131I-labeled Anti-HER2 Camelid sdAb as a Theranostic Tool in Cancer Treatment

Optimizing the Therapeutic Index of sdAb-Based Radiopharmaceuticals Using Pretargeting

Single-domain antibodies (sdAbs) demonstrate favorable pharmacokinetic profiles for molecular imaging applications. However, their renal excretion and retention are obstacles for applications in targeted radionuclide therapy (TRT). Methods: Using a click-chemistry-based pretargeting approach, we aimed to reduce kidney retention of a fibroblast activation protein α (FAP)-targeted sdAb, 4AH29, for 177Lu-TRT. Key pretargeting parameters (sdAb-injected mass and lag time) were optimized in healthy mice and U87MG (FAP+) xenografts. A TRT study in a pancreatic ductal adenocarcinoma (PDAC) patient-derived xenograft (PDX) model was performed as a pilot study for sdAb-based pretargeting applications. Results: Modification of 4AH29 with trans-cyclooctene (TCO) moieties did not modify the sdAb pharmacokinetic profile. A 200-µg injected mass of 4AH29-TCO and an 8-h lag time for the injection of [177Lu]Lu-DOTA-PEG7-tetrazine resulted in the highest kidney therapeutic index (2.0 ± 0.4), which was 5-fold higher than that of [177Lu]Lu-DOTA-4AH29 (0.4 ± 0.1). FAP expression in the tumor microenvironment was validated in a PDAC PDX model with both immunohistochemistry and PET/CT imaging. Mice treated with the pretargeting high-activity approach (4AH29-TCO + [177Lu]Lu-DOTA-PEG7-tetrazine; 3 × 88 MBq, 1 injection per week for 3 wk) demonstrated prolonged survival compared with the vehicle control and conventionally treated ([177Lu]Lu-DOTA-4AH29; 3 × 37 MBq, 1 injection per week for 3 wk) mice. Mesangial expansion was reported in 7 of 10 mice in the conventional cohort, suggesting treatment-related kidney morphologic changes, but was not observed in the pretargeting cohort. Conclusion: This study validates pretargeting to mitigate sdAbs' kidney retention with no observation of morphologic changes on therapy regimen at early time points. Clinical translation of click-chemistry-based pre-TRT is warranted on the basis of its ability to alleviate toxicities related to biovectors' intrinsic pharmacokinetic profiles. The absence of representative animal models with extensive stroma and high FAP expression on cancer-associated fibroblasts led to a low mean tumor-absorbed dose even with high injected activity and consequently to modest survival benefit in this PDAC PDX.

Plug-and-play nucleic acid-mediated multimerization of biparatopic nanobodies for molecular imaging

In cancer molecular imaging, selecting binders with high specificity and affinity for biomarkers is paramount for achieving high-contrast imaging within clinical time frames. Nanobodies have emerged as potent candidates, surpassing antibodies in pre-clinical imaging due to their convenient production, rapid renal clearance, and deeper tissue penetration. Multimerization of nanobodies is a popular strategy to enhance their affinity and pharmacokinetics; however, traditional methods are laborious and may yield heterogeneous products. In this study, we employ a Holliday junction (HJ)-like nucleic acid-based scaffold to create homogeneous nanostructures with precise multivalent and multiparatopic nanobody displays. The plug-and-play assembly allowed the screening of several nanobody multimer configurations for the detection of the breast cancer biomarker, human epidermal growth factor receptor 2 (HER2). In vitro studies demonstrated significant improvements in binding avidity, particularly with the biparatopic construct exhibiting high sensitivity, surpassing that of traditional antibody-based cell binding. Furthermore, our HJ platform allowed for adaptation from fluorescence-based to nuclear imaging, as demonstrated in xenografted mice, thereby allowing for future in vivo applications. This work highlights the potential of nucleic acid-mediated multimerization to markedly enhance nanobody binding, by exploring synergistic combinations and offering versatility for both in vitro diagnostics and cancer molecular imaging with prospects for future theranostic applications.

Efficient α and β- radionuclide therapy targeting fibroblast activation protein-α in an aggressive preclinical mouse tumour model

PURPOSE

Targeted radionuclide therapy (TRT) is a cancer treatment with relative therapeutic efficacy across various cancer types. We studied the therapeutic potential of TRT using fibroblast activation protein-α (FAP) targeting sdAbs (4AH29) labelled with 225Ac or 131I in immunocompetent mice in a human FAP (hFAP) expressing lung cancer mouse model. We further explored the combination of TRT with programmed cell death ligand 1 (PD-L1) immune checkpoint blockade (ICB).

METHODS

We studied the biodistribution and tumour uptake of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 by ex vivo γ-counting. Therapeutic efficacy of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 was evaluated in an immunocompetent mouse model. Flow cytometry analysis of tumours from [225Ac]Ac-DOTA-4AH29 treated mice was performed. Treatment with [225Ac]Ac-DOTA-4AH29 was repeated in combination with PD-L1 ICB.

RESULTS

The biodistribution showed high tumour uptake of [131I]I-GMIB-4AH29 with 3.5 ± 0.5% IA/g 1 h post-injection (p.i.) decreasing to 0.9 ± 0.1% IA/g after 24 h. Tumour uptake of [225Ac]Ac-DOTA-4AH29 was also relevant with 2.1 ± 0.5% IA/g 1 h p.i. with a less steep decrease to 1.7 ± 0.2% IA/g after 24 h. Survival was significantly improved after treatment with low and high doses [131I]I-GMIB-4AH29 or [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Moreover, we observed significantly higher PD-L1 expression in tumours of mice treated with [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Therefore, we combined high dose [225Ac]Ac-DOTA-4AH29 with PD-L1 ICB showing therapeutic synergy.

CONCLUSION

[225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit high and persistent tumour targeting, translating into prolonged survival in mice bearing aggressive tumours. Moreover, we demonstrate that the combination of PD-L1 ICB with [225Ac]Ac-DOTA-4AH29 TRT enhances its therapeutic efficacy.

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.

Radioimmunotheragnosis in Cancer Research

The combination of immunoPET-where an antibody (Ab) is labeled with an isotope for PET imaging-and radioimmunotherapy (RIT), using the same antibody with a therapeutic isotope, offers significant advantages in cancer management. ImmunoPET allows non-invasive imaging of antigen expression, which aids in patient selection for subsequent radioimmunotherapy. It also facilitates the assessment of tumor response to therapy, allowing for treatment adjustments if necessary. In addition, immunoPET provides critical pharmacokinetic data, including antibody biodistribution and clearance rates, which are essential for dosimetry calculations and treatment protocol optimization. There are still challenges to overcome. Identifying appropriate target antigens that are selectively expressed on cancer cells while minimally expressed on normal tissues remains a major hurdle to reduce off-target toxicity. In addition, it is critical to optimize the pharmacokinetics of radiolabeled antibodies to maximize tumor uptake and minimize normal tissue uptake, particularly in vital organs such as the liver and kidney. This approach offers the potential for targeted and personalized cancer therapy with reduced systemic toxicity by exploiting the specificity of monoclonal antibodies and the cytotoxic effects of radiation. However, further research is needed to address remaining challenges and to optimize these technologies for clinical use.

Single-Domain Antibodies as Antibody-Drug Conjugates: From Promise to Practice-A Systematic Review

BACKGROUND

Antibody-drug conjugates (ADCs) represent potent cancer therapies that deliver highly toxic drugs to tumor cells precisely, thus allowing for targeted treatment and significantly reducing off-target effects. Despite their effectiveness, ADCs can face limitations due to acquired resistance and potential side effects.

OBJECTIVES

This study focuses on advances in various ADC components to improve both the efficacy and safety of these agents, and includes the analysis of several novel ADC formats. This work assesses whether the unique features of VHHs-such as their small size, enhanced tissue penetration, stability, and cost-effectiveness-make them a viable alternative to conventional antibodies for ADCs and reviews their current status in ADC development.

METHODS

Following PRISMA guidelines, this study focused on VHHs as components of ADCs, examining advancements and prospects from 1 January 2014 to 30 June 2024. Searches were conducted in PubMed, Cochrane Library, ScienceDirect and LILACS using specific terms related to ADCs and single-domain antibodies. Retrieved articles were rigorously evaluated, excluding duplicates and non-qualifying studies. The selected peer-reviewed articles were analyzed for quality and synthesized to highlight advancements, methods, payloads, and future directions in ADC research.

RESULTS

VHHs offer significant advantages for drug conjugation over conventional antibodies due to their smaller size and structure, which enhance tissue penetration and enable access to previously inaccessible epitopes. Their superior stability, solubility, and manufacturability facilitate cost-effective production and expand the range of targetable antigens. Additionally, some VHHs can naturally cross the blood-brain barrier or be easily modified to favor their penetration, making them promising for targeting brain tumors and metastases. Although no VHH-drug conjugates (nADC or nanoADC) are currently in the clinical arena, preclinical studies have explored various conjugation methods and linkers.

CONCLUSIONS

While ADCs are transforming cancer treatment, their unique mechanisms and associated toxicities challenge traditional views on bioavailability and vary with different tumor types. Severe toxicities, often linked to compound instability, off-target effects, and nonspecific blood cell interactions, highlight the need for better understanding. Conversely, the rapid distribution, tumor penetration, and clearance of VHHs could be advantageous, potentially reducing toxicity by minimizing prolonged exposure. These attributes make single-domain antibodies strong candidates for the next generation of ADCs, potentially enhancing both efficacy and safety.

Preclinical research progress in HER2-targeted small-molecule probes for breast cancer

Breast cancer is a malignant tumor that has the highest morbidity and mortality in women worldwide. Human epidermal growth factor receptor 2 (HER2) is a key driver of breast cancer development. Therefore, accurate assessment of HER2 expression in cancer patients and timely initiation or termination of anti-HER2 treatment are crucial for the prognosis of breast cancer patients. The emergence of radiolabeled molecular probes targeting HER2 makes this assessment possible. This article describes different types of small-molecule probes that target HER2 and are used in current preclinical applications and summarizes their advantages and disadvantages.

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.

PSMA-reactive NB7 single domain antibody fragment: A potential scaffold for developing prostate cancer theranostics

INTRODUCTION

Single domain antibody fragments (sdAbs) are an appealing scaffold for radiopharmaceutical development due to their small size (~15 kDa), high solubility, high stability, and excellent tumor penetration. Previously, we developed NB7 sdAb, which has very high affinity for an epitope on PSMA that is different from those targeted by small molecule PSMA inhibitors. Herein, we evaluated NB7 after radioiodination using [*I]SGMIB (1,3,4-isomer) and iso-[*I]SGMIB (1,3,5-isomer), as well as their 211At-labeled analogues.

METHODS

[*I]SGMIB, iso-[*I]SGMIB, [211At]SAGMB, and iso-[211At]SAGMB conjugates of NB7 sdAb were synthesized and their binding affinity, cell uptake and internalization were assessed in PSMA+ PC3 PIP and PSMA- PC3 flu cells. Biodistribution studies were performed in mice bearing PSMA+ PC3 PIP xenografts. First, a single-label experiment evaluated the tissue distribution of a NB7 bearing a His6-tag (NB7H6) and labeled with iso-[125I]SGMIB. Three paired-label experiments then were performed to compare: a) NB7 labeled using [*I]SGMIB and iso-[*I]SGMIB, b) 131I- vs 211At-labeled NB7 conjugates and c) [125I]SGMIB-NB7H6 to the small molecule PSMA inhibitor [131I]YF2.

RESULTS

All NB7 radioconjugates bound specifically to PSMA with dissociation constants, Kd, in the low nM range (1.4-6.4 nM). An initial biodistribution study demonstrated good tumor uptake for iso-[125I]SGMIB-NB7H6 (7.2 ± 1.5 % ID/g at 1 h) and no deleterious effect of the His6-tag on renal activity levels, which declined to 3.1 ± 1.1 % ID/g by 4 h. Paired-label biodistribution found no distinction between the two SGMIB isomer NB7 conjugates with the [131I]SGMIB-NB7-to-iso-[125I]SGMIB-NB7 tumor uptake ratios not significantly different from unity: 1.06 ± 0.08 at 1 h, 1.04 ± 0.12 at 4 h, and 1.07 ± 0.09 at 24 h. Both isomer conjugates cleared rapidly from normal tissues and exhibited very low uptake in thyroid, lacrimal and salivary glands. Paired-label biodistribution of [131I]SGMIB-NB7H6 and [211At]SAGMB-NB7H6 demonstrated similar tumor uptake and kidney clearance for the two radioconjugates. However, levels of 211At in thyroid, stomach, salivary and lacrimal glands were significantly higher (P < 0.05) that those for 131I suggesting greater dehalogenation for [211At]SAGMB-NB7H6. Finally, co-administration of [125I]SGMIB-NB7H6 and [131I]YF2 demonstrated good tumor uptake for both with considerably more rapid renal clearance for the NB7 radioconjugate.

CONCLUSION

NB7 radioconjugates exhibited good accumulation in PSMA-positive xenografts with rapid clearance from kidney and other normal tissues. We conclude that NB7 is a potentially useful scaffold for developing PSMA-targeted theranostics with different characteristics than current small molecule and antibody-based approaches.

Dual-Targeting of the HER2 Cancer Receptor with an Antibody-Directed Enzyme and a Nanobody-Guided MMAE Prodrug Scaffold

Antibody-enzyme conjugates have shown potential as tissue-specific prodrug activators by antibody-directed enzyme prodrug therapy (ADEPT), but the approach met challenges clinically due to systemic drug release. Here, we report a novel dual-targeting ADEPT system (DuADEPT) which is based on active cancer receptor targeting of both a trastuzumab-sialidase conjugate (Tz-Sia) and a highly potent sialidase-activated monomethyl auristatin E (MMAE) prodrug scaffold. The scaffold is based on a four-way junction of the artificial nucleic acid analog acyclic (L)-threoninol nucleic acid ((L)-aTNA) which at the ends of its four arms carries one nanobody targeting HER2 and three copies of the prodrug. Dual-targeting of the constructs to two proximal epitopes of HER2 was shown by flow cytometry, and a dual-targeted enzymatic drug release assay revealed cytotoxicity upon prodrug activation specifically for HER2-positive cancer cells. The specific delivery and activation of prodrugs in this way could potentially be used to decrease systemic side effects and increase drug efficacy, and utilization of Tz-Sia provides an opportunity to combine the local chemotherapeutic effect of the DuADEPT with an anticancer immune response.

Novel OX40 and 4-1BB derived spacers enhance CD30 CAR activity and safety in CD30 positive lymphoma models

The chimeric antigen receptor (CAR) derived from the CD30 specific murine antibody, HRS-3, has produced promising clinical efficacy with a favorable safety profile in the treatment of relapsed or refractory CD30-positive lymphomas. However, persistence of the autologous CAR-T cells was brief, and many patients relapsed a year after treatment. The lack of persistence may be attributed to the use of a wild-type immunoglobulin (Ig)G1 spacer that can associate with Fc receptors. We first identified the cysteine-rich domain (CRD) 5 of CD30 as the primary binding epitope of HRS-3 and armed with this insight, attempted to improve the HRS-3 CAR functionality with a panel of novel spacer designs. We demonstrate that HRS-3 CARs with OX40 and 4-1BB derived spacers exhibited similar anti-tumor efficacy, circumvented interactions with Fc receptors, and secreted lower levels of cytokines in vitro than a CAR employing the IgG1 spacer. Humanization of the HRS-3 scFv coupled with the 4-1BB spacer preserved potent on-target, on-tumor efficacy, and on-target, off-tumor safety. In a lymphoma mouse model of high tumor burden, T cells expressing humanized HRS-3 CD30.CARs with the 4-1BB spacer potently killed tumors with low levels of circulating inflammatory cytokines, providing a promising candidate for future clinical development in the treatment of CD30-positive malignancies.

TransGEM: a molecule generation model based on Transformer with gene expression data

MOTIVATION

It is difficult to generate new molecules with desirable bioactivity through ligand-based de novo drug design, and receptor-based de novo drug design is constrained by disease target information availability. The combination of artificial intelligence and phenotype-based de novo drug design can generate new bioactive molecules, independent from disease target information. Gene expression profiles can be used to characterize biological phenotypes. The Transformer model can be utilized to capture the associations between gene expression profiles and molecular structures due to its remarkable ability in processing contextual information.

RESULTS

We propose TransGEM (Transformer-based model from gene expression to molecules), which is a phenotype-based de novo drug design model. A specialized gene expression encoder is used to embed gene expression difference values between diseased cell lines and their corresponding normal tissue cells into TransGEM model. The results demonstrate that the TransGEM model can generate molecules with desirable evaluation metrics and property distributions. Case studies illustrate that TransGEM model can generate structurally novel molecules with good binding affinity to disease target proteins. The majority of genes with high attention scores obtained from TransGEM model are associated with the onset of the disease, indicating the potential of these genes as disease targets. Therefore, this study provides a new paradigm for de novo drug design, and it will promote phenotype-based drug discovery.

AVAILABILITY AND IMPLEMENTATION

The code is available at https://github.com/hzauzqy/TransGEM.

Detection of HER2 expression using 99mTc-NM-02 nanobody in patients with breast cancer: a non-randomized, non-blinded clinical trial

BACKGROUND

99mTc radiolabeled nanobody NM-02 (99mTc-NM-02) is a novel single photon emission computed tomography (SPECT) probe with a high affinity and specificity for human epidermal growth factor receptor 2 (HER2). In this study, a clinical imaging trial was conducted to investigate the relationship between 99mTc-NM-02 uptake and HER2 expression in patients with breast cancer.

METHODS

Thirty patients with pathologically confirmed breast cancer were recruited and imaged with both 99mTc-NM-02 SPECT/computed tomography (CT) and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/CT. According to the treatment conditions before recruitment, patients were divided into two groups, the newly diagnosed group (n = 24) and the treated group (n = 6). The maximal standard uptake value (SUVmax) of 18F-FDG and SUVmax and mean SUV (SUVmean) of 99mTc-NM-02 in the lesions were determined to analyze the relationship with HER2 expression.

RESULTS

No meaningful relationship was observed between 18F-FDG uptake and HER2 expression in 30 patients with breast cancer. 99mTc-NM-02 uptake was positively correlated with HER2 expression in the newly diagnosed group, but no correlation was observed in the treated group. 99mTc-NM-02 uptake in HER2-positive lesions was lower in those with effective HER2-targeted therapy compared with the newly diagnosed group. 99mTc-NM-02 SPECT/CT detected brain and bone metastases of breast cancer with a different imaging pattern from 18F-FDG PET/CT. 99mTc-NM-02 showed no non-specific uptake in inflamed tissues and revealed intra- and intertumoral HER2 heterogeneity by SPECT/CT imaging in 9 of the 30 patients with breast cancer.

CONCLUSIONS

99mTc-NM-02 SPECT/CT has the potential for visualizing whole-body HER2 overexpression in untreated patients, making it a promising method for HER2 assessment in patients with breast cancer.

TRIAL REGISTRATION

NCT04674722, Date of registration: December 19, 2020.

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.

Antibody-mimetic drug conjugate with efficient internalization activity using anti-HER2 VHH and duocarmycin

Antibody-mimetic drug conjugate (AMDC) is a cancer cell-targeted drug delivery system based on the non-covalent binding of mutated streptavidin and modified biotin, namely Cupid and Psyche. However, the development of AMDCs is hampered by difficulties in post-translational modification or poor internalization activity. Here, we report an expression, refolding, and purification method for AMDC using a variable heavy chain of heavy chain-only antibodies (VHHs). Monomeric anti-HER2 VHH fused to Cupid was expressed in Escherichia coli inclusion bodies. Solubilization and refolding at optimized reducing conditions and pH levels were selected to form a functional, tetrameric protein (anti-HER2 VHH-Cupid) that can be easily purified based on molecular weight. Anti-HER2 VHH-Cupid non-covalently creates a tight complex with Psyche linked to a potent DNA-alkylating agent, duocarmycin. This complex can be absorbed by the HER2-expressing human breast cancer cell line, KPL-4, and kills KPL-4 cells in vitro and in vivo. The production of a targeting protein with internalizing activity, combined with the non-covalent conjugation of a highly potent payload, renders AMDC a promising platform for developing cancer-targeted therapy.

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.

Physicochemical differences between camelid single-domain antibodies and mammalian antibodies

Background/aim

In recent years, single-domain antibodies, also known as nanobodies, have emerged as an alternative to full immunoglobulin Gs (IgGs), due to their various advantages, including increased solubility, faster clearance, and cheaper production. Nanobodies are generally derived from the variable domain of the camelid heavy-chain-only immunoglobulin Gs (hcIgGs). Due to the high sequence homology between variable heavy chains of camelids (VHHs) and humans (VHs), hcIgGs are ideal candidates for nanobody development. However, further examination is needed to understand the structural differences between VHs and VHHs. This analysis is essential for nanobody engineering to mitigate potential immunogenicity, while preserving stability, functionality, and antigen specificity.

Materials and methods

We obtained the VH and VHH sequences of various camelid and non-camelid mammalian antibodies from public databases and used multiple sequence alignment based on the Chothia numbering scheme. Aligned sequences were subjected to diverse analyses encompassing paratope length, binding prediction, motif, disulfide bridge, salt bridge profiling, and physicochemical characteristic distribution. Logistic Regression coupled with the Boruta - Random Forest algorithm facilitated the comprehensive examination of physicochemical properties.

Results

Our findings revealed longer, less variable paratope sequences in VHHs, along with specific antigen binding residues with increased binding potential compared to VHs. Although the VHs showed more heterogeneous noncanonical disulfide bond patterns, the VHHs had a higher number of noncanonical disulfide bridges. Intriguingly, a typical salt bridge between the 94th and 101st positions in the VHs had a very low encounter rate in the VHHs. Surprisingly, we also identified notable differences in the physicochemical patterns of mostly conserved frameworks (FWs), especially the FW2 and FW3 regions, between VHs and VHHs.

Conclusion

Our findings point to possible key sites in VHHs as candidate residues for nanobody engineering efforts.

Preclinical Evaluation of a Radiotheranostic Single-Domain Antibody Against Fibroblast Activation Protein α

Fibroblast activation protein α (FAP) is highly expressed on cancer-associated fibroblasts of epithelial-derived cancers. Breast, colon, and pancreatic tumors often show strong desmoplastic reactions, which result in a dominant presence of stromal cells. FAP has gained interest as a target for molecular imaging and targeted therapies. Single-domain antibodies (sdAbs) are the smallest antibody-derived fragments with beneficial pharmacokinetic properties for molecular imaging and targeted therapy. Methods: We describe the generation, selection, and characterization of a sdAb against FAP. In mice, we assessed its imaging and therapeutic potential after radiolabeling with tracer-dose 131I and 68Ga for SPECT and PET imaging, respectively, and with 131I and 225Ac for targeted radionuclide therapy. Results: The lead sdAb, 4AH29, exhibiting picomolar affinity for a distinct FAP epitope, recognized both purified and membrane-bound FAP protein. Radiolabeled versions, including [68Ga]Ga-DOTA-4AH29, [225Ac]Ac-DOTA-4AH29, and [131I]I-guanidinomethyl iodobenzoate (GMIB)-4AH29, displayed radiochemical purities exceeding 95% and effectively bound to recombinant human FAP protein and FAP-positive GM05389 human fibroblasts. These radiolabeled compounds exhibited rapid and specific accumulation in human FAP-positive U87-MG glioblastoma tumors, with low but specific uptake in lymph nodes, uterus, bone, and skin (∼2-3 percentage injected activity per gram of tissue [%IA/g]). Kidney clearance of unbound [131I]I-GMIB-4AH29 was fast (<1 %IA/g after 24 h), whereas [225Ac]Ac-DOTA-4AH29 exhibited slower clearance (8.07 ± 1.39 %IA/g after 24 h and 2.47 ± 0.18 %IA/g after 96 h). Mice treated with [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 demonstrated prolonged survival compared with those receiving vehicle solution. Conclusion: [68Ga]Ga-DOTA-4AH29 and [131I]I-GMIB-4AH29 enable precise FAP-positive tumor detection in mice. Therapeutic [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit strong and sustained tumor targeting, resulting in dose-dependent therapeutic effects in FAP-positive tumor-bearing mice, albeit with kidney toxicity observed later for [225Ac]Ac-DOTA-4AH29. This study confirms the potential of radiolabeled sdAb 4AH29 as a radiotheranostic agent for FAP-positive cancers, warranting clinical evaluation.

Design principles for engineering light-controlled antibodies

Engineered antibodies are essential tools for research and advanced pharmacy. In the development of therapeutics, antibodies are excellent candidates as they offer both target recognition and modulation. Thanks to the latest advances in biotechnology, light-activated antibody fragments can be constructed to control spontaneous antigen interaction with high spatiotemporal precision. To implement conditional antigen binding, several optogenetic and optochemical engineering concepts have recently been developed. Here, we highlight the various strategies and discuss the features of opto-conditional antibodies. Each concept offers intrinsic advantages beneficial to different applications. In summary, the novel design approaches constitute a complementary toolset to promote current and upcoming antibody technologies with ultimate precision.

Shape complementarity processes for ultrashort-burst sensitive M13-PEG-WS2-powered MCF-7 cancer cell sensors

Biomarkers have the potential to be utilized in disease diagnosis, prediction and monitoring. The cancer cell type is a leading candidate for next-generation biomarkers. Although traditional digital biomolecular sensor (DBS) technology has shown to be effective in assessing cell-based interactions, low cell-population detection of cancer cell types is extremely challenging. Here, we controlled the electrical signature of a two-dimensional (2D) nanomaterial, tungsten disulfide (WS2), by utilizing a combination of the Phage-integrated Polymer and the Nanosheet (PPN), viz., the integration of the M13-conjugated polyethylene glycol (PEG) and the WS2, through shape-complementarity phenomena, and developed a sensor system, i.e., the Phage-based DBS (P-DBS), for the specific, rapid, sensitive detection of clinically-relevant MCF-7 cells. The P-DBS attains a detection limit of 12 cells per μL, as well as a contrast of 1.25 between the MCF-10A sample signal and the MCF-7 sample signal. A reading length of 200 μs was further achieved, along with a relative cell viability of ∼100% for both MCF-7 and MCF-10A cells and with the PNN. Atomistic simulations reveal the structural origin of the shape complementarity-facilitated decrease in the output impedance of the P-DBS. The combination of previously unreported exotic sensing materials and digital sensor design represents an approach to unlocking the ultra-sensitive detection of cancer cell types and provides a promising avenue for early cancer diagnosis, staging and monitoring.

Development of Novel Trifunctional Chelating Agents That Enhance Tumor Retention of Radioimmunoconjugates

Chelator-containing radioimmunoconjugates (RICs) composed of monoclonal antibodies, chelators, and radiometals exhibit broad potential for cancer diagnosis or therapy. In this study, we developed novel trifunctional chelating agents that enhance the tumor retention of RICs, MDPEI2, and MDPEI4, which contain the metal chelator DOTA, a maleimide moiety, and diethylenetriamine (PEI2) or tetraethylenepentamine (PEI4), respectively, as a poly(ethylenimine) (PEI) scaffold for the addition of positive charges to the radiometabolites of RICs to reduce their release from tumor cells. Trastuzumab radiolabeled by [111In]In-MDPEI2 ([111In]In-TMDPEI2) or [111In]In-MDPEI4 ([111In]In-TMDPEI4) showed high immunoreactivity and lower rates of exportations of their radiometabolites from tumor cells than RICs without PEI scaffolds. The tumor uptake of [111In]In-TMDPEI2 and [111In]In-TMDPEI4 was enhanced compared with RICs without PEI scaffolds, and [111In]In-TMDPEI2 exhibited the highest tumor/blood ratio. These results indicate the utility of MDPEI2 to synthesize RICs with favorable tumor-targeting properties in vivo by controlling the radioactivity distribution in tumor cells.

Single domain Camelid antibody fragments for molecular imaging and therapy of cancer

Despite innovations in cancer therapeutics, cancer remains associated with high mortality and is one of biggest health challenges worldwide. Therefore, developing precise cancer imaging and effective treatments is an unmet clinical need. A relatively novel type of therapeutics are heavy chain variable domain antibody fragments (VHHs) derived from llamas. Here, we explored the suitability of VHHs for cancer imaging and therapy through reviewing the existing literature. We searched the MEDLINE, EMBASE and Cochrane databases and identified 32 papers on molecular imaging and 41 papers on therapy that were suitable for comprehensive reviewing. We found that VHHs harbor a higher specificity and affinity compared to mAbs, which contributes to high-quality imaging and less side-effects on healthy cells. The employment of VHHs in cancer imaging showed remarkably shorter times between administration and imaging. Studies showed that 18F and 99mTc are two optimal radionuclides for imaging with VHHs and that site-specific labelling is the optimal conjugation modality for VHHs with radionuclide or fluorescent molecules. We found different solutions for reducing kidney retention and immunogenicity of VHHs. VHHs as anticancer therapeutics have been tested in photodynamic therapy, targeted radionuclide therapy, immunotherapy and molecular targeted therapy. These studies showed that VHHs target unique antigen epitopes, which are distinct from the ones recognized by mAbs. This advantage means that VHHs may be more effective for targeted anticancer therapy and can be combined with mAbs. We found that high cellular internalization and specificity of VHHs contributes to the effectiveness and safety of VHHs as anticancer therapeutics. Two clinical trials have confirmed that VHHs are effective and safe for cancer imaging and therapy. Together, VHHs seem to harbor several advantages compared to mAbs and show potential for application in personalized treatment for cancer patients. VHH-based imaging and therapy are promising options for improving outcomes of cancer patients.

Nanomaterials: a promising multimodal theranostics platform for thyroid cancer

Thyroid cancer is the most prevalent malignant neoplasm of the cervical region and endocrine system, characterized by a discernible upward trend in incidence over recent years. Ultrasound-guided fine needle aspiration is the current standard for preoperative diagnosis of thyroid cancer, albeit with limitations and a certain degree of false-negative outcomes. Although differentiated thyroid carcinoma generally exhibits a favorable prognosis, dedifferentiation is associated with an unfavorable clinical course. Anaplastic thyroid cancer, characterized by high malignancy and aggressiveness, remains an unmet clinical need with no effective treatments available. The emergence of nanomedicine has opened new avenues for cancer theranostics. The unique features of nanomaterials, including multifunctionality, modifiability, and various detection modes, enable non-invasive and convenient thyroid cancer diagnosis through multimodal imaging. For thyroid cancer treatment, nanomaterial-based photothermal therapy or photodynamic therapy, combined with chemotherapy, radiotherapy, or gene therapy, holds promise in reducing invasiveness and prolonging patient survival or alleviating pain in individuals with anaplastic thyroid carcinoma. Furthermore, nanomaterials enable simultaneous diagnosis and treatment of thyroid cancer. This review aims to provide a comprehensive survey of the latest developments in nanomaterials for thyroid cancer diagnosis and treatment and encourage further research in developing innovative and effective theranostic approaches for thyroid cancer.

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.

Co-crystallisation and humanisation of an anti-HER2 single-domain antibody as a theranostic tool

Human epidermal growth factor receptor-2 (HER2) is a well-recognised biomarker associated with 25% of breast cancers. In most cases, early detection and/or treatment correlates with an increased chance of survival. This study, has identified and characterised a highly specific anti-HER2 single-domain antibody (sdAb), NM-02, as a potential theranostic tool. Complete structural description by X-ray crystallography has revealed a non-overlapping epitope with current anti-HER2 antibodies. To reduce the immunogenicity risk, NM-02 underwent a humanisation process and retained wild type-like binding properties. To further de-risk the progression towards chemistry, manufacturing and control (CMC) we performed full developability profiling revealing favourable thermal and physical biochemical 'drug-like' properties. Finally, the application of the lead humanised NM-02 candidate (variant K) for HER2-specific imaging purposes was demonstrated using breast cancer HER2+/BT474 xenograft mice.

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.

Current status and future expectations of nanobodies in oncology trials

INTRODUCTION

Monoclonal antibodies have revolutionized personalized medicine for cancer in recent decades. Despite their broad application in oncology, their large size and complexity may interfere with successful tumor targeting for certain applications of cancer diagnosis and therapy. Nanobodies have unique structural and pharmacological features compared to monoclonal antibodies and have successfully been used as complementary anti-cancer diagnostic and/or therapeutic tools.

AREAS COVERED

Here, an overview is given of the nanobody-based diagnostics and therapeutics that have been or are currently being tested in oncological clinical trials. Furthermore, preclinical developments, which are likely to be translated into the clinic in the near future, are highlighted.

EXPERT OPINION

Overall, the presented studies show the application potential of nanobodies in the field of oncology, making it likely that more nanobodies will be clinically approved in the upcoming future.

Covalent Proteins as Targeted Radionuclide Therapies Enhance Antitumor Effects

Molecularly targeted radionuclide therapies (TRTs) struggle with balancing efficacy and safety, as current strategies to increase tumor absorption often alter drug pharmacokinetics to prolong circulation and normal tissue irradiation. Here we report the first covalent protein TRT, which, through reacting with the target irreversibly, increases radioactive dose to the tumor without altering the drug's pharmacokinetic profile or normal tissue biodistribution. Through genetic code expansion, we engineered a latent bioreactive amino acid into a nanobody, which binds to its target protein and forms a covalent linkage via the proximity-enabled reactivity, cross-linking the target irreversibly in vitro, on cancer cells, and on tumors in vivo. The radiolabeled covalent nanobody markedly increases radioisotope levels in tumors and extends tumor residence time while maintaining rapid systemic clearance. Furthermore, the covalent nanobody conjugated to the α-emitter actinium-225 inhibits tumor growth more effectively than the noncovalent nanobody without causing tissue toxicity. Shifting the protein-based TRT from noncovalent to covalent mode, this chemical strategy improves tumor responses to TRTs and can be readily scaled to diverse protein radiopharmaceuticals engaging broad tumor targets.

Reviving a Classic Antigen with a Cutting-Edge Approach: Nanobodies for HER2+ Breast Cancer

The serendipitous discovery of nanobodies (NBs) around two decades ago opened the door to new possibilities for innovative strategies, particularly in cancer treatment. These antigen-binding fragments are derived from heavy-chain-only antibodies naturally found in the serum of camelids and sharks. NBs are an appealing agent for the progress of innovative therapeutic strategies because they combine the advantageous assets of smaller molecules and conventional monoclonal antibodies (mAbs). Moreover, the possibility to produce NBs using bacterial systems reduces manufacturing expenses and speeds up the production process, making them a feasible option for the development of new bio-drugs. Several NBs have been developed over the past 10 years and are currently being tested in clinical trials for various human targets. Here, we provide an overview of the notable structural and biochemical characteristics of NBs, particularly in their application against HER2, an extracellular receptor that often gets aberrantly activated during breast cancer tumorigenesis. The focus is on the recent advancements in diagnostic and therapeutic research up to the present date.

Biased activation of the receptor tyrosine kinase HER2

HER2 belongs to the ErbB sub-family of receptor tyrosine kinases and regulates cellular proliferation and growth. Different from other ErbB receptors, HER2 has no known ligand. Activation occurs through heterodimerization with other ErbB receptors and their cognate ligands. This suggests several possible activation paths of HER2 with ligand-specific, differential response, which has so far remained unexplored. Using single-molecule tracking and the diffusion profile of HER2 as a proxy for activity, we measured the activation strength and temporal profile in live cells. We found that HER2 is strongly activated by EGFR-targeting ligands EGF and TGFα, yet with a distinguishable temporal fingerprint. The HER4-targeting ligands EREG and NRGβ1 showed weaker activation of HER2, a preference for EREG, and a delayed response to NRGβ1. Our results indicate a selective ligand response of HER2 that may serve as a regulatory element. Our experimental approach is easily transferable to other membrane receptors targeted by multiple ligands.

Smaller size packs a stronger punch - Recent advances in small antibody fragments targeting tumour-associated carbohydrate antigens

Attached to proteins, lipids, or forming long, complex chains, glycans represent the most versatile post-translational modification in nature and surround all human cells. Unique glycan structures are monitored by the immune system and differentiate self from non-self and healthy from malignant cells. Aberrant glycosylations, termed tumour-associated carbohydrate antigens (TACAs), are a hallmark of cancer and are correlated with all aspects of cancer biology. Therefore, TACAs represent attractive targets for monoclonal antibodies for cancer diagnosis and therapy. However, due to the thick and dense glycocalyx as well as the tumour micro-environment, conventional antibodies often suffer from restricted access and limited effectiveness in vivo. To overcome this issue, many small antibody fragments have come forth, showing similar affinity with better efficiency than their full-length counterparts. Here we review small antibody fragments against specific glycans on tumour cells and highlight their advantages over conventional antibodies.

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.

Engineering a modular double-transmembrane synthetic receptor system for customizing cellular programs

Implementation of designer receptors in engineered cells confers them to sense a particular physiological or disease state and respond with user-defined programs. To expand the therapeutic application scope of engineered cells, synthetic receptors realized through different strategies are in great demand. Here, we develop a synthetic receptor system that exerts dual control by incorporating two transmembrane helices for the signal chain. Together with a sensor-actuator device with minimal background signals and a positive loop circuit, this receptor system can sensitively respond to extracellular protein signals. We demonstrate that this synthetic receptor system can be readily adapted to respond to various inputs, such as interleukin-1 (IL-1), programmed death ligand 1 (PD-L1), and HER2, and release customized outputs, including fluorescence signals and the therapeutic molecule IL-2. The robust signaling ability and generality of this receptor system promise it to be a useful tool in the field of cell engineering for fundamental research and translational applications.

Nanobodies: Robust miniprotein binders in biomedicine

Variable domains of heavy chain-only antibodies (VHH), also known as nanobodies (Nbs), are monomeric antigen-binding domains derived from the camelid heavy chain-only antibodies. Nbs are characterized by small size, high target selectivity, and marked solubility and stability, which collectively facilitate high-quality drug development. In addition, Nbs are readily expressed from various expression systems, including E. coli and yeast cells. For these reasons, Nbs have emerged as preferred antibody fragments for protein engineering, disease diagnosis, and treatment. To date, two Nb-based therapies have been approved by the U.S. Food and Drug Administration (FDA). Numerous candidates spanning a wide spectrum of diseases such as cancer, immune disorders, infectious diseases, and neurodegenerative disorders are under preclinical and clinical investigation. Here, we discuss the structural features of Nbs that allow for specific, versatile, and strong target binding. We also summarize emerging technologies for identification, structural analysis, and humanization of Nbs. Our main focus is to review recent advances in using Nbs as a modular scaffold to facilitate the engineering of multivalent polymers for cutting-edge applications. Finally, we discuss remaining challenges for Nb development and envision new opportunities in Nb-based research.

High in-vivo stability in preclinical and first-in-human experiments with [18F]AlF-RESCA-MIRC213: a 18F-labeled nanobody as PET radiotracer for diagnosis of HER2-positive cancers

PURPOSE

[18F]AlF-RESCA was introduced as a core particularly useful for 18F-labeling of heat-sensitive biomolecules. However, no translational studies have been reported up to now. Herein, we reported the first-in-human evaluation of an 18F-labeled anti-HER2 nanobody MIRC213 as a PET radiotracer for imaging HER2-positive cancers.

METHODS

MIRC213 was produced by E. coli and conjugated with ( ±)-H3RESCA-Mal. [18F]AlF-RESCA-MIRC213 was prepared at room temperature. Its radiochemical purity and stability of were determined by radio-HPLC with the size-exclusion chromatographic column. Cell uptake was performed in NCI-N87 (HER2 +) and MCF-7 (HER2-) cells and the cell-binding affinity was verified in SK-OV-3 (HER2 +) cells. Small-animal PET/CT was performed using SK-OV-3, NCI-N87, and MCF-7 tumor-bearing mice at 30 min, 1 h, and 2 h post-injection. For blocking experiment, excess MIRC213 was co-injected with radiotracer. Biodistribution were performed on SKOV-3 and MCF-7 tumor-bearing mice at 2 h post-injection. For clinical study, PET/CT images were acquired at 2 h and 4 h after injection of [18F]AlF-RESCA-MIRC213 (1.85-3.7 MBq/kg) in six breast cancer patients (3 HER2-positive and 3 HER2-negative). All patients underwent [18F]-FDG PET/CT within a week for tissue selection purpose. Distribution and dosimetry were calculated. Standardized uptake values (SUV) were measured in tumors and normal organs.

RESULTS

MIRC213 was produced with > 95% purity and modified with RESCA to obtain RESCA-MIRC213. [18F]AlF-RESCA-MIRC213 was prepared within 20 min at room temperature with the radiochemical yield of 50.48 ± 7.6% and radiochemical purity of > 98% (n > 10), and remained stable in both PBS (88%) and 5% HSA (92%) after 6 h. The 2 h cellular uptake of [18F]AlF-RESCA-MIRC213 in NCI-N87 cells was 11.22 ± 0.60 AD%/105 cells. Its binding affinity Kd value was determined to be 1.23 ± 0.58 nM. Small-animal PET/CT with [18F]AlF-RESCA-MIRC213 can clearly differentiate SK-OV-3 and NCI-N87 tumors from MCF-7 tumors and background with a high uptake of 4.73 ± 1.18 ID%/g and substantially reduced to 1.70 ± 0.13 ID%/g for the blocking group (p < 0.05) in SK-OV-3 tumors at 2 h post-injection. No significant bone radioactivity was seen in the tumor-bearing animals. In all six breast cancer patients, there was no adverse reaction during study. The uptake of [18F]AlF-RESCA-MIRC213 was mainly in lacrimal gland, parotid gland, submandibular gland, thyroid gland, gallbladder, kidneys, liver, and intestines. There was no significant bone radioactivity accumulation in cancer patients. [18F]AlF-RESCA-MIRC213 had significantly higher tumor uptake in lesions from HER2-positive patients than that lesions from HER2-negative patients (SUVmax of 3.62 ± 1.56 vs. 1.41 ± 0.41, p = 0.0012) at 2 h post-injection. The kidneys received the highest radiation dose of 2.42 × 10-1 mGy/MBq, and the effective dose was 1.56 × 10-2 mSv/MBq.

CONCLUSIONS

[18F]AlF-RESCA-MIRC213 could be prepared with high radiolabeling yield under mild conditions. [18F]AlF-RESCA-MIRC213 has relatively high stability both in vitro and in vivo. The results from clinical transformation suggest that [18F]AlF-RESCA-MIRC213 PET/CT is a safe procedure with favorable pharmacokinetics and dosimetry profile, and it is a promising new PET radiotracer for noninvasive diagnosis of HER2-positive cancers.

131I-Labeled Anti-HER2 Nanobody for Targeted Radionuclide Therapy of HER2-Positive Breast Cancer

Purpose

The unique structure of nanobodies is advantageous for the development of radiopharmaceuticals for nuclear medicine. Nanobodies targeted to human epidermal growth factor receptor 2 (HER2) can be used as tools for the imaging and therapy of HER2-overexpressing tumors. In this study, we aimed to describe the generation of a ¹³¹I-labeled anti-HER2 nanobody as a targeted radionuclide therapy (TRNT) agent for HER2-positive breast cancer.

Methods

The anti-HER2 nanobody NM-02 was labeled with ¹³¹I using the iodogen method, and its radiochemical purity and stability in vitro were assessed. The pharmacokinetic profile of ¹³¹I-NM-02 was investigated in normal mice. Tumor accumulation, biodistribution, and therapeutic potential of ¹³¹I-NM-02 were evaluated in HER2-positive SKBR3 xenografts; HER2-negative MB-MDA-231 xenografts were used as the control group.

Results

¹³¹I-NM-02 could be readily prepared with satisfactory radiochemical purity and stability in vitro. Apparent tumor uptake was observed in HER2-positive tumor-bearing mice with rapid blood clearance and favorable biodistribution. ¹³¹I-NM-02 could significantly inhibit tumor growth and extend the life of these mice with good organ compatibility. Negligible tumor accumulation and inhibitory effects of ¹³¹I-NM-02 were observed in the negative control group.

Conclusion

¹³¹I-NM-02 has the potential to be explored as a novel tool for TRNT of HER2-positive breast cancer.

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.

Nanobodies for the Early Detection of Ovarian Cancer

Ovarian cancer ranks fifth in cancer-related deaths among women. Since ovarian cancer patients are often asymptomatic, most patients are diagnosed only at an advanced stage of disease. This results in a 5-year survival rate below 50%, which is in strong contrast to a survival rate as high as 94% if detected and treated at an early stage. Monitoring serum biomarkers offers new possibilities to diagnose ovarian cancer at an early stage. In this study, nanobodies targeting the ovarian cancer biomarkers human epididymis protein 4 (HE4), secretory leukocyte protease inhibitor (SLPI), and progranulin (PGRN) were evaluated regarding their expression levels in bacterial systems, epitope binning, and antigen-binding affinity by enzyme-linked immunosorbent assay and surface plasmon resonance. The selected nanobodies possess strong binding affinities for their cognate antigens (K_D~0.1-10 nM) and therefore have a pronounced potential to detect ovarian cancer at an early stage. Moreover, it is of utmost importance that the limits of detection (LOD) for these biomarkers are in the pM range, implying high specificity and sensitivity, as demonstrated by values in human serum of 37 pM for HE4, 163 pM for SLPI, and 195 pM for PGRN. These nanobody candidates could thus pave the way towards multiplexed biosensors.

Evaluation of 68Ga-Radiolabeled Peptides for HER2 PET Imaging

One in eight women will be diagnosed with breast cancer in their lifetime and approximately 25% of those cases will be HER2-positive. Current methods for diagnosing HER2-positive breast cancer involve using IHC and FISH from suspected cancer biopsies to quantify HER2 expression. HER2 PET imaging could potentially increase accuracy and improve the diagnosis of lesions that are not available for biopsies. Using two previously discovered HER2-targeting peptides, we modified each peptide with the chelator DOTA and a PEG2 linker resulting in DOTA-PEG2-GSGKCCYSL (P5) and DOTA-PEG2-DTFPYLGWWNPNEYRY (P6). Each peptide was labeled with 68Ga and was evaluated for HER2 binding using in vitro cell studies and in vivo tumor xenograft models. Both [68Ga]P5 and [68Ga]P6 showed significant binding to HER2-positive BT474 cells versus HER2-negative MDA-MB-231 cells ([68Ga]P5; 0.68 ± 0.20 versus 0.47 ± 0.05 p < 0.05 and [68Ga]P6; 0.55 ± 0.21 versus 0.34 ± 0.12 p < 0.01). [68Ga]P5 showed a higher percent injected dose per gram (%ID/g) binding to HER2-positive tumors two hours post-injection compared to HER2-negative tumors (0.24 ± 0.04 versus 0.12 ± 0.06; p < 0.05), while the [68Ga]P6 peptide showed significant binding (0.98 ± 0.22 versus 0.51 ± 0.08; p < 0.05) one hour post-injection. These results lay the groundwork for the use of peptides to image HER2-positive breast cancer.

Immobilization-Free Binding and Affinity Characterization of Higher Order Bispecific Antibody Complexes Using Size-Based Microfluidics

Simultaneous targeting of different antigens by bispecific antibodies (bsAbs) is permitting synergistic binding functionalities with high therapeutic potential, but is also rendering their analysis challenging. We introduce flow-induced dispersion analysis (FIDA) for the in-depth characterization of bsAbs with diverse molecular architectures and valencies under near-native conditions without potentially obstructive surface immobilization. Individual equilibrium dissociation constants are determined in solution, even in higher-order complexes with both antigens involved, hereby allowing the analysis of binding cooperativity and elucidation of a potential interference between the interactions. We further illustrate bispecific binding functionality as incremental increases in complex sizes when the bsAbs are exposed to one or two antigens. The possibility for comprehensive binding analysis with low material consumption and high matrix tolerability irrespective of molecular format and with little optimization renders FIDA a versatile tool for format selection and characterization of complex bi/multispecific protein therapeutics throughout the drug development and biomanufacturing pipeline.

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.

Encoding latent SuFEx reactive meta-fluorosulfate tyrosine to expand covalent bonding of proteins

The introduction of new covalent bonds into proteins is affording novel avenues for protein research and applications, yet it remains difficult to generate covalent linkages at all possible sites and across diverse protein classes. Herein, we genetically encoded meta-fluorosulfate-L-tyrosine (mFSY) to selectively react with lysine, tyrosine, and histidine via proximity-enabled SuFEx reaction. mFSY was able to target residues that were elusive for previous unnatural amino acids, and permitted engineering of various proteins including affibody, nanobody, and Fab into covalent binders that irreversibly cross-linked EGFR and HER2. mFSY is thus valuable for developing covalent proteins for biological research, synthetic biology, and biotherapeutics.

An overview on display systems (phage, bacterial, and yeast display) for production of anticancer antibodies; advantages and disadvantages

Antibodies as ideal therapeutic and diagnostic molecules are among the top-selling drugs providing considerable efficacy in disease treatment, especially in cancer therapy. Limitations of the hybridoma technology as routine antibody generation method in conjunction with numerous developments in molecular biology led to the development of alternative approaches for the streamlined identification of most effective antibodies. In this regard, display selection technologies such as phage display, bacterial display, and yeast display have been widely promoted over the past three decades as ideal alternatives to traditional methods. The display of antibodies on phages is probably the most widespread of these methods, although surface display on bacteria or yeast have been employed successfully, as well. These methods using various sizes of combinatorial antibody libraries and different selection strategies possessing benefits in screening potency, generating, and isolation of high affinity antibodies with low risk of immunogenicity. Knowing the basics of each method assists in the design and retrieval process of antibodies suitable for different diseases, including cancer. In this review, we aim to outline the basics of each library construction and its display method, screening and selection steps. The advantages and disadvantages in comparison to alternative methods, and their applications in antibody engineering will be explained. Finally, we will review approved or non-approved therapeutic antibodies developed by employing these methods, which may serve as therapeutic antibodies in cancer therapy.

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.

Engineering nanobodies for next-generation molecular imaging

In recent years, nanobodies have emerged as ideal imaging agents for molecular imaging. Molecular nanobody imaging combines the specificity of nanobodies with the sensitivity of state-of-the-art molecular imaging modalities, such as positron emission tomography (PET). Given that modifications of nanobodies alter their pharmacokinetics (PK), the engineering strategies that combine nanobodies with radionuclides determine the effectiveness, reliability, and safety of the molecular imaging probes. In this review, we introduce conjugation strategies that have been applied to nanobodies, including random conjugation, ^99mTc tricarbonyl chemistry, sortase A-mediated site-specific conjugation, maleimide-cysteine chemistry, and click chemistries. We also summarize the latest advances in nanobody tracers, emphasizing their preclinical and clinical use. In addition, we elaborate on nanobody-based near-infrared fluorescence (NIRF) imaging and image-guided surgery.

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.

Assessment of mouse-specific pharmacokinetics in kidneys based on 131I activity measurements using micro-SPECT

BACKGROUND

In order to acquire accurate drug pharmacokinetic information, which is required for tissue dosimetry, micro-SPECT must be quantitative to allow for an accurate assessment of radioligand activity in the relevant tissue. This study investigates the feasibility of deriving accurate mouse-specific time-integrated drug pharmacokinetic data in mouse kidneys from activity measurements using micro-SPECT.

METHODS

An animal experiment was carried out to evaluate the accuracy of ¹³¹I activity quantification in mouse kidneys (mean tissue volume of 0.140 mL) using a micro-SPECT system against conventional ex vivo gamma counting (GC) in a NaI(Tl) detector. The imaging setting investigated was that of the mouse biodistribution of a ¹³¹I-labelled single-domain antibody fragment (sdAb), currently being investigated for targeted radionuclide therapy of HER2-expressing cancer. SPECT imaging of ¹³¹I 365-keV photons was done with a VECTor/CT system (MILabs, Netherlands) using a high-energy mouse collimator with 1.6-mm-diameter pinholes. For both activity quantification techniques, the pharmacokinetic profile of the radioligand from approximately 1-73 h p.i. was derived and the time-integrated activity coefficient per gram of tissue (ã/M) was estimated. Additionally, SPECT activity recovery coefficients were determined in a phantom setting.

RESULTS

SPECT activities underestimate the reference activities by an amount that is dependent on the ¹³¹I activity concentration in the kidney, and thus on the time point of the pharmacokinetic profile. This underestimation is around - 12% at 1.5 h (2.89 MBq mL^-1 mean reference activity concentration), - 13% at 6.6 h (149 kBq mL^-1), - 40% at 24 h (17.6 kBq mL^-1) and - 46% at 73 h (5.2 kBq mL^-1) p.i. The ã/M value estimated from SPECT activities is, nevertheless, within - 14% from the reference (GC) ã/M value. Furthermore, better quantitative accuracy (within 2% from GC) in the SPECT ã/M value is achieved when SPECT activities are compensated for partial recovery with a phantom-based recovery coefficient of 0.85.

CONCLUSION

The SPECT imaging system used, together with a robust activity quantification methodology, allows an accurate estimation of time-integrated pharmacokinetic information of the ¹³¹I-labelled sdAb in mouse kidneys. This opens the possibility to perform mouse-specific kidney-tissue dosimetry based on pharmacokinetic data acquired in vivo on the same mice used in nephrotoxicity studies.

Evaluation of an 131I-labeled HER2-specific single domain antibody fragment for the radiopharmaceutical therapy of HER2-expressing cancers

Radiopharmaceutical therapy (RPT) is an attractive strategy for treatment of disseminated cancers including those overexpressing the HER2 receptor including breast, ovarian and gastroesophageal carcinomas. Single-domain antibody fragments (sdAbs) exemplified by the HER2-targeted VHH_1028 evaluated herein are attractive for RPT because they rapidly accumulate in tumor and clear faster from normal tissues than intact antibodies. In this study, VHH_1028 was labeled using the residualizing prosthetic agent N-succinimidyl 3-guanidinomethyl 5-[¹³¹I]iodobenzoate (iso-[¹³¹I]SGMIB) and its tissue distribution evaluated in the HER2-expressing SKOV-3 ovarian and BT474 breast carcinoma xenograft models. In head-to-head comparisons to [¹³¹I]SGMIB-2Rs15d, a HER2-targeted radiopharmaceutical currently under clinical investigation, iso-[¹³¹I]SGMIB-VHH_1028 exhibited significantly higher tumor uptake and significantly lower kidney accumulation. The results demonstrated 2.9 and 6.3 times more favorable tumor-to-kidney radiation dose ratios in the SKOV-3 and BT474 xenograft models, respectively. Iso-[¹³¹I]SGMIB-VHH_1028 was prepared using a solid-phase extraction method for purification of the prosthetic agent intermediate Boc₂-iso-[¹³¹I]SGMIB that reproducibly scaled to therapeutic-level doses and obviated the need for its HPLC purification. Single-dose (SKOV-3) and multiple-dose (BT474) treatment regimens demonstrated that iso-[¹³¹I]SGMIB-VHH_1028 was well tolerated and provided significant tumor growth delay and survival prolongation. This study suggests that iso-[¹³¹I]SGMIB-VHH_1028 is a promising candidate for RPT of HER2-expressing cancers and further development is warranted.