Imaging of EGFR expression in murine xenografts using site-specifically labelled anti-EGFR 111In-DOTA-Z EGFR:2377 Affibody molecule: aspect of the injected tracer amount

Oncolytic herpes simplex viruses designed for targeted treatment of EGFR-bearing tumors

Oncolytic herpes simplex viruses (oHSVs) have emerged as leading cancer therapeutic agents. Effective oHSV virotherapy may ultimately require both intratumoral and systemic vector administration to target the primary tumor and distant metastases. An attractive approach to enhancing oHSV tumor specificity is engineering the virus envelope glycoproteins for selective recognition of and infection via tumor-specific cell surface proteins. We previously demonstrated that oHSVs could be retargeted to EGFR-expressing cells by the incorporation of a single-chain antibody (scFv) at the N terminus of glycoprotein D (gD). Here, we compared retargeted oHSVs generated by the insertion of scFv, affibody molecule, or VHH antibody ligands at different positions within the N terminus of gD. When compared to the scFv-directed oHSVs, VHH and affibody molecules mediated enhanced EGFR-specific tumor cell entry, spread and cell killing in vitro, and enabled long-term tumor-specific virus replication following intravenous delivery in vivo. Moreover, oHSVs retargeted via a VHH ligand reduced tumor growth upon intravenous injection and achieved complete tumor destruction after intratumoral injection. Systemic oHSV delivery is important for the treatment of metastatic disease, and our enhancements in targeted oHSV design are a critical step in creating an effective tumor-specific oHSVs for safe administration via the bloodstream.

Recent Advances on Affibody- and DARPin-Conjugated Nanomaterials in Cancer Therapy

Affibodies and designed ankyrin repeat proteins (DARPins) are synthetic proteins originally derived from the Staphylococcus aureus virulence factor protein A and the human ankyrin repeat proteins, respectively. The use of these molecules in healthcare has been recently proposed as they are endowed with biochemical and biophysical features heavily demanded to target and fight diseases, as they have a strong binding affinity, solubility, small size, multiple functionalization sites, biocompatibility, and are easy to produce; furthermore, impressive chemical and thermal stability can be achieved. especially when using affibodies. In this sense, several examples reporting on affibodies and DARPins conjugated to nanomaterials have been published, demonstrating their suitability and feasibility in nanomedicine for cancer therapy. This minireview provides a survey of the most recent studies describing affibody- and DARPin-conjugated zero-dimensional nanomaterials, including inorganic, organic, and biological nanoparticles, nanorods, quantum dots, liposomes, and protein- and DNA-based assemblies for targeted cancer therapy in vitro and in vivo.

"PFH/AGM-CBA/HSV-TK/LIPOSOME-Affibody": Novel Targeted Nano Ultrasound Contrast Agents for Ultrasound Imaging and Inhibited the Growth of ErbB2-Overexpressing Gastric Cancer Cells

Objective

Gastric cancer is one of the most lethal malignancies in the world. However, the current research on the diagnosis and treatment of nano-ultrasound contrast agents in the field of tumor is mostly focused on breast cancer, ovarian cancer, prostate cancer, liver cancer, etc. Due to the interference of gas in the stomach, there is no report on the treatment of gastric cancer. Herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) therapy system is the most mature tumor suicide gene in cancer treatment. At the same time, in order to improve its safety and efficiency, we designed a gastric tumor targeted ultrasound-triggered phase-transition nano ultrasound contrast agent PFH/AGM-CBA/HSV-TK/Liposome (PAHL)-Affibody complex.

Methods

In our study, guanidinylated SS-PAAs polymer poly(agmatine/N, N′-cystamine-bis-acrylamide) (AGM-CBA) was used as a nuclear localization vector of suicide gene to form a polyplex, perfluorohexane (PFH) was used as ultrasound contrast agent, liposomes were used to encapsulate perfluorohexane droplets and the polyplexes of AGM-CBA/HSV-TK, and affibody molecules were conjugated to the prepared PAHL in order to obtain a specific targeting affinity to human epidermal growth factor receptor type 2 (ErbB2) at gastric cancer cells. With the aid of ultrasound targeted microbubble destruction technology and the nuclear localization effect of AGM-CBA vector, the transfection efficiency of the suicide gene in gastric cancer cells was significantly increased, leading to significant apoptosis of gastric cancer cells.

Results

It was shown that PAHL-Affibody complex was nearly spherical with an average diameter of 560 ± 28.9 nm, having higher and specific affinity to ErbB2 (+) gastric cells. In vitro experiments further confirmed that PAHL could target gastric cancer cells expressing ErbB2. In a contrast-enhanced ultrasound scanning study, the prepared ultrasound-triggered phase-change nano-ultrasound contrast agent, PAHL, showed improved ultrasound enhancement effects. With the application of the low-frequency ultrasound, the gene transfection efficiency of PAHL was significantly improved,  thereby inducing significant apoptosis in gastric cancer cells.

Conclusion

This study constructs PFH/AGM-CBA/HSV-TK/Liposome-Affibody nano ultrasound contrast agent, which provides new ideas for the treatment strategy of ErbB2-positive gastric cancer and provides some preliminary experimental basis for its inhibitory effect.

Recombinant immunotoxins development for HER2-based targeted cancer therapies

Understanding the molecular mechanisms of cancer biology introduces targeted therapy as a complementary method along with other conventional therapies. Recombinant immunotoxins are tumor specific antibodies that their recognizing fragment is utilized for delivering modified toxins into tumor cells. These molecules have been considered as a targeted strategy in the treatment of human cancers. HER2 tumor biomarker is a transmembrane tyrosine kinase receptor that can be used for targeted therapies in the forms of anti-HER2 monoclonal antibodies, antibody-drug conjugates and immunotoxins. There have been many studies on HER2-based immunotoxins in recent years, however, little progress has been made in the clinical field which demanded more improvements. Here, we summarized the HER2 signaling and it's targeting using immunotherapeutic agents in human cancers. Then, we specifically reviewed anti-HER2 immunotoxins, and their strengths and drawbacks to highlight their promising clinical impact.

Production, Purification, and Applications of a Potential Theranostic Pair: Cobalt-55 and Cobalt-58m

The emerging success of [⁶⁸Ga/¹⁷⁷Lu]Ga/Lu-DOTATATE as a theranostic pair has spurred interest in other isotopes as potential theranostic combinations. Here, we review cobalt-55 and cobalt-58m as a potential theranostic pair. Radionuclidically pure cobalt-55 and cobalt-58m have been produced on small cyclotrons with high molar activity. In vitro, DOTATOC labeled with cobalt has shown greater affinity for SSTR2 than DOTATOC labeled with gallium and yttrium. Similarly, [^58mCo]Co-DOTATATE has shown improved cell-killing capabilities as compared to DOTATATE labeled with either indium-111 or lutetium-177. Finally, PET imaging with an isotope such as cobalt-55 allows for image acquisition at much later timepoints than gallium, allowing for an increased degree of biological clearance of non-bound radiotracer. We discuss the accelerator targetry and radiochemistry used to produce cobalt-55,58m, emphasizing the implications of these techniques to downstream radiotracers being developed for imaging and therapy.

PET and SPECT Imaging of the EGFR Family (RTK Class I) in Oncology

The human epidermal growth factor receptor family (EGFR-family, other designations: HER family, RTK Class I) is strongly linked to oncogenic transformation. Its members are frequently overexpressed in cancer and have become attractive targets for cancer therapy. To ensure effective patient care, potential responders to HER-targeted therapy need to be identified. Radionuclide molecular imaging can be a key asset for the detection of overexpression of EGFR-family members. It meets the need for repeatable whole-body assessment of the molecular disease profile, solving problems of heterogeneity and expression alterations over time. Tracer development is a multifactorial process. The optimal tracer design depends on the application and the particular challenges of the molecular target (target expression in tumors, endogenous expression in healthy tissue, accessibility). We have herein summarized the recent preclinical and clinical data on agents for Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) imaging of EGFR-family receptors in oncology. Antibody-based tracers are still extensively investigated. However, their dominance starts to be challenged by a number of tracers based on different classes of targeting proteins. Among these, engineered scaffold proteins (ESP) and single domain antibodies (sdAb) show highly encouraging results in clinical studies marking a noticeable trend towards the use of smaller sized agents for HER imaging.

The Use of a Non-Conventional Long-Lived Gallium Radioisotope 66Ga Improves Imaging Contrast of EGFR Expression in Malignant Tumours Using DFO-ZEGFR:2377 Affibody Molecule

Epidermal growth factor receptor (EGFR) is overexpressed in many malignancies. EGFR-targeted therapy extends survival of patients with disseminated cancers. Radionuclide molecular imaging of EGFR expression would make EGFR-directed treatment more personalized and therefore more efficient. A previous study demonstrated that affibody molecule [⁶⁸Ga]Ga-DFO-ZEGFR:2377 permits specific positron-emission tomography (PET) imaging of EGFR expression in xenografts at 3 h after injection. We anticipated that imaging at 24 h after injection would provide higher contrast, but this is prevented by the short half-life of ⁶⁸Ga (67.6 min). Here, we therefore tested the hypothesis that the use of the non-conventional long-lived positron emitter ⁶⁶Ga (T_1/2 = 9.49 h, β⁺ = 56.5%) would permit imaging with higher contrast. ⁶⁶Ga was produced by the ⁶⁶Zn(p,n)⁶⁶Ga nuclear reaction and DFO-ZEGFR:2377 was efficiently labelled with ⁶⁶Ga with preserved binding specificity in vitro and in vivo. At 24 h after injection, [⁶⁶Ga]Ga-DFO-ZEGFR:2377 provided 3.9-fold higher tumor-to-blood ratio and 2.3-fold higher tumor-to-liver ratio than [⁶⁸Ga]Ga-DFO-ZEGFR:2377 at 3 h after injection. At the same time point, [⁶⁶Ga]Ga-DFO-ZEGFR:2377 provided 1.8-fold higher tumor-to-blood ratio, 3-fold higher tumor-to-liver ratio, 1.9-fold higher tumor-to-muscle ratio and 2.3-fold higher tumor-to-bone ratio than [⁸⁹Zr]Zr-DFO-ZEGFR:2377. Biodistribution data were confirmed by whole body PET combined with magnetic resonance imaging (PET/MRI). The use of the positron emitter ⁶⁶Ga for labelling of DFO-ZEGFR:2377 permits PET imaging of EGFR expression at 24 h after injection and improves imaging contrast.

Detection of Tumor-Associated Membrane Receptors on Extracellular Vesicles from Non-Small Cell Lung Cancer Patients via Immuno-PCR

Simple Summary

Lung cancer is often detected at late stages when metastases are present and the genomic make-ups of the tumors are heterogeneous. Analyses of genomic alterations in non-small-cell lung cancer (NSCLC) have revealed mutated tumor-associated membrane receptors and fusion proteins, which can be targeted via tyrosine kinase inhibitors (TKIs). TKIs initially often have a good effect, but a fraction of the tumor lesions may develop resistance through additional mutations in the targeted kinases or by increased expression/function of other membrane receptors. Detection of TKI-bypassing mechanisms is difficult in tissue biopsies as these analyze only a subpart of tumors or lesions. Liquid biopsies based on tumor-secreted small extracellular vesicles (sEVs) into body fluids can assess tumor heterogeneity. We present an immuno-PCR method for the detection of the epidermal growth factor receptor (EGFR), the human epidermal growth factor receptor 2 (HER2), and the insulin-like growth factor 1 receptor (IGF-1R) on sEVs. Initial investigations of sEVs from EGFR-mutant NSCLC tumor cells or pleural effusion (PE) fluid from patients with NSCLC or benign diseases showed different protein profiles for individual sEV samples. Further development of the immuno-PCR could complement DNA/mRNA-based assays detecting kinase mutations to allow longitudinal treatment monitoring of diverse TKI-bypassing mechanisms.

Abstract

Precision cancer medicine for non-small-cell lung cancer (NSCLC) has increased patient survival. Nevertheless, targeted agents towards tumor-associated membrane receptors only result in partial remission for a limited time, calling for approaches which allow longitudinal treatment monitoring. Rebiopsy of tumors in the lung is challenging, and metastatic lesions may have heterogeneous signaling. One way ahead is to use liquid biopsies such as circulating tumor DNA or small extracellular vesicles (sEVs) secreted by the tumor into blood or other body fluids. Herein, an immuno-PCR-based detection of the tumor-associated membrane receptors EGFR, HER2, and IGF-1R on CD9-positive sEVs from NSCLC cells and pleural effusion fluid (PE) of NSCLC patients is developed utilizing DNA conjugates of antibody mimetics and affibodies, as detection agents. Results on sEVs purified from culture media of NSCLC cells treated with anti-EGFR siRNA, showed that the reduction of EGFR expression can be detected via immuno-PCR. Protein profiling of sEVs from NSCLC patient PE samples revealed the capacity to monitor EGFR, HER2, and IGF-1R with the immuno-PCR method. We detected a significantly higher EGFR level in sEVs derived from a PE sample of a patient with an EGFR-driven NSCLC adenocarcinoma than in sEVs from PE samples of non-EGFR driven adenocarcinoma patients or in samples from patients with benign lung disease. In summary, we have developed a diagnostic method for sEVs in liquid biopsies of cancer patients which may be used for longitudinal treatment monitoring to detect emerging bypassing resistance mechanisms in a noninvasive way.

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.

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.

Influence of Residualizing Properties of the Radiolabel on Radionuclide Molecular Imaging of HER3 Using Affibody Molecules

Human epidermal growth factor receptor type 3 (HER3) is an emerging therapeutic target in several malignancies. To select potential responders to HER3-targeted therapy, radionuclide molecular imaging of HER3 expression using affibody molecules could be performed. Due to physiological expression of HER3 in normal organs, high imaging contrast remains challenging. Due to slow internalization of affibody molecules by cancer cells, we hypothesized that labeling (HE)₃-Z_HER3:08698-DOTAGA affibody molecule with non-residualizing [¹²⁵I]-N-succinimidyl-4-iodobenzoate (PIB) label would improve the tumor-to-normal organs ratios compared to previously reported residualizing radiometal labels. The [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA was compared side-by-side with [¹¹¹In]In-(HE)₃-Z_HER3:08698-DOTAGA. Both conjugates demonstrated specific high-affinity binding to HER3-expressing BxPC-3 and DU145 cancer cells. Biodistribution in mice bearing BxPC-3 xenografts at 4 and 24 h pi showed faster clearance of the [¹²⁵I]I-PIB label compared to the indium-111 label from most tissues, except blood. This resulted in higher tumor-to-organ ratios in HER3-expressing organs for [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA at 4 h, providing the tumor-to-liver ratio of 2.4 ± 0.3. The tumor uptake of both conjugates was specific, however, it was lower for the [¹²⁵I]I-PIB label. In conclusion, the use of non-residualizing [¹²⁵I]I-PIB label for HER3-targeting affibody molecule provided higher tumor-to-liver ratio than the indium-111 label, however, further improvement in tumor uptake and retention is needed.

Radiometal-labeled anti-VCAM-1 nanobodies as molecular tracers for atherosclerosis - impact of radiochemistry on pharmacokinetics

Radiolabeling of nanobodies with radiometals by chelation has the advantage of being simple, fast and easy to implement in clinical routine. In this study, we validated 68Ga/111In-labeled anti-VCAM-1 nanobodies as potential radiometal-based tracers for molecular imaging of atherosclerosis. Both showed specific targeting of atherosclerotic lesions in ApoE-/- mice. Nevertheless, uptake in lesions and constitutively VCAM-1 expressing organs was lower than previously reported for the 99mTc-labeled analog. We further investigated the impact of different radiolabeling strategies on the in vivo biodistribution of nanobody-based tracers. Comparison of the pharmacokinetics between 68Ga-, 18F-, 111In- and 99mTc-labeled anti-VCAM-1 nanobodies showed highest specific uptake for 99mTc-nanobody at all time-points, followed by the 68Ga-, 111In- and 18F-labeled tracer. No correlation was found with the estimated number of radioisotopes per nanobody, and mimicking specific activity of other radiolabeling methods did not result in an analogous biodistribution. We also demonstrated specificity of the tracer using mice with a VCAM-1 knocked-down phenotype, while showing for the first time the in vivo visualization of a protein knock-down using intrabodies. Conclusively, the chosen radiochemistry does have an important impact on the biodistribution of nanobodies, in particular on the specific targeting, but differences are not purely due to the tracer's specific activity.

Analysis of Progress and Challenges of EGFR-Targeted Molecular Imaging in Cancer With a Focus on Affibody Molecules

Epidermal growth factor receptor (EGFR)-targeted cancer therapy requires an accurate estimation of EGFR expression in tumors to identify responsive patients, monitor therapeutic effect, and estimate prognosis. The EGFR molecular imaging is an optimal method for evaluating EGFR expression in vivo accurately and noninvasively. In this review, we discuss the recent advances in EGFR-targeted molecular imaging in cancer, with a special focus on the development of imaging agents, including epidermal growth factor (EGF) ligand, monoclonal antibodies, antibody fragments, Affibody, and small molecules. Each substrate or probe, whether it is an endogenous ligand, antibody, peptide, or small molecule labeled with fluorochrome or radionuclide, has unique advantages and limitations. Antibody-based probes have high affinity but a long metabolic cycle and therefore offer poor imaging quality. Affibody molecules promise to surpass antibody-based probes due to their small size, stable chemical properties, and high affinity to the target. Small-molecule probes are safe, have favorable pharmacokinetics, and show high affinity and specificity, in addition to having an ideal size, but are inadequate for delayed imaging after injection due to their fast clearance.

Label-Free Surface Protein Profiling of Extracellular Vesicles by an Electrokinetic Sensor

Small extracellular vesicles (sEVs) generated from the endolysosomal system, often referred to as exosomes, have attracted interest as a suitable biomarker for cancer diagnostics, as they carry valuable biological information and reflect their cells of origin. Herein, we propose a simple and inexpensive electrical method for label-free detection and profiling of sEVs in the size range of exosomes. The detection method is based on the electrokinetic principle, where the change in the streaming current is monitored as the surface markers of the sEVs interact with the affinity reagents immobilized on the inner surface of a silica microcapillary. As a proof-of-concept, we detected sEVs derived from the non-small-cell lung cancer (NSCLC) cell line H1975 for a set of representative surface markers, such as epidermal growth factor receptor (EGFR), CD9, and CD63. The detection sensitivity was estimated to be ∼175000 sEVs, which represents a sensor surface coverage of only 0.04%. We further validated the ability of the sensor to measure the expression level of a membrane protein by using sEVs displaying artificially altered expressions of EGFR and CD63, which were derived from NSCLC and human embryonic kidney (HEK) 293T cells, respectively. The analysis revealed that the changes in EGFR and CD63 expressions in sEVs can be detected with a sensitivity in the order of 10% and 3%, respectively, of their parental cell expressions. The method can be easily parallelized and combined with existing microfluidic-based EV isolation technologies, allowing for rapid detection and monitoring of sEVs for cancer diagnosis.

Improved contrast of affibody-mediated imaging of HER3 expression in mouse xenograft model through co-injection of a trivalent affibody for in vivo blocking of hepatic uptake

Human epidermal growth factor receptor type 3 (HER3) plays a crucial role in the progression of many cancer types. In vivo radionuclide imaging could be a reliable method for repetitive detection of HER3-expression in tumors. The main challenge of HER3-imaging is the low expression in tumors together with endogenous receptor expression in normal tissues, particularly the liver. A HER3-targeting affibody molecule labeled with radiocobalt via a NOTA chelator [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉ has demonstrated the most favorable biodistribution profile with the lowest unspecific hepatic uptake and high activity uptake in tumors. We hypothesized that specific uptake of labeled affibody monomer might be selectively blocked in the liver but not in tumors by a co-injection of non-labeled corresponding trivalent affibody (Z₀₈₆₉₉)₃. Biodistribution of [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉ and [¹¹¹In]In-DOTA-(Z₀₈₆₉₉)₃ was studied in BxPC-3 xenografted mice. [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉ was co-injected with unlabeled trivalent affibody DOTA-(Z₀₈₆₉₉)₃ at different monomer:trimer molar ratios. HER3-expression in xenografts was imaged using [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉ and [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉: DOTA-(Z₀₈₆₉₉)₃. Hepatic activity uptake of [⁵⁷Co]Co-NOTA-Z₀₈₆₉₉: DOTA-(Z₀₈₆₉₉)₃ decreased with increasing monomer:trimer molar ratio. The tumor activity uptake and tumor-to-liver ratios were the highest for the 1:3 ratio. SPECT/CT images confirmed the biodistribution data. Imaging of HER3 expression can be improved by co-injection of a radiolabeled monomeric affibody-based imaging probe together with a trivalent affibody.

Affibody-Based PET Imaging to Guide EGFR-Targeted Cancer Therapy in Head and Neck Squamous Cell Cancer Models

In head and neck squamous cell cancer, the human epidermal growth factor receptor 1 (EGFR) is the dominant signaling molecule among all members of the family. So far, cetuximab is the only approved anti-EGFR monoclonal antibody used for the treatment of head and neck squamous cell cancer, but despite the benefits of adding it to standard treatment regimens, attempts to define a predictive biomarker to stratify patients for cetuximab treatment have been unsuccessful. We hypothesized that imaging with EGFR-specific radioligands may facilitate noninvasive measurement of EGFR expression across the entire tumor burden and allow for dynamic monitoring of cetuximab-mediated changes in receptor expression. Methods: EGFR-specific Affibody molecule (ZEGFR:03115) was radiolabeled with 89Zr and 18F. The radioligands were characterized in vitro and in mice bearing subcutaneous tumors with varying levels of EGFR expression. The protein dose for imaging studies was assessed by injecting 89Zr-deferoxamine-ZEGFR:03115 (2.4-3.6 MBq, 2 μg) either together with or 30 min after increasing amounts of unlabeled ZEGFR:03115 (1, 5, 10, 15, and 20 μg). PET images were acquired at 3, 24, and 48 h after injection, and the image quantification data were correlated with the biodistribution results. The EGFR expression and biodistribution of the tracer were assessed ex vivo by immunohistochemistry, Western blot, and autoradiography. To downregulate the EGFR level, treatment with cetuximab was performed, and 18F-aluminium fluoride-NOTA-ZEGFR:03115 (12 μg, 1.5-2 MBq/mouse) was used to monitor receptor changes. Results: In vivo studies demonstrated that coinjecting 10 μg of nonlabeled molecules with 89Zr-deferoxamine-ZEGFR:03115 allows for clear tumor visualization 3 h after injection. The radioconjugate tumor accumulation was EGFR-specific, and PET imaging data showed a clear differentiation between xenografts with varying EGFR expression levels. A strong correlation was observed between PET analysis, ex vivo estimates of tracer concentration, and receptor expression in tumor tissues. Additionally, 18F-aluminium fluoride-NOTA-ZEGFR:03115 could measure receptor downregulation in response to EGFR inhibition. Conclusion: ZEGFR:03115-based radioconjugates can assess different levels of EGFR level in vivo and measure receptor expression changes in response to cetuximab, indicating a potential for assessment of adequate treatment dosing with anti-EGFR antibodies.

A Novel Engineered Small Protein for Positron Emission Tomography Imaging of Human Programmed Death Ligand-1: Validation in Mouse Models and Human Cancer Tissues

PURPOSE

To design and evaluate a small engineered protein binder targeting human programmed death-1 ligand (hPD-L1) in vivo for PET imaging in four mouse tumor models, and in situ in human cancer specimens.Experimental Design: The hPD-L1 protein binder, FN3_hPD-L1, was engineered using a 12-kDa human fibronectin type-3 domain (FN3) scaffold. The binder's affinity was assayed in CT26 mouse colon carcinoma cells stably expressing hPD-L1 (CT26/hPD-L1). ⁶⁴Cu-FN3_hPD-L1 was assayed for purity, specific activity, and immunoreactivity. Four groups of NSG mice (n = 3-5/group) were imaged with ⁶⁴Cu-FN3_hPD-L1 PET imaging (1-24 hours postinjection of 3.7 MBq/7 μg of Do-FN3 in 200 μL PBS): Nod SCID Gamma (NSG) mice bearing (i) syngeneic CT26/hPD-L1tumors, (ii) CT26/hPD-L1 tumors blocked (blk) by preinjected nonradioactive FN3_hPD-L1 binder, (iii) hPD-L1-negative Raji xenografts, and (iv) MDA-MB-231 xenografts. The FN3_hPD-L1 binder staining was evaluated against validated hPD-L1 antibodies by immunostaining in human cancer specimens.

RESULTS

FN3_hPD-L1 bound hPD-L1 with 1.4 ± 0.3 nmol/L affinity in CT26/hPD-L1 cells. ⁶⁴Cu-FN3_hPD-L1 radiotracer showed >70% yield and >95% purity. ⁶⁴Cu-FN3_hPD-L1 PET imaging of mice bearing CT26/hPD-L1 tumors showed tumor-to-muscle ratios of 5.6 ± 0.9 and 13.1 ± 2.3 at 1 and 4 hours postinjection, respectively. The FN3_hPD-L1 binder detected hPD-L1 expression in human tissues with known hPD-L1 expression status based on two validated antibodies.

CONCLUSIONS

The ⁶⁴Cu-FN3_hPD-L1 radiotracer represents a novel, small, and high-affinity binder for imaging hPD-L1 in tumors. Our data support further exploration and clinical translation of this binder for noninvasive identification of cancer patients who may respond to immune checkpoint blockade therapies.

Anti-EGFR Affibodies with Site-Specific Photo-Cross-Linker Incorporation Show Both Directed Target-Specific Photoconjugation and Increased Retention in Tumors

A significant challenge for solid tumor treatment is ensuring that a sufficient concentration of therapeutic agent is delivered to the tumor site at doses that can be tolerated by the patient. Biomolecular targeting can bias accumulation in tumors by taking advantage of specific interactions with receptors overexpressed on cancerous cells. However, while antibody-based immunoconjugates show high binding to specific cells, their low dissociation constants ( KD) and large Stokes radii hinder their ability to penetrate deep into tumor tissue, leading to incomplete cell killing and tumor recurrence. To address this, we demonstrate the design and production of a photo-cross-linkable affibody that can form a covalent bond to epidermal growth factor receptor (EGFR) under near UV irradiation. Twelve cysteine mutations were created of an EGFR affibody and conjugated with maleimide-benzophenone. Of these only one exhibited photoconjugation to EGFR, as demonstrated by SDS-PAGE and Western blot. Next this modified affibody was shown to not only bind EGFR expressing cells but also show enhanced retention in a 3D tumor spheroid model, with minimal loss up to 24 h as compared to either unmodified EGFR-binding affibodies or nonbinding, photo-cross-linkable affibodies. Finally, in order to show utility of photo-cross-linking at clinically relevant wavelengths, upconverting nanoparticles (UCNPs) were synthesized that could convert 980 nm light to UV and blue light. In the presence of UCNPs, both direct photoconjugation to EGFR and enhanced retention in tumor spheroids could be obtained using near-infrared illumination. Thus, the photoactive affibodies developed here may be utilized as a platform technology for engineering new therapy conjugates that can penetrate deep into tumor tissue and be retained long enough for effective tumor therapy.

Preclinical Evaluation of [68Ga]Ga-DFO-ZEGFR:2377: A Promising Affibody-Based Probe for Noninvasive PET Imaging of EGFR Expression in Tumors

Radionuclide imaging of epidermal growth factor receptor (EGFR) expression in tumors may stratify patients for EGFR-targeting therapies and predict response or resistance to certain treatments. Affibody molecules, which are nonimmunoglobulin scaffold proteins, have a high potential as probes for molecular imaging. In this study, maleimido derivative of desferrioxamine B (DFO) chelator was site-specifically coupled to the C-terminal cysteine of the anti-EGFR affibody molecule ZEGFR:2377, and the DFO-ZEGFR:2377 conjugate was labeled with the generator-produced positron-emitting radionuclide ⁶⁸Ga. Stability, specificity of binding to EGFR-expressing cells, and processing of [⁶⁸Ga]Ga-DFO-ZEGFR:2377 by cancer cells after binding were evaluated in vitro. In vivo studies were performed in nude mice bearing human EGFR-expressing A431 epidermoid cancer xenografts. The biodistribution of [⁶⁸Ga]Ga-DFO-ZEGFR:2377 was directly compared with the biodistribution of [⁸⁹Zr]Zr-DFO-ZEGFR:2377. DFO-ZEGFR:2377 was efficiently (isolated yield of 73 ± 3%) and stably labeled with ⁶⁸Ga. Binding of [⁶⁸Ga]Ga-DFO-ZEGFR:2377 to EGFR-expressing cells in vitro was receptor-specific and proportional to the EGFR expression level. In vivo saturation experiment demonstrated EGFR-specific accumulation of [⁶⁸Ga]Ga-DFO-ZEGFR:2377 in A431 xenografts. Compared to [⁸⁹Zr]Zr-DFO-ZEGFR:2377, [⁶⁸Ga]Ga-DFO-ZEGFR:2377 demonstrated significantly (p < 0.05) higher uptake in tumors and lower uptake in spleen and bones. This resulted in significantly higher tumor-to-organ ratios for [⁶⁸Ga]Ga-DFO-ZEGFR:2377. In conclusion, [⁶⁸Ga]Ga-DFO-ZEGFR:2377 is a promising probe for imaging of EGFR expression.

Radionuclide imaging of VEGFR2 in glioma vasculature using biparatopic affibody conjugate: proof-of-principle in a murine model

Vascular endothelial growth factor receptor-2 (VEGFR2) is a key mediator of angiogenesis and therefore a promising therapeutic target in malignancies including glioblastoma multiforme (GBM). Molecular imaging of VEGFR2 expression may enable patient stratification for antiangiogenic therapy. The goal of the current study was to evaluate the capacity of the novel anti-VEGFR2 biparatopic affibody conjugate (Z_VEGFR2-Bp₂) for in vivo visualization of VEGFR2 expression in GBM. Methods: Z_VEGFR2-Bp₂ coupled to a NODAGA chelator was generated and radiolabeled with indium-111. The VEGFR2-expressing murine endothelial cell line MS1 was used to evaluate in vitro binding specificity and affinity, cellular processing and targeting specificity in mice. Further tumor targeting was studied in vivo in GL261 glioblastoma orthotopic tumors. Experimental imaging was performed. Results: [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ bound specifically to VEGFR2 (K_D=33±18 pM). VEGFR2-mediated accumulation was observed in liver, spleen and lungs. The tumor-to-organ ratios 2 h post injection for mice bearing MS1 tumors were approximately 11 for blood, 15 for muscles and 78 for brain. Intracranial GL261 glioblastoma was visualized using SPECT/CT. The activity uptake in tumors was significantly higher than in normal brain tissue. The tumor-to-cerebellum ratios after injection of 4 µg [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were significantly higher than the ratios observed for the 40 µg injected dose and for the non-VEGFR2 binding size-matched conjugate, demonstrating target specificity. Microautoradiography of cryosectioned CNS tissue was in good agreement with the SPECT/CT images. Conclusion: The anti-VEGFR2 affibody conjugate [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ specifically targeted VEGFR2 in vivo and visualized its expression in a murine GBM orthotopic model. Tumor-to-blood ratios for [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ were higher compared to other VEGFR2 imaging probes. [¹¹¹In]In-NODAGA-Z_VEGFR2-Bp₂ appears to be a promising probe for in vivo noninvasive visualization of tumor angiogenesis in glioblastoma.

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.

Influence of composition of cysteine-containing peptide-based chelators on biodistribution of 99mTc-labeled anti-EGFR affibody molecules

Epidermal growth factor receptor (EGFR) is overexpressed in a number of cancers and is the molecular target for several anti-cancer therapeutics. Radionuclide molecular imaging of EGFR expression should enable personalization of anti-cancer treatment. Affibody molecule is a promising type of high-affinity imaging probes based on a non-immunoglobulin scaffold. A series of derivatives of the anti-EGFR affibody molecule ZEGFR:2377, having peptide-based cysteine-containing chelators for conjugation of ^99mTc, was designed and evaluated. It was found that glutamate-containing chelators Gly-Gly-Glu-Cys (GGEC), Gly-Glu-Glu-Cys (GEEC) and Glu-Glu-Glu-Cys (EEEC) provide the best labeling stability. The glutamate containing conjugates bound to EGFR-expressing cells specifically and with high affinity. Specific targeting of EGFR-expressing xenografts in mice was demonstrated. The number of glutamate residues in the chelator had strong influence on biodistribution of radiolabeled affibody molecules. Increase of glutamate content was associated with lower uptake in normal tissues. The ^99mTc-labeled variant containing the EEEC chelator provided the highest tumor-to-organ ratios. In conclusion, optimizing the composition of peptide-based chelators enhances contrast of imaging of EGFR-expression using affibody molecules.

Molecular design of radiocopper-labelled Affibody molecules

The use of long-lived positron emitters ⁶⁴Cu or ⁶¹Cu for labelling of Affibody molecules may improve breast cancer patients’ stratification for HER-targeted therapy. Previous animal studies have shown that the use of triaza chelators for ⁶⁴Cu labelling of synthetic Affibody molecules is suboptimal. In this study, we tested a hypothesis that the use of cross-bridged chelator, CB-TE2A, in combination with Gly-Glu-Glu-Glu spacer for labelling of Affibody molecules with radiocopper would improve imaging contrast. CB-TE2A was coupled to the N-terminus of synthetic Affibody molecules extended either with a glycine (designation CB-TE2A-G-ZHER2:342) or Gly-Glu-Glu-Glu spacer (CB-TE2A-GEEE-ZHER2:342). Biodistribution and targeting properties of ⁶⁴Cu-CB-TE2A-G-ZHER2:342 and ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 were compared in tumor-bearing mice with the properties of ⁶⁴Cu-NODAGA-ZHER2:S1, which had the best targeting properties in the previous study. ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 provided appreciably lower uptake in normal tissues and higher tumor-to-organ ratios than ⁶⁴Cu-CB-TE2A-G-ZHER2:342 and ⁶⁴Cu-NODAGA-ZHER2:S1. The most pronounced was a several-fold difference in the hepatic uptake. At the optimal time point, 6 h after injection, the tumor uptake of ⁶⁴Cu-CB-TE2A-GEEE-ZHER2:342 was 16 ± 6%ID/g and tumor-to-blood ratio was 181 ± 52. In conclusion, a combination of the cross-bridged CB-TE2A chelator and Gly-Glu-Glu-Glu spacer is preferable for radiocopper labelling of Affibody molecules and, possibly, other scaffold proteins having high renal re-absorption.

Evaluation of affibody charge modification identified by synthetic consensus design in molecular PET imaging of epidermal growth factor receptor

Tumor overexpression of epidermal growth factor receptor (EGFR) correlates to therapeutic response in select patient populations. Thus, molecular positron emission tomography (PET) imaging of EGFR could stratify responders versus non-responders. We previously demonstrated effectiveness of a "synthetic consensus" design principle to identify six neutralizing mutations within a 58-amino acid EGFR-targeted affibody domain. Herein, we extend the approach to identify additional neutralized variants that vary net charge from -2 to either -4 or +4 while retaining high affinity (1.6 ± 1.2 nM and 2.5 ± 0.7 nM), specific binding to EGFR, secondary structure, and stability (T_m = 68 °C and 59 °C). We radiolabeled the resultant collection of five charge variants with ⁶⁴Cu and evaluated PET imaging performance in murine models with subcutaneously xenografted EGFR^high and EGFR^low tumors. All variants exhibited good EGFR^high tumor imaging as early as 1 h, with EA35S (+3/-5) achieving 7.7 ± 1.4 %ID/g tumor at 4 h with 1.5 ± 0.3%ID/g EGFR^low tumor, 34 ± 5 tumor:muscle and 12 ± 3 tumor:blood ratios. The positively charged EA62S mutant (+6/-2) exhibited 2.2-3.3-fold higher liver signal than the other variants (p<0.01). The EA68 variant with higher charge density was more stable to human and mouse serum than neutralized variants. In a comparison of radiometal chelators, 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA) exhibited superior physiological specificity to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). In total, these studies comparatively evaluated a set of EGFR-targeted affibodies varying in net charge and charge density, which revealed functional variations that are useful in engineering an ideal probe for translational studies.

Dual-Nuclide Radiopharmaceuticals for Positron Emission Tomography Based Dosimetry in Radiotherapy

Improvement of the accuracy of dosimetry in radionuclide therapy has the potential to increase patient safety and therapeutic outcomes. Although positron emission tomography (PET) is ideally suited for acquisition of dosimetric data because PET is inherently quantitative and offers high sensitivity and spatial resolution, it is not directly applicable for this purpose because common therapeutic radionuclides lack the necessary positron emission. This work reports on the synthesis of dual-nuclide labeled radiopharmaceuticals with therapeutic and PET functionality, which are based on common and widely available metal radionuclides. Dual-chelator conjugates, featuring interlinked cyclen- and triazacyclononane-based polyphosphinates DOTPI and TRAP, allow for strictly regioselective complexation of therapeutic (e.g., 177 Lu, 90 Y, or 213 Bi) and PET (e.g., 68 Ga) radiometals in the same molecular framework by exploiting the orthogonal metal ion selectivity of these chelators (DOTPI: large cations, such as lanthanide(III) ions; TRAP: small trivalent ions, such as GaIII ). Such DOTPI-TRAP conjugates were decorated with 3 Gly-urea-Lys (KuE) motifs for targeting prostate-specific membrane antigen (PSMA), employing Cu-catalyzed (CuAAC) as well as strain-promoted (SPAAC) click chemistry. These were labeled with 177 Lu or 213 Bi and 68 Ga and used for in vivo imaging of LNCaP (human prostate carcinoma) tumor xenografts in SCID mice by PET, thus proving practical applicability of the concept.

Cyclic versus Noncyclic Chelating Scaffold for 89Zr-Labeled ZEGFR:2377 Affibody Bioconjugates Targeting Epidermal Growth Factor Receptor Overexpression

Zirconium-89 is an emerging radionuclide for positron emission tomography (PET) especially for biomolecules with slow pharmacokinetics as due to its longer half-life, in comparison to fluorine-18 and gallium-68, imaging at late time points is feasible. Desferrioxamine B (DFO), a linear bifunctional chelator (BFC) is mostly used for this radionuclide so far but shows limitations regarding stability. Our group recently reported on fusarinine C (FSC) with similar zirconium-89 complexing properties but potentially higher stability related to its cyclic structure. This study was designed to compare FSC and DFO head-to-head as bifunctional chelators for ⁸⁹Zr-radiolabeled EGFR-targeting ZEGFR:2377 affibody bioconjugates. FSC-ZEGFR:2377 and DFO-ZEGFR:2377 were evaluated regarding radiolabeling, in vitro stability, specificity, cell uptake, receptor affinity, biodistribution, and microPET-CT imaging. Both conjugates were efficiently labeled with zirconium-89 at room temperature but radiochemical yields increased substantially at elevated temperature, 85 °C. Both ⁸⁹Zr-FSC-ZEGFR:2377 and ⁸⁹Zr-DFO-ZEGFR:2377 revealed remarkable specificity, affinity and slow cell-line dependent internalization. Radiolabeling at 85 °C showed comparable results in A431 tumor xenografted mice with minor differences regarding blood clearance, tumor and liver uptake. In comparison ⁸⁹Zr-DFO-ZEGFR:2377, radiolabeled at room temperature, showed a significant difference regarding tumor-to-organ ratios. MicroPET-CT imaging studies of ⁸⁹Zr-FSC-ZEGFR:2377 as well as ⁸⁹Zr-DFO-ZEGFR:2377 confirmed these findings. In summary we were able to show that FSC is a suitable alternative to DFO for radiolabeling of biomolecules with zirconium-89. Furthermore, our findings indicate that ⁸⁹Zr-radiolabeling of DFO conjugates at higher temperature reduces off-chelate binding leading to significantly improved tumor-to-organ ratios and therefore enhancing image contrast.

Fluorescent Affibody Molecule Administered In Vivo at a Microdose Level Labels EGFR Expressing Glioma Tumor Regions

Purpose

Fluorescence guidance in surgical oncology provides the potential to realize enhanced molecular tumor contrast with dedicated targeted tracers, potentially with a microdose injection level. For most glioma tumors, the blood brain barrier is compromised allowing some exogenous drug/molecule delivery and accumulation for imaging. The aberrant overexpression and/or activation of epidermal growth factor receptor (EGFR) is associated with many types of cancers, including glioblastoma, and so the use of a near-infrared (NIR) fluorescent molecule targeted to the EGFR receptor provides the potential for improving tumor contrast during surgery. Fluorescently labeled affibody molecule (ABY-029) has high EGFR affinity and high potential specificity with reasonably fast plasma clearance. In this study, ABY-29 was evaluated in glioma versus normal brain uptake from intravenous injection at a range of doses, down to a microdose injection level.

Procedure

Nude rats were inoculated with the U251 human glioma cell line in the brain. Tumors were allowed to grow for 3–4 weeks. ABY-029 fluorescence ex vivo imaging of brain slices was acquired at different time points (1–48 h) and varying injection doses from 25 to 122 μg/kg (from human protein microdose equivalent to five times microdose levels).

Results

The tumor was most clearly visualized at 1-h post-injection with 8- to 16-fold average contrast relative to normal brain. However, the tumor still could be identified after 48 h. In all cases, the ABY-029 fluorescence appeared to localize preferentially in EGFR-positive regions. Increasing the injected dose from a microdose level to five times, a microdose level increased the signal by 10-fold, and the contrast was from 8 to 16, showing that there was value in doses slightly higher than the microdose restriction. Normal tissue uptake was found to be affected by the tumor size, indicating that edema was a likely factor affecting the expected tumor to normal tissue contrast.

Conclusion

These results suggest that the NIR-labeled affibody molecules provide an excellent potential to increase surgical visualization of EGFR-positive tumor regions.

Evaluation of a radiocobalt-labelled affibody molecule for imaging of human epidermal growth factor receptor 3 expression

The human epidermal growth factor receptor 3 (HER3) is involved in the development of cancer resistance towards tyrosine kinase-targeted therapies. Several HER3‑targeting therapeutics are currently under clinical evaluation. Non-invasive imaging of HER3 expression could improve patient management. Affibody molecules are small engineered scaffold proteins demonstrating superior properties as targeting probes for molecular imaging compared with monoclonal antibodies. Feasibility of in vivo HER3 imaging using affibody molecules has been previously demonstrated. Preclinical studies have shown that the contrast when imaging using anti-HER3 affibody molecules can be improved over time. We aim to develop an agent for PET imaging of HER3 expression using the long-lived positron-emitting radionuclide cobalt-55 (55Co) (T1/2=17.5 h). A long-lived cobalt isotope 57Co was used as a surrogate for 55Co in this study. The anti-HER3 affibody molecule HEHEHE-ZHER3-NOTA was labelled with radiocobalt with high yield, purity and stability. Biodistribution of 57Co-HEHEHE-ZHER3-NOTA was measured in mice bearing DU145 (prostate carcinoma) and LS174T (colorectal carcinoma) xenografts at 3 and 24 h post injection (p.i.). Tumour-to-blood ratios significantly increased between 3 and 24 h p.i. (p<0.05). At 24 h p.i., tumour-to-blood ratios were 6 for DU145 and 8 for LS174T xenografts, respectively. HER3‑expressing xenografts were clearly visualized in a preclinical imaging setting already 3 h p.i., and contrast further improved at 24 h p.i. In conclusion, the radiocobalt-labelled anti-HER3 affibody molecule, HEHEHE-ZHER3-NOTA, is a promising tracer for imaging of HER3 expression in tumours.

Affibody molecules for molecular imaging and targeted drug delivery in the management of breast cancer

Breast cancer is one of the leading reasons for the morbidity and mortality of cancer related death globally. The modern therapies are basically the combination of the breast-preserving surgeries or ablation with or without node biopsy or destroying the carcinoma cells adjuvant with chemotherapy, radiotherapy, hormonal or biological therapies depending upon the nature of the receptor of the cancerous cells, nature of the lymph node, as well as the tendency of the recurrence. For decade's carcinoma management suffered by the limitation of imagining, targeting and penetrability problem associated with management and cure of this deadly disease leads to unwanted chemo-toxicity and side effects. Alike other antibody mimetics, affibodies are designed with the combinatorial protein engineering approaches which are small and robust protein scaffolds retaining the favorable folding and stability. Affibody is one of the significantly important tools for imaging and diagnosis of the affinity specific over expressed proteins in the breast cancer management. The review summarizes the various affibody strategies uses in the management of breast cancer.

Facilitating in vivo tumor localization by principal component analysis based on dynamic fluorescence molecular imaging

Fluorescence molecular imaging has been used to target tumors in mice with xenograft tumors. However, tumor imaging is largely distorted by the aggregation of fluorescent probes in the liver. A principal component analysis (PCA)-based strategy was applied on the in vivo dynamic fluorescence imaging results of three mice with xenograft tumors to facilitate tumor imaging, with the help of a tumor-specific fluorescent probe. Tumor-relevant features were extracted from the original images by PCA and represented by the principal component (PC) maps. The second principal component (PC2) map represented the tumor-related features, and the first principal component (PC1) map retained the original pharmacokinetic profiles, especially of the liver. The distribution patterns of the PC2 map of the tumor-bearing mice were in good agreement with the actual tumor location. The tumor-to-liver ratio and contrast-to-noise ratio were significantly higher on the PC2 map than on the original images, thus distinguishing the tumor from its nearby fluorescence noise of liver. The results suggest that the PC2 map could serve as a bioimaging marker to facilitate in vivo tumor localization, and dynamic fluorescence molecular imaging with PCA could be a valuable tool for future studies of in vivo tumor metabolism and progression.

The use of radiocobalt as a label improves imaging of EGFR using DOTA-conjugated Affibody molecule

Several anti-cancer therapies target the epidermal growth factor receptor (EGFR). Radionuclide imaging of EGFR expression in tumours may aid in selection of optimal cancer therapy. The ¹¹¹In-labelled DOTA-conjugated Z_EGFR:2377 Affibody molecule was successfully used for imaging of EGFR-expressing xenografts in mice. An optimal combination of radionuclide, chelator and targeting protein may further improve the contrast of radionuclide imaging. The aim of this study was to evaluate the targeting properties of radiocobalt-labelled DOTA-Z_EGFR:2377. DOTA-Z_EGFR:2377 was labelled with ⁵⁷Co (T_1/2 = 271.8 d), ⁵⁵Co (T_1/2 = 17.5 h), and, for comparison, with the positron-emitting radionuclide ⁶⁸Ga (T_1/2 = 67.6 min) with preserved specificity of binding to EGFR-expressing A431 cells. The long-lived cobalt radioisotope ⁵⁷Co was used in animal studies. Both ⁵⁷Co-DOTA-Z_EGFR:2377 and ⁶⁸Ga-DOTA-Z_EGFR:2377 demonstrated EGFR-specific accumulation in A431 xenografts and EGFR-expressing tissues in mice. Tumour-to-organ ratios for the radiocobalt-labelled DOTA-Z_EGFR:2377 were significantly higher than for the gallium-labelled counterpart already at 3 h after injection. Importantly, ⁵⁷Co-DOTA-Z_EGFR:2377 demonstrated a tumour-to-liver ratio of 3, which is 7-fold higher than the tumour-to-liver ratio for ⁶⁸Ga-DOTA-Z_EGFR:2377. The results of this study suggest that the positron-emitting cobalt isotope ⁵⁵Co would be an optimal label for DOTA-Z_EGFR:2377 and further development should concentrate on this radionuclide as a label.

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.

PET imaging of zirconium-89 labelled cetuximab: A phase I trial in patients with head and neck and lung cancer

BACKGROUND AND PURPOSE

PET imaging of cetuximab uptake may help selecting cancer patients with the highest chance of benefit. The aim of this phase I trial was to determine the safety of the tracer ⁸⁹Zr-cetuximab and to assess tumour uptake.

METHODS

Two dose schedules were used; two consecutive doses of 60MBq ⁸⁹Zr-cetuximab or a single dose of 120MBq, both preceded by 400mg/m² of unlabelled cetuximab. Toxicity (CTCAE 3.0) was scored twice weekly. PET-CT scans were acquired on days 4, 5 and 6 (step 1) or 5, 6, 7 (step 2). Because tumour uptake could not be assessed satisfactorily, a third step was added including EGFR overexpressing tumours.

RESULTS

Nine patients were included (6 NSCLC; 3 HNC). No additional toxicity was associated with administration of ⁸⁹Zr-cetuximab compared to standard cetuximab. A tumour to blood ratio (TBR)>1 was observed in all but one patient, with a maximum of 4.56. TBR was not different between dose schedules. There was a trend for higher TBR at intervals>5days after injection.

CONCLUSIONS

Both presented ⁸⁹Zr-cetuximab administration schedules are safe. The recommended dose for future trials is 60MBq, with a minimum time interval for scanning of 6days.

Simultaneous In Vivo Fluorescent Markers for Perfusion, Protoporphyrin Metabolism, and EGFR Expression for Optically Guided Identification of Orthotopic Glioma

Purpose: While extent of tumor resection is an important predictor of outcome in glioma, margin delineation remains challenging due to lack of inherent contrast between tumor and normal parenchyma. Fluorescence-guided surgery is promising for its ability to enhance contrast through exogenous fluorophores; however, the specificity and sensitivity of the underlying contrast mechanism and tumor delivery and uptake vary widely across approved and emerging agents.Experimental Design: Rats with orthotopic F98 wild-type and F98 EGFR-positive (EGFR⁺) gliomas received in vivo administration of IRDye680RD, 5-aminioleuvulinic acid, and ABY-029-markers of perfusion, protoporphyrin metabolism, and EGFR expression, respectively. Ex vivo imaging demonstrates the contrast mechanism-dependent spatial heterogeneity and enables within-animal comparisons of tumor-to-background ratio (TBR).Results: Generally, ABY-029 outperformed PpIX in F98_EGFR orthotopic tumor margins and core (50% and 60% higher TBR, respectively). PpIX outperformed ABY-029 in F98_wt margins by 60% but provided equivalent contrast in the bulk tumor. IRDye680RD provided little contrast, having an average TBR of 1.7 ± 0.2. The unique spatial patterns of each agent were combined into a single metric, the multimechanistic fluorescence-contrast index (MFCI). ABY-029 performed best in EGFR⁺ tumors (91% accuracy), while PpIX performed best in wild-type tumors (87% accuracy). Across all groups, ABY-029 and PpIX performed similarly (80% and 84%, respectively) but MFCI was 91% accurate, supporting multiagent imaging when tumor genotype was unknown.Conclusions: Human use of ABY-029 for glioma resection should enhance excision of EGFR⁺ tumors and could be incorporated into current PpIX strategies to further enhance treatment in the general glioma case. Clin Cancer Res; 23(9); 2203-12. ©2016 AACR.

64Cu-Labeled Gp2 Domain for PET Imaging of Epidermal Growth Factor Receptor

This purpose of this study is to determine the efficacy of a 45-amino acid Gp2 domain, engineered to bind to epidermal growth factor receptor (EGFR), as a positron emission tomography (PET) probe of EGFR in a xenograft mouse model. The EGFR-targeted Gp2 (Gp2-EGFR) and a nonbinding control were site-specifically labeled with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator. Binding affinity was tested toward human EGFR and mouse EGFR. Biological activity on downstream EGFR signaling was examined in cell culture. DOTA-Gp2 molecules were labeled with ⁶⁴Cu and intravenously injected (0.6-2.3 MBq) into mice bearing EGFR^high (n = 7) and EGFR^low (n = 4) xenografted tumors. PET/computed tomography (CT) images were acquired at 45 min, 2 h, and 24 h. Dynamic PET (25 min) was also acquired. Tomography results were verified with gamma counting of resected tissues. Two-tailed t tests with unequal variances provided statistical comparison. DOTA-Gp2-EGFR bound strongly to human (K_D = 7 ± 5 nM) and murine (K_D = 29 ± 6 nM) EGFR, and nontargeted Gp2 had no detectable binding. Gp2-EGFR did not agonize EGFR nor antagonize EGF-EGFR. ⁶⁴Cu-Gp2-EGFR tracer effectively localized to EGFR^high tumors at 45 min (3.2 ± 0.5%ID/g). High specificity was observed with significantly lower uptake in EGFR^low tumors (0.9 ± 0.3%ID/g, p < 0.001), high tumor-to-background ratios (11 ± 6 tumor/muscle, p < 0.001). Nontargeted Gp2 tracer had low uptake in EGFR^high tumors (0.5 ± 0.3%ID/g, p < 0.001). Similar data was observed at 2 h, and tumor signal was retained at 24 h (2.9 ± 0.3%ID/g). An engineered Gp2 PET imaging probe exhibited low background and target-specific EGFR^high tumor uptake at 45 min, with tumor signal retained at 24 h postinjection, and compared favorably with published EGFR PET probes for alternative protein scaffolds. These beneficial in vivo characteristics, combined with thermal stability, efficient evolution, and small size of the Gp2 domain validate its use as a future class of molecular imaging agents.

Feasibility of imaging of epidermal growth factor receptor expression with ZEGFR:2377 affibody molecule labeled with 99mTc using a peptide-based cysteine-containing chelator

The epidermal growth factor receptor (EGFR) is overexpressed in a number of malignant tumors and is a molecular target for several specific anticancer antibodies and tyrosine kinase inhibitors. The overexpression of EGFR is a predictive biomarker for response to several therapy regimens. Radionuclide molecular imaging might enable detection of EGFR overexpression by a non-invasive procedure and could be used repeatedly. Affibody molecules are engineered scaffold proteins, which could be selected to have a high affinity and selectivity to predetermined targets. The anti-EGFR ZEGFR:2377 affibody molecule is a potential imaging probe for EGFR detection. The use of the generator-produced radionuclide ^99mTc should facilitate clinical translation of an imaging probe due to its low price, availability and favorable dosimetry of the radionuclide. In the present study, we evaluated feasibility of ZEGFR:2377 labeling with ^99mTc using a peptide-based cysteine-containing chelator expressed at the C-terminus of ZEGFR:2377. The label was stable in vitro under cysteine challenge. In addition, ^99mTc-ZEGFR:2377 was capable of specific binding to EGFR-expressing cells with high affinity (274 pM). Studies in BALB/C nu/nu mice bearing A431 xenografts demonstrated that ^99mTc-ZEGFR:2377 accumulates in tumors in an EGFR-specific manner. The tumor uptake values were 3.6±1 and 2.5±0.4% ID/g at 3 and 24 h after injection, respectively. The corresponding tumor-to-blood ratios were 1.8±0.4 and 8±3. The xenografts were clearly visualized at both time-points. This study demonstrated the potential of ^99mTc-labeled ZEGFR:2377 for imaging of EGFR in vivo.

Recent progress in protein-protein interaction study for EGFR-targeted therapeutics

INTRODUCTION

Epidermal growth factor receptor (EGFR) expression is upregulated in many tumors and its aberrant signaling drives progression of many cancer types. Consequently, EGFR has become a clinically validated target as extracellular tumor marker for antibodies as well as for tyrosine kinase inhibitors. Within the last years, new mechanistic insights were uncovered and, based on clinical experience as well as progress in protein engineering, novel bio-therapeutic approaches were developed and tested.

AREAS COVERED

The potential therapeutic targeting arsenal in the fight against cancer now encompasses bispecific or biparatopic antibodies, DARPins, Adnectins, Affibodies, peptides and combinations of these binding molecules with viral- and nano-particles. We review past and recent binding proteins from the literature and include a brief description of the various targeting approaches. Special attention is given to the binding modes with the EGFR. Expert commentary: Clinical data from the three approved anti EGFR antibodies indicate that there is room for improved therapeutic efficacy. Having choices in size, affinity, avidity and the mode of EGFR binding as well as the possibility to combine various effector functions opens the possibility to rationally design more effective therapeutics.

Preclinical PET imaging of EGFR levels: pairing a targeting with a non-targeting Sel-tagged Affibody-based tracer to estimate the specific uptake

Background

Though overexpression of epidermal growth factor receptor (EGFR) in several forms of cancer is considered to be an important prognostic biomarker related to poor prognosis, clear correlations between biomarker assays and patient management have been difficult to establish. Here, we utilize a targeting directly followed by a non-targeting tracer-based positron emission tomography (PET) method to examine some of the aspects of determining specific EGFR binding in tumors.

Methods

The EGFR-binding Affibody molecule Z_EGFR:2377 and its size-matched non-binding control Z_Taq:3638 were recombinantly fused with a C-terminal selenocysteine-containing Sel-tag (Z_EGFR:2377-ST and Z_Taq:3638-ST). The proteins were site-specifically labeled with DyLight488 for flow cytometry and ex vivo tissue analyses or with ¹¹C for in vivo PET studies. Kinetic scans with the ¹¹C-labeled proteins were performed in healthy mice and in mice bearing xenografts from human FaDu (squamous cell carcinoma) and A431 (epidermoid carcinoma) cell lines. Changes in tracer uptake in A431 xenografts over time were also monitored, followed by ex vivo proximity ligation assays (PLA) of EGFR expressions.

Results

Flow cytometry and ex vivo tissue analyses confirmed EGFR targeting by Z_EGFR:2377-ST-DyLight488. [Methyl-¹¹C]-labeled Z_EGFR:2377-ST-CH₃ and Z_Taq:3638-ST-CH₃ showed similar distributions in vivo, except for notably higher concentrations of the former in particularly the liver and the blood. [Methyl-¹¹C]-Z_EGFR:2377-ST-CH₃ successfully visualized FaDu and A431 xenografts with moderate and high EGFR expression levels, respectively. However, in FaDu tumors, the non-specific uptake was large and sometimes equally large, illustrating the importance of proper controls. In the A431 group observed longitudinally, non-specific uptake remained at same level over the observation period. Specific uptake increased with tumor size, but changes varied widely over time in individual tumors. Total (membranous and cytoplasmic) EGFR in excised sections increased with tumor growth. There was no positive correlation between total EGFR and specific tracer uptake, which, since Z_EGFR:2377 binds extracellularly and is slowly internalized, indicates a discordance between available membranous and total EGFR expression levels.

Conclusions

Same-day in vivo dual tracer imaging enabled by the Sel-tag technology and ¹¹C-labeling provides a method to non-invasively monitor membrane-localized EGFR as well as factors affecting non-specific uptake of the PET ligand.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0213-8) contains supplementary material, which is available to authorized users.

Purification-Free Method for Preparing Technetium-99m-Labeled Multivalent Probes for Enhanced in Vivo Imaging of Saturable Systems

Metallic radionuclides provide target-specific radiolabeled probes for molecular imaging in radiochemical yields sufficient for administration to subjects without purification. However, unlabeled ligands in the injectate can compete for targeted molecules with radiolabeled probes, which eventually necessitates postlabeling purification. Herein we describe a "1 to 3" design to circumvent the issue by taking advantage of inherent coordination properties of technetium-99m ((99m)Tc). A monovalent RGD ligand possessing an isonitrile as a coordinating moiety (CN-RGD) was reacted with [(99m)Tc(CO)3(OH2)3](+) to prepare [(99m)Tc(CO)3(CN-RGD)3](+) in over 95% radiochemical yields. This complex exhibited higher integrin αvβ3 binding affinity than its unlabeled monovalent ligand, primarily due to its multivalency. This compound visualized a murine tumor without removing unlabeled ligands, while a (99m)Tc-labeled monovalent probe derived from a monovalent ligand could not. The metal coordination-mediated synthesis of radiolabeled multivalent probes thereby can be a useful approach for preparing ready-to-use target-specific probes labeled with (99m)Tc and other metallic radionuclides of interest.

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.

Feasibility of Affibody-Based Bioorthogonal Chemistry-Mediated Radionuclide Pretargeting

Affibody molecules constitute a new class of probes for radionuclide tumor targeting. The small size of Affibody molecules is favorable for rapid localization in tumors and clearance from circulation. However, high renal reabsorption of Affibody molecules prevents the use of residualizing radiometals, including several promising low-energy β- and α-emitters, for radionuclide therapy. We tested a hypothesis that Affibody-based pretargeting mediated by a bioorthogonal interaction between trans-cyclooctene (TCO) and tetrazine would provide higher accumulation of radiometals in tumor xenografts than in the kidneys.

METHODS

TCO was conjugated to the anti-human epidermal growth factor receptor 2 (HER2) Affibody molecule Z2395. DOTA-tetrazine was labeled with (111)In and (177)Lu. In vitro pretargeting was studied in HER2-expressing SKOV-3 and BT474 cell lines. In vivo studies were performed on BALB/C nu/nu mice bearing SKOV-3 xenografts.

RESULTS

(125)I-Z2395-TCO bound specifically to HER2-expressing cells in vitro with an affinity of 45 ± 16 pM. (111)In-tetrazine bound specifically and selectively to Z2395-TCO pretreated cells. In vivo studies demonstrated HER2-specific (125)I-Z2395-TCO accumulation in xenografts. TCO-mediated (111)In-tetrazine localization was shown in tumors, when the radiolabeled tracer was injected 4 h after an injection of Z2395-TCO. At 1 h after injection, the tumor uptake of (111)In-tetrazine and (177)Lu-tetrazine was approximately 2-fold higher than the renal uptake. Pretargeting provided more than a 56-fold reduction of renal uptake of (111)In in comparison with direct targeting.

CONCLUSION

The feasibility of Affibody-based bioorthogonal chemistry-mediated pretargeting was demonstrated. The use of pretargeting provides a substantial reduction of radiometal accumulation in kidneys, creating preconditions for palliative radionuclide therapy.

Affibody-mediated PET imaging of HER3 expression in malignant tumours

Human epidermal growth factor receptor 3 (HER3) is involved in the progression of various cancers and in resistance to therapies targeting the HER family. In vivo imaging of HER3 expression would enable patient stratification for anti-HER3 immunotherapy. Key challenges with HER3-targeting are the relatively low expression in HER3-positive tumours and HER3 expression in normal tissues. The use of positron-emission tomography (PET) provides advantages of high resolution, sensitivity and quantification accuracy compared to SPECT. Affibody molecules, imaging probes based on a non-immunoglobulin scaffold, provide high imaging contrast shortly after injection. The aim of this study was to evaluate feasibility of PET imaging of HER3 expression using (68)Ga-labeled affibody molecules. The anti-HER3 affibody molecule HEHEHE-Z08698-NOTA was successfully labelled with (68)Ga with high yield, purity and stability. The agent bound specifically to HER3-expressing cancer cells in vitro and in vivo. At 3 h pi, uptake of (68)Ga-HEHEHE-Z08698-NOTA was significantly higher in xenografts with high HER3 expression (BT474, BxPC-3) than in xenografts with low HER3 expression (A431). In xenografts with high expression, tumour-to-blood ratios were >20, tumour-to-muscle >15, and tumour-to-bone >7. HER3-positive xenografts were visualised using microPET 3 h pi. In conclusion, PET imaging of HER3 expression is feasible using (68)Ga-HEHEHE-Z08698-NOTA shortly after administration.

ADAPT, a Novel Scaffold Protein-Based Probe for Radionuclide Imaging of Molecular Targets That Are Expressed in Disseminated Cancers

Small engineered scaffold proteins have attracted attention as probes for radionuclide-based molecular imaging. One class of these imaging probes, termed ABD-Derived Affinity Proteins (ADAPT), has been created using the albumin-binding domain (ABD) of streptococcal protein G as a stable protein scaffold. In this study, we report the development of a clinical lead probe termed ADAPT6 that binds HER2, an oncoprotein overexpressed in many breast cancers that serves as a theranostic biomarker for several approved targeting therapies. Surface-exposed amino acids of ABD were randomized to create a combinatorial library enabling selection of high-affinity binders to various proteins. Furthermore, ABD was engineered to enable rapid purification, to eradicate its binding to albumin, and to enable rapid blood clearance. Incorporation of a unique cysteine allowed site-specific conjugation to a maleimido derivative of a DOTA chelator, enabling radionuclide labeling, ¹¹¹In for SPECT imaging and ⁶⁸Ga for PET imaging. Pharmacologic studies in mice demonstrated that the fully engineered molecule (111)In/⁶⁸Ga-DOTA-(HE)3-ADAPT6 was specifically bound and taken up by HER2-expressing tumors, with a high tumor-to-normal tissue ratio in xenograft models of human cancer. Unbound tracer underwent rapid renal clearance followed by high renal reabsorption. HER2-expressing xenografts were visualized by gamma-camera or PET at 1 hour after infusion. PET experiments demonstrated feasibility for discrimination of xenografts with high or low HER2 expression. Our results offer a preclinical proof of concept for the use of ADAPT probes for noninvasive in vivo imaging.

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.