ImmunoPET imaging of B-cell lymphoma using 124I-anti-CD20 scFv dimers (diabodies)

Molecular Imaging of Innate Immunity and Immunotherapy

The innate immune system plays a key role as the first line of defense in various human diseases including cancer, cardiovascular and inflammatory diseases. In contrast to tissue biopsies and blood biopsies, in vivo imaging of the innate immune system can provide whole body measurements of immune cell location and function and changes in response to disease progression and therapy. Rationally developed molecular imaging strategies can be used in evaluating the status and spatio-temporal distributions of the innate immune cells in near real-time, mapping the biodistribution of novel innate immunotherapies, monitoring their efficacy and potential toxicities, and eventually for stratifying patients that are likely to benefit from these immunotherapies. In this review, we will highlight the current state-of-the-art in noninvasive imaging techniques for preclinical imaging of the innate immune system particularly focusing on cell trafficking, biodistribution, as well as pharmacokinetics and dynamics of promising immunotherapies in cancer and other diseases; discuss the unmet needs and current challenges in integrating imaging modalities and immunology and suggest potential solutions to overcome these barriers.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

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.

Outcome Prediction and Evaluation by Imaging the Key Elements of Therapeutic Responses to Cancer Immunotherapies Using PET

Cancer immunotherapy (also known as immuno-oncology), a promising anti-cancer strategy by harnessing the body's own immune system against cancer, has emerged as the "fifth therapeutic pilla" in the field of cancer treatment since surgery, chemotherapy, radiation and targeted therapy. Clinical efficacy of several immunotherapies has been demonstrated in clinical settings, however, only a small subset of patients exhibit dramatic or durable responses, with the highest reported frequency about 10-40% from single-agent PD-L1/PD-1 inhibitors, suggesting the urgent need of consistent objective response biomarkers for monitoring therapeutic response accurately, predicting therapeutic efficacy and selecting responders. Key elements of therapeutic responses to cancer immunotherapies contain the cancer cell response and the alternation of inherent immunological characteristics. Here, we document the literature regarding imaging the key elements of therapeutic responses to cancer immunotherapies using PET. We discussed PET imaging approaches according to different response mechanisms underlying diverse immune-therapeutic categories, and also highlight the ongoing efforts to identify novel immunotherapeutic PET imaging biomarkers. In this article, we show that PET imaging of the key elements of therapeutic responses to cancer immunotherapies using PET can allow for more precise prediction, earlier therapy response monitoring, and improved management. However, all of these strategies need more preclinical study and clinical validation before further development as imaging indicators of the immune response.

Production of Stabilized Antibody Fragments in the E. coli Bacterial Cytoplasm and in Transiently Transfected Mammalian Cells

Monoclonal antibodies (mAbs) are currently the fastest growing class of therapeutic proteins. Parallel to full-length IgG format the development of recombinant technologies provided the production of smaller recombinant antibody variants. The single-chain variable fragment (scFv) antibody is a minimal form of functional antibody comprised of the variable domains of immunoglobulin light and heavy chains connected by a flexible linker. In most cases, scFvs are expressed in the periplasm bacterium E. coli. The production of soluble scFvs is more effective in quantity, however, under the reducing conditions of the E. coli bacterial cytoplasm it is inefficient because of the inability of the disulfide bonds to form. Hence, scFvs are either secreted to the periplasm as soluble proteins or expressed in the cytoplasm as insoluble inclusion bodies and recovered by refolding. The cytoplasmic expression of scFvs as a C-terminal fusion to maltose-binding protein (MBP) provided the high-level production of stable, soluble, and functional fusion protein. The below protocol provides the detailed description of MBP-scFv production in E. coli utilizing two expression systems: pMALc-TNN and pMALc-NHNN. Although the MBP tag does not disrupt the most of antibody activities, the MBP-TNN-scFv product can be cleaved by Tobacco Etch Virus (TEV) protease in order to obtain untagged scFv.The second protocol is for efficient production of Fab antibody fragments as MBP fusion proteins secreted by transiently transfected mammalian cells. While transient transfection is a fast and effective way of obtaining several mgs of antibody for initial screening and validation of antibodies, some antibody sequences express poorly or not at all. For such antibodies, fusion to MBP provides an effective approach for solving the expression problem.

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.

The Immunoimaging Toolbox

The recent clinical success of cancer immunotherapy has renewed interest in the development of tools to image the immune system. In general, immunotherapies attempt to enable the body's own immune cells to seek out and destroy malignant disease. Molecular imaging of the cells and molecules that regulate immunity could provide unique insight into the mechanisms of action, and failure, of immunotherapies. In this article, we will provide a comprehensive overview of the current state-of-the-art immunoimaging toolbox with a focus on imaging strategies and their applications toward immunotherapy.

Pharmacokinetics of radiolabeled dimeric sdAbs constructs targeting human CD20

Single-domain antibody fragments (sdAbs) are the smallest functional antigen-binding fragments, derived from heavy chain-only camelid antibodies. When designed as radiolabeled monomeric probes for imaging and therapy of cancer, their fast and specific targeting results in high tumor-to-background ratios early after injection. However, their moderate absolute uptake into tumors might not always be sufficient to treat cancerous lesions. We have evaluated the pharmacokinetics of seven constructs derived from a CD20-targeting monomeric sdAb (αCD20). The constructs differed in affinity or avidity towards CD20 (dimeric αCD20-αCD20 and αCD20 fused to a non-targeting control sdAb, referred to as αCD20-ctrl) and blood half-lives (αCD20 fused to an albumin-targeting sdAb (αAlb) = αCD20-αAlb). The constructs were radiolabeled with ¹¹¹In (imaging) and ¹⁷⁷Lu (therapy) using the bifunctional chelator CHX-A"-DTPA and evaluated in vitro and in vivo. In mice, tumor uptake of ¹⁷⁷Lu-DTPA-αCD20 decreased from 4.82 ± 1.80 to 0.13 ± 0.05% IA/g over 72 h. Due to its rapid blood clearance, tumor-to-blood (T/B) ratios of >100 were obtained within 24 h. Although in vitro internalization indicated that dimeric ¹⁷⁷Lu-DTPA-αCD20-αCD20 was superior in terms of total cell-associated radioactivity, this was not confirmed in vivo. Blood clearance was slower and absolute tumor uptake became significantly higher for αCD20-αAlb. Blood levels of ¹⁷⁷Lu-DTPA-αCD20-αAlb decreased from 68.30 ± 10.53 to 3.58 ± 0.66% IA/g over 120 h, while tumor uptake increased from 6.21 ± 0.94 to 24.90 ± 2.83% IA/g, resulting in lower T/B ratios. Taken together, these results indicate that the increased size of dimeric αCD20-αCD20 or the fusion of monomeric αCD20 to an albumin-targeting moiety (αAlb) counterbalance their improved tumor targeting capacity compared to monomeric αCD20.

Theranostic Radiolabeled Anti-CD20 sdAb for Targeted Radionuclide Therapy of Non-Hodgkin Lymphoma

Anti-CD20 radioimmunotherapy is an effective approach for therapy of relapsed or refractory CD20^pos lymphomas, but faces limitations due to poor tumor penetration and undesirable pharmacokinetics of full antibodies. Camelid single-domain Ab fragments (sdAb) might circumvent some of the limitations of radiolabeled full antibodies. In this study, a set of hCD20-targeting sdAbs was generated, and their capacity to bind hCD20 was evaluated in vitro and in vivo A lead sdAb, sdAb 9079, was selected on the basis of its specific tumor targeting and significant lower kidney accumulation compared with other sdAbs. SdAb 9079 was then radiolabeled with ⁶⁸Ga and ¹⁷⁷Lu for PET imaging and targeted therapy. The therapeutic potential of ¹⁷⁷Lu-DTPA-sdAb was compared with that of ¹⁷⁷Lu-DTPA-rituximab and unlabeled rituximab in mice bearing hCD20^pos tumors. Radiolabeled with ⁶⁸Ga, sdAb 9079 showed specific tumor uptake, with very low accumulation in nontarget organs, except kidneys. The tumor uptake of ¹⁷⁷Lu-DTPA-sdAb 9079 after 1.5 h was 3.4 ± 1.3% ID/g, with T/B and T/M ratios of 13.3 ± 4.6 and 32.9 ± 15.6. Peak tumor accumulation of ¹⁷⁷Lu-DTPA-rituximab was about 9 times higher, but concomitantly with high accumulation in nontarget organs and very low T/B and T/M ratios (0.8 ± 0.1 and 7.1 ± 2.4). Treatment of mice with ¹⁷⁷Lu-DTPA-sdAb 9079 significantly prolonged median survival compared with control groups and was as effective as treatment with rituximab or its ¹⁷⁷Lu-labeled variant. Taken together, sdAb 9079 displays promising features as a theranostic drug to treat CD20^pos lymphomas. Mol Cancer Ther; 16(12); 2828-39. ©2017 AACR.

ImmunoPET of Malignant and Normal B Cells with 89Zr- and 124I-Labeled Obinutuzumab Antibody Fragments Reveals Differential CD20 Internalization In Vivo

Purpose: The B-cell antigen CD20 provides a target for antibody-based positron emission tomography (immunoPET). We engineered antibody fragments targeting human CD20 and studied their potential as immunoPET tracers in transgenic mice (huCD20TM) and in a murine lymphoma model expressing human CD20.Experimental Design: Anti-CD20 cys-diabody (cDb) and cys-minibody (cMb) based on rituximab and obinutuzumab (GA101) were radioiodinated and used for immunoPET imaging of a murine lymphoma model. Pairwise comparison of obinutuzumab-based antibody fragments labeled with residualizing (⁸⁹Zr) versus non-residualizing (¹²⁴I) radionuclides by region of interest analysis of serial PET images was conducted both in the murine lymphoma model and in huCD20TM to assess antigen modulation in vivoResults:¹²⁴I-GAcDb and ¹²⁴I-GAcMb produced high-contrast immunoPET images of B-cell lymphoma and outperformed the respective rituximab-based tracers. ImmunoPET imaging of huCD20TM showed specific uptake in lymphoid tissues. The use of the radiometal ⁸⁹Zr as alternative label for GAcDb and GAcMb yielded greater target-specific uptake and retention compared with ¹²⁴I-labeled tracers. Pairwise comparison of ⁸⁹Zr- and ¹²⁴I-labeled GAcDb and GAcMb allowed assessment of in vivo internalization of CD20/antibody complexes and revealed that CD20 internalization differs between malignant and endogenous B cells.Conclusions: These obinutuzumab-based PET tracers have the ability to noninvasively and quantitatively monitor CD20-expression and have revealed insights into CD20 internalization upon antibody binding in vivo Because they are based on a humanized mAb they have the potential for direct clinical translation and could improve patient selection for targeted therapy, dosimetry prior to radioimmunotherapy, and prediction of response to therapy. Clin Cancer Res; 23(23); 7242-52. ©2017 AACR.

Lymphoma: current status of clinical and preclinical imaging with radiolabeled antibodies

Lymphoma is a complex disease that arises from cells of the immune system with an intricate pathology. While lymphoma may be classified as Hodgkin or non-Hodgkin, each type of tumor is genetically and phenotypically different and highly invasive tissue biopsies are the only method to investigate these differences. Noninvasive imaging strategies, such as immunoPET, can provide a vital insight into disease staging, monitoring treatment response in patients, and dose planning in radioimmunotherapy. ImmunoPET imaging with radiolabeled antibody-based tracers may also assist physicians in optimizing treatment strategies and enhancing patient stratification. Currently, there are two common biomarkers for molecular imaging of lymphoma, CD20 and CD30, both of which have been considered for investigation in preclinical imaging studies. In this review, we examine the current status of both preclinical and clinical imaging of lymphoma using radiolabeled antibodies. Additionally, we briefly investigate the role of radiolabeled antibodies in lymphoma therapy. As radiolabeled antibodies play critical roles in both imaging and therapy of lymphoma, the development of novel antibodies and the discovery of new biomarkers may greatly affect lymphoma imaging and therapy in the future.

Radiation Dosimetry Study of [(89)Zr]rituximab Tracer for Clinical Translation of B cell NHL Imaging using Positron Emission Tomography

PURPOSE

We evaluated the dosimetry of [(89)Zr]rituximab, an anti-CD20 immunoPET tracer to image B cell non-Hodgkin's lymphoma (NHL) using a humanized transgenic mouse model that expresses human CD20 transgenic mice (huCD20TM).

PROCEDURES

Rituximab was conjugated to desferrioxamine (Df) for radiolabeling of Zirconium-89. [(89)Zr]rituximab (2.8 ± 0.2 MBq) was tail vein-injected into huCD20T mice. Positron emission tomography (PET)/CT imaging was performed on the two groups of mice (blocking = 2 mg/kg pre-dose of rituximab and non-blocking; n = 5) at eight time points (1, 4, 24, 48, 72, 96, 120, and 168 h) post injection.

RESULTS

The novel [(89)Zr]rituximab PET tracer had good immunoreactivity, was stable in human serum, and was able to specifically target human CD20 in mice. The human equivalents of highest dose (mean ± SD) organs with and without pre-dose are liver (345 ± 284 μSv/MBq) and spleen (1165 ± 149 μSv/MBq), respectively.

CONCLUSIONS

Dosimetry of the human patient whole-body dose was found to be 145 MBq per annum, and the patient dose-limiting organ will be the liver (with rituximab pre-dose blocking) and spleen for non-blocking. The [(89)Zr]rituximab (t½ = 78.4 h) imaging of B cell NHL patients could permit the observation of targeting lesions in NHL patients over an extended period due to longer half-life as compared to the [(64)Cu] rituximab (t½ = 12.7 h).

Fluorescent Image-Guided Surgery with an Anti-Prostate Stem Cell Antigen (PSCA) Diabody Enables Targeted Resection of Mouse Prostate Cancer Xenografts in Real Time

PURPOSE

The inability to visualize cancer during prostatectomy contributes to positive margins, cancer recurrence, and surgical side effects. A molecularly targeted fluorescent probe offers the potential for real-time intraoperative imaging. The goal of this study was to develop a probe for image-guided prostate cancer surgery.

EXPERIMENTAL DESIGN

An antibody fragment (cys-diabody, cDb) against prostate stem cell antigen (PSCA) was conjugated to a far-red fluorophore, Cy5. The integrity and binding of the probe to PSCA was confirmed by gel electrophoresis, size exclusion, and flow cytometry, respectively. Subcutaneous models of PSCA-expressing xenografts were used to assess the biodistribution and in vivo kinetics, whereas an invasive intramuscular model was utilized to explore the performance of Cy5-cDb-mediated fluorescence guidance in representative surgical scenarios. Finally, a prospective, randomized study comparing surgical resection with and without fluorescent guidance was performed to determine whether this probe could reduce the incidence of positive margins.

RESULTS

Cy5-cDb demonstrated excellent purity, stability, and specific binding to PSCA. In vivo imaging showed maximal signal-to-background ratios at 6 hours. In mice carrying PSCA(+) and negative (-) dual xenografts, the mean fluorescence ratio of PSCA(+/-) tumors was 4.4:1. In surgical resection experiments, residual tumors <1 mm that were missed on white light surgery were identified and resected using fluorescence guidance, which reduced the incidence of positive surgical margins (0/8) compared with white light surgery alone (7/7).

CONCLUSIONS

Fluorescently labeled cDb enables real-time in vivo imaging of prostate cancer xenografts in mice, and facilitates more complete tumor removal than conventional white light surgery alone.

High contrast tumor imaging with radio-labeled antibody Fab fragments tailored for optimized pharmacokinetics via PASylation

Although antigen-binding fragments (Fabs) of antibodies constitute established tracers for in vivo radiodiagnostics, their functionality is hampered by a very short circulation half-life. PASylation, the genetic fusion with a long, conformationally disordered amino acid chain comprising Pro, Ala and Ser, provides a convenient way to expand protein size and, consequently, retard renal filtration. Humanized αHER2 and αCD20 Fabs were systematically fused with 100 to 600 PAS residues and produced in E. coli. Cytofluorimetric titration analysis on tumor cell lines confirmed that antigen-binding activities of the parental antibodies were retained. The radio-iodinated PASylated Fabs were studied by positron emission tomography (PET) imaging and biodistribution analysis in mouse tumor xenograft models. While the unmodified αHER2 and αCD20 Fabs showed weak tumor uptake (0.8% and 0.2% ID/g, respectively; 24 h p.i.) tumor-associated radioactivity was boosted with increasing PAS length (up to 9 and 26-fold, respectively), approaching an optimum for Fab-PAS₄₀₀. Remarkably, 6- and 5-fold higher tumor-to-blood ratios compared with the unmodified Fabs were measured in the biodistribution analysis (48 h p.i.) for αHER2 Fab-PAS₁₀₀ and Fab-PAS₂₀₀, respectively. These findings were confirmed by PET studies, showing high imaging contrast in line with tumor-to-blood ratios of 12.2 and 5.7 (24 h p.i.) for αHER2 Fab-PAS₁₀₀ and Fab-PAS₂₀₀. Even stronger tumor signals were obtained with the corresponding αCD20 Fabs, both in PET imaging and biodistribution analysis, with an uptake of 2.8% ID/g for Fab-PAS₁₀₀vs. 0.24% ID/g for the unmodified Fab. Hence, by engineering Fabs via PASylation, plasma half-life can be tailored to significantly improve tracer uptake and tumor contrast, thus optimally matching reagent/target interactions.

Near-infrared fluorescence imaging of non-Hodgkin's lymphoma CD20 expression using Cy7-conjugated obinutuzumab

PURPOSE

Obinutuzumab is the first fully humanized and glycoengineered monoclonal antibody (mAb) directly targeting CD20 antigen, which is expressed on B cell lymphocytes and the majority of non-Hodgkin's lymphoma (NHL). This study aims to design a diagnostic molecular probe, Cy7-Obinutuzumab (Cy7-Obi), in which Cy7 is a near-infrared fluorescent dye. This probe is used to noninvasively image CD20 antigen expressed in NHL cells.

PROCEDURES

Cy7-Obi probe was synthesized through nucleophilic substitution reaction between NHS-Cy7 and obinutuzumab. After purification, the conjugate was fully characterized by a series of methods. The immunoreactivity and molecular specificity of the probe were confirmed using flow cytometry and in vitro microscopy on Raji (CD20-positive) cells. For in vivo imaging, Cy7-Obi probe (1 nmol) was injected intravenously in severe combined immunodeficiency (SCID) mice bearing Raji tumors which overexpress CD20 (n = 3) and was imaged with near-infrared fluorescence (NIRF) at 6, 9, 12, 24, 60, and 96 h post-probe injection. For pre-block, obinutuzumab (3.25 mg) was injected intravenously in tumor-bearing mice 6 h before the administration of Cy7-Obi probe.

RESULTS

The synthesized Cy7-Obi probe in this paper mimics obinutuzumab in both structure and function. Flow cytometry analysis of the probe and obinutuzumab on Raji cells showed minor difference in binding affinity/specificity with CD20. The probe showed significant fluorescence signal when it was examined on Raji cells using in vitro microscopy. The fluorescence signal can be blocked by pretreatment with obinutuzumab. The probe Cy7-Obi also showed high tumor uptake when it was examined by in vivo optical imaging on Raji tumor-bearing mice. The tumor uptake can be blocked by pretreatment with obinutuzumab (n = 3, p < 0.05). The in vivo imaging results were also confirmed by ex vivo imaging of dissected organs. Finally, the probe Cy7-Obi has shown excellent tumor targeting and specificity through immunofluorescence analysis.

CONCLUSIONS

We have shown that humanized Cy7-Obi probe can be used for NIRF imaging successfully. The probe may be an effective and noninvasive diagnostic molecular probe capable of tracking CD20 overexpression in NHL.

Development, optimization, and validation of novel anti-TEM1/CD248 affinity agent for optical imaging in cancer

Tumor Endothelial Marker-1 (TEM1/CD248) is a tumor vascular marker with high therapeutic and diagnostic potentials. Immuno-imaging with TEM1-specific antibodies can help to detect cancerous lesions, monitor tumor responses, and select patients that are most likely to benefit from TEM1-targeted therapies. In particular, near infrared(NIR) optical imaging with biomarker-specific antibodies can provide real-time, tomographic information without exposing the subjects to radioactivity. To maximize the theranostic potential of TEM1, we developed a panel of all human, multivalent Fc-fusion proteins based on a previously identified single chain antibody (scFv78) that recognizes both human and mouse TEM1. By characterizing avidity, stability, and pharmacokinectics, we identified one fusion protein, 78Fc, with desirable characteristics for immuno-imaging applications. The biodistribution of radiolabeled 78Fc showed that this antibody had minimal binding to normal organs, which have low expression of TEM1. Next, we developed a 78Fc-based tracer and tested its performance in different TEM1-expressing mouse models. The NIR imaging and tomography results suggest that the 78Fc-NIR tracer performs well in distinguishing mouse- or human-TEM1 expressing tumor grafts from normal organs and control grafts in vivo. From these results we conclude that further development and optimization of 78Fc as a TEM1-targeted imaging agent for use in clinical settings is warranted.

Enhanced immunoPET of ALCAM-positive colorectal carcinoma using site-specific ⁶⁴Cu-DOTA conjugation

Activated leukocyte cell adhesion molecule (ALCAM) is an immunoglobulin superfamily cell adhesion molecule that is aberrantly expressed in a wide variety of human tumors, including melanoma, prostate cancer, breast cancer, colorectal carcinoma, bladder cancer and pancreatic adenocarcinoma. This wide spectrum of human malignancies makes ALCAM a prospective pan-cancer immunoPET target to aid in detection and diagnosis in multiple malignancies. In this study, we assess site-specific versus non-site-specific conjugation strategies for (64)Cu-DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) immunoPET imaging of a fully human ALCAM cys-diabody (cDb) with a reduced linker length that retains its bivalent binding ability. ALCAM constructs with linker lengths of eight, five and three amino acids were produced to make true non-covalent site-specifically modified cDbs. Characterization by gel electrophoresis, size exclusion chromatography, flow cytometry and mass spectrometry of the various constructs was performed. To demonstrate the increased utility of targeting multiple malignancies expressing ALCAM, we compare the targeting of the site-specific versus non-site-specific conjugated cDbs to the human colorectal cancer xenograft LS174T. Interestingly, the conjugation strategy not only affects tumor targeting but also hepatic and renal uptake/clearance.

Tandem myeloablative 131I-rituximab radioimmunotherapy and high-dose chemotherapy in refractory/relapsed non-Hodgkin lymphoma patients

This Phase I/II study investigated myeloablative (131)I-rituximab radioimmunotherapy (RIT) and high-dose chemotherapy supported by one or two autologous stem cell transplantations in heavily pretreated patients with relapsed or refractory B cell non-Hodgkin lymphoma. Myeloablative RIT was safe and feasible when followed by autologous stem cell transplantation with low incidence of secondary late effects and could be a reasonable alternative regimen especially in elderly patients and in patients who have concerns about high-dose chemotherapy. Tandem myeloablative (131)I-rituximab RIT and high-dose chemotherapy supported by two autologous stem cell transplantations was also feasible. However, the toxicity was higher than after myeloablative RIT, therefore it might be recommended to restrict the tandem approach to lymphoma with poor prognosis.

Engineered antibody fragments for immuno-PET imaging of endogenous CD8+ T cells in vivo

The noninvasive detection and quantification of CD8(+) T cells in vivo are important for both the detection and staging of CD8(+) lymphomas and for the monitoring of successful cancer immunotherapies, such as adoptive cell transfer and antibody-based immunotherapeutics. Here, antibody fragments are constructed to target murine CD8 to obtain rapid, high-contrast immuno-positron emission tomography (immuno-PET) images for the detection of CD8 expression in vivo. The variable regions of two anti-murine CD8-depleting antibodies (clones 2.43 and YTS169.4.2.1) were sequenced and reformatted into minibody (Mb) fragments (scFv-CH3). After production and purification, the Mbs retained their antigen specificity and bound primary CD8(+) T cells from the thymus, spleen, lymph nodes, and peripheral blood. Importantly, engineering of the parental antibodies into Mbs abolished the ability to deplete CD8(+) T cells in vivo. The Mbs were subsequently conjugated to S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid for (64)Cu radiolabeling. The radiotracers were injected i.v. into antigen-positive, antigen-negative, immunodeficient, antigen-blocked, and antigen-depleted mice to evaluate specificity of uptake in lymphoid tissues by immuno-PET imaging and ex vivo biodistribution. Both (64)Cu-radiolabeled Mbs produced high-contrast immuno-PET images 4 h postinjection and showed specific uptake in the spleen and lymph nodes of antigen-positive mice.

Production of stabilized scFv antibody fragments in the E. coli bacterial cytoplasm

Monoclonal antibodies (mAbs) are currently the fastest growing class of therapeutic proteins. Parallel to full-length IgG format the development of recombinant technologies provided the production of smaller recombinant antibody variants. The single-chain variable fragment (scFv) antibody is a minimal form of functional antibody comprised of the variable domains of immunoglobulin light and heavy chains connected by a flexible linker. In most cases, scFvs are expressed in the bacterium E. coli. The production of soluble scFvs under the reducing conditions of the E. coli bacterial cytoplasm is inefficient because of the inability of the disulfide bonds to form. Hence, scFvs are either secreted to the periplasm as soluble proteins or expressed in the cytoplasm as insoluble inclusion bodies and recovered by refolding. The cytoplasmic expression of scFvs as a C-terminal fusion to maltose-binding protein (MBP) provided the high-level production of stable, soluble, and functional fusion protein. The below protocol provides the detailed description of MBP-scFv production in E. coli utilizing two expression systems: pMalc-TNN and pMalc-NHNN. Although the MBP tag does not disrupt the most of antibody activities, the MBP-TNN-scFv product can be cleaved by TEV protease in order to obtain untagged scFv.

A novel engineered anti-CD20 tracer enables early time PET imaging in a humanized transgenic mouse model of B-cell non-Hodgkins lymphoma

PURPOSE

The aim of this article was to evaluate the use of a novel engineered anti-CD20 protein based on the 10 kDa human fibronectin type 3 domain (FN3) and subsequently compare with (64)Cu-rituximab for positron emission tomography (PET) imaging of CD20.

EXPERIMENTAL DESIGN

The engineered FN3(CD20) and FN3(WT) were produced in Escherichia coli cells at 2 to 5 mg/L, conjugated to DOTA, labeled with (64)Cu, and used for PET imaging of huCD20 expression in B cells. Humanized transgenic mice and subcutaneously xenografted mice each received intravenous (64)Cu-FN3(CD20) or FN3(WT) (3.7 MBq/4 μg Do-FN3 in 200 μL PBS). Control group received a blocking dose (50-fold excess) of unconjugated FN3(CD20) two hours before radiotracer injection. PET imaging was carried out at 1 to 24 hours postinjections.

RESULTS

In vitro assay demonstrated FN3 binds CD20 with 20 nmol/L affinity on CD20-expressing cells. (64)Cu-FN3(CD20) showed clear, high-contrast visualization of huCD20-expressing B cells in the spleen of transgenic mice as early as 1 hour postinjection [38 ± 3% injected dose (ID)/g] and exhibited a spleen-to-blood ratio of 13 by 4 hours. This is higher uptake (P = 0.04) and 10-fold greater signal-to-background (P = 0.04) than the (64)Cu-rituximab antibody radiotracer. Tumor uptake (16.8 ± 1.6 vs. 5.6 ± 1.4%ID/g) and tumor:background ratios were superior for FN3CD20 relative to rituximab in xenograft studies as well.

CONCLUSIONS

The (64)Cu-Do-FN3(CD20) radiotracer represents a novel small, high-affinity binder for imaging human CD20, which may be well suited for B-cell non-Hodgkin's lymphoma imaging in patients at early time points.

Radretumab radioimmunotherapy in patients with brain metastasis: a 124I-L19SIP dosimetric PET study

Radioimmunotherapy (RIT) with (131)I-labeled L19SIP (radretumab; a small immunoprotein format antibody directed against the ED-B domain of fibronectin; ∼ 80 kDa molecular weight) has been investigated in several clinical trials. Here, we describe the use of immuno-PET imaging with iodine-124 ((124)I)-labeled L19SIP to predict doses delivered to tumor lesions and healthy organs by a subsequent radretumab RIT in patients with brain metastases from solid cancer. Bone marrow doses were evaluated both during the diagnostic phase and posttherapy, measuring activities in blood (germanium detector) and whole body (lanthanum bromide detector). Expected doses for radretumab administration (4,107 MBq/m(2)) were calculated from data obtained after administration of an average of 167 MBq (124)I-L19SIP to 6 patients. To assess lesion average doses, the positron emission tomography (PET) scanner was calibrated for the use of (124)I with an International Electrotechnical Commission (IEC) Body Phantom and recovery coefficients were calculated. The average dose to bone red marrow was 0.21 Gy/GBq, with high correlation between provisional and actual posttherapy doses. Although the fraction of injected activity in normal organs was similar in different patients, the antibody uptake in the neoplastic lesions varied by as much as a factor of 60. Immuno-PET with (124)I-labeled L19SIP offers significant advantages over conventional (131)I imaging, in particular accuracy of dosimetric results. Furthermore, the study indicates that antibody uptake can be highly variable even in different lesions of the same patient and that immuno-PET procedures may guide product development with armed antibodies.

Positron emission tomography of 64Cu-DOTA-Rituximab in a transgenic mouse model expressing human CD20 for clinical translation to image NHL

PURPOSE

This study aims to evaluate (64)Cu-DOTA-rituximab (PETRIT) in a preclinical transgenic mouse model expressing human CD20 for potential clinical translation.

PROCEDURES

(64)Cu was chelated to DOTA-rituximab. Multiple radiolabeling, quality assurance, and imaging experiments were performed. The human CD20 antigen was expressed in B cells of transgenic mice (CD20TM). The mice groups studied were: (a) control (nude mice, n = 3) that received 7.4 MBq/dose, (b) with pre-dose (CD20TM, n = 6) received 2 mg/kg pre-dose of cold rituximab prior to PETRIT of 7.4 MBq/dose, and (c) without pre-dose (CD20TM, n = 6) PETRIT alone received 7.4 MBq/dose. Small animal PET was used to image mice at various time points (0, 1, 2, 4, 24, 48, and 72 h). The OLINDA/EXM software was used to determine the human equivalent dose for individual organs.

RESULTS

PETRIT was obtained with a specific activity of 545 ± 38.91 MBq/nmole, radiochemical purity >95%, and immunoreactivity >75%. At 24 h, spleenic uptake of PETRIT%ID/g (mean ± STD) with and without pre-dose was 1.76 ± 0.43% and 16.5 ± 0.45%, respectively (P value = 0.01). Liver uptake with and without pre-dose was 0.41 ± 0.51% and 0.52 ± 0.17% (P value = 0.86), respectively. The human equivalents of highest dose organs with and without pre-dose are osteogenic cells at 30.8 ± 0.4 μSv/MBq and the spleen at 99 ± 4 μSv/MBq, respectively.

CONCLUSIONS

PET imaging with PETRIT in huCD20 transgenic mice provided human dosimetry data for eventual applications in non-Hodgkins lymphoma patients.

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.

Analysis of the relationship between end-to-end distance and activity of single-chain antibody against colorectal carcinoma

We investigated the relationship of End-to-end distance between VH and VL with different peptide linkers and the activity of single-chain antibodies by computer-aided simulation. First, we developed (G4S)n (where n = 1-9) as the linker to connect VH and VL, and estimated the 3D structure of single-chain Fv antibody (scFv) by homologous modeling. After molecular models were evaluated and optimized, the coordinate system of every protein was built and unified into one coordinate system, and End-to-end distances calculated using 3D space coordinates. After expression and purification of scFv-n with (G4S)n as n = 1, 3, 5, 7 or 9, the immunoreactivity of purified ND-1 scFv-n was determined by ELISA. A multi-factorial relationship model was employed to analyze the structural factors affecting scFv: rn=ABn-ABO2+CDn-CDO2+BCn-BCst2. The relationship between immunoreactivity and r-values revealed that fusion protein structure approached the desired state when the r-value = 3. The immunoreactivity declined as the r-value increased, but when the r-value exceeded a certain threshold, it stabilized. We used a linear relationship to analyze structural factors affecting scFv immunoreactivity.

Anti-carcinoembryonic antigen single-chain variable fragment antibody variants bind mouse and human neonatal Fc receptor with different affinities that reveal distinct cross-species differences in serum half-life

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.

Design and production of multimeric antibody fragments, focused on diabodies with enhanced clinical efficacy

Multimeric antibody fragments, particularly dimers (diabodies), trimers (triabodies), and tetramers (tetrabodies) of single-chain Fv molecules (scFv), provide high avidity through multivalent binding to the target antigen. The combination of their smaller size and avid binding can provide desirable biological characteristics for tumor targeting applications in vivo; for example, diabodies can have greater tumor penetration and faster blood clearance rates compared to intact full-size antibodies (IgGs). The pharmacokinetic and biodistribution characteristics can further be optimized by the addition of specific thiolation sites for conjugation of PEG molecules to regulate molecular weight and reduce kidney uptake. Thiolation sites can also be used for precise loading of therapeutic payloads. This protocol describes our method for construction and bacterial production of soluble multimeric antibody scFv fragments, focusing on diabodies (scFv dimers).

IBC's 21st Annual Antibody Engineering and 8th Annual Antibody Therapeutics International Conferences and 2010 Annual Meeting of the Antibody Society. December 5-9, 2010, San Diego, CA USA

The 21^st Annual Antibody Engineering and 8^th Annual Antibody Therapeutics international conferences, and the 2010 Annual Meeting of The Antibody Society, organized by IBC Life Sciences with contributions from The Antibody Society and two Scientific Advisory Boards, was held December 5–9, 2010 in San Diego, CA. The conferences featured over 100 presentations and 100 posters, and included a pre-conference workshop on deep-sequencing of antibody genes. The total number of delegates exceeded 800, which set a new attendance record for the conference.

The conferences were organized with a focus on antibody engineering only on the first day and a joint engineering/therapeutics session on the last day. Delegates could select from presentations that occurred in two simultaneous sessions on days 2 and 3. Day 1 included presentations on neutralizing antibodies and the identification of vaccine targets, as well as a historical overview of 20 years of phage display utilization. Topics presented in the Antibody Engineering sessions on day 2 and 3 included antibody biosynthesis, structure and stability; antibodies in a complex environment; antibody half-life; and targeted nanoparticle therapeutics. In the Antibody Therapeutics sessions on days 2 and 3, preclinical and early stage development and clinical updates of antibody therapeutics, including TRX518, SYM004, MM111, PRO140, CVX-241, ASG-5ME, U3-1287 (AMG888), R1507 and trastuzumab emtansine, were discussed and perspectives were provided on the development of biosimilar and biobetter antibodies, including coverage of regulatory and intellectual property issues. The joint engineering/therapeutics session on the last day focused on bispecific and next-generation antibodies. Summaries of most of the presentations are included here, but, due to the large number of speakers, it was not possible to include summaries for every presentation.

Delegates enjoyed the splendid views of the San Diego Bay and proximity to the Gaslamp Quarter provided by the venue. The 22^nd Annual Antibody Engineering and 9th Annual Antibody Therapeutics conferences, and the 2011 Annual Meeting of The Antibody Society, are planned for December 5–8, 2011 at the same location in San Diego, and will include two two-day short courses on Introduction to Antibody Engineering and Protein Characterization for Biotechnology Product Development.

Bioinformatics-led design of single-chain antibody molecules targeting DNA sequences for retinoblastoma

AIM

To analyze the relationship between the structure and function of single-chain Fv antibody (scFv) with bioinformatics methods, so as to provide theoretical basis for retinoblastoma targeted therapy.

METHODS

Single-chain antibodies are reconstructed for cancer-targeted therapy to provide good penetration into tumor tissue and to improve their pharmacokinetics in vivo, offering a clinically valuable application. The relationship needs to be analyzed that there may be some variations between the structure and function of the fusion proteins, and the relationship between the structure and function of protein molecules was obtained through analyzing relevant literature at home and abroad as well as modeling analysis.

RESULTS

Through our analysis of the interaction region between the antibody and the antigen, and of the binding sites for molecular conformation, it is clear that existing antibodies need to be modified at the DNA sequence level, enhancing the biological activity of the antibodies. Based on the view that bio-molecular computer models are closely integrated with biological experiments, a bio-molecular structure-activity relationship model can be established in terms of molecular conformation, physical and chemical properties and the biological activity of single-chain antibodies. Two enlightenments are obtained from our analysis. On the one hand, the structure-activity relationship is clear for new immune molecules at the gene expression level. On the other hand, a single-chain antibody molecule can be designed and optimized for the cancer-oriented treatment.

CONCLUSION

In this article, we provide the theoretical and experimental basis for the development of single-chain antibodies appropriate for retinoblastoma therapy.

Chemically modified antibodies as diagnostic imaging agents

Notable new applications of antibodies for imaging involve genetically extracting the essential molecular recognition properties of an antibody, and in some cases enhancing them by mutation, before protein expression. The classic paradigm of intravenous administration of a labeled antibody to image not only its target but also its metabolism can be improved on. Protocols involving molecular targeting with an engineered unlabeled protein derived from an antibody, followed by capture of a small probe molecule that provides a signal, are being developed to a high level of utility. This is accompanied by new strategies for probe capture such as irreversible binding, incorporation of engineered enzyme active sites, and antibody-ligand systems that generate a signal only upon binding or uptake.

Antibody vectors for imaging

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.