Quantitative immuno-positron emission tomography imaging of HER2-positive tumor xenografts with an iodine-124 labeled anti-HER2 diabody

Covalent Proteins as Targeted Radionuclide Therapies Enhance Antitumor Effects

Molecularly targeted radionuclide therapies (TRTs) struggle with balancing efficacy and safety, as current strategies to increase tumor absorption often alter drug pharmacokinetics to prolong circulation and normal tissue irradiation. Here we report the first covalent protein TRT, which, through reacting with the target irreversibly, increases radioactive dose to the tumor without altering the drug's pharmacokinetic profile or normal tissue biodistribution. Through genetic code expansion, we engineered a latent bioreactive amino acid into a nanobody, which binds to its target protein and forms a covalent linkage via the proximity-enabled reactivity, cross-linking the target irreversibly in vitro, on cancer cells, and on tumors in vivo. The radiolabeled covalent nanobody markedly increases radioisotope levels in tumors and extends tumor residence time while maintaining rapid systemic clearance. Furthermore, the covalent nanobody conjugated to the α-emitter actinium-225 inhibits tumor growth more effectively than the noncovalent nanobody without causing tissue toxicity. Shifting the protein-based TRT from noncovalent to covalent mode, this chemical strategy improves tumor responses to TRTs and can be readily scaled to diverse protein radiopharmaceuticals engaging broad tumor targets.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Molecular and functional imaging in cancer-targeted therapy: current applications and future directions

Targeted anticancer drugs block cancer cell growth by interfering with specific signaling pathways vital to carcinogenesis and tumor growth rather than harming all rapidly dividing cells as in cytotoxic chemotherapy. The Response Evaluation Criteria in Solid Tumor (RECIST) system has been used to assess tumor response to therapy via changes in the size of target lesions as measured by calipers, conventional anatomically based imaging modalities such as computed tomography (CT), and magnetic resonance imaging (MRI), and other imaging methods. However, RECIST is sometimes inaccurate in assessing the efficacy of targeted therapy drugs because of the poor correlation between tumor size and treatment-induced tumor necrosis or shrinkage. This approach might also result in delayed identification of response when the therapy does confer a reduction in tumor size. Innovative molecular imaging techniques have rapidly gained importance in the dawning era of targeted therapy as they can visualize, characterize, and quantify biological processes at the cellular, subcellular, or even molecular level rather than at the anatomical level. This review summarizes different targeted cell signaling pathways, various molecular imaging techniques, and developed probes. Moreover, the application of molecular imaging for evaluating treatment response and related clinical outcome is also systematically outlined. In the future, more attention should be paid to promoting the clinical translation of molecular imaging in evaluating the sensitivity to targeted therapy with biocompatible probes. In particular, multimodal imaging technologies incorporating advanced artificial intelligence should be developed to comprehensively and accurately assess cancer-targeted therapy, in addition to RECIST-based methods.

Synthesis of Radioiodinated Compounds. Classical Approaches and Achievements of Recent Years

This review demonstrates the progress in the synthesis of radioiodinated compounds over the past decade. The possibilities and limitations of radiopharmaceuticals with different iodine isotopes, as well as the synthesis of low and high molecular weight compounds containing radioiodine, are discussed. An analysis of synthesis strategies, substrate frameworks, isolation methods, and metabolic stability, and the possibility of industrial production of radioiodinated organic derivatives which can find applications in the synthesis of drugs and diagnostics are presented.

Tracking Uptake and Metabolism of Xenometallomycins Using a Multi-Isotope Tagging Strategy

Synthetic and naturally occurring siderophores and their conjugates provide access to the bacterial cytoplasm via active membrane transport. Previously, we displaced iron with the radioactive isotope ⁶⁷Ga to quantify and track in vitro and in vivo uptake and distribution of siderophore Trojan Horse antibiotic conjugates. Here, we introduce a multi-isotope tagging strategy to individually elucidate the fate of metal cargo and the ligand construct with radioisotopes ⁶⁷Ga and ¹²⁴I. We synthesized gallium(III) model complexes of a ciprofloxacin-functionalized linear desferrichrome (Ga-D6) and deferoxamine (Ga-D7) incorporating an iodo-tyrosine linker to enable radiolabeling using the metal-binding (⁶⁷Ga) and the cargo-conjugation site (¹²⁴I). Radiochemical experiments with Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa wt strains show that ⁶⁷Ga-D6/D7 and Ga-D6-¹²⁴I/D7-¹²⁴I have comparable uptake, indicating intact complex import and siderophore-mediated uptake. In naive mice, ⁶⁷Ga-D6/D7 and Ga-D6-¹²⁴I/D7-¹²⁴I demonstrate predominantly renal clearance; urine metabolite analysis indicates in vivo dissociation of Ga(III) is a likely mechanism of degradation for ⁶⁷Ga-D6/D7 when compared to ligand radiolabeled compounds, Ga-D6-¹²⁴I/D7-¹²⁴I, which remain >60% intact in urine. Cumulatively, this work demonstrates that a multi-isotope tagging strategy effectively elucidates the in vitro uptake, pharmacokinetics, and in vivo stability of xenometallomycins with modular chemical structures.

Radiochemistry, Production Processes, Labeling Methods, and ImmunoPET Imaging Pharmaceuticals of Iodine-124

Target-specific biomolecules, monoclonal antibodies (mAb), proteins, and protein fragments are known to have high specificity and affinity for receptors associated with tumors and other pathological conditions. However, the large biomolecules have relatively intermediate to long circulation half-lives (>day) and tumor localization times. Combining superior target specificity of mAbs and high sensitivity and resolution of the PET (Positron Emission Tomography) imaging technique has created a paradigm-shifting imaging modality, ImmunoPET. In addition to metallic PET radionuclides, 124I is an attractive radionuclide for radiolabeling of mAbs as potential immunoPET imaging pharmaceuticals due to its physical properties (decay characteristics and half-life), easy and routine production by cyclotrons, and well-established methodologies for radioiodination. The objective of this report is to provide a comprehensive review of the physical properties of iodine and iodine radionuclides, production processes of 124I, various 124I-labeling methodologies for large biomolecules, mAbs, and the development of 124I-labeled immunoPET imaging pharmaceuticals for various cancer targets in preclinical and clinical environments. A summary of several production processes, including 123Te(d,n)124I, 124Te(d,2n)124I, 121Sb(α,n)124I, 123Sb(α,3n)124I, 123Sb(3He,2n)124I, natSb(α, xn)124I, natSb(3He,n)124I reactions, a detailed overview of the 124Te(p,n)124I reaction (including target selection, preparation, processing, and recovery of 124I), and a fully automated process that can be scaled up for GMP (Good Manufacturing Practices) production of large quantities of 124I is provided. Direct, using inorganic and organic oxidizing agents and enzyme catalysis, and indirect, using prosthetic groups, 124I-labeling techniques have been discussed. Significant research has been conducted, in more than the last two decades, in the development of 124I-labeled immunoPET imaging pharmaceuticals for target-specific cancer detection. Details of preclinical and clinical evaluations of the potential 124I-labeled immunoPET imaging pharmaceuticals are described here.

Functional Domain Order of an Anti-EGFR × Anti-CD16 Bispecific Diabody Involving NK Cell Activation

Bispecific antibodies (bsAbs) have emerged as promising therapeutics. A bispecific diabody (bsDb) is a small bsAb consisting of two distinct chimeric single-chain components, with two possible arrangements of the domains. We previously reported the effect of domain order on the function of a humanized bsDb targeting the epidermal growth factor receptor (EGFR) on cancer cells, and CD3 on T cells. Notably, the co-localization of a T-cell receptor (TCR) with CD3 is bulky, potentially affecting the cross-linking ability of bsDbs, due to steric hindrance. Here, we constructed and evaluated humanized bsDbs, with different domain orders, targeting EGFR and CD16 on natural killer (NK) cells (hEx16-Dbs). We predicted minimal effects due to steric hindrance, as CD16 lacks accessory molecules. Interestingly, one domain arrangement displayed superior cytotoxicity in growth inhibition assays, despite similar cross-linking abilities for both domain orders tested. In hEx16-Dbs specifically, domain order might affect the agonistic activity of the anti-CD16 portion, which was supported by a cytokine production test, and likely contributed to the superiority of one of the hEx16-Dbs. Our results indicate that both the target antigen and mode of action of an antibody must be considered in the construction of highly functional bsAbs.

Brain Delivery of Single-Domain Antibodies: A Focus on VHH and VNAR

Passive immunotherapy, i.e., treatment with therapeutic antibodies, has been increasingly used over the last decade in several diseases such as cancers or inflammation. However, these proteins have some limitations that single-domain antibodies could potentially solve. One of the main issues of conventional antibodies is their limited brain penetration because of the blood-brain barrier (BBB). In this review, we aim at exploring the different options single-domain antibodies (sDAbs) such as variable domain of heavy-chain antibodies (VHHs) and variable new antigen receptors (VNARs) have already taken to reach the brain allowing them to be used as therapeutic, diagnosis or transporter tools.

First clinical study of a pegylated diabody 124I-labeled PEG-AVP0458 in patients with tumor-associated glycoprotein 72 positive cancers

Through protein engineering and a novel pegylation strategy, a diabody specific to tumor-associated glycoprotein 72 (TAG-72) (PEG-AVP0458) has been created to optimize pharmacokinetics and bioavailability to tumor. We report the preclinical and clinical translation of PEG-AVP0458 to a first-in-human clinical trial of a diabody. Methods: Clinical translation followed characterization of PEG-AVP0458 drug product and preclinical biodistribution and imaging assessments of Iodine-124 trace labeled PEG-AVP0458 (¹²⁴I-PEG-AVP0458). The primary study objective of the first-in-human study was the safety of a single protein dose of 1.0 or 10 mg/m^{2 124}I-PEG-AVP0458 in patients with TAG-72 positive relapsed/ metastatic prostate or ovarian cancer. Secondary study objectives were evaluation of the biodistribution, tumor uptake, pharmacokinetics and immunogenicity. Patients were infused with a single-dose of ¹²⁴I labeled PEG-AVP0458 (3-5 mCi (111-185 MBq) for positron emission tomography (PET) imaging, performed sequentially over a one-week period. Safety, pharmacokinetics, biodistribution, and immunogenicity were assessed up to 28 days after infusion. Results: PEG-AVP0458 was radiolabeled with ¹²⁴I and shown to retain high TAG-72 affinity and excellent targeting of TAG-72 positive xenografts by biodistribution analysis and PET imaging. In the first-in-human trial, no adverse events or toxicity attributable to ¹²⁴I-PEG-AVP0458 were observed. Imaging was evaluable in 5 patients, with rapid and highly specific targeting of tumor and minimal normal organ uptake, leading to high tumor:blood ratios. Serum concentration values of ¹²⁴I-PEG-AVP0458 showed consistent values between patients, and there was no significant difference in T½α and T½β between dose levels with mean (± SD) results of T½α = 5.10 ± 4.58 hours, T½β = 46.19 ± 13.06 hours. Conclusions: These data demonstrates the safety and feasibility of using pegylated diabodies for selective tumor imaging and potential delivery of therapeutic payloads in cancer patients.

Construction and Preclinical Evaluation of 211At Labeled Anti-mesothelin Antibodies as Potential Targeted Alpha Therapy Drugs

Targeted alpha therapy (TAT) is a promising tumor therapy that can specifically transport α particle to the vicinity of tumor cells while the normal cells are only slightly irradiated. Mesothelin is a highly promising molecular signature for many types of solid tumors including malignant mesothelioma, pancreatic cancer, ovarian cancer and lung adenocarcinoma etc., while the expression in normal human tissues are limited, thus making mesothelin a promising antigen for TAT. Previously we developed a theoretical model that could predict and optimize in vitro screening of potential TAT drugs. The aim of the study is construction and preclinical evaluation of 211At labeled anti-mesothelin antibodies as potential TAT drugs. Mesothelin expression of two tumor cell lines were confirmed by flow cytometry, and their radiosensitivities were also evaluated. We used two kinds of anti-mesothelin antibodies, ET210-6 and ET210-28, to construct TAT drugs. Then, radiochemical purity, stability in vitro, affinity of the conjugates and mesothelin expression level were assessed. The specific killing of mesothelin-positive cancer cells treated by 211At-ET210-28 and 211At-ET210-6 were studied via Cell Counting Kit-8 assay and colony formation assay. 211At-ET210-28 and 211At-ET210-6 revealed excellent affinity and stability in both phosphate buffer saline and fetal bovine serum environment. Radiolabeled antibody conjugates bound specifically to mesothelin-positive cells in vitro. Both 211At-ET210-28 and 211At-ET210-6 could specifically kill mesothelin-positive cells with negligible damages to mesothelin-negative cells. Our findings provide initial proof-of-concept for the potential use of 211At labeled ET210-28/ET210-6 anti-mesothelin antibody in specific killings of mesothelin-positive tumor cells.

Construction and Preclinical Evaluation of a 124/131I-Labeled Radiotracer for the Detection of Mesothelin-Overexpressing Cancer

Mesothelin is a molecular biomarker of many types of solid cancers, which may represent a highly promising new target in the development of cancer-targeted diagnostic agents. A human anti-mesothelin antibody with a low molecular weight, ET210sc, was applied; this antibody has potent affinity and can penetrate tissue quickly and stably without causing immunoreactions. We developed a new ^124/131I-labeled radiotracer of ET210sc. The ^124/131I-labeled ET210sc radiotracer showed excellent radiochemical quality (with over 99% radiolabeling yield, 0.07 GBq/μmol specific activity) and remarkable stability in phosphate-buffered saline (>95% at 3 days). The radiotracer retained its potent affinity (dissociation constant, K_d = 0.101 nM). The radiotracer specifically bound to mesothelin-positive cells in vitro. Interestingly, the radiotracer exhibited significant positive-to-negative tumor uptake ratios (1.5:1) 3 days postinjection. The estimated absorbed doses of each organ (e.g., 0.704 mGy/MBq for the rectum; 0.341 mGy/MBq for the spleen) met the medical safety standards for further clinical applications. Our findings provide an initial proof of concept for the potential use of ^124/131I-labeled ET210sc radiotracers to detect mesothelin-overexpressing cancer. ¹²⁴I-ET210sc is proposed to be an ideal imaging agent for further clinical applications.

Build-up functionalization of anti-EGFR × anti-CD3 bispecific diabodies by integrating high-affinity mutants and functional molecular formats

Designing non-natural antibody formats is a practical method for developing highly functional next-generation antibody drugs, particularly for improving the therapeutic efficacy of cancer treatments. One approach is constructing bispecific antibodies (bsAbs). We previously reported a functional humanized bispecific diabody (bsDb) that targeted epidermal growth factor receptor and CD3 (hEx3-Db). We enhanced its cytotoxicity by constructing an Fc fusion protein and rearranging order of the V domain. In this study, we created an additional functional bsAb, by integrating the molecular formats of bsAb and high-affinity mutants previously isolated by phage display in the form of Fv. Introducing the high-affinity mutations into bsDbs successfully increased their affinities and enhanced their cytotoxicity in vitro and in vivo. However, there were some limitations to affinity maturation of bsDb by integrating high-affinity Fv mutants, particularly in Fc-fused bsDb with intrinsic high affinity, because of their bivalency. The tetramers fractionated from the bsDb mutant exhibited the highest in vitro growth inhibition among the small bsAbs and was comparable to the in vivo anti-tumor effects of Fc-fused bsDbs. This molecule shows cost-efficient bacterial production and high therapeutic potential.

Tri-functional platform for construction of modular antibody fragments for in vivo 18F-PET or NIRF molecular imaging

Positron emission tomography (PET) molecular imaging is a powerful tool for interrogating physiological and biochemical processes to understand the biology of disease and advance therapeutic developments. Near-infrared fluorescence (NIRF) optical imaging has become increasingly popular for intraoperative staging to enable cellular resolution imaging of tumor margins during surgical resection. In addition, engineered antibody fragments have emerged as promising molecular imaging agents given their exquisite target selectivity, rapid systemic clearance and site-selective chemical modification. We report a tri-functional platform for construction of a modular antibody fragment that can rapidly be labeled with radionuclides or fluorophores for PET or NIRF molecular imaging of prostate stem cell antigen (PSCA).

To provide a universal approach towards the targeted delivery of PET and optical imaging agents, we have developed a tri-functional platform (TFP) for the facile construction of modular, target-specific tracers.

Preclinical Imaging in Targeted Cancer Therapies

Preclinical imaging with radiolabeled probes can provide noninvasive tools to test the efficacy of targeted agents in tumors harboring specific genetic alterations and to identify imaging parameters that can be used as pharmacodynamics markers in cancer patients. The present review will primarily focus on preclinical imaging studies that can accelerate the clinical approval of targeted agents and promote the development of imaging biomarkers for clinical applications. Since only subgroups of patients may benefit from treatment with targeted anticancer agents, the identification of a patient population expressing the target is of primary importance for the success of clinical trials. Preclinical imaging studies tested the ability of new radiolabeled compounds to recognize mutant, amplified, or overexpressed targets and some of these tracers were transferred to the clinical setting. More common tracers such as ¹⁸F-Fluorothymidine and ¹⁸F-Fluorodeoxyglucose were employed in animal models to test the inhibition of the target and downstream pathways through the evaluation of early changes of proliferation and glucose metabolism allowing the identification of sensitive and resistant tumors. Furthermore, since the majority of patients treated with targeted anticancer agents will invariably develop resistance, preclinical imaging studies were performed to test the efficacy of reversal agents to overcome resistance. These studies provided consistent evidence that imaging with radiolabeled probes can monitor the reversal of drug resistance by newly designed alternative compounds. Finally, despite many difficulties and challenges, preclinical imaging studies targeting the expression of immune checkpoints proved the principle that it is feasible to select patients for immunotherapy based on imaging findings. In conclusion, preclinical imaging can be considered as an integral part of the complex translational process that moves a newly developed targeted agent from laboratory to clinical application intervening in all clinically relevant steps including patient selection, early monitoring of drug effects and reversal of drug resistance.

Combining Diagnostic Imaging and Pathology for Improving Diagnosis and Prognosis of Cancer

In the era of personalized medicine, the management of oncological patients requires a translational and multidisciplinary approach. During early phases of cancer development, biochemical alterations of cell metabolism occur much before the formation of detectable tumour masses. Current molecular imaging techniques, targeted to the study of molecular kinetics, employ molecular tracers capable of detecting cancer lesions with both high sensitivity and specificity while also providing essential information for both prognosis and therapy. On the contrary, complementary and crucial information is provided by histopathological examination and ancillary techniques such as immunohistochemistry. Thus, the successful collaboration between diagnostic imaging and anatomic pathology can represent a fundamental step in the "tortuous" but decisive path towards personalized medicine.

Immuno-PET Imaging and Pharmacokinetics of an Anti-CEA scFv-based Trimerbody and Its Monomeric Counterpart in Human Gastric Carcinoma-Bearing Mice

Monoclonal antibodies (mAbs) are currently used as therapeutic agents in different types of cancer. However, mAbs and antibody fragments developed so far show suboptimal properties in terms of circulation time and tumor penetration/retention. Here, we report the radiolabeling, pharmacokinetic evaluation, and determination of tumor targeting capacity of the previously validated anti-CEA MFE23-scFv-based N-terminal trimerbody (MFE23^N-trimerbody), and the results are compared to those obtained for the monomeric MFE23-scFv. Dissection and gamma-counting studies performed with the ¹³¹I-labeled protein scaffolds in normal mice showed slower blood clearance for the trimerbody, and accumulation in the kidneys, the spleen, and the liver for both species. These, together with a progressive uptake in the small intestine, confirm a combined elimination scheme with hepatobiliary and urinary excretion. Positron emission tomography studies performed in a xenograft mouse model of human gastric adenocarcinoma, generated by subcutaneous administration of CEA-positive human MKN45 cells, showed higher tumor accumulation and tumor-to-muscle (T/M) ratios for ¹²⁴I-labeled MFE23^N-trimerbody than for MFE23-scFv. Specific uptake was not detected with PET imaging in CEA negative xenografts as indicated by low T/M ratios. Our data suggest that engineered intermediate-sized trivalent antibody fragments could be promising candidates for targeted therapy and imaging of CEA-positive tumors.

F8-IL10: A New Potential Antirheumatic Drug Evaluated by a PET-Guided Translational Approach

Antibody fragment F8-mediated interleukin 10 (IL10) delivery is a novel treatment for rheumatoid arthritis (RA). F8 binds to the extra-domain-A of fibronectin (ED-A). In this study, in vivo biodistribution and arthritis targeting of radiolabeled F8-IL10 were investigated in RA patients, followed by further animal studies. Therefore, three RA patients (DAS28 > 3.2) received 0.4 mg of 30–74 megabecquerel [¹²⁴I]I–F8–IL10 for PET-CT and blood sampling. In visually identified PET-positive joints, target-to-background was calculated. Healthy mice, rats, and arthritic rats were injected with iodinated F8-IL10 or KSF-IL10 control antibody. Various organs were excised, weighed, and counted for radioactivity. Tissue sections were stained for fibronectin ED-A. In RA patients, [¹²⁴I]I–F8–IL10 was cleared rapidly from the circulation with less than 1% present in blood after 5 min. PET-CT showed targeting in 38 joints (11–15 per patient) and high uptake in the liver and spleen. Mean target-to-background ratios of PET-positive joints were 2.5 ± 1.2, 1.5 times higher for clinically active than clinically silent joints. Biodistribution of radioiodinated F8-IL10 in healthy mice showed no effect of the radioiodination method. [¹²⁴I]I–F8–IL10 joint uptake was also demonstrated in arthritic rats, ∼14-fold higher than that of the control antibody [¹²⁴I]I-KSF-IL10 (p < 0.001). Interestingly, liver and spleen uptake were twice as high in arthritic than in healthy rats and were related to increased (∼7×) fibronectin ED-A expression in these tissues. In conclusion, [¹²⁴I]I–F8–IL10 uptake was observed in arthritic joints in RA patients holding promise for visualization of inflamed joints by PET-CT imaging and therapeutic targeting. Patient observations and, subsequently, arthritic animal studies pointed to awareness of increased [¹²⁴I]I–F8–IL10 uptake in the liver and spleen associated with moderate systemic inflammation. This translational study demonstrated the value of in vivo biodistribution and PET-CT-guided imaging in development of new and potential antirheumatic drugs’.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications

Antibodies have long been recognised as potent vectors for carrying diagnostic medical radionuclides, contrast agents and optical probes to diseased tissue for imaging. The area of ImmunoPET combines the use of positron emission tomography (PET) imaging with antibodies to improve the diagnosis, staging and monitoring of diseases. Recent developments in antibody engineering and PET radiochemistry have led to a new wave of experimental ImmunoPET imaging agents that are based on a range of antibody fragments and affibodies. In contrast to full antibodies, engineered affibody proteins and antibody fragments such as minibodies, diabodies, single-chain variable region fragments (scFvs), and nanobodies are much smaller but retain the essential specificities and affinities of full antibodies in addition to more desirable pharmacokinetics for imaging. Herein, recent key developments in the PET radiolabelling strategies of antibody fragments and related affibody molecules are highlighted, along with the main PET imaging applications of overexpressed antigen-associated tumours and immune cells.

Engineering the hinge region of human IgG1 Fc-fused bispecific antibodies to improve fragmentation resistance

Fc domain fusion can improve the therapeutic effects of relatively small biological molecules such as peptides, cytokines, and antibody fragments. Fc fusion proteins can also be used to enhance the cytotoxic effects of small bispecific antibodies (bsAbs). However, fragmentation of Fc fusion proteins, which mainly occurs around the hinge regions during production, storage, and circulation in the blood, is a major issue. In this study, we first investigated the mechanisms of fragmentation around the hinge region during storage using Fc-fused bsAbs with specificity for epidermal growth factor receptor and CD3 as a model. The fragmentation peaks generated by gel filtration analysis indicated that both contaminating proteases and dissolved active oxygen should be considered causes of fragmentation. We designed and constructed variants by introducing a point mutation into the upper hinge region, which reduced the cleavage caused by dissolved active oxygen, and shortened the hinge region to restrict access of proteases. These hinge modifications improved fragmentation resistance and did not affect the biological activity of the bsAbs in vitro. We confirmed the versatility of the hinge modifications using another Fc-fused bsAb. Our results show that hinge modifications to the Fc fusion protein, especially the introduction of a point mutation into the upper hinge region, can reduce fragmentation substantially, and these modifications can be used to improve the fragmentation resistance of other recombinant Fc fusion proteins.

Dual-Modality ImmunoPET/Fluorescence Imaging of Prostate Cancer with an Anti-PSCA Cys-Minibody

Inadequate diagnostic methods for prostate cancer lead to over- and undertreatment, and the inability to intraoperatively visualize positive margins may limit the success of surgical resection. Prostate cancer visualization could be improved by combining the complementary modalities of immuno-positron emission tomography (immunoPET) for preoperative disease detection, and fluorescence imaging-guided surgery (FIGS) for real-time intraoperative tumor margin identification. Here, we report on the evaluation of dual-labeled humanized anti-prostate stem cell antigen (PSCA) cys-minibody (A11 cMb) for immunoPET/fluorescence imaging in subcutaneous and orthotopic prostate cancer models.

Methods: A11 cMb was site-specifically conjugated with the near-infrared fluorophore Cy5.5 and radiolabeled with ¹²⁴I or ⁸⁹Zr. ¹²⁴I-A11 cMb-Cy5.5 was used for successive immunoPET/fluorescence imaging of prostate cancer xenografts expressing high or moderate levels of PSCA (22Rv1-PSCA and PC3-PSCA). ⁸⁹Zr-A11 cMb-Cy5.5 dual-modality imaging was evaluated in an orthotopic model. Ex vivo biodistribution at 24 h was used to confirm the uptake values, and tumors were visualized by post-mortem fluorescence imaging.

Results: A11 cMb-Cy5.5 retained low nanomolar affinity for PSCA-positive cells. Conjugation conditions were established (dye-to-protein ratio of 0.7:1) that did not affect the biodistribution, pharmacokinetics, or clearance of A11 cMb. ImmunoPET using dual-labeled ¹²⁴I-A11 cMb-Cy5.5 showed specific targeting to both 22Rv1-PSCA and PC3-PSCA s.c. xenografts in nude mice. Ex vivo biodistribution confirmed specific uptake to PSCA-expressing tumors with 22Rv1-PSCA:22Rv1 and PC3-PSCA:PC3 ratios of 13:1 and 5.6:1, respectively. Consistent with the immunoPET, fluorescence imaging showed a strong signal from both 22Rv1-PSCA and PC3-PSCA tumors compared with non-PSCA expressing tumors. In an orthotopic model, ⁸⁹Zr-A11 cMb-Cy5.5 immunoPET was able to detect intraprostatically implanted 22Rv1-PSCA cells. Importantly, fluorescence imaging clearly distinguished the prostate tumor from surrounding seminal vesicles.

Conclusion: Dual-labeled A11 cMb specifically visualized PSCA-positive tumor by successive immunoPET/fluorescence, which can potentially be translated for preoperative whole-body prostate cancer detection and intraoperative surgical guidance in patients.

Comprehensive study of domain rearrangements of single-chain bispecific antibodies to determine the best combination of configurations and microbial host cells

Small bispecific antibodies (bsAbs) are important therapeutic molecules and represent the first bsAb format approved by the United States Food and Drug Administration. Diabody (Db), a small bsAb format, has four possible domain orders; we previously reported the differences in the expression levels and cancer growth inhibition effects upon rearranging the domain order of this format. However, there have been no comprehensive reports on domain rearrangements of bispecific single-chain Db (scDb) and tandem single-chain Fv (taFv), which are widely used bsAb formats. In this study, we designed all possible domain orders for scDb and taFv (each with eight variants) with identical Fv pairs and individually expressed all 16 variants using Escherichia coli, Pichia pastoris, and Brevibacillus choshinensis. Comprehensive investigations showed that the intrinsic functions of the variants were similar to each other, regardless of the expression host system, but expression levels varied depending on the format as well as on the host cell. Among the 16 variants, we found a promising candidate that exhibited high activity and productivity. Furthermore, we determined that B. choshinensis is an attractive expression host because of its secretory production of recombinant proteins.

Evaluation of HER2-specific peptide ligand for its employment as radiolabeled imaging probe

HER2 transmembrane receptor is an important target in immunotherapy treatment of breast and gastroesophageal cancer. Molecular imaging of HER2 expression may provide essential prognostic and predictive information concerning disseminated cancer and aid in selection of an optimal therapy. Radiolabeled low molecular weight peptide ligands are particularly attractive as probes for molecular imaging, since they reach and bind to the target and clear from non-target organs and blood stream faster than bulky antibodies. In this study, we evaluated a potential HER2-imaging probe, an A9 nonapeptide, derived from the trastuzumab-Fab portion. Its cellular uptake was investigated by mass spectrometry analysis of the cytoplasmic cellular extracts. Moreover, based on in-silico modeling, DTPA chelator was conjugated to N-terminus of A9. ¹¹¹In-labeled A9 demonstrated nanomolar affinity to HER2-expressing BT474 cells and favorable biodistribution profile in NMRI mice. This study suggests that the peptide A9 represents a good lead candidate for development of molecular probe, to be used for imaging purposes and for the delivery of cytotoxic agents.

Novel PSCA targeting scFv-fusion proteins for diagnosis and immunotherapy of prostate cancer

PURPOSE

Despite great progress in the diagnosis and treatment of localized prostate cancer (PCa), there remains a need for new diagnostic markers that can accurately distinguish indolent and aggressive variants. One promising approach is the antibody-based targeting of prostate stem cell antigen (PSCA), which is frequently overexpressed in PCa. Here, we show the construction of a molecular imaging probe comprising a humanized scFv fragment recognizing PSCA genetically fused to an engineered version of the human DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT), the SNAP-tag, enabling specific covalent coupling to various fluorophores for diagnosis of PCa. Furthermore, the recombinant immunotoxin (IT) PSCA(scFv)-ETA' comprising the PSCA(scFv) and a truncated version of Pseudomonas exotoxin A (PE, ETA') was generated.

METHODS

We analyzed the specific binding and internalization behavior of the molecular imaging probe PSCA(scFv)-SNAP in vitro by flow cytometry and live cell imaging, compared to the corresponding IT PSCA(scFv)-ETA'. The cytotoxic activity of PSCA(scFv)-ETA' was tested using cell viability assays. Specific binding was confirmed on formalin-fixed paraffin-embedded tissue specimen of early and advanced PCa.

RESULTS

Alexa Fluor® 647 labeling of PSCA(scFv)-SNAP confirmed selective binding to PSCA, leading to rapid internalization into the target cells. The recombinant IT PSCA(scFv)-ETA' showed selective binding leading to internalization and efficient elimination of target cells.

CONCLUSIONS

Our data demonstrate, for the first time, the specific binding, internalization, and cytotoxicity of a scFv-based fusion protein targeting PSCA. Immunohistochemical staining confirmed the specific ex vivo binding to primary PCa material.

Multifunctional Desferrichrome Analogues as Versatile 89Zr(IV) Chelators for ImmunoPET Probe Development

New bifunctional hexa- and octadentate analogues of the hydroxamate-containing siderophore desferrichrome (DFC) have been synthesized and evaluated as ⁸⁹Zr-chelating agents for immunoPET applications. The in vitro and in vivo inertness of these new ligands, Orn3-hx (hexadentate) and Orn-4hx derivatives (octadentate), was compared to the gold standard hexadentate, hydroxamate-containing chelator for ⁸⁹Zr desferrioxamine (DFO). Density functional theory was employed to model the geometries of the resulting Zr(IV) complexes and to predict their relative stabilities as follows: Zr(Orn4-hx) > Zr(DFC) > Zr(Orn3-hx). Transchelation challenge experiments of the corresponding radiochemical complexes with excess ethylenediaminetetraacetate (EDTA) indicated complex stability in accordance with DFT calculations. Radiolabeling of these ligands with ⁸⁹Zr was quantitative (0.25 μmol of ligand, pH 7.4, room temperature, 20 min). For antibody conjugation, the isothiocyanate (NCS) functional group was introduced to the N terminus of Orn3-hx and Orn-4hx. An additional trifunctional derivative that bears a silicon-rhodamine fluorophore on the C-terminus and NCS on the N terminus was also furnished. As proof of concept, all NCS derivatives were conjugated to the HER2-targeting antibody, trastuzumab. Radiolabeling of immunoconjugates with ⁸⁹Zr was accomplished with radiochemical yields of 16 ± 2% to 95 ± 2%. These constructs were administered to naive mice (male, C57BL/6J, n = 4) to assess in vivo inertness, which is inversely correlated with uptake of ⁸⁹Zr in bone, after 96 h circulation time. We found bone uptake to range from 7.0 ± 2.2 to 10.7 ± 1.3% ID/g, values that compare well to the corresponding DFO conjugate (7.1 ± 0.8% ID/g). In conclusion, we have rationally designed linear, bifunctional and trifunctional desferrichrome analogues suitable for the mild and inert radiolabeling of antibodies with the radionuclide ⁸⁹Zr.

Comparison of a mouse and a novel human scFv-SNAP-auristatin F drug conjugate with potent activity against EGFR-overexpressing human solid tumor cells

Antibody–drug conjugates (ADCs) can deliver toxins to specific targets such as tumor cells. They have shown promise in preclinical/clinical development but feature stoichiometrically undefined chemical linkages, and those based on full-size antibodies achieve only limited tumor penetration. SNAP-tag technology can overcome these challenges by conjugating benzylguanine-modified toxins to single-chain fragment variables (scFvs) with 1:1 stoichiometry while preserving antigen binding. Two (human and mouse) scFv-SNAP fusion proteins recognizing the epidermal growth factor receptor (EGFR) were expressed in HEK 293T cells. The purified fusion proteins were conjugated to auristatin F (AURIF). Binding activity was confirmed by flow cytometry/immunohistochemistry, and cytotoxic activity was confirmed by cell viability/apoptosis and cell cycle arrest assays, and a novel microtubule dynamics disassembly assay was performed. Both ADCs bound specifically to their target cells in vitro and ex vivo, indicating that the binding activity of the scFv-SNAP fusions was unaffected by conjugation to AURIF. Cytotoxic assays confirmed that the ADCs induced apoptosis and cell cycle arrest at nanomolar concentrations and microtubule disassembly. The SNAP-tag technology provides a platform for the development of novel ADCs with defined conjugation sites and stoichiometry. We achieved the stable and efficient linkage of AURIF to human or murine scFvs using the SNAP-tag technology, offering a strategy to improve the development of personalized medicines.

Cerenkov Luminescence Imaging as a Modality to Evaluate Antibody-Based PET Radiotracers

Antibodies, and engineered antibody fragments, labeled with radioisotopes are being developed as radiotracers for the detection and phenotyping of diseases such as cancer. The development of antibody-based radiotracers requires extensive characterization of their in vitro and in vivo properties, including their ability to target tumors in an antigen-selective manner. In this study, we investigated the use of Cerenkov luminescence imaging (CLI) as compared with PET as a modality for evaluating the in vivo behavior of antibody-based radiotracers.

METHODS

The anti-prostate-specific membrane antigen (PSMA) huJ591 antibody (IgG; 150 kDa) and its minibody (Mb; 80 kDa) format were functionalized with the chelator 1,4,7-triazacyclononane-1-glutaric acid-4,7-diacetic acid (NODAGA) and radiolabeled with the positron-emitting radionuclide ⁶⁴Cu (half-life, 12.7 h). Immunoreactive preparations of the radiolabeled antibodies were injected into NCr nu/nu mice harboring PSMA-positive CWR22Rv1 and PSMA-negative PC-3 tumor xenografts. Tumor targeting was evaluated by both PET and CLI.

RESULTS

⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb retained the ability to bind cell surface PSMA, and both radiotracers exhibited selective uptake into PSMA-positive tumors. Under the experimental conditions used, PSMA-selective uptake of ⁶⁴Cu-NODAGA-PSMA-IgG and ⁶⁴Cu-NODAGA-PSMA-Mb was observed by CLI as early as 3 h after injection, with tumor-to-background ratios peaking at 24 (IgG) and 16 (Mb) h after injection. Targeting data generated by CLI correlated with that generated by PET and necropsy.

CONCLUSION

CLI provided a rapid and simple assessment of the targeting specificity and pharmacokinetics of the antibody-based PET radiotracers that correlated well with the behavior observed by standard PET imaging. Moreover, CLI provided clear discrimination between uptake kinetics of an intact IgG and its small-molecular-weight derivative Mb. These data support the use of CLI for the evaluation of radiotracer performance.

New Dioxaborolane Chemistry Enables [(18)F]-Positron-Emitting, Fluorescent [(18)F]-Multimodality Biomolecule Generation from the Solid Phase

New protecting group chemistry is used to greatly simplify imaging probe production. Temperature and organic solvent-sensitive biomolecules are covalently attached to a biotin-bearing dioxaborolane, which facilitates antibody immobilization on a streptavidin-agarose solid-phase support. Treatment with aqueous fluoride triggers fluoride-labeled antibody release from the solid phase, separated from unlabeled antibody, and creates [(18)F]-trifluoroborate-antibody for positron emission tomography and near-infrared fluorescent (PET/NIRF) multimodality imaging. This dioxaborolane-fluoride reaction is bioorthogonal, does not inhibit antigen binding, and increases [(18)F]-specific activity relative to solution-based radiosyntheses. Two applications are investigated: an anti-epithelial cell adhesion molecule (EpCAM) monoclonal antibody (mAb) that labels prostate tumors and Cetuximab, an anti-epidermal growth factor receptor (EGFR) mAb (FDA approved) that labels lung adenocarcinoma tumors. Colocalized, tumor-specific NIRF and PET imaging confirm utility of the new technology. The described chemistry should allow labeling of many commercial systems, diabodies, nanoparticles, and small molecules for dual modality imaging of many diseases.

Process development of a human recombinant diabody expressed in E. coli: engagement of CD99-induced apoptosis for target therapy in Ewing's sarcoma

Ewing's sarcoma (EWS) is the second most common primary bone tumor in pediatric patients characterized by over expression of CD99. Current management consists in extensive chemotherapy in addition to surgical resection and/or radiation. Recent improvements in treatment are still overshadowed by severe side effects such as toxicity and risk of secondary malignancies; therefore, more effective strategies are urgently needed. The goal of this work was to develop a rapid, inexpensive, and "up-scalable" process of a novel human bivalent single-chain fragment variable diabody (C7 dAbd) directed against CD99, as a new therapeutic approach for EWS. We first investigated different Escherichia coli constructs of C7 dAbd in small-scale studies. Starting from 60 % soluble fraction, we obtained a yield of 25 mg C7 dAbd per liter of bacterial culture with the construct containing pelB signal sequence. In contrast, a low recovery of C7 dAbd was achieved starting from periplasmic inclusion bodies. In order to maximize the yield of C7 dAbd, large-scale fermentation was optimized. We obtained from 75 % soluble fraction 35 mg C7 dAbd per L of cell culture grown in a synthetic media containing 3 g/L of vegetable peptone and 1 g/L of yeast extract. Furthermore, we demonstrated the better efficacy of the cell lysis by homogenization versus periplasmic extraction, in reducing endotoxin level of the C7 dAbd. For gram-scale purification, a direct aligned two-step chromatography cascade based on binding selectivity was developed. Finally, we recovered C7 dAbd with low residual process-related impurities, excellent reactivity, and apoptotic ability against EWS cells.

Stability engineering of anti-EGFR scFv antibodies by rational design of a lambda-to-kappa swap of the VL framework using a structure-guided approach

Phage-display technology facilitates rapid selection of antigen-specific single-chain variable fragment (scFv) antibodies from large recombinant libraries. ScFv antibodies, composed of a VH and VL domain, are readily engineered into multimeric formats for the development of diagnostics and targeted therapies. However, the recombinant nature of the selection strategy can result in VH and VL domains with sub-optimal biophysical properties, such as reduced thermodynamic stability and enhanced aggregation propensity, which lead to poor production and limited application. We found that the C10 anti-epidermal growth factor receptor (EGFR) scFv, and its affinity mutant, P2224, exhibit weak production from E. coli. Interestingly, these scFv contain a fusion of lambda3 and lambda1 V-region (LV3 and LV1) genes, most likely the result of a PCR aberration during library construction. To enhance the biophysical properties of these scFvs, we utilized a structure-based approach to replace and redesign the pre-existing framework of the VL domain to one that best pairs with the existing VH. We describe a method to exchange lambda sequences with a more stable kappa3 framework (KV3) within the VL domain that incorporates the original lambda DE-loop. The resulting scFvs, C10KV3_LV1DE and P2224KV3_LV1DE, are more thermodynamically stable and easier to produce from bacterial culture. Additionally, C10KV3_LV1DE and P2224KV3_LV1DE retain binding affinity to EGFR, suggesting that such a dramatic framework swap does not significantly affect scFv binding. We provide here a novel strategy for redesigning the light chain of problematic scFvs to enhance their stability and therapeutic applicability.

A fully human scFv phage display library for rapid antibody fragment reformatting

Phage display libraries of human single-chain variable fragments (scFvs) are a reliable source of fully human antibodies for scientific and clinical applications. Frequently, scFvs form the basis of larger, bivalent formats to increase valency and avidity. A small and versatile bivalent antibody fragment is the diabody, a cross-paired scFv dimer (∼55 kDa). However, generation of diabodies from selected scFvs requires decreasing the length of the interdomain scFv linker, typically by overlap PCR. To simplify this process, we designed two scFv linkers with integrated restriction sites for easy linker length reduction (17-residue to 7-residue or 18-residue to 5-residue, respectively) and generated two fully human scFv phage display libraries. The larger library (9 × 10(9) functional members) was employed for selection against a model antigen, human N-cadherin, yielding novel scFv clones with low nanomolar monovalent affinities. ScFv clones from both libraries were reformatted into diabodies by restriction enzyme digestion and re-ligation. Size-exclusion chromatography analysis confirmed the proper dimerization of most of the diabodies. In conclusion, these specially designed scFv phage display libraries allow us to rapidly reformat the selected scFvs into diabodies, which can greatly accelerate early stage antibody development when bivalent fragments are needed for candidate screening.

(64)Cu-DOTA-trastuzumab PET imaging and HER2 specificity of brain metastases in HER2-positive breast cancer patients

Background

The purpose of this study was to determine whether brain metastases from HER2-positive breast cancer could be detected noninvasively using positron emission tomography (PET) with ⁶⁴Cu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-trastuzumab.

Methods

PET was performed on five patients with brain metastases from HER2-positive breast cancer, at 24 or 48 h after the injection of approximately 130 MBq of the probe ⁶⁴Cu-DOTA-trastuzumab. Radioactivity in metastatic brain tumors was evaluated based on PET images in five patients. Autoradiography, immunohistochemistry (IHC), and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis were performed in one surgical case to confirm HER2 specificity of ⁶⁴Cu-DOTA-trastuzumab.

Results

Metastatic brain lesions could be visualized by ⁶⁴Cu-DOTA-trastuzumab PET in all of five cases, which might indicated that trastuzumab passes through the blood-brain barrier (BBB). The HER2 specificity of ⁶⁴Cu-DOTA-trastuzumab was demonstrated in one patient by autoradiography, immunohistochemistry, and LC-MS/MS.

Conclusions

Cu-DOTA-trastuzumab PET could be a potential noninvasive procedure for serial identification of metastatic brain lesions in patients with HER2-positive breast cancer.

Trial registration

UMIN000004170

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.

Anti-MET immunoPET for non-small cell lung cancer using novel fully human antibody fragments

MET, the receptor of hepatocyte growth factor, plays important roles in tumorigenesis and drug resistance in numerous cancers, including non-small cell lung cancer (NSCLC). As increasing numbers of MET inhibitors are being developed for clinical applications, antibody fragment-based immunopositron emission tomography (immunoPET) has the potential to rapidly quantify in vivo MET expression levels for drug response evaluation and patient stratification for these targeted therapies. Here, fully human single-chain variable fragments (scFvs) isolated from a phage display library were reformatted into bivalent cys-diabodies (scFv-cys dimers) with affinities to MET ranging from 0.7 to 5.1 nmol/L. The candidate with the highest affinity, H2, was radiolabeled with (89)Zr for immunoPET studies targeting NSCLC xenografts: low MET-expressing Hcc827 and the gefitinib-resistant Hcc827-GR6 with 4-fold MET overexpression. ImmunoPET at as early as 4 hours after injection produced high-contrast images, and ex vivo biodistribution analysis at 20 hours after injection showed about 2-fold difference in tracer uptake levels between the parental and resistant tumors (P < 0.01). Further immunoPET studies using a larger fragment, the H2 minibody (scFv-CH3 dimer), produced similar results at later time points. Two of the antibody clones (H2 and H5) showed in vitro growth inhibitory effects on MET-dependent gefitinib-resistant cell lines, whereas no effects were observed on resistant lines lacking MET activation. In conclusion, these fully human antibody fragments inhibit MET-dependent cancer cells and enable rapid immunoPET imaging to assess MET expression levels, showing potential for both therapeutic and diagnostic applications.

Rearranging the domain order of a diabody-based IgG-like bispecific antibody enhances its antitumor activity and improves its degradation resistance and pharmacokinetics

One approach to creating more beneficial therapeutic antibodies is to develop bispecific antibodies (bsAbs), particularly IgG-like formats with tetravalency, which may provide several advantages such as multivalent binding to each target antigen. Although the effects of configuration and antibody-fragment type on the function of IgG-like bsAbs have been studied, there have been only a few detailed studies of the influence of the variable fragment domain order. Here, we prepared four types of hEx3-scDb-Fc, IgG-like bsAbs, built from a single-chain hEx3-Db (humanized bispecific diabody [bsDb] that targets epidermal growth factor receptor and CD3), to investigate the influence of domain order and fusion manner on the function of a bsDb with an Fc fusion format. Higher cytotoxicities were observed with hEx3-scDb-Fcs with a variable light domain (VL)-variable heavy domain (VH) order (hEx3-scDb-Fc-LHs) compared with a VH-VL order, indicating that differences in the Fc fusion manner do not affect bsDb activity. In addition, flow cytometry suggested that the higher cytotoxicities of hEx3-scDb-Fc-LH may be attributable to structural superiority in cross-linking. Interestingly, enhanced degradation resistance and prolonged in vivo half-life were also observed with hEx3-scDb-Fc-LH. hEx3-scDb-Fc-LH and its IgG2 variant exhibited intense in vivo antitumor effects, suggesting that Fc-mediated effector functions are dispensable for effective anti-tumor activities, which may cause fewer side effects. Our results show that merely rearranging the domain order of IgG-like bsAbs can enhance not only their antitumor activity, but also their degradation resistance and in vivo half-life, and that hEx3-scDb-Fc-LHs are potent candidates for next-generation therapeutic antibodies.

The development of immunoconjugates for targeted cancer therapy

Immunoconjugates are specific, highly effective, minimally toxic anticancer therapies that are beginning to show promise in the clinic. Immunoconjugates consist of three separate components: an antibody that binds to a cancer cell antigen with high specificity, an effector molecule that has a high capacity to kill the cancer cell, and a linker that will ensure the effector does not separate from the antibody during transit and will reliably release the effector to the cancer cell or tumour stroma. The high affinity antibody-antigen interaction allows specific and selective delivery of a range of effectors, including pharmacologic agents, radioisotopes, and toxins, to cancer cells. Some anticancer molecules are not well tolerated when administered systemically owing to unacceptable toxicity to the host. However, this limitation can be overcome through the linking of such cytotoxins to specific antibodies, which mask the toxic effects of the drug until it reaches its target. Conversely, many unconjugated antibodies are highly specific for a cancer target, but have low therapeutic potential and can be repurposed as delivery vehicles for highly potent effectors. In this Review, we summarize the successes and shortcomings of immunoconjugates, and discuss the future potential for the development of these therapies.

Antibody-based imaging of HER-2: moving into the clinic

Human epidermal growth factor receptor-2 (HER-2) mediates a number of important cellular activities, and is up-regulated in a diverse set of cancer cell lines, especially breast cancer. Accordingly, HER-2 has been regarded as a common drug target in cancer therapy. Antibodies can serve as ideal candidates for targeted tumor imaging and drug delivery, due to their inherent affinity and specificity. Advanced by the development of a wide variety of imaging techniques, antibody-based imaging of HER-2 can allow for early detection and localization of tumors, as well as monitoring of drug delivery and tissue's response to drug treatment. In this review article, antibody-based imaging of HER-2 are summarized and discussed, with an emphasis on the involved imaging methods.

Armed therapeutic viruses - a disruptive therapy on the horizon of cancer immunotherapy

For the past 150 years cancer immunotherapy has been largely a theoretical hope that recently has begun to show potential as a highly impactful treatment for various cancers. In particular, the identification and targeting of immune checkpoints have given rise to exciting data suggesting that this strategy has the potential to activate sustained antitumor immunity. It is likely that this approach, like other anti-cancer strategies before it, will benefit from co-administration with an additional therapeutic and that it is this combination therapy that may generate the greatest clinical outcome for the patient. In this regard, oncolytic viruses are a therapeutic moiety that is well suited to deliver and augment these immune-modulating therapies in a highly targeted and economically advantageous way over current treatment. In this review, we discuss the blockade of immune checkpoints, how oncolytic viruses complement and extend these therapies, and speculate on how this combination will uniquely impact the future of cancer immunotherapy.

Anti-CEA antibody fragments labeled with [(18)F]AlF for PET imaging of CEA-expressing tumors

A facile and rapid method to label peptides with (18)F based on chelation of [(18)F]AlF has been developed recently. Since this method requires heating to 100 °C, it cannot be used to label heat-sensitive proteins. Here, we used a two-step procedure to prepare (18)F-labeled heat-labile proteins using the [(18)F]AlF method based on hot maleimide conjugation. 1,4,7-Triazacyclononae-1,4-diacetate (NODA) containing a methyl phenylacetic acid group (MPA) functionalized with N-(2-aminoethyl)maleimide (EM) was used as a ligand which was labeled with [(18)F]AlF and then conjugated to the humanized anti-CEA antibody derivatives hMN-14-Fab', hMN-14-(scFv)2 (diabody), and a Dock-and-Lock engineered dimeric fragment hMN-14 Fab-AD2 at room temperature. The in vivo tumor targeting characteristics of the (18)F-labeled antibody derivatives were determined by PET imaging of mice with s.c. xenografts. NODA-MPAEM was radiolabeled with [(18)F]AlF at a specific activity of 29-39 MBq/nmol and a labeling efficiency of 94 ± 2%. The labeling efficiencies of the maleimide conjugation ranged from 70% to 77%, resulting in [(18)F]AlF-labeled hMN14-Fab', hMN14-Fab-AD2, or hMN14-diabody with a specific activity of 15-17 MBq/nmol. The radiolabeled conjugates were purified by gel filtration. For biodistribution and microPET imaging, antibody fragments were injected intravenously into BALB/c nude mice with s.c. CEA-expressing LS174T xenografts (right flank) and CEA-negative SK-RC-52 xenografts (left flank). All [(18)F]AlF-labeled conjugates showed specific uptake in the LS174T xenografts with a maximal tumor uptake of 4.73% ID/g at 4 h after injection. Uptake in CEA-negative SK-RC-52 xenografts was significantly lower. Tumors were clearly visualized on microPET images. Using a [(18)F]AlF-labeled maleimide functionalized chelator, antibody fragments could be radiofluorinated within 4 h at high specific activity. Here, we translated this method to preclinical PET imaging studies and showed feasibility of [(18)F]AlF-fluorinated hMN-14-Fab', [(18)F]AlF-hMN-14-Fab-AD2, and [(18)F]AlF-hMN-14-diabody for microPET imaging of CEA-expressing colonic cancer.

Advances in immuno-positron emission tomography: antibodies for molecular imaging in oncology

Identification of cancer cell-surface biomarkers and advances in antibody engineering have led to a sharp increase in the development of therapeutic antibodies. These same advances have led to a new generation of radiolabeled antibodies and antibody fragments that can be used as cancer-specific imaging agents, allowing quantitative imaging of cell-surface protein expression in vivo. Immuno-positron emission tomography (immunoPET) imaging with intact antibodies has shown success clinically in diagnosing and staging cancer. Engineered antibody fragments, such as diabodies, minibodies, and single-chain Fv (scFv) -Fc, have been successfully employed for immunoPET imaging of cancer cell-surface biomarkers in preclinical models and are poised to bring same-day imaging into clinical development. ImmunoPET can potentially provide a noninvasive approach for obtaining target-specific information useful for titrating doses for radioimmunotherapy, for patient risk stratification and selection of targeted therapies, for evaluating response to therapy, and for predicting adverse effects, thus contributing to the ongoing development of personalized cancer treatment.