Imaging of HER2-expressing tumours using a synthetic Affibody molecule containing the 99mTc-chelating mercaptoacetyl-glycyl-glycyl-glycyl (MAG3) sequence

Radiolabeled HER2-targeted molecular probes in breast cancer imaging: current knowledge and future prospective

Breast cancer is the most frequent non-dermatologic malignancy in women. Breast cancer is characterized by the expression of the human epidermal growth factor receptor type 2 (HER2), and the presence or lack of estrogen receptor (ER) and progesterone receptor (PR) expression. HER2 overexpression is reported in about 20 to 25% of breast cancer patients, which is usually linked to cancer progression, metastases, and poor survival. Immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) are the gold standards for determining HER2 status, even though IHC has largely focused on quantifying HER2+ status versus "other" HER2 status (including variants with low or no expression). Recent findings regarding the beneficial therapeutic effects of anti-HER2 monoclonal antibodies (mAb) in HER2low metastatic patients lead to changes in the classic definition of advanced breast cancer, and methods for precise assessment of HER2 status are being developed. As a result, various radiolabeled HER-targeted mAbs and antibody fragments have been designed to avoid repeated biopsies with potential bias due to tumor heterogeneity, including single-chain variable fragment (scFv), F(ab')2, affibody, and nanobody. These small targeting radiotracers displayed favorable biodistributions, clearance, and stability, allowing for higher image quality, shorter circulation half-life, and lower immunogenicity. This study aimed to comprehensively review the application of radiolabeled anti-HER2 antibody fragments in breast cancer in vivo imaging and provide a better understanding of targeted HER2 quantification.

Unlocking the power of affibody conjugated radioactive metallopharmaceuticals for targeted cancer diagnosis and therapy

Cancer is the second-largest death-causing disease after cardiovascular diseases. Effective research on cancer diagnosis and subsequent elimination plays a vital role in reducing the cancer-related death toll. Radiotherapy is one of the best strategies that could kill masses of solid tumour tissues; however, the efficacy is limited due to the bystander effect. This issue could be solved by the emergence of targeted delivery of radiometallic complexes, enabling clinicians to monitor the tumour regions and effectively destroy the tumour. Affibody® molecules are a class of synthetic peptides known as antibody mimics having the binding sites of an antibody. The specificity of affibodies is found to be greater than that of antibodies due to their small size. This review intends to highlight the recent developments in the field of affibody-targeted radiometallopharmaceuticals. These approaches could be essential for early cancer detection, tumour staging, and monitoring the response to therapy and could produce better therapeutic outcomes. In an attempt to provide ideas and inspiration for the researchers to design affibody-conjugated radiopharmaceuticals that are clinically applicable, we have provided an in-depth exploration of the various types of affibody-conjugated radiopharmaceuticals that are currently in clinical trials and various other pre-clinically tested conjugates in this article. Only a few review reports on affibody-conjugated radiometallopharmaceuticals, typically focusing on a specific molecular target or radionuclides reported. In this review, we provide a comprehensive overview of most radiometals, such as 111In, 68Ga, 64Cu, 55Co, 57Co, 44Sc, 99mTc, 89Zr, 90Y, 211At, 188Re, and 177Lu, choice of chelators, and potential cancer-associated molecular targets such HER2, EGFR or HER1, HER3, IGF-1R, PDGFRβ, VEGFR2, PD-L1, CAIX, PD-L1, neonatal Fc receptor (FcRn) and B7-H3. This approach highlights the advancements made over the past twenty years in affibody conjugates for radio imaging and therapy in oncology.

Affibody-based molecular probe 99mTc-(HE)3ZHER2:V2 for non-invasive HER2 detection in ovarian and breast cancer xenografts

Purpose

This study aimed to assess the biodistribution and bioactivity of the affibody molecular probe 99mTc-(HE)3ZHER2:V2, prepared by genetic recombination, and to investigate its potential for targeted human epidermal growth factor receptor 2 (HER2) imaging in SKOV3 ovarian cancer and MDA-MB-361 breast cancer xenografts.

Methods

Affibody molecules were generated through genetic recombination. The radiochemical purity of the 99mTc-labeled HER2 affibody was determined using reverse phase high performance liquid chromatography (RP-HPLC). Evaluation of HER2 affinity in SKOV3 ovarian cancer cells and MDA-MB-361 breast cancer cells (HER2-positive) was conducted by calculating equilibrium dissociation constants. Biodistribution of the 99mTc-labeled affibody molecular probe was assessed in Balb/c mice bearing SKOV3 tumors. Tumor targeting specificity was evaluated in Balb/c mice using SKOV3, MDA-MB-361, and AT-3 (HER2-negative) xenografts.

Results

Affibody (HE)3ZHER2:V2, generated through recombinant gene expression, was successfully labeled with 99mTc, achieving a radiochemical purity of (96.0 ± 1.7)% (n = 3) as determined by RP-HPLC. This molecular probe exhibited specific binding to HER2-positive SKOV3 cells, demonstrating intense radioactive uptake. Biodistribution analysis showed rapid accumulation of 99mTc-(HE)3ZHER2:V2 in HER2-positive tumors post-administration, primarily clearing through the urinary system. Single-photon emission computed tomography imaging conducted 1-3 h after intravenous injection of 99mTc-(HE)3ZHER2:V2 into HER2-positive SKOV3 and MDA-MB-361 nude mouse models confirmed targeted uptake of the molecular probe by the tumors.

Conclusions

The molecular probe 99mTc-(HE)3ZHER2:V2 developed in this study effectively targets HER2 for imaging HER2-positive SKOV3 and MDA-MB-361 xenografts in vivo. It exhibits rapid blood clearance without evident toxic effects, suggesting its potential as a valuable marker for detecting HER2 expression in tumor cells.

Influence of Molecular Design on the Tumor Targeting and Biodistribution of PSMA-Binding Tracers Labeled with Technetium-99m

Previously, we designed the EuK-based PSMA ligand BQ0413 with an maE3 chelator for labeling with technetium-99m. It showed efficient tumor targeting, but our preclinical data and preliminary clinical results indicated that the renal excretion levels need to be decreased. We hypothesized that this could be achieved by a decrease in the ligand's total negative charge, achieved by substituting negatively charged glutamate residues in the chelator with glycine. The purpose of this study was to evaluate the tumor targeting and biodistribution of two new PSMA inhibitors, BQ0411 and BQ0412, compared to BQ0413. Conjugates were radiolabeled with Tc-99m and characterized in vitro, using PC3-pip cells, and in vivo, using NMRI and PC3-pip tumor-bearing mice. [99mTc]Tc-BQ0411 and [99mTc]Tc-BQ0412 demonstrated PSMA-specific binding to PC3-pip cells with picomolar affinity. The biodistribution pattern for the new conjugates was characterized by rapid excretion. The tumor uptake for [99mTc]Tc-BQ0411 was 1.6-fold higher compared to [99mTc]Tc-BQ0412 and [99mTc]Tc-BQ0413. [99mTc]Tc-BQ0413 has demonstrated predominantly renal excretion, while the new conjugates underwent both renal and hepatobiliary excretion. In this study, we have demonstrated that in such small targeting ligands as PSMA-binding EuK-based pseudopeptides, the structural blocks that do not participate in binding could have a crucial role in tumor targeting and biodistribution. The presence of a glycine-based coupling linker in BQ0411 and BQ0413 seems to optimize biodistribution. In conclusion, the substitution of amino acids in the chelating sequence is a promising method to alter the biodistribution of [99mTc]Tc-labeled small-molecule PSMA inhibitors. Further improvement of the biodistribution properties of BQ0413 is needed.

Advances in the Application of Radionuclide-Labeled HER2 Affibody for the Diagnosis and Treatment of Ovarian Cancer

Human epidermal growth factor receptor 2 (HER2) is a highly expressed tumor marker in epithelial ovarian cancer, and its overexpression is considered to be a potential factor of poor prognosis. Therefore, monitoring the expression of HER2 receptor in tumor tissue provides favorable conditions for accurate localization, diagnosis, targeted therapy, and prognosis evaluation of cancer foci. Affibody has the advantages of high affinity, small molecular weight, and stable biochemical properties. The molecular probes of radionuclide-labeled HER2 affibody have recently shown broad application prospects in the diagnosis and treatment of ovarian cancer; the aim is to introduce radionuclides into the cancer foci, display systemic lesions, and kill tumor cells through the radioactivity of the radionuclides. This process seamlessly integrates the diagnosis and treatment of ovarian cancer. Current research and development of new molecular probes of radionuclide-labeled HER2 affibody should focus on overcoming the deficiencies of non-specific uptake in the kidney, bone marrow, liver, and gastrointestinal tract, and on reducing the background of the image to improve image quality. By modifying the amino acid sequence; changing the hydrophilicity, surface charge, and lipid solubility of the affibody molecule; and using different radionuclides, chelating agents, and labeling conditions to optimize the labeling method of molecular probes, the specific uptake of molecular probes at tumor sites will be improved, while reducing radioactive retention in non-target organs and obtaining the best target/non-target value. These measures will enable the clinical use of radionuclide-labeled HER2 affibody molecular probes as soon as possible, providing a new clinical path for tumor-specific diagnosis, targeted therapy, and efficacy evaluation. The purpose of this review is to describe the application of radionuclide-labeled HER2 affibody in the imaging and treatment of ovarian cancer, including its potential clinical value and dilemmas.

99mTc-labeled peptide targeting interleukin 13 receptor α 2 for tumor imaging in a cervical cancer mouse model

OBJECTIVE

Pep-1 (CGEMGWVRC) can potently bind to interleukin 13 receptor α 2 (IL-13Rα2), a tumor-restricted receptor found to be expressed in various malignancies. In this study, we intended to prepare a 99mTc-labeled probe and evaluate its in vivo tumor accumulation properties in a cervical cancer xenograft model.

METHODS

The Pep-1 was designed and radiolabeled with 99mTc by conjugation with mercaptoacetyl-triglycine (MAG3). The labeling yield, radiochemical purity and stability were characterized in vitro. Cell uptake assays and fluorescence imaging were conducted for qualitative and quantitative evaluation of the specificity and affinity of Pep-1. Flow cytometry and tissue immunofluorescence were used to confirm the IL-13Rα2 expression in cervical cancer. Biodistribution and in vivo imaging were performed periodically to evaluate the imaging value of 99mTc-MAG3-Pep-1 in cervical cancer xenograft model.

RESULTS

99mTc-MAG3-Pep-1 was successfully prepared with a high labeling yield and radiochemical purity (> 95%). Specific cell uptake was demonstrated by scramble control and unlabeled MAG3-Pep-1 blockade. Flow cytometry and tissue immunofluorescence also confirmed the mild IL-13Rα2 expression of HeLa. In the gamma imaging study and biodistribution, the tumors were imaged clearly at 2-6 h after injection of 99mTc-MAG3-Pep-1 and the accumulation of 99mTc-MAG3-Pep-1 in tumor was significantly higher than that in the blocking and scramble controls, demonstrating ligand-receptor binding specificity.

CONCLUSIONS

This work demonstrated that 99mTc-MAG3-Pep-1 can bind to cervical cancer with high affinity and specificity. MAG3-Pep-1 may be a prospective precursor for IL-13Rα2-expressing cancer therapy.

Arginine-Arginine-Leucine Peptide Targeting Heat Shock Protein 70 for Cancer Imaging

Arg-Arg-Leu (RRL) is a potent tumor-homing tripeptide. However, the binding target is unclear. In this study, we intended to identify the binding target of RRL and evaluate the tumor targeting of ^99mTc-MAG₃-RRL in vivo. Biotin-RRL, 5-TAMRA-RRL, and ^99mTc-MAG₃-RRL were designed to trace the binding target and tumor lesion. Immunoprecipitation-mass spectrometry was conducted to identify the candidate proteins and determination of the subcellular localization was also performed. A pull-down assay was performed to demonstrate the immunoprecipitate. Fluorescence colocalization and cell uptake assays were performed to elucidate the correlation between the selected binding protein and RRL, and the internalization mechanism of RRL. Biodistribution and in vivo imaging were performed to evaluate the tumor accumulation and targeting of ^99mTc-MAG₃-RRL. The target for RRL was screened to be heat shock protein 70 (HSP70). The prominent uptake distribution of RRL was concentrated in the membrane and cytoplasm. A pull-down assay demonstrated the existence of HSP70 in the biotin-RRL captured complex. Regarding fluorescence colocalization and cell uptake assays, RRL may interact with HSP70 at the nucleotide-binding domain (NBD). Clathrin-dependent endocytosis and macropinocytosis could be a vital internalization mechanism of RRL. In vivo imaging and biodistribution both demonstrated that ^99mTc-MAG₃-RRL can trace tumors with satisfactory accumulation in hepatoma xenograft mice. The radioactive signals accumulated in tumor lesions can be blocked by VER-155008, which can bind to the NBD of HSP70. Our findings revealed that RRL may interact with HSP70 and that ^99mTc-MAG₃-RRL could be a prospective probe for visualizing overexpressed HSP70 tumor sections.

Stability Enhancement of a Dimeric HER2-Specific Affibody Molecule through Sortase A-Catalyzed Head-to-Tail Cyclization

Natural backbone-cyclized proteins have an increased thermostability and resistance towards proteases, characteristics that have sparked interest in head-to-tail cyclization as a method to stability-enhance proteins used in diagnostics and therapeutic applications, for example. In this proof-of principle study, we have produced and investigated a head-to-tail cyclized and HER2-specific ZHER2:342 Affibody dimer. The sortase A-mediated cyclization reaction is highly efficient (>95%) under optimized conditions, and renders a cyclic ZHER3:342-dimer with an apparent melting temperature, Tm, of 68 °C, which is 3 °C higher than that of its linear counterpart. Circular dichroism spectra of the linear and cyclic dimers looked very similar in the far-UV range, both before and after thermal unfolding to 90 °C, which suggests that cyclization does not negatively impact the helicity or folding of the cyclic protein. The cyclic dimer had an apparent sub-nanomolar affinity (Kd ~750 pM) to the HER2-receptor, which is a ~150-fold reduction in affinity relative to the linear dimer (Kd ~5 pM), but the anti-HER2 Affibody dimer remained a high-affinity binder even after cyclization. No apparent difference in proteolytic stability was detected in an endopeptidase degradation assay for the cyclic and linear dimers. In contrast, in an exopeptidase degradation assay, the linear dimer was shown to be completely degraded after 5 min, while the cyclic dimer showed no detectable degradation even after 60 min. We further demonstrate that a site-specifically DyLight 594-labeled cyclic dimer shows specific binding to HER2-overexpressing cells. Taken together, the results presented here demonstrate that head-to-tail cyclization can be an effective strategy to increase the stability of an Affibody dimer.

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

PURPOSE

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

RESULTS

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

CONCLUSION

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

Preclinical Evaluation of 99mTc-Labeled GRPR Antagonists maSSS/SES-PEG2-RM26 for Imaging of Prostate Cancer

BACKGROUND

Gastrin-releasing peptide receptor (GRPR) is an important target for imaging of prostate cancer. The wide availability of single-photon emission computed tomography/computed tomography (SPECT/CT) and the generator-produced ^99mTc can be utilized to facilitate the use of GRPR-targeting radiotracers for diagnostics of prostate cancers.

METHODS

Synthetically produced mercaptoacetyl-Ser-Ser-Ser (maSSS)-PEG₂-RM26 and mercaptoacetyl-Ser-Glu-Ser (maSES)-PEG₂-RM26 (RM26 = d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂) were radiolabeled with ^99mTc and characterized in vitro using PC-3 cells and in vivo, using NMRI or PC-3 tumor bearing mice. SPECT/CT imaging and dosimetry calculations were performed for [^99mTc]Tc-maSSS-PEG₂-RM26.

RESULTS

Peptides were radiolabeled with high yields (>98%), demonstrating GRPR specific binding and slow internalization in PC-3 cells. [^99mTc]Tc-maSSS-PEG₂-RM26 outperformed [^99mTc]Tc-maSES-PEG_2-RM26 in terms of GRPR affinity, with a lower dissociation constant (61 pM vs 849 pM) and demonstrating higher tumor uptake. [^99mTc]Tc-maSSS-PEG₂-RM26 had tumor-to-blood, tumor-to-muscle, and tumor-to-bone ratios of 97 ± 56, 188 ± 32, and 177 ± 79, respectively. SPECT/CT images of [^99mTc]Tc-maSSS-PEG₂-RM26 clearly visualized the GRPR-overexpressing tumors. The dosimetry estimated for [^99mTc]Tc-maSSS-PEG₂-RM26 showed the highest absorbed dose in the small intestine (1.65 × 10^-3 mGy/MBq), and the effective dose is 3.49 × 10^-3 mSv/MBq.

CONCLUSION

The GRPR antagonist maSSS-PEG₂-RM26 is a promising GRPR-targeting agent that can be radiolabeled through a single-step with the generator-produced ^99mTc and used for imaging of GRPR-expressing prostate cancer.

Improvement of Targeted Chemotherapy of HER2-positive Ovarian Malignant Cell Line by ZHER2-Idarubicin Conjugate: An in vitro Study

Background & Objective:

Overexpression of human epidermal growth factor receptor 2 (HER2) causes cell transformation and development of various types of malignancies. Idarubicin is an effective anti-neoplastic drug but its specific delivery to the targeted cells is still a great challenge. Affibody as a cost-effective peptide molecule with low molecular weight has a high affinity for HER2 receptors. Breast and ovarian cancers as wide speared types of malignancies are associated with high expression of HER2. In the current study, we assessed the cytotoxic effects of idarubicin-Z_HER2 affibody conjugate on the positive-HER2 cancer cell lines.

Methods:

The cytotoxic effects of constructed idarubicin-Z_HER2 affibody conjugate on the SK-BR-3, SK-OV-3, and MCF-7 cells with various levels of HER2 expression were evaluated by MTT assay following 48 hours of incubation.

Results:

Idarubicin showed a potent and dose-dependent cytotoxic effect against all treated cell lines while the SK-OV-3 cells were significantly more sensitive. The dimeric form of the Z_HER2 affibody molecule showed a mild effect on the cell viability of all treated cells at its optimum concentration. The constructed Idarubicin-Z_HER2 affibody conjugate decreased the viability of SK-OV-3 cells at its optimal concentration, more efficiently and specifically than other treated cells.

Conclusion:

The Z_HER2-affibody conjugate of idarubicin has a more specific cytotoxic effect compared with idarubicin alone against HER2-overexpressing ovarian cancerous cells. It appears the Z_HER2-affibody conjugate of idarubicin has great potential to be implicated as an innovative anti-cancer agent in future clinical trials in patients with HER2-overexpressing ovarian cancer.

Influence of Several Compounds and Drugs on the Renal Uptake of Radiolabeled Affibody Molecules

Affibody molecules are the most studied class of engineered scaffold proteins (ESPs) in radionuclide molecular imaging. Attempts to use affibody molecules directly labelled with radiometals for targeted radionuclide therapy were hampered by the high uptake and retention of radioactivity in kidneys. Several promising strategies have been implemented to circumvent this problem. Here, we investigated whether a pharmacological approach targeting different components of the reabsorption system could be used to lower the uptake of [^99mTc]Tc-Z_HER:2395 affibody molecule in kidneys. Pre-injection of probenecid, furosemide, mannitol or colchicine had no influence on activity uptake in kidneys compared to the control group. Mice pre-injected with maleate and fructose had 33% and 51% reduction in the kidney-associated activity, respectively, compared to the control group. Autoradiography images showed that the accumulation of activity after [^99mTc]Tc-Z_HER2:2395 injection was in the renal cortex and that both maleate and fructose could significantly reduce it. Results from this study demonstrate that pharmacological intervention with maleate and fructose was effective in reducing the kidney uptake of affibody molecules. A presumable mechanism is the disruption of ATP-mediated cellular uptake and endocytosis processes of affibody molecules by tubular cells.

On the prevention of kidney uptake of radiolabeled DARPins

Background

Designed ankyrin repeat proteins (DARPins) are small engineered scaffold proteins (14–18 kDa) that demonstrated promising tumor-targeting properties in preclinical studies. However, high renal accumulation of activity for DARPins labeled with residualizing labels is a limitation for targeted radionuclide therapy. A better understanding of the mechanisms behind the kidney uptake of DARPins could aid the development of strategies to reduce it. In this study, we have investigated whether the renal uptake of [^99mTc]Tc(CO)₃-G3 DARPin could be reduced by administration of compounds that act on various parts of the reabsorption system in the kidney.

Results

Co-injection of lysine or Gelofusine was not effective for the reduction of kidney uptake of [^99mTc]Tc(CO)₃-G3. Administration of sodium maleate before the injection of [^99mTc]Tc(CO)₃-G3 reduced the kidney-associated activity by 60.4 ± 10.3%, while administration of fructose reduced it by 46.9 ± 7.6% compared with the control. The decrease in the kidney uptake provided by sodium maleate was also observed for [^99mTc]Tc(CO)₃-9_29 DARPin. Preinjection of colchicine, probenecid, mannitol, or furosemide had no effect on the kidney uptake of [^99mTc]Tc(CO)₃-G3. Kidney autoradiography showed mainly cortical accumulation of activity for all studied groups.

Conclusion

Common clinical strategies were not effective for the reduction of kidney uptake of [^99mTc]Tc(CO)₃-G3. Both fructose and maleate lower the cellular ATP level in the proximal tubule cells and their reduction of the kidney reuptake indicates the involvement of an ATP-driven uptake mechanism. The decrease provided by maleate for both G3 and 9_29 DARPins indicates that their uptake proceeds through a mechanism independent of DARPin structure and binding site composition.

Comparison of tumor‑targeting properties of directly and indirectly radioiodinated designed ankyrin repeat protein (DARPin) G3 variants for molecular imaging of HER2

Evaluation of human epidermal growth factor receptor 2 (HER2) expression levels in breast and gastroesophageal cancer is used for the stratification of patients for HER2‑targeting therapies. The use of radionuclide molecular imaging may facilitate such evaluation in a non‑invasive way. Designed ankyrin repeat proteins (DARPins) are engineered scaffold proteins with high potential as probes for radionuclide molecular imaging. DARPin G3 binds with high affinity to HER2 and may be used to visualize this important therapeutic target. Studies on other engineered scaffold proteins have demonstrated that selection of the optimal labeling approach improves the sensitivity and specificity of radionuclide imaging. The present study compared two methods of labeling G3, direct and indirect radioiodination, to select an approach providing the best imaging contrast. G3‑H6 was labeled with iodine‑124, iodine‑125 and iodine‑131 using a direct method. A novel construct bearing a C‑terminal cysteine, G3‑GGGC, was site‑specifically labeled using [125I]I‑iodo‑[(4‑hydroxyphenyl)ethyl]maleimide (HPEM). The two radiolabeled G3 variants preserved binding specificity and high affinity to HER2‑expressing cells. The specificity of tumor targeting in vivo was demonstrated. Biodistribution comparison of [131I]I‑G3‑H6 and [125I]I‑HPEM‑G3‑GGGC in mice, bearing HER2‑expressing SKOV3 xenografts, demonstrated an appreciable contribution of hepatobiliary excretion to the clearance of [125I]I‑HPEM‑G3‑GGGC and a decreased tumor uptake compared to [131I]I‑G3‑H6. The direct label provided higher tumor‑to‑blood and tumor‑to‑organ ratios compared with the indirect label at 4 h post‑injection. The feasibility of high contrast PET/CT imaging of HER2 expression in SKOV3 xenografts in mice using [124I]I‑G3‑H6 was demonstrated. In conclusion, direct radioiodination is the preferable approach for labeling DARPin G3 with iodine‑123 and iodine‑124 for clinical single photon emission computed tomography and positron emission tomography imaging.

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.

Preclinical evaluation of 99mTc direct labeling ZHER2:V2 for HER2 positive tumors imaging

The present study aimed to label Z_HER2:V2 with technetium-99m (^99mTc) using a simple method and to evaluate its clinical potential as a diagnostic probe for human epidermal growth factor receptor type 2 (HER2)-positive tumors. The ZHER2:V2 (Affibody molecule of ZHER2:2395-C, which is based on the ZHER2:342 binding sequence with C-terminal engineered cysteine) with C-terminal chelating sequence GGGC was designed and labeled with 99mTc. The 99mTc-ZHER2:V2 labeling efficiency was analyzed. The cellular uptake, retention and binding affinity, and the stability of the probe were examined in vitro. 99mTc-ZHER2:V2 biodistribution analysis and imaging were performed in BALB/c nude mice bearing SKOV3 (HER2-overexpression) xenografts. Furthermore, imaging of the probe was performed in MCF-7 (HER2 low-expression) xenografts. The 99mTc-ZHER2:V2 labeling efficiency was identified as 98.99±0.99% (n=6), and was stable in physiological saline and fresh human serum at 37°C in vitro. The cellular uptake peak of SKOV3 cells at 24 h was 6.15±0.18%, the cellular retention ratio of the probe was 48.58±4.52% at 6 h following interrupted incubation, and ~70% of 99mTc-ZHER2:V2 was membrane bound following 24 h. 99mTc-ZHER2:V2 was blocked by excess amounts of unlabeled ZHER2:V2 in SKOV3 cells. 99mTc-ZHER2:V2 exhibited high distribution (10.07% ID/g) in SKOV3 ×enografts at 6 h following injection. The single photon emission computed tomography (SPECT) imaging revealed clear localization of 99mTc-ZHER2:V2 in the SKOV3 ×enografts at 4 h. However, there was low uptake in MCF-7 tumors on the SPECT images. The SKOV3 ×enograft imaging could be blocked by excess amounts unlabelled ZHER2:V2. 99mTc-ZHER2:V2 is an easy and quick labeling method, with high labeling yields, and radiochemical purity. 99mTc-ZHER2:V2 is a promising probe for the diagnosis of HER2-overexpression tumors and the monitoring of therapy response.

Influence of protein properties and protein modification on biodistribution and tumor uptake of anticancer antibodies, antibody derivatives, and non-Ig scaffolds

Newly developed protein drugs that target tumor-associated antigens are often modified in order to increase their therapeutic effect, tumor exposure, and safety profile. During the development of protein drugs, molecular imaging is increasingly used to provide additional information on their in vivo behavior. As a result, there are increasing numbers of studies that demonstrate the effect of protein modification on whole body distribution and tumor uptake of protein drugs. However, much still remains unclear about how to interpret obtained biodistribution data correctly. Consequently, there is a need for more insight in the correct way of interpreting preclinical and clinical imaging data. Summarizing the knowledge gained to date may facilitate this interpretation. This review therefore provides an overview of specific protein properties and modifications that can affect biodistribution and tumor uptake of anticancer antibodies, antibody fragments, and nonimmunoglobulin scaffolds. Protein properties that are discussed in this review are molecular size, target interaction, FcRn binding, and charge. Protein modifications that are discussed are radiolabeling, fluorescent labeling drug conjugation, glycosylation, humanization, albumin binding, and polyethylene glycolation.

Preparation and Identification of HER2 Radioactive Ligands and Imaging Study of Breast Cancer-Bearing Nude Mice

OBJECTIVE: A micro-molecule peptide TP1623 of ^99mTc-human epithelial growth factor receptor 2 (HER2) was prepared and the feasibility of using it as a HER2-positive molecular imaging agent for breast cancer was evaluated. METHODS: TP1623 was chemically synthesized and labeled with ^99mTc. The labeling ratio and stability were detected. HER2 expression levels of breast cancer cells (SKBR3 and MDA-MB-231) and cell binding activity were measured. Biodistribution of ^99mTC-TP1623 in normal mice was detected. SKBR3/MDA-MB-231-bearing nude mice models with high/low expressions of HER2 were established. Tumor tissues were stained with hematoxylin–eosin (HE) and measured by immunohistochemistry to confirm the formation of tumors and HER2 expression. SPECT imaging was conducted for HER2-overexpressing SKBR3-bearing nude mice. The T/NT ratio was calculated and compared with that of MDA-MB-231-bearing nude mice with low HER2 expression. The competitive inhibition image was used to discuss the specific binding of ^99mTc- TP1623 and the tumor. RESULTS: The labeling ratio of ^99mTc-TP1623, specific activity, and radiochemical purity (RCP) after 6 h at room temperature were (97.39 ± 0.23)%, (24.61 ± 0.06) TBq/mmol, and (93.25 ± 0.06)%, respectively. HER2 of SKBR3 and MDA-MB-231 cells showed high and low expression levels by immunohistochemistry, respectively. The in vitro receptor assays indicated that specific binding of TP1623 and HER2 was retained. Radioactivity in the brain was always at the lowest level, while the clearance rate of blood and the excretion rate of the kidneys were fast. HE staining showed that tumor cells were observed in SKBR3- and MDA-MB-231-bearing nude mice, with significant heteromorphism and increased mitotic count. The imaging of mice showed that targeted images could be made of ^99mTc-TP1623 in high HER2-expressing tumors, while no obvious development was shown in tumors in low HER2-expressing nude mice. No development was visible in tumors in competitive inhibition of imaging, which indicates the combination of ^99mTc-TP1623 and tumor was mediated by HER2. CONCLUSION: High labeling ratio and specific activity of ^99mTc-TP1623 is successfully prepared; it is a molecular imaging agent for HER2-positive tumors that has potential applicative value.

99mTc-radiolabeled GE11-modified peptide for ovarian tumor targeting

Background

Ovarian cancer is a serious threat for women health and the early diagnosis of this cancer might improves the survival rate of patients. The use of the targeted radiopharmaceuticals could be a non-invasive and logical method for tumor imaging. The aim of this study was to radiolabel GE11 peptide as a new specific probe for imaging of ovarian tumor.

Methods

HYNIC-SSS-GE11 peptide was labeled with ^99mTc using tricine as a coligand. The ^99mTc-tricine-HYNIC-SSS-GE11 peptide was evaluated for specific cellular binding in three cell lines with different levels of EGFR expression. Tumor targeting was assessed in SKOV3 tumor bearing mice.

Results

By using tricine as a coligand, labeling yield was more than 98% and the stability of the radiolabelled peptide in human serum up to 4 h was 96%. The in vitro cell uptake test showed that this radiolabeled peptide had a good affinity to SKOV3 cells with dissociation constant of 73 nM. The in vivo results showed a tumor/muscle ratio of 3.2 at 4 h following injection of ^99mTc-tricine-HYNIC-SSS-GE11 peptide.

Conclusions

Results of this study showed that ^99mTc-tricine-HYNIC-SSS-GE11 peptide could be a promising tool for diagnosis and staging of ovarian cancer.

Graphical Abstract

^99mTc-tricine-HYNIC-SSS-GE11, a novl targeted agent for ovarian tumor imaging

Increasing the Net Negative Charge by Replacement of DOTA Chelator with DOTAGA Improves the Biodistribution of Radiolabeled Second-Generation Synthetic Affibody Molecules

A promising strategy to enable patient stratification for targeted therapies is to monitor the target expression in a tumor by radionuclide molecular imaging. Affibody molecules (7 kDa) are nonimmunoglobulin scaffold proteins with a 25-fold smaller size than intact antibodies. They have shown an apparent potential as molecular imaging probes both in preclinical and clinical studies. Earlier, we found that hepatic uptake can be reduced by the incorporation of negatively charged purification tags at the N-terminus of Affibody molecules. We hypothesized that liver uptake might similarly be reduced by positioning the chelator at the N-terminus, where the chelator-radionuclide complex will provide negative charges. To test this hypothesis, a second generation synthetic anti-HER2 ZHER2:2891 Affibody molecule was synthesized and labeled with (111)In and (68)Ga using DOTAGA and DOTA chelators. The chelators were manually coupled to the N-terminus of ZHER2:2891 forming an amide bond. Labeling DOTAGA-ZHER2:2891 and DOTA-ZHER2:2891 with (68)Ga and (111)In resulted in stable radioconjugates. The tumor-targeting and biodistribution properties of the (111)In- and (68)Ga-labeled conjugates were compared in SKOV-3 tumor-bearing nude mice at 2 h postinjection. The HER2-specific binding of the radioconjugates was verified both in vitro and in vivo. Using the DOTAGA chelator gave significantly lower radioactivity in liver and blood for both radionuclides. The (111)In-labeled conjugates showed more rapid blood clearance than the (68)Ga-labeled conjugates. The most pronounced influence of the chelators was found when they were labeled with (68)Ga. The DOTAGA chelator gave significantly higher tumor-to-blood (61 ± 6 vs 23 ± 5, p < 0.05) and tumor-to-liver (10.4 ± 0.6 vs 4.5 ± 0.5, p < 0.05) ratios than the DOTA chelator. This study demonstrated that chelators may be used to alter the uptake of Affibody molecules, and most likely other scaffold-based imaging probes, for improvement of imaging contrast.

Monitoring therapeutic response of human ovarian cancer to trastuzumab by SPECT imaging with (99m)Tc-peptide-Z(HER2:342)

INTRODUCTION

Patients with human epidermal growth factor receptor 2 (HER2)-positive cancer are candidates for treatment with the anti-HER2 antibody trastuzumab. How to systemically assess tumor HER2 expression and identifying appropriate use of anti-HER2 therapies by noninvasive imaging in vivo is an urgent issue. The purpose of this study was to evaluate SPECT imaging of (99m)Tc-Gly-(D)Ala-Gly-Gly-Z(HER2:342) ((99m)Tc-peptide-Z(HER2:342)) for monitoring therapeutic response to trastuzumab in nude mice bearing HER2-positive SKOV-3 xenografts.

METHODS

Nude mice bearing HER2-positive SKOV-3 xenografts were treated with trastuzumab (treatment group) or saline (control) with ten mice in each group. Mice in trastuzumab-treated group were given trastuzumab intraperiotoneally 4 mg/kg on day 1 and 2 mg/kg on day 8; Mice in control group were given physiological saline on day 1 and 8. Mice body weights and tumour volume were monitored every three days during treatment. In vivo SPECT imaging was performed in mice of the two groups using (99m)Tc-peptide-Z(HER2:342) before treatment, on day 8 and 15 after treatment. Radiolabeled probe uptake in tumours was measured as the ratio of radioactive counts in the tumour to that in the contralateral equivalent region (T/NT). After SPECT imaging on day 15, all the mice were euthanized, biodistribution studies of the SKOV-3 xenografts were carried out to validate the imaging results and HER2 expression of the transplanted tumours was analyzed by immunohistochemistry (IHC). Correlation analysis was performed between T/NT ratios acquired by in vivo SPECT imaging on day 15 and the HER2 level of tumours. In vitro cell binding capacity of (99m)Tc-Z(HER2:342) with SKOV-3 cells in the absence and presence of varying amount of trastuzumab were also conducted in the study.

RESULTS

Twenty mice body weight in the two groups gradually increased during treatment, but there was no statistical difference (p > 0.05). Though volumes of SKOV-3 xenografts gradually increased in each group during the treatment, the transplanted tumours in trastuzumab-treated group had a slower growth than those in control group (p < 0.05). Compared with the baseline, the results of in vivo imaging showed that radionuclide accumulation in transplanted tumours reduced significantly in trastuzumab-treated group after treatment (p < 0.05), whereas the tumour accumulation in control group increased after treatment. Biodistribution studies demonstrated that the results corresponded well with in vivo imaging data. Immunohistochemical staining confirmed the significant reduction in tumor HER2 level upon trastuzumab treatment, and there was an obviously positive correlation between T/NT ratios and HER2 level of tumours with correlation coefficient rs = 0.919, p < 0.05. There was no significant significance in cell binding ratios between varying amount of trastuzumab and the absence of trastuzumab (p > 0.05).

CONCLUSIONS

The early response to trastuzumab in mice bearing SKOV-3 xenografts was successfully monitored by SPECT imaging using (99m)Tc-peptide-Z(HER2:342). This approach may be valuable in monitoring the therapeutic response in HER 2-positive tumours under HER2-targeted therapy.

Selection of an optimal cysteine-containing peptide-based chelator for labeling of affibody molecules with (188)Re

Affibody molecules constitute a class of small (7 kDa) scaffold proteins that can be engineered to have excellent tumor targeting properties. High reabsorption in kidneys complicates development of affibody molecules for radionuclide therapy. In this study, we evaluated the influence of the composition of cysteine-containing C-terminal peptide-based chelators on the biodistribution and renal retention of (188)Re-labeled anti-HER2 affibody molecules. Biodistribution of affibody molecules containing GGXC or GXGC peptide chelators (where X is G, S, E or K) was compared with biodistribution of a parental affibody molecule ZHER2:2395 having a KVDC peptide chelator. All constructs retained low picomolar affinity to HER2-expressing cells after labeling. The biodistribution of all (188)Re-labeled affibody molecules was in general comparable, with the main observed difference found in the uptake and retention of radioactivity in excretory organs. The (188)Re-ZHER2:V2 affibody molecule with a GGGC chelator provided the lowest uptake in all organs and tissues. The renal retention of (188)Re-ZHER2:V2 (3.1 ± 0.5 %ID/g at 4 h after injection) was 55-fold lower than retention of the parental (188)Re-ZHER2:2395 (172 ± 32 %ID/g). We show that engineering of cysteine-containing peptide-based chelators can be used for significant improvement of biodistribution of (188)Re-labeled scaffold proteins, particularly reduction of their uptake in excretory organs.

Design and evaluation of radiolabeled tracers for tumor imaging

The growing understanding of tumor biology and the identification of tumor-specific genetic and molecular alterations, such as the overexpression of membrane receptors and other proteins, allows for personalization of patient management using targeted therapies. However, this puts stringent demands on the diagnostic tools used to identify patients who are likely to respond to a particular treatment. Radionuclide molecular imaging is a promising noninvasive method to visualize and characterize the expression of such targets. A number of different proteins, from full-length antibodies and their derivatives to small scaffold proteins and peptide receptor-ligands, have been applied to molecular imaging, each demonstrating strengths and weaknesses. Here, we discuss the concept of molecular targeting and, in particular, molecular imaging of cancer-associated targets. Additionally, we describe important biotechnological considerations and desired features when designing and developing tracers for radionuclide molecular imaging.

Evaluation of 99mTc-peptide-ZHER2:342 Affibody® molecule for in vivo molecular imaging

OBJECTIVE

The aim of this study was to develop an improved method for labelling ZHER2:342 with Technetium-99m ((99m)Tc) using Gly-(d) Ala-Gly-Gly as a chelator and to evaluate the feasibility of its use for visualization of HER2 expression in vivo.

METHODS

The Affibody® molecule ZHER2:342 was synthesized by Fmoc/tBu solid phase synthesis. The chelator, Gly-(d) Ala-Gly-Gly, was introduced by manual synthesis as the N-terminal extensions of ZHER2:342. ZHER2:342 was labelled with (99m)Tc. The labelling efficiency, radiochemical purity and in vitro stability of the labelled molecular probe were analysed by reversed-phase high performance liquid chromatography. Biodistribution and molecular imaging using (99m)Tc-peptide-ZHER2:342 were performed.

RESULTS

The molecular probe was successfully synthesized and labelled with (99m)Tc with the labelling efficiency of 98.10 ± 1.73% (n=5). The radiolabelled molecular probe remained highly stable in vitro. The molecular imaging showed high uptake in HER2-expressing SKOV-3 xenografts, whereas the MDA-MB-231 xenografts with low HER2 expression were not clearly imaged at any time after the injection of (99m)Tc-peptide-ZHER2:342. The predominant clearance pathway for (99m)Tc-peptide-ZHER2:342 was through the kidneys. Conculsion: (99m)Tc-peptide-ZHER2:342 using Gly-(d) Ala-Gly-Gly as a chelator is a promising tracer agent with favourable biodistribution and imaging properties that may be developed as a radiopharmaceutical for the detection of HER2-positive malignant tumours.

ADVANCES IN KNOWLEDGE

The (99m)Tc-peptide-ZHER2:342 molecular probe is a promising tracer agent, and the results in this study provide a foundation for future development of protocols for earlier visual detection of cancer in the clinical setting.

An improved radiolabelled RNA aptamer molecule for HER2 imaging in cancers

Human epidermal growth factor receptor 2 (HER2) expression has been shown to be increased in several types of human tumours. In this study, for the imaging of HER2-related tumours, a modified RNA aptamer with HER2-specific targeting was labelled with (99m)Tc, by using hydrazino nicotinamide (HYNIC) as the chelator in the presence of tricine or ethylenediamine-N,N'-diacetic acid (EDDA) as the co-ligand. Stability testing of the radiolabelled aptamers in the serum was performed through SDS-PAGE. The aptamer-radionuclide conjugate was evaluated for its cellular HER2-specific binding in ovarian cancer cells (SKOV-3), and its biodistribution properties were assessed in normal and SKOV-3 tumour-bearing mice. In the presence of either tricine or EDDA, the HYNIC-RNA aptamers were labelled with (99m)Tc at a high yield and radiochemical purity. Cellular experiments confirmed the specific binding of the RNA aptamer to the HER2 receptor. In the animal biodistribution study, uptake of the EDDA-co-liganded (99m)Tc-HYNIC-RNA aptamer by the liver and spleen was remarkably lower than that of the aptamer with tricine. Tumours also showed a higher accumulation of radioactivity with the EDDA-co-liganded aptamer complex. This study demonstrated EDDA to be better than tricine for use as a co-ligand with the RNA aptamer, which can be a potential tool for the molecular imaging of HER2-overexpressing cancers.

Imaging EGFR and HER2 by PET and SPECT: a review

In an effort to discover a noninvasive method for predicting which cancer patients will benefit from therapy targeting the EGFR and HER2 proteins, a large body of the research has been conducted toward the development of PET and SPECT imaging agents, which selectively target these receptors. We provide a general overview of the advances made toward imaging EGFR and HER2, detailing the investigation of PET and SPECT imaging agents ranging in size from small molecules to monoclonal antibodies.

PET of EGFR expression with an 18F-labeled affibody molecule

Epidermal growth factor receptor (EGFR) is often overexpressed in a variety of human cancers, and its expression is associated with poor prognosis for many cancer types. However, an accurate technique to noninvasively image EGFR expression in vivo is not available in the clinical setting. In this research, an Affibody analog, anti-EGFR Ac-Cys-Z(EGFR:1907), was successfully site-specifically (18)F-labeled for PET of EGFR expression.

METHODS

The prosthetic group N-[2-(4-(18)F-fluorobenzamido) ethyl] maleimide ((18)F-FBEM) was conjugated to Ac-Cys-Z(EGFR:1907) under mild conditions (pH 7) to produce the probe (18)F-FBEM-Cys-Z(EGFR:1907). The binding affinity and specificity tests of (18)F-FBEM-Cys-Z(EGFR:1907) to EGFR were conducted using A431 cancer cells. Small-animal PET and biodistribution studies were conducted on various mice tumor xenograft models with EGFR overexpression (6 types) after injection of approximately 2.0 MBq of (18)F-FBEM-Cys-Z(EGFR:1907) with or without coinjection of unlabeled Ac-Cys-Z(EGFR:1907) for up to 3 h after injection. A correlation study between (18)F-FBEM-Cys-Z(EGFR:1907) small- animal PET quantification and ex vivo Western blot analysis of tumor EGFR expression was conducted in those 6 types of tumor models.

RESULTS

(18)F-FBEM-Cys-Z(EGFR:1907) binds to EGFR with low nanomolar affinity (37 nM) in A431 cells. (18)F-FBEM-Cys-Z(EGFR:1907) rapidly accumulated in the tumor and cleared from most of the normal organs except the liver and kidneys at 3 h after injection, allowing excellent tumor-to-normal tissue contrast to be obtained. In the A431 tumor xenograft model, coinjection of the PET probe with 45 μg of Ac-Cys-Z(EGFR:1907) was able to improve the tumor uptake (3.9 vs. 8.1 percentage of the injected radioactive dose per gram of tissue, at 3 h after injection) and tumor imaging contrast, whereas coinjection with 500 μg of Ac-Cys-Z(EGFR:1907) successfully blocked the tumor uptake significantly (8.1 vs. 1.0 percentage of the injected radioactive dose per gram of tissue, at 3 h after injection, 88% inhibition, P < 0.05). Moderate correlation was found between the tumor tracer uptake at 3 h after injection quantified by PET and EGFR expression levels measured by Western blot assay (P = 0.007, R = 0.59).

CONCLUSION

(18)F-FBEM-Cys-Z(EGFR:1907) is a novel protein scaffold-based PET probe for imaging EGFR overexpression of tumors, and its ability to differentiate tumors with high and low EGFR expression in vivo holds promise for future clinical translation.

Evaluation of a HER2-targeting affibody molecule combining an N-terminal HEHEHE-tag with a GGGC chelator for 99mTc-labelling at the C terminus

Affibody molecules are a class of small (ca.7 kDa) robust scaffold proteins with high potential as tracers for radionuclide molecular imaging in vivo. Incorporation of a cysteine-containing peptide-based chelator at the C terminus provides an opportunity for stable labelling with the radionuclide (99m)Tc. The use of a GGGC chelator at the C terminus has provided the lowest renal radioactivity retention of the previously investigated peptide-based chelators. Previously, it has also been demonstrated that replacement of the His(6)-tag with the negatively charged histidine-glutamate-histidine-glutamate-histidine-glutamate (HEHEHE)-tag permits purification of affibody molecules by immobilized metal ion affinity chromatography (IMAC) and provides low hepatic accumulation of radioactivity of conjugates site-specifically labelled at the C terminus using several different nuclides. We hypothesized that the combination of a HEHEHE-tag at the N terminus and a GGGC chelator at the C terminus of an affibody molecule would be a favourable format permitting IMAC purification and providing low uptake in excretory organs. To investigate this hypothesis, a (HE)(3)-Z(HER2:342)-GGGC affibody molecule was generated. It could be efficiently purified by IMAC and stably labelled with (99m)Tc. (99m)Tc-(HE)(3)-Z(HER2:342)-GGGC preserved specific binding to HER2-expressing cells. In NMRI mice, hepatic uptake of (99m)Tc-(HE)(3)-Z(HER2:342)-GGGC was lower than the uptake of the control affibody molecules, (99m)Tc-Z(HER2:2395)-VDC and (99m)Tc-Z(HER2:342)-GGGC. At 1 and 4 h after injection, the renal uptake of (99m)Tc-(HE)(3)-Z(HER2:342)-GGGC was 2-3-fold lower than uptake of (99m)Tc-Z(HER2:2395)-VDC, but it was substantially higher than uptake of (99m)Tc-Z(HER2:342)-GGGC. Further investigation indicated that a fraction of (99m)Tc was chelated by the HEHEHE-tag which caused a higher accumulation of radioactivity in the kidneys. Thus, a combination of a HEHEHE-tag and the GGGC chelator in targeting scaffold proteins was found to be undesirable in the case of (99m)Tc labelling due to a partial loss of site-specificity of nuclide chelation.

Engineering of affibody molecules for therapy and diagnostics

Affibody molecules are small and robust non-immunoglobulin affinity ligands capable of binding to a wide range of protein targets. They are selected from combinatorial libraries based on a 58 amino acid, three-alpha-helical Z-domain scaffold. They share no sequence or structural homologies to antibodies and in contrast to antibodies they can be functionally produced both by peptide synthesis and by recombinant expression in Escherichia coli. Protein engineering is used to adapt Affibody molecules binding to a target of interest to the specific demands imposed by the intended application. Obviously, the optimal molecule for molecular imaging will be different from the optimal molecule for therapy. Here, we describe general strategies to optimize Affibody molecules for diagnostic imaging and therapy applications.

Protein-based tumor molecular imaging probes

Molecular imaging is an emerging discipline which plays critical roles in diagnosis and therapeutics. It visualizes and quantifies markers that are aberrantly expressed during the disease origin and development. Protein molecules remain to be one major class of imaging probes, and the option has been widely diversified due to the recent advances in protein engineering techniques. Antibodies are part of the immunosystem which interact with target antigens with high specificity and affinity. They have long been investigated as imaging probes and were coupled with imaging motifs such as radioisotopes for that purpose. However, the relatively large size of antibodies leads to a half-life that is too long for common imaging purposes. Besides, it may also cause a poor tissue penetration rate and thus compromise some medical applications. It is under this context that various engineered protein probes, essentially antibody fragments, protein scaffolds, and natural ligands have been developed. Compared to intact antibodies, they possess more compact size, shorter clearance time, and better tumor penetration. One major challenge of using protein probes in molecular imaging is the affected biological activity resulted from random labeling. Site-specific modification, however, allows conjugation happening in a stoichiometric fashion with little perturbation of protein activity. The present review will discuss protein-based probes with focus on their application and related site-specific conjugation strategies in tumor imaging.

Applications of molecular imaging

Today molecular imaging technologies play a central role in clinical oncology. The use of imaging techniques in early cancer detection, treatment response, and new therapy development is steadily growing and has already significantly impacted on clinical management of cancer. In this chapter, we overview three different molecular imaging technologies used for the understanding of disease biomarkers, drug development, or monitoring therapeutic outcome. They are (1) optical imaging (bioluminescence and fluorescence imaging), (2) magnetic resonance imaging (MRI), and (3) nuclear imaging (e.g., single-photon emission computed tomography (SPECT) and positron emission tomography (PET)). We review the use of molecular reporters of biological processes (e.g., apoptosis and protein kinase activity) for high-throughput drug screening and new cancer therapies, diffusion MRI as a biomarker for early treatment response and PET and SPECT radioligands in oncology.

A novel 18F-labeled two-helix scaffold protein for PET imaging of HER2-positive tumor

PURPOSE

Two-helix scaffold proteins (~ 5 kDa) against human epidermal growth factor receptor type 2 (HER2) have been discovered in our previous work. In this research we aimed to develop an (18)F-labeled two-helix scaffold protein for positron emission tomography (PET) imaging of HER2-positive tumors.

METHODS

An aminooxy-functionalized two-helix peptide (AO-MUT-DS) with high HER2 binding affinity was synthesized through conventional solid phase peptide synthesis. The purified linear peptide was cyclized by I(2) oxidation to form a disulfide bridge. The cyclic peptide was then conjugated with a radiofluorination synthon, 4-(18)F-fluorobenzyl aldehyde ((18)F-FBA), through the aminooxy functional group at the peptide N terminus (30% yield, non-decay corrected). The binding affinities of the peptides were analyzed by Biacore analysis. Cell uptake assay of the resulting PET probe, (18)F-FBO-MUT-DS, was performed at 37°C. (18)F-FBO-MUT-DS with high specific activity (20-32 MBq/nmol, 88-140 μCi/μg, end of synthesis) was injected into mice xenograft model bearing SKOV3 tumor. MicroPET and biodistribution and metabolic stability studies were then conducted.

RESULTS

Cell uptake assays showed high and specific cell uptake (~12% applied activity at 1 h) by incubation of (18)F-FBO-MUT-DS with HER2 high-expressing SKOV3 ovarian cancer cells. The affinities (K(D)) of AO-MUT-DS and FBO-MUT-DS as tested by Biacore analysis were 2 and 1 nM, respectively. In vivo small animal PET demonstrated fast tumor targeting, high tumor accumulation, and good tumor to normal tissue contrast of (18)F-FBO-MUT-DS. Biodistribution studies further revealed that the probe had excellent tumor uptake (6.9%ID/g at 1 h post-injection) and was cleared through both liver and kidneys. Co-injection of the probe with 500 μg of HER2 Affibody protein reduced the tumor uptake (6.9 vs 1.8%ID/g, p < 0.05).

CONCLUSION

F-FBO-MUT-DS displays excellent HER2 targeting ability and tumor PET imaging quality. The two-helix scaffold proteins are suitable for development of (18)F-based PET probes.

Order of amino acids in C-terminal cysteine-containing peptide-based chelators influences cellular processing and biodistribution of 99mTc-labeled recombinant Affibody molecules

Affibody molecules constitute a novel class of molecular display selected affinity proteins based on non-immunoglobulin scaffold. Preclinical investigations and pilot clinical data have demonstrated that Affibody molecules provide high contrast imaging of tumor-associated molecular targets shortly after injection. The use of cysteine-containing peptide-based chelators at the C-terminus of recombinant Affibody molecules enabled site-specific labeling with the radionuclide 99mTc. Earlier studies have demonstrated that position, composition and the order of amino acids in peptide-based chelators influence labeling stability, cellular processing and biodistribution of Affibody molecules. To investigate the influence of the amino acid order, a series of anti-HER2 Affibody molecules, containing GSGC, GEGC and GKGC chelators have been prepared and characterized. The affinity to HER2, cellular processing of 99mTc-labeled Affibody molecules and their biodistribution were investigated. These properties were compared with that of the previously studied 99mTc-labeled Affibody molecules containing GGSC, GGEC and GGKC chelators. All variants displayed picomolar affinities to HER2. The substitution of a single amino acid in the chelator had an appreciable influence on the cellular processing of 99mTc. The biodistribution of all 99mTc-labeled Affibody molecules was in general comparable, with the main difference in uptake and retention of radioactivity in excretory organs. The hepatic accumulation of radioactivity was higher for the lysine-containing chelators and the renal retention of 99mTc was significantly affected by the amino acid composition of chelators. The order of amino acids influenced renal uptake of some conjugates at 1 h after injection, but the difference decreased at later time points. Such information can be helpful for the development of other scaffold protein-based imaging and therapeutic radiolabeled conjugates.

Radiolabeled affibody-albumin bioconjugates for HER2-positive cancer targeting

Affibody molecules have received significant attention in the fields of molecular imaging and drug development. However, Affibody scaffolds display an extremely high renal uptake, especially when modified with chelators and then labeled with radiometals. This unfavorable property may impact their use as radiotherapeutic agents in general and as imaging probes for the detection of tumors adjacent to kidneys in particular. Herein, we present a simple and generalizable strategy for reducing the renal uptake of Affibody molecules while maintaining their tumor uptake. Human serum albumin (HSA) was consecutively modified by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid mono-N-hydroxysuccinimide ester (DOTA-NHS ester) and the bifunctional cross-linker sulfosuccinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (Sulfo-SMCC). The HER2 Affibody analogue, Ac-Cys-Z(HER2:342), was covalently conjugated with HSA, and the resulting bioconjugate DOTA-HSA-Z(HER2:342) was further radiolabeled with ⁶⁴Cu and ¹¹¹In and evaluated in vitro and in vivo. Radiolabeled DOTA-HSA-Z(HER2:342) conjugates displayed a significant and specific cell uptake into SKOV3 cell cultures. Positron emission tomography (PET) investigations using ⁶⁴Cu-DOTA-HSA-Z(HER2:342) were performed in SKOV3 tumor-bearing nude mice. High tumor uptake values (>14% ID/g at 24 and 48 h) and high liver accumulations but low kidney accumulations were observed. Biodistribution studies and single-photon emission computed tomography (SPECT) investigations using ¹¹¹In-DOTA-HSA-Z(HER2:342) validated these results. At 24 h post injection, the biodistribution data revealed high tumor (16.26% ID/g) and liver (14.11% ID/g) uptake but relatively low kidney uptake (6.06% ID/g). Blocking studies with coinjected, nonlabeled Ac-Cys-Z(HER2:342) confirmed the in vivo specificity of HER2. Radiolabeled DOTA-HSA-Z(HER2:342) Affibody conjugates are promising SPECT and PET-type probes for the imaging of HER2 positive cancer. More importantly, DOTA-HSA-Z(HER2:342) is suitable for labeling with therapeutic radionuclides (e.g., ⁹⁰Y or ¹⁷⁷Lu) for treatment studies. The approach of using HSA to optimize the pharmacokinetics and biodistribution profile of Affibodies may be extended to the design of many other targeting molecules.

Miniproteins as phage display-scaffolds for clinical applications

Miniproteins are currently developed as alternative, non-immunoglobin proteins for the generation of novel binding motifs. Miniproteins are rigid scaffolds that are stabilised by alpha-helices, beta-sheets and disulfide-constrained secondary structural elements. They are tolerant to multiple amino acid substitutions, which allow for the integration of a randomised affinity function into the stably folded framework. These properties classify miniprotein scaffolds as promising tools for lead structure generation using phage display technologies. Owing to their high enzymatic resistance and structural stability, miniproteins are ideal templates to display binding epitopes for medical applications in vivo. This review summarises the characteristics and the engineering of miniproteins as a novel class of scaffolds to generate of alternative binding agents using phage display screening. Moreover, recent developments for therapeutic and especially diagnostic applications of miniproteins are reviewed.

Molecular design and optimization of 99mTc-labeled recombinant affibody molecules improves their biodistribution and imaging properties

Affibody molecules are a recently developed class of targeting proteins based on a nonimmunoglobulin scaffold. The small size (7 kDa) and subnanomolar affinity of Affibody molecules enables high-contrast imaging of tumor-associated molecular targets, particularly human epidermal growth factor receptor type 2 (HER2). (99m)Tc as a label offers advantages in clinical practice, and earlier studies demonstrated that (99m)Tc-labeled recombinant Affibody molecules with a C-terminal cysteine could be used for HER2 imaging. However, the renal retention of radioactivity exceeded tumor uptake, which might complicate imaging of metastases in the lumbar region. The aim of this study was to develop an agent with low renal uptake and preserved tumor targeting.

METHODS

A series of recombinant derivatives of the HER2-binding Z(HER2)(:342) Affibody molecule with a C-terminal chelating sequence, -GXXC (X denoting glycine, serine, lysine, or glutamate), was designed. The constructs were labeled with (99m)Tc and evaluated in vitro and in vivo.

RESULTS

All variants were stably labeled with (99m)Tc, with preserved capacity to bind specifically to HER2-expressing cells in vitro and in vivo. The composition of the chelating sequence had a clear influence on the cellular processing and biodistribution properties of the Affibody molecules. The best variant, (99m)Tc-Z(HER2)(:V2), with the C-terminal chelating sequence -GGGC, provided the lowest radioactivity retention in all normal organs and tissues including the kidneys. (99m)Tc-Z(HER2)(:V2) displayed high uptake of radioactivity in HER2-expressing xenografts, 22.6 ± 4.0 and 7.7 ± 1.5 percentage injected activity per gram of tissue at 4 h after injection in SKOV-3 (high HER2 expression) and DU-145 (low HER2 expression) tumors, respectively. In both models, the tumor uptake exceeded the renal uptake.

CONCLUSION

These results demonstrate that the biodistribution properties of recombinant (99m)Tc-labeled Affibody molecules can be optimized by modification of the C-terminal cysteine-containing chelating sequence. (99m)Tc-Z(HER2)(:V2) is a promising candidate for further development as a diagnostic radiopharmaceutical for imaging of HER2-expressing tumors. These results may be useful for the development of imaging agents based on other Affibody molecules and, hopefully, other scaffolds.

An engineered knottin peptide labeled with 18F for PET imaging of integrin expression

Knottins are small constrained polypeptides that share a common disulfide-bonded framework and a triple-stranded beta-sheet fold. Previously, directed evolution of the Ecballium elaterium trypsin inhibitor (EETI-II) knottin led to the identification of a mutant that bound to tumor-specific alpha(v)beta(3) and alpha(v)beta(5) integrin receptors with low nanomolar affinity. The objective of this study was to prepare and evaluate a radiofluorinated version of this knottin (termed 2.5D) for microPET imaging of integrin positive tumors in living subjects. Knottin peptide 2.5D was prepared by solid-phase synthesis and folded in vitro, and its free N-terminal amine was reacted with N-succinimidyl-4-18/19F-fluorobenzoate (18/19F-SFB) to produce the fluorinated peptide 18/19F-FB-2.5D. The binding affinities of unlabeled knottin peptide 2.5D and 19F-FB-2.5D to U87MG glioblastoma cells were measured by competition binding assay using 125I-labeled echistatin. It was found that unlabeled 2.5D and 19F-FB-2.5D competed with 125I-echistatin for binding to cell surface integrins with IC(50) values of 20.3 +/- 7.3 and 13.2 +/- 5.4 nM, respectively. Radiosynthesis of 18F-FB-2.5D resulted in a product with high specific activity (ca. 100 GBq/micromol). Next, biodistribution and positron emission tomography (PET) imaging studies were performed to evaluate the in vivo behavior of 18F-FB-2.5D. Approximately 3.7 MBq 18F-FB-2.5D was injected into U87MG tumor-bearing mice via the tail vein. Biodistribution studies demonstrated that 18F-FB-2.5D had moderate tumor uptake at 0.5 h post injection, and coinjection of a large excess of the unlabeled peptidomimetic c(RGDyK) as a blocking agent significantly reduced tumor uptake (1.90 +/- 1.15 vs 0.57 +/- 0.14%ID/g, 70% inhibition, P < 0.05). In vivo microPET imaging showed that 18F-FB-2.5D rapidly accumulated in the tumor and quickly cleared from the blood through the kidneys, allowing excellent tumor-to-normal tissue contrast to be obtained. Collectively, 18F-FB-2.5D allows integrin-specific PET imaging of U87MG tumors with good contrast and further demonstrates that knottins are excellent peptide scaffolds for development of PET probes with potential for clinical translation.

64Cu-labeled affibody molecules for imaging of HER2 expressing tumors

INTRODUCTION

The development of molecular probes based on novel engineered protein constructs is under active investigation due to the great potential of this generalizable strategy for imaging a variety of tumor targets.

DISCUSSION

In this report, human epidermal growth factor receptor type 2 (HER2)-binding Affibody molecules were radiolabeled with (64)Cu and their imaging ability was further evaluated in tumor mice models to understand the promise and limitations of such probes. The anti-HER2 Affibody molecules in monomeric (Z(HER2:477)) and dimeric [(Z(HER2:477))(2)] forms were site specifically modified with the maleimide-functionalized chelator, 1,4,7,10-tetraazacyclododecane-1,4,7-tris(acetic acid)-10-acetate mono (N-ethylmaleimide amide) (Mal-DOTA). The resulting DOTA-Affibody conjugates were radiolabeled with (64)Cu and evaluated in nude mice bearing subcutaneous SKOV3 tumors. Biodistribution experiments showed that tumor uptake values of (64)Cu-DOTA-Z(HER2:477) and (64)Cu-DOTA-(Z(HER2:477))(2) were 6.12 +/- 1.44% and 1.46 +/- 0.50% ID/g, respectively, in nude mice (n = 3 each) at 4 h postinjection. Moreover, (64)Cu-labeled monomer exhibited significantly higher tumor/blood ratio than that of radiolabeled dimeric counterpart at all time points examined in this study. MicroPET imaging of (64)Cu-DOTA-Z(HER2:477) in SKOV3 tumor mice clearly showed good and specific tumor localization. This study demonstrates that (64)Cu-labeled Z(HER2:477) is a promising targeted molecular probe for imaging HER2 receptor expression in living mice. Further work is needed to improve the excretion properties, hence dosimetry and imaging efficacy, of the radiometal-based probe.

Engineering and characterization of a bispecific HER2 x EGFR-binding affibody molecule

HER2 (human epidermal-growth-factor receptor-2; ErbB2) and EGFR (epidermal-growth-factor receptor) are overexpressed in various forms of cancer, and the co-expression of both HER2 and EGFR has been reported in a number of studies. The simultaneous targeting of HER2 and EGFR has been discussed as a strategy with which to potentially increase efficiency and selectivity in molecular imaging and therapy of certain cancers. In an effort to generate a molecule capable of bispecifically targeting HER2 and EGFR, a gene fragment encoding a bivalent HER2-binding affibody molecule was genetically fused in-frame with a bivalent EGFR-binding affibody molecule via a (G4S)3 [(Gly4-Ser)3]-encoding gene fragment. The encoded 30 kDa affibody construct (ZHER2)2-(G4S)3-(ZEGFR)2, with potential for bs (bispecific) binding to HER2 and EGFR, was expressed in Escherichia coli and characterized in terms of its binding capabilities. The retained ability to bind HER2 and EGFR separately was demonstrated using both biosensor technology and flow-cytometric analysis, the latter using HER2- and EGFR-overexpressing cells. Furthermore, simultaneous binding to HER2 and EGFR was demonstrated in: (i) a sandwich format employing real-time biospecific interaction analysis where the bs affibody molecule bound immobilized EGFR and soluble HER2; (ii) immunofluorescence microscopy, where the bs affibody molecule bound EGFR-overexpressing cells and soluble HER2; and (iii) a cell-cell interaction analysis where the bs affibody molecule bound HER2-overexpressing SKBR-3 cells and EGFR-overexpressing A-431 cells. This is, to our knowledge, the first reported bs affinity protein with potential ability for the simultaneous targeting of HER2 and EGFR. The potential future use of this and similar constructs, capable of bs targeting of receptors to increase the efficacy and selectivity in imaging and therapy, is discussed.