Evaluation of an Anti-HER2 Nanobody Labeled with 225Ac for Targeted α-Particle Therapy of Cancer

Nanobody-based immunotoxins: A precision tool in the treatment of solid tumors

Solid tumors, the main cause of cancer-related death, represent a significant therapeutic challenge due to the high-density microenvironment and intolerance to conventional treatments. Nanobody-based immunotoxins (NbITs) are an exciting candidate, combining the ultimate specificity of nanobodies (single-domain antibody fragments of camelid antibodies) and detrimental effects of the toxin. These nanobodies are small (one-tenth of conventional antibody size), thermostable with high specificity, high antigen binding affinity which give it the ability to penetrate into solid tumors. Specific delivery to tumor cells is achieved through conjugating nanobodies with cytotoxic agents of bacterial origin or synthetic drugs. This phenomenon is initially attracted to the cells by the antigen-antibody interaction that is further enhanced by receptor-mediated internalization and cytotoxic payload release that subdues essential cellular processes and, as a consequence, damages the cells. This review discusses the mechanisms that underlie the effectiveness of NbITs, such as tumor antigen recognition, toxin release, and cellular signaling pathways elicited by the internalized toxins. We also discuss the application of NbITs in treating cancers such as HER2-positive breast cancer and EGFR-overexpressing lung cancer, and other cancers, highlighting their ability to address limitations of conventional therapies. Key challenges in NbIT development, including stability, immunogenicity, and efficient delivery, are critically evaluated. Current advances such as the creation of bispecific nanobody constructs, optimization of linker strategies, as well as the incorporation of nanoparticle-based delivery systems are maximizing the therapeutic potential of these molecules. This review synthesizes recent progress and addresses current obstacles in NbIT development, showcasing their transformative potential as a targeted therapeutic approach for solid tumors. It also covers future opportunities to develop and advance this emerging treatment strategy.

Trends in nanobody radiotheranostics

As the smallest antibody fragment with specific binding affinity, nanobody-based nuclear medicine has demonstrated significant potential to revolutionize the field of precision medicine, supported by burgeoning preclinical investigations and accumulating clinical evidence. However, the visualization of nanobodies has also exposed their suboptimal biodistribution patterns, which has spurred collaborative efforts to refine their pharmacokinetic and pharmacodynamic profiles for improved therapeutic efficacy. In this review, we present clinical results that exemplify the benefits of nanobody-based molecular imaging in cancer diagnosis. Moreover, we emphasize the indispensable role of molecular imaging as a tool for evaluating and optimizing nanobodies, thereby expanding their therapeutic potential in cancer treatment in the foreseeable future.

In vitro and in vivo evaluation of anti-HER2 antibody conjugates labelled with 225Ac

BACKGROUND

Overexpression of human epidermal growth factor receptor type 2 (HER2) occurs in multiple carcinomas. For example, up to 20% of breast cancer cases are classified as HER2 positive (HER2+). Treatment of this condition typically involves immunotherapy using monoclonal antibodies, such as trastuzumab or pertuzumab. The precise targeting of monoclonal antibodies to HER2+ tumour lesions can be used as well in radioimmunotherapy to deliver medical radionuclides exactly to the afflicted area and therefore minimize radiation exposure of healthy tissues. In this study, DOTA conjugates of monoclonal antibodies trastuzumab and pertuzumab were prepared and tested in vitro. One of these, 225Ac-DOTA-pertuzumab, was also the subject of an ex vivo biodistribution study with normal as well as HER2+ and HER2- tumour-xenografted mice. This radioconjugate has not been previously described.

RESULTS

Three DOTA-conjugates of HER2 targeting monoclonal antibodies, one of trastuzumab and two of pertuzumab, were prepared and radiolabelled with 225Ac in different molar ratios. This procedure led to an optimisation of the preparation and radiolabelling process. The radioconjugates were shown to be highly stable in vitro in both fetal bovine serum and phosphate buffered saline under room temperature and decreased temperature for 10 days. In vitro cell studies with HER2-overexpressing cell-line (SKOV-3) and low HER2-expressing cell line (MDA-MB-231) proved that radioconjugates of both antibodies have high binding specificity and affinity towards HER2 receptors. These findings were confirmed for a novel radioconjugate 225Ac-DOTA-pertuzumab in an ex vivo biodistribution study, where uptake in HER2+ tumour was 50 ± 14% ID/g and HER2- tumour showed uptake comparable with healthy tissues (max. 5.0 ± 1.7% ID/g). The high uptake observed in the spleen can be attributed to the elimination of the antibody, as well as the use of an immunedeficient mouse strain (SCID).

CONCLUSIONS

During this study, the optimization of preparation and radiolabelling of HER2 targeting antibodies with 225Ac was accomplished. Furthermore, the radioconjugate 225Ac-DOTA-pertuzumab was prepared and evaluated for the first time. The radioconjugates of both tested antibodies demonstrated excellent qualities in terms of stability and HER2 receptor affinity. Initial ex vivo studies indicated that especially the radioconjugate 225Ac-DOTA-pertuzumab is a very promising candidate for further more detailed in vivo studies.

A HER2 Specific Nanobody-Drug Conjugate: Site-Selective Bioconjugation and In Vitro Evaluation in Breast Cancer Models

A human epidermal growth factor receptor 2 (HER2)-specific nanobody called 2Rs15d, containing a His3LysHis6 segment at the C-terminus, was recombinantly produced. Subsequent site-selective acylation on the C-terminally activated lysine residue allowed installation of the cytotoxin monomethyl auristatin E-functionalized cathepsin B-sensitive payload to provide a highly homogenous nanobody-drug conjugate (NBC), which demonstrated high potency and selectivity for HER2-positive breast cancer models.

Development and evaluation of a single domain antibody targeting folate receptor alpha for radioligand therapy

BACKGROUND

Folate receptor alpha (FRα) overexpression is seen in many cancers. Radioligand therapy (RLT) has emerged as a promising tool to target FRα and has been investigated previously, but further progression was limited due to high kidney retention and, subsequently, toxicity. To circumvent this, we present here the development of a [131I]I-GMIB-conjugated anti-human FRα (hFRα) single-domain antibody (sdAb), with intrinsically fast renal clearance and concomitant low kidney retention. We report the hit-to-lead development of an anti-hFRα sdAb. We evaluated its potential in vitro and assessed its targeting ability using SPECT imaging in hFRα-knockin and tumour-bearing mice. The toxicity and therapeutic efficacy of the [131I]I-GMIB-sdAb were investigated in mouse models.

RESULTS

The lead anti-hFRα sdAb 2BD42 was developed with picomolar affinities, low koff, and radiolabelled using [131I]I with yields of > 41% and purity > 99%. [131I]I-GMIB-2BD42 retained tumour uptake (> 5%IA/g at 1 h p.i. and > 1.5%IA/g at 24 h p.i.) and fast kidney clearance (< 1%IA/g at 24 h p.i.) in athymic and hFRα-knock-in mice. Athymic mice bearing hFRα-positive xenografts treated with [131I]I-GMIB-2BD42 showed prolonged survival without toxicity compared to animals that received the vehicle solution or radioactive control.

CONCLUSION

The therapeutic lead radiopharmaceutical [131I]I-GMIB-2BD42 showed fast pharmacokinetics with specific retention in hFRα + tumours. In addition, we report therapeutic efficacy with no signs of toxicity. In this study, we successfully designed a new drug for RLT, overcoming previous limitations, such as high kidney retention, which could aid in revitalising FRα-targeted radiotherapy.

Astatine-211 and actinium-225: two promising nuclides in targeted alpha therapy

Nuclear medicine therapy offers a promising approach for tumor treatment, as the energy emitted during radionuclide decay causes irreparable damage to tumor cells. Notably, α-decay exhibits an even more significant destructive potential. By conjugating α-nuclides with antibodies or small-molecule inhibitors, targeted alpha therapy (TAT) can enhance tumor destruction while minimizing toxic side effects, making TAT an increasingly attractive antineoplastic strategy. Astatine-211 ( 211At) and actinium-225 ( 225Ac) have emerged as highly effective agents in TAT due to their exceptional physicochemical properties and biological effects. In this review, we highlight the applications of 211At-/ 225Ac-radiopharmaceuticals, particularly in specific tumor targets, such as prostate-specific membrane antigen (PSMA) in prostate cancers, cluster of differentiation (CD) in hematological malignancies, human epidermal growth factor receptor-2 (HER2) in ovarian cancers, and somatostatin receptor (SSTR) in neuroendocrine tumors. We synthesize the progress from preclinical and clinical trials to provide insights into the promising potential of 211At-/ 225Ac-radiopharmaceuticals for future treatments.

Click Chemistry Enables [89Zr]Zr-DOTA Radioimmunoconjugation for Theranostic 89Zr-immunoPET

There have been predictions that the use of the macrocyclic chelating agent 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) in zirconium-89 (89Zr) immuno-positron emission tomography (89Zr-immunoPET) could enhance the in vivo stability of 89Zr radioimmunoconjugates. However, conjugating [89Zr]Zr-DOTA to a monoclonal antibody (mAb) remains a challenge as the heat treatment required for [89Zr]Zr-DOTA chelation can lead to thermal denaturation of the mAb moieties. We developed a method for synthesizing [89Zr]Zr-DOTA-mAb based on a tetrazine (Tz)-conjugated bifunctional DOTA derivative 2,2',2″-(10-(1-(4-(1,2,4,5-tetrazin-3-yl)phenyl)-3,21,26-trioxo-6,9,12,15,18-pentaoxa-29-carboxy-2,22,25-triazanonacosane-29-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetic acid (DOTAGA-Tz) and the inverse electron-demand Diels-Alder (IEDDA) click chemistry reaction where trans-cyclooctene-modified mAbs are conjugated to [89Zr]Zr-DOTAGA without being exposed to heat. The stability of IEDDA-derived [89Zr]Zr-DOTAGA-trastuzumab was confirmed by in vitro, ex vivo, and in vivo testing and comparative analysis against the conventional deferoxamine (DFO) counterpart [89Zr]Zr-DFO-trastuzumab. The in vivo immunoPET imaging using [89Zr]Zr-DOTAGA-trastuzumab clearly visualized human epidermal growth factor receptor 2-positive malignancies in murine xenograft models. Greater tumor contrast was observed from [89Zr]Zr-DOTAGA-trastuzumab at a 72-h delayed scan compared with [89Zr]Zr-DFO-trastuzumab. These findings suggest that our IEDDA ligation approach can be an effective means of synthesizing [89Zr]Zr-DOTA-mAb and can enhance the theranostic potential of 89Zr-immunoPET in DOTA-mediated radioimmunotherapy.

Targeting HER2 in solid tumors: Unveiling the structure and novel epitopes

Human epidermal growth factor receptor-2 (HER2) is overexpressed in various solid tumor types, acting as an established therapeutic target. Over the last three decades, the fast-paced development of diverse HER2-targeted agents, notably marked by the introduction of the antibody-drug conjugate (ADC), yielding substantial improvements in survival rates. However, resistance to anti-HER2 treatments continues to pose formidable challenges. The complex structure and dynamic dimerization properties of HER2 create significant hurdles in the development of novel targeted therapeutics. In this review, we synthesize the latest insights into the structural intricacies of HER2 and present an unprecedented overview of the epitope characteristics of HER2-targeted antibodies and their derivatives. Furthermore, we delve into the correlation between anti-HER2 antibody binding epitopes and their respective functions, with a particular focus on their efficacy against resistant tumors. In addition, we highlight the potential of emerging anti-HER2 agents that target specific sites or non-overlapping epitopes, poised to transform the therapeutic landscape for HER2-positive tumors in the foreseeable future.

Single-domain antibodies as therapeutics for solid tumor treatment

Single-domain antibodies (sdAbs), initially identified in camelids or sharks and commonly referred to as nanobodies or VNARs, have emerged as a promising alternative to conventional therapeutic antibodies. These sdAbs have many superior physicochemical and pharmacological properties, including small size, good solubility and thermostability, easier accessible epitopes, and strong tissue penetration. However, the inherent challenges associated with the animal origin of sdAbs limit their clinical use. In recent years, various innovative humanization technologies, including complementarity-determining region (CDR) grafting or complete engineering of fully human sdAbs, have been developed to mitigate potential immunogenicity issues and enhance their compatibility. This review provides a comprehensive exploration of sdAbs, emphasizing their distinctive features and the progress in humanization methodologies. In addition, we provide an overview of the recent progress in developing drugs and therapeutic strategies based on sdAbs and their potential in solid tumor treatment, such as sdAb-drug conjugates, multispecific sdAbs, sdAb-based delivery systems, and sdAb-based cell therapy.

Plant-Derived Anti-Human Epidermal Growth Factor Receptor 2 Antibody Suppresses Trastuzumab-Resistant Breast Cancer with Enhanced Nanoscale Binding

Traditional monoclonal antibodies such as Trastuzumab encounter limitations when treating Human Epidermal Growth Factor Receptor 2 (HER2)-positive breast cancer, particularly in cases that develop resistance. This study introduces plant-derived anti-HER2 variable fragments of camelid heavy chain domain (VHH) fragment crystallizable region (Fc) KEDL(K) antibody as a potent alternative for overcoming these limitations. A variety of biophysical techniques, in vitro assays, and in vivo experiments uncover the antibody's nanoscale binding dynamics with transmembrane HER2 on living cells. Single-molecule force spectroscopy reveals the rapid formation of two robust bonds, exhibiting approximately 50 pN force resistance and bond lifetimes in the second range. The antibody demonstrates a specific affinity for HER2-positive breast cancer cells, including those that are Trastuzumab-resistant. Moreover, in immune-deficient mice, the plant-derived anti-HER2 VHH-FcK antibody exhibits superior antitumor activity, especially against tumors that are resistant to Trastuzumab. These findings underscore the plant-derived antibody's potential as an impactful immunotherapeutic strategy for treating Trastuzumab-resistant HER2-positive breast cancer.

Efficacy and Safety of 225 Ac-DOTATATE in the Treatment of Neuroendocrine Neoplasms With High SSTR Expression

PURPOSE

We aimed to evaluate the efficacy and safety of 225 Ac-DOTATATE targeted α therapy (TAT) in various neuroendocrine neoplasms (NENs) with high somatostatin receptor (SSTR) expression.

PATIENTS AND METHODS

This single-center prospective study included 10 patients with histologically diagnosed NENs that exhibited increased SSTR expression on 68 Ga-DOTATATE PET/CT imaging. All patients received 225 Ac-DOTATATE TAT. The primary end points were molecular imaging-based response and disease control rate (DCR), measured using the slightly modified Positron Emission Tomography Response Criteria in Solid Tumors 1.0. The secondary end points were adverse event profiles and clinical responses. The adverse event profile was determined according to the Common Terminology Criteria for Adverse Events version 5.0. Clinical response was assessed using the EORTC QLQ-C30 v3.0 (European Organization for Research and Treatment of Cancer Core Quality of Life questionnaire version 3.0).

RESULTS

A molecular imaging-based partial response was observed in 40% of all patients, SD in 40%, PD in 20%, and DCR in 80%. The DCR was 83.3% (5/6) in patients who were previously treated with 177 Lu-DOTATATE. According to the EORTC QLQ-C30 v3.0 score, most symptoms improved after 225 Ac-DOTATATE treatment, with only diarrhea showing no improvement. Grade III/IV hematological, kidney, and liver toxicities were not observed. The median follow-up time was 14 months (7-22 months), and no deaths were reported.

CONCLUSIONS

This initial study suggests that 225 Ac-DOTATATE is a potentially promising option for treating NENs with elevated SSTR expression, with an acceptable toxicity profile and well-tolerated adverse effects.

A modular approach to enhancing cell membrane-coated nanoparticle functionality using genetic engineering

Since their initial development, cell membrane-coated nanoparticles (CNPs) have become increasingly popular in the biomedical field. Despite their inherent versatility and ability to enable complex biological applications, there is considerable interest in augmenting the performance of CNPs through the introduction of additional functionalities. Here we demonstrate a genetic-engineering-based modular approach to CNP functionalization that can encompass a wide range of ligands onto the nanoparticle surface. The cell membrane coating is engineered to express a SpyCatcher membrane anchor that can readily form a covalent bond with any moiety modified with SpyTag. To demonstrate the broad utility of this technique, three unique targeted CNP formulations are generated using different classes of targeting ligands, including a designed ankyrin repeat protein, an affibody and a single-chain variable fragment. In vitro, the modified nanoparticles exhibit enhanced affinity towards cell lines overexpressing the cognate receptors for each ligand. When formulated with a chemotherapeutic payload, the modularly functionalized nanoparticles display strong targeting ability and growth suppression in a murine tumour xenograft model of ovarian cancer. Our data suggest genetic engineering offers a feasible approach for accelerating the development of multifunctional CNPs for a broad range of biomedical applications.

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.

Targeted drug delivery using nanobodies to deliver effective molecules to breast cancer cells: the most attractive application of nanobodies

Targeted drug delivery is one of the attractive ways in which cancer treatment can significantly reduce side effects. In the last two decades, the use of antibodies as a tool for accurate detection of cancer has been noted. On the other hand, the binding of drugs and carriers containing drugs to the specific antibodies of cancer cells can specifically target only these cells. However, the use of whole antibodies brings challenges, including their large size, the complexity of conjugation, the high cost of production, and the creation of immunogenic reactions in the body. The use of nanobodies, or VHHs, which are a small part of camel heavy chain antibodies, is very popular due to their small size, high craftsmanship, and low production cost. In this article, in addition to a brief overview of the structure and characteristics of nanobodies, the use of this molecule in the targeted drug delivery of breast cancer has been reviewed.

A high affinity and specificity anti-HER2 single-domain antibody (VHH) that targets trastuzumab's epitope with versatile biochemical, biological, and medical applications

In the past decade, various single-domain antibodies from llamas, also known as VHH or nanobody, have been discovered with applications in tumor imaging and cancer therapy. However, the potential application of anti-HER2 VHHs as a diagnostic tool suitable for ELISA, flow cytometry, cell imaging, bispecific antibody engineering, and immunohistochemistry has not been fully elucidated. To investigate this potential, HER2 antigen was expressed in HEK293 F cells, purified, and used to immunize llama. Using phage display, anti-HER2 VHHs with high affinity and specificity were isolated, sequenced, and constructed with a Histag and c-Myc tag. The constructed anti-HER2 VHHs were then expressed in E. coli, purified, and evaluated for their use in ELISA, flow cytometry, cell imaging, and immunohistochemistry. The affinities of the anti-HER2 VHHs toward the HER2 antigen were determined using biolayer interferometry. Furthermore, the binding sites of the anti-HER2 VHHs were evaluated by epitope mapping and in silico modeling and docking. Here, we report the sequence of an anti-HER2 VHH with high affinity (sub-nanomolar), specificity, and selectivity. This VHH binds to the same epitope as trastuzumab and can be utilized to generate bispecific antibodies or used as a diagnostic tool to differentiate HER2+ from HER2- antigens on plates, cells, and tissues. This discovery has broad applications in biochemical, biological, and medical sciences.

Promising Diagnostic and Therapeutic Approaches Based on VHHs for Cancer Management

The discovery of the distinctive structure of heavy chain-only antibodies in species belonging to the Camelidae family has elicited significant interest in their variable antigen binding domain (VHH) and gained attention for various applications, such as cancer diagnosis and treatment. This article presents an overview of the characteristics, advantages, and disadvantages of VHHs as compared to conventional antibodies, and their usage in diverse applications. The singular properties of VHHs are explained, and several strategies that can augment their utility are outlined. The preclinical studies illustrating the diagnostic and therapeutic efficacy of distinct VHHs in diverse formats against solid cancers are summarized, and an overview of the clinical trials assessing VHH-based agents in oncology is provided. These investigations demonstrate the enormous potential of VHHs for medical research and healthcare.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

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.

Evaluation of [89Zr]Zr-DFO-2Rs15d Nanobody for Imaging of HER2-Positive Breast Cancer

One of the most aggressive forms of breast cancer involves the overexpression of human epidermal growth factor receptor 2 (HER2). HER2 is overexpressed in ∼25% of all breast cancers and is associated with increased proliferation, increased rates of metastasis, and poor prognosis. Treatment for HER2-positive breast cancer has vastly improved since the development of the monoclonal antibody trastuzumab (Herceptin) as well as other biological constructs. However, patients still commonly develop resistance, illustrating the need for newer therapies. Nanobodies have become an important focus for potential development as HER2-targeting imaging agents and therapeutics. Nanobodies have many favorable characteristics, including high stability in heat and nonphysiological pH, while maintaining their low-nanomolar affinity for their designed targets. Specifically, the 2Rs15d nanobody has been developed for targeting HER2 and has been evaluated as a diagnostic imaging agent for single-photon emission computed tomography (SPECT) and positron emission tomography (PET). While a construct of 2Rs15d with the positron emitter 68Ga is currently in phase I clinical trials, the only PET images acquired in preclinical or clinical research have been within 3 h postinjection. We evaluated our in-house produced 2Rs15d nanobody, conjugated with the chelator deferoxamine (DFO), and radiolabeled with 89Zr for PET imaging up to 72 h postinjection. [89Zr]Zr-DFO-2Rs15d demonstrated high stability in both phosphate-buffered saline (PBS) and human serum. Cell binding studies showed high binding and specificity for HER2, as well as prominent internalization. Our in vivo PET imaging confirmed high-quality visualization of HER2-positive tumors up to 72 h postinjection, whereas HER2-negative tumors were not visualized. Subsequent biodistribution studies quantitatively supported the significant HER2-positive tumor uptake compared to the negative control. Our studies fill an important gap in understanding the imaging and binding properties of the 2Rs15d nanobody at extended time points. As many therapeutic radioisotopes have single or multiday half-lives, this information will directly benefit the potential of the radiotherapy development of 2Rs15d for HER2-positive breast cancer patients.

Targeted Alpha Therapy (TAT) with Single-Domain Antibodies (Nanobodies)

The persistent threat of cancer necessitates the development of improved and more efficient therapeutic strategies that limit damage to healthy tissues. Targeted alpha therapy (TαT), a novel form of radioimmuno-therapy (RIT), utilizes a targeting vehicle, commonly antibodies, to deliver high-energy, but short-range, alpha-emitting particles specifically to cancer cells, thereby reducing toxicity to surrounding normal tissues. Although full-length antibodies are often employed as targeting vehicles for TαT, their high molecular weight and the presence of an Fc-region lead to a long blood half-life, increased bone marrow toxicity, and accumulation in other tissues such as the kidney, liver, and spleen. The discovery of single-domain antibodies (sdAbs), or nanobodies, naturally occurring in camelids and sharks, has introduced a novel antigen-specific vehicle for molecular imaging and TαT. Given that nanobodies are the smallest naturally occurring antigen-binding fragments, they exhibit shorter relative blood half-lives, enhanced tumor uptake, and equivalent or superior binding affinity and specificity. Nanobody technology could provide a viable solution for the off-target toxicity observed with full-length antibody-based TαT. Notably, the pharmacokinetic properties of nanobodies align better with the decay characteristics of many short-lived α-emitting radionuclides. This review aims to encapsulate recent advancements in the use of nanobodies as a vehicle for TαT.

Application of Magnetic Particles for Fast Determination of Immunoreactive Fraction of 68Ga-Labelled VHH Antibodies to PD-L1

Quantification of the immunoreactive fraction (IRF) of radioactive isotope-labeled antibodies or their fragments is necessary to assess the specific activity of radiopharmaceuticals. Traditionally, cells expressing the target molecules on their surface are used to determine IRF, but such analysis is time-consuming and has difficulties with standardization. The aim of the study was to develop a fast and reliable method for quantitative determination of IRF by 68Ga-labeled VHH antibodies to PD-L1 based on the use of magnetic particles coated with antigen molecules.

Materials and Methods

Commercially available magnetic particles coated with protein A have been used in our study. The antigen conjugated with the Fc fragment (PD-L1-Fc) was immobilized on the particles. The IRF value of 68Ga radionuclide-labeled nanobodies (VHH) against PD-L1 (68Ga-VHH-PD-L1) was determined using magnetic particles coated with antigen molecules and cells expressing the antigen on their surface. When VHH antibodies were conjugated to 68Ga radionuclide, protein molecules were modified using bifunctional chelating agents: tetraazacyclododecanetetraacetic acid (DOTA) or deferoxamine (DFO). The magnitude of IRF was defined as the ratio of radioactivity specifically bound to particles or cells to the total radioactivity added to the sample.

Results

The specificity of the 68Ga-VHH-PD-L1 radioimmunoconjugate binding to the antigen-coated magnetic particles has been proved. Some special aspects, which should be taken into consideration when using this method, have been established. The comparison of the IRF estimates using the antigen-expressing cells and magnetic particles has not revealed any significant differences in the results obtained in our study. Nevertheless, the presented method based on magnetic particles with immobilized antigen molecules requires only 15 min to determine the radioimmunoconjugate IRF, which is of fundamental importance for the routine assessment of the specificity of radiopharmaceuticals containing short-lived isotopes.

225Ac-Labeled Somatostatin Analogs in the Management of Neuroendocrine Tumors: From Radiochemistry to Clinic

The widespread use of peptide receptor radionuclide therapy (PRRT) represents a major therapeutic breakthrough in nuclear medicine, particularly since the introduction of 177Lu-radiolabeled somatostatin analogs. These radiopharmaceuticals have especially improved progression-free survival and quality of life in patients with inoperable metastatic gastroenteropancreatic neuroendocrine tumors expressing somatostatin receptors. In the case of aggressive or resistant disease, the use of somatostatin derivatives radiolabeled with an alpha-emitter could provide a promising alternative. Among the currently available alpha-emitting radioelements, actinium-225 has emerged as the most suitable candidate, especially regarding its physical and radiochemical properties. Nevertheless, preclinical and clinical studies on these radiopharmaceuticals are still few and heterogeneous, despite the growing momentum for their future use on a larger scale. In this context, this report provides a comprehensive and extensive overview of the development of 225Ac-labeled somatostatin analogs; particular emphasis is placed on the challenges associated with the production of 225Ac, its physical and radiochemical properties, as well as the place of 225Ac-DOTATOC and 225Ac-DOTATATE in the management of patients with advanced metastatic neuroendocrine tumors.

Doxorubicin- and Trastuzumab-Modified Gold Nanoparticles as Potential Multimodal Agents for Targeted Therapy of HER2+ Cancers

Recently, targeted nanoparticles (NPs) have attracted much attention in cancer treatment due to their high potential as carriers for drug delivery. In this article, we present a novel bioconjugate (DOX–AuNPs–Tmab) consisting of gold nanoparticles (AuNPs, 30 nm) attached to chemotherapeutic agent doxorubicin (DOX) and a monoclonal antibody, trastuzumab (Tmab), which exhibited specific binding to HER2 receptors. The size and shape of synthesized AuNPs, as well as their surface modification, were analyzed by the TEM (transmission electron microscopy) and DLS (dynamic light scattering) methods. Biological studies were performed on the SKOV-3 cell line (HER2+) and showed high specificity of binding to the receptors and internalization capabilities, whereas MDA-MB-231 cells (HER2−) did not. Cytotoxicity experiments revealed a decrease in the metabolic activity of cancer cells and surface area reduction of spheroids treated with DOX–AuNPs–Tmab. The bioconjugate induced mainly cell cycle G2/M-phase arrest and late apoptosis. Our results suggest that DOX–AuNPs–Tmab has great potential for targeted therapy of HER2-positive tumors.

Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose 211At-Labeled Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with ²¹¹At, because of their rapid accumulation in tumor and clearance from normal tissues. Here, we evaluate the therapeutic potential of this strategy with 5F7 and VHH_1028-2 sdAbs that bind with high affinity to domain IV of human epidermal growth factor receptor type 2 (HER2). Methods: The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using N-succinimidyl-3-²¹¹At-astato-5-guanidinomethyl benzoate (iso-²¹¹At-SAGMB). The cytotoxicity of iso- ²¹¹At-SAGMB-5F7 and iso- ²¹¹At-SAGMB-VHH_2001 were compared on HER2-expressing BT474 breast carcinoma cells. Three experiments in mice with subcutaneous BT474 xenografts were performed to evaluate the therapeutic effectiveness of single doses of iso- ²¹¹At-SAGMB-5F7 (0.7-3.0 MBq), iso- ²¹¹At-SAGMB-VHH_1028 (1.0-3.0 MBq), and iso- ²¹¹At-SAGMB-VHH_1028 and iso- ²¹¹At-SAGMB-VHH_2001 (∼1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso- ²¹¹At-SAGMB-5F7 (D₀ = 1.313 kBq/mL) whereas iso- ²¹¹At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with ²¹¹At-labeled HER2-specific 5F7 and VHH_1028 but not with HER2-irrelevant VHH_2001. At the 3.0-MBq dose, complete tumor regression was seen in 3 of 4 mice treated with iso- ²¹¹At-SAGMB-5F7 and 8 of 11 mice treated with iso- ²¹¹At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Conclusion: Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the iso- ²¹¹At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.

Preclinical Evaluation of 225Ac-Labeled Single-Domain Antibody for the Treatment of HER2pos Cancer

Human epidermal growth factor receptor type 2 (HER2) is overexpressed in various cancers; thus, HER2-targeting single-domain antibodies (sdAb) could offer a useful platform for radioimmunotherapy. In this study, we optimized the labeling of an anti-HER2-sdAb with the α-particle-emitter 225Ac through a DOTA-derivative. The formed radioconjugate was tested for binding affinity, specificity and internalization properties, whereas cytotoxicity was evaluated by clonogenic and DNA double-strand-breaks assays. Biodistribution studies were performed in mice bearing subcutaneous HER2pos tumors to estimate absorbed doses delivered to organs and tissues. Therapeutic efficacy and potential toxicity were assessed in HER2pos intraperitoneal ovarian cancer model and in healthy C57Bl/6 mice. [225Ac]Ac-DOTA-2Rs15d exhibited specific cell uptake and cell-killing capacity in HER2pos cells (EC50 = 3.9 ± 1.1 kBq/mL). Uptake in HER2pos lesions peaked at 3 hours (9.64 ± 1.69% IA/g), with very low accumulation in other organs (<1% IA/g) except for kidneys (11.69 ± 1.10% IA/g). α-camera imaging presented homogeneous uptake of radioactivity in tumors, although heterogeneous in kidneys, with a higher signal density in cortex versus medulla. In mice with HER2pos disseminated tumors, repeated administration of [225Ac]Ac-DOTA-2Rs15d significantly prolonged survival (143 days) compared to control groups (56 and 61 days) and to the group treated with HER2-targeting mAb trastuzumab (100 days). Histopathologic evaluation revealed signs of kidney toxicity after repeated administration of [225Ac]Ac-DOTA-2Rs15d. [225Ac]Ac-DOTA-2Rs15d efficiently targeted HER2pos cells and was effective in treatment of intraperitoneal disseminated tumors, both alone and as an add-on combination with trastuzumab, albeit with substantial signs of inflammation in kidneys. This study warrants further development of [225Ac]Ac-DOTA-2Rs15d.

Site-Specific Radiohalogenation of a HER2-Targeted Single-Domain Antibody Fragment Using a Novel Residualizing Prosthetic Agent

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter ²¹¹At. Herein, we have evaluated iso-[²¹¹At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. Iso-[²¹¹At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail. Both radiohalogenated PODS-5F7GGC conjugates were synthesized in good radiochemical yields and retained high binding affinity on HER2-positive BT474 breast carcinoma cells. Iso-[²¹¹At]AGMB-PODS-5F7GGC was considerably more stable in vitro than its maleimide analogue in the presence of cysteine and human serum albumin (HSA) and exhibited excellent tumor uptake and high in vivo stability. Superior tumor-to-kidney activity ratios were seen for both radiohalogenated PODS-5F7GGC conjugates compared with [¹⁷⁷Lu]Lu-DOTA-PODS-5F7GGC. These results suggest that iso-[²¹¹At]AGMB-PODS-5F7GGC warrants further evaluation for the treatment of HER2-expressing malignancies.

Biologicals as theranostic vehicles in paediatric oncology

Biologicals, such as antibodies or antibody-fragments e.g. nanobodies, have changed the landscape of cancer therapy and can be used in combination with traditional cancer treatments. They have been demonstrated to be excellent vehicles for molecular imaging. Several biologicals for nuclear imaging of adult cancer may be used in combination with (nuclear) therapy. Though it's great potential, molecular imaging using biologicals is rarely applied in paediatric oncology. This paper describes the current status of biologicals as radiopharmaceuticals for childhood cancer. Furthermore, the importance and potential for developing additional biological theranostics as opportunity to image and treat childhood cancer is discussed.

Antibody Engineering for Nuclear Imaging and Radioimmunotherapy

Radiolabeled antibodies have become indispensable tools in nuclear medicine. However, the natural roles of antibodies within the immune system mean that they have several intrinsic limitations as a platform for radiopharmaceuticals. In recent years, the field has increasingly turned to antibody engineering to circumvent these issues while retaining the manifold benefits of the immunoglobulin framework. In this "Focus on Molecular Imaging" review, we cover recent advances in the application of antibody engineering to immunoPET, immunoSPECT, and radioimmunotherapy. Specifically, we address how antibody engineering has been used to improve radioimmunoconjugates on four fronts: optimizing pharmacokinetics, facilitating site-specific bioconjugation, modulating Fc interactions, and creating bispecific constructs.

Challenges and opportunities in developing Actinium-225 radiopharmaceuticals

Actinium-225 (225Ac) has emerged as a promising therapeutic radioisotope for targeted alpha therapy. It emits net four alpha particles during its decay to stable daughter bismuth-209, rightly called an in-vivo nano-generator. Compared to the worldwide demand of 225Ac, the amount produced via depleted thorium-229 sources is minimal, making it an expensive radionuclide. However, many research groups are working on optimizing the parameters for the production of 225Ac via different routes, including cyclotrons, reactors and high-energy linear accelerators. The present review article focuses on the various aspects associated with the development of 225Ac radiopharmaceuticals. It includes the challenges and opportunities associated with the production methods, labeling chemistry, in-vivo kinetics and dosimetry of 225Ac radiopharmaceuticals. A brief description is also given about the 225Ac radiopharmaceuticals at preclinical stages, clinical trials and used routinely.

Rhenium Radioisotopes for Medicine, a Focus on Production and Applications

In recent decades, the use of alpha; pure beta; or beta/gamma emitters in oncology, endocrinology, and interventional cardiology rheumatology, has proved to be an important alternative to the most common therapeutic regimens. Among radionuclides used for therapy in nuclear medicine, two rhenium radioisotopes are of particular relevance: rhenium-186 and rhenium-188. The first is routinely produced in nuclear reactors by direct neutron activation of rhenium-186 via 185Re(n,γ)186Re nuclear reaction. Rhenium-188 is produced by the decay of the parent tungsten-188. Separation of rhenium-188 is mainly performed using a chromatographic 188W/188Re generator in which tungsten-188 is adsorbed on the alumina column, similar to the 99Mo/99mTc generator system, and the radionuclide eluted in saline solution. The application of rhenium-186 and rhenium-188 depends on their specific activity. Rhenium-186 is produced in low specific activity and is mainly used for labeling particles or diphosphonates for bone pain palliation. Whereas, rhenium-188 of high specific activity can be used for labeling peptides or bioactive molecules. One of the advantages of rhenium is its chemical similarity with technetium. So, diagnostic technetium analogs labeled with radiorhenium can be developed for therapeutic applications. Clinical trials promoting the use of 186/188Re-radiopharmaceuticals is, in particular, are discussed.

Radiotheranostic Agents in Hematological Malignancies

Radioimmunotherapy (RIT) is a cancer treatment that combines radiation therapy with tumor-directed monoclonal antibodies (Abs). Although RIT had been introduced for the treatment of CD20 positive non-Hodgkin lymphoma decades ago, it never found a broad clinical application. In recent years, researchers have developed theranostic agents based on Ab fragments or small Ab mimetics such as peptides, affibodies or single-chain Abs with improved tumor-targeting capacities. Theranostics combine diagnostic and therapeutic capabilities into a single pharmaceutical agent; this dual application can be easily achieved after conjugation to radionuclides. The past decade has seen a trend to increased specificity, fastened pharmacokinetics, and personalized medicine. In this review, we discuss the different strategies introduced for the noninvasive detection and treatment of hematological malignancies by radiopharmaceuticals. We also discuss the future applications of these radiotheranostic agents.

Matrices and Affinity Ligands for Antibody Purification and Corresponding Applications in Radiotherapy

Antibodies have become an important class of biological products in cancer treatments such as radiotherapy. The growing therapeutic applications have driven a demand for high-purity antibodies. Affinity chromatography with a high affinity and specificity has always been utilized to separate antibodies from complex mixtures. Quality chromatographic components (matrices and affinity ligands) have either been found or generated to increase the purity and yield of antibodies. More importantly, some matrices (mainly particles) and affinity ligands (including design protocols) for antibody purification can act as radiosensitizers or carriers for therapeutic radionuclides (or for radiosensitizers) either directly or indirectly to improve the therapeutic efficiency of radiotherapy. This paper provides a brief overview on the matrices and ligands used in affinity chromatography that are involved in antibody purification and emphasizes their applications in radiotherapy to enrich potential approaches for improving the efficacy of radiotherapy.

Emerging applications of nanobodies in cancer therapy

Cancer is a heterogeneous disease, requiring treatment tailored to the unique phenotype of the patient's tumor. Monoclonal antibodies (mAbs) and variants thereof have enabled targeted therapies to selectively target cancer cells. Cancer cell-specific mAbs have been used for image-guided surgery and targeted delivery of radionuclides or toxic agents, improving classical treatment strategies. Cancer cell-specific mAbs can further inhibit tumor cell growth or can stimulate immune-mediated destruction of cancer cells, a feature that has also been achieved through mAb-mediated manipulation of immune cells and pathways. Drawbacks of mAbs and their variants, together with the discovery of camelid heavy chain-only antibodies and the many advantageous features of their variable domains, referred to as V_HHs, single domain antibodies or nanobodies (Nbs), resulted in the exploration of Nbs as an alternative targeting moiety. We therefore review the state-of-the-art as well as novel exploitation strategies of Nbs for targeted cancer therapy.

Nanobodies as molecular imaging probes

Camelidae derived single-domain antibodies (sdAbs), commonly known as nanobodies (Nbs), are the smallest antibody fragments with full antigen-binding capacity. Owing to their desirable properties such as small size, high specificity, strong affinity, excellent stability, and modularity, nanobodies are on their way to overtake conventional antibodies in terms of popularity. To date, a broad range of nanobodies have been generated against different molecular targets with applications spanning basic research, diagnostics, and therapeutics. In the field of molecular imaging, nanobody-based probes have emerged as a powerful tool. Radioactive or fluorescently labeled nanobodies are now used to detect and track many targets in different biological systems using imaging techniques. In this review, we provide an overview of the use of nanobodies as molecular probes. Additionally, we discuss current techniques for the generation, conjugation, and intracellular delivery of nanobodies.

Targeted α-therapy in non-prostate malignancies

Progress in unraveling the complex biology of cancer, novel developments in radiochemistry, and availability of relevant α-emitters for targeted therapy have provided innovative approaches to precision cancer management. The approval of 223Ra dichloride for treatment of men with osseous metastatic castrate-resistant prostate cancer unleashed targeted α-therapy as a safe and effective cancer management strategy. While there is currently active research on new α-therapy regimens for prostate cancer based on the prostate-specific membrane antigen, there is emerging development of radiopharmaceutical therapy with a range of biological targets and α-emitting radioisotopes for malignancies other than the prostate cancer. This article provides a brief review of preclinical and first-in-human studies of targeted α-therapy in the cancers of brain, breast, lung, gastrointestinal, pancreas, ovary, and the urinary bladder. The data on leukemia, melanoma, myeloma, and neuroendocrine tumors will also be presented. It is anticipated that with further research the emerging role of targeted α-therapy in cancer management will be defined and validated.

Nanobody-Based Theranostic Agents for HER2-Positive Breast Cancer: Radiolabeling Strategies

The overexpression of human epidermal growth factor 2 (HER2) in breast cancer (BC) has been associated with a more aggressive tumor subtype, poorer prognosis and shorter overall survival. In this context, the development of HER2-targeted radiotracers is crucial to provide a non-invasive assessment of HER2 expression to select patients for HER2-targeted therapies, monitor response and identify those who become resistant. Antibodies represent ideal candidates for this purpose, as they provide high contrast images for diagnosis and low toxicity in the therapeutic setting. Of those, nanobodies (Nb) are of particular interest considering their favorable kinetics, crossing of relevant biological membranes and intratumoral distribution. The purpose of this review is to highlight the unique characteristics and advantages of Nb-based radiotracers in BC imaging and therapy. Additionally, radiolabeling methods for Nb including direct labeling, indirect labeling via prosthetic group and indirect labeling via complexation will be discussed, reporting advantages and drawbacks. Furthermore, the preclinical to clinical translation of radiolabeled Nbs as promising theranostic agents will be reported.

Labeling single domain antibody fragments with 18F using a novel residualizing prosthetic agent - N-succinimidyl 3-(1-(2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate

INTRODUCTION

Labeling single domain antibody fragments (sdAbs) with 18F is an attractive strategy for immunoPET. Earlier, we developed a residualizing label, N-succinimidyl 3-((4-(4-fluorobutyl)-1H-1,2,3-triazol-1-yl)methyl)-5-(guanidinomethyl)benzoate ([18F]RL-I), synthesized via a click reaction for labeling sdAbs with 18F, that has attractive features but suffered from modest radiochemical yields and suboptimal hydrophobicity. Herein, we have evaluated the potential utility of an analogous agent, N-succinimidyl 3-(1-(2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)-5-(guanidinomethyl)benzoate ([18F]SFETGMB; [18F]RL-III) designed to address these limitations.

METHODS

[18F]RL-III was synthesized by the click reaction between 3-((2,3-bis(tert-butoxycarbonyl)guanidino)methyl)-5-ethynylbenzoate and 1-azido-2-(2-(2-(2-[18F]fluoroethoxy)ethoxy)ethoxy)ethane and subsequent deprotection. The anti-HER2 sdAbs 5F7 and 2Rs15d were labeled by conjugation with [18F]RL-III and compared in a paired-label fashion to the sdAbs labeled using N-succinimidyl 4-guanidinomethyl-3-[125I]iodobenzoate ([125I]SGMIB) or N-succinimidyl 3-guanidinomethyl-5-[125I]iodobenzoate (iso-[125I]SGMIB). The 18F-labeled sdAbs were evaluated in vitro using HER2-expressing breast and ovarian carcinoma cells (BT474/BT474M1 and SKOV-3) and in vivo in athymic mice bearing subcutaneous SKOV-3 or BT474 xenografts. PET imaging of athymic mice bearing either subcutaneous BT474 or intracranial BT474M1Br-Fluc xenografts after administration of [18F]RL-III-5F7 also was performed.

RESULTS

Radiochemical yields for the synthesis of Boc2-[18F]RL-III (21.5 ± 3.4%) were significantly higher than reported for Boc2-[18F]RL-I. The overall radiochemical yields for the synthesis of [18F]RL-III-2Rs15d and [18F]RL-III-5F7 from aqueous [18F]fluoride were 1.7 ± 0.7% and 3.8 ± 2.3%, respectively. Both sdAbs, labeled using [18F]RL-III, retained affinity and immunoreactivity to HER2. Uptake and internalization of [18F]RL-III-5F7 in HER2-expressing cells was higher than that seen for [18F]RL-III-2Rs15d. Although different xenograft models were used, [18F]RL-III-2Rs15d showed relatively high uptake in a number of normal tissues, while uptake of [18F]RL-III-5F7 was mainly in tumor and kidneys with minimal background activity. Concordant with the necropsy experiments, microPET imaging with [18F]RL-III-5F7 in the BT474 subcutaneous model demonstrated clear delineation of the tumor (12.2 ± 5.1% ID/g) with minimal background activity except in kidneys. A tumor uptake (max) of 0.98%ID/g and a tumor-to-normal brain ratio of 9.8:1 were observed for [18F]RL-III-5F7 in the intracranial model.

CONCLUSIONS

Although higher radiochemical yields than that reported for [18F]RL-I were obtained, considerable improvements are needed for this method to be of practical utility. Despite clear tumor delineation with [18F]RL-III-5F7 as early as 1 h, high activity levels in the kidneys remain a concern.

Development of a 99mTc-Labeled Single-Domain Antibody for SPECT/CT Assessment of HER2 Expression in Breast Cancer

Accurate determination of human epidermal growth factor receptor 2 (HER2) expression is essential for HER2-targeted therapy in patients with cancer. HER2 expression in a complex environment, such as in a heterogeneous tumor, makes the precise assessment of the HER2 status difficult using current methods. In this study, we developed a novel ^99mTc-labeled anti-HER2 single-domain antibody (^99mTc-NM-02) as a molecular imaging tracer for the noninvasive detection of HER2 expression and investigated its safety, radiation dosimetry, biodistribution, and tumor-targeting potential in 10 patients with breast cancer. Our data showed that no drug-related adverse reactions occurred. The tracer mainly accumulated in the kidneys and liver with mild uptake in the spleen, intestines, and thyroid; however, only background tracer levels were observed in other organs where primary tumors and metastases typically occurred. The mean effective dose was 6.56 × 10^-3 mSv/MBq, and tracer uptake was visually observed in the primary tumors and metastases. A maximal standard uptake value of 1.5 was determined as a reasonable cutoff for identifying HER2 positivity using SPECT/CT imaging. Our ^99mTc-NM-02 tracer is safe for use in breast cancer imaging, with reasonable radiation doses, favorable biodistribution, and imaging characteristics. ^99mTc-NM-02 SPECT imaging may be an accurate and noninvasive method to detect the HER2 status in patients with breast cancer.

Lyophilization of NOTA-sdAbs: First step towards a cold diagnostic kit for 68Ga-labeling

Lyophilization is commonly used in the production of pharmaceutical compounds to increase the stability of the Active Pharmaceutical Ingredient (API) by removing solvents. This study investigates the possibility to lyophilize an anti-HER2 and an anti-MMR single-domain antibody fragment (sdAb)-based precursor as a first step in the development of a diagnostic kit for PET imaging.

METHODS

NOTA-sdAb precursors have been lyophilized with the following formulation: 100 µg NOTA-sdAb in 0.1 M NaOAc (NaOAc), 5% (w/v%) mannitol-sucrose mix at a 2:1 ratio and 0.1 mg/mL polysorbate 80. During development of the formulation and drying cycle, factors such as cake appearance, glass transition temperature and residual moisture were analyzed to ensure qualitative and stable lyophilized samples. Stability studies of lyophilized precursor were conducted up to 18 months after storage at 2-8 °C by evaluating the precursor integrity, aggregation, functionality and ⁶⁸Ga-labeling efficiency. A comparative biodistribution study (lyophilized vs non-lyophilized precursor) was conducted in wild type mice (n = 3) and in tumor bearing mice (n = 6).

RESULTS

The lyophilized NOTA-anti-HER2 precursor shows consistent stability data in vitro for up to 12 months at 2-8 °C in three separate batches, with results indicating stability even for up to T18m. No aggregation, degradation or activity loss was observed. Radiochemical purity after ⁶⁸Ga-labeling is consistent over a period of 12 months (RCP ≥ 95% at T12m). In vivo biodistribution analyses show a typical [⁶⁸Ga]Ga-NOTA-anti-HER2 sdAb distribution profile and a comparable tumor uptake for the lyophilized compound vs non-lyophilized (5.5% vs 5.7 %IA/g, respectively). In vitro results of lyophilized NOTA-anti-MMR precursor indicates stability for up to 18 months, while in vivo data show a comparable tumor uptake (2.5% vs 2.8 %IA/g, respectively) and no significant difference in kidney retention (49.4% vs 47.5 %IA/g, respectively).

CONCLUSION

A formulation and specific freeze-drying cycle were successfully developed to lyophilize NOTA-sdAb precursors for long-term storage at 2-8 °C. In vivo data show no negative impact of the lyophilization process on the in vivo behavior or functionality of the lyophilized precursor. These results highlight the potential to develop a kit for the preparation of ⁶⁸Ga-sdAb-based radiopharmaceuticals.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

Calcium Carbonate Core-Shell Particles for Incorporation of 225Ac and Their Application in Local α-Radionuclide Therapy

Actinium-225 (225Ac) radiolabeled submicrometric core-shell particles (SPs) made of calcium carbonate (CaCO3) coated with biocompatible polymers [tannic acid-human serum albumin (TA/HSA)] have been developed to improve the efficiency of local α-radionuclide therapy in melanoma models (B16-F10 tumor-bearing mice). The developed 225Ac-SPs possess radiochemical stability and demonstrate effective retention of 225Ac and its daughter isotopes. The SPs have been additionally labeled with zirconium-89 (89Zr) to perform the biodistribution studies using positron emission tomography-computerized tomography (PET/CT) imaging for 14 days after intratumoral injection. According to the PET/CT analysis, a significant accumulation of 89Zr-SPs in the tumor area is revealed for the whole investigation period, which correlates with the direct radiometry analysis after intratumoral administration of 225Ac-SPs. The histological analysis has revealed no abnormal changes in healthy tissue organs after treatment with 225Ac-SPs (e.g., no acute pathologic findings are detected in the liver and kidneys). At the same time, the inhibition of tumor growth has been observed as compared with control samples [nonradiolabeled SPs and phosphate-buffered saline (PBS)]. The treatment of mice with 225Ac-SPs has resulted in prolonged survival compared to the control samples. Thus, our study validates the application of 225Ac-doped core-shell submicron CaCO3 particles for local α-radionuclide therapy.

Diagnostics and Therapeutics in Targeting HER2 Breast Cancer: A Novel Approach

Breast cancer is one of the most commonly occurring cancers in women globally and is the primary cause of cancer mortality in females. BC is highly heterogeneous with various phenotypic expressions. The overexpression of HER2 is responsible for 15-30% of all invasive BC and is strongly associated with malignant behaviours, poor prognosis and decline in overall survival. Molecular imaging offers advantages over conventional imaging modalities, as it provides more sensitive and specific detection of tumours, as these techniques measure the biological and physiological processes at the cellular level to visualise the disease. Early detection and diagnosis of BC is crucial to improving clinical outcomes and prognosis. While HER2-specific antibodies and nanobodies may improve the sensitivity and specificity of molecular imaging, the radioisotope conjugation process may interfere with and may compromise their binding functionalities. Aptamers are single-stranded oligonucleotides capable of targeting biomarkers with remarkable binding specificity and affinity. Aptamers can be functionalised with radioisotopes without compromising target specificity. The attachment of different radioisotopes can determine the aptamer's functionality in the treatment of HER2(+) BC. Several HER2 aptamers and investigations of them have been described and evaluated in this paper. We also provide recommendations for future studies with HER2 aptamers to target HER2(+) BC.

HER2-directed antibodies, affibodies and nanobodies as drug-delivery vehicles in breast cancer with a specific focus on radioimmunotherapy and radioimmunoimaging

PURPOSE

The aim of the present paper is to review the role of HER2 antibodies, affibodies and nanobodies as vehicles for imaging and therapy approaches in breast cancer, including a detailed look at recent clinical data from antibody drug conjugates and nanobodies as well as affibodies that are currently under development.

RESULTS

Clinical and preclinical studies have shown that the use of monoclonal antibodies in molecular imaging is impaired by slow blood clearance, associated with slow and low tumor uptake and with limited tumor penetration potential. Antibody fragments, such as nanobodies, on the other hand, can be radiolabelled with short-lived radioisotopes and provide high-contrast images within a few hours after injection, allowing early diagnosis and reduced radiation exposure of patients. Even in therapy, the small radioactively labeled nanobodies prove to be superior to radioactively labeled monoclonal antibodies due to their higher specificity and their ability to penetrate the tumor.

CONCLUSION

While monoclonal antibodies are well established drug delivery vehicles, the current literature on molecular imaging supports the notion that antibody fragments, such as affibodies or nanobodies, might be superior in this approach.

Perspectives on metals-based radioimmunotherapy (RIT): moving forward

Radioimmunotherapy (RIT) is FDA-approved for the clinical management of liquid malignancies, however, its use for solid malignancies remains a challenge. The putative benefit of RIT lies in selective targeting of antigens expressed on the tumor surface using monoclonal antibodies, to systemically deliver cytotoxic radionuclides. The past several decades yielded dramatic improvements in the quality, quantity, recent commercial availability of alpha-, beta- and Auger Electron-emitting therapeutic radiometals. Investigators have created new or improved existing bifunctional chelators. These bifunctional chelators bind radiometals and can be coupled to antigen-specific antibodies. In this review, we discuss approaches to develop radiometal-based RITs, including the selection of radiometals, chelators and antibody platforms (i.e. full-length, F(ab')2, Fab, minibodies, diabodies, scFv-Fc and nanobodies). We cite examples of the performance of RIT in the clinic, describe challenges to its implementation, and offer insights to address gaps toward translation.

Advancing understanding of actinide(iii) (Ac, Am, Cm) aqueous complexation chemistry

The positive impact of having access to well-defined starting materials for applied actinide technologies - and for technologies based on other elements - cannot be overstated. Of numerous relevant 5f-element starting materials, those in complexing aqueous media find widespread use. Consider acetic acid/acetate buffered solutions as an example. These solutions provide entry into diverse technologies, from small-scale production of actinide metal to preparing radiolabeled chelates for medical applications. However, like so many aqueous solutions that contain actinides and complexing agents, 5f-element speciation in acetic acid/acetate cocktails is poorly defined. Herein, we address this problem and characterize Ac3+ and Cm3+ speciation as a function of increasing acetic acid/acetate concentrations (0.1 to 15 M, pH = 5.5). Results obtained via X-ray absorption and optical spectroscopy show the aquo ion dominated in dilute acetic acid/acetate solutions (0.1 M). Increasing acetic acid/acetate concentrations to 15 M increased complexation and revealed divergent reactivity between early and late actinides. A neutral Ac(H2O)6 (1)(O2CMe)3 (1) compound was the major species in solution for the large Ac3+. In contrast, smaller Cm3+ preferred forming an anion. There were approximately four bound O2CMe1- ligands and one to two inner sphere H2O ligands. The conclusion that increasing acetic acid/acetate concentrations increased acetate complexation was corroborated by characterizing (NH4)2M(O2CMe)5 (M = Eu3+, Am3+ and Cm3+) using single crystal X-ray diffraction and optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements).

Radiolabeled nanobodies for tumor targeting: From bioengineering to imaging and therapy

So far, numerous molecules and biomolecules have been evaluated for tumor targeting purposes for radionuclide-based imaging and therapy modalities. Due to the high affinity and specificity against tumor antigens, monoclonal antibodies are appropriate candidates for tumor targeting. However, their large size prevents their comprehensive application in radionuclide-based tumor imaging or therapy, since it leads to their low tumor penetration, low blood clearance, and thus inappropriate tumor-to-background ratio. Nowadays, the variable domain of heavy-chain antibodies from the Camelidae family, known as nanobodies (Nbs), turn into exciting candidates for medical research. Considering several innate advantages of these new tumor-targeting agents, including excellent affinity and specificity toward antigen, high solubility, high stability, fast washout from blood, convenient production, ease of selection, and low immunogenicity, it assumes that they may overcome generic problems of monoclonal antibodies, their fragments, and other vectors used for tumor imaging/therapy. After three decades of Nbs discovery, the increasing number of their preclinical and clinical investigations, which have led to outstanding results, confirm their application for tumor targeting purposes. This review describes Nbs characteristics, the diagnostic and therapeutic application of their radioconjugates, and their recent advances.

Radiochemical separation of 224Ra from 232U and 228Th sources for 224Ra/212Pb/212Bi generator

The application of diagnostic and therapeutic radionuclides in nuclear medicine has grown significantly and has translated into the increased interest in radionuclide generators and their development. ²²⁴Ra and its shorter-lived daughters, ²¹²Pb and ²¹²Bi, are very interesting radionuclides from Targeted Alpha Therapy point of view for treatment of small cancers or metastatic forms. The purpose of the present work was to develop a simple generator for rapid elution of carrier-free ²²⁴Ra from ²³²U or ²²⁸Th sources by radiochemical separation based on extraction chromatography with the utilization of a home-made material. The bis(2-ethylhexyl) hydrogen phosphate (HDEHP) extractant was immobilized on polytetrafluroethylene (PTFE) grains and its ability to selectively adsorb ²³²U and ²²⁸Th, with simultaneous high elution recovery of ²²⁴Ra, was checked over few years. The ²²⁴Ra was quantitatively eluted with small volume (3-5 mL) of 0.1 M HNO₃ with low breakthrough (<0.005%) and was used for further milking of ²¹²Bi and ²¹²Pb from DOWEX 50WX12 by 0.75 M and 2.0 M HCl, respectively. The elaborated here methods allowed high recovery of ²²⁴Ra, ²¹²Pb and ²¹²Bi radionuclides and their application in radiolabeling of various biomolecules.

Generation of a nanobody against HER2 tyrosine kinase using phage display library screening for HER2-positive breast cancer therapy development

Human epidermal growth factor receptor 2 (HER2) protein overexpression is found in ~30% of invasive breast carcinomas and in a high proportion of noninvasive ductal carcinomas in situ. Targeted cancer therapy is based on monoclonal antibodies and kinase inhibitors and reflects a new era of cancer therapy. However, delivery to tumor cells in vivo is hampered by the large size (150 kDa) of conventional antibodies. Furthermore, there are many disadvantages with the current anti-HER2 drug, including drug resistance and adverse effects. Nanobodies (15 kDa), single-domain antibody (sdAb) fragments, can overcome these limitations. This study produced the recombinant sdAb against the HER2-tyrosine kinase (HER2-TK) domain using phage display technology. Three specific anti-HER2-TK sdAbs were selected for further characterization. Hallmark VHH residue identification and amino acid sequence analysis revealed that clone numbers 4 and 22 were VH antibodies, whereas clone number 17 was a VH H antibody (nanobody). The half-maximal inhibitory concentration of VHH17 exhibited significantly greater HER2 kinase-inhibition activity than the other clones. Consistent with these results, several charges and polar residues of the HER2-TK activation loop that were predicted based on mimotope analysis also appeared in the docking result and interacted via the CDR1, CDR2 and CDR3 loops of VHH17. Furthermore, the cell-penetrable VHH17 (R9 VHH17) showed cell-penetrability and significantly decreased HER2-positive cancer cell viability. Thus, the VH H17 could be developed as an effective therapeutic agent to treat HER2-positive breast cancer.