly compare these two ABMs when conjugated to an integrin αvβ6 binding peptide (αvβ6-BP); a peptide that is currently being used for positron emission tomography (PET) imaging in patients with metastatic cancer. The ABM-modified αvβ6-BP peptides were synthesized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid (DOTA) chelator for radiolabeling with copper-64 to yield [64Cu]Cu DOTA-EB-αvβ6-BP ([64Cu]1) and [64Cu]Cu DOTA-IP-αvβ6-BP ([64Cu]2). Both peptides were evaluated in vitro for serum albumin binding, serum stability, and cell binding and internalization in the paired engineered melanoma cells DX3puroβ6 (αvβ6 +) and DX3puro (αvβ6 −), and pancreatic BxPC-3 (αvβ6 +) cells and in vivo in a BxPC-3 xenograft mouse model. Serum albumin binding for [64Cu]1 and [64Cu]2 was 53−63% and 42−44%, respectively, with good human serum stability (24 h: [64Cu]1 76%, [64Cu]2 90%). Selective αvβ6 cell binding was observed for both [64Cu]1 and [64Cu]2 (αvβ6 (+) cells: 30.3−55.8% and 48.5−60.2%, respectively, vs. αvβ6 (−) cells <3.1% for both). In vivo BxPC-3 tumor uptake for both peptides at 4 h was 5.29 ± 0.59 and 7.60 ± 0.43% ID/g ([64Cu]1 and [64Cu]2, respectively), and remained at 3.32 ± 0.46 and 4.91 ± 1.19% ID/g, respectively, at 72 h, representing a >3-fold improvement over the non-ABM parent peptide and thereby providing improved PET images. Comparing [64Cu]1 and [64Cu]2, the IP-ABM-αvβ6-BP [64Cu]2 displayed higher serum stability, higher tumor accumulation, and lower kidney and liver accumulation, resulting in better tumor-to-organ ratios for high contrast visualization of the αvβ6 (+) tumor by PET imaging.

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

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

Fatty acid-conjugated radiopharmaceuticals for fibroblast activation protein-targeted radiotherapy

INTRODUCTION

Radiopharmaceuticals that target cancer-associated fibroblasts (CAFs) have become an increasingly attractive strategy for cancer theranostics. Recently, a series of fibroblast activation protein inhibitor (FAPI)-based radiopharmaceuticals have been successfully applied to the diagnosis of a variety of cancers and exhibited excellent tumor selectivity. Nevertheless, CAF-targeted radionuclide therapy encounters difficulties in cancer treatment, as the tumor uptake and retention of FAPIs are insufficient. To meet this challenge, we tried to conjugate albumin-binding moiety to FAPI molecule for prolonged circulation that may increase the accumulation and retention of radiopharmaceuticals in tumor.

METHODS

Two fatty acids, lauric acid (C12) and palmitic acid (C16), were conjugated to FAPI-04 to give two albumin-binding FAPI radiopharmaceuticals, denoted as FAPI-C12 and FAPI-C16, respectively. They had been radiolabeled with gallium-68, yttrium-86, and lutecium-177 for stability study, binding affinity assay, PET and SPECT imaging, biodistribution, and radionuclide therapy study to systematically evaluate their potential for CAF-targeted radionuclide therapy.

RESULTS

FAPI-C12 and FAPI-C16 showed high binding affinity to FAP with the IC₅₀ of 6.80 ± 0.58 nM and 5.06 ± 0.69 nM, respectively. They were stable in both saline and plasma. The tumor uptake of [⁶⁸Ga]Ga-FAPI-04 decreased by 56.9% until 30 h after treated with FAPI-C16 before, and the uptakes of [⁸⁶Y]Y-FAPI-C12 and [⁸⁶Y]Y-FAPI-C16 in HT-1080-FAP tumor were both much higher than that of HT-1080-Vehicle tumor which identified the high FAP specific of these two radiopharmaceuticals. Both FAPI-C12 and FAPI-C16 showed notably longer circulation and significantly enhanced tumor uptake than those of FAPI-04. [¹⁷⁷Lu]Lu-FAPI-C16 had the higher tumor uptake at both 24 h (11.22 ± 1.18%IA/g) and 72 h (6.50 ± 1.19%IA/g) than that of [¹⁷⁷Lu]Lu-FAPI-C12 (24 h, 7.54 ± 0.97%IA/g; 72 h, 2.62 ± 0.65%IA/g); both of them were much higher than [¹⁷⁷Lu]Lu-FAPI-04 with the value of 1.24 ± 0.54%IA/g at 24 h after injection. Significant tumor volume inhibition of [¹⁷⁷Lu]Lu-FAPI-C16 at the high activity of 29.6 MBq was observed, and the median survival was 28 days which was much longer than that of the [¹⁷⁷Lu]Lu-FAPI-04 treated group of which the median survival was only 10 days.

CONCLUSION

This proof-of-concept study validates the hypothesis that conjugation of albumin binders may shift the pharmacokinetics and enhance the tumor uptake of FAPI-based radiopharmaceuticals. This could be a general strategy to transform the diagnostic FAP-targeted radiopharmaceuticals into their therapeutic pairs.

Rational Design, Pharmacomodulation, and Synthesis of [68Ga]Ga-Alb-FAPtp-01, a Selective Tumor-Associated Fibroblast Activation Protein Tracer for PET Imaging of Glioma

Dynamic changes in the tumor-associated fibroblast activation protein (FAP) expression in tumors of different stages may be helpful for prognostic evaluation and treatment response monitoring, making this protein a promising surveillance biomarker for timely diagnosis of malignant tumors and effective planning of patient care. To prospectively verify the diagnostic efficacy value of the developed FAP tracers, [⁶⁸Ga]Ga-FAPtp and [⁶⁸Ga]Ga-Alb-FAPtp-01, dynamic/static positron emission tomography (PET)/computed tomography scans were acquired for tumor-targeting studies in vivo and in comparison with the well-established clinically used tracer [⁶⁸Ga]Ga-FAPI-04. The optimized rationally designed FAP-targeting PET tracer, [⁶⁸Ga]Ga-Alb-FAPtp-01, with albumin-binding capability demonstrated prominent tumor uptake over time. The mean standard uptake value (SUV) and the tumor/muscle (T/M) ratio were as high as 1.775 ± 0.179 SUV and T/M = 5.9, 1.533 ± 0.222 SUV and T/M = 6.7, and 1.425 ± 0.204 SUV and T/M = 9.5, respectively, at 1, 2, and 3 h. Its improved tumor uptake and pharmacokinetics suggest that the [⁶⁸Ga]Ga-Alb-FAPtp-01 tracer can noninvasively detect FAP activation in vivo, permitting a precise definition of its roles in tumors of different stages and yielding insights regarding FAP-targeted radiotherapeutic strategies at the molecular level.

PSMA-Targeting Radiopharmaceuticals for Prostate Cancer Therapy: Recent Developments and Future Perspectives

Simple Summary

One of the most frequently diagnosed cancer in men is adenocarcinoma of the prostate. Once the disease is metastatic, only very limited treatment options are available, resulting in a very short median survival time of 13 months; however, this reality is gradually changing due to the discovery of prostate-specific membrane antigen (PSMA), a protein that is present in cancerous prostate tissue. Researchers have developed pharmaceuticals specific for PSMA, ranging from antibodies (mAb) to low-molecular weight molecules coupled to beta minus and alpha-emitting radionuclides for their use in targeted radionuclide therapy (TRT). TRT offers the possibility of selectively removing cancer tissue via the emission of radiation or radioactive particles within the tumour. In this article, the major milestones in PSMA ligand research and the therapeutic developments are summarised, together with a future perspective on the enhancement of current therapeutic approaches.

Abstract

Prostate cancer (PC) is the second most common cancer among men, with 1.3 million yearly cases worldwide. Among those cancer-afflicted men, 30% will develop metastases and some will progress into metastatic castration-resistant prostate cancer (mCRPC), which is associated with a poor prognosis and median survival time that ranges from nine to 13 months. Nevertheless, the discovery of prostate specific membrane antigen (PSMA), a marker overexpressed in the majority of prostatic cancerous tissue, revolutionised PC care. Ever since, PSMA-targeted radionuclide therapy has gained remarkable international visibility in translational oncology. Furthermore, on first clinical application, it has shown significant influence on therapeutic management and patient care in metastatic and hormone-refractory prostate cancer, a disease that previously had remained immedicable. In this article, we provide a general overview of the main milestones in the development of ligands for PSMA-targeted radionuclide therapy, ranging from the firstly developed monoclonal antibodies to the current state-of-the-art low molecular weight entities conjugated with various radionuclides, as well as potential future efforts related to PSMA-targeted radionuclide therapy.

Synthesis and Evaluation of 68Ga- and 177Lu-Labeled (R)- vs (S)-DOTAGA Prostate-Specific Membrane Antigen-Targeting Derivatives

Prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer cells and therefore is an attractive target for prostate cancer diagnosis and radionuclide therapy. Recently, published results from clinical studies using a new PSMA-targeting PET imaging agent, [⁶⁸Ga]Ga-PSMA-093 ([⁶⁸Ga]Ga-HBED-CC-O-carboxymethyl-Tyr-CO-NH-Glu), support the development of this agent for the diagnosis of prostate cancer. In this study, the HBED-CC chelating group in PSMA-093 was replaced by stereoselective (R)- or (S)-DOTAGA. This chelating group serves not only for chelating ⁶⁸Ga but is also amendable for complexing other radioactive metals for radionuclide therapy. The corresponding optically pure (R)- and (S)-[⁶⁸Ga/¹⁷⁷Lu]-DOTAGA derivatives, (R)-[⁶⁸Ga/¹⁷⁷Lu]-13 and (S)-[⁶⁸Ga/¹⁷⁷Lu]-13, were successfully prepared. Comparison of radiolabeling, binding affinity, cell uptake, and biodistribution between the two isomers was performed. Radiolabeling of (R)-[¹⁷⁷Lu]Lu-13 and (S)-[¹⁷⁷Lu]Lu-13 at 50 °C suggested that rates of complex formation were time-dependent and the formation of (S)-[¹⁷⁷Lu]Lu-13 was distinctly faster. The rates of complex formation for the corresponding ⁶⁸Ga agents were comparable between structural isomers. The ^natGa and ^natLu equivalents showed high binding PSMA affinity (IC₅₀ = 24-111 nM), comparable to that of the parent agent, [^natGa]Ga-PSMA-093 (IC₅₀ = 34.0 nM). Results of cell uptake and biodistribution studies in PSMA-expressing PC3-PIP tumor-bearing mice appeared to show no difference between the labeled (R)- and (S)-isomers. This is the first time that a pair of [⁶⁸Ga/¹⁷⁷Lu]-(R)- and (S)-DOTAGA isomers of PSMA agents were evaluated. Results of biological studies between the isomers showed no noticeable difference; however, the distinctions on the rate of Lu complex formation should be considered in the development of new ¹⁷⁷Lu-DOTAGA-based radionuclide therapy agents in the future.