Rapid optical imaging of human breast tumour xenografts using anti-HER2 VHHs site-directly conjugated to IRDye 800CW for image-guided surgery

Development of a novel anti-CEACAM5 VHH for SPECT imaging and potential cancer therapy applications

PURPOSE

In this study, we investigated the utility of a novel developed anti-CEACAM5 VHH for cancer diagnosis and its potential of being a targeting-moiety of VHH-drug conjugates for cancer therapy.

METHODS

Anti-CEACAM5 VHH (6B11) affinity and specific cellular binding was confirmed by ELISA, FACS and immunofluorescence in cancer cell lines with varying CEACAM5 expression levels. Intracellular penetration ability within tumor spheroids was tested with Oregon Green 488 labeled 6B11 (OG488-6B11). Biodistribution and binding specificity of 99mTc-radiolabeled 6B11 was tested in A549 CEACAM5 overexpressing (A549-CEA5-OV) and knockout (A549-CEA5-KO) tumor-bearing mice upon SPECT/CT imaging, γ-counting and autoradiography. The therapeutic efficacy of 6B11 and 6F8 (anti-CEACAM5 VHH with lower binding affinity) was tested by viability, wound healing and adhesion assays. To verify the potential of VHHs as a warhead for VHH-drug conjugation, an internalization assay with OG488 labeled VHH was performed.

RESULT

6B11 demonstrated high binding affinity (EC50 0.5nM) and cellular binding. OG488-6B11 penetrated tumor spheroids completely at 24 h, while a conventional antibody was only visible at the spheroid periphery. SPECT imaging indicated higher uptake (p < 0.05) in A549-CEA5-OV tumors, resulting in increased tumor-to-blood ratios especially at 4 (2.0016 ± 1.1893, p = 0.035) and 24 (2.9371 ± 2.0683, p = 0.003) hpi compared to A549-CEA5-KO tumors at 4 (0.5640 ± 0.3576) and 24 (0.8051 ± 0.4351) hpi. 99mTc-6B11 was predominantly renally cleared. Autoradiography and immunohistochemistry confirmed these uptake patterns. 6B11 nor 6F8 did exhibit significant anti-cancer therapeutic efficacy in vitro. OG488-6B11 was effectively internalized and accumulated in cells in a time-dependent manner, to end up in the lysosomes.

CONCLUSION

The anti-CEACAM5 VHH 6B11 is a good candidate for SPECT-based cancer diagnosis and can be potentially used as targeting moiety in the development of VHH-based drug conjugates for cancer treatments.

Research trends in the use of nanobodies for cancer therapy

Although there are many challenges in using nanobodies for treating various complex tumor diseases, including rapid renal clearance and the complex blood-brain barrier environment, nanobodies have shown great potential due to their high antigen affinity, excellent tumor penetration ability, and favorable safety profile. Since the discovery of the variable domain (VHH) of camelid heavy-chain antibodies in 1993, nanobodies have been progressively applied to various cancer therapy platforms, such as antagonistic drugs and targeting agents for effector domains. In recent years, several nanobody-based drugs, including Caplacizumab, KN-035, and Ozoralizumab, have been approved for clinical use. Among them, KN-035 is used for treating advanced solid tumors, and these advancements have propelled nanobody development to new heights. Currently, nanobodies are being rapidly applied to the treatment of a wide range of diseases, from viral infections to cancer, demonstrating strong advantages in areas such as targeted protein degradation, bioimaging, nanobody-drug conjugation, bispecific T-cell engagers, and vaccine applications. Bibliometric tools, including CiteSpace, HisCite Pro, and Alluvial Generator, were employed to trace the historical development of nanobodies in cancer research. The contributions of authors, countries, and institutions in this field were analyzed, and research hotspots and emerging trends were identified through keyword analysis and influential articles. Future trends were also predicted. This study provides a unique, comprehensive, and objective perspective on the use of nanobodies in tumor research, laying a foundation for future research directions and offering valuable insights for researchers in the field.

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.

Trop2-Targeted Molecular Imaging in Solid Tumors: Current Advances and Future Outlook

Trophoblast cell surface antigen 2 (Trop2), a transmembrane glycoprotein, plays a dual role in physiological and pathological processes. In healthy tissues, Trop2 facilitates development and orchestrates intracellular calcium signaling. However, its overexpression in numerous solid tumors shifts its function toward driving cell proliferation and metastasis, thus leading to a poor prognosis. The clinical relevance of Trop2 is underscored by its utility as both a biomarker for diagnostic imaging and a target for therapy. Notably, the U.S. Food and Drug Administration (FDA) has approved sacituzumab govitecan (SG), a novel Trop2-targeted agent, for treating triple-negative breast cancer (TNBC) and refractory urothelial cancer, highlighting the significance of Trop2 in clinical oncology. Molecular imaging, a powerful tool for visualizing and quantifying biological phenomena at the molecular and cellular levels, has emerged as a critical technique for studying Trop2. This approach encompasses various modalities, including optical imaging, positron emission tomography (PET), single photon emission computed tomography (SPECT), and targeted antibodies labeled with radioactive isotopes. Incorporating Trop2-targeted molecular imaging into clinical practice is vital for the early detection, prognostic assessment, and treatment planning of a broad spectrum of solid tumors. Our review captures the latest progress in Trop2-targeted molecular imaging, focusing on both diagnostic and therapeutic applications across diverse tumor types, including lung, breast, gastric, pancreatic, prostate, and cervical cancers, as well as salivary gland carcinomas. We critically evaluate the current state by examining the relevant applications, diagnostic accuracy, therapeutic efficacy, and inherent limitations. Finally, we analyze the challenges impeding widespread clinical application and offer insights into strategies for advancing the field, thereby guiding future research endeavors.

Single domain antibody: Development and application in biotechnology and biopharma

Heavy-chain antibodies (HCAbs) are a unique type of antibodies devoid of light chains, and comprised of two heavy chains-only that recognize their cognate antigen by virtue of a single variable domain also referred to as VHH, single domain antibody (sdAb), or nanobody (Nb). These functional HCAbs, serendipitous discovered about three decades ago, are exclusively found in camelids, comprising dromedaries, camels, llamas, and vicugnas. Nanobodies have become an essential tool in biomedical research and medicine, both in diagnostics and therapeutics due to their beneficial properties: small size, high stability, strong antigen-binding affinity, low immunogenicity, low production cost, and straightforward engineering into more potent affinity reagents. The occurrence of HCAbs in camelids remains intriguing. It is believed to be an evolutionary adaptation, equipping camelids with a robust adaptive immune defense suitable to respond to the pressure from a pathogenic invasion necessitating a more profound antigen recognition and neutralization. This evolutionary innovation led to a simplified HCAb structure, possibly supported by genetic mutations and drift, allowing adaptive mutation and diversification in the heavy chain variable gene and constant gene regions. Beyond understanding their origins, the application of nanobodies has significantly advanced over the past 30 years. Alongside expanding laboratory research, there has been a rapid increase in patent application for nanobodies. The introduction of commercial nanobody drugs such as Cablivi, Nanozora, Envafolimab, and Carvykti has boosted confidence among in their potential. This review explores the evolutionary history of HCAbs, their ontogeny, and applications in biotechnology and pharmaceuticals, focusing on approved and ongoing medical research pipelines.

Epidermal Growth Factor Receptor (EGFR) Inhibitors Screened from Autodisplayed Fv-Antibody Library

Inhibitors of the epithermal growth factor receptor (EGFR) were screened from an autodisplayed Fv-antibody library using an anti-EGF antibody. The Fv-antibody library was expressed on the outer membrane of Escherichia coli, which corresponds to the heavy chain VH region of immunoglobulin G. The library was constructed by randomizing the CDR3 region of expressed VH regions (11 amino acid residues) by site-directed mutagenesis. Using an anti-EGF antibody as a screening probe, amino acid sequences (CDR3 region) with antibody binding affinity were screened from the Fv-antibody library. These amino acid sequences were considered to have similar chemical properties to EGF, which can bind to EGFR. Two autodisplayed clones with Fv-antibodies against EGFR were screened from the Fv-antibody library, and the screened Fv-antibodies were expressed as soluble proteins. The binding affinity (KD) was estimated using an SPR biosensor, and the inhibitory activity of expressed Fv-antibodies was observed for PANC-1 pancreatic tumor cells and T98G glioblastoma cells using Western blot analysis of proteins in the EGFR-mediated signaling pathway. The viability of PANC-1 and T98G cells was observed to decrease via the inhibitory activity of expressed Fv-antibodies. Finally, interactions between Fv-antibodies and EGFR were analyzed by using molecular docking simulations.

Structural insights into the role and targeting of EGFRvIII

The epidermal growth factor receptor (EGFR) is a well-known oncogenic driver in lung and other cancers. In glioblastoma multiforme (GBM), the EGFR deletion variant III (EGFRvIII) is frequently found alongside EGFR amplification. Agents targeting the EGFR axis have shown limited clinical benefits in GBM and the role of EGFRvIII in GBM is poorly understood. To shed light on the role of EGFRvIII and its potential as a therapeutic target, we determined X-ray crystal structures of a monomeric EGFRvIII extracellular region (ECR). The EGFRvIII ECR resembles the unliganded conformation of EGFR, including the orientation of the C-terminal region of domain II. Domain II is mostly disordered, but the ECR structure is compact. We selected a nanobody with preferential binding to EGFRvIII relative to EGFR and structurally defined an epitope on domain IV that is occluded in the unliganded intact EGFR. These findings suggest new avenues for EGFRvIII targeting in GBM.

Self-reporting photodynamic nanobody conjugate for precise and sustainable large-volume tumor treatment

Nanobodies (Nbs), the smallest antigen-binding fragments with high stability and affinity derived from the variable domain of naturally occurring heavy-chain-only antibodies in camelids, have been shown as an efficient way to improve the specificity to tumors for photodynamic therapy (PDT). Nonetheless, the rapid clearance of Nbs in vivo restricts the accumulation and retention of the photosensitizer at the tumor site causing insufficient therapeutic outcome, especially in large-volume tumors. Herein, we develop photodynamic conjugates, MNB-Pyra Nbs, through site-specific conjugation between 7D12 Nbs and type I photosensitizer MNB-Pyra (morpholine-modified nile blue structure connected to pyrazolinone) in a 1:2 ratio. The photosensitizers with long-term retention can be released at the tumor site by reactive oxygen species cleavage after illumination, accompanied with fluorescence recovery for self-reporting the occurrence of PDT. Ultimately, a single dose of MNB-Pyra Nbs demonstrate highly effective tumor suppression with high biosafety in the large-volume tumor models after three rounds of PDT. This nanobody conjugate provides a paradigm for the design of precise long-time retention photosensitizers and is expected to promote the development of PDT.

Molecular and functional insight into anti-EGFR nanobody: Theranostic implications for malignancies

Targeted therapy and imaging are the most popular techniques for the intervention and diagnosis of cancer. A potential therapeutic target for the treatment of cancer is the epidermal growth factor receptor (EGFR), primarily for glioblastoma, lung, and breast cancer. Over-production of ligand, transcriptional up-regulation due to autocrine/paracrine signalling, or point mutations at the genomic locus may contribute to the malfunction of EGFR in malignancies. This exploit makes use of EGFR, an established biomarker for cancer diagnostics and treatment. Despite considerable development in the last several decades in making EGFR inhibitors, they are still not free from limitations like toxicity and a short serum half-life. Nanobodies and antibodies share similar binding properties, but nanobodies have the additional advantage that they can bind to antigenic epitopes deep inside the target that conventional antibodies are unable to access. For targeted therapy, anti-EGFR nanobodies can be conjugated to various molecules such as drugs, peptides, toxins and photosensitizers. These nanobodies can be designed as novel immunoconjugates using the universal modular antibody-based platform technology (UniCAR). Furthermore, Anti-EGFR nanobodies can be expressed in neural stem cells and visualised by effective fluorescent and radioisotope labelling.

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.

Fluorescent Probes for Imaging in Humans: Where Are We Now?

Optical imaging has become an indispensable technology in the clinic. The molecular design of cell-targeted and highly sensitive materials, the validation of specific disease biomarkers, and the rapid growth of clinically compatible instrumentation have altogether revolutionized the way we use optical imaging in clinical settings. One prime example is the application of cancer-targeted molecular imaging agents in both trials and routine clinical use to define the margins of tumors and to detect lesions that are "invisible" to the surgeons, leading to improved resection of malignant tissues without compromising viable structures. In this Perspective, we summarize some of the key research advances in chemistry, biology, and engineering that have accelerated the translation of optical imaging technologies for use in human patients. Finally, our paper comments on several research areas where further work will likely render the next generation of technologies for translational optical imaging.

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.

Identification of a CEACAM5 targeted nanobody for positron emission tomography imaging and near-infrared fluorescence imaging of colorectal cancer

PURPOSE

Here, we aim to identify a CEACAM5-targeted nanobody and demonstrate its application in positron emission tomography (PET) imaging and near-infrared (NIR) fluorescence imaging in colorectal cancer (CRC).

METHODS

Immunohistochemistry was applied to verify CEACAM5 expression in CRC and metastatic lymph nodes (mLNs). CEACAM5-targeted nanobodies were obtained by immunization of human CEACAM5 protein in a dromedary, followed by several rounds of phage screenings. Immunofluorescence staining and flow cytometry was carried out to determine the binding affinity of the nanobodies. The nanobodies were radiolabeled by coupling 18F-SFB for PET imaging of CRC subcutaneous xenografts and lymph node metastasis (LNM). IRDye800CW (IR800) were conjugated to form NIR probes for NIR imaging in CRC subcutaneous models.

RESULTS

CEACAM5 was overexpressed in either human CRC tissues or mLNs. A CEACAM5 targeted nanobody, Nb41 was successfully generated, with excellent in vitro binding properties. Incorporation of albumin binding domain (ABD) did not affect the affinity of Nb41. In vivo imaging showed that both 18F-FB-Nb41 and 18F-FB-Nb41-ABD showed obvious accumulation in the tumor. Due to the longer retention in the blood, 18F-FB-Nb41-ABD enrichment in tumors was significantly delayed but higher compared to 18F-FB-Nb41. Both 18F-FB-Nb41 and 18F-FB-Nb41-ABD showed prominent LNM enrichment. Similarly, the IR800-conjugated nanobodies Nb41-IR800 and Nb41-ABD-IR800 exhibited superior imaging effects in subcutaneous models, while Nb41-ABD-IR800 exhibited higher fluorescence intensity in the tumor accompanied with a remarkedly delay compared to Nb41-IR800.

CONCLUSION

Collectively, we presented the identification and in vivo validation of a CEACAM5-targeted nanobody and a fused nanobody with an ABD, which enabled to the non-invasive visualization of malignancy of CRC using PET imaging and NIR imaging in subcutaneous models as well as LNM models.

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.

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.

CDH17 nanobodies facilitate rapid imaging of gastric cancer and efficient delivery of immunotoxin

BACKGROUND

It is highly desirable to develop new therapeutic strategies for gastric cancer given the low survival rate despite improvement in the past decades. Cadherin 17 (CDH17) is a membrane protein highly expressed in cancers of digestive system. Nanobody represents a novel antibody format for cancer targeted imaging and drug delivery. Nanobody targeting CHD17 as an imaging probe and a delivery vehicle of toxin remains to be explored for its theragnostic potential in gastric cancer.

METHODS

Naïve nanobody phage library was screened against CDH17 Domain 1-3 and identified nanobodies were extensively characterized with various assays. Nanobodies labeled with imaging probe were tested in vitro and in vivo for gastric cancer detection. A CDH17 Nanobody fused with toxin PE38 was evaluated for gastric cancer inhibition in vitro and in vivo.

RESULTS

Two nanobodies (A1 and E8) against human CDH17 with high affinity and high specificity were successfully obtained. These nanobodies could specifically bind to CDH17 protein and CDH17-positive gastric cancer cells. E8 nanobody as a lead was extensively determined for tumor imaging and drug delivery. It could efficiently co-localize with CDH17-positive gastric cancer cells in zebrafish embryos and rapidly visualize the tumor mass in mice within 3 h when conjugated with imaging dyes. E8 nanobody fused with toxin PE38 showed excellent anti-tumor effect and remarkably improved the mice survival in cell-derived (CDX) and patient-derived xenograft (PDX) models. The immunotoxin also enhanced the anti-tumor effect of clinical drug 5-Fluorouracil.

CONCLUSIONS

The study presents a novel imaging and drug delivery strategy by targeting CDH17. CDH17 nanobody-based immunotoxin is potentially a promising therapeutic modality for clinical translation against gastric cancer.

Modular Site-Specific Conjugation of Nanobodies Using Two Co-Associating Tags

The homogeneous labeling of antibodies and their fragments is a critical step for the generation of robust probes used in immuno-detection applications. To date, numerous chemical, genetic and peptide-based site-specific coupling methods have been developed. Among these methods, co-assembling peptide-tags is one of the most straightforward and versatile solutions. Here, we describe site-specific labeling of nanobodies through the use of two co-associating peptides tags, E3 and K3, originating from the tetramerization domain of p53. These E3 and K3-tags provide a simple and robust method for associating stoichiometric amount of V_HH and fluorescent probes, either fluorescent proteins or fluorochromes, at specific positions. As a proof of concept, a nanobody targeting the human epidermal growth factor receptor 2 (HER2), the nano-HER2 was genetically fused to the E3 and associated with different fluorescent K3-derivates. Entities were produced separately in Escherichia coli in soluble forms at high yields and co-assembled in vitro. These molecular probes present high binding specificity on HER2-overexpressing cells in flow-cytometry with relative binding constants in the low nanomolar range and are stable enough to stain HER2-receptor on living cells followed detection using fluorescent confocal microscopy. Altogether, our results demonstrate that the non-covalent conjugation method using these two co-associating peptides can be easily implemented for the modular engineering of molecular probes for cell immuno-staining.

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.

The current status and future prospects for molecular imaging-guided precision surgery

Molecular imaging technologies are increasingly used to diagnose, monitor, and guide treatment of i.e., cancer. In this review, the current status and future prospects of the use of molecular imaging as an instrument to help realize precision surgery is addressed with focus on the main components that form the conceptual basis of intraoperative molecular imaging. Paramount for successful interventions is the relevance and accessibility of surgical targets. In addition, selection of the correct combination of imaging agents and modalities is critical to visualize both microscopic and bulk disease sites with high affinity and specificity. In this context developments within engineering/imaging physics continue to drive the growth of image-guided surgery. Particularly important herein is enhancement of sensitivity through improved contrast and spatial resolution, features that are critical if sites of cancer involvement are not to be overlooked during surgery. By facilitating the connection between surgical planning and surgical execution, digital surgery technologies such as computer-aided visualization nicely complement these technologies. The complexity of image guidance, combined with the plurality of technologies that are becoming available, also drives the need for evaluation mechanisms that can objectively score the impact that technologies exert on the performance of healthcare professionals and outcome improvement for patients.

The Potential of Antibody Technology and Silver Nanoparticles for Enhancing Photodynamic Therapy for Melanoma

Melanoma is highly aggressive and is known to be efficient at resisting drug-induced apoptotic signals. Resection is currently the gold standard for melanoma management, but it only offers local control of the early stage of the disease. Metastatic melanoma is prone to recurrence, and has a poor prognosis and treatment response. Thus, the need for advanced theranostic alternatives is evident. Photodynamic therapy has been increasingly studied for melanoma treatment; however, it relies on passive drug accumulation, leading to off-target effects. Nanoparticles enhance drug biodistribution, uptake and intra-tumoural concentration and can be functionalised with monoclonal antibodies that offer selective biorecognition. Antibody-drug conjugates reduce passive drug accumulation and off-target effects. Nonetheless, one limitation of monoclonal antibodies and antibody-drug conjugates is their lack of versatility, given cancer's heterogeneity. Monoclonal antibodies suffer several additional limitations that make recombinant antibody fragments more desirable. SNAP-tag is a modified version of the human DNA-repair enzyme, O6-alkylguanine-DNA alkyltransferase. It reacts in an autocatalytic and covalent manner with benzylguanine-modified substrates, providing a simple protein labelling system. SNAP-tag can be genetically fused with antibody fragments, creating fusion proteins that can be easily labelled with benzylguanine-modified payloads for site-directed delivery. This review aims to highlight the benefits and limitations of the abovementioned approaches and to outline how their combination could enhance photodynamic therapy for melanoma.

Generation of a High-Affinity Nanobody Against CD147 for Tumor Targeting and Therapeutic Efficacy Through Conjugating Doxorubicin

CD147, a glycosylated transmembrane protein in the immunoglobulin superfamily, is overexpressed on the surfaces of various tumor cells and promotes cancer cell proliferation, invasion, and metastasis. Nanobodies, characterized by small sizes, high affinities and specificities, and low immunogenicities, are promising diagnostic and therapeutic tools. However, there are few reports on nanobodies that specifically target CD147. In this work, a specific anti-CD147 nanobody has been successfully identified using phage display technology. The tumor target and antitumor effects have also been detected in different CD147-positive tumors in in vitro and in vivo assays, respectively. Meanwhile, it has a synergistic effect for inhibiting 4T1-bearing mice through conjugating doxorubicin. It may afford new strategies for cancer therapies.

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.

Conjugation of IRDye Photosensitizers or Fluorophores to Nanobodies

Fluorophores have been conjugated to nanobodies for approximately a decade, for several applications in molecular biology. More recently, photosensitizers have been conjugated to nanobodies for targeted photodynamic therapy (PDT). The most common chemistry is the random conjugation in which commercial fluorophores or photosensitizers contain a N-hydroxysuccinimide ester (NHS ester) group that reacts specifically and efficiently with lysines in the amino acid sequence of the nanobody and with the N-terminal amino groups to form a stable amide bond. Alternatively, maleimide-containing fluorophores or photosensitizers can be used for conjugation to thiols, in a site-directed manner through a cysteine incorporated at the C-terminal of the nanobody. This chapter addresses both conjugation strategies, providing details on the reaction conditions, purification, and characterization of the conjugates obtained.

Site-Specific and Residualizing Linker for 18F Labeling with Enhanced Renal Clearance: Application to an Anti-HER2 Single-Domain Antibody Fragment

Single-domain antibody fragments (sdAbs) are promising vectors for immuno-PET; however, better methods for labeling sdAbs with 18F are needed. Herein, we evaluate a site-specific strategy using an 18F residualizing motif and the anti-epidermal growth factor receptor 2 (HER2) sdAb 5F7 bearing an engineered C-terminal GGC tail (5F7GGC). Methods: 5F7GGC was site-specifically attached with a tetrazine-bearing agent via thiol-maleimide reaction. The resultant conjugate was labeled with 18F by inverse electron demand Diels-Alder cycloaddition with a trans-cyclooctene attached to 6-18F-fluoronicotinoyl moiety via a renal brush border enzyme-cleavable linker and a PEG4 chain (18F-5F7GGC). For comparisons, 5F7 sdAb was labeled using the prototypical residualizing agent, N-succinimidyl 3-(guanidinomethyl)-5-125I-iodobenzoate (iso-125I-SGMIB). The 2 labeled sdAbs were compared in paired-label studies performed in the HER2-expressing BT474M1 breast carcinoma cell line and athymic mice bearing BT474M1 subcutaneous xenografts. Small-animal PET/CT imaging after administration of 18F-5F7GGC in the above mouse model was also performed. Results:18F-5F7GGC was synthesized in an overall radiochemical yield of 8.9% ± 3.2% with retention of HER2 binding affinity and immunoreactivity. The total cell-associated and intracellular activity for 18F-5F7GGC was similar to that for coincubated iso-125I-SGMIB-5F7. Likewise, the uptake of 18F-5F7GGC in BT474M1 xenografts in mice was similar to that for iso-125I-SGMIB-5F7; however, 18F-5F7GGC exhibited significantly more rapid clearance from the kidney. Small-animal PET/CT imaging confirmed high uptake and retention in the tumor with very little background activity at 3 h except in the bladder. Conclusion: This site-specific and residualizing 18F-labeling strategy could facilitate clinical translation of 5F7 anti-HER2 sdAb as well as other sdAbs for immuno-PET.

Nanobodies as versatile tools: A focus on targeted tumor therapy, tumor imaging and diagnostics

Monoclonal antibodies and vaccines have widely been studied for the immunotherapy of cancer, though their large size appears to limit their functionality in solid tumors, in large part due to unique properties of tumor microenvironment. Smaller formats of antibodies have been developed to throw such restrictions. These small format antibodies include antigen binding fragments, single-chain variable fragments, single variable domain of camelid antibody (so-called nanobody (Nb) or VHH). Since their serendipitous discovery, nanobodies have been studies at length in the fields of research, diagnostics and therapy. These antigen binding fragments, originating from camelid heavy-chain antibodies, possess unusual hallmarks in terms of (small) size, stability, solubility and specificity, hence allowing cost-effective production and sometimes out performing monoclonal antibodies. In addition, these small camelid heavy-chain antibodies are highly adaptable tools for cancer research as they enable specific modulation of targets, enzymatic and non-enzymatic proteins alike. Molecular imaging studies benefit from the rapid, homogeneous tumor accumulation of nanobodies and their fast blood clearance, permitting previously unattainable fast tumor visualization. Moreover, they are endowed with considerable therapeutic potential as inhibitors of receptor-ligand pairs and deliverers of drugs or drug-loaded nanoparticles towards tumors. In this review, we shed light on the current status of nanobodies in diagnosis and imaging of tumor and exploiting nanobodies revert immunosuppressive events, modulation of immune checkpoints, and as deliverers of drugs for targeted tumor therapy.

Nanobodies Enhancing Cancer Visualization, Diagnosis and Therapeutics

Worldwide, cancer is a serious health concern due to the increasing rates of incidence and mortality. Conventional cancer imaging, diagnosis and treatment practices continue to substantially contribute to the fight against cancer. However, these practices do have some risks, adverse effects and limitations, which can affect patient outcomes. Although antibodies have been developed, successfully used and proven beneficial in various oncology practices, the use of antibodies also comes with certain challenges and limitations (large in size, poor tumor penetration, high immunogenicity and a long half-life). Therefore, it is vital to develop new ways to visualize, diagnose and treat cancer. Nanobodies are novel antigen-binding fragments that possess many advantageous properties (small in size, low immunogenicity and a short half-life). Thus, the use of nanobodies in cancer practices may overcome the challenges experienced with using traditional antibodies. In this review, we discuss (1) the challenges with antibody usage and the superior qualities of nanobodies; (2) the use of antibodies and nanobodies in cancer imaging, diagnosis, drug delivery and therapy (surgery, radiotherapy, chemotherapy and immunotherapy); and (3) the potential improvements in oncology practices due to the use of nanobodies as compared to antibodies.

Radiomics, aptamers and nanobodies: New insights in cancer diagnostics and imaging

At present, cancer is a major health issue and the second leading cause of mortality worldwide. Researchers have been working hard on investigating not only improved therapeutics but also on early detection methods, both critical to increasing treatment efficacy and developing methods for disease prevention. Diagnosis of cancers at an early stage can promote timely medical intervention and effective treatment and will result in inhibiting tumor growth and development. Several advances have been made in the diagnostics and imagining technologies for early tumor detection and deciding an effective therapy these include radiomics, nanobodies, and aptamers. Here in this review, we summarize the main applications of radiomics, aptamers, and the use of nanobody-based probes for molecular imaging applications in diagnosis, treatment planning, and evaluations in the field of oncology to develop quantitative and personalized medicine. The preclinical data reported to date are quite promising, and it is predicted that nanobody-based molecular imaging agents will play an important role in the diagnosis and management of different cancer types in near future.

Biochemistry, structure, and cellular internalization of a four nanobody-bearing Fc dimer

VHH stands for the variable regions of heavy chain only of camelid IgGs. The VHH family forms a set of interesting proteins derived from antibodies that maintain their capacity to recognize the antigen, despite their relatively small molecular weight (in the 12,000 Da range). Continuing our exploration of the possibilities of those molecules, we chose to design alternative molecules with maintained antigen recognition, but enhanced capacity, by fusing four VHH with one Fc, the fragment crystallizable region of antibodies. In doing so, we aimed at having a molecule with superior quantitative antigen recognition (×4) while maintaining its size below the 110 kDa. In the present paper, we described the building of those molecules that we coined VHH₂ -Fc-VHH₂ . The structure of VHH₂ -Fc-VHH₂ in complex with HER2 antigen was determined using electronic microscopy and modeling. The molecule is shown to bind four HER2 proteins at the end of its flexible arms. VHH₂ -Fc-VHH₂ also shows an internalization capacity via HER2 receptor superior to the reference anti-HER2 monoclonal antibody, Herceptin®, and to a simple fusion of two VHH with one Fc (VHH₂ -Fc). This new type of molecules, VHH₂ -Fc-VHH₂ , could be an interesting addition to the therapeutic arsenal with multiple applications, from diagnostic to therapy.

Nanobody Conjugates for Targeted Cancer Therapy and Imaging

Conventional antibody-based targeted cancer therapy is one of the most promising avenues of successful cancer treatment, with the potential to reduce toxic side effects to healthy cells surrounding tumor cells. However, the full potential of antibodies is severely limited due to their large size, low stability, slow clearance, and high immunogenicity. Alternatively, recently discovered nanobodies, which are the smallest naturally occurring antigen-binding format, have shown great potential for addressing these limitations. Bioconjugation of nanobodies to functional groups such as toxins, enzymes, radionucleotides, and fluorophores can improve the efficacy and potency of nanobodies, enhance their in vivo pharmacokinetics, and expand the range of potential applications. Herein, we review the superior characteristics of nanobodies in comparison to conventional antibodies and provide insight into recent developments in nanobody conjugates for targeted cancer therapy and imaging.

Homogeneous tumor targeting with a single dose of HER2-targeted albumin-binding domain-fused nanobody-drug conjugates results in long-lasting tumor remission in mice

Background: The non-homogenous distribution of antibody-drug conjugates (ADCs) within solid tumors is a major limiting factor for their wide clinical application. Nanobodies have been shown to rapidly penetrate into xenografts, achieving more homogeneous tumor targeting. However, their rapid renal clearance can hamper their application as nanobody drug conjugates (NDCs). Here, we evaluate whether half-life extension via non-covalent interaction with albumin can benefit the efficacy of a HER2-targeted NDC.

Methods: HER2-targeted nanobody 11A4 and the irrelevant nanobody R2 were genetically fused to an albumin-binding domain (ABD) at their C-terminus. Binding to both albumin and tumor cells was determined by ELISA-based assays. The internalization potential as well as the in vitro efficacy of NDCs were tested on HER2 expressing cells. Serum half-life of iodinated R2 and R2-ABD was studied in tumor-free mice. The distribution of fluorescently labelled 11A4 and 11A4-ABD was assessed in vitro in 3D spheroids. Subsequently, the in vivo distribution was evaluated by optical molecular imaging and ex vivo by tissue biodistribution and tumor immunohistochemical analysis after intravenous injection of IRDye800-conjugated nanobodies in mice bearing HER2-positive subcutaneous xenografts. Finally, efficacy studies were performed in HER2-positive NCI-N87 xenograft-bearing mice intravenously injected with a single dose (250 nmol/kg) of nanobodies conjugated to auristatin F (AF) either via a maleimide or the organic Pt(II)‑based linker, coined Lx^®.

Results: 11A4-ABD was able to bind albumin and HER2 and was internalized by HER2 expressing cells, irrespective of albumin presence. Interaction with albumin did not alter its distribution through 3D spheroids. Fusion to ABD resulted in a 14.8-fold increase in the serum half-life, as illustrated with the irrelevant nanobody. Furthermore, ABD fusion prolonged the accumulation of 11A4-ABD in HER2-expressing xenografts without affecting the expected homogenous intratumoral distribution. Next to that, reduced kidney retention of ABD-fused nanobodies was observed. Finally, a single dose administration of either 11A4-ABD-maleimide-AF or 11A4-ABD-Lx-AF led to long-lasting tumor remission in HER2-positive NCI-N87 xenograft-bearing mice.

Conclusion: Our results demonstrate that genetic fusion of a nanobody to ABD can significantly extend serum half-life, resulting in prolonged and homogenous tumor accumulation. Most importantly, as supported by the impressive anti-tumor efficacy observed after a single dose administration of 11A4-ABD-AF, our data reveal that monovalent internalizing ABD-fused nanobodies have potential for the development of highly effective NDCs.

Nanobodies as in vivo, non-invasive, imaging agents

In vivo imaging has become in recent years an incredible tool to study biological events and has found critical applications in diagnostic medicine. Although a lot of efforts and applications have been achieved using monoclonal antibodies, other types of delivery agents are being developed. Among them, VHHs, antigen binding fragments derived from camelid heavy chain-only antibodies, also known as nanobodies, have particularly attracted attention. Indeed, their stability, fast clearance, good tissue penetration, high solubility, simple cloning and recombinant production make them attractive targeting agents for imaging modalities such as PET, SPECT or Infra-Red. In this review, we discuss the pioneering work that has been carried out using VHHs and summarize the recent developments that have been made using nanobodies for in vivo, non-invasive, imaging.

Not so innocent: Impact of fluorophore chemistry on the in vivo properties of bioconjugates

The combination of targeting ligands and fluorescent dyes is a powerful strategy to observe cell types and tissues of interest. Conjugates of peptides, proteins, and, in particular, monoclonal antibodies (mAbs) exhibit excellent tumor targeting in various contexts. This approach has been translated to a clinical setting to provide real-time molecular insights during the surgical resection of solid tumors. A critical element of this approach is the generation of highly fluorescent bioconjugates that maintain the properties of the parent targeting ligand. A number of studies have found that fluorophores can dramatically impact the pharmacokinetic and tumor-targeting properties of the bioconjugates they are meant to only innocently observe. In this review, we summarize several examples of these effects and highlight strategies that have been used to mitigate them. These include the application of site-specific labeling chemistries, modulating label density, and altering the structure of the fluorescent probe itself. In particular, we point out the significant potential of fluorophores with hydrophilic but net-neutral structures. Overall, this review highlights recent progress in refining the in vivo properties of fluorescent bioconjugates, and we hope, will inform future efforts in this area.

Unique Benefits of Tumor-Specific Nanobodies for Fluorescence Guided Surgery

Tumor-specific fluorescence labeling is promising for real-time visualization of solid malignancies during surgery. There are a number of technologies to confer tumor-specific fluorescence. Antibodies have traditionally been used due to their versatility in modifications; however, their large size hampers efficient fluorophore delivery. Nanobodies are a novel class of molecules, derived from camelid heavy-chain only antibodies, that have shown promise for tumor-specific fluorescence labeling. Nanobodies are ten times smaller than standard antibodies, while maintaining antigen-binding capacity and have advantageous features, including rapidity of tumor labeling, that are reviewed in the present report. The present report reviews special considerations needed in developing nanobody probes, the status of current literature on the use of nanobody probes in fluorescence guided surgery, and potential challenges to be addressed for clinical translation.

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.

Comparison of HER2-Targeted Antibodies for Fluorescence-Guided Surgery in Breast Cancer

Background

Although therapeutic advances have led to enhanced survival in patients with human epidermal growth factor receptor 2 (HER2)-positive breast cancer, detection of residual disease remains challenging. Here, we examine two approved anti-HER2 monoclonal antibodies (mAbs), trastuzumab and pertuzumab, as potential candidates for the development of immunoconjugates for fluorescence-guided surgery (FGS).

Methods

mAbs were conjugated to the near-infrared fluorescent (NIRF) dye, IRDye800, and for quantitative in vitro assessment, to the radiometal chelator, desferrioxamine, to enable dual labeling with ⁸⁹Zr. In vitro binding was evaluated in HER2-overexpressing (BT474, SKBR3) and HER2-negative (MCF7) cell lines. BT474 and MCF7 xenografts were used for in vivo and ex vivo fluorescence imaging.

Results

In vitro findings demonstrated HER2-mediated binding for both fluorescent immunoconjugates and were in agreement with radioligand assays using dual-labeled immunoconjugates. In vivo and ex vivo studies showed preferential accumulation of the fluorescently-labeled mAbs in tumors and similar tumor-to-background ratios. In vivo HER2 specificity was confirmed by immunohistochemical staining of resected tumors and normal tissues.

Conclusions

We showed for the first time that fluorescent trastuzumab and pertuzumab immunoconjugates have similar NIRF imaging performance and demonstrated the possibility of performing HER2-targeted FGS with agents that possess distinct epitope specificity.

Site-Specific Dual-Labeling of a VHH with a Chelator and a Photosensitizer for Nuclear Imaging and Targeted Photodynamic Therapy of EGFR-Positive Tumors

Variable domains of heavy chain only antibodies (VHHs) are valuable agents for application in tumor theranostics upon conjugation to both a diagnostic probe and a therapeutic compound. Here, we optimized site-specific conjugation of the chelator DTPA and the photosensitizer IRDye700DX to anti-epidermal growth factor receptor (EGFR) VHH 7D12, for applications in nuclear imaging and photodynamic therapy. 7D12 was site-specifically equipped with bimodal probe DTPA-tetrazine-IRDye700DX using the dichlorotetrazine conjugation platform. Binding, internalization and light-induced toxicity of DTPA-IRDye700DX-7D12 were determined using EGFR-overexpressing A431 cells. Finally, ex vivo biodistribution of DTPA-IRDye700DX-7D12 in A431 tumor-bearing mice was performed, and tumor homing was visualized with SPECT and fluorescence imaging. DTPA-IRDye700DX-7D12 was retrieved with a protein recovery of 43%, and a degree of labeling of 0.56. Spectral properties of the IRDye700DX were retained upon conjugation. 111In-labeled DTPA-IRDye700DX-7D12 bound specifically to A431 cells, and they were effectively killed upon illumination. DTPA-IRDye700DX-7D12 homed to A431 xenografts in vivo, and this could be visualized with both SPECT and fluorescence imaging. In conclusion, the dichlorotetrazine platform offers a feasible method for site-specific dual-labeling of VHH 7D12, retaining binding affinity and therapeutic efficacy. The flexibility of the described approach makes it easy to vary the nature of the probes for other combinations of diagnostic and therapeutic compounds.

Nanobody-A versatile tool for cancer diagnosis and therapeutics

In spite of the successful use of monoclonal antibodies (mAbs) in clinic for tumor treatment, their applications are still hampered in therapeutic development due to limitations, such as tumor penetration and high cost of manufacture. Nanobody, a single domain antibody that holds the strong antigen targeting and binding capacity, has demonstrated various advantages relative to antibody. Nanobody is considered as a next-generation of antibody-derived tool in the antigen related recognition and modulation. A number of nanobodies have been developed and evaluated in different stages of clinical trials for cancer treatment. Here we summarized the current progress of nanobody in tumor diagnosis and therapeutics, particularly on the conjugation of nanobody with functional moieties. The nanobody conjugation of diagnostic agents, such as radionuclide and optical tracers, can achieve specific tumor imaging. The nanobody-drug conjugates can enhance the therapeutic efficacy of anti-tumor drugs and reduce the adverse effects. The decoration of nanobody on nanodrug delivery systems can further improve the drug targeting to specific tumors. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Nanobody: a promising toolkit for molecular imaging and disease therapy

Nanobodies are the recombinant variable domains of heavy-chain-only antibodies, with many unique properties such as small size, excellent solubility, superior stability, quick clearance from blood, and deep tissue penetration. As a result, nanobodies have become a promising tool for the diagnosis and therapy of diseases. As imaging tracers, nanobodies allow an early acquisition of high-quality images, provide a comprehensive evaluation of the disease, and subsequently enable a personalized precision therapy. As therapeutic agents, nanobodies enable a targeted therapy by lesion-specific delivery of drugs and effector domains, thereby improving the specificity and efficacy of the therapy. Up to date, a wide variety of nanobodies have been developed for a broad range of molecular targets and have played a significant role in patients with a broad spectrum of diseases. In this review, we aim to outline the current state-of-the-art research on the nanobodies for medical applications and then discuss the challenges and strategies for their further clinical translation.

Site-specific radioiodination of an anti-HER2 single domain antibody fragment with a residualizing prosthetic agent

INTRODUCTION

As a consequence of their small size, high stability and high affinity, single domain antibody fragments (sdAbs) are appealing targeting vectors for radiopharmaceutical development. With sdAbs binding to internalizing receptors like HER2, residualizing prosthetic agents can enhance tumor retention of radioiodine, which until now has been done with random labeling approaches. Herein we evaluate a site-specific strategy utilizing a radioiodinated, residualizing maleimido moiety and the anti-HER2 sdAb 5F7 bearing a GGC tail for conjugation.

METHODS

Maleimidoethyl 3-(guanidinomethyl)-5-iodobenzoate ([131I]MEGMB) and its N-succinimidyl ester analogue, iso-[125I]SGMIB, were labeled by halodestannylation and conjugated with 5F7GGC and 5F7, respectively. Radiochemical purity, immunoreactivity and binding affinity were determined. Paired-label experiments directly compared iso-[125I]SGMIB-5F7 and [131I]MEGMIB-5F7GGC with regard to internalization/residualization and affinity on HER2-expressing SKOV-3 ovarian carcinoma cells as well as biodistribution and metabolite distribution in athymic mice with subcutaneous SKOV-3 xenografts.

RESULTS

[131I]MEGMIB-5F7GGC had an immunoreactivity of 81.3% and Kd = 0.94 ± 0.27 nM. Internalization assays demonstrated high intracellular trapping for both conjugates, For example, at 1 h, intracellular retention was 50.30 ± 3.36% for [131I]MEGMIB-5F7GGC and 55.95 ± 3.27% for iso-[125I]SGMIB-5F7, while higher retention was seen for iso-[125I]SGMIB-5F7 at later time points. Peak tumor uptake was similar for both conjugates (8.35 ± 2.66%ID/g and 8.43 ± 2.84%ID/g for iso-[125I]SGMIB-5F7 and [131I]MEGMIB-5F7GGC at 1 h, respectively); however, more rapid normal tissue clearance was seen for [131I]MEGMIB-5F7GGC, with a 2-fold higher tumor-to-kidney ratio and a 3-fold higher tumor-to-liver ratio compared with co-injected iso-[125I]SGMIB-5F7. Consisted with this, generation of labeled catabolites in the kidneys was higher for [131I]MEGMIB-5F7GGC.

CONCLUSION

[131I]MEGMIB-5F7GGC offers similar tumor targeting as iso-[125I]SGMIB-5F7 but with generally lower normal tissue uptake.

ADVANCES IN KNOWLEDGE AND IMPLICATION FOR PATIENT CARE

The site specific nature of the [131I]MEGMIB reagent may facilitate clinical translation, particularly for sdAb with compromised affinity after random labeling.

Nanobodies as Versatile Tool for Multiscale Imaging Modalities

Molecular imaging is constantly growing in different areas of preclinical biomedical research. Several imaging methods have been developed and are continuously updated for both in vivo and in vitro applications, in order to increase the information about the structure, localization and function of molecules involved in physiology and disease. Along with these progresses, there is a continuous need for improving labeling strategies. In the last decades, the single domain antigen-binding fragments nanobodies (Nbs) emerged as important molecular imaging probes. Indeed, their small size (~15 kDa), high stability, affinity and modularity represent desirable features for imaging applications, providing higher tissue penetration, rapid targeting, increased spatial resolution and fast clearance. Accordingly, several Nb-based probes have been generated and applied to a variety of imaging modalities, ranging from in vivo and in vitro preclinical imaging to super-resolution microscopy. In this review, we will provide an overview of the state-of-the-art regarding the use of Nbs in several imaging modalities, underlining their extreme versatility and their enormous potential in targeting molecules and cells of interest in both preclinical and clinical studies.

Nanobodies in cancer

For treatment and diagnosis of cancer, antibodies have proven their value and now serve as a first line of therapy for certain cancers. A unique class of antibody fragments called nanobodies, derived from camelid heavy chain-only antibodies, are gaining increasing acceptance as diagnostic tools and are considered also as building blocks for chimeric antigen receptors as well as for targeted drug delivery. The small size of nanobodies (∼15 kDa), their stability, ease of manufacture and modification for diverse formats, short circulatory half-life, and high tissue penetration, coupled with excellent specificity and affinity, account for their attractiveness. Here we review applications of nanobodies in the sphere of tumor biology.

Nanobody-Based Probes for Subcellular Protein Identification and Visualization

Understanding how building blocks of life contribute to physiology is greatly aided by protein identification and cellular localization. The two main labeling approaches developed over the past decades are labeling with antibodies such as immunoglobulin G (IgGs) or use of genetically encoded tags such as fluorescent proteins. However, IgGs are large proteins (150 kDa), which limits penetration depth and uncertainty of target position caused by up to ∼25 nm distance of the label created by the chosen targeting approach. Additionally, IgGs cannot be easily recombinantly modulated and engineered as part of fusion proteins because they consist of multiple independent translated chains. In the last decade single domain antigen binding proteins are being explored in bioscience as a tool in revealing molecular identity and localization to overcome limitations by IgGs. These nanobodies have several potential benefits over routine applications. Because of their small size (15 kDa), nanobodies better penetrate during labeling procedures and improve resolution. Moreover, nanobodies cDNA can easily be fused with other cDNA. Multidomain proteins can thus be easily engineered consisting of domains for targeting (nanobodies) and visualization by fluorescence microscopy (fluorescent proteins) or electron microscopy (based on certain enzymes). Additional modules for e.g., purification are also easily added. These nanobody-based probes can be applied in cells for live-cell endogenous protein detection or may be purified prior to use on molecules, cells or tissues. Here, we present the current state of nanobody-based probes and their implementation in microscopy, including pitfalls and potential future opportunities.

Modular Conjugation of a Potent Anti-HER2 Immunotoxin Using Coassociating Peptides

Immunotoxins are emerging candidates for cancer therapeutics. These biomolecules consist of a cell-targeting protein combined to a polypeptide toxin. Associations of both entities can be achieved either chemically by covalent bonds or genetically creating fusion proteins. However, chemical agents can affect the activity and/or stability of the conjugate proteins, and additional purification steps are often required to isolate the final conjugate from unwanted byproducts. As for fusion proteins, they often suffer from low solubility and yield. In this report, we describe a straightforward conjugation process to generate an immunotoxin using coassociating peptides (named K3 and E3), originating from the tetramerization domain of p53. To that end, a nanobody targeting the human epidermal growth factor receptor 2 (nano-HER2) and a protein toxin fragment from Pseudomonas aeruginosa exotoxin A (TOX) were genetically fused to the E3 and K3 peptides. Entities were produced separately in Escherichia coli in soluble forms and at high yields. The nano-HER2 fused to the E3 or K3 helixes (nano-HER2-E3 and nano-HER2-K3) and the coassembled immunotoxins (nano-HER2-K3E3-TOX and nano-HER2-E3K3-TOX) presented binding specificity on HER2-overexpressing cells with relative binding constants in the low nanomolar to picomolar range. Both toxin modules (E3-TOX and K3-TOX) and the combined immunotoxins exhibited similar cytotoxicity levels compared to the toxin alone (TOX). Finally, nano-HER2-K3E3-TOX and nano-HER2-E3K3-TOX evaluated on various breast cancer cells were highly potent and specific to killing HER2-overexpressing breast cancer cells with IC50 values in the picomolar range. Altogether, we demonstrate that this noncovalent conjugation method using two coassembling peptides can be easily implemented for the modular engineering of immunotoxins targeting different types of cancers.

Dual Targeting of Endothelial and Cancer Cells Potentiates In Vitro Nanobody-Targeted Photodynamic Therapy

Photodynamic therapy (PDT) induces cell death through local light activation of a photosensitizer, although sub-optimal tumor specificity and side effects have hindered its clinical application. We introduced a new strategy named nanobody-targeted PDT in which photosensitizers are delivered to tumor cells by means of nanobodies. As efficacy of targeted PDT can be hampered by heterogeneity of target expression and/or moderate/low target expression levels, we explored the possibility of combined targeting of endothelial and cancer cells in vitro. We developed nanobodies binding to the mouse VEGFR2, which is overexpressed on tumor vasculature, and combined these with nanobodies specific for the cancer cell target EGFR. The nanobodies were conjugated to the photosensitizer IRDye700DX and specificity of the newly developed nanobodies was verified using several endothelial cell lines. The cytotoxicity of these conjugates was assessed in monocultures and in co-cultures with cancer cells, after illumination with an appropriate laser. The results show that the anti-VEGFR2 conjugates are specific and potent PDT agents. Nanobody-targeted PDT on co-culture of endothelial and cancer cells showed improved efficacy, when VEGFR2 and EGFR targeting nanobodies were applied simultaneously. Altogether, dual targeting of endothelial and cancer cells is a promising novel therapeutic strategy for more effective nanobody-targeted PDT.

A Facile and Reproducible Synthesis of Near-Infrared Fluorescent Conjugates with Small Targeting Molecules for Microbial Infection Imaging

Optical imaging of microbial infections, based on the detection of targeted fluorescent probes, offers high sensitivity and resolution with a relatively simple and portable setup. As the absorbance of near-infrared (NIR) light by human tissues is minimal, using respective tracers, such as IRdye800CW, enables imaging deeper target sites in the body. Herein, we present a general strategy for the conjugation of IRdye800CW and IRdye700DX to small molecules (vancomycin and amphotericin B) to provide conjugates targeted toward bacterial and fungal infections for optical imaging and photodynamic therapy. In particular, we present how the use of coupling agents (such as HBTU or HATU) leads to high yields (over 50%) in the reactions of amines and IRDye-NHS esters and how precipitation can be used as a convenient purification strategy to remove excess of the targeting molecule after the reaction. The high selectivity of the synthesized model compound Vanco-800CW has been proven in vitro, and the development of analogous agents opens up new possibilities for diagnostic and theranostic purposes. In times of increasing microbial resistance, this research gives us access to a platform of new fluorescent tracers for the imaging of infections, enabling early diagnosis and respective treatment.