Radiotheranostics Using a Novel 225Ac-Labeled Radioligand with Improved Pharmacokinetics Targeting Prostate-Specific Membrane Antigen

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.

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.

Fundamental evaluation regarding the relationship between albumin-binding and tumor accumulation of PSMA-targeting radioligands

OBJECTIVE

The marked success of prostate-specific membrane antigen (PSMA)-targeting radioligands with albumin binder (ALB) is attributed to the improvement of blood retention and tumor accumulation. [111In]In-PNT-DA1, our PSMA-targeting radioligand with ALB, also achieved improved tumor accumulation due to its prolonged blood retention. Although the advantage of ALBs is related to their reversible binding to albumin, the relationship between albumin-binding and tumor accumulation of PSMA-targeting radioligands remains unclear because of the lack of information about radioligands with stronger albumin-binding than ALBs. In this study, we designed and synthesized [111In]In-PNT-DM-HSA, a new radioligand that consists of a PSMA-targeting radioligand covalently bound to albumin. The pharmacokinetics of [111In]In-PNT-DM-HSA was compared with those of [111In]In-PNT-DA1 and [111In]In-PSMA-617, a non-ALB-conjugated radioligand, to evaluate the relationship between albumin-binding and tumor accumulation.

METHOD

The [111In]In-PNT-DM-HSA was prepared by incubation of [111In]In-PNT-DM, a PSMA-targeting radioligand including a maleimide group, and human serum albumin (HSA). The ability of [111In]In-PNT-DM-HSA was evaluated by in vitro assays. A biodistribution study using LNCaP tumor-bearing mice was conducted to compare the pharmacokinetics of [111In]In-PNT-DM-HSA, [111In]In-PNT-DA1, and [111In]In-PSMA-617.

RESULTS

The [111In]In-PNT-DM-HSA was obtained at a favorable radiochemical yield and high radiochemical purity. In vitro assays revealed that [111In]In-PNT-DM-HSA had fundamental characteristics as a PSMA-targeting radioligand interacting with albumin covalently. In a biodistribution study, [111In]In-PNT-DM-HSA and [111In]In-PNT-DA1 showed higher blood retention than [111In]In-PSMA-617. On the other hand, the tumor accumulation of [111In]In-PNT-DA1 was much higher than [111In]In-PNT-DM-HSA and [111In]In-PSMA-617.

CONCLUSIONS

These results indicate that the moderate reversible binding of ALB with albumin, not covalent binding, may play a critical role in enhancing the tumor accumulation of PSMA-targeting radioligands.

Tailoring Fibroblast-Activation Protein Targeting for Theranostics: A Comparative Preclinical Evaluation of the 68Ga- and 177Lu-Labeled Monomeric and Dimeric Fibroblast-Activation Protein Inhibitors DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2

BACKGROUND

FAP radiopharmaceuticals show promise for cancer diagnosis; however, their limited tumor residency hinders treatment. This study compared two FAPi derivatives, DOTA.SA.FAPi and DOTAGA.(SA.FAPi)2, labeled with gallium-68 and lutetium-177, aiming to determine an optimum combination for creating theranostic pairs.

METHODS

The radiotracers were studied for lipophilicity, binding to human serum proteins, and binding to human cancer-associated fibroblasts (CAFs) in vitro, including saturation and internalization/externalization studies. PET/SPECT/CT and biodistribution studies were conducted in PC3 and U87MG xenografts for [68Ga]Ga-DOTA.SA.FAPi and [68Ga]Ga-DOTAGA.(SA.FAPi)2. [177Lu]Lu-DOTA.SA.FAPi and [177Lu]Lu-DOTAGA.(SA.FAPi)2, were evaluated in PC3 xenografts. Biodistribution studies of [68Ga]Ga-DOTA.SA.FAPi were performed in healthy male and female mice.

RESULTS

All radiotracers exhibited strong binding to FAP. Their internalization rate was fast while only [177Lu]Lu-DOTAGA.(SA.FAPi)2 was retained longer in CAFs. [68Ga]Ga-DOTAGA.(SA.FAPi)2 and [177Lu]Lu-DOTAGA.(SA.FAPi)2 displayed elevated lipophilicity and affinity for human serum proteins compared to [68Ga]Ga-DOTA.SA.FAPi and [177Lu]Lu-DOTA.SA.FAPi. In vivo studies revealed slower washout of [68Ga]Ga-DOTAGA.(SA.FAPi)2 within 3 h compared to [68Ga]Ga-DOTA.SA.FAPi. The tumor-to-tissue ratios of [68Ga]Ga-DOTAGA.(SA.FAPi)2 versus [68Ga]Ga-DOTA.SA.FAPi did not exhibit any significant differences. [177Lu]Lu-DOTAGA.(SA.FAPi)2 maintained a significant tumor uptake even after 96 h p.i. compared to [177Lu]Lu-DOTA.SA.FAPi.

CONCLUSIONS

Dimeric compounds hold promise for therapy, while monomers are better suited for diagnostics. Finding the right combination is essential for effective disease management.

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.

[Radiotheranostics Based on Chemical Control of Radioactivity Pharmacokinetics]

Recently, radiotheranostics, which systematically combines diagnosis by nuclear medicine imaging and treatment by internal radiotherapy, constitutes a new modality in cancer treatment, with some clinical reports showing marked effects on cancer. We have been developing multifunctional chelates containing a target recognition unit, a radiation release unit, and a radioactivity pharmacokinetics control unit in the same molecule to develop efficient agents for cancer radiotheranostics based on chemical control of radioactivity pharmacokinetics. Using these compounds, we have achieved improved cancer accumulation and reduced renal accumulation in tumor-bearing mice, and have developed novel hybrid radiotheranostic agents that can be applied to simultaneously perform target-specific molecular imaging using γ-ray emitting radionuclides and internal radiotherapy using α-particle-emitting radionuclides. For example, 111In/225Ac-labeled PSMA-DA1, which targets prostate-specific membrane antigen (PSMA) for radiotheranostics, achieved clear in vivo imaging of PSMA in tumor-bearing mice and showed marked tumor growth inhibition. In addition to PSMA, this platform for radiotheranostics has also shown efficacy against various cancer target molecules, including carbonic anhydrase IX (CA-IX), which is highly expressed in hypoxic regions of cancer, and glucagon-like peptide-1 receptor (GLP-1R), which is highly expressed in insulinomas. This review presents these recent results of our studies on radiotheranostics for cancer.

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.

A review of recent advancements in Actinium-225 labeled compounds and biomolecules for therapeutic purposes

In nuclear medicine, cancers that cannot be cured or can only be treated partially by traditional techniques like surgery or chemotherapy are killed by ionizing radiation as a form of therapeutic treatment. Actinium-225 is an alpha-emitting radionuclide that is highly encouraging as a therapeutic approach and more promising for targeted alpha therapy (TAT). Actinium-225 is the best candidate for tumor cells treatment and has physical characteristics such as high (LET) linear energy transfer (150 keV per μm), half-life (t1/2  = 9.92d), and short ranges (400-100 μm) which prevent the damage of normal healthy tissues. The introduction of various new radiopharmaceuticals and radioisotopes has significantly assisted the advancement of nuclear medicine. Ac-225 radiopharmaceuticals continuously demonstrate their potential as targeted alpha therapeutics. 225 Ac-labeled radiopharmaceuticals have confirmed their importance in medical and clinical areas by introducing [225 Ac]Ac-PSMA-617, [225 Ac]Ac-DOTATOC, [225 Ac]Ac-DOTA-substance-P, reported significantly improved response in patients with prostate cancer, neuroendocrine, and glioma, respectively. The development of these radiopharmaceuticals required a suitable buffer, incubation time, optimal pH, and reaction temperature. There is a growing need to standardize quality control (QC) testing techniques such as radiochemical purity (RCP). This review aims to summarize the development of the Ac-225 labeled compounds and biomolecules. The current state of their reported resulting clinical applications is also summarized as well.

Recent Pre-Clinical Advancements in Nuclear Medicine: Pioneering the Path to a Limitless Future

The theranostic approach in oncology holds significant importance in personalized medicine and stands as an exciting field of molecular medicine. Significant achievements have been made in this field in recent decades, particularly in treating neuroendocrine tumors using 177-Lu-radiolabeled somatostatin analogs and, more recently, in addressing prostate cancer through prostate-specific-membrane-antigen targeted radionuclide therapy. The promising clinical results obtained in these indications paved the way for the further development of this approach. With the continuous discovery of new molecular players in tumorigenesis, the development of novel radiopharmaceuticals, and the potential combination of theranostics agents with immunotherapy, nuclear medicine is poised for significant advancements. The strategy of theranostics in oncology can be categorized into (1) repurposing nuclear medicine agents for other indications, (2) improving existing radiopharmaceuticals, and (3) developing new theranostics agents for tumor-specific antigens. In this review, we provide an overview of theranostic development and shed light on its potential integration into combined treatment strategies.

Synthesis and evaluation of 111 In-labeled tetrapeptide-based compounds as single-photon emission computed tomography imaging probes targeting granzyme B

Granzyme B is an attractive target as a biomarker for contributing to improve the treatment with immune checkpoint inhibitor (ICI). In this study, we designed novel 111 In-labeled granzyme B-targeting single-photon emission computed tomography (SPECT) imaging probes, [111 In]IDT and [111 In]IDAT. Nonradioactive In-labeled granzyme B-targeting compounds ([nat In]IDT, [nat In]IDAT) showed the affinity for recombinant mouse granzyme B. [111 In]IDT and [111 In]IDAT were obtained with moderate radiochemical yield and high stability in mouse plasma (>95%). In a biodistribution experiment using tumor-bearing mice, [111 In]IDT and [111 In]IDAT showed moderate accumulation in tumor. Ex vivo autoradiography (ARG) indicated that the accumulation of radioactivity in tumor was correlated to expression of granzyme B confirmed by the immunohistochemical staining. These results indicated that [111 In]IDT and [111 In]IDAT showed the basic properties as granzyme B-targeting SPECT probes.

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.

Recent advances in the molecular targeted drugs for prostate cancer

CONTEXT

Prostate cancer (PCa) is the second largest male tumor in the world and one of the most common malignant tumors in the urinary system. In recent years, the incidence rate of PCa in China has been increasing year by year. Meanwhile, refractory hormone resistance and adverse drug reactions of advanced PCa cause serious harm to patients.

OBJECTIVE

The present study aims to systematically review the recent advances in molecularly targeted drugs for prostate cancer and to use the retrieval and analysis of the literature library to summarize the adverse effects of different drugs so as to maximize the treatment benefits of targeted therapies.

EVIDENCE ACQUISITION

We performed a systematic literature search of the Medline, EMBASE, PubMed, and Cochrane databases up to March 2022 in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement. Medical Subject Heading (MeSH) terms and keywords such as (prostate cancer) AND (molecular target drugs) AND (side effect) were used. No language restrictions were set on the search process, and all these results were processed independently by two authors. Consensus was reached through discussion once met with any disagreements. The primary endpoint was differential features between different molecular targeted drugs. Secondary endpoints were side effects of different drugs on the body and corresponding prognostic values.

EVIDENCE SYNTHESIS

The Cochrane Collaboration risk of bias tool was used to assess the study quality in terms of sequence generation, allocation concealment, blinding, the completeness of outcome data, selective reporting and other biases. We retrieved 332 articles, of which 49 met the criteria for inclusion. Included studies show that prostatic tumor cells, tumor neovascularization and immune checkpoints are the main means for targeted therapy. Common drugs include 177 Lu-PSMA, Olaparib, Rucaparib, Bevacizumab, Pazopanib, Sorafenib, Cabozantinib, Aflibercept, Ipilimumab, Atezolizumab, Avelumab, Durvalumab. A series of publicly available data suitable for further analysis of side effects. An over-representation analysis of these datasets revealed reasonable dosage and usage is the key to controlling the side effects of targeted drugs. Important information such as the publication year, the first author, location and outcome observation of adverse effects was extracted from the original article. If the study data has some insufficient data, contacting the corresponding authors is necessary. All the studies included prospective nonrandomized and randomized research. Retrospective reviews were also screened according to the relevant to the purpose of this study. Meeting abstracts as well as letters to the editor and editorials were excluded.

STATISTICAL ANALYSIS

Data analysis was based on Cochrane's risk of bias tools to obtain the quality assessment. The included randomized studies used RoB2 and non-randomized ones corresponded to ROBINS-I. Standardized mean differences (SMD) were used to determine relative risk (RR) and side effects between groups. The eggers' test was used to check the publication bias from variable information in the included studies. All p < 0.05 were considered to be significant, and 95% was set as the confidence interval.

CONCLUSIONS

With the approval of a variety of targeted drugs, targeted therapy will be widely used in the treatment of advanced or metastatic prostate cancer. Despite the existence of adverse reactions related to targeted drug treatment, it is still meaningful to adjust the drug dosage or treatment cycle to reduce the occurrence of adverse reactions, improving the treatment benefits of patients.

[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.

Application of the Chelator-Based Clickable Radiotheranostic Platform to Moderate-Molecular-Weight Ligands

We have reported that the chelator-based clickable radiotheranostic platform, ADIBO-DOTADG-ALB (ADA), has favorable properties as a radiotheranostic platform for low-molecular-weight ligands. In this study, we evaluated the applicability of ADA to moderate-molecular-weight ligands to expand the utility of the ADA platform. As a moderate-molecular-weight ligand, we selected exendin-4, a peptide-based agonist to glucagon-like peptide-1 receptor (GLP-1R). An exendin-4-incorporated ADA derivative, exendin-4-Cys40-triazole-DOTADG-ALB (EtDA), was radiolabeled with 111In by the conjugation of exendin-4-Cys40 azide to [111In]In-ADA. The click ligation of exendin-4-Cys40 azide to [111In]In-ADA was quantitatively completed in 10 min under ambient conditions. In the in vitro cell-binding assay and albumin-binding assay, [111In]In-EtDA showed strong binding to both a GLP-1R-expressing cell and albumin. In the biodistribution assay, [111In]In-EtDA showed markedly protracted tumor uptake, which was significantly decreased by the coinjection of exendin-4-Cys40. The single photon emission computed tomography (SPECT) image of [111In]In-EtDA visualized the tumor clearly. These results indicated the utility of [111In]In-EtDA as a radiotheranostic agent, suggesting the applicability of the ADA platform to moderate-molecular-weight ligands.

Structure-Activity Relationships and Pharmacokinetics of 111In-Labeled Glucagon-like Peptide-1 Receptor-Targeting Exendin-4 Derivatives Conjugated with Albumin Binder Moieties

Insulinomas are neuroendocrine tumors that are derived from pancreatic β-cells, and they often overexpress the glucagon-like peptide-1 receptor (GLP-1R). Radiolabeled exendin-4 derivatives have been used to noninvasively detect the GLP-1R during the diagnosis and preoperative localization of insulinomas; however, their marked renal accumulation can hinder the imaging of pancreatic tail lesions. In this study, we designed and synthesized ¹¹¹In-labeled exendin-4 derivatives that possessed 4-(4-substituted phenyl)-moieties as albumin binder (ALB) moieties ([¹¹¹In]In-E4DA2-4), and studied their structure-activity relationships and pharmacokinetics (as well as those of [¹¹¹In]In-E4DA1, which we previously reported) to determine their usefulness as radioligands for GLP-1R imaging. ¹¹¹In-labeling was performed by reacting maleimide precursors with [¹¹¹In]InCl₃ in 2-(N-morpholino)ethanesulfonic acid buffer, and then, the products were conjugated with exendin-4-Cys⁴⁰. A saturation binding assay using GLP-1R-expressing INS-1 cells was carried out to evaluate the in vitro affinity of the radioligands for the cells. In addition, the affinity of the ¹¹¹In-labeled derivatives for human serum albumin (HSA) was evaluated in an HSA-binding assay. Furthermore, an in vivo biodistribution study and single-photon emission computed tomography (SPECT) imaging were performed using INS-1 tumor-bearing mice. [¹¹¹In]In-E4DA1-4 were prepared at radiochemical yields of 6-17%. In the saturation binding assay, [¹¹¹In]In-E4DA1-4 showed a similar affinity for the INS-1 cells, indicating that the kind of ALB moiety used had no effect on the affinity of the exendin-4 derivatives for the cells. In the HSA-binding assay, [¹¹¹In]In-E4DA1-4 all bound to HSA. In the biodistribution assay, [¹¹¹In]In-E4DA1-4 exhibited marked tumor accumulation and retention. In addition, they showed lower renal accumulation than previously reported exendin-4-based radioligands without ALB moieties. The pharmacokinetics of the ¹¹¹In-labeled exendin-4 derivatives varied markedly according to the kind of ALB moiety used. In particular, [¹¹¹In]In-E4DA2, which contained a 4-(4-bromophenyl)butyric acid derivative as an ALB moiety, showed the highest tumor accumulation. SPECT imaging with [¹¹¹In]In-E4DA2 clearly visualized INS-1 tumors with no marked accumulation in normal organs. These results provide important information that will aid the design of novel exendin-4-based radioligands targeting the GLP-1R.

Synthesis and Evaluation of Novel 111In-Labeled Picolinic Acid-Based Radioligands Containing an Albumin Binder for Development of a Radiotheranostic Platform

Picolinic acid-based metallic chelators, e.g., neunpa and octapa, have attracted much attention as promising scaffolds for radiotheranostic agents, particularly those containing larger α-emitting radiometals. Furthermore, albumin binder (ALB) moieties, which noncovalently bind to albumin, have been utilized to improve the pharmacokinetics of radioligands targeting various biomolecules. In this study, we designed and synthesized novel neunpa and octapa derivatives (Neunpa-2 and Octapa-2, respectively), which contained a prostate-specific membrane antigen (PSMA)-binding moiety (model targeting vector) and an ALB moiety. We evaluated the fundamental properties of these derivatives as radiotheranostic agents using ¹¹¹In. In a cell-binding assay using LNCaP (PSMA-positive) cells, [¹¹¹In]In-Neunpa-2 and [¹¹¹In]In-Octapa-2 specifically bound to the LNCaP cells. In addition, a human serum albumin (HSA)-binding assay revealed that [¹¹¹In]In-Neunpa-2 and [¹¹¹In]In-Octapa-2 exhibited greater binding to HSA than their non-ALB-conjugated counterparts ([¹¹¹In]In-Neunpa-1 and [¹¹¹In]In-Octapa-1, respectively). A biodistribution assay conducted in LNCaP tumor-bearing mice showed that the introduction of the ALB moiety into the ¹¹¹In-labeled neunpa and octapa derivatives resulted in markedly enhanced tumor uptake and retention of the radioligands. Furthermore, single-photon emission computed tomography imaging of LNCaP tumor-bearing mice with [¹¹¹In]In-Octapa-2 produced tumor images. These results indicate that [¹¹¹In]In-Octapa-2 may be a useful PSMA imaging probe and that picolinic acid-based ALB-conjugated radiometallic complexes may be promising candidates as radiotheranostic agents.

Rational Design and Pharmacomodulation of Protein-Binding Theranostic Radioligands for Targeting the Fibroblast Activation Protein

The fibroblast activation protein (FAP), overexpressed on cancer-associated fibroblasts (CAFs), has become a valuable target for tumor diagnosis and therapy. However, most FAP-based radioligands show insufficient tumor uptake and retention. In this study, three novel albumin-binding FAP ligands (denoted as FSDD₀I, FSDD₁I, and FSDD₃I) were labeled with ⁶⁸Ga and ¹⁷⁷Lu to overcome these limitations. Cell-based studies and molecular docking assays were performed to identify the specificity and protein-binding properties for FAP. Positron emission tomography (PET) scans in human hepatocellular carcinoma patient-derived xenografts (HCC-PDXs) animal models revealed longer blood retention of ⁶⁸Ga-FSDD₀I than ⁶⁸Ga-FAPI-04, ⁶⁸Ga-FSDD₁I, and ⁶⁸Ga-FSDD₃I. Remarkably, ⁶⁸Ga-FSDD₃I had prominent tumor-to-nontarget (T/NT) ratios. The prominent tumor retention properties of ¹⁷⁷Lu-FSDD₀I in single photon emission computed tomography (SPECT) imaging and biodistribution studies were demonstrated. In summary, this study reports a proof-of-concept study of albumin-binding radioligands for FAP-targeted imaging and targeted radionuclide therapy (TRT).

Development of an 111In-Labeled Glucagon-Like Peptide-1 Receptor-Targeting Exendin-4 Derivative that Exhibits Reduced Renal Uptake

Insulinomas are neuroendocrine tumors that are mainly found in the pancreas. Surgical resection is currently the first-line treatment for insulinomas; thus, it is vital to preoperatively determine their locations. The marked expression of the glucagon-like peptide-1 receptor (GLP-1R) is seen in pancreatic β-cells and almost all insulinomas. Radiolabeled derivatives of exendin-4, a GLP-1R agonist, have been used with nuclear medicine imaging techniques for the in vivo detection of the GLP-1R; however, their marked renal accumulation can hinder the imaging of pancreatic tail lesions. To develop a GLP-1R imaging probe that exhibits reduced renal accumulation, we designed and synthesized a straight-chain GLP-1R-targeting radioligand, [¹¹¹In]In-E4DA1, which consisted of exendin-4, DOTADG (a chelator), and an (iodophenyl)butyric acid derivative (an albumin binder [ALB]). We performed preclinical evaluations of [¹¹¹In]In-E4DA1 to investigate its utility as a GLP-1R imaging probe. [¹¹¹In]In-E4DA1 and [¹¹¹In]In-E4D (a control compound lacking the ALB moiety) were prepared by reacting the corresponding precursors with [¹¹¹In]InCl₃ in buffer. Cell-binding and human serum albumin (HSA)-binding assays were performed to assess the in vitro affinity of the molecules for INS-1 (GLP-1R-positive) cells and albumin, respectively. A biodistribution assay and single-photon emission computed tomography imaging were carried out using INS-1 tumor-bearing mice. In the cell-binding assay, [¹¹¹In]In-E4DA1 and [¹¹¹In]In-E4D exhibited in vitro binding to INS-1 cells. In the HSA-binding assay, [¹¹¹In]In-E4DA1 bound to HSA, while [¹¹¹In]In-E4D showed little HSA binding. The in vivo experiments involving INS-1 tumor-bearing mice revealed that the introduction of an ALB moiety into the DOTADG-based exendin-4 derivative markedly increased the molecule's tumor accumulation while decreasing its renal accumulation. In addition, [¹¹¹In]In-E4DA1 exhibited greater tumor accumulation than renal accumulation, whereas previously reported radiolabeled exendin-4 derivatives demonstrated much higher accumulation in the kidneys than in tumors. These results indicate that [¹¹¹In]In-E4DA1 may be a useful GLP-1R imaging probe, as it demonstrates only slight renal accumulation.

Legumain-mediated self-assembly of 131I-labelled agent for targeted radiotherapy of tumor

Targeted radionuclide therapy (TRT) has been a promising strategy for cancer therapy, which can inhibit or kill cancer cells by selectively delivering radionuclide to target tissues. Herein, a legumain-targeted therapeutic...