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

Noninvasive molecular imaging approaches include nuclear, optical, magnetic resonance imaging, computed tomography, ultrasound, and photoacoustic imaging, which require accumulation of a signal delivered by a probe at the target site. Monoclonal antibodies are high affinity molecules that can be used for specific, high signal delivery to cell surface molecules. However, their long circulation time in blood makes them unsuitable as imaging probes. Efforts to improve antibodies pharmacokinetics without compromising affinity and specificity have been made through protein engineering. Antibody variants that differ in antigen binding sites and size have been generated and evaluated as imaging probes to target tissues of interest. Fast clearing fragments, such as single-chain variable fragment (scFv; 25 kDa), with 1 antigen-binding site (monovalent) demonstrated low accumulation in tumors because of the low exposure time to the target. Using scFv as building block to produce larger, bivalent fragments, such as scFv dimers (diabodies, 50 kDa) and scFv-fusion proteins (80 kDa minibodies and 105 kDa scFv-Fc), resulted in higher tumor accumulation because of their longer residence time in blood. Imaging studies with these fragments after radiolabeling have demonstrated excellent, high-contrast images in gamma cameras and positron emission tomography scanners. Several studies have also investigated antibody fragments conjugated to fluorescence (near infrared dyes), bioluminescence (luciferases), and quantum dots for optical imaging and iron oxides nanoparticles for magnetic resonance imaging. However, these studies indicate that there are several factors that influence successful targeting and imaging. These include stability of the antibody fragment, the labeling chemistry (direct or indirect), whether critical residues are modified, the number of antigen expressed on the cell, and whether the target has a rapid recycling rate or internalizes upon binding. The preclinical data presented are compelling, and it is evident that antibody-based molecular imaging tracers will play an important future role in the diagnosis and management of cancer and other diseases. [Copyright &y& Elsevier]

As the immuno-oncology field continues the rapid growth witnessed over the past decade, optimising patient outcomes requires an evolution in the current response-assessment guidelines for phase 2 and 3 immunotherapy clinical trials and clinical care. Additionally, investigational tools—including image analysis of standard-of-care scans (such as CT, magnetic resonance, and PET) with analytics, such as radiomics, functional magnetic resonance agents, and novel molecular-imaging PET agents—offer promising advancements for assessment of immunotherapy. To document current challenges and opportunities and identify next steps in immunotherapy diagnostic imaging, the National Cancer Institute Clinical Imaging Steering Committee convened a meeting with diverse representation among imaging experts and oncologists to generate a comprehensive review of the state of the field. [ABSTRACT FROM AUTHOR]

Introduction This work describes the development and characterization of two antibody fragments that specifically target the α v β 6 integrin, a non-covalent diabody and a disulfide-stabilized cys-diabody. The diabodies were analyzed for their ability to bind both immobilized and cell surface-bound α v β 6 . Radiolabeling was done using non-site-specific and site-specific conjugation approaches with N -succinimidyl 4-[ 18 F]fluorobenzoate ([ 18 F]-SFB) and the bifunctional chelator 1,4,7-triazacyclononane-triacetic acid maleimide (NOTA-maleimide) and copper-64 ([ 64 Cu]), respectively. The affects of each radiolabeling method on RCY, RCP, and immunoreactivity were analyzed for the [ 18 F]-FB-α v β 6 diabody, [ 18 F]-FB-α v β 6 cys-diabody, and the [ 64 Cu]-NOTA-α v β 6 cys-diabody. Methods Diabodies were constructed from the variable domains of the humanized 6.3G9 anti-α v β 6 intact antibody. The anti-α v β 6 cys-diabody was engineered with C-terminal cysteines to enable covalent dimerization and site-specific modification. Biochemical characterization included SDS-PAGE, Western blot, and electrospray ionization to confirm MW, and flow cytometry and ELISA experiments were used to determine binding affinity and specificity to α v β 6 . The diabodies were radiolabeled with [ 18 F]-SFB and in addition, the anti-α v β 6 cys-diabody was also radiolabeled site-specifically using NOTA-maleimide and [ 64 Cu]. Immunoreactivities were confirmed using in vitro cell binding to DX3Puroβ 6 (α v β 6 +) and DX3Puro (α v β 6 −) cell lines. Results The diabodies were purified from cell culture supernatants with purities >98%. Subnanomolar binding affinity towards α v β 6 was confirmed by ELISA (diabody IC 50 = 0.8 nM, cys-diabody IC 50 = 0.6 nM) and flow cytometry revealed high specificity only to the DX3Puroβ 6 cell line for both diabodies. RCYs were 22.6% ± 3.6% for the [ 18 F]-FB-α v β 6 diabody, 8.3% ± 1.7% for the [ 18 F]-FB-α v β 6 cys-diabody and 43.5% ± 5.5% for the [ 64 Cu]-NOTA-α v β 6 cys-diabody. In vitro cell binding assays revealed excellent specificity and retention of immunoreactivity ([ 18 F]-FB-α v β 6 diabody = 58.7% ± 6.7%, [ 18 F]-FB-α v β 6 cys-diabody = 80.4% ± 4.4%, [ 64 Cu]-NOTA-α v β 6 cys-diabody = 59.4% ± 0.6%) regardless of the radiolabeling method used. Conclusions Two novel diabodies with excellent binding affinity and specificity for the α v β 6 integrin in vitro were developed. Radiolabeling of the diabodies with fluorine-18 ([ 18 F]) and [ 64 Cu] revealed advantages and disadvantages with regards to methodologies and RCYs, however immunoreactivities were well preserved regardless of radiolabeling approach. [ABSTRACT FROM AUTHOR]

Serum half-life of IgG is controlled by the neonatal Fc receptor (FcRn) that interacts with the IgG Fc region and may be increased or decreased as a function of altered FcRn binding. Preclinical evaluations of modified IgGs are frequently carried out in mice, but such IgGs may bind differently to mouse and human FcRn (mFcRn and hFcRn). Here, we report a detailed characterization of a matched set of mouse-human chimeric T84.66 scFv-Fc variants with specificity for the tumor carcinoembryonic antigen and mutations in the FcRn-binding site. Binding to soluble mFcRn and hFcRn was measured using in vitro assays, and the results were compared with blood clearance in vivo in normal (mFcRn bearing) and hFcRn transgenic mice. All variants bound better to mFcRn than to hFcRn. The loss of affinity varied among the mutants, however, and also the hierarchy of binding differed depending on the receptor. The mutations had no major impact on binding to the classical Fcγ receptors. Importantly, the trend of blood clearance in both strains of mice correlated with the hierarchy of binding obtained using soluble FcRn. Consequently, in vitro interaction analysis of engineered IgGs regarding their cross-species FcRn binding ability provides information for prediction of in vivo pharmacokinetics. [ABSTRACT FROM AUTHOR]

Glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1), a GPI-anchored endothelial cell protein, binds lipoprotein lipase (LPL) and transports it into the lumen of capillaries where it hydrolyzes triglycerides in lipoproteins. GPIHBP1 is assumed to be expressed mainly within the heart, skeletal muscle, and adipose tissue, the sites where most lipolysis occurs, but the tissue pattern of GPIHBP1 expression has never been evaluated systematically. Because GPIHBP1 is found on the luminal face of capillaries, we predicted that it would be possible to define GPIHBP1 expression patterns with radiolabeled GPIHBP1-specific antibodies and positron emission tomography (PET) scanning. In Gpihbp1-/- mice, GPIHBP1-specific antibodies were cleared slowly from the blood, and PET imaging showed retention of the antibodies in the blood pools (heart and great vessels). In Gpihbp1+/+ mice, the antibodies were cleared extremely rapidly from the blood and, to our surprise, were taken up mainly by lung and liver. Immunofluorescence microscopy confirmed the presence of GPIHBP1 in the capillary endothelium of both lung and liver. In most tissues with high levels of Gpihbp1 expression, Lpl expression was also high, but the lung was an exception (very high Gpihbp1 expression and extremely low Lpl expression). Despite low Lpl transcript levels, however, LPL protein was readily detectable in the lung, suggesting that some of that LPL originates elsewhere and then is captured by GPIHBP1 in the lung. In support of this concept, lung LPL levels were significantly lower in Gpihbp1-/- mice than in Gpihbp1+/+ mice. In addition, Lpl-/- mice expressing human LPL exclusively in muscle contained high levels of human LPL in the lung. [ABSTRACT FROM AUTHOR]

Abstract: Introduction: A 105-kDa double mutant single-chain Fv-Fc fragment (scFv-Fc DM) derived from the anti-p185HER2 hu4D5v8 antibody (trastuzumab; Herceptin) has been described recently. The goal of this study was to investigate whether improved tumor targeting could be achieved with this fragment through the use of residualizing radioiodination methods. Methods: The scFv-Fc DM fragment was radioiodinated using N-succinimidyl 4-guanidinomethyl 3-[131I]iodobenzoate ([131I]SGMIB) and N ɛ-(3-[131I]iodobenzoyl)-Lys5-N α- maleimido-Gly1-GEEEK ([131I]IB-Mal-d-GEEEK), two residualizing radioiodination agents that have been used successfully with intact antibodies. Paired-label internalization assays of the labeled fragments were performed in vitro using MCF7 human breast cancer cells transfected to express HER2 (MCF7-HER2); comparisons were made to scFv-Fc DM directly radioiodinated using Iodogen. The tissue distribution of the scFv-Fc DM labeled with [125I]IB-Mal-d-GEEEK and [131I]SGMIB was compared in athymic mice bearing MCF7-HER2 xenografts. Results: The scFv-Fc DM fragment was labeled with [131I]SGMIB and [131I]IB-Mal-d-GEEEK in conjugation yields of 53% and 25%, respectively, with preservation of immunoreactivity for HER2. Internalization assays indicated that labeling via SGMIB resulted in a 1.6- to 3.5-fold higher (P<.05) retention of radioactivity, compared to that from the directly labeled fragment, in HER2-expressing cells during a 24-h observation period. Likewise, the amount of radioactivity retained in cells from the IB-Mal-d-GEEEK-labeled fragment was 1.4- to 3.3-fold higher (P<.05). Tumor uptake of radioiodine activity in athymic mice bearing MCF7-HER2 xenografts in vivo was significantly higher for the [125I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment compared with that of the [131I]SGMIB-labeled fragment, particularly at later time points. The uptake of 125I was threefold (3.6±1.1 %ID/g vs. 1.2±0.4 %ID/g) and fourfold (3.1±1.7 %ID/g vs. 0.8±0.4 %ID/g) higher than that for 131I at 24 and 48 h, respectively. However, the [125I]IB-Mal-d-GEEEK-labeled scFv-Fc DM fragment also exhibited considerably higher levels of radioiodine activity in liver, spleen and kidney. Conclusions: The overall results further demonstrate the potential utility of these two prosthetic groups for the radiohalogenation of internalizing monoclonal antibodies and their fragments. Specifically, the trastuzumab-derived double mutant fragment in combination with these residualizing agents warrants further evaluation for imaging and possibly treatment of HER2 expressing malignancies. [Copyright &y& Elsevier]