Modulation of the Pharmacokinetics of a Radioligand Targeting Carbonic Anhydrase-IX with Albumin-Binding Moieties

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.

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.

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.

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

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

[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.

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.

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.

New Developments in Carbonic Anhydrase IX-Targeted Fluorescence and Nuclear Imaging Agents

Carbonic anhydrase IX (CAIX) is a tumor-specific and hypoxia-induced biomarker for the molecular imaging of solid malignancies. The nuclear- and optical-imaging of CAIX-expressing tumors have received great attention due to their potential for clinical applications. Nuclear imaging is a powerful tool for the non-invasive diagnosis of primary and metastatic CAIX-positive tumors and for the assessment of responses to antineoplastic treatment. Intraoperative optical fluorescence imaging provides improved visualization for surgeons to increase the discrimination of tumor lesions, allowing for safer surgical treatment. Over the past decades, many CAIX-targeted molecular imaging probes, based on monoclonal antibodies, antibody fragments, peptides, and small molecules, have been reported. In this review, we outline the recent development of CAIX-targeted probes for single-photon emission computerized tomography (SPECT), positron emission tomography (PET), and near-infrared fluorescence imaging (NIRF), and we discuss issues yet to be addressed.

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.

Carbonic anhydrase inhibitors: an update on experimental agents for the treatment and imaging of hypoxic tumors

INTRODUCTION

Hypoxic tumors, unlike normal tissues, overexpress proteins involved in oxygen sensing, metabolism, pH regulation, angiogenesis, immunological response, and other survival mechanisms, which are under investigation as antitumor drug targets.

AREAS COVERED

Carbonic anhydrase (CA) isoforms CA IX and XII are among these validated antitumor/antimetastatic drug targets, with several of their inhibitors undergoing preclinical or clinical-stage investigations. Alone or in combination with other chemotherapeutic agents or radiotherapy, CA IX/XII inhibitors, such as SLC-0111, SLC-149, S4, 6A10, etc., were shown to inhibit the growth of the primary tumor, metastases, and invasiveness of many tumor types, being also amenable for the development of imaging agents.

EXPERT OPINION

SLC-0111 is the most investigated agent, being in Phase Ib/II clinical trials. In addition to its interference with extracellular acidifications, it has been shown to promote ferroptosis in cancer cells, another antitumor mechanism of this compound and the entire class. A large number sulfonamide and non-sulfonamide inhibitors have been developed using SLC-0111 as lead in the last three years, together with hybrid agents incorporating CA inhibitors and other anticancer chemotypes, including cytotoxins, telomerase, thioredoxin or P-glycoprotein inhibitors, adenosine A₂A receptor antagonists, pyrophosphatase/phosphodiesterase-3 inhibitors or antimetabolites. All of them showed significant antitumor activity.

Feasibility of using a 99mTc-hydroxamamide complex containing an albumin binder moiety for in vivo albumin labeling-based tumor imaging

Human serum albumin (HSA), which is distributed throughout the blood, is used as a carrier for transporting drugs to tumors based on the enhanced permeability and retention (EPR) effect. To develop an agent for the in vivo radiolabeling of endogenous albumin, we designed and synthesized novel hydroxamamide (Ham)-based technetium-99m (^99mTc) complexes, which contained a monovalent or bivalent 4-(4-iodophenyl)butyric acid (IA) derivative as an albumin binder (ALB) moiety ([^99mTc]AB2 and [^99mTc]ALB2, respectively), and evaluated their utility for in vivo tumor imaging. In an in vitro HSA-binding assay, [^99mTc]AB2 and [^99mTc]ALB2 showed greater binding to HSA than [^99mTc]BHam, a ^99mTc-Ham complex without an ALB moiety. In an in vivo biodistribution assay, [^99mTc]ALB2 showed marked blood and tumor retention (25.13 and 4.61% injected dose (ID)/g, respectively, at 1 h postinjection), suggesting that the EPR effect had been induced. However, [^99mTc]AB2 showed no marked blood or tumor retention (4.16 and 0.75% ID/g, respectively, at 1 h postinjection), probably because the affinity of the monovalent IA derivative for albumin was insufficient to induce the EPR effect. These findings indicated that the multivalent interactions of [^99mTc]ALB2 had enhanced its affinity for albumin. ^99mTc-complexes containing multivalent ALB moieties may be useful for tumor imaging.

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

225Ac-based radiotheranostics targeting prostate-specific membrane antigen (PSMA) has induced impressive responses in patients with metastatic castration-resistant prostate cancer. To enhance the therapeutic effects of radioligands labeled with 225Ac (half-life: 10 days), a radioligand that shows longer tumor retention would be useful. Here, we designed and synthesized a straight-chain PSMA-targeting radioligand, PSMA-DA1, which includes an (iodophenyl)butyric acid derivative as an albumin binder (ALB). We performed preclinical evaluations of PSMA-DA1 as a tool for PSMA-targeting radiotheranostics using 111In, 90Y, and 225Ac. [111In]In-PSMA-DA1 demonstrated significantly greater tumor uptake and retention than a corresponding non-ALB-conjugated compound. In mice, single-photon emission computed tomography performed with [111In]In-PSMA-DA1 produced clear tumor images, and the administration of [90Y]Y-PSMA-DA1 or [225Ac]Ac-PSMA-DA1 inhibited tumor growth. [225Ac]Ac-PSMA-DA1 had antitumor effects in mice at a lower radioactivity level than [225Ac]Ac-PSMA-617, which has been reported to be clinically useful. These results indicate that PSMA-DA1 may be a useful PSMA-targeting radiotheranostic agent.

Development of a novel radiotheranostic platform with a DOTA-based trifunctional chelating agent

Radiotheranostics has attracted attention as a powerful strategy for treating cancer patients with precision medicine. We designed and synthesized a novel DOTA-based trifunctional agent, ADIBO-DOTADG-ALB (ADA), which allowed compounds with targeting ligands, radiometals, and an albumin binder to be readily prepared. ADA exhibited promising properties as a theranostic platform.