99mTc(CO)3+ labeled domain I/II-specific anti-EGFR (scFv)2 antibody fragment for imaging EGFR expression

Preparation, Characterization, and Radiolabeling of Anti-HER2 scFv With Technetium Tricarbonyl and Stability Studies

Breast cancer is the most common diagnosed cancer, and the second cause of cancer death among women, worldwide. HER2 overexpression occurred in approximately 15% to 20% of breast cancers. Invasive biopsy method has been used for detection of HER2 overexpression. HER2-targeted imaging via an appropriate radionuclide is a promising method for sensitive and accurate identification of HER2+ primary and metastatic lesions. 99mTc-anti-HER2 scFv can specifically target malignancies and be used for diagnosis of the cancer type and metastasis as well as treatment of breast cancer. We radiolabeled anti-HER2 scFv that was expressed in Escherichia coli and purified through Ni-NTA resin under native condition with 99mTc-tricarbonyl formed from boranocarbonate. HER2-based ELISA, BCA, TLC, and HPLC were used in this study. In the current study, anti-HER2 scFv was lyophilized before radiolabeling. It was found that freeze-drying did not change the binding activity of anti-HER2 scFv to HER2. Results demonstrated direct anti-HER2 scFv radiolabeling by 99mTc-tricarbonyl to hexahistidine sequence (His-tag) without any changes in biological activity and radiochemical purity of around 98%. Stability analysis revealed that 99mTc-anti-HER2 scFv is stable for at least 24 h in PBS buffer, normal saline, human plasma proteins, and histidine solution.

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.

Radiolabeled nanobodies for tumor targeting: From bioengineering to imaging and therapy

So far, numerous molecules and biomolecules have been evaluated for tumor targeting purposes for radionuclide-based imaging and therapy modalities. Due to the high affinity and specificity against tumor antigens, monoclonal antibodies are appropriate candidates for tumor targeting. However, their large size prevents their comprehensive application in radionuclide-based tumor imaging or therapy, since it leads to their low tumor penetration, low blood clearance, and thus inappropriate tumor-to-background ratio. Nowadays, the variable domain of heavy-chain antibodies from the Camelidae family, known as nanobodies (Nbs), turn into exciting candidates for medical research. Considering several innate advantages of these new tumor-targeting agents, including excellent affinity and specificity toward antigen, high solubility, high stability, fast washout from blood, convenient production, ease of selection, and low immunogenicity, it assumes that they may overcome generic problems of monoclonal antibodies, their fragments, and other vectors used for tumor imaging/therapy. After three decades of Nbs discovery, the increasing number of their preclinical and clinical investigations, which have led to outstanding results, confirm their application for tumor targeting purposes. This review describes Nbs characteristics, the diagnostic and therapeutic application of their radioconjugates, and their recent advances.

Nimotuzumab Site-Specifically Labeled with 89Zr and 225Ac Using SpyTag/SpyCatcher for PET Imaging and Alpha Particle Radioimmunotherapy of Epidermal Growth Factor Receptor Positive Cancers

Simple Summary

Monoclonal antibodies (IgG) are excellent probes for targeting cell surface receptors for imaging and therapeutic applications. These theranostic agents are often developed by randomly conjugating radioisotopes/drugs/chelators to the primary amine of lysine or the sulfhydryl groups of cysteine on the antibody. Random conjugation often alters the properties of the antibody. We have site-specifically radiolabeled nimotuzumab an anti-epidermal growth factor receptor (EGFR) monoclonal antibody with 89Zr and 225Ac using SpyTag: ∆N-SpyCatcher for positron emission tomography (PET) imaging and alpha particle radiotherapy, and evaluated these agents in a model of EGFR-positive triple negative breast cancer (TNBC). Nimotuzumab-SpyTag-∆N-SpyCatcher constructs showed improved binding in vitro compared with randomly conjugated constructs. 89Zr-nimotuzumab-SpyTag-∆N-SpyCatcher specifically delineated EGFR-positive xenograft in vivo using microPET/CT imaging. Compared with control treatment groups, 225Ac-nimotuzumab-SpyTag-∆N-SpyCatcher more than doubled the survival of mice bearing EGFR-positive MDA-MB-231 TNBC xenograft. This work highlights a facile method to site-specifically radiolabel antibodies using SpyTag: ∆N-SpyCatcher.

Abstract

To develop imaging and therapeutic agents, antibodies are often conjugated randomly to a chelator/radioisotope or drug using a primary amine (NH₂) of lysine or sulfhydryl (SH) of cysteine. Random conjugation to NH₂ or SH groups can require extreme conditions and may affect target recognition/binding and must therefore be tested. In the present study, nimotuzumab was site-specifically labeled using ∆N-SpyCatcher/SpyTag with different chelators and radiometals. Nimotuzumab is a well-tolerated anti-EGFR antibody with low skin toxicities. First, ΔN-SpyCatcher was reduced using tris(2-carboxyethyl)phosphine (TCEP), which was followed by desferoxamine-maleimide (DFO-mal) conjugation to yield a reactive ΔN-SpyCatcher-DFO. The ΔN-SpyCatcher-DFO was reacted with nimotuzumab-SpyTag to obtain stable nimotuzumab-SpyTag-∆N-SpyCatcher-DFO. Radiolabeling was performed with ⁸⁹Zr, and the conjugate was used for the in vivo microPET imaging of EGFR-positive MDA-MB-468 xenografts. Similarly, ∆N-SpyCatcher was conjugated to an eighteen-membered macrocyclic chelator macropa-maleimide and used to radiolabel nimotuzumab-SpyTag with actinium-225 (²²⁵Ac) for in vivo radiotherapy studies. All constructs were characterized using biolayer interferometry, flow cytometry, radioligand binding assays, HPLC, and bioanalyzer. MicroPET/CT imaging showed a good tumor uptake of ⁸⁹Zr-nimotuzumab-SpyTag-∆N-SpyCatcher with 6.0 ± 0.6%IA/cc (n = 3) at 48 h post injection. The EC₅₀ of ²²⁵Ac-nimotuzumab-SpyTag-∆N-SpyCatcher and ²²⁵Ac-control-IgG-SpyTag-∆N-SpyCatcher against an EGFR-positive cell-line (MDA-MB-468) was 3.7 ± 3.3 Bq/mL (0.04 ± 0.03 nM) and 18.5 ± 4.4 Bq/mL (0.2 ± 0.04 nM), respectively. In mice bearing MDA-MB-468 EGFR-positive xenografts, ²²⁵Ac-nimotuzumab-SpyTag-∆N-SpyCatcher significantly (p = 0.0017) prolonged the survival of mice (64 days) compared to ²²⁵Ac-control IgG (28.5 days), nimotuzumab (28.5 days), or PBS-treated mice (30 days). The results showed that the conjugation and labeling using SpyTag/∆N-SpyCatcher to nimotuzumab did not significantly (p > 0.05) alter the receptor binding of nimotuzumab compared with a non-specific conjugation approach. ²²⁵Ac-nimotuzumab-SpyTag-∆N-SpyCatcher was effective in vitro and in an EGFR-positive triple negative breast cancer xenograft model.

Development and preclinical evaluation of cixutumumab drug conjugates in a model of insulin growth factor receptor I (IGF-1R) positive cancer

Overexpression of insulin growth factor receptor type 1 (IGF-1R) is observed in many cancers. Antibody drug conjugates (ADCs) with PEGylated maytansine (PEG₆-DM1) show promise in vitro. We developed PEG₆-DM1 ADCs with low and high drug to antibody ratios (DAR) using an anti-IGF-1R antibody cixutumumab (IMC-A12). Conjugates with low (cixutumumab-PEG₆-DM1-Low) and high (cixutumumab-PEG₆-DM1-High) DAR as 3.4 and 7.2, respectively, were generated. QC was performed by UV spectrophotometry, HPLC, bioanalyzer, and biolayer-interferometry. We compared the in vitro binding and internalization rates of the ADCs in IGF-1R-positive MCF-7/Her18 cells. We radiolabeled the ADCs with ¹¹¹In and used microSPECT/CT imaging and ex vivo biodistribution to understand their in vivo behavior in MCF-7/Her18 xenograft mice. The therapeutic potential of the ADC was studied in vitro and in mouse xenograft. Internalization rates of all ADCs was high and increased over 48 h and EC₅₀ was in the low nanomolar range. MicroSPECT/CT imaging and ex vivo biodistribution showed significantly lower tumor uptake of ¹¹¹In-cixutumumab-PEG₆-DM1-High compared to ¹¹¹In-cixutumumab-PEG₆-DM1-Low and ¹¹¹In-cixutumumab. Cixutumumab-PEG₆-DM1-Low significantly prolonged the survival of mice bearing MCF-7/Her18 xenograft compared with cixutumumab, cixutumumab-PEG₆-DM1-High, or the PBS control group. Cixutumumab-PEG₆-DM1-Low ADC was more effective. The study highlights the potential utility of cixutumumab-ADCs as theranostics against IGF-1R positive cancers.

Quantitative 99mTc Labeling Kit for HYNIC-Conjugated Single Chain Antibody Fragments Targeting Malignant Mesothelioma

Single chain antibody fragment (scFv) is a promising agent for imaging and targeted therapy. The objective of the study is to evaluate a kit formulation for ^99mTc labeling of scFv for tumor imaging. The scFv was engineered to contain a cysteine tag to accommodate the specific conjugation of HYNIC and subsequent ^99mTc labeling. The labeling conditions were formulated to allow instantaneous one-pot quantitative labeling. The reproducibility of labeling was evaluated at various time points during kit storage at -20 °C. In vitro cell binding experiments and HPLC analysis were performed to assess binding affinity and radiolabel stability, respectively. In vivo tumor targeting study was performed in xenograft models with biodistribution studied at 1, 3, and 24 h post-injection. The optimized kit with 5 μg SnF₂, pH 5.5, and 50 μg GH along with as low as 15 μg of HYNIC-cys-scFv provided high labeling yield (>95%), high specific activity (1.8 × 10⁷ Ci/Mol), and robust reproducibility with shelf life up to 90 days when stored at -20 °C. The in vitro cell binding study showed the labeled scFv maintained the binding capability with an apparent K_D of ∼27 nM. The animal study using tumor-bearing mice showed high tumor uptake at 16.9%ID/g 24 h post-injection along with rapid blood clearance (0.18%ID/g) and kidney excretion (44%ID/g), resulting in very high contrast (tumor/muscle >200:1). A kit formulation for ^99mTc labeling of scFvs targeting mesothelioma was developed based on specific HYNIC conjugation and GH (Glucoheptonate) as a coligand, producing not only high specific activity, but also improved tumor uptake. This convenient one-pot labeling method has the potential for translation into clinical use and is applicable to other scFvs as well.

A promising cancer diagnosis and treatment strategy: targeted cancer therapy and imaging based on antibody fragment

With the arrival of the precision medicine and personalized treatment era, targeted therapy that improves efficacy and reduces side effects has become the mainstream approach of cancer treatment. Antibody fragments that further enhance penetration and retain the most critical antigen-specific binding functions are considered the focus of research targeting cancer imaging and therapy. Thanks to the superior penetration and rapid blood clearance of antibody fragments, antibody fragment-based imaging agents enable efficient and sensitive imaging of tumour sites. In tumour-targeted therapy, antibody fragments can directly inhibit tumour proliferation and growth, serve as an ideal carrier for delivery of anti-tumour drugs, or manipulate the immune system to eliminate tumour cells. In this review, the excellent physicochemical properties and the basic structure of antibody fragments are expressly depicted depicted, the progress of antibody fragments in cancer therapy and imaging are thoroughly summarized, and the future development of antibody fragments is predicted.

MicroSPECT/CT Imaging of Cell-Line and Patient-Derived EGFR-Positive Tumor Xenografts in Mice with Panitumumab Fab Modified with Hexahistidine Peptides To Enable Labeling with 99mTc(I) Tricarbonyl Complex

We aimed to investigate the feasibility of conjugating synthetic hexahistidine peptides (His₆) peptides to panitumumab Fab (PmFab) to enable labeling with [^99mTc(H₂O)₃(CO)₃]⁺ complex and study these radioimmunoconjugates for imaging EGFR-overexpressing tumor xenografts in mice by microSPECT/CT. Fab were reacted with a 10-fold excess of sulfo-SMCC to introduce maleimide functional groups for reaction with the terminal thiol on peptides [CGYGGHHHHHH] that harbored the His₆ motif. Modification of Fab with His₆ peptides was assessed by SDS-PAGE/Western blot, and the number of His₆ peptides introduced was quantified by a radiometric assay incorporating ¹²³I-labeled peptides into the conjugation reaction. Radiolabeling was achieved by incubation of PmFab-His₆ in PBS, pH 7.0, with [^99mTc(H₂O)₃(CO)₃]⁺ in a 1.4 MBq/μg ratio. The complex was prepared by adding [^99mTcO₄]^- to an Isolink kit (Paul Scherrer Institute). Immunoreactivity was assessed in a direct (saturation) binding assay using MDA-MB-468 human triple-negative breast cancer (TNBC) cells. Tumor and normal tissue uptake and imaging properties of ^99mTc-PmFab-His₆ (70 μg; 35-40 MBq) injected i.v. (tail vein) were compared to irrelevant ^99mTc-Fab 3913 in NOD/SCID mice engrafted subcutaneously (s.c.) with EGFR-overexpressing MDA-MB-468 or PANC-1 human pancreatic ductal carcinoma (PDCa) cell-line derived xenografts (CLX) at 4 and 24 h post injection (p.i.). In addition, tumor imaging studies were performed with ^99mTc-PmFab-His₆ in mice with patient-derived tumor xenografts (PDX) of TNBC, PDCa, and head and neck squamous cell carcinoma (HNSCC). Biodistribution studies in nontumor bearing Balb/c mice were performed to project the radiation absorbed doses for imaging studies in humans with ^99mTc-PmFab-His₆. PmFab was derivatized with 0.80 ± 0.03 His₆ peptides. Western blot and SDS-PAGE confirmed the presence of His₆ peptides. ^99mTc-PmFab-His₆ was labeled to high radiochemical purity (≥95%), and the K_d for binding to EGFR on MDA-MB-468 cells was 5.5 ± 0.4 × 10^-8 mol/L. Tumor uptake of ^99mTc-PmFab-His₆ at 24 h p.i. was significantly (P < 0.05) higher than irrelevant ^99mTc-Fab 3913 in mice with MDA-MB-468 tumors (14.9 ± 3.1%ID/g vs 3.0 ± 0.9%ID/g) and in mice with PANC-1 tumors (5.6 ± 0.6 vs 0.5 ± 0.1%ID/g). In mice implanted orthotopically in the pancreas with the same PDCa PDX, tumor uptake at 24 h p.i. was 4.2 ± 0.2%ID/g. Locoregional metastases of these PDCa tumors in the peritoneum exhibited slightly and significantly lower uptake than the primary tumors (3.1 ± 0.3 vs 4.2 ± 0.3%ID/g; P = 0.02). In mice implanted with different TNBC or HNSCC PDX, tumor uptake at 24 h p.i. was variable and ranged from 3.7 to 11.4%ID/g and 3.8-14.5%ID/g, respectively. MicroSPECT/CT visualized all CLX and PDX tumor xenografts at 4 and 24 h p.i. Dosimetry estimates revealed that in humans, the whole body dose from administration of 740-1110 MBq of ^99mTc-PmFab-His₆ would be 2-3 mSv, which is less than for a ^99mTc-medronate bone scan (4 mSv).