Preclinical evaluation of multistep targeting of diasialoganglioside GD2 using an IgG-scFv bispecific antibody with high affinity for GD2 and DOTA metal complex

Theranostic GPA33-Pretargeted Radioimmunotherapy of Human Colorectal Carcinoma with a Bivalent 177Lu-Labeled Radiohapten

Radiolabeled small-molecule DOTA-haptens can be combined with antitumor/anti-DOTA bispecific antibodies (BsAbs) for pretargeted radioimmunotherapy (PRIT). For optimized delivery of the theranostic γ- and β-emitting isotope 177Lu with DOTA-based PRIT (DOTA-PRIT), bivalent Gemini (DOTA-Bn-thiourea-PEG4-thiourea-Bn-DOTA, aka (3,6,9,12-tetraoxatetradecane-1,14-diyl)bis(DOTA-benzyl thiourea)) was developed. Methods: Gemini was synthesized by linking 2 S-2-(4-isothiocyanatobenzyl)-DOTA molecules together via a 1,14-diamino-PEG4 linker. [177Lu]Lu-Gemini was prepared with no-carrier-added 177LuCl3 to a molar-specific activity of 123 GBq/μmol and radiochemical purity of more than 99%. The specificity of BsAb-177Lu-Gemini was verified in vitro. Subsequently, we evaluated biodistribution and whole-body clearance for [177Lu]Lu-Gemini and, for comparison, our gold-standard monovalent [177Lu]Lu-S-2-(4-aminobenzyl)-DOTA ([177Lu]Lu-DOTA-Bn) in naïve (tumor-free) athymic nude mice. For our proof-of-concept system, a 3-step pretargeting approach was performed with an established DOTA-PRIT regimen (anti-GPA33/anti-DOTA IgG-scFv BsAb, a clearing agent, and [177Lu]Lu-Gemini) in mouse models. Results: Initial in vivo studies showed that [177Lu]Lu-Gemini behaved similarly to [177Lu]Lu-DOTA-Bn, with almost identical blood and whole-body clearance kinetics, as well as biodistribution and mouse kidney dosimetry. Pretargeting [177Lu]Lu-Gemini to GPA33-expressing SW1222 human colorectal xenografts was highly effective, leading to absorbed doses of [177Lu]Lu-Gemini for blood, tumor, liver, spleen, and kidneys of 3.99, 455, 6.93, 5.36, and 14.0 cGy/MBq, respectively. Tumor-to-normal tissue absorbed-dose ratios (i.e., therapeutic indices [TIs]) for the blood and kidneys were 114 and 33, respectively. In addition, we demonstrate that the use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT leads to improved TIs and augmented [177Lu]Lu-Gemini tumor uptake and retention in comparison to monovalent [177Lu]Lu-DOTA-Bn. Finally, we established efficacy in SW1222 tumor-bearing mice, demonstrating that a single injection of anti-GPA33 DOTA-PRIT with 44 MBq (1.2 mCi) of [177Lu]Lu-Gemini (estimated tumor-absorbed dose, 200 Gy) induced complete responses in 5 of 5 animals and a histologic cure in 2 of 5 (40%) animals. Moreover, a significant increase in survival compared with nontreated controls was noted (maximum tolerated dose not reached). Conclusion: We have developed a bivalent DOTA-radiohapten, [177Lu]Lu-Gemini, that showed improved radiopharmacology for DOTA-PRIT application. The use of bivalent [177Lu]Lu-Gemini in DOTA-PRIT, as opposed to monovalent [177Lu]Lu-DOTA-Bn, allows curative treatments with considerably less administered 177Lu activity while still achieving high TIs for both the blood (>100) and the kidneys (>30).

GD2-targeting therapy: a comparative analysis of approaches and promising directions

Disialoganglioside GD2 is a promising target for immunotherapy with expression primarily restricted to neuroectodermal and epithelial tumor cells. Although its role in the maintenance and repair of neural tissue is well-established, its functions during normal organism development remain understudied. Meanwhile, studies have shown that GD2 plays an important role in tumorigenesis. Its functions include proliferation, invasion, motility, and metastasis, and its high expression and ability to transform the tumor microenvironment may be associated with a malignant phenotype. Structurally, GD2 is a glycosphingolipid that is stably expressed on the surface of tumor cells, making it a suitable candidate for targeting by antibodies or chimeric antigen receptors. Based on mouse monoclonal antibodies, chimeric and humanized antibodies and their combinations with cytokines, toxins, drugs, radionuclides, nanoparticles as well as chimeric antigen receptor have been developed. Furthermore, vaccines and photoimmunotherapy are being used to treat GD2-positive tumors, and GD2 aptamers can be used for targeting. In the field of cell therapy, allogeneic immunocompetent cells are also being utilized to enhance GD2 therapy. Efforts are currently being made to optimize the chimeric antigen receptor by modifying its design or by transducing not only αβ T cells, but also γδ T cells, NK cells, NKT cells, and macrophages. In addition, immunotherapy can combine both diagnostic and therapeutic methods, allowing for early detection of disease and minimal residual disease. This review discusses each immunotherapy method and strategy, its advantages and disadvantages, and highlights future directions for GD2 therapy.

Efficacy of HER2-Targeted Intraperitoneal 225Ac α-Pretargeted Radioimmunotherapy for Small-Volume Ovarian Peritoneal Carcinomatosis

Epithelial ovarian cancer (EOC) is often asymptomatic and presents clinically in an advanced stage as widespread peritoneal microscopic disease that is generally considered to be surgically incurable. Targeted α-therapy with the α-particle-emitting radionuclide 225Ac (half-life, 9.92 d) is a high-linear-energy-transfer treatment approach effective for small-volume disease and even single cells. Here, we report the use of human epidermal growth factor receptor 2 (HER2) 225Ac-pretargeted radioimmunotherapy (PRIT) to treat a mouse model of human EOC SKOV3 xenografts growing as peritoneal carcinomatosis (PC). Methods: On day 0, 105 SKOV3 cells transduced with a luciferase reporter gene were implanted intraperitoneally in nude mice, and tumor engraftment was verified by bioluminescent imaging (BLI). On day 15, treatment was started using 1 or 2 cycles of 3-step anti-HER2 225Ac-PRIT (37 kBq/cycle as 225Ac-Proteus DOTA), separated by a 1-wk interval. Efficacy and toxicity were monitored for up to 154 d. Results: Untreated PC-tumor-bearing nude mice showed a median survival of 112 d. We used 2 independent measures of response to evaluate the efficacy of 225Ac-PRIT. First, a greater proportion of the treated mice (9/10 1-cycle and 8/10 2-cycle; total, 17/20; 85%) survived long-term compared with controls (9/27, 33%), and significantly prolonged survival was documented (log-rank [Mantel-Cox] P = 0.0042). Second, using BLI, a significant difference in the integrated BLI signal area to 98 d was noted between controls and treated groups (P = 0.0354). Of a total of 8 mice from the 2-cycle treatment group (74 kBq total) that were evaluated by necropsy, kidney radiotoxicity was mild and did not manifest itself clinically (normal serum blood urea nitrogen and creatinine). Dosimetry estimates (relative biological effectiveness-weighted dose, where relative biological effectiveness = 5) per 37 kBq administered for tumors and kidneys were 56.9 and 16.1 Gy, respectively. One-cycle and 2-cycle treatments were equally effective. With immunohistology, mild tubular changes attributable to α-toxicity were observed in both therapeutic groups. Conclusion: Treatment of EOC PC-tumor-bearing mice with anti-HER2 225Ac-PRIT resulted in histologic cures and prolonged survival with minimal toxicity. Targeted α-therapy using the anti-HER2 225Ac-PRIT system is a potential treatment for otherwise incurable EOC.

Current Status and Future Perspective on Molecular Imaging and Treatment of Neuroblastoma

Neuroblastoma is the most common extracranial solid tumor in children and arises from anywhere along the sympathetic nervous system. It is a highly heterogeneous disease with a wide range of prognosis, from spontaneous regression or maturing to highly aggressive. About half of pediatric neuroblastoma patients develop the metastatic disease at diagnosis, which carries a poor prognosis. Nuclear medicine plays a pivotal role in the diagnosis, staging, response assessment, and long-term follow-up of neuroblastoma. And it has also played a prominent role in the treatment of neuroblastoma. Because the structure of metaiodobenzylguanidine (MIBG) is similar to that of norepinephrine, 90% of neuroblastomas are MIBG-avid. 123I-MIBG whole-body scintigraphy is the standard nuclear imaging technique for neuroblastoma, usually in combination with SPECT/CT. However, approximately 10% of neuroblastomas are MIBG nonavid. PET imaging has many technical advantages over SPECT imaging, such as higher spatial and temporal resolution, higher sensitivity, superior quantitative capability, and whole-body tomographic imaging. In recent years, various tracers have been used for imaging neuroblastoma with PET. The importance of patient-specific targeted radionuclide therapy for neuroblastoma therapy has also increased. 131I-MIBG therapy is part of the front-line treatment for children with high-risk neuroblastoma. And peptide receptor radionuclide therapy with radionuclide-labeled somatostatin analogues has been successfully used in the therapy of neuroblastoma. Moreover, radioimmunoimaging has important applications in the diagnosis of neuroblastoma, and radioimmunotherapy may provide a novel treatment modality against neuroblastoma. This review discusses the use of current and novel radiopharmaceuticals in nuclear medicine imaging and therapy of neuroblastoma.

“Click-to-Clear”: A Strategy to Minimize Radioactivity from the Blood Pool Utilizing Staudinger Ligation

The availability of several bioorthogonal reactions that can proceed selectively and efficiently under physiologically relevant conditions has garnered the interest of biochemists and organic chemists alike. Bioorthogonal cleavage reactions represent the latest innovation in click chemistry. Here, we employed the Staudinger ligation reaction to release radioactivity from immunoconjugates, improving target-to-background ratios. In this proof-of-concept study, model systems, including the anti-HER2 antibody trastuzumab, radioisotope I-131, and a newly synthesized bifunctional phosphine, were used. Staudinger ligation occurred when biocompatible N-glycosyl azides reacted with this radiolabeled immunoconjugate, leading to cleavage of the radioactive label from the molecule. We demonstrated this click cleavage in vitro and in vivo. Biodistribution studies in tumor models showed that radioactivity was eliminated from the bloodstream, thereby improving tumor-to-blood ratios. SPECT imaging revealed that tumors could be visualized with enhanced clarity. Our simple approach represents a novel application of bioorthogonal click chemistry in the development of antibody-based theranostics.

A rotaxane-based platform for tailoring the pharmacokinetics of cancer-targeted radiotracers

Radiolabelled monoclonal antibodies (mAbs) are a cornerstone of molecular diagnostic imaging and targeted radioimmunotherapy in nuclear medicine, but one of the major challenges in the field is to identify ways of reducing the radiation burden to patients. We reasoned that a rotaxane-based platform featuring a non-covalent mechanical bond between the radionuclide complex and the biologically active mAb could offer new ways of controlling the biophysical properties of cancer-specific radiotracers for positron emission tomography (PET). Herein, we present the photoradiosynthesis and characterisation of [⁸⁹Zr]ZrFe-[4]rotaxane-azepin-onartuzumab ([⁸⁹Zr]ZrFe-2), a unique rotaxane-antibody conjugate for PET imaging and quantification of the human hepatocyte growth factor receptor (c-MET). Multiple component self-assembly reactions were combined with simultaneous ⁸⁹Zr-radiolabelling and light-induced bioconjugation methods to give [⁸⁹Zr]ZrFe-2 in 15 ± 1% (n = 3) decay-corrected radiochemical yield, with >90% radiochemical purity, and molar activities suitable for PET imaging studies (>6.1 MBq mg^-1 of protein). Cellular assays confirmed the specificity of [⁸⁹Zr]ZrFe-2 binding to the c-MET receptor. Temporal PET imaging in athymic nude mice bearing subcutaneous MKN-45 gastric adenocarcinoma xenografts demonstrated specific binding of [⁸⁹Zr]ZrFe-2 toward c-MET in vivo, where tumour uptake reached 9.8 ± 1.3 %ID g^-1 (72 h, n = 5) in a normal group and was reduced by ∼56% in a control (blocking) group. Head-to-head comparison of the biodistribution and excretion profile of [⁸⁹Zr]ZrFe-2versus two control compounds, alongside characterisation of two potential metabolites, showed that the rotaxane-radiotracer has an improved clearance profile with higher tumour-to-tissue contrast ratios and reduced radiation exposure to critical (dose-limiting) organs including liver, spleen, and kidneys. Collectively, the experimental results suggested that non-covalent mechanical bonds between the radionuclide and mAb can be used to fine-tune the pharmacokinetic profile of supramolecular radiopharmaceuticals in ways that are simply not accessible when using traditional covalent design.

Pretargeting: A Path Forward for Radioimmunotherapy

Pretargeted radioimmunodiagnosis and radioimmunotherapy aim to efficiently combine antitumor antibodies and medicinal radioisotopes for high-contrast imaging and high-therapeutic-index (TI) tumor targeting, respectively. As opposed to conventional radioimmunoconjugates, pretargeted approaches separate the tumor-targeting step from the payload step, thereby amplifying tumor uptake while reducing normal-tissue exposure. Alongside contrast and TI, critical parameters include antibody immunogenicity and specificity, availability of radioisotopes, and ease of use in the clinic. Each of the steps can be optimized separately; as modular systems, they can find broad applications irrespective of tumor target, tumor type, or radioisotopes. Although this versatility presents enormous opportunity, pretargeting is complex and presents unique challenges for clinical translation and optimal use in patients. The purpose of this article is to provide a brief historical perspective on the origins and development of pretargeting strategies in nuclear medicine, emphasizing 2 protein delivery systems that have been extensively evaluated (i.e., biotin-streptavidin and hapten-bispecific monoclonal antibodies), as well as radiohaptens and radioisotopes. We also highlight recent innovations, including pretargeting with bioorthogonal chemistry and novel protein vectors (such as self-assembling and disassembling proteins and Affibody molecules). We caution the reader that this is by no means a comprehensive review of the past 3 decades of pretargeted radioimmunodiagnosis and pretargeted radioimmunotherapy. But we do aim to highlight major developmental milestones and to identify benchmarks for success with regard to TI and toxicity in preclinical models and clinically. We believe this approach will lead to the identification of key obstacles to clinical success, revive interest in the utility of radiotheranostics applications, and guide development of the next generation of pretargeted theranostics.

Bispecific antibodies for the treatment of neuroblastoma

Bispecific antibodies (BsAb) are a new generation of antibody-based therapy, conveying artificial specificity to polyclonal T cells or radiohaptens. These drugs have been successfully implemented to cure hematologic malignancies and are under clinical investigation for solid tumors including HRNB. BsAbs designed to engage T cells or increase the therapeutic index of radiotherapy hold the potential to significantly improve the long-term survival of HRNB patients by shrinking bulky tumors and more effectively eliminating micrometastases and preventing relapse. BsAbs can also be used to arm T cells, yielding a product analogous to CAR T cells, possibly with an improved safety profile. A thoughtful and realistic integration of these therapies into the standard of care should benefit more patients worldwide. Here we describe the history of development of BsAbs for HRNB, which dates back almost three decades. We discuss the merits and pitfalls of all relevant BsAbs, including T cell-engagers and agents used for radioimmunotherapy, highlighting the importance of structural design and interdomain spacing for anti-tumor efficacy.

Variable Expression of the Disialoganglioside GD2 in Breast Cancer Molecular Subtypes

Simple Summary

GD2 is an antigen that is tumor-specific and can be used as a target for specific immunotherapies. Since the knowledge about GD2 in breast cancer is limited, we analyzed the frequency of GD2 expression in breast cancer using two different staining methods and the impact of GD2 expression on the survival of breast cancer patients. GD2 expression was found in more than 50% of breast cancer cases, with the highest frequency in hormone receptor-positive tumors. GD2 expression was not significantly associated with patient outcome. Unlike previous studies with smaller sample sizes that lacked correlation with clinical data, this study includes a larger cohort and associations with survival data and shows that GD2 is expressed on human breast cancer cells, providing a potential target for immunotherapies (e.g., anti-GD2 antibodies or GD2 CAR T cells), that are currently undergoing clinical testing.

Abstract

The disialoganglioside GD2 is a tumor-associated antigen that may allow for the application of targeted immunotherapies (anti-GD2 antibodies, GD2 CAR T cells) in patients with neuroblastoma and other solid tumors. We retrospectively investigated GD2 expression in a breast cancer cohort, using immunohistochemistry (IHC) and immunofluorescence (IF) on tissue microarrays (TMAs), and its impact on survival. GD2 expression on IHC (n = 568) and IF (n = 503) was investigated in relation to subtypes and patient outcome. Overall, 50.2% of the 568 IHC-assessed samples and 69.8% of the 503 IF-assessed samples were GD2-positive. The highest proportion of GD2-positive tumors was observed in luminal tumors. Significantly fewer GD2-positive cases were detected in triple-negative breast cancer (TNBC) compared with other subtypes. The proportion of GD2-expressing tumors were significantly lower in HER2-positive breast cancer in comparison with luminal tumors on IF staining (but not IHC). GD2 expression of IHC or IF was not significantly associated with disease-free or overall survival, in either the overall cohort or in individual subtypes. However, GD2 expression can be seen in more than 50% of breast cancer cases, with the highest frequency in hormone receptor-positive tumors. With this high expression frequency, patients with GD2-positive advanced breast cancer of all subtypes may benefit from GD2-targeting immunotherapies, which are currently subject to clinical testing.

Interdomain spacing and spatial configuration drive the potency of IgG-[L]-scFv T cell bispecific antibodies

T cell-bispecific antibodies (BsAbs) couple cytotoxic T lymphocytes to tumor cells, inducing their destruction. Although there are more than 60 classes of BsAbs in development, the relative importance of parameters such as interdomain spacing or spatial configuration is largely unknown. Here, we dissected a symmetric dual bivalent BsAb platform (IgG-[L]-scFv: antitumor IgG with anti-CD3 scFv fused to the light chains) to explore the importance of valency and spatial configuration for BsAb-induced T cell cytotoxicity. Our results revealed that placing tumor and T cell binding domains on the same side of a BsAb (cis-configuration) elicited substantially stronger antitumor activity, in vitro and in vivo, compared to positioning them on opposite sides (trans-configuration). Moreover, using two cis-modules in the same BsAb further improved cytotoxicity (up to 2000-fold). In addition, separating antigen-binding components with a single Ig domain (C_L) markedly enhanced cytokine release and in vivo tumor responses compared to smaller (G₄S₁) or larger (C_H1-C_H2-C_H3) spacers. These findings provide guidelines for improving BsAb function and highlight the importance of spatial configuration and dual bivalency as development parameters.

Engineered Cells as a Test Platform for Radiohaptens in Pretargeted Imaging and Radioimmunotherapy Applications

Pretargeted imaging and radioimmunotherapy approaches are designed to have superior targeting properties over directly targeted antibodies but impose more complex pharmacology, which hinders efforts to optimize the ligands prior to human applications. Human embryonic kidney 293T cells expressing the humanized single-chain variable fragment (scFv) C825 (huC825) with high-affinity for DOTA-haptens (293T-huC825) in a transmembrane-anchored format eliminated the requirement to use other pretargeting reagents and provided a simplified, accelerated assay of radiohapten capture while offering normalized cell surface expression of the molecular target of interest. Using binding assays, ex vivo biodistribution, and in vivo imaging, we demonstrated that radiohaptens based on benzyl-DOTA and a second generation "Proteus" DOTA-platform effectively and specifically engaged membrane-bound huC825, achieving favorable tumor-to-normal tissue uptake ratios in mice. Furthermore, [⁸⁶Y]Y-DOTA-Bn predicted absorbed dose to critical organs with reasonable accuracy for both [¹⁷⁷Lu]Lu-DOTA-Bn and [²²⁵Ac]Ac-Pr, which highlights the benefit of a dosimetry-based treatment approach.

Paediatric Molecular Radiotherapy: Challenges and Opportunities

The common contemporary indications for paediatric molecular radiotherapy (pMRT) are differentiated thyroid cancer and neuroblastoma. It may also have value in neuroendocrine cancers, and it is being investigated in clinical trials for other diseases. pMRT is the prototypical biomarker-driven, precision therapy, with a unique mode of delivery and mechanism of action. It is safe and well tolerated, compared with other treatments. However, its full potential has not yet been achieved, and its wider use faces a number of challenges and obstacles. Paradoxically, the success of radioactive iodine as a curative treatment for metastatic thyroid cancer has led to a 'one size fits all' approach and limited academic enquiry into optimisation of the conventional treatment regimen, until very recently. Second, the specialised requirements for the delivery of pMRT are available in only a very limited number of centres. This limited capacity and geographical coverage results in reduced accessibility. With few enthusiastic advocates for this treatment modality, investment in research to improve treatments and broaden indications from both industry and national and charitable research funders has historically been suboptimal. Nonetheless, there is now an increasing interest in the opportunities offered by pMRT. Increased research funding has been allocated, and technical developments that will permit innovative approaches in pMRT are available for exploration. A new portfolio of clinical trials is being assembled. These studies should help to move at least some paediatric treatments from simply palliative use into potentially curative protocols. Therapeutic strategies require modification and optimisation to achieve this. The delivery should be personalised and tailored appropriately, with a comprehensive evaluation of tumour and organ-at-risk dosimetry, in alignment with the external beam model of radiotherapy. This article gives an overview of the current status of pMRT, indicating the barriers to progress and identifying ways in which these may be overcome.

Immunotherapy of Pediatric Solid Tumors: Treatments at a Crossroads, with an Emphasis on Antibodies

Over the last decade, immunotherapy has rapidly changed the therapeutic landscape and prognosis for many hematologic malignancies and adult solid tumors. Despite this success, immunotherapy for pediatric solid tumors remains in the early stages of development, and significant clinical benefit has yet to be realized, with anti-GD2 for neuroblastoma being the exception. The limited neoepitope expression and paucity of T-cell infiltration into the immunosuppressive tumor microenvironment have hampered current established immunotherapies. Emerging approaches to recruit T cells, to convert phenotypically "cold" into "inflamed" tumors, and to vastly improve therapeutic indices hold exceptional promise. Here, we review these approaches, highlighting the role of the tumor microenvironment and novel antibody platforms to maximize the full clinical potential of immunotherapy in pediatric oncology.

A Self-Assembling and Disassembling (SADA) Bispecific Antibody (BsAb) Platform for Curative Two-step Pretargeted Radioimmunotherapy

PURPOSE

Many cancer treatments suffer from dose-limiting toxicities to vital organs due to poor therapeutic indices. To overcome these challenges we developed a novel multimerization platform that rapidly removes tumor-targeting proteins from the blood to substantially improve therapeutic index.

EXPERIMENTAL DESIGN

The platform was designed as a fusion of a self-assembling and disassembling (SADA) domain to a tandem single-chain bispecific antibody (BsAb, anti-ganglioside GD2 × anti-DOTA). SADA-BsAbs were assessed with multiple in vivo tumor models using two-step pretargeted radioimmunotherapy (PRIT) to evaluate tumor uptake, dosimetry, and antitumor responses.

RESULTS

SADA-BsAbs self-assembled into stable tetramers (220 kDa), but could also disassemble into dimers or monomers (55 kDa) that rapidly cleared via renal filtration and substantially reduced immunogenicity in mice. When used with rapidly clearing DOTA-caged PET isotopes, SADA-BsAbs demonstrated accurate tumor localization, dosimetry, and improved imaging contrast by PET/CT. When combined with therapeutic isotopes, two-step SADA-PRIT safely delivered massive doses of alpha-emitting (²²⁵Ac, 1.48 MBq/kg) or beta-emitting (¹⁷⁷Lu, 6,660 MBq/kg) S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA) payloads to tumors, ablating them without any short-term or long-term toxicities to the bone marrow, kidneys, or liver.

CONCLUSIONS

The SADA-BsAb platform safely delivered large doses of radioisotopes to tumors and demonstrated no toxicities to the bone marrow, kidneys, or liver. Because of its modularity, SADA-BsAbs can be easily adapted to most tumor antigens, tumor types, or drug delivery approaches to improve therapeutic index and maximize the delivered dose.See related commentary by Capala and Kunos, p. 377.

Alpha radioimmunotherapy using 225Ac-proteus-DOTA for solid tumors - safety at curative doses

This is the initial report of an α-based pre-targeted radioimmunotherapy (PRIT) using ²²⁵Ac and its theranostic pair, ¹¹¹In. We call our novel tumor-targeting DOTA-hapten PRIT system "proteus-DOTA" or "Pr." Herein we report the first results of radiochemistry development, radiopharmacology, and stoichiometry of tumor antigen binding, including the role of specific activity, anti-tumor efficacy, and normal tissue toxicity with the Pr-PRIT approach (as α-DOTA-PRIT). A series of α-DOTA-PRIT therapy studies were performed in three solid human cancer xenograft models of colorectal cancer (GPA33), breast cancer (HER2), and neuroblastoma (GD2), including evaluation of chronic toxicity at ~20 weeks of select survivors. Methods: Preliminary biodistribution experiments in SW1222 tumor-bearing mice revealed that ²²⁵Ac could not be efficiently pretargeted with current DOTA-Bn hapten utilized for ¹⁷⁷Lu or ⁹⁰Y, leading to poor tumor uptake in vivo. Therefore, we synthesized Pr consisting of an empty DOTA-chelate for ²²⁵Ac, tethered via a short polyethylene glycol linker to a lutetium-complexed DOTA for picomolar anti-DOTA chelate single-chain variable fragment (scFv) binding. Pr was radiolabeled with ²²⁵Ac and its imaging surrogate, ¹¹¹In. In vitro studies verified anti-DOTA scFv recognition of [²²⁵Ac]Pr, and in vivo biodistribution and clearance studies were performed to evaluate hapten suitability and in vivo targeting efficiency. Results: Intravenously (i.v.) administered ²²⁵Ac- or ¹¹¹In-radiolabeled Pr in mice showed rapid renal clearance and minimal normal tissue retention. In vivo pretargeting studies show high tumor accumulation of Pr (16.71 ± 5.11 %IA/g or 13.19 ± 3.88 %IA/g at 24 h p.i. for [²²⁵Ac]Pr and [¹¹¹In]Pr, respectively) and relatively low uptake in normal tissues (all average ≤ 1.4 %IA/g at 24 h p.i.). Maximum tolerated dose (MTD) was not reached for either [²²⁵Ac]Pr alone or pretargeted [²²⁵Ac]Pr at administered activities up to 296 kBq/mouse. Single-cycle treatment consisting of α-DOTA-PRIT with either huA33-C825 bispecific anti-tumor/anti-DOTA-hapten antibody (BsAb), anti-HER2-C825 BsAb, or hu3F8-C825 BsAb for targeting GPA33, HER2, or GD2, respectively, was highly effective. In the GPA33 model, no complete responses (CRs) were observed but prolonged overall survival of treated animals was 42 d for α-DOTA-PRIT vs. 25 d for [²²⁵Ac]Pr only (P < 0.0001); for GD2, CRs (7/7, 100%) and histologic cures (4/7, 57%); and for HER2, CRs (7/19, 37%) and histologic cures (10/19, 56%) with no acute or chronic toxicity. Conclusions: [²²⁵Ac]Pr and its imaging biomarker [¹¹¹In]Pr demonstrate optimal radiopharmacologic behavior for theranostic applications of α-DOTA-PRIT. For this initial evaluation of efficacy and toxicity, single-cycle treatment regimens were performed in all three systems. Histologic toxicity was not observed, so MTD was not observed. Prolonged overall survival, CRs, and histologic cures were observed in treated animals. In comparison to RIT with anti-tumor IgG antibodies, [²²⁵Ac]Pr has a much improved safety profile. Ultimately, these data will be used to guide clinical development of toxicity and efficacy studies of [²²⁵Ac]Pr, with the goal of delivering massive lethal doses of radiation to achieve a high probability of cure without toxicity.

Disialoganglioside GD2 Expression in Solid Tumors and Role as a Target for Cancer Therapy

Gangliosides are carbohydrate-containing sphingolipids that are widely expressed in normal tissues, making most subtypes unsuitable as targets for cancer therapy. However, the disialoganglioside GD2 subtype has limited expression in normal tissues but is overexpressed across a wide range of tumors. Disialoganglioside GD2 can be considered a tumor-associated antigen and well-suited as a target for cancer therapy. Disialoganglioside GD2 is implicated in tumor development and malignant phenotypes through enhanced cell proliferation, motility, migration, adhesion, and invasion, depending on the tumor type. This provides a rationale for targeting disialoganglioside GD2 in cancer therapy with the development of anti-GD2 monoclonal antibodies and other therapeutic approaches. Anti-GD2 monoclonal antibodies target GD2-expressing tumor cells, leading to phagocytosis and destruction by means of antibody-dependent cell-mediated cytotoxicity, lysis by complement-dependent cytotoxicity, and apoptosis and necrosis through direct induction of cell death. Anti-GD2 monoclonal antibodies may also prevent homing and adhesion of circulating malignant cells to the extracellular matrix. Disialoganglioside GD2 is highly expressed by almost all neuroblastomas, by most melanomas and retinoblastomas, and by many Ewing sarcomas and, to a more variable degree, by small cell lung cancer, gliomas, osteosarcomas, and soft tissue sarcomas. Successful treatment of disialoganglioside GD2-expressing tumors with anti-GD2 monoclonal antibodies is hindered by pharmacologic factors such as insufficient antibody affinity to mediate antibody-dependent cell-mediated cytotoxicity, inadequate penetration of antibody into the tumor microenvironment, and toxicity related to disialoganglioside GD2 expression by normal tissues such as peripheral sensory nerve fibers. Nonetheless, anti-GD2 monoclonal antibody dinutuximab (ch14.18) has been approved by the U.S. Food and Drug Administration and dinutuximab beta (ch14.18/CHO) has been approved by the European Medicines Agency for the treatment of high-risk neuroblastoma in pediatric patients. Clinical trials of anti-GD2 therapy are currently ongoing in patients with other types of disialoganglioside GD2-expressing tumors as well as neuroblastoma. In addition to anti-GD2 monoclonal antibodies, anti-GD2 therapeutic approaches include chimeric antigen receptor T-cell therapy, disialoganglioside GD2 vaccines, immunocytokines, immunotoxins, antibody-drug conjugates, radiolabeled antibodies, targeted nanoparticles, and T-cell engaging bispecific antibodies. Clinical trials should clarify further the potential of anti-GD2 therapy for disialoganglioside GD2-expressing malignant tumors.

Design and preclinical evaluation of nanostars for the passive pretargeting of tumor tissue

INTRODUCTION

Pretargeting strategies that do not rely on the expression of molecular targets have expanded imaging and therapy options for cancer patients. Nanostars with designed multivalency and which highly accumulate in tumor tissue via the enhanced permeability and retention (EPR) effect may therefore be the ideal vectors for the development of a passive pretargeting approach.

METHODS

Nanostars were synthesized, consisting of 7-8 center-cross-linked arms that were modified with trans-cyclooctene (TCO) using poly(ethylene glycol) (PEG) linkers of 12 or 106 monomer units or without linker. The bioorthogonal click reaction with radiofluorinated 2,2'-(7-(2-(tetrazine-poly(ethyleneglycol)11-amino)-2-oxoethyl)-1,4,7-triazonane-1,4-diyl)diacetic acid ([18F]F-Tz-PEG11-NODA) or 2,2'-(7-(2-(tetrazine-amino)-2-oxoethyl)-1,4,7-triazonane-1,4-diyl)diacetic acid ([18F]F-Tz-NODA) was measured by ex vivo biodistribution studies and positron emission tomography (PET) in mice bearing tumors with high EPR characteristics. Bioorthogonal masking was performed using a tetrazine-functionalized dextran polymer (Tz-DP).

RESULTS

Highest tumor accumulation of [18F]F-Tz-PEG11-NODA was observed for nanostars functionalized with TCO without linker, with a tumor uptake of 3.2 ± 0.4%ID/g and a tumor-to-muscle ratio of 12.8 ± 4.2, tumor-to-large intestine ratio of 0.5 ± 0.3 and tumor-to-kidney ratio of 2.0 ± 0.3, being significantly higher than for nanostars functionalized with TCO-PEG12 (P < 0.05) or TCO-PEG106 (P < 0.05). Tumor uptake and tumor-to-tissue ratios did not improve upon bioorthogonal masking with Tz-DP or when using a smaller, more lipophilic tetrazine([18F]F-Tz-NODA).

CONCLUSIONS

A pretargeting strategy was developed based on the passive delivery of TCO-functionalized nanostars. Such a strategy would allow for the imaging and treatment of tumors with apparent EPR characteristics, with high radioactive tumor doses and minimal doses to off-target tissues.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as 89Zr-Df-pertuzumab and 89Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

Targets and Antibody Formats for Immunotherapy of Neuroblastoma

Neuroblastoma (NB) is a malignant embryonal tumor of the sympathetic nervous system that is most commonly diagnosed in the abdomen, often presenting with signs and symptoms of metastatic spread. Three decades ago, high-risk NB metastatic to bone and bone marrow in children was not curable. Today, with multimodality treatment, 50% of these patients will survive, but most suffer from debilitating treatment-related complications. Novel targeted therapies to improve cure rates while minimizing toxicities are urgently needed. Recent molecular discoveries in oncology have spawned the development of an impressive array of targeted therapies for adult cancers, yet the paucity of recurrent somatic mutations or activated oncogenes in pediatric cancers poses a major challenge to the evolving paradigm of personalized medicine. Although low tumor mutational burden is a major hurdle for immune checkpoint inhibitors, an immature or impaired immune system and inhibitory tumor microenvironment can further complicate the prospects for successful immunotherapy. In this regard, despite the poor immunogenic properties of NB, the success of antibody-based immunotherapy and radioimmunotherapy directed at single targets (eg, GD2 and B7-H3) is both encouraging and surprising, given that most solid tumor antibodies that use Fc-dependent mechanisms or radioimmunotargeting have largely failed. Here, we summarize the current information on the immunologic properties of this tumor, its potential immunotherapeutic targets, and novel antibody-based strategies on the horizon.

An N-Acetylgalactosamino Dendron-Clearing Agent for High-Therapeutic-Index DOTA-Hapten Pretargeted Radioimmunotherapy

Clearing agents (CAs) can rapidly remove nonlocalized targeting biomolecules from circulation for hepatic catabolism, thereby enhancing the therapeutic index (TI), especially for blood (marrow), of the subsequently administered radioisotope in any multistep pretargeting strategy. Herein we describe the synthesis and in vivo evaluation of a fully synthetic glycodendrimer-based CA for DOTA-based pretargeted radioimmunotherapy (DOTA-PRIT). The novel dendron-CA consists of a nonradioactive yttrium-DOTA-Bn molecule attached via a linker to a glycodendron displaying 16 terminal α-thio-N-acetylgalactosamine (α-SGalNAc) units (CCA α-16-DOTA-Y³⁺; molecular weight: 9059 Da). Pretargeting [¹⁷⁷Lu]LuDOTA-Bn with CCA α-16-DOTA-Y³⁺ to GPA33-expressing SW1222 human colorectal xenografts was highly effective, leading to absorbed doses of [¹⁷⁷Lu]LuDOTA-Bn for blood, tumor, liver, spleen, and kidneys of 11.7, 468, 9.97, 5.49, and 13.3 cGy/MBq, respectively. Tumor-to-normal tissues absorbed-dose ratios (i.e., TIs) ranged from 40 (e.g., for blood and kidney) to about 550 for stomach.

Therapeutic Applications of Pretargeting

Targeted therapies, such as radioimmunotherapy (RIT), present a promising treatment option for the eradication of tumor lesions. RIT has shown promising results especially for hematologic malignancies, but the therapeutic efficacy is limited by unfavorable tumor-to-background ratios resulting in high radiotoxicity. Pretargeting strategies can play an important role in addressing the high toxicity profile of RIT. Key to pretargeting is the concept of decoupling the targeting vehicle from the cytotoxic agent and administrating them separately. Studies have shown that this approach has the ability to enhance the therapeutic index as it can reduce side effects caused by off-target irradiation and thereby increase curative effects due to higher tolerated doses. Pretargeted RIT (PRIT) has been explored for imaging and treatment of different cancer types over the years. This review will give an overview of the various targeted therapies in which pretargeting has been applied, discussing PRIT with alpha- and beta-emitters and as part of combination therapy, plus its use in drug delivery systems.

Design and Production of Bispecific Antibodies

With the current biotherapeutic market dominated by antibody molecules, bispecific antibodies represent a key component of the next-generation of antibody therapy. Bispecific antibodies can target two different antigens at the same time, such as simultaneously binding tumor cell receptors and recruiting cytotoxic immune cells. Structural diversity has been fast-growing in the bispecific antibody field, creating a plethora of novel bispecific antibody scaffolds, which provide great functional variety. Two common formats of bispecific antibodies on the market are the single-chain variable fragment (scFv)-based (no Fc fragment) antibody and the full-length IgG-like asymmetric antibody. Unlike the conventional monoclonal antibodies, great production challenges with respect to the quantity, quality, and stability of bispecific antibodies have hampered their wider clinical application and acceptance. In this review, we focus on these two major bispecific types and describe recent advances in the design, production, and quality of these molecules, which will enable this important class of biologics to reach their therapeutic potential.

A review of bispecific antibodies and antibody constructs in oncology and clinical challenges

Bispecific antibodies (bsAbs) are antibodies that bind two distinct epitopes to cancer.. For use in oncology, one bsAb has been approved and 57 bsAbs are in clinical trials, none of which has reached phase 3. These bsAbs show great variability in design and mechanism of action. The various designs are often linked to the mechanisms of actions. The majority of bsAbs engage immune cells to destroy tumor cells. However, some bsAbs are also used to deliver payloads to tumors or to block tumor signaling pathways. This review provides insight into the choice of construct for bsAbs, summarizes the clinical development of bsAbs in oncology and identifies subsequent challenges.

Advances in Anti-GD2 Immunotherapy for Treatment of High-risk Neuroblastoma

Neuroblastoma (NBL) is the most common extracranial solid tumor in pediatrics, yet overall survival is poor for high-risk cases. Immunotherapy regimens using a tumor-selective antidisialoganglioside (anti-GD2) monoclonal antibody (mAb) have been studied for several decades now, but have only recently been incorporated into standard of care treatment for patients with high-risk NBL with clear benefit. Here we review a brief history of anti-GD2-based immunotherapy, current areas of neuroblastoma research targeting GD2, and potential diagnostic and therapeutic uses targeting GD2.

Theranostic pretargeted radioimmunotherapy of internalizing solid tumor antigens in human tumor xenografts in mice: Curative treatment of HER2-positive breast carcinoma

In recent reports, we have shown that optimized pretargeted radioimmunotherapy (PRIT) based on molecularly engineered antibody conjugates and ¹⁷⁷Lu-DOTA chelate (DOTA-PRIT) can be used to cure mice bearing human solid tumor xenografts using antitumor antibodies to minimally internalizing membrane antigens, GPA33 (colon) and GD2 (neuroblastoma). However, many solid tumor membrane antigens are internalized after antibody binding and it is generally believed that internalizing tumor membrane antigens are not suitable targets for PRIT. In this study, we tested the hypothesis that DOTA-PRIT can be performed successfully to target HER2, an internalizing membrane antigen widely expressed in breast, ovarian, and gastroesophageal junction cancers.

Methods: DOTA-PRIT was carried out in athymic nude mice bearing BT-474 xenografts, a HER2-expressing human breast cancer, using a three-step dosing regimen consisting of sequential intravenous administrations of: 1) a bispecific IgG-scFv (210 kD) format (BsAb) carrying the IgG sequence of the anti-HER2 antibody trastuzumab and the scFv “C825” with high-affinity, hapten-binding antibody for Bn-DOTA (metal) (BsAb: anti-HER2-C825), 2) a 500 kD dextran-based clearing agent, followed by 3) ¹⁷⁷Lu-DOTA-Bn. At the time of treatment, athymic nude mice bearing established subcutaneous BT-474 tumors (medium- and smaller-sized tumors with tumor volumes of 209 ± 101 mm³ and ranging from palpable to 30 mm³, respectively), were studied along with controls. We studied single- and multi-dose regimens. For groups receiving fractionated treatment, we verified quantitative tumor targeting during each treatment cycle using non-invasive imaging with single-photon emission computed tomography/computed tomography (SPECT/CT).

Results: We achieved high therapeutic indices (TI, the ratio of radiation-absorbed dose in tumor to radiation-absorbed dose to critical organs, such as bone marrow) for targeting in blood (TI = 28) and kidney (TI = 7), while delivering average radiation-absorbed doses of 39.9 cGy/MBq to tumor. Based on dosimetry estimates, we implemented a curative fractionated therapeutic regimen for medium-sized tumors that would deliver approximately 70 Gy to tumors, which required treatment with a total of 167 MBq ¹⁷⁷Lu-DOTA-Bn/mouse (estimated absorbed tumor dose: 66 Gy). This regimen was well tolerated and achieved 100% complete responses (CRs; defined herein as tumor volume equal to or smaller than 4.2 mm³), including 62.5% histologic cure (5/8) and 37.5% microscopic residual disease (3/8) at 85 days (d). Treatment controls showed tumor progression to 207 ± 201% of pre-treatment volume at 85 d and no CRs. Finally, we show that treatment with this curative ¹⁷⁷Lu regimen leads to a very low incidence of histopathologic abnormalities in critical organs such as bone marrow and kidney among survivors compared with non-treated controls.

Conclusion: Contrary to popular belief, we demonstrate that DOTA-PRIT can be successfully adapted to an internalizing antigen-antibody system such as HER2, with sufficient TIs and absorbed tumor doses to achieve a high probability of cures of established human breast cancer xenografts while sparing critical organs of significant radiotoxicity.

Building blocks for bispecific and trispecific antibodies

Bispecific antibodies (BsAbs), which target two antigens or epitopes, incorporate the specificities and properties of two distinct monoclonal antibodies (mAbs) into a single molecule. As such, BsAbs can elicit synergistic activities and provide the capacity for enhanced therapeutic efficacy and/or safety compared to what can be achieved with conventional monospecific IgGs. There are many building block formats to generate BsAbs and Trispecific antibodies (TsAbs) based on combining the antigen recognition domains of monoclonal antibodies (mAbs). This review describes the many and varied antibody-based building blocks used to achieve multivalency and multispecificity. These diverse building blocks provide opportunities to tailor the design of BsAbs and TsAbs to match the desired applications.

Bioorthogonal Masking of Circulating Antibody-TCO Groups Using Tetrazine-Functionalized Dextran Polymers

Pretargeting strategies have gained popularity for the in vivo imaging and therapy of cancer by combining antibodies with small molecule radioligands. In vivo recombination of both moieties can be achieved using the bioorthogonal inverse electron demand Diels-Alder (IEDDA) chemistry between tetrazine (Tz) and trans-cyclooctene (TCO). An issue that arises with pretargeting strategies is that while part of the antibody dose accumulates at antigen-expressing tumor tissue, there is a significant portion of the injected antibody that remains in circulation, causing a reduction in target-to-background ratios. Herein, we report the development of a novel TCO scavenger, the masking agent DP-Tz. DP-Tz is based on Tz-modified dextran polymers (DP, MW = 0.5-2 MDa). Large dextran polymers were reported to exhibit low penetration of tumor vasculature and appeared nontoxic, nonimmunogenic, and easily modifiable. Our newly developed masking agent deactivates the remaining TCO-moieties on the circulating mAbs yet does not impact the tumor uptake of the Tz-radioligand. In pretargeting studies utilizing a ⁶⁸Ga-labeled tetrazine radioligand ([⁶⁸Ga]Ga-NOTA-PEG₁₁-tetrazine), DP-Tz constructs (Tz/DP ratios of 62-254) significantly increased TTB ratios from 0.8 ± 0.3 (control cohorts) to up to 5.8 ± 2.3 at 2 h postinjection. Tumor tissue delineation in PET imaging experiments employing DP-Tz is significantly increased compared to control. Uptake values of other significant organs, such as heart, lungs, pancreas, and stomach, were decreased on average by 2-fold when using DP-Tz. Overall, pretargeting experiments utilizing DP-Tz showed significantly improved tumor delineation, enhanced PET image quality, and reduced uptake in vital organs. We believe that this new masking agent is a powerful new addition to the IEDDA-based pretargeting tool box and, due to its properties, an excellent candidate for clinical translation.

The making of bispecific antibodies

During the past two decades we have seen a phenomenal evolution of bispecific antibodies for therapeutic applications. The 'zoo' of bispecific antibodies is populated by many different species, comprising around 100 different formats, including small molecules composed solely of the antigen-binding sites of two antibodies, molecules with an IgG structure, and large complex molecules composed of different antigen-binding moieties often combined with dimerization modules. The application of sophisticated molecular design and genetic engineering has solved many of the technical problems associated with the formation of bispecific antibodies such as stability, solubility and other parameters that confer drug properties. These parameters may be summarized under the term 'developability'. In addition, different 'target product profiles', i.e., desired features of the bispecific antibody to be generated, mandates the need for access to a diverse panel of formats. These may vary in size, arrangement, valencies, flexibility and geometry of their binding modules, as well as in their distribution and pharmacokinetic properties. There is not 'one best format' for generating bispecific antibodies, and no single format is suitable for all, or even most of, the desired applications. Instead, the bispecific formats collectively serve as a valuable source of diversity that can be applied to the development of therapeutics for various indications. Here, a comprehensive overview of the different bispecific antibody formats is provided.

Anti-GD2 immunotherapy for neuroblastoma

INTRODUCTION

Current therapeutic approaches for high-risk neuroblastoma (HR-NB) include high-dose chemotherapy, surgery and radiotherapy; interventions that are associated with long and short-term toxicities. Effective immunotherapy holds particular promise for improving survival and quality of life by reducing exposure to cytotoxic agents. GD2, a surface glycolipid is the most common target for immunotherapy. Areas covered: We review the status of anti-GD2 immunotherapies currently in clinical use for neuroblastomas and novel GD2-targeted strategies in preclinical development. Expert commentary: Anti-GD2 monoclonal antibodies are associated with improved survival in patients in their first remission and are increasingly being used for chemorefractory and relapsed neuroblastoma. As protein engineering technology has become more accessible, newer antibody constructs are being tested. GD2 is also being targeted by natural killer cells and T-cells. Active immunity can be elicited by anti-GD2 vaccines. The rational combination of currently available and soon-to-emerge immunotherapeutic approaches, and their integration into conventional multimodality therapies will require further investigation to optimize their use for HR-NB.

Pretargeted Imaging and Therapy

In vivo pretargeting stands as a promising approach to harnessing the exquisite tumor-targeting properties of antibodies for nuclear imaging and therapy while simultaneously skirting their pharmacokinetic limitations. The core premise of pretargeting lies in administering the targeting vector and radioisotope separately and having the 2 components combine within the body. In this manner, pretargeting strategies decrease the circulation time of the radioactivity, reduce the uptake of the radionuclide in healthy nontarget tissues, and facilitate the use of short-lived radionuclides that would otherwise be incompatible with antibody-based vectors. In this short review, we seek to provide a brief yet informative survey of the 4 preeminent mechanistic approaches to pretargeting, strategies predicated on streptavidin and biotin, bispecific antibodies, complementary oligonucleotides, and bioorthogonal click chemistry.

Overcoming resistance to HER2-targeted therapy with a novel HER2/CD3 bispecific antibody

T-cell-based therapies have emerged as one of the most clinically effective ways to target solid and non-solid tumors. HER2 is responsible for the oncogenesis and treatment resistance of several human solid tumors. As a member of the HER family of tyrosine kinase receptors, its over-activity confers unfavorable clinical outcome. Targeted therapies directed at this receptor have achieved responses, although development of resistance is common. We explored a novel HER2/CD3 bispecific antibody (HER2-BsAb) platform that while preserving the anti-proliferative effects of trastuzumab, it recruits and activates non-specific circulating T-cells, promoting T cell tumor infiltration and ablating HER2(+) tumors, even when these are resistant to standard HER2-targeted therapies. Its in vitro tumor cytotoxicity, when expressed as EC50, correlated with the surface HER2 expression in a large panel of human tumor cell lines, irrespective of lineage or tumor type. HER2-BsAb-mediated cytotoxicity was relatively insensitive to PD-1/PD-L1 immune checkpoint inhibition. In four separate humanized mouse models of human breast cancer and ovarian cancer cell line xenografts, as well as human breast cancer and gastric cancer patient-derived xenografts (PDXs), HER2-BsAb was highly effective in promoting T cell infiltration and suppressing tumor growth when used in the presence of human peripheral blood mononuclear cells (PBMC) or activated T cells (ATC). The in vivo and in vitro antitumor properties of this BsAb support its further clinical development as a cancer immunotherapeutic.

Bispecific Antibodies as a Development Platform for New Concepts and Treatment Strategies

With the development of molecular cloning technology and the deep understanding of antibody engineering, there are diverse bispecific antibody formats from which to choose to pursue the optimal biological activity and clinical purpose. The single-chain-based bispecific antibodies usually bridge tumor cells with immune cells and form an immunological synapse because of their relatively small size. Bispecific antibodies in the IgG format include asymmetric bispecific antibodies and homodimerized bispecific antibodies, all of which have an extended blood half-life and their own crystalline fragment (Fc)-mediated functions. Besides retargeting effector cells to the site of cancer, new applications were established for bispecific antibodies. Bispecific antibodies that can simultaneously bind to cell surface antigens and payloads are a very ideal delivery system for therapeutic use. Bispecific antibodies that can inhibit two correlated signaling molecules at the same time can be developed to overcome inherent or acquired resistance and to be more efficient angiogenesis inhibitors. Bispecific antibodies can also be used to treat hemophilia A by mimicking the function of factor VIII. Bispecific antibodies also have broad application prospects in bone disorders and infections and diseases of the central nervous system. The latest developments of the formats and application of bispecific antibodies will be reviewed. Furthermore, the challenges and perspectives are summarized in this review.

Establishment of the In Vivo Efficacy of Pretargeted Radioimmunotherapy Utilizing Inverse Electron Demand Diels-Alder Click Chemistry

The pretargeting system based on the inverse electron demand Diels-Alder reaction (IEDDA) between trans-cyclooctene (TCO) and tetrazine (Tz) combines the favorable pharmacokinetic properties of radiolabeled small molecules with the affinity and specificity of antibodies. This strategy has proven to be an efficient method for the molecularly targeted delivery of pharmaceuticals, including isotopes for radiological imaging. Despite encouraging results from in vivo PET imaging studies, this promising system has yet to be thoroughly evaluated for pretargeted radioimmunotherapy (PRIT). Toward that end, we synthesized two novel ¹⁷⁷Lu-labeled tetrazine-bearing radioligands. Next, we compared the usefulness of our ligands for PRIT when paired with TCO-modified 5B1-a human, anti-CA19.9 mAb-in preclinical murine models of pancreatic cancer. The exemplary ligand, ¹⁷⁷Lu-DOTA-PEG₇-Tz, showed rapid (4.6 ± 0.8% ID/g at 4 hours) and persistent (16.8 ± 3.9% ID/g at 120 hours) uptake in tumors while concurrently clearing from blood and nontarget tissues. Single-dose therapy studies using 5B1-TCO and varying amounts of ¹⁷⁷Lu-DOTA-PEG₇-Tz (400, 800, and 1,200 μCi) showed that our system elicits a dose-dependent therapeutic response in mice bearing human xenografts. Furthermore, dosimetry calculations suggest that our approach is amenable to clinical applications with its excellent dosimetric profile in organs of clearance (i.e., liver and kidneys) as well as in dose-limiting tissues, such as red marrow. This study established that a pretargeted methodology utilizing the IEDDA reaction can rapidly and specifically deliver a radiotherapeutic payload to tumor tissue, thus illustrating its excellent potential for clinical translation. Mol Cancer Ther; 16(1); 124-33. ©2016 AACR.

Engineering anti-Lewis-Y hu3S193 antibodies with improved therapeutic ratio for radioimmunotherapy of epithelial cancers

Background

The aim of the study was to explore Fc mutations of a humanised anti-Lewis-Y antibody (IgG1) hu3S193 as a strategy to improve therapeutic ratios for therapeutic payload delivery.

Methods

Four hu3S193 variants (I253A, H310A, H435A and I253A/H310A) were generated via site-directed mutagenesis and radiolabelled with diagnostic isotopes iodine-125 or indium-111. Biodistribution studies in Lewis-Y-positive tumour-bearing mice were used to calculate the dose in tumours and organs for therapeutic isotopes (iodine-131, yttrium-90 and lutetium-177).

Results

¹¹¹In-labelled I253A and H435A showed similar slow kinetics (t_1/2β, 63.2 and 62.2 h, respectively) and a maximum tumour uptake of 33.11 ± 4.05 and 33.69 ± 3.77 percentage injected dose per gramme (%ID/g), respectively. ¹¹¹In-labelled I253A/H310A cleared fastest (t_1/2β, 9.1 h) with the lowest maximum tumour uptake (23.72 ± 0.85 %ID/g). The highest increase in tumour-to-blood area under the curve (AUC) ratio was observed with the metal-labelled mutants (⁹⁰Y and ¹⁷⁷Lu). ¹⁷⁷Lu-CHX-A" DTPA-hu3S193 I253A/H310A (6:1) showed the highest tumour-to-blood AUC ratio compared to wild type (3:1) and other variants and doubling of calculated dose to tumour based on red marrow dose constraints.

Conclusions

These results suggest that hu3S193 Fc can be engineered with improved therapeutic ratios for ⁹⁰Y- and ¹⁷⁷Lu-based therapy, with the best candidate being hu3S193 I253A/H310A for ¹⁷⁷Lu-based therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0180-0) contains supplementary material, which is available to authorized users.

Theranostic pretargeted radioimmunotherapy of colorectal cancer xenografts in mice using picomolar affinity ⁸⁶Y- or ¹⁷⁷Lu-DOTA-Bn binding scFv C825/GPA33 IgG bispecific immunoconjugates

PURPOSE

GPA33 is a colorectal cancer (CRC) antigen with unique retention properties after huA33-mediated tumor targeting. We tested a pretargeted radioimmunotherapy (PRIT) approach for CRC using a tetravalent bispecific antibody with dual specificity for GPA33 tumor antigen and DOTA-Bn-(radiolanthanide metal) complex.

METHODS

PRIT was optimized in vivo by titrating sequential intravenous doses of huA33-C825, the dextran-based clearing agent, and the C825 haptens (177)Lu-or (86)Y-DOTA-Bn in mice bearing the SW1222 subcutaneous (s.c.) CRC xenograft model.

RESULTS

Using optimized PRIT, therapeutic indices (TIs) for tumor radiation-absorbed dose of 73 (tumor/blood) and 12 (tumor/kidney) were achieved. Estimated absorbed doses (cGy/MBq) to tumor, blood, liver, spleen, and kidney for single-cycle PRIT were 65.8, 0.9 (TI 73), 6.3 (TI 10), 6.6 (TI 10), and 5.3 (TI 12), respectively. Two cycles of PRIT (66.6 or 111 MBq (177)Lu-DOTA-Bn) were safe and effective, with a complete response of established s.c. tumors (100 - 700 mm(3)) in nine of nine mice, with two mice alive without recurrence at >140 days. Tumor log kill in this model was estimated to be 2.1 - 3.0 based on time to 500-mm(3) tumor recurrence. In addition, PRIT dosimetry/diagnosis was performed by PET imaging of the positron-emitting DOTA hapten (86)Y-DOTA-Bn.

CONCLUSION

We have developed anti-GPA33 PRIT as a triple-step theranostic strategy for preclinical detection, dosimetry, and safe targeted radiotherapy of established human colorectal mouse xenografts.

Immunotherapy of Childhood Sarcomas

Pediatric sarcomas are a heterogeneous group of malignant tumors of bone and soft tissue origin. Although more than 100 different histologic subtypes have been described, the majority of pediatric cases belong to the Ewing’s family of tumors, rhabdomyosarcoma and osteosarcoma. Most patients that present with localized stage are curable with surgery and/or chemotherapy; however, those with metastatic disease at diagnosis or those who experience a relapse continue to have a very poor prognosis. New therapies for these patients are urgently needed. Immunotherapy is an established treatment modality for both liquid and solid tumors, and in pediatrics, most notably for neuroblastoma and osteosarcoma. In the past, immunomodulatory agents such as interferon, interleukin-2, and liposomal-muramyl tripeptide phosphatidyl-ethanolamine have been tried, with some activity seen in subsets of patients; additionally, various cancer vaccines have been studied with possible benefit. Monoclonal antibody therapies against tumor antigens such as disialoganglioside GD2 or immune checkpoint targets such as CTLA-4 and PD-1 are being actively explored in pediatric sarcomas. Building on the success of adoptive T cell therapy for EBV-related lymphoma, strategies to redirect T cells using chimeric antigen receptors and bispecific antibodies are rapidly evolving with potential for the treatment of sarcomas. This review will focus on recent preclinical and clinical developments in targeted agents for pediatric sarcomas with emphasis on the immunobiology of immune checkpoints, immunoediting, tumor microenvironment, antibody engineering, cell engineering, and tumor vaccines. The future integration of antibody-based and cell-based therapies into an overall treatment strategy of sarcoma will be discussed.

Chimeric antigen receptors and bispecific antibodies to retarget T cells in pediatric oncology

Cancer immunotherapy using antigen-specific T cells has broad therapeutic potential. Chimeric antigen receptors and bispecific antibodies can redirect T cells to kill tumors without human leukocyte antigens (HLA) restriction. Key determinants of clinical potential include the choice of target antigen, antibody specificity, antibody affinity, tumor accessibility, T cell persistence, and tumor immune evasion. For pediatric cancers, additional constraints include their propensity for bulky metastatic disease and the concern for late toxicities from treatment. Nonetheless, the recent preclinical and clinical developments of these T cell based therapies are highly encouraging.

Radioimmunotherapy of human tumours

The eradication of cancer remains a vexing problem despite recent advances in our understanding of the molecular basis of neoplasia. One therapeutic approach that has demonstrated potential involves the selective targeting of radionuclides to cancer-associated cell surface antigens using monoclonal antibodies. Such radioimmunotherapy (RIT) permits the delivery of a high dose of therapeutic radiation to cancer cells, while minimizing the exposure of normal cells. Although this approach has been investigated for several decades, the cumulative advances in cancer biology, antibody engineering and radiochemistry in the past decade have markedly enhanced the ability of RIT to produce durable remissions of multiple cancer types.

Disialoganglioside GD2 as a therapeutic target for human diseases

INTRODUCTION

Ganglioside GD2 is found in vertebrates and invertebrates, overexpressed among pediatric and adult solid tumors, including neuroblastoma, glioma, retinoblastoma, Ewing's family of tumors, rhabdomyosarcoma, osteosarcoma, leiomyosarcoma, liposarcoma, fibrosarcoma, small cell lung cancer and melanoma. It is also found on stem cells, neurons, some nerve fibers and basal layer of the skin.

AREAS COVERED

GD2 provides a promising clinical target for radiolabeled antibodies, bispecific antibodies, chimeric antigen receptor (CAR)-modified T cells, drug conjugates, nanoparticles and vaccines. Here, we review its biochemistry, normal physiology, role in tumorigenesis, important characteristics as a target, as well as anti-GD2-targeted strategies.

EXPERT OPINION

Bridging the knowledge gaps in understanding the interactions of GD2 with signaling molecules within the glycosynapses, and the regulation of its cellular expression should improve therapeutic strategies targeting this ganglioside. In addition to anti-GD2 IgG mAbs, their drug conjugates, radiolabeled forms especially when genetically engineered to improve therapeutic index and novel bispecific forms or CARs to retarget T-cells are promising candidates for treating metastatic cancers.

Retargeting T cells to GD2 pentasaccharide on human tumors using Bispecific humanized antibody

Anti-disialoganglioside GD2 IgG antibodies have shown clinical efficacy in solid tumors that lack human leukocyte antigens (e.g., neuroblastoma) by relying on Fc-dependent cytotoxicity. However, there are pain side effects secondary to complement activation. T-cell retargeting bispecific antibodies (BsAb) also have clinical potential, but it is thus far only effective against liquid tumors. In this study, a fully humanized hu3F8-BsAb was developed, in which the anti-CD3 huOKT3 single-chain Fv fragment (ScFv) was linked to the carboxyl end of the anti-GD2 hu3F8 IgG1 light chain, and was aglycosylated at N297 of Fc to prevent complement activation and cytokine storm. In vitro, hu3F8-BsAb activated T cells through classic immunologic synapses, inducing GD2-specific tumor cytotoxicity at femtomolar EC50 with >10⁵-fold selectivity over normal tissues, releasing Th1 cytokines (TNFα, IFNγ, and IL2) when GD2⁺ tumors were present. In separate murine neuroblastoma and melanoma xenograft models, intravenous hu3F8-BsAb activated T cells in situ and recruited intravenous T cells for tumor ablation, significantly prolonging survival from local recurrence or from metastatic disease. Hu3F8-BsAb, but not control BsAb, drove T cells and monocytes to infiltrate tumor stroma. These monocytes were necessary for sustained T-cell proliferation and/or survival and contributed significantly to the antitumor effect. The in vitro and in vivo antitumor properties of hu3F8-BsAb and its safety profile support its further clinical development as a cancer therapeutic, and provide the rationale for exploring aglycosylated IgG-scFv as a structural platform for retargeting human T cells.

GD2-targeted immunotherapy and radioimmunotherapy

Ganglioside GD2 is a tumor-associated surface antigen found in a broad spectrum of human cancers and stem cells. They include pediatric embryonal tumors (neuroblastoma, retinoblastoma, brain tumors, osteosarcoma, Ewing sarcoma, rhabdomyosarcoma), as well as adult cancers (small cell lung cancer, melanoma, soft tissue sarcomas). Because of its restricted normal tissue distribution, GD2 has been proven safe for antibody targeting. Anti-GD2 antibody is now incorporated into the standard of care for the treatment of high-risk metastatic neuroblastoma. Building on this experience, novel combinations of antibodies, cytokines, cells, and genetically engineered products all directed at GD2 are rapidly moving into the clinic. In this review, past and present immunotherapy trials directed at GD2 will be summarized, highlighting the lessons learned and the future directions.