Copper-64-Labeled Antibody Fragments for Immuno-PET/Radioimmunotherapy with Low Renal Radioactivity Levels and Amplified Tumor-Kidney Ratios

Comparative Kidney Uptake of Nanobody-Based PET Tracers Labeled with Various Fluorine-18-Labeled Prosthetic Groups

Nanobodies, or single-domain antibody fragments, are promising candidates for molecular imaging due to their small size, rapid tissue penetration, and high target specificity. However, a significant challenge in their use is high renal uptake and retention, which can limit the therapeutic efficacy and complicate image interpretation. This study compares five different fluorine-18-labeled prosthetic groups for nanobodies, aiming to optimize pharmacokinetics and minimize kidney retention while maintaining tumor targeting. Using an epidermal growth factor receptor (EGFR) targeting nanobody as a model, two labeling approaches were evaluated; direct labeling of RESCA (with and without polyethylene glycol (PEG))-conjugated nanobody using Al[18F]F and indirect labeling using ([18F]F-fluoropyridine ([18F]F-FPy)-based prosthetic groups (site-specific and nonsite-specific). Labeled nanobodies were characterized in vitro for binding affinity and cell uptake with in vivo behavior assessed in EGFR + A431 tumor-bearing mice using PET imaging and biodistribution studies. Labeling with Al[18F]F showed high renal retention, which was partially mitigated by PEGylation. However, PEGylation also led to a decreased tumor uptake, particularly with longer PEG chains. Labeling using [18F]F-FPy prosthetic groups exhibited the most favorable pharmacokinetics, with rapid renal clearance and minimal kidney retention while maintaining high tumor uptake. These constructs showed excellent tumor-to-background contrast as early as 1 h postinjection. The study confirms that the selection of the prosthetic group significantly impacts the in vivo behavior of nanobodies, particularly regarding kidney accumulation. [18F]F-FPy-based prosthetic groups show the most promising results, with high tumor and minimal kidney uptake. Robust production of [18F]F-FPy on Sep-Pak is adaptable to clinical translation. Moreover, the potential substitution of 18F with therapeutic radioisotopes such as 131I or 211At could expand the application of these nanobodies from diagnostics to targeted radionuclide therapy while maintaining a low kidney exposure. These findings have important implications for optimizing nanobody-based radiopharmaceuticals for molecular imaging and targeted radionuclide therapy.

Recent advances in copper homeostasis-involved tumor theranostics

As the third essential trace element in the human body, copper plays a crucial role in various physiological processes, which lays the foundation for its broad applications in cancer treatments. The overview of copper, including pharmacokinetics, signaling pathways, and homeostasis dysregulation, is hereby discussed. Additionally, cuproptosis, as a newly proposed cell death mechanism associated with copper accumulation, is analyzed and further developed for efficient cancer treatment. Different forms of Cu-based nanoparticles and their advantages, as well as limiting factors, are introduced. Moreover, the unique characteristics of Cu-based nanoparticles give rise to their applications in various imaging modalities. In addition, Cu-based nanomaterials are featured by their excellent photothermal property and ROS-associated tumor-killing potential, which are widely explored in diverse cancer therapies and combined therapies. Reducing the concentration of Cu2+/Cu+ is another cancer-killing method, and chelators can meet this need. More importantly, challenges and future prospects are identified for further research.

Reduction of the hepatic radioactivity levels of [111In]In-DOTA-labeled antibodies via cleavage of a linkage metabolized in lysosomes

INTRODUCTION

Radiolabeled antibodies are promising tools for cancer diagnosis using nuclear medicine. A DOTA-chelating system is useful for preparing immuno-positron emission tomography and immuno-single-photon emission computed tomography probes with various radiometals. Radiolabeled antibodies are generally metabolized in the reticuloendothelial system, producing radiometabolites after proteolysis in hepatic lysosomes. Because of the bulkiness and extremely high hydrophilicity of DOTA, radiometabolites containing a radiometal-DOTA complex typically exhibit high and persistent localization in hepatic lysosomes. Radioactivity in the liver impairs the accurate diagnosis of cancer surrounding the liver and liver metastasis, and a high tumor/liver ratio is desirable. In this study, we reduced the hepatic radioactivity of radiometal-labeled antibodies containing a DOTA-chelating system. A cleavable linkage was inserted to liberate the radiometabolite, which exhibited a short residence time in hepatocytes.

METHODS

Using indium-111 (111In)-labeled antibodies, we prepared 111In-labeled galactosyl-neoglycoalbumins (NGAs) because they are useful for evaluating the residence time of radiometabolites in the liver. An 111In-labeled NGA with a cleavable linkage ([111In]In-DO3AiBu-Bn-FGK-NGA) was administered to normal mice, and biodistribution studies and metabolic analyses of urinary and fecal samples were performed with comparison to an 111In-labeled NGA prepared by a conventional method ([111In]In-DOTA-Bn-SCN-NGA). Then, 111In-labeled antibodies ([111In]In-DO3AiBu-Bn-FGK-IgG and [111In]In-DOTA-Bn-SCN-IgG) were prepared using a procedure similar to that for 111In-labeled NGAs. In vitro plasma stability and biodistribution were investigated for both 111In-labeled antibodies in U87MG tumor-bearing mice.

RESULTS

Through the liberation of radiometabolites including [111In]In-DO3AiBu-Bn-F, [111In]In-DO3AiBu-Bn-FGK-NGA was cleared more rapidly from the liver than [111In]In-DOTA-Bn-SCN-NGA (4.07 ± 1.54%ID VS 71.68 ± 3.03%ID at 6 h postinjection). [111In]In-DO3AiBu-Bn-FGK-IgG exhibited lower tumor accumulation (8.83 ± 1.48%ID/g) but a significantly higher tumor/liver ratio (2.21 ± 0.53) than [111In]In-DOTA-Bn-SCN-IgG (11.65 ± 2.17%ID/g in the tumor and a tumor/liver ratio of 0.85 ± 0.18) at 72 h after injection.

CONCLUSION

A molecular design that reduces the high and persistent hepatic radioactivity of radiolabeled antibodies by liberating radiometabolites with a short hepatic residence time in lysosomes would be applicable for radiometal-labeled antibodies using a DOTA-chelating system.

Approaches to Reducing Normal Tissue Radiation from Radiolabeled Antibodies

Radiolabeled antibodies are powerful tools for both imaging and therapy in the field of nuclear medicine. Radiolabeling methods that do not release radionuclides from parent antibodies are essential for radiolabeling antibodies, and practical radiolabeling protocols that provide high in vivo stability have been established for many radionuclides, with a few exceptions. However, several limitations remain, including undesirable side effects on the biodistribution profiles of antibodies. This review summarizes the numerous efforts made to tackle this problem and the recent advances, mainly in preclinical studies. These include pretargeting approaches, engineered antibody fragments and constructs, the secondary injection of clearing agents, and the insertion of metabolizable linkages. Finally, we discuss the potential of these approaches and their prospects for further clinical application.

Radiometals in Imaging and Therapy: Highlighting Two Decades of Research

The present article highlights the important progress made in the last two decades in the fields of molecular imaging and radionuclide therapy. Advancements in radiometal-based positron emission tomography, single photon emission computerized tomography, and radionuclide therapy are illustrated in terms of their production routes and ease of radiolabeling. Applications in clinical diagnostic and radionuclide therapy are considered, including human studies under clinical trials; their current stages of clinical translations and findings are summarized. Because the metalloid astatine is used for imaging and radionuclide therapy, it is included in this review. In regard to radionuclide therapy, both beta-minus (β-) and alpha (α)-emitting radionuclides are discussed by highlighting their production routes, targeted radiopharmaceuticals, and current clinical translation stage.

Applications of radiolabeled antibodies in neuroscience and neuro-oncology

Positron emission tomography (PET) is a powerful tool in medicine and drug development, allowing for non-invasive imaging and quantitation of biological processes in live organisms. Targets are often probed with small molecules, but antibody-based PET is expanding because of many benefits, including ease of design of new antibodies toward targets, as well as the very strong affinities that can be expected. Application of antibodies to PET imaging of targets in the central nervous system (CNS) is a particularly nascent field, but one with tremendous potential. In this review, we discuss the growth of PET in imaging of CNS targets, present the promises and progress in antibody-based CNS PET, explore challenges faced by the field, and discuss questions that this promising approach will need to answer moving forward for imaging and perhaps even radiotherapy.

New Long-Acting [89Zr]Zr-DFO GLP-1 PET Tracers with Increased Molar Activity and Reduced Kidney Accumulation

Positron emission tomography (PET) imaging is used in drug development to noninvasively measure biodistribution and receptor occupancy. Ideally, PET tracers retain target binding and biodistribution properties of the investigated drug. Previously, we developed a zirconium-89 PET tracer based on a long-circulating glucagon-like peptide 1 receptor agonist (GLP-1RA) using desferrioxamine (DFO) as a chelator. Here, we aimed to develop an improved zirconium-89-labeled GLP-1RA with increased molar activity to increase the uptake in low receptor density tissues, such as brain. Furthermore, we aimed at reducing tracer accumulation in the kidneys. Introducing up to four additional Zr-DFOs resulted in higher molar activity and stability, while retaining potency. Branched placement of DFOs was especially beneficial. Tracers with either two or four DFOs had similar biodistribution as the tracer with one DFO in vivo, albeit increased kidney and liver uptake. Reduced kidney accumulation was achieved by introducing an enzymatically cleavable Met-Val-Lys (MVK) linker motif between the chelator and the peptide.

Antibody and Nanobody Radiolabeling with Copper-64: Solid vs. Liquid Target Approach

Antibody and nanobody-based copper-64 radiopharmaceuticals are increasingly being proposed as theranostic tools in multiple human diseases. While the production of copper-64 using solid targets has been established for many years, its use is limited due to the complexity of solid target systems, which are available in only a few cyclotrons worldwide. In contrast, liquid targets, available in virtually in all cyclotrons, constitute a practical and reliable alternative. In this study, we discuss the production, purification, and radiolabeling of antibodies and nanobodies using copper-64 obtained from both solid and liquid targets. Copper-64 production from solid targets was performed on a TR-19 cyclotron with an energy of 11.7 MeV, while liquid target production was obtained by bombarding a nickel-64 solution using an IBA Cyclone Kiube cyclotron with 16.9 MeV on target. Copper-64 was purified from both solid and liquid targets and used to radiolabel NODAGA-Nb, NOTA-Nb, and DOTA-Trastuzumab conjugates. Stability studies were conducted on all radioimmunoconjugates in mouse serum, PBS, and DTPA. Irradiation of the solid target yielded 13.5 ± 0.5 GBq with a beam current of 25 ± 1.2 μA and an irradiation time of 6 h. On the other hand, irradiation of the liquid target resulted in 2.8 ± 1.3 GBq at the end of bombardment (EOB) with a beam current of 54.5 ± 7.8 μA and an irradiation time of 4.1 ± 1.3 h. Successful radiolabeling of NODAGA-Nb, NOTA-Nb, and DOTA-Trastuzumab with copper-64 from both solid and liquid targets was achieved. Specific activities (SA) obtained with the solid target were 0.11, 0.19, and 0.33 MBq/μg for NODAGA-Nb, NOTA-Nb, and DOTA-trastuzumab, respectively. For the liquid target, the corresponding SA values were 0.15, 0.12, and 0.30 MBq/μg. Furthermore, all three radiopharmaceuticals demonstrated stability under the testing conditions. While solid targets have the potential to produce significantly higher activity in a single run, the liquid process offers advantages such as speed, ease of automation, and the feasibility of back-to-back production using a medical cyclotron. In this study, successful radiolabeling of antibodies and nanobodies was achieved using both solid and liquid targets approaches. The radiolabeled compounds exhibited high radiochemical purity and specific activity, rendering them suitable for subsequent in vivo pre-clinical imaging studies.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Enzymological Characterization of 64Cu-Labeled Neprilysin Substrates and Their Application for Modulating the Renal Clearance of Targeted Radiopharmaceuticals

The applicability of radioligands for targeted endoradionuclide therapy is limited due to radiation-induced toxicity to healthy tissues, in particular to the kidneys as primary organs of elimination. The targeting of enzymes of the renal brush border membrane by cleavable linkers that permit the formation of fast eliminating radionuclide-carrying cleavage fragments gains increasing interest. Herein, we synthesized a small library of ⁶⁴Cu-labeled cleavable linkers and quantified their substrate potentials toward neprilysin (NEP), a highly abundant peptidase at the renal brush border membrane. This allowed for the derivation of structure-activity relationships, and selected cleavable linkers were attached to the somatostatin receptor subtype 2 ligand [Tyr³]octreotate. Radiopharmacological characterization revealed that a substrate-based targeting of NEP in the kidneys with small peptides entails their premature cleavage in the blood circulation by soluble and endothelium-derived NEP. However, for a kidney-specific targeting of NEP, the additional targeting of albumin in the blood is highlighted.

Lasso peptide microcin J25 variant containing RGD motif as a PET probe for integrin a v ß 3 in tumor imaging

Microcin J25 (MccJ25), a lasso peptide, has a unique 3-D interlocked structure that provides high stability under acidic conditions, at high temperatures, and in the presence of proteases. In this study, we generated a positron emission tomography (PET) probe based on MccJ25 analog with an RGD motif and investigated their pharmacokinetics and utility for integrin α_vβ₃ imaging in tumors. The MccJ25 variant with an RGD motif in the loop region and a lysine substitution at the C-terminus (MccJ25(RGDF)GtoK) was produced in E. coli transfected with plasmid DNA containing the MccJ25 biosynthetic gene cluster (mcjABCD). [⁶⁴Cu]Cu-MccJ25(RGDF)GtoK was synthesized using the C-terminal lysine labeled with copper-64 (t_1/2 = 12.7 h) via a bifunctional chelator; it showed stability in 90% mouse plasma for 45 min. Using PET imaging for integrin α_vβ₃ positive U87MG tumor bearing mice, [⁶⁴Cu]Cu-MccJ25(RGDF)GtoK could clearly distinguish the tumor, and its accumulation was significantly higher than that of MccJ25(GIGT)GtoK without the binding motif for integrin α_vβ₃. Furthermore, MccJ25(RGDF)GtoK enabled visualization of only U87MG tumors but not MCF-7 tumors with low integrin α_vβ₃ expression in double tumor-bearing mice. In ex vivo biodistribution analysis, the integrin α_vβ₃ non-specific accumulation of [⁶⁴Cu]Cu-MccJ25(RGDF)GtoK was significantly lower in various tissues, except for the kidneys, as compared to the control probe ([⁶⁴Cu]Cu-cyclic RGD peptide). These results of the present study indicate that ⁶⁴Cu-labeling methods are appropriate for the synthesis of MccJ25-based PET probes, and [⁶⁴Cu]Cu-MccJ25 variants are useful tools for cancer molecular imaging.

Immuno-PET: Design options and clinical proof-of-concept

Radioimmunoconjugates have been used for over 30 years in nuclear medicine applications. In the last few years, advances in cancer biology knowledge have led to the identification of new molecular targets specific to certain patient subgroups. The use of these targets in targeted therapies approaches has allowed the developments of specifically tailored therapeutics for patients. As consequence of the PET-imaging progresses, nuclear medicine has developed powerful imaging tools, based on monoclonal antibodies, to in vivo characterization of these tumor biomarkers. This imaging modality known as immuno-positron emission tomography (immuno-PET) is currently in fastest-growing and its medical value lies in its ability to give a non-invasive method to assess the in vivo target expression and distribution and provide key-information on the tumor targeting. Currently, immuno-PET presents promising probes for different nuclear medicine topics as staging/stratification tool, theranostic approaches or predictive/prognostic biomarkers. To develop a radiopharmaceutical drug that can be used in immuno-PET approach, it is necessary to find the best compromise between the isotope choice and the immunologic structure (full monoclonal antibody or derivatives). Through some clinical applications, this paper review aims to discuss the most important aspects of the isotope choice and the usable proteic structure that can be used to meet the clinical needs.

Application of Cleavable Linkers to Improve Therapeutic Index of Radioligand Therapies

Radioligand therapy (RLT) is an emergent drug class for cancer treatment. The dose administered to cancer patients is constrained by the radiation exposure to normal tissues to maintain an appropriate therapeutic index. When a radiopharmaceutical or its radiometabolite is retained in the kidneys, radiation dose deposition in the kidneys can become a dose-limiting factor. A good exemplar is [¹⁷⁷Lu]Lu-DOTATATE, where patients receive a co-infusion of basic amino acids for nephroprotection. Besides peptides, there are other classes of targeting vectors like antibody fragments, antibody mimetics, peptidomimetics, and small molecules that clear through the renal pathway. In this review, we will review established and emerging strategies that can be used to mitigate radiation-induced nephrotoxicity, with a focus on the development and incorporation of cleavable linkers for radiopharmaceutical designs. Finally, we offer our perspectives on cleavable linkers for RLT, highlighting future areas of research that will help advance the technology.

Production of GMP-Compliant Clinical Amounts of Copper-61 Radiopharmaceuticals from Liquid Targets

PET imaging has gained significant momentum in the last few years, especially in the area of oncology, with an increasing focus on metal radioisotopes owing to their versatile chemistry and favourable physical properties. Copper-61 (t_1/2 = 3.33 h, 61% β⁺, E_max = 1.216 MeV) provides unique advantages versus the current clinical standard (i.e., gallium-68) even though, until now, no clinical amounts of ⁶¹Cu-based radiopharmaceuticals, other than thiosemicarbazone-based molecules, have been produced. This study aimed to establish a routine production, using a standard medical cyclotron, for a series of widely used somatostatin analogues, currently labelled with gallium-68, that could benefit from the improved characteristics of copper-61. We describe two possible routes to produce the radiopharmaceutical precursor, either from natural zinc or enriched zinc-64 liquid targets and further synthesis of [⁶¹Cu]Cu-DOTA-NOC, [⁶¹Cu]Cu-DOTA-TOC and [⁶¹Cu]Cu-DOTA-TATE with a fully automated GMP-compliant process. The production from enriched targets leads to twice the amount of activity (3.28 ± 0.41 GBq vs. 1.84 ± 0.24 GBq at EOB) and higher radionuclidic purity (99.97% vs. 98.49% at EOB). Our results demonstrate, for the first time, that clinical doses of ⁶¹Cu-based radiopharmaceuticals can easily be obtained in centres with a typical biomedical cyclotron optimised to produce ¹⁸F-based radiopharmaceuticals.