Synthesis and evaluation of indium-111-labeled imidazothiadiazole sulfonamide derivative for single photon emission computed tomography imaging targeting carbonic anhydrase-IX

High-Affinity NIR-Fluorescent Inhibitors for Tumor Imaging via Carbonic Anhydrase IX

Tumor imaging and delivery of therapeutic agents may be achieved by designing high-affinity and high-selectivity compounds recognizing a tumor cell-expressing biomarker, such as carbonic anhydrase IX (CA IX). The CAIX, overexpressed in many hypoxic solid tumors, helps adjust to the energy requirements of the hypoxic environment, reduces intracellular acidification, and participates in the metastatic invasion of adjacent tissues. Here, we designed a series of sulfonamide compounds bearing CAIX-recognizing, high-affinity, and high-selectivity groups conjugated via a PEG linker to near-infrared (NIR) fluorescent probes used in the clinic for optically guided cancer surgery. We determined compound affinities for CAIX and other 11 catalytically active CA isozymes by the thermal shift assay and showed that the affinity Kd value of CAIX was in the subnanomolar range, hundred to thousand-fold higher than those of other CA isozymes. Similar affinities were also observed for CAIX expressed on the cancer cell surface in live HeLa cell cultures, as determined by the competition assay. The NIR-fluorescent compounds showed excellent properties in visualizing CAIX-positive tumors but not CAIX-negative knockout tumors in a nude mice xenograft model. These compounds would therefore be helpful in optically guided cancer surgery and could potentially be developed for anticancer treatment by radiotherapy.

[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.

Five-Membered Heterocyclic Sulfonamides as Carbonic Anhydrase Inhibitors

The development of heterocyclic derivatives has progressed considerably over the past decades, and many new carbonic anhydrase inhibitors (CAIs) fall into this field. In particular, five-membered heterocyclic sulfonamides have been generally shown to be more effective inhibitors compared to six-membered rings ones. Despite the importance of oxygen and nitrogen five-membered heterocyclic aromatic rings in medicinal chemistry, the installation of sulfonamide moiety on such heterocycles has not received much attention. On the other hand, 1,3,4-thiadiazole/thiadiazoline ring-bearing sulfonamides are the scaffolds which have been widely used in a variety of pharmaceutically important CAIs such as acetazolamide, metazolamide and their many derivatives obtained by using the tail approach. Here, we reviewed the field focusing on the diverse biological activities of these CAIs, such as antiglaucoma, antiepileptic, antitumor and antiinfective properties. This review highlights developments involving five-membered heterocyclic sulfonamides over the last years, with a focus on their pharmacological/clinical applications.

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.

Advances in Development of Radiometal Labeled Amino Acid-Based Compounds for Cancer Imaging and Diagnostics

Radiolabeled biomolecules targeted at tumor-specific enzymes, receptors, and transporters in cancer cells represent an intensively investigated and promising class of molecular tools for the cancer diagnosis and therapy. High specificity of such biomolecules is a prerequisite for the treatment with a lower burden to normal cells and for the effective and targeted imaging and diagnosis. Undoubtedly, early detection is a key factor in efficient dealing with many severe tumor types. This review provides an overview and critical evaluation of novel approaches in the designing of target-specific probes labeled with metal radionuclides for the diagnosis of most common death-causing cancers, published mainly within the last three years. Advances are discussed such traditional peptide radiolabeling approaches, and click and nanoparticle chemistry. The progress of radiolabeled peptide based ligands as potential radiopharmaceuticals is illustrated via novel structure and application studies, showing how the molecular modifications reflect their binding selectivity to significant onco-receptors, toxicity, and, by that, practical utilization. The most impressive outputs in categories of newly developed structures, as well as imaging and diagnosis approaches, and the most intensively studied oncological diseases in this context, are emphasized in order to show future perspectives of radiometal labeled amino acid-based compounds in nuclear medicine.

Synthesis and evaluation of 68Ga-labeled imidazothiadiazole sulfonamide derivatives for PET imaging of carbonic anhydrase-IX

INTRODUCTION

Carbonic anhydrase-IX (CA-IX) is markedly overexpressed in many types of solid tumors promoting tumorigenicity and tumor growth. We synthesized novel 68Ga-labeled imidazothiadiazole sulfonamide (IS) derivatives ([68Ga]Ga-DO3A-IS1 and [68Ga]Ga-DO2A-IS2), and evaluated their utility as positron emission tomography (PET) probes targeting CA-IX.

METHODS

[67/68Ga]Ga-DO3A-IS1 and [67/68Ga]Ga-DO2A-IS2 were synthesized from corresponding precursors by ligand substitution reaction in acetate buffer. Cell binding assays were performed using HT-29 cells, which highly express CA-IX, and MDA-MB-231 cells, which show lower-level expression of CA-IX, and a biodistribution assay with model mice bearing the HT-29 or MDA-MB-231 tumor was performed. [68Ga]Ga-DO3A-IS1 was further evaluated by PET/CT.

RESULTS

To evaluate their fundamental properties, [67Ga]Ga-DO3A-IS1 and [67Ga]Ga-DO2A-IS2 were synthesized by conjugation with 67Ga, which has a much longer decay half-life and can be utilized more easily than 68Ga. [67/68Ga]Ga-DO3A-IS1 and [67/68Ga]Ga-DO2A-IS2 were prepared from corresponding precursors with preferable yield and purity. [67Ga]Ga-DO3A-IS1 and [67Ga]Ga-DO2A-IS2 showed significantly greater binding to HT-29 cells than MDA-MB-231 cells in vitro and the binding of [67Ga]Ga-DO2A-IS2 to HT-29 cells was much greater than that of [67Ga]Ga-DO3A-IS1, suggesting multivalent interactions. [67Ga]Ga-DO3A-IS1 and [67Ga]Ga-DO2A-IS2 showed significant selectivity for the HT-29 tumor in vivo, while tumor uptake of [67Ga]Ga-DO3A-IS1 was greater than that of [67Ga]Ga-DO2A-IS2. PET/CT of [68Ga]Ga-DO3A-IS1 showed selectivity for the HT-29 tumor, although [68Ga]Ga-DO3A-IS1 could not be used to visualize the HT-29 tumor clearly because of its strong background signals.

CONCLUSION

These results indicate that 68Ga-labeled IS derivatives may be useful 68Ga-PET probes targeting CA-IX with further structural modifications.

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

The expression of carbonic anhydrase-IX (CA-IX) in tumors can lead to a poor prognosis; thus, CA-IX has attracted much attention as a target molecule for cancer diagnosis and treatment. An ¹¹¹In-labeled imidazothiadiazole sulfonamide (IS) derivative, [¹¹¹In]In-DO3A-IS1, exhibited marked tumor accumulation but also marked renal accumulation, raising concerns about it producing a low signal/background ratio and a high radiation burden on the kidneys. In this study, four ¹¹¹In-labeled IS derivatives, IS-[¹¹¹In]In-DO2A-ALB1-4, which contained four different kinds of albumin binder (ALB) moieties, were designed and synthesized with the aim of improving the pharmacokinetics of [¹¹¹In]In-DO3A-IS1. Their utility for imaging tumors that strongly express CA-IX was evaluated in mice. An in vitro binding assay of cells that strongly expressed CA-IX (HT-29 cells) was performed using acetazolamide as a competitor against CA-IX, and IS-[¹¹¹In]In-DO2A-ALB1-4 did not exhibit reduced binding to HT-29 cells compared with [¹¹¹In]In-DO3A-IS1. In contrast, IS-[¹¹¹In]In-DO2A-ALB1-4 showed a greater ability to bind to human serum albumin than [¹¹¹In]In-DO3A-IS1 in vitro. In an in vivo biodistribution study, the introduction of an ALB moiety into the ¹¹¹In-labeled IS derivative markedly decreased renal accumulation and increased HT-29 tumor accumulation and blood retention. The pharmacokinetics of the IS derivatives varied depending on the substituted group within the ALB moiety. Single-photon emission computed tomography imaging with IS-[¹¹¹In]In-DO2A-ALB1, which showed the highest tumor/kidney ratio in the biodistribution study, facilitated clear HT-29 tumor imaging, and no strong signals were observed in the normal organs. These results indicate that IS-[¹¹¹In]In-DO2A-ALB1 may be an effective CA-IX imaging probe and that the introduction of ALB moieties may improve the pharmacokinetics of CA-IX ligands.

PET imaging and pharmacological therapy targeting carbonic anhydrase-IX high-expressing tumors using US2 platform based on bivalent ureidosulfonamide

Carbonic anhydrase-IX (CA-IX) is attracting much attention as a target molecule for cancer treatment since high expression of CA-IX can lead to a poor prognosis of patients. We previously reported low-molecular-weight ¹¹¹In/⁹⁰Y complexes with a bivalent ureidosulfonamide scaffold ([¹¹¹In/⁹⁰Y]In/Y-US2) as cancer radiotheranostic agents for single photon emission computed tomography and radionuclide-based therapy targeting CA-IX. Here, we applied the US2 platform to positron emission tomography (PET) imaging and pharmacological therapy targeting CA-IX high-expressing tumors by introducing ⁶⁸Ga and ^natIn, respectively. In an in vitro cell binding assay, [⁶⁷Ga]Ga-US2, an alternative complex of [⁶⁸Ga]Ga-US2 with a longer half-life, markedly bound to CA-IX high-expressing (HT-29) cells compared with low-expressing (MDA-MB-231) cells. In a biodistribution study with HT-29 and MDA-MB-231 tumor-bearing mice, [⁶⁷Ga]Ga-US2 showed accumulation in the HT-29 tumor (3.81% injected dose/g at 60 min postinjection) and clearance from the blood pool with time. PET with [⁶⁸Ga]Ga-US2 clearly visualized the HT-29 tumor in model mice at 60 min postinjection. In addition, the administration of [^natIn]In-US2 to HT-29 tumor-bearing mice led to tumor growth delay and prolonged mouse survival, while no critical toxicity was observed. These results indicate that [⁶⁸Ga]Ga-US2 and [^natIn]In-US2 may be useful imaging and therapeutic agents targeting CA-IX, respectively, and that US2 may serve as an effective cancer theranostic platform utilizing CA-IX.