Gallium-68-labeled affibody molecule for PET imaging of PDGFRβ expression in vivo

Comparison of approaches for increasing affinity of affibody molecules for imaging of B7-H3: dimerization and affinity maturation

BACKGROUND

Radionuclide molecular imaging can be used to visualize the expression levels of molecular targets. Affibody molecules, small and high affinity non-immunoglobulin scaffold-based proteins, have demonstrated promising properties as targeting vectors for radionuclide tumour imaging of different molecular targets. B7-H3 (CD276), an immune checkpoint protein belonging to the B7 family, is overexpressed in different types of human malignancies. Visualization of overexpression of B7-H3 in malignancies enables stratification of patients for personalized therapies. Affinity maturation of anti-B7-H3 Affibody molecules as an approach to improve the binding affinity and targeting properties was recently investigated. In this study, we tested the hypothesis that a dimeric format may be an alternative option to increase the apparent affinity of Affibody molecules to B7-H3 and accordingly improve imaging contrast.

RESULTS

Two dimeric variants of anti-B7-H3 Affibody molecules were produced (designated ZAC12*-ZAC12*-GGGC and ZAC12*-ZTaq_3-GGGC). Both variants were labelled with Tc-99m (99mTc) and demonstrated specific binding to B7-H3-expressing cells in vitro. [99mTc]Tc-ZAC12*-ZAC12*-GGGC showed subnanomolar affinity (KD1=0.28 ± 0.10 nM, weight = 68%), which was 7.6-fold higher than for [99mTc]Tc-ZAC12*-ZTaq_3-GGGC (KD=2.1 ± 0.9 nM). Head-to-head biodistribution of both dimeric variants of Affibody molecules compared with monomeric affinity matured SYNT-179 (all labelled with 99mTc) in mice bearing B7-H3-expressing SKOV-3 xenografts demonstrates that both dimers have lower tumour uptake and lower tumour-to-organ ratios compared to the SYNT-179 Affibody molecule.

CONCLUSION

The improved functional affinity by dimerization does not compensate the disadvantage of increased molecular size for imaging purposes.

Radiolabeled Affibody Molecules for PET Imaging

Owing to their ease of engineering and production, chemical stability, size, and high target affinity and specificity, radiolabeled affibody molecules have been recognized as very promising molecular imaging probes in both preclinical and clinical settings. Herein we describe the methods for the preparation of affibody-chelator conjugates and their subsequent radiolabeling with 18F-AlF, 68Ga, 89Zr.

Optimized, automated and cGMP-compliant synthesis of the HER2 targeting [68Ga]Ga-ABY-025 tracer

BACKGROUND

The Affibody molecule, ABY-025, has demonstrated utility to detect human epidermal growth factor receptor 2 (HER2) in vivo, either radiolabelled with indium-111 (111In) or gallium-68 (68Ga). Using the latter, 68Ga, is preferred due to its use in positron emission tomography with superior resolution and quantifying capabilities in the clinical setting compared to 111In. For an ongoing phase II study (NCT05619016) evaluating ABY-025 for detecting HER2-low lesions and selection of patients for HER2-targeted treatment, the aim was to optimize an automated and cGMP-compliant radiosynthesis of [68Ga]Ga-ABY-025. [68Ga]Ga-ABY-025 was produced on a synthesis module, Modular-Lab PharmTracer (Eckert & Ziegler), commonly used for 68Ga-labelings. The radiotracer has previously been radiolabeled on this module, but to streamline the production, the method was optimized. Steps requiring manual interactions to the radiolabeling procedure were minimized including a convenient and automated pre-concentration of the 68Ga-eluate and a simplified automated final formulation procedure. Every part of the radiopharmaceutical production was carefully developed to gain robustness and to avoid any operator bound variations to the manufacturing. The optimized production method was successfully applied for 68Ga-labeling of another radiotracer, verifying its versatility as a universal and robust method for radiosynthesis of Affibody-based peptides.

RESULTS

A simplified and optimized automated cGMP-compliant radiosynthesis method of [68Ga]Ga-ABY-025 was developed. With a decay corrected radiochemical yield of 44 ± 2%, a radiochemical purity (RCP) of 98 ± 1%, and with an RCP stability of 98 ± 1% at 2 h after production, the method was found highly reproducible. The production method also showed comparable results when implemented for radiolabeling another similar peptide.

CONCLUSION

The improvements made for the radiosynthesis of [68Ga]Ga-ABY-025, including introducing a pre-concentration of the 68Ga-eluate, aimed to utilize the full potential of the 68Ge/68Ga generator radioactivity output, thereby reducing radioactivity wastage. Furthermore, reducing the number of manually performed preparative steps prior to the radiosynthesis, not only minimized the risk of potential human/operator errors but also enhanced the process' robustness. The successful application of this optimized radiosynthesis method to another similar peptide underscores its versatility, suggesting that our method can be adopted for 68Ga-labeling radiotracers based on Affibody molecules in general.

TRIAL REGISTRATION

NCT, NCT05619016, Registered 7 November 2022, https://clinicaltrials.gov/study/NCT05619016?term=HER2&cond=ABY025&rank=1.

High-precision targeting and destruction of cancer-associated PDGFR-β+ stromal fibroblasts through self-assembling, protein-only nanoparticles

The need for more effective and precision medicines for cancer has pushed the exploration of new materials appropriate for drug delivery and imaging, and alternative receptors for targeting. Among the most promising strategies, finding suitable cell surface receptors and targeting agents for cancer-associated platelet derived growth factor receptor β (PDGFR-β)+ stromal fibroblasts is highly appealing. As a neglected target, this cell type mechanically and biologically supports the growth, progression, and infiltration of solid tumors in non-small cell lung, breast, pancreatic, and colorectal cancers. We have developed a family of PDGFR-β-targeted nanoparticles based on biofabricated, self-assembling proteins, upon hierarchical and iterative selective processes starting from four initial candidates. The modular protein PDGFD-GFP-H6 is well produced in recombinant bacteria, resulting in structurally robust oligomeric particles that selectively penetrates into PDGFR-β+ stromal fibroblasts in a dose-dependent manner, by means of the PDGFR-β ligand PDGFD. Upon in vivo administration, these GFP-carrying protein nanoparticles precisely accumulate in tumor tissues and enlighten them for IVIS observation. When GFP is replaced by a microbial toxin, selective tumor tissue destruction is observed associated with a significant reduction in tumor volume growth. The presented data validate the PDGFR-β/PDGFD pair as a promising toolbox for targeted drug delivery in the tumor microenvironment and oligomeric protein nanoparticles as a powerful instrument to mediate highly selective biosafe targeting in cancer through non-cancer cells. STATEMENT OF SIGNIFICANCE: We have developed a transversal platform for nanoparticle-based drug delivery into cancer-associated fibroblasts. This is based on the engineered modular protein PDGFD-GFP-H6 that spontaneously self-assemble and selectively penetrates into PDGFR-β+ stromal fibroblasts in a dose-dependent manner, by means of the PDGFR-β ligand PDGFD. In vivo, these protein nanoparticles accumulate in tumor and when incorporating a microbial toxin, they destroy tumor tissues with a significant reduction in tumor volume, in absence of side toxicities. The data presented here validate the PDGFR-β/PDGFD pair as a fully versatile toolbox for targeted drug delivery in the tumor microenvironment intended as a synergistic treatment.

Imaging of fibrogenesis in the liver by [18F]TZ-Z09591, an Affibody molecule targeting platelet derived growth factor receptor β

BACKGROUND

Platelet-derived growth factor receptor beta (PDGFRβ) is a receptor overexpressed on activated hepatic stellate cells (aHSCs). Positron emission tomography (PET) imaging of PDGFRβ could potentially allow the quantification of fibrogenesis in fibrotic livers. This study aims to evaluate a fluorine-18 radiolabeled Affibody molecule ([18F]TZ-Z09591) as a PET tracer for imaging liver fibrogenesis.

RESULTS

In vitro specificity studies demonstrated that the trans-Cyclooctenes (TCO) conjugated Z09591 Affibody molecule had a picomolar affinity for human PDGFRβ. Biodistribution performed on healthy rats showed rapid clearance of [18F]TZ-Z09591 through the kidneys and low liver background uptake. Autoradiography (ARG) studies on fibrotic livers from mice or humans correlated with histopathology results. Ex vivo biodistribution and ARG revealed that [18F]TZ-Z09591 binding in the liver was increased in fibrotic livers (p = 0.02) and corresponded to binding in fibrotic scars.

CONCLUSIONS

Our study highlights [18F]TZ-Z09591 as a specific tracer for fibrogenic cells in the fibrotic liver, thus offering the potential to assess fibrogenesis clearly.

Evaluation of affinity matured Affibody molecules for imaging of the immune checkpoint protein B7-H3

B7-H3 (CD276), an immune checkpoint protein, is a promising molecular target for immune therapy of malignant tumours. Sufficient B7-H3 expression level is a precondition for successful therapy. Radionuclide molecular imaging is a powerful technique for visualization of expression levels of molecular targets in vivo. Use of small radiolabelled targeting proteins would enable high-contrast radionuclide imaging of molecular targets if adequate binding affinity and specificity of an imaging probe could be provided. Affibody molecules, small engineered affinity proteins based on a non-immunoglobulin scaffold, have demonstrated an appreciable potential in radionuclide imaging. Proof-of principle of radionuclide visualization of expression levels of B7-H3 in vivo was demonstrated using the [99mTc]Tc-AC12-GGGC Affibody molecule. We performed an affinity maturation of AC12, enabling selection of clones with higher affinity. Three most promising clones were expressed with a -GGGC (triglycine-cysteine) chelating sequence at the C-terminus and labelled with technetium-99m (99mTc). 99mTc-labelled conjugates bound to B7-H3-expressing cells specifically in vitro and in vivo. Biodistribution in mice bearing B7-H3-expressing SKOV-3 xenografts demonstrated improved imaging properties of the new conjugates compared with the parental variant [99mTc]Tc-AC12-GGGC. [99mTc]Tc-SYNT-179 provided the strongest improvement of tumour-to-organ ratios. Thus, affinity maturation of B7-H3 Affibody molecules could improve biodistribution and targeting properties for imaging of B7-H3-expressing tumours.

Detection of Experimental Colorectal Peritoneal Metastases by a Novel PDGFRβ-Targeting Nanobody

Simple Summary

Colorectal cancer can metastasize to multiple distant sites. Metastases growing within the peritoneal cavity cause a high degree of morbidity and are associated with very poor survival. Moreover, peritoneal metastases are difficult to detect using conventional imaging methods. Consequently, peritoneal metastases are generally under-diagnosed and their response to therapy is difficult to assess. An extensive molecular and cellular analysis of colorectal peritoneal metastases revealed that these lesions express very high levels of specific markers that could serve as targets for imaging-based diagnosis and treatment. In the present report, we explore the potential value of one such marker, PDGFRB, to serve as a target for peritoneal metastasis detection by molecular imaging. Therefore, we generated a PDGFRB-binding llama nanobody and demonstrate its utility in detecting peritoneal metastases in mice. The clinical development of PDGFRB-targeting tracers may help to improve the diagnosis of peritoneal metastases and the clinical management of this highly aggressive disease entity.

Abstract

Peritoneal metastases in colorectal cancer (CRC) belong to Consensus Molecular Subtype 4 (CMS4) and are associated with poor prognosis. Conventional imaging modalities, such as Computed Tomography (CT) and Fluorodeoxyglucose-Positron Emission Tomography (FDG-PET), perform very poorly in the detection of peritoneal metastases. However, the stroma-rich nature of these lesions provides a basis for developing molecular imaging strategies. In this study, conducted from 2019 to 2021, we aimed to generate a Platelet-Derived Growth Factor Receptor beta (PDGFRB)-binding molecular imaging tracer for the detection of CMS4 CRC, including peritoneal metastases. The expression of PDGFRB mRNA discriminated CMS4 from CMS1-3 (AUROC = 0.86 (95% CI 0.85–0.88)) and was associated with poor relapse-free survival. PDGFRB mRNA and protein levels were very high in all human peritoneal metastases examined (n = 66). Therefore, we generated a PDGFRB-targeting llama nanobody (VHH1E12). Biotin-labelled VHH1E12 bound to immobilized human and mouse PDGFRB with high affinity (EC50 human PDGFRB = 7 nM; EC50 murine PDGFRB = 0.8 nM), and to PDGFRB-expressing HEK293 cells grown in vitro. A pharmacokinetic analysis of IRDye-800CW-conjugated VHH1E12 in mice showed that the plasma half-life was 6 min. IRDye-800CW-conjugated VHH1E12 specifically accumulated in experimentally induced colorectal cancer peritoneal metastases in mice. A tissue analysis subsequently demonstrated co-localization of the nanobody with PDGFRB expression in the tumour stroma. Our results demonstrate the potential value of PDGFRB-targeted molecular imaging as a novel strategy for the non-invasive detection of CMS4 CRC, in particular, peritoneal metastases.

ImmunoPET: Antibody-Based PET Imaging in Solid Tumors

Immuno-positron emission tomography (immunoPET) is a molecular imaging modality combining the high sensitivity of PET with the specific targeting ability of monoclonal antibodies. Various radioimmunotracers have been successfully developed to target a broad spectrum of molecules expressed by malignant cells or tumor microenvironments. Only a few are translated into clinical studies and barely into clinical practices. Some drawbacks include slow radioimmunotracer kinetics, high physiologic uptake in lymphoid organs, and heterogeneous activity in tumoral lesions. Measures are taken to overcome the disadvantages, and new tracers are being developed. In this review, we aim to mention the fundamental components of immunoPET imaging, explore the groundbreaking success achieved using this new technique, and review different radioimmunotracers employed in various solid tumors to elaborate on this relatively new imaging modality.

Clinical Evaluation of Nuclear Imaging Agents in Breast Cancer

Precision medicine is the customization of therapy for specific groups of patients using genetic or molecular profiling. Noninvasive imaging is one strategy for molecular profiling and is the focus of this review. The combination of imaging and therapy for precision medicine gave rise to the field of theranostics. In breast cancer, the detection and quantification of therapeutic targets can help assess their heterogeneity, especially in metastatic disease, and may help guide clinical decisions for targeted treatments. Positron emission tomography (PET) or single-photon emission tomography (SPECT) imaging has the potential to play an important role in the molecular profiling of therapeutic targets in vivo for the selection of patients who are likely to respond to corresponding targeted therapy. In this review, we discuss the state-of-the-art nuclear imaging agents in clinical research for breast cancer. We reviewed 17 clinical studies on PET or SPECT agents that target 10 different receptors in breast cancer. We also discuss the limitations of the study designs and of the imaging agents in these studies. Finally, we offer our perspective on which imaging agents have the highest potential to be used in clinical practice in the future.

Fibroblast activation protein identifies Consensus Molecular Subtype 4 in colorectal cancer and allows its detection by 68Ga-FAPI-PET imaging

Background

In colorectal cancer (CRC), the consensus molecular subtype 4 (CMS4) is associated with therapy resistance and poor prognosis. Clinical diagnosis of CMS4 is hampered by locoregional and temporal variables influencing CMS classification. Diagnostic tools that comprehensively detect CMS4 are therefore urgently needed.

Methods

To identify targets for molecular CMS4 imaging, RNA sequencing data of 3232 primary CRC patients were explored. Heterogeneity of marker expression in relation to CMS4 status was assessed by analysing 3–5 tumour regions and 91.103 single-tumour cells (7 and 29 tumours, respectively). Candidate marker expression was validated in CMS4 peritoneal metastases (PM; n = 59). Molecular imaging was performed using the ⁶⁸Ga-DOTA-FAPI-46 PET tracer.

Results

Fibroblast activation protein (FAP) mRNA identified CMS4 with very high sensitivity and specificity (AUROC > 0.91), and was associated with significantly shorter relapse-free survival (P = 0.0038). Heterogeneous expression of FAP among and within tumour lesions correlated with CMS4 heterogeneity (AUROC = 1.00). FAP expression was homogeneously high in PM, a near-homogeneous CMS4 entity. FAPI-PET identified focal and diffuse PM that were missed using conventional imaging. Extra-peritoneal metastases displayed extensive heterogeneity of tracer uptake.

Conclusion

FAP expression identifies CMS4 CRC. FAPI-PET may have value in the comprehensive detection of CMS4 tumours in CRC. This is especially relevant in patients with PM, for whom effective imaging tools are currently lacking.

Novel Receptor Tyrosine Kinase Pathway Inhibitors for Targeted Radionuclide Therapy of Glioblastoma

Glioblastoma (GB) remains the most fatal brain tumor characterized by a high infiltration rate and treatment resistance. Overexpression and/or mutation of receptor tyrosine kinases is common in GB, which subsequently leads to the activation of many downstream pathways that have a critical impact on tumor progression and therapy resistance. Therefore, receptor tyrosine kinase inhibitors (RTKIs) have been investigated to improve the dismal prognosis of GB in an effort to evolve into a personalized targeted therapy strategy with a better treatment outcome. Numerous RTKIs have been approved in the clinic and several radiopharmaceuticals are part of (pre)clinical trials as a non-invasive method to identify patients who could benefit from RTKI. The latter opens up the scope for theranostic applications. In this review, the present status of RTKIs for the treatment, nuclear imaging and targeted radionuclide therapy of GB is presented. The focus will be on seven tyrosine kinase receptors, based on their central role in GB: EGFR, VEGFR, MET, PDGFR, FGFR, Eph receptor and IGF1R. Finally, by way of analyzing structural and physiological characteristics of the TKIs with promising clinical trial results, four small molecule RTKIs were selected based on their potential to become new therapeutic GB radiopharmaceuticals.

Development of Radiogallium-Labeled Peptides for Platelet-Derived Growth Factor Receptor β (PDGFRβ) Imaging: Influence of Different Linkers

The purpose of this study is to develop peptide-based platelet-derived growth factor receptor β (PDGFRβ) imaging probes and examine the effects of several linkers, namely un-natural amino acids (D-alanine and β-alanine) and ethylene-glycol (EG), on the properties of Ga-DOTA-(linker)-IPLPPPRRPFFK peptides. Seven radiotracers, ⁶⁷Ga-DOTA-(linker)-IPLPPPRRPFFK peptides, were designed, synthesized, and evaluated. The stability and cell uptake in PDGFRβ positive peptide cells were evaluated in vitro. The biodistribution of [⁶⁷Ga]Ga-DOTA-EG₂-IPLPPPRRPFFK ([⁶⁷Ga]27) and [⁶⁷Ga]Ga-DOTA-EG₄-IPLPPPRRPFFK ([⁶⁷Ga]28), which were selected based on in vitro stability in murine plasma and cell uptake rates, were determined in BxPC3-luc-bearing nu/nu mice. Seven ⁶⁷Ga-labeled peptides were successfully synthesized with high radiochemical yields (>85%) and purities (>99%). All evaluated radiotracers were stable in PBS (pH 7.4) at 37 °C. However, only [⁶⁷Ga]27 and [⁶⁷Ga]28 remained more than 75% after incubation in murine plasma at 37 °C for 1 h. [⁶⁷Ga]27 exhibited the highest BxPC3-luc cell uptake among the prepared radiolabeled peptides. As regards the results of the biodistribution experiments, the tumor-to-blood ratios of [⁶⁷Ga]27 and [⁶⁷Ga]28 at 1 h post-injection were 2.61 ± 0.75 and 2.05 ± 0.77, respectively. Co-injection of [⁶⁷Ga]27 and an excess amount of IPLPPPRRPFFK peptide as a blocking agent can significantly decrease this ratio. However, tumor accumulation was not considered sufficient. Therefore, further probe modification is required to assess tumor accumulation for in vivo imaging.

First-in-Humans Trial of Dasatinib-Derivative Tracer for Tumor Kinase-Targeted PET

We developed a first-of-kind dasatinib-derivative imaging agent, 18F-SKI-249380 (18F-SKI), and validated its use for noninvasive in vivo tyrosine kinase-targeted tumor detection in preclinical models. In this study, we assessed the feasibility of using 18F-SKI for PET imaging in patients with malignancies. Methods: Five patients with a prior diagnosis of breast cancer, renal cell cancer, or leukemia underwent whole-body PET/CT imaging 90 min after injection of 18F-SKI (mean, 241.24 ± 116.36 MBq) as part of a prospective study. In addition, patients underwent either a 30-min dynamic scan of the upper abdomen including, at least partly, cardiac left ventricle, liver, spleen, and kidney (n = 2) or three 10-min whole-body PET/CT scans (n = 3) immediately after injection and blood-based radioactivity measurements to determine the time course of tracer distribution and facilitate radiation dose estimates. A subset of 3 patients had a delayed whole-body PET/CT scan at 180 min. Biodistribution, dosimetry, and tumor uptake were quantified. Absorbed doses were calculated using OLINDA/EXM 1.0. Results: No adverse events occurred after injection of 18F-SKI. In total, 27 tumor lesions were analyzed, with a median SUVpeak of 1.4 (range, 0.7-2.3) and tumor-to-blood ratios of 1.6 (range, 0.8-2.5) at 90 min after injection. The intratumoral drug concentrations calculated for 4 reference lesions ranged from 0.03 to 0.07 nM. In all reference lesions, constant tracer accumulation was observed between 30 and 90 min after injection. A blood radioassay indicated that radiotracer clearance from blood and plasma was initially rapid (blood half-time, 1.31 ± 0.81 min; plasma, 1.07 ± 0.66 min; n = 4), followed variably by either a prolonged terminal phase (blood half-time, 285 ± 148.49 min; plasma, 240 ± 84.85 min; n = 2) or a small rise to a plateau (n = 2). Like dasatinib, 18F-SKI underwent extensive metabolism after administration, as evidenced by metabolite analysis. Radioactivity was predominantly cleared via the hepatobiliary route. The highest absorbed dose estimates (mGy/MBq) in normal tissues were to the right colon (0.167 ± 0.04) and small intestine (0.153 ± 0.03). The effective dose was 0.0258 mSv/MBq (SD, 0.0034 mSv/MBq). Conclusion:18F-SKI demonstrated significant tumor uptake, distinct image contrast despite low injected doses, and rapid clearance from blood.

Imaging using radiolabelled targeted proteins: radioimmunodetection and beyond

The use of radiolabelled antibodies was proposed in 1970s for staging of malignant tumours. Intensive research established chemistry for radiolabelling of proteins and understanding of factors determining biodistribution and targeting properties. The use of radioimmunodetection for staging of cancer was not established as common practice due to approval and widespread use of [¹⁸F]-FDG, which provided a more general diagnostic use than antibodies or their fragments. Expanded application of antibody-based therapeutics renewed the interest in radiolabelled antibodies. RadioimmunoPET emerged as a powerful tool for evaluation of pharmacokinetics of and target engagement by biotherapeutics. In addition to monoclonal antibodies, new radiolabelled engineered proteins have recently appeared, offering high-contrast imaging of expression of therapeutic molecular targets in tumours shortly after injection. This creates preconditions for noninvasive determination of a target expression level and stratification of patients for targeted therapies. Radiolabelled proteins hold great promise to play an important role in development and implementation of personalised targeted treatment of malignant tumours. This article provides an overview of biodistribution and tumour-seeking features of major classes of targeting proteins currently utilized for molecular imaging. Such information might be useful for researchers entering the field of the protein-based radionuclide molecular imaging.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as 89Zr-Df-pertuzumab and 89Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

Affibody Molecules as Targeting Vectors for PET Imaging

Affibody molecules are small (58 amino acids) engineered scaffold proteins that can be selected to bind to a large variety of proteins with a high affinity. Their small size and high affinity make them attractive as targeting vectors for molecular imaging. High-affinity affibody binders have been selected for several cancer-associated molecular targets. Preclinical studies have shown that radiolabeled affibody molecules can provide highly specific and sensitive imaging on the day of injection; however, for a few targets, imaging on the next day further increased the imaging sensitivity. A phase I/II clinical trial showed that ⁶⁸Ga-labeled affibody molecules permit an accurate and specific measurement of HER2 expression in breast cancer metastases. This paper provides an overview of the factors influencing the biodistribution and targeting properties of affibody molecules and the chemistry of their labeling using positron emitters.

Alternative scaffolds in radionuclide diagnosis of malignancies

This review discusses a relatively new class of targeted molecules that is being actively studied for radionuclide diagnosis and treatment of malignancies. The full-size antibodies used so far have non-optimal pharmacological properties, slow distribution in the body, poor penetration into the tissue and kidney excretion, and high immunogenicity, which significantly complicates their use in clinical practice. Over the past decade, a new class of targeted molecules, called “non-immunoglobulin scaffolds” have become popular; they have all the requirements for optimal delivery of a radionuclide to tumor cells. Scaffolds usually are smaller in size in comparison with antibodies, but they are larger than peptides, and are characterized by high affinity and optimal biochemical, biophysical, biological, and economic features. The advantages of such proteins are their stable structure, good penetration into tissues, the possibility of additional functionalization and expression in the bacterial system, which ensures low production costs.

The results of preclinical and clinical studies for diagnosis of malignancies using such proteins as affibody, adnectin, DARPins, etc., have demonstrated their high specificity, affinity, good tolerance and low immunogenicity. 

Dysregulated integrin αVβ3 and CD47 signaling promotes joint inflammation, cartilage breakdown, and progression of osteoarthritis

Osteoarthritis (OA) is the leading cause of joint failure, yet the underlying mechanisms remain elusive, and no approved therapies that slow progression exist. Dysregulated integrin function was previously implicated in OA pathogenesis. However, the roles of integrin αVβ3 and the integrin-associated receptor CD47 in OA remain largely unknown. Here, transcriptomic and proteomic analyses of human and murine osteoarthritic tissues revealed dysregulated expression of αVβ3, CD47, and their ligands. Using genetically deficient mice and pharmacologic inhibitors, we showed that αVβ3, CD47, and the downstream signaling molecules Fyn and FAK are crucial to OA pathogenesis. MicroPET/CT imaging of a mouse model showed elevated ligand-binding capacities of integrin αVβ3 and CD47 in osteoarthritic joints. Further, our in vitro studies demonstrated that chondrocyte breakdown products, derived from articular cartilage of individuals with OA, induced αVβ3/CD47-dependent expression of inflammatory and degradative mediators, and revealed the downstream signaling network. Our findings identify a central role for dysregulated αVβ3 and CD47 signaling in OA pathogenesis and suggest that activation of αVβ3 and CD47 signaling in many articular cell types contributes to inflammation and joint destruction in OA. Thus, the data presented here provide a rationale for targeting αVβ3, CD47, and their signaling pathways as a disease-modifying therapy.

Targeting HER2-positive gastric cancer with a novel 18F-labeled ZHER2:342 probe

To realize the diagnosis of HER2-positive gastric cancer via PET imaging, herein, a new kind of 18F-labeled HER2 affibody probe was created; the bifunctional maleimide derivative 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA-MAL) was first coupled to a polypeptide, and the resulting compound was subsequently labeled with the 18FAl complex. The binding characteristics of the probe were assessed using both in vitro studies and in vivo microPET imaging and biodistribution experiments. Immunohistochemical staining was performed to confirm the expression level of HER2 in the studied cell lines and tumors. The probe was successfully produced with the radiochemical purity of more than 95%. The NCI N87 cell-associated radioactivity was 19.31 ± 1.01% AD, and it decreased to 0.83 ± 0.04% AD per 106 cells after blocking HER2 as early as 15 minutes post-incubation (p < 0.05). A competition binding assay between radiolabeled and non-radioactive affibody molecules with NCI N87 indicated that the IC50 was 8.10 nM. The microPET imaging and biodistribution of human gastric cancer xenografts demonstrated that the probe could specifically accumulate in tumors at early time points. Protein detection confirmed a strong HER2 expression in NCIN87 and a weak HER2 expression in SGC7901. In conclusion, 18FAl-NOTA-MAL-Cys-GGGRDN(M0)-ZHER2:342 was successfully prepared via a one-step method. The favorable preclinical data showed specific and effective tumor targeting capacity of the proposed probe; this revealed that the probe proposed herein might have potential application in gastric cancer imaging.

Combination of ribosome display and next generation sequencing as a powerful method for identification of affibody binders against β-lactamase CTX-M15

CTX-M15 is one of the most widespread, extended spectrum β-lactamases, a major determinant of antibiotic resistance representing urgent public health threats, among enterobacterial strains infecting humans and animals. Here we describe the selection of binders to CTX-M15 from a combinatorial affibody library displayed on ribosomes. Upon three increasingly selective ribosome display iterations, selected variants were identified by next generation sequencing (NGS). Nine affibody variants with high relative abundance bearing QRP and QLH amino acid motifs at residues 9-11 were produced and characterized in terms of stability, affinity and specificity. All affibodies were correctly folded, with affinities ranging from 0.04 to 2 μM towards CTX-M15, and successfully recognized CTX-M15 in bacterial lysates, culture supernatants and on whole bacteria. It was further demonstrated that the binding of affibody molecules to CTX-M15 modulated the enzyme's kinetic parameters. This work provides an approach using ribosome display coupled to NGS for the rapid generation of protein ligands of interest in diagnostic and research applications.

Radiobrominated benzimidazole-quinoline derivatives as Platelet-derived growth factor receptor beta (PDGFRβ) imaging probes

Platelet-derived growth factor receptor beta (PDGFRβ) affects in numerous human cancers and has been recognized as a promising molecular target for cancer therapies. The overexpression of PDGFRβ could be a biomarker for cancer diagnosis. Radiolabeled ligands having high affinity for the molecular target could be useful tools for the imaging of overexpressed receptors in tumors. In this study, we aimed to develop radiobrominated PDGFRβ ligands and evaluate their effectiveness as PDGFRβ imaging probes. The radiolabeled ligands were designed by modification of 1-{2-[5-(2-methoxyethoxy)-1H- benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine (1), which shows selective inhibition profile toward PDGFRβ. The bromine atom was introduced directly into C-5 of the quinoline group of 1, or indirectly by the conjugation of 1 with the 3-bromo benzoyl group. [77Br]1-{5-Bromo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinoline-8-yl}piperidin-4-amine ([77Br]2) and [77Br]-N-3-bromobenzoyl-1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}-piperidin-4-amine ([77Br]3) were prepared using a bromodestannylation reaction. In a cellular uptake study, [77Br]2 and [77Br]3 more highly accumulatd in BxPC3-luc cells (PDGFRβ-positive) than in MCF7 cells (PDGFRβ-negative), and their accumulation was significantly reduced by pretreatment with inhibitors. In biodistribution experiments, [77Br]2 accumulation was higher than [77Br]3 accumulation at 1 h postinjection. These findings suggest that [76Br]2 is more promising for positron emission tomography (PET) imaging of PDGFRβ than [76Br]3.

Optimized Molecular Design of ADAPT-Based HER2-Imaging Probes Labeled with 111In and 68Ga

Radionuclide molecular imaging is a promising tool for visualization of cancer associated molecular abnormalities in vivo and stratification of patients for specific therapies. ADAPT is a new type of small engineered proteins based on the scaffold of an albumin binding domain of protein G. ADAPTs have been utilized to select and develop high affinity binders to different proteinaceous targets. ADAPT6 binds to human epidermal growth factor 2 (HER2) with low nanomolar affinity and can be used for its in vivo visualization. Molecular design of ¹¹¹In-labeled anti-HER2 ADAPT has been optimized in several earlier studies. In this study, we made a direct comparison of two of the most promising variants, having either a DEAVDANS or a (HE)₃DANS sequence at the N-terminus, conjugated with a maleimido derivative of DOTA to a GSSC amino acids sequence at the C-terminus. The variants (designated DOTA-C⁵⁹-DEAVDANS-ADAPT6-GSSC and DOTA-C⁶¹-(HE)₃DANS-ADAPT6-GSSC) were stably labeled with ¹¹¹In for SPECT and ⁶⁸Ga for PET. Biodistribution of labeled ADAPT variants was evaluated in nude mice bearing human tumor xenografts with different levels of HER2 expression. Both variants enabled clear discrimination between tumors with high and low levels of HER2 expression. ¹¹¹In-labeled ADAPT6 derivatives provided higher tumor-to-organ ratios compared to ⁶⁸Ga-labeled counterparts. The best performing variant was DOTA-C⁶¹-(HE)₃DANS-ADAPT6-GSSC, which provided tumor-to-blood ratios of 208 ± 36 and 109 ± 17 at 3 h for ¹¹¹In and ⁶⁸Ga labels, respectively.

Same-Day Imaging Using Small Proteins: Clinical Experience and Translational Prospects in Oncology

Imaging of expression of therapeutic targets may enable stratification of patients for targeted treatments. The use of small radiolabeled probes based on the heavy-chain variable region of heavy-chain-only immunoglobulins or nonimmunoglobulin scaffolds permits rapid localization of radiotracers in tumors and rapid clearance from normal tissues. This makes high-contrast imaging possible on the day of injection. This mini review focuses on small proteins for radionuclide-based imaging that would allow same-day imaging, with the emphasis on clinical applications and promising preclinical developments within the field of oncology.

Molecular imaging of platelet-derived growth factor receptor-alpha (PDGFRα) in papillary thyroid cancer using immuno-PET

INTRODUCTION

Receptor tyrosine kinase (RTK) platelet-derived growth factor receptor-alpha (PDGFRα) was recently identified as a molecular switch for dedifferentiation in thyroid cancer that predicts resistance to therapy as well as recurrence of disease in papillary thyroid cancer. Here we describe the radiolabeling and functional characterization of an imaging probe based on a PDGFRα-specific monoclonal antibody (mAb) for immuno-PET imaging of PDGFRα in papillary thyroid cancer.

METHODS

Antibody D13C6 (Cell Signaling) was decorated with chelator NOTA using bioconjugation reaction with 2-(p-NCS-Bz)-NOTA. Radiolabeling was carried out using 40 μg of antibody-NOTA conjugate with 143-223 MBq of [⁶⁴Cu]CuCl₂ in 0.25 M NaOAc (pH 5.5) at 30 °C for 1 h. The reaction mixture was purified with size-exclusion chromatography (PD-10 column). PDGFRα and mock transfected B-CPAP thyroid cancer cells lines for validation of ⁶⁴Cu-labeled immuno-conjugates were generated using LVX-Tet-On technology. PET imaging was performed in NSG mice bearing bilaterally-induced PDGFRα (+/-) B-CPAP tumors.

RESULTS

Bioconjugation of NOTA chelator to monoclonal antibody D13C6 resulted in 2.8 ± 1.3 chelator molecules per antibody as determined by radiometric titration with ⁶⁴Cu. [⁶⁴Cu]Cu-NOTA-D13C6 was isolated in high radiochemical purity (>98%) and good radiochemical yields (19-61%). The specific activity was 0.9-5.1 MBq/μg. Cellular uptake studies revealed a specific radiotracer uptake in PDGFRα expressing cells compared to control cells. PET imaging resulted in SUV_mean values of ~5.5 for PDGFRα (+) and ~2 for PDGFRα (-) tumors, after 48 h p.i.. After 1 h, radiotracer uptake was also observed in the bone marrow (SUV_mean ~5) and spleen (SUV_mean ~8.5).

CONCLUSION

Radiolabeled antibody [⁶⁴Cu]Cu-NOTA-D13C6 represents a novel and promising radiotracer for immuno-PET imaging of PDGFRα in metastatic papillary thyroid cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The presented work has the potential to allow physicians to identify papillary thyroid cancer patients at risk of metastases by using the novel immuno-PET imaging assay based on PDGFRα-targeting antibody [⁶⁴Cu]Cu-NOTA-D13C6.

Comparative evaluation of tumor targeting using the anti-HER2 ADAPT scaffold protein labeled at the C-terminus with indium-111 or technetium-99m

ABD-Derived Affinity Proteins (ADAPTs) is a novel class of engineered scaffold proteins derived from an albumin-binding domain of protein G. The use of ADAPT6 derivatives as targeting moiety have provided excellent preclinical radionuclide imaging of human epidermal growth factor 2 (HER2) tumor xenografts. Previous studies have demonstrated that selection of nuclide and chelator for its conjugation has an appreciable effect on imaging properties of scaffold proteins. In this study we performed a comparative evaluation of the anti-HER2 ADAPT having an aspartate-glutamate-alanine-valine-aspartate-alanine-asparagine-serine (DEAVDANS) N-terminal sequence and labeled at C-terminus with ^99mTc using a cysteine-containing peptide based chelator, glycine-serine-serine-cysteine (GSSC), and a similar variant labeled with ¹¹¹In using a maleimido derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator. Both ^99mTc-DEAVDANS-ADAPT6-GSSC and ¹¹¹In-DEAVDANS-ADAPT6-GSSC-DOTA accumulated specifically in HER2-expressing SKOV3 xenografts. The tumor uptake of both variants did not differ significantly and average values were in the range of 19–21%ID/g. However, there was an appreciable variation in uptake of conjugates in normal tissues that resulted in a notable difference in the tumor-to-organ ratios. The ¹¹¹In-DOTA label provided 2–6 fold higher tumor-to-organ ratios than ^99mTc-GSSC and is therefore the preferable label for ADAPTs.

PDGF receptors in tumor stroma: Biological effects and associations with prognosis and response to treatment

Platelet-derived growth factor (PDGF) ligands and their receptors (PDGFRα and PDGFRβ) regulate mesenchymal cells, such as fibroblasts and pericytes. These cells are important constituents of tumor stroma where they impact on tumor growth, metastasis and drug response. Studies in model systems have demonstrated ability of the PDGF system to regulate the tumor-stimulatory effects of fibroblasts, as well as their ability to promote cancer cell migration and invasion. Animal studies imply PDGFR-signaling as a regulator of tumor drug uptake. Emerging correlative analyses of different tumor collections are identifying clinically relevant variations in stromal PDGFR status, and associations between PDGFR status in tumor stroma and survival. These associations could either relate to effects of stromal PDGFR signaling on the natural course of the disease or response to treatment. The availability of clinically approved PDGFR-inhibitory drugs suggest interesting possibilities for novel clinical studies, performed on selected patient sub-groups, which further exploits tumor stroma-derived PDGFR signaling.

Synthesis and evaluation of radioiodinated 1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine derivatives for platelet-derived growth factor receptor β (PDGFRβ) imaging

Platelet-derived growth factor receptor β (PDGFRβ) is a transmembrane tyrosine kinase receptor and it is upregulated in various malignant tumors. Radiolabeled PDGFRβ inhibitors can be a convenient tool for the imaging of tumors overexpressing PDGFRβ. In this study, [¹²⁵I]-1-{5-iodo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinoline-8-yl}piperidin-4-amine ([¹²⁵I]IIQP) and [¹²⁵I]-N-3-iodobenzoyl-1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}-piperidin-4-amine ([¹²⁵I]IB-IQP) were designed and synthesized, and their potential as PDGFRβ imaging agents was evaluated. In cellular uptake experiments, [¹²⁵I]IIQP and [¹²⁵I]IB-IQP showed higher uptake by PDGFRβ-positive cells than by PDGFRβ-negative cells, and the uptake in PDGFRβ-positive cells was inhibited by co-culture with PDGFRβ ligands. The biodistribution of both radiotracers in normal mice exhibited hepatobiliary excretion as the main route. In mice inoculated with BxPC3-luc (PDGFRβ-positive), the tumor uptake of radioactivity at 1h after the injection of [¹²⁵I]IIQP was significantly higher than that after the injection of [¹²⁵I]IB-IQP. These results indicated that [¹²⁵I]IIQP can be a suitable PDGFRβ imaging agent. However, further modification of its structure will be required to obtain a more appropriate PDGFRβ-targeted imaging agent with a higher signal/noise ratio.

Affibody Molecules in Biotechnological and Medical Applications

Affibody molecules are small (6.5-kDa) affinity proteins based on a three-helix bundle domain framework. Since their introduction 20 years ago as an alternative to antibodies for biotechnological applications, the first therapeutic affibody molecules have now entered clinical development and more than 400 studies have been published in which affibody molecules have been developed and used in a variety of contexts. In this review, we focus primarily on efforts over the past 5 years to explore the potential of affibody molecules for medical applications in oncology, neurodegenerative, and inflammation disorders, including molecular imaging, receptor signal blocking, and delivery of toxic payloads. In addition, we describe recent examples of biotechnological applications, in which affibody molecules have been exploited as modular affinity fusion partners.

Affibody molecules as engineered protein drugs

Affibody molecules can be used as tools for molecular recognition in diagnostic and therapeutic applications. There are several preclinical studies reported on diagnostic and therapeutic use of this molecular class of alternative scaffolds, and early clinical evidence is now beginning to accumulate that suggests the Affibody molecules to be efficacious and safe in man. The small size and ease of engineering make Affibody molecules suitable for use in multispecific constructs where AffiMabs is one such that offers the option to potentiate antibodies for use in complex disease.

Comparative Evaluation of Anti-HER2 Affibody Molecules Labeled with 64Cu Using NOTA and NODAGA

Imaging using affibody molecules enables discrimination between breast cancer metastases with high and low expression of HER2, making appropriate therapy selection possible. This study aimed to evaluate if the longer half-life of ⁶⁴Cu (T_1/2 = 12.7 h) would make ⁶⁴Cu a superior nuclide compared to ⁶⁸Ga for PET imaging of HER2 expression using affibody molecules. The synthetic ZHER2:S1 affibody molecule was conjugated with the chelators NOTA or NODAGA and labeled with ⁶⁴Cu. The tumor-targeting properties of ⁶⁴Cu-NOTA-ZHER2:S1 and ⁶⁴Cu-NODAGA-ZHER2:S1 were evaluated and compared with the targeting properties of ⁶⁸Ga-NODAGA-ZHER2:S1 in mice. Both ⁶⁴Cu-NOTA-ZHER2:S1 and ⁶⁴Cu-NODAGA-ZHER2:S1 demonstrated specific targeting of HER2-expressing xenografts. At 2 h after injection of ⁶⁴Cu-NOTA-ZHER2:S1, ⁶⁴Cu-NODAGA-ZHER2:S1, and ⁶⁸Ga-NODAGA-ZHER2:S1, tumor uptakes did not differ significantly. Renal uptake of ⁶⁴Cu-labeled conjugates was dramatically reduced at 6 and 24 h after injection. Notably, radioactivity uptake concomitantly increased in blood, lung, liver, spleen, and intestines, which resulted in decreased tumor-to-organ ratios compared to 2 h postinjection. Organ uptake was lower for ⁶⁴Cu-NODAGA-ZHER2:S1. The most probable explanation for this biodistribution pattern was the release and redistribution of renal radiometabolites. In conclusion, monoamide derivatives of NOTA and NODAGA may be suboptimal chelators for radiocopper labeling of anti-HER2 affibody molecules and, possibly, other scaffold proteins with high renal uptake.

Influence of the N-Terminal Composition on Targeting Properties of Radiometal-Labeled Anti-HER2 Scaffold Protein ADAPT6

Radionuclide-imaging-based stratification of patients to targeted therapies makes cancer treatment more personalized and therefore more efficient. Albumin-binding domain derived affinity proteins (ADAPTs) constitute a novel group of imaging probes based on the scaffold of an albumin-binding domain (ABD). To evaluate how different compositions of the N-terminal sequence of ADAPTs influence their biodistribution, a series of human epidermal growth factor receptor type 2 (HER2)-binding ADAPT6 derivatives with different N-terminal sequences were created: GCH₆DANS (2), GC(HE)₃DANS (3), GCDEAVDANS (4), and GCVDANS(5). These were compared with the parental variant: GCSS(HE)₃DEAVDANS (1). All variants were site-specifically conjugated with a maleimido-derivative of a DOTA chelator and labeled with ¹¹¹In. Binding to HER2-expressing cells in vitro, in vivo biodistribution as well as targeting properties of the new variants were compared with properties of the ¹¹¹In-labeled parental ADAPT variant 1 (¹¹¹In-DOTA-1). The composition of the N-terminal sequence had an apparent influence on biodistribution of ADAPT6 in mice. The use of a hexahistidine tag in ¹¹¹In-DOTA-2 was associated with elevated hepatic uptake compared to the (HE)₃-containing counterpart, ¹¹¹In-DOTA-3. All new variants without a hexahistidine tag demonstrated lower uptake in blood, lung, spleen, and muscle compared to uptake in the parental variant. The best new variants, ¹¹¹In-DOTA-3 and ¹¹¹In-DOTA-5, provided tumor uptakes of 14.6 ± 2.4 and 12.5 ± 1.3% ID/g at 4 h after injection, respectively. The tumor uptake of ¹¹¹In-DOTA-3 was significantly higher than the uptake of the parental ¹¹¹In-DOTA-1 (9.1 ± 2.0% ID/g). The tumor-to-blood ratios of 395 ± 75 and 419 ± 91 at 4 h after injection were obtained for ¹¹¹In-DOTA-5 and ¹¹¹In-DOTA-3, respectively. In conclusion, the N-terminal sequence composition affects the biodistribution and targeting properties of ADAPT-based imaging probes, and its optimization may improve imaging contrast.

Evaluation of the first 44Sc-labeled Affibody molecule for imaging of HER2-expressing tumors

INTRODUCTION

Affibody molecules are small (58 amino acids) high-affinity proteins based on a tri-helix non-immunoglobulin scaffold. A clinical study has demonstrated that PET imaging using Affibody molecules labeled with ⁶⁸Ga (T_½=68min) can visualize metastases of breast cancer expressing human epidermal growth factor receptor type 2 (HER2) and provide discrimination between tumors with high and low expression level. This may help to identify breast cancer patients benefiting from HER2-targeting therapies. The best discrimination was at 4h post injection. Due to longer half-life, a positron-emitting radionuclide ⁴⁴Sc (T_½=4.04h) might be a preferable label for Affibody molecules for imaging at several hours after injection.

METHODS

A synthetic second-generation anti-HER2 Affibody molecule Z_HER2:2891 was labeled with ⁴⁴Sc via a DOTA-chelator conjugated to the N-terminal amino group. Binding specificity, affinity and cellular processing ⁴⁴Sc-DOTA-Z_HER2:2891 and ⁶⁸Ga-DOTA-Z_HER2:2891 were compared in vitro using HER2-expressing cells. Biodistribution and imaging properties of ⁴⁴Sc-DOTA-Z_HER2:2891 and ⁶⁸Ga-DOTA-Z_HER2:2891 were evaluated in Balb/c nude mice bearing HER2-expression xenografts.

RESULTS

The labeling yield of 98±2% and specific activity of 7.8GBq/μmol were obtained. The conjugate demonstrated specific binding to HER2-expressing SKOV3.ip cells in vitro and to SKOV3.ip xenografts in nude mice. The distribution of radioactivity at 3h post injection was similar for ⁴⁴Sc-DOTA-Z_HER2:2891 and ⁶⁸Ga-DOTA-Z_HER2:2891, but the blood clearance of the ⁴⁴Sc-labeled variant was slower and the tumor-to-blood ratio was reduced (15±2 for ⁴⁴Sc-DOTA-Z_HER2:2891 vs 46±9 for ⁶⁸Ga-DOTA-Z_HER2:2891). At 6h after injection of ⁴⁴Sc-DOTA-Z_HER2:2891 the tumor uptake was 8±2% IA/g and the tumor-to-blood ratio was 51±8. Imaging using small-animal PET/CT demonstrated that ⁴⁴Sc-DOTA-Z_HER2:2891 provides specific and high-contrast imaging of HER2-expressing xenografts.

CONCLUSION

The ⁴⁴Sc- DOTA-Z_HER2:2891 Affibody molecule is a promising probe for imaging of HER2-expression in malignant tumors.

Aptamer-Functionalized Nanoparticles as "Smart Bombs": The Unrealized Potential for Personalized Medicine and Targeted Cancer Treatment

Conventional delivery of chemotherapeutic agents leads to multiple systemic side effects and toxicity, limiting the doses that can be used. The development of targeted therapies to selectively deliver anti-cancer agents to tumor cells without damaging neighboring unaffected cells would lead to higher effective local doses and improved response rates. Aptamers are single-stranded oligonucleotides that bind to target molecules with both high affinity and high specificity. The high specificity exhibited by aptamers promotes localization and uptake by specific cell populations, such as tumor cells, and their conjugation to anti-cancer drugs has been explored for targeted therapy. Advancements in the development of polymeric nanoparticles allow anti-cancer drugs to be encapsulated in protective nonreactive shells for controlled drug delivery with reduced toxicity. The conjugation of aptamers to nanoparticle-based therapeutics may further enhance direct targeting and personalized medicine. Here we present how the combinatorial use of aptamer and nanoparticle technologies has the potential to develop "smart bombs" for targeted cancer treatment, highlighting recent pre-clinical studies demonstrating efficacy for the direct targeting to particular tumor cell populations. However, despite these pre-clinical promising results, there has been little progress in moving this technology to the bedside.

Comparative Evaluation of Affibody Molecules for Radionuclide Imaging of in Vivo Expression of Carbonic Anhydrase IX

Overexpression of the enzyme carbonic anhydrase IX (CAIX) is documented for chronically hypoxic malignant tumors as well as for normoxic renal cell carcinoma. Radionuclide molecular imaging of CAIX would be useful for detection of hypoxic areas in malignant tumors, for patients' stratification for CAIX-targeted therapies, and for discrimination of primary malignant and benign renal tumors. Earlier, we have reported feasibility of in vivo radionuclide based imaging of CAIX expressing tumors using Affibody molecules, small affinity proteins based on a nonimmunoglobulin scaffold. In this study, we compared imaging properties of several anti-CAIX Affibody molecules having identical scaffold parts and competing for the same epitope on CAIX, but having different binding paratopes. Four variants were labeled using residualizing ^99mTc and nonresidualizing ¹²⁵I labels. All radiolabeled variants demonstrated high-affinity detection of CAIX-expressing cell line SK-RC-52 in vitro and specific accumulation in SK-RC-52 xenografts in vivo. ¹²⁵I-labeled conjugates demonstrated much lower radioactivity uptake in kidneys but higher radioactivity concentration in blood compared with ^99mTc-labeled counterparts. Although all variants cleared rapidly from blood and nonspecific compartments, there was noticeable difference in their biodistribution. The best variant for imaging of expression of CAIX in disseminated cancer was ^99mTc-(HE)₃-ZCAIX:2 providing tumor uptake of 16.3 ± 0.9% ID/g and tumor-to-blood ratio of 44 ± 7 at 4 h after injection. For primary renal cell carcinoma, the most promising imaging candidate was ¹²⁵I-ZCAIX:4 providing tumor-kidney ratio of 2.1 ± 0.5. In conclusion, several clones of scaffold proteins should be evaluated to select the best variant for development of an imaging probe with optimal sensitivity for the intended application.

PET imaging of epidermal growth factor receptor expression in tumours using 89Zr-labelled ZEGFR:2377 affibody molecules

Epidermal growth factor receptor (EGFR) is a transmembrane tyrosine kinase receptor, which is overexpressed in many types of cancer. The use of EGFR-targeting monoclonal antibodies and tyrosine-kinase inhibitors improves significantly survival of patients with colorectal, non-small cell lung cancer and head and neck squamous cell carcinoma. Detection of EGFR overexpression provides important prognostic and predictive information influencing management of the patients. The use of radionuclide molecular imaging would enable non-invasive repeatable determination of EGFR expression in disseminated cancer. Moreover, positron emission tomography (PET) would provide superior sensitivity and quantitation accuracy in EGFR expression imaging. Affibody molecules are a new type of imaging probes, providing high contrast in molecular imaging. In the present study, an EGFR-binding affibody molecule (ZEGFR:2377) was site-specifically conjugated with a deferoxamine (DFO) chelator and labelled under mild conditions (room temperature and neutral pH) with a positron-emitting radionuclide ⁸⁹Zr. The ⁸⁹Zr-DFO-ZEGFR:2377 tracer demonstrated specific high affinity (160±60 pM) binding to EGFR-expressing A431 epidermoid carcinoma cell line. In mice bearing A431 xenografts, ⁸⁹Zr-DFO-ZEGFR:2377 demonstrated specific uptake in tumours and EGFR-expressing tissues. The tracer provided tumour uptake of 2.6±0.5% ID/g and tumour-to-blood ratio of 3.7±0.6 at 24 h after injection. ⁸⁹Zr-DFO-ZEGFR:2377 provides higher tumour-to-organ ratios than anti-EGFR antibody ⁸⁹Zr-DFO-cetuximab at 48 h after injection. EGFR-expressing tumours were clearly visualized by microPET using ⁸⁹Zr-DFO-ZEGFR:2377 at both 3 and 24 h after injection. In conclusion, ⁸⁹Zr-DFO-ZEGFR:2377 is a potential probe for PET imaging of EGFR-expression in vivo.

Stromal Targets for Fluorescent-Guided Oncologic Surgery

Pre-operative imaging techniques are essential for tumor detection and diagnosis, but offer limited help during surgery. Recently, the applicability of imaging during oncologic surgery has been recognized, using near-infrared fluorescent dyes conjugated to targeting antibodies, peptides, or other vehicles. Image-guided oncologic surgery (IGOS) assists the surgeFon to distinguish tumor from normal tissue during operation, and can aid in recognizing vital structures. IGOS relies on an optimized combination of a dedicated fluorescent camera system and specific probes for targeting. IGOS probes for clinical use are not widely available yet, but numerous pre-clinical studies have been published and clinical trials are being established or prepared. Most of the investigated probes are based on antibodies or peptides against proteins on the membranes of malignant cells, whereas others are directed against stromal cells. Targeting stroma cells for IGOS has several advantages. Besides the high stromal content in more aggressive tumor types, the stroma is often primarily located at the periphery/invasive front of the tumor, which makes stromal targets particularly suited for imaging purposes. Moreover, because stroma up-regulation is a physiological reaction, most proteins to be targeted on these cells are “universal” and not derived from a specific genetic variation, as is the case with many upregulated proteins on malignant cancer cells.

In vivo imaging with antibodies and engineered fragments

Antibodies have clearly demonstrated their utility as therapeutics, providing highly selective and effective drugs to treat diseases in oncology, hematology, cardiology, immunology and autoimmunity, and infectious diseases. More recently, a pressing need for equally specific and targeted imaging agents for assessing disease in vivo, in preclinical models and patients, has emerged. This review summarizes strategies for developing and optimizing antibodies as targeted probes for use in non-invasive imaging using radioactive, optical, magnetic resonance, and ultrasound approaches. Recent advances in engineered antibody fragments and scaffolds, conjugation and labeling methods, and multimodality probes are highlighted. Importantly, antibody-based imaging probes are seeing new applications in detection and quantitation of cell surface biomarkers, imaging specific responses to targeted therapies, and monitoring immune responses in oncology and other diseases. Antibody-based imaging will provide essential tools to facilitate the transition to truly precision medicine.

Site-Specific Radioiodination of HER2-Targeting Affibody Molecules using 4-Iodophenethylmaleimide Decreases Renal Uptake of Radioactivity

Affibody molecules are small scaffold-based affinity proteins with promising properties as probes for radionuclide-based molecular imaging. However, a high reabsorption of radiolabeled Affibody molecules in kidneys is an issue. We have shown that the use of ¹²⁵I-3-iodo-((4-hydroxyphenyl)ethyl)maleimide (IHPEM) for site-specific labeling of cysteine-containing Affibody molecules provides high tumor uptake but low radioactivity retention in kidneys. We hypothesized that the use of 4-iodophenethylmaleimide (IPEM) would further reduce renal retention of radioactivity because of higher lipophilicity of radiometabolites. An anti-human epidermal growth factor receptor type 2 (HER2) Affibody molecule (Z_HER2:2395) was labeled using ¹²⁵I-IPEM with an overall yield of 45±3 %. ¹²⁵I-IPEM-Z_HER2:2395 bound specifically to HER2-expressing human ovarian carcinoma cells (SKOV-3 cell line). In NMRI mice, the renal uptake of ¹²⁵I-IPEM-Z_HER2:2395 (24±2 and 5.7±0.3 % IA g⁻¹at 1 and 4 h after injection, respectively) was significantly lower than uptake of ¹²⁵I-IHPEM-Z_HER2:2395 (50±8 and 12±2 % IA g⁻¹at 1 and 4 h after injection, respectively). In conclusion, the use of a more lipophilic linker for the radioiodination of Affibody molecules reduces renal radioactivity.