Improved therapeutic results by pretargeted radioimmunotherapy of non-Hodgkin's lymphoma with a new recombinant, trivalent, anti-CD20, bispecific antibody

Recent preclinical and clinical advances in radioimmunotherapy for non-Hodgkin's lymphoma

Radioimmunotherapy (RIT) is a therapy that combines a radioactive nucleotide with a monoclonal antibody (mAb). RIT enhances the therapeutic effect of mAb and reduces toxicity compared with conventional treatment. The purpose of this review is to summarize the current progress of RIT for treating non-Hodgkin's lymphoma (NHL) based on recent preclinical and clinical studies. The efficacy of RIT targeting the B-lymphocyte antigen cluster of differentiation 20 (CD20) has been demonstrated in clinical trials. Two radioimmunoconjugates targeting CD20, yttrium-90 (90Y)-ibritumomab-tiuxetan (Zevalin) and iodine-131 (131I)-tositumomab (Bexxar), have been approved in the USA Food and Drug Administration (FDA) for treating relapsed/refractory indolent or transformed NHL in 2002 and 2003, respectively. Although these two radioimmunoconjugates are effective and least toxic, they have not achieved popularity due to increasing access to novel therapies and the complexity of their delivery process. RIT is constantly evolving with the identification of novel targets and novel therapeutic strategies using newer radionuclides such as alpha-particle isotopes. Alpha-particles show very short path lengths and high linear energy transfer. These characteristics provide increased tumor cell-killing activities and reduced non-specific bystander responses on normal tissue. This review also discusses reviewed pre-targeted RIT (PRIT) and immuno-positron emission tomography (PET). PRIT potentially increases the dose of radionuclide delivered to tumors while toxicities to normal tissues are limited. Immuno-PET is a molecular imaging tracer that combines the high sensitivity of PET with the specific targeting capability of mAb. Immuno-PET strategies targeting CD20 and other antigens are currently being developed. The theragnostic approach by immuno-PET will be useful in monitoring the treatment response.

Carrier systems of radiopharmaceuticals and the application in cancer therapy

Radiopharmaceuticals play a vital role in cancer therapy. The carrier of radiopharmaceuticals can precisely locate and guide radionuclides to the target, where radionuclides kill surrounding tumor cells. Effective application of radiopharmaceuticals depends on the selection of an appropriate carrier. Herein, different types of carriers of radiopharmaceuticals and the characteristics are briefly described. Subsequently, we review radiolabeled monoclonal antibodies (mAbs) and their derivatives, and novel strategies of radiolabeled mAbs and their derivatives in the treatment of lymphoma and colorectal cancer. Furthermore, this review outlines radiolabeled peptides, and novel strategies of radiolabeled peptides in the treatment of neuroendocrine neoplasms, prostate cancer, and gliomas. The emphasis is given to heterodimers, bicyclic peptides, and peptide-modified nanoparticles. Last, the latest developments and applications of radiolabeled nucleic acids and small molecules in cancer therapy are discussed. Thus, this review will contribute to a better understanding of the carrier of radiopharmaceuticals and the application in cancer therapy.

Radioimmunotherapy of Non-Hodgkin B-cell Lymphoma: An update

Systemic radioimmunotherapy (RIT) is arguably the most effective and least toxic anticancer treatment for non-Hodgkin lymphoma (NHL). In treatment-naïve patients with indolent NHL, the efficacy of a single injection of RIT compares with that of multiple cycles of combination chemotherapy. However, 20 years following the approval of the first CD20-targeting radioimmunoconjugates ⁹⁰Y-Ibritumomab-tiuxetan (Zevalin) and ¹³¹I-tositumomab (Bexxar), the number of patients referred for RIT in western countries has dramatically decreased. Notwithstanding this, the development of RIT has continued. Therapeutic targets other than CD20 have been identified, new vector molecules have been produced allowing for faster delivery of RIT to the target, and innovative radionuclides with favorable physical characteristics such as alpha emitters have been more widely available. In this article, we reviewed the current status of RIT in NHL, with particular focus on recent clinical and preclinical developments.

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.

A pretargeted multimodal approach for image-guided resection in a xenograft model of colorectal cancer

Background

Image-guided surgery may improve surgical outcome for colorectal cancer patients. Here, we evaluated the feasibility of a pretargeting strategy for multimodal imaging in colorectal cancer using an anti-carcinoembryonic antigen (CEA) x anti-histamine-succinyl-glycine (HSG) bispecific antibody (TF2) in conjunction with the dual-labeled diHSG peptide (RDC018), using both a fluorophore for near-infrared fluorescence imaging and a chelator for radiolabeling.

Methods

Nude mice with subcutaneous (s.c) CEA-expressing LS174T human colonic tumors and CEA-negative control tumors were injected with TF2. After 16 h, different doses of ¹¹¹In-labeled IMP-288 (non-fluorescent) or its fluorescent derivative RDC018 were administered to compare biodistributions. MicroSPECT/CT and near-infrared fluorescence imaging were performed 2 and 24 h after injection. Next, the biodistribution of the dual-labeled humanized anti-CEA IgG antibody [¹¹¹In]In-DTPA-hMN-14-IRDye800CW (direct targeting) was compared with the biodistribution of ¹¹¹In-RDC018 in mice with TF2-pretargeted tumors, using fluorescence imaging and gamma counting. Lastly, mice with intraperitoneal LS174T tumors underwent near-infrared fluorescence image-guided resection combined with pre- and post-resection microSPECT/CT imaging.

Results

¹¹¹In-RDC018 showed specific tumor targeting in pretargeted CEA-positive tumors (21.9 ± 4.5 and 10.0 ± 4.7% injected activity per gram (mean ± SD %IA/g), at 2 and 24 hours post-injection (p.i.), respectively) and a biodistribution similar to ¹¹¹In-IMP288. Both fluorescence and microSPECT/CT images confirmed preferential tumor accumulation. At post mortem dissection, intraperitoneal tumors were successfully identified and removed using pretargeting with TF2 and ¹¹¹In-RDC018.

Conclusion

A pretargeted approach for multimodal image-guided resection of colorectal cancer in a preclinical xenograft model is feasible, enables preoperative SPECT/CT, and might facilitate intraoperative fluorescence imaging.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0551-4) contains supplementary material, which is available to authorized users.

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.

Novel Methods to Improve the Efficiency of Radioimmunotherapy for Non-Hodgkin Lymphoma

Radioimmunotherapy (RIT) is a novel strategy for treating non-Hodgkin lymphoma (NHL). Several studies have shown the promising results of using RIT in NHL, which have led to FDA approval for two RIT agents in treating low grade NHL. In spite of these favorable results in low-grade NHL, most of the aggressive or relapsed/refractory NHL subjects experience relapses following RIT. Although more aggressive treatments such as myeloablative doses of RIT followed by stem cell transplantation appear to be able to provide a longer survival for some patients these approaches are associated with significant treatment-related adverse events and challenging to deliver in most centers. Therefore, it seems reasonable to develop treatment approaches that enhance the efficiency of RIT, while reducing its toxicity. In this paper, novel methods that improve the efficiency of RIT and reduce its toxicity through various mechanisms are reviewed. Further clinical development of these methods could expand the NHL patient groups eligible for receiving RIT, and even extend the use of RIT to new indications and disease groups in future.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

Prospective of 68Ga Radionuclide Contribution to the Development of Imaging Agents for Infection and Inflammation

During the last decade, the utilization of ⁶⁸Ga for the development of imaging agents has increased considerably with the leading position in the oncology. The imaging of infection and inflammation is lagging despite strong unmet medical needs. This review presents the potential routes for the development of ⁶⁸Ga-based agents for the imaging and quantification of infection and inflammation in various diseases and connection of the diagnosis to the treatment for the individualized patient management.

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.

Radioimmunotherapy of cancer with high linear energy transfer (LET) radiation delivered by radionuclides emitting α-particles or Auger electrons

Radioimmunotherapy (RIT) aims to selectively deliver radionuclides emitting α-particles, β-particles or Auger electrons to tumors by conjugation to monoclonal antibodies (mAbs) that recognize tumor-associated antigens/receptors. The approach has been most successful for treatment of non-Hodgkin's B-cell lymphoma but challenges have been encountered in extending these promising results to the treatment of solid malignancies. These challenges include the low potency of β-particle emitters such as ¹³¹I, ¹⁷⁷Lu or ⁹⁰Y which have been commonly conjugated to the mAbs, due to their low linear energy transfer (LET=0.1-1.0keV/μm). Furthermore, since the β-particles have a 2-10mm range, there has been dose-limiting non-specific toxicity to hematopoietic stem cells in the bone marrow (BM) due to the cross-fire effect. Conjugation of mAbs to α-particle-emitters (e.g. ²²⁵Ac, ²¹³Bi, ²¹²Pb or ²¹¹At) or Auger electron-emitters (e.g. ¹¹¹In, ⁶⁷Ga, ¹²³I or ¹²⁵I) would increase the potency of RIT due to their high LET (50-230keV/μm and 4 to 26keV/μm, respectively). In addition, α-particles have a range in tissues of 28-100μm and Auger electrons are nanometer in range which greatly reduces or eliminates the cross-fire effect compared to β-particles, potentially reducing their non-specific toxicity to the BM. In this review, we describe the results of preclinical and clinical studies of RIT of cancer using radioimmunoconjugates emitting α-particles or Auger electrons, and discuss the potential of these high LET forms of radiation to improve the outcome of cancer patients.

Radioactive antibodies: a historical review of selective targeting and treatment of cancer

Radioactive antibodies have served as imaging and therapeutic agents for several decades, but recent developments raise enthusiasm that a new generation of cancer therapeutics and diverse molecular imaging agents for various cancers are more likely than ever before. This article traces the development of tumor-targeting antibodies labeled with diagnostic or therapeutic radionuclides, and describes the problems encountered and the clinical advances made. We also emphasize recent attempts to improve both molecular imaging and radioimmunotherapy with multistep pretargeting methods that separate the delivery of the tumor-binding, bispecific antibody given in the first step from the radionuclide carrier, which, in the second step, will localize to the "anti-carrier" binding arm of the pretargeted bispecific antibody.

Addressing challenges of heterogeneous tumor treatment through bispecific protein-mediated pretargeted drug delivery

Tumors are frequently characterized by genomically and phenotypically distinct cancer cell subpopulations within the same tumor or between tumor lesions, a phenomenon termed tumor heterogeneity. These diverse cancer cell populations pose a major challenge to targeted delivery of diagnostic and/or therapeutic agents, as the conventional approach of conjugating individual ligands to nanoparticles is often unable to facilitate intracellular delivery to the full spectrum of cancer cells present in a given tumor lesion or patient. As a result, many cancers are only partially suppressed, leading to eventual tumor regrowth and/or the development of drug-resistant tumors. Pretargeting (multistep targeting) approaches involving the administration of 1) a cocktail of bispecific proteins that can collectively bind to the entirety of a mixed tumor population followed by 2) nanoparticles containing therapeutic and/or diagnostic agents that can bind to the bispecific proteins accumulated on the surface of target cells offer the potential to overcome many of the challenges associated with drug delivery to heterogeneous tumors. Despite its considerable success in improving the efficacy of radioimmunotherapy, the pretargeting strategy remains underexplored for a majority of nanoparticle therapeutic applications, especially for targeted delivery to heterogeneous tumors. In this review, we will present concepts in tumor heterogeneity, the shortcomings of conventional targeted systems, lessons learned from pretargeted radioimmunotherapy, and important considerations for harnessing the pretargeting strategy to improve nanoparticle delivery to heterogeneous tumors.

Antibody Positron Emission Tomography Imaging in Anticancer Drug Development

More than 50 monoclonal antibodies (mAbs), including several antibody–drug conjugates, are in advanced clinical development, forming an important part of the many molecularly targeted anticancer therapeutics currently in development. Drug development is a relatively slow and expensive process, limiting the number of drugs that can be brought into late-stage trials. Development decisions could benefit from quantitative biomarkers, enabling visualization of the tissue distribution of (potentially modified) therapeutic mAbs to confirm effective whole-body target expression, engagement, and modulation and to evaluate heterogeneity across lesions and patients. Such biomarkers may be realized with positron emission tomography imaging of radioactively labeled antibodies, a process called immunoPET. This approach could potentially increase the power and value of early trials by improving patient selection, optimizing dose and schedule, and rationalizing observed drug responses. In this review, we summarize the available literature and the status of clinical trials regarding the potential of immunoPET during early anticancer drug development.

Pretargeted imaging and radioimmunotherapy of cancer using antibodies and bioorthogonal chemistry

Selective delivery of radionuclides to tumors may be accomplished using a two-step approach, in which in the first step the tumor is pretargeted with an unlabeled antibody construct and in the second step the tumor is targeted with a radiolabeled small molecule. This results in a more rapid clearance of the radioactivity from normal tissues due to the fast pharmacokinetics of the small molecule as compared to antibodies. In the last decade, several pretargeting approaches have been tested, which have shown improved tumor-to-background ratios and thus improved imaging and therapy as compared to directly labeled antibodies. In this review, we will discuss the strategies and applications in (pre-)clinical studies of pretargeting concepts based on the use of bispecific antibodies, which are capable of binding to both a target antigen and a radiolabeled peptide. So far, three generations of the bispecific antibody-based pretargeting approach have been studied. The first clinical studies have shown the feasibility and potential for these pretargeting systems to detect and treat tumor lesions. However, to fully integrate the pretargeting approach in clinic, further research should focus on the best regime and pretargeting protocol. Additionally, recent developments in the use of bioorthogonal chemistry for pretargeting of tumors suggest that this chemical pretargeting approach is an attractive alternative strategy for the detection and treatment of tumor lesions.

Radioimmunotherapy: a specific treatment protocol for cancer by cytotoxic radioisotopes conjugated to antibodies

Radioimmunotherapy (RIT) represents a selective internal radiation therapy, that is, the use of radionuclides conjugated to tumor-directed monoclonal antibodies (including those fragments) or peptides. In a clinical field, two successful examples of this treatment protocol are currently extended by ⁹⁰Y-ibritumomab tiuxetan (Zevalin) and ¹³¹I-tositumomab (Bexxar), both of which are anti-CD20 monoclonal antibodies coupled to cytotoxic radioisotopes and are approved for the treatment of non-Hodgkin lymphoma patients. In addition, some beneficial observations are obtained in preclinical studies targeting solid tumors. To date, in order to reduce the unnecessary exposure and to enhance the therapeutic efficacy, various biological, chemical, and treatment procedural improvements have been investigated in RIT. This review outlines the fundamentals of RIT and current knowledge of the preclinical/clinical trials for cancer treatment.

Predictive patient-specific dosimetry and individualized dosing of pretargeted radioimmunotherapy in patients with advanced colorectal cancer

PURPOSE

Pretargeted radioimmunotherapy (PRIT) with bispecific antibodies (bsMAb) and a radiolabeled peptide reduces the radiation dose to normal tissues. Here we report the accuracy of an (111)In-labeled pretherapy test dose for personalized dosing of (177)Lu-labeled IMP288 following pretargeting with the anti-CEA × anti-hapten bsMAb, TF2, in patients with metastatic colorectal cancer (CRC).

METHODS

In 20 patients bone marrow absorbed doses (BMD) and doses to the kidneys were predicted based on blood samples and scintigrams acquired after (111)In-IMP288 injection for individualized dosing of PRIT with (177)Lu-IMP288. Different dose schedules were studied, varying the interval between the bsMAb and peptide administration (5 days vs. 1 day), increasing the bsMAb dose (75 mg vs. 150 mg), and lowering the peptide dose (100 μg vs. 25 μg).

RESULTS

TF2 and (111)In/(177)Lu-IMP288 clearance was highly variable. A strong correlation was observed between peptide residence times and individual TF2 blood concentrations at the time of peptide injection (Spearman's ρ = 0.94, P < 0.0001). PRIT with 7.4 GBq (177)Lu-IMP288 resulted in low radiation doses to normal tissues (BMD <0.5 Gy, kidney dose <3 Gy). Predicted (177)Lu-IMP288 BMD were in good agreement with the actual measured doses (mean ± SD difference -0.0026 ± 0.028 mGy/MBq). Hematological toxicity was mild in most patients, with only two (10 %) having grade 3-4 thrombocytopenia. A correlation was found between platelet toxicity and BMD (Spearman's ρ = 0.58, P = 0.008). No nonhematological toxicity was observed.

CONCLUSION

These results show that individual high activity doses in PRIT in patients with CEA-expressing CRC could be safely administered by predicting the radiation dose to red marrow and kidneys, based on dosimetric analysis of a test dose of TF2 and (111)In-IMP288.

A preclinical model of CD38-pretargeted radioimmunotherapy for plasma cell malignancies

The vast majority of patients with plasma cell neoplasms die of progressive disease despite high response rates to novel agents. Malignant plasma cells are very radiosensitive, but the potential role of radioimmunotherapy (RIT) in the management of plasmacytomas and multiple myeloma has undergone only limited evaluation. Furthermore, CD38 has not been explored as a RIT target despite its uniform high expression on malignant plasma cells. In this report, both conventional RIT (directly radiolabeled antibody) and streptavidin-biotin pretargeted RIT (PRIT) directed against the CD38 antigen were assessed as approaches to deliver radiation doses sufficient for multiple myeloma cell eradication. PRIT demonstrated biodistributions that were markedly superior to conventional RIT. Tumor-to-blood ratios as high as 638:1 were seen 24 hours after PRIT, whereas ratios never exceeded 1:1 with conventional RIT. (90)Yttrium absorbed dose estimates demonstrated excellent target-to-normal organ ratios (6:1 for the kidney, lung, liver; 10:1 for the whole body). Objective remissions were observed within 7 days in 100% of the mice treated with doses ranging from 800 to 1,200 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin, including 100% complete remissions (no detectable tumor in treated mice compared with tumors that were 2,982% ± 2,834% of initial tumor volume in control animals) by day 23. Furthermore, 100% of animals bearing NCI-H929 multiple myeloma tumor xenografts treated with 800 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin achieved long-term myeloma-free survival (>70 days) compared with none (0%) of the control animals.

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.

Diels-Alder reaction for tumor pretargeting: in vivo chemistry can boost tumor radiation dose compared with directly labeled antibody

Current pretargeting systems use noncovalent biologic interactions, which are prone to immunogenicity. We previously developed a novel approach based on the bioorthogonal reaction between a radiolabeled tetrazine and an antibody-conjugated trans-cyclooctene (TCO). However, the tumor-to-blood ratio was low due to reaction with freely circulating antibody-TCO.

METHODS

Here we developed 2 tetrazine-functionalized clearing agents that enable rapid reaction with and removal of a TCO-tagged antibody (CC49) from blood. Next, we incorporated this approach into an optimized pretargeting protocol in LS174T-bearing mice. Then we compared the pretargeted (177)Lu-labeled tetrazine with (177)Lu-labeled CC49. The biodistribution data were used for mouse and human dosimetry calculations.

RESULTS

The use of a clearing agent led to a doubling of the tetrazine tumor uptake and a 125-fold improvement of the tumor-to-blood ratio at 3 h after tetrazine injection. Mouse dosimetry suggested that this should allow for an 8-fold higher tumor dose than is possible with nonpretargeted radioimmunotherapy. Also, humans treated with CC49-TCO-pretargeted (177)Lu-tetrazine would receive a dose to nontarget tissues 1 to 2 orders of magnitude lower than with directly labeled CC49.

CONCLUSION

The in vivo performance of chemical pretargeting falls within the range of results obtained for the clinically validated pretargeting approaches in mice, with the advantage of potentially allowing for fractionated radiotherapy as a result of a lower likelihood of immunogenicity. These findings demonstrate that biologic pretargeting concepts can be translated to rapid bioorthogonal chemical approaches with retained potential.

Development of an imaging-guided CEA-pretargeted radionuclide treatment of advanced colorectal cancer: first clinical results

Background:

Radiolabelled antibody targeting of cancer is limited by slow blood clearance. Pretargeting with a non-radiolabelled bispecific monoclonal antibody (bsMAb) followed by a rapidly clearing radiolabelled hapten peptide improves tumour localisation. The primary goals of this first pretargeting study in patients with the anti-CEACAM5 × anti-hapten (HSG) bsMAb, TF2, and the radiolabelled hapten-peptide, IMP288, were to assess optimal pretargeting conditions and safety in patients with metastatic colorectal cancer (CRC).

Methods:

Different dose schedules were studied in four cohorts of five patients: (1) shortening the interval between the bsMAb and peptide administration (5 days vs 1 day), (2) escalating the TF2 dose (from 75 to 150 mg), and (3) reducing the peptide dose (from 100 to 25 μg). After confirmation of tumour targeting by ¹¹¹In-IMP288, patients were treated with a bsMAb/¹⁷⁷Lu-IMP288 cycle.

Results:

Rapid and selective tumour targeting of the radiolabelled peptide was visualised within 1 h, with high tumour-to-tissue ratios (>20 at 24 h). Improved tumour targeting was achieved with a 1-day interval between the administration of the bsMAb and the peptide and with the 25-μg peptide dose. High ¹⁷⁷Lu-IMP288 doses (2.5–7.4 GBq) were well tolerated, with some manageable TF2 infusion reactions, and transient grades 3–4 thrombocytopaenia in 10% of the patients who received ¹⁷⁷Lu-IMP288.

Conclusion:

This phase I study demonstrates for the first time that pretargeting with bsMAb TF2 and radiolabelled IMP288 in patients with CEA-expressing CRC is feasible and safe. With this pretargeting method, tumours are specifically and rapidly targeted.

Radioimmunotherapy of B-Cell Non-Hodgkin's Lymphoma

This manuscript reviews current advances in the use of radioimmunotherapy (RIT) for the treatment of B-cell non-Hodgkin's lymphoma (NHL). RIT has been in use for more than 20 years and has progressed significantly with the discovery of new molecular targets, the development of new stable chelates, the humanization of monoclonal antibodies (MAbs), and the use of pretargeting techniques. Today, two products targeting the CD20 antigen are approved: (131)I-tositumomab (Bexxar(®)), and (90)Y-ibritumomab tiuxetan (Zevalin(®)). (131)I-tositumomab is available in the United States, and (90)Y-ibritumumab tiuxetan in Europe, the United States, Asia, and Africa. RIT can be integrated in clinical practice using non-ablative activities for treatment of patients with relapsed or refractory follicular lymphoma (FL) or as consolidation after induction chemotherapy in front-line treatment in FL patients. Despite the lack of phase III studies to clearly define the efficacy of RIT in the management of B lymphoma in the era of rituximab-based therapy, RIT efficacy in NHL has been demonstrated. In relapsing refractory FL and transformed NHL, RIT as a monotherapy induces around 30% complete response with a possibility of durable remissions. RIT consolidation after induction therapy significantly improves the quality of the response. Dose-limiting toxicity of RIT is hematological, depending on bone marrow involvement and prior treatment. Non-hematological toxicity is generally low. Different studies have been published assessing innovative protocols of RIT or new indications, in particular treatment in patients with aggressive lymphomas. High-dose treatment, RIT as consolidation after different therapeutic induction modalities, RIT in first-line treatment or fractionated RIT showed promising results. New MAbs, in particular humanized MAbs, or combinations of naked and radiolabeled MAbs, also appear promising. Personalized dosimetry protocols should be developed to determine injected activity.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Using antibodies to target cancer therapeutics

INTRODUCTION

Over a half a century ago, radiolabeled antibodies were shown to localize selectively in tissues based on the expression of unique antigens. Antibodies have since become the de facto targeting agent, even inspiring the development of non-antibody compounds for targeting purposes.

AREAS COVERED

In this article, we review various aspects of how antibodies are transforming the way cancer is being detected and treated, with the growing demand for unconjugated and many new antibody conjugates. While unconjugated antibodies continue to garner most of the attention, interest in new antibody drug conjugates and immunotoxins has expanded over the past few years. However, there continues to be active research with new radioimmunoconjugates for imaging and therapy, particularly with α-emitters, as well as antibody-targeted cytokines and other biological response modifiers.

EXPERT OPINION

The increasing number of new agents being developed and tested clinically suggests that antibody-targeted compounds will have an expanding role in the future.

Pretargeted molecular imaging and radioimmunotherapy

Pretargeting is a multi-step process that first has an unlabeled bispecific antibody (bsMAb) localize within a tumor by virtue of its anti-tumor binding site(s) before administering a small, fast-clearing radiolabeled compound that then attaches to the other portion of the bsMAb. The compound's rapid clearance significantly reduces radiation exposure outside of the tumor and its small size permits speedy delivery to the tumor, creating excellent tumor/nontumor ratios in less than 1 hour. Haptens that bind to an anti-hapten antibody, biotin that binds to streptavidin, or an oligonucleotide binding to a complementary oligonucleotide sequence have all been radiolabeled for use by pretargeting. This review will focus on a highly flexible anti-hapten bsMAb platform that has been used to target a variety of radionuclides to image (SPECT and PET) as well as treat tumors.

Engineering antibodies for cancer therapy

The advent of modern antibody engineering has led to numerous successes in the application of these proteins for cancer therapy in the 13 years since the first Food and Drug Administration approval, which has stimulated active interest in developing more and better drugs based on these molecules. A wide range of tools for discovering and engineering antibodies has been brought to bear on this challenge in the past two decades. Here, we summarize mechanisms of monoclonal antibody therapeutic activity, challenges to effective antibody-based treatment, existing technologies for antibody engineering, and current concepts for engineering new antibody formats and antibody alternatives as next generation biopharmaceuticals for cancer treatment.

Cancer radioimmunotherapy

Targeting of radionuclides with antibodies, or radioimmunotherapy, has been an active field of research spanning nearly 50 years, evolving with advancing technologies in molecular biology and chemistry, and with many important preclinical and clinical studies illustrating the benefits, but also the challenges, which all forms of targeted therapies face. There are currently two radiolabeled antibodies approved for the treatment of non-Hodgkin lymphoma, but radioimmunotherapy of solid tumors remains a challenge. Novel antibody constructs, focusing on treatment of localized and minimal disease, and pretargeting are all promising new approaches that are currently under investigation.

The dock-and-lock method combines recombinant engineering with site-specific covalent conjugation to generate multifunctional structures

Advances in recombinant protein technology have facilitated the production of increasingly complex fusion proteins with multivalent, multifunctional designs for use in various in vitro and in vivo applications. In addition, traditional chemical conjugation remains a primary choice for linking proteins with polyethylene glycol (PEG), biotin, fluorescent markers, drugs, and others. More recently, site-specific conjugation of two or more interactive modules has emerged as a valid approach to expand the existing repertoires produced by either recombinant engineering or chemical conjugation alone, thus advancing the range of potential applications. Five such methods, each involving a specific binding event, are highlighted in this review, with a particular focus on the Dock-and-Lock (DNL) method, which exploits the natural interaction between the dimerization and docking domain (DDD) of cAMP-dependent protein kinase (PKA) and the anchoring domain (AD) of A-kinase anchoring proteins (AKAP). The various enablements of DNL to date include trivalent, tetravalent, pentavalent, and hexavalent antibodies of monospecificity or bispecificity; immnocytokines comprising multiple copies of interferon-alpha (IFNα); and site-specific PEGylation. These achievements attest to the power of the DNL platform technology to develop novel therapeutic and diagnostic agents from both proteins and nonproteins for unmet medical needs.

Pretargeted radioimmunotherapy of colorectal cancer metastases: models and pharmacokinetics predict influence of the physical and radiochemical properties of the radionuclide

PURPOSE

We investigated influences of pretargeting variables, tumor location, and radionuclides in pretargeted radioimmunotherapy (PRIT) as well as estimated tumor absorbed doses.

METHODS

LS-174T human colonic carcinoma cells expressing carcinoembryonic antigen (CEA) were inoculated in nude mice. Biodistribution of a bispecific anti-CEA x anti-hapten antibody, TF2, and of a TF2-pretargeted peptide was assessed and a multi-compartment pharmacokinetic model was devised. Tissue absorbed doses were calculated for (131)I, (177)Lu, (90)Y, (211)At, and (213)Bi using realistic specific activities.

RESULTS

Under conditions optimized for tumor imaging (10:1 TF2 to peptide molar ratio, interval time 15-24 h), tumor uptake reached ∼9 ID/g in subcutaneous tumors at 2 h with very low accretion in normal tissues (tumor to blood ratio >20:1 after 2 h). For a low dose of peptide (0.04 nmol), (211)At is predicted to deliver a high absorbed dose to tumors [41.5 Gy considering a relative biologic effect (RBE) of 5], kidneys being dose-limiting. (90)Y and (213)Bi would also deliver high absorbed doses to tumor (18.6 for (90)Y and 26.5 Gy for (213)Bi, taking RBE into account, for 0.1 nmol) and acceptable absorbed doses to kidneys. With hepatic metastases, a twofold higher tumor absorbed dose is expected. Owing to the low activities measured in blood, the bone marrow absorbed dose is expected to be without significant toxicity.

CONCLUSION

Pretargeting achieves high tumor uptake and higher tumor to background ratios compared to direct RIT. Short-lived radionuclides are predicted to deliver high tumor absorbed doses especially (211)At, with kidneys being the dose-limiting organ. (177)Lu and (131)I should be considered for repeated injections.

Coiled-coil based drug-free macromolecular therapeutics: in vivo efficacy

We evaluated a new concept in cancer therapy, coiled-coil mediated induction of apoptosis in Raji B cells, for treatment of human B-cell lymphoma in a preclinical animal model. The system is composed of a pair of complementary coiled-coil peptides, CCE and CCK, forming antiparallel heterodimers; Fab' fragment of the 1F5 anti-CD20 antibody; and N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer. One peptide is conjugated to the Fab' fragment (Fab'-CCE), the other is conjugated in multiple grafts to polyHPMA (CCK-P; P is the HPMA copolymer backbone). Intravenous administration of Fab'-CCE conjugate, followed by the administration of CCK-P produced long-term survivors in SCID (C.B.-17) mice bearing human B-lymphoma xenografts. The rationale of the design is the absence of low molecular weight drugs and the fact that crosslinking of CD20 at B-cell surface results in apoptosis. This approach creates a new paradigm for manipulating molecular recognition principles in the design of improved cancer treatment.

A new mammalian host cell with enhanced survival enables completely serum-free development of high-level protein production cell lines

With over 25 monoclonal antibodies (mAbs) currently approved and many more in development, there is considerable interest in gaining improved productivity by increasing cell density and enhancing cell survival of production cell lines. In addition, high costs and growing safety concerns with use of animal products have made the availability of serum-free cell lines more appealing. We elected to transfect the myeloma cell line Sp2/0-Ag14 with Bcl2-EEE, the constitutively active phosphomimetic mutant of Bcl2, for extended cell survival. After adaptation of the initial transfectants to serum-independent growth, a clone with superior growth properties, referred to as SpESF, was isolated and further subjected to iterative rounds of stressful growth over a period of 4 months. The effort resulted in the selection of a promising clone, designated SpESFX-10, which was shown to exhibit robust growth and resist apoptosis induced by sodium butyrate or glutamine deprivation. The advantage of SpESFX-10 as a host for generating mAb-production cell lines was demonstrated by its increased transfection efficiency, culture longevity, and mAb productivity, as well as by the feasibility of accomplishing the entire cell line development process, including transfection, subcloning, and cryopreservation, in the complete absence of serum.

A pretargeted nanoparticle system for tumor cell labeling

Nanoparticle-based cancer diagnostics and therapeutics can be significantly enhanced by selective tissue localization, but the strategy can be complicated by the requirement of a targeting ligand conjugated on nanoparticles, that is specific to only one or a limited few types of neoplastic cells, necessitating the development of multiple nanoparticle systems for different diseases. Here, we present a new nanoparticle system that capitalizes on a targeting pretreatment strategy, where a circulating fusion protein (FP) selectively prelabels the targeted cellular epitope, and a biotinylated iron oxide nanoparticle serves as a secondary label that binds to the FP on the target cell. This approach enables a single nanoparticle formulation to be used with any one of existing fusion proteins to bind a variety of target cells. We demonstrated this approach with two fusion proteins against two model cancer cell lines: lymphoma (Ramos) and leukemia (Jurkat), which showed 72.2% and 91.1% positive labeling, respectively. Notably, TEM analysis showed that a large nanoparticle population was endocytosed via attachment to the non-internalizing CD20 epitope.

Pretargeted 177Lu radioimmunotherapy of carcinoembryonic antigen-expressing human colonic tumors in mice

Pretargeted radioimmunotherapy (PRIT) with bispecific antibodies in combination with a radiolabeled peptide reduces the radiation dose to normal tissues, especially the bone marrow. In this study, the optimization, therapeutic efficacy, and toxicity of PRIT of colon cancer with a (177)Lu-labeled peptide was determined in mice with carcinoembryonic antigen (CEA)-expressing human tumors.

METHODS

To obtain the optimal therapeutic efficacy, several strategies were evaluated to increase the total amount of radioactivity targeted to subcutaneous LS174T colon cancer tumors in BALB/c nude mice. First, the maximum amount of bispecific anti-CEA and antihapten antibody TF2 and the peptide IMP288 that could be targeted was determined. Second, the tumor targeting of repeated administrations of radiolabeled IMP288 was investigated. Mice received 1 TF2 injection, followed by multiple IMP288 injections (3-h interval) or multiple cycles, with each IMP288 administration preceded by a new TF2 injection (72-h interval). PRIT was administered at maximum doses of TF2 and (177)Lu-labeled IMP288 in groups of 9 mice with subcutaneous LS174T tumors. Mice received 1, 2, or 3 successive cycles of treatment (26 MBq/mouse/cycle) or carrier only. The primary endpoint was survival; secondary endpoints were tumor growth, body weight, bone marrow, and renal toxicity.

RESULTS

The highest amount of radioactivity delivered to a subcutaneous colon tumor was achieved by the administration of 5.0 nmol of TF2 and 0.28 nmol of IMP288 in 3 successive cycles, with each IMP288 preceded by a new TF2 injection (72-h interval). PRIT effectively delayed tumor growth and prolonged survival significantly. Higher activity doses, administered in successive cycles, correlated with longer survival: the median survival of untreated mice was 13 d (range, 6-20 d), whereas that of mice treated with 1, 2, or 3 cycles of PRIT was 24 (range, 24-31 d), 45 (range, 38 ≥ 130 d), and 65 (range, 48 ≥ 130 d) days, respectively. Toxicity was limited: no significant changes in mean body weight were measured. Minimal changes in leukocyte counts were measured at 2 and 3 wk after injection, with full recovery within 7 wk after treatment. Platelet counts were unaffected. Serum creatinine levels were not increased significantly; thus, there was no indication of acute renal toxicity.

CONCLUSION

This study indicates that PRIT in mice is an effective treatment modality against colon cancer, with limited toxicity.

Bispecific antibodies for cancer therapy

Bispecific antibodies, in contrast to conventional monoclonal antibodies, can bind simultaneously two different antigens. Taking advantage of this virtue, they are mostly designed for immune effector cell redirection to tumors and for radionuclide pretargeting to tumors. Bispecific antibodies of the first generation were produced by chemical cross-linking or cell-fusion technologies. More recently, the application of genetic engineering technologies gave rise to numerous formats of bispecific antibody fragments and whole IgG molecules. Because bispecific antibodies enable therapeutic strategies that are not possible with conventional monoclonal antibodies, they attract strong interest. Several bispecific antibody formats have already shown clinical efficacy in cancer patients, catalyzing efforts to translate the imaginative bispecific antibody concepts into effective therapies.

Improved cancer therapy and molecular imaging with multivalent, multispecific antibodies

Antibodies are highly versatile proteins with the ability to be used to target diverse compounds, such as radionuclides for imaging and therapy, or drugs and toxins for therapy, but also can be used unconjugated to elicit therapeutically beneficial responses, usually with minimal toxicity. This update describes a new procedure for forming multivalent and/or multispecific proteins, known as the dock-and-lock (DNL) technique. Developed as a procedure for preparing bispecific antibodies capable of binding divalently to a tumor antigen and monovalently to a radiolabeled hapten-peptide for pretargeted imaging and therapy, this methodology has the flexibility to create a number of other biologic agents of therapeutic interest. A variety of constructs, based on anti-CD20 and CD22 antibodies, have been made, with results showing that multispecific antibodies have very different properties from the respective parental monospecific antibodies. The technique is not restricted to antibody combination, but other biologics, such as interferon-alpha2b, have been prepared. These types of constructs not only allow small biologics to be sustained in the blood longer, but also to be selectively targeted. Thus, DNL technology is a highly flexible platform that can be used to prepare many different types of agents that could further improve cancer detection and therapy.

Pretargeted radioimmunotherapy for hematologic and other malignancies

Radioimmunotherapy (RIT) has emerged as one of the most promising treatment options, particularly for hematologic malignancies. However, this approach has generally been limited by a suboptimal therapeutic index (target-to-nontarget ratio) and an inability to deliver sufficient radiation doses to tumors selectively. Pretargeted RIT (PRIT) circumvents these limitations by separating the targeting vehicle from the subsequently administered therapeutic radioisotope, which binds to the tumor-localized antibody or is quickly excreted if unbound. A growing number of preclinical proof-of-principle studies demonstrate that PRIT is feasible and safe and provides improved directed radionuclide delivery to malignant cells compared with conventional RIT while sparing normal cells from nonspecific radiotoxicity. Early phase clinical studies corroborate these preclinical findings and suggest better efficacy and lesser toxicities in patients with hematologic and other malignancies. With continued research, PRIT-based treatment strategies promise to become cornerstones to improved outcomes for cancer patients despite their complexities.

Improved 18F labeling of peptides with a fluoride-aluminum-chelate complex

We reported previously the feasibility to radiolabel peptides with fluorine-18 ((18)F) using a rapid one-pot method that first mixes (18)F(-) with Al(3+) and then binds the (Al(18)F)(2+) complex to a NOTA ligand on the peptide. In this report, we examined several new NOTA ligands and determined how temperature, reaction time, and reagent concentration affected the radiolabeling yield. Four structural variations of the NOTA ligand had isolated radiolabeling yields ranging from 5.8% to 87% under similar reaction conditions. All of the Al(18)F NOTA complexes were stable in vitro in human serum, and those that were tested in vivo also were stable. The radiolabeling reactions were performed at 100 degrees C, and the peptides could be labeled in as little as 5 min. The IMP467 peptide could be labeled up to 115 GBq/micromol (3100 Ci/mmol), with a total reaction and purification time of 30 min without chromatographic purification.

Recombinant bispecific monoclonal antibodies prepared by the dock-and-lock strategy for pretargeted radioimmunotherapy

The selective delivery of therapeutic radionuclides is a promising approach for treating cancer. Antibody-targeted radionuclides are of particular interest, with 2 products approved for the treatment of certain forms of non-Hodgkin lymphoma. However, for many other cancers, radioimmunotherapy has been ineffective, being limited by prolonged exposure to the highly radiosensitive bone marrow. An alternative approach, known as pretargeting, separates radionuclide from the antibody, allowing the radiation to be delivered on a small molecule that can quickly and efficiently migrate into the tumor, and then rapidly clear from the body with minimal retention in tissues. Several pretargeting methods have been developed that differ in the way they selectively capture the radionuclide. This review focuses on the development of a novel form of bispecific monoclonal antibody (bsMAb) pretargeting that uses a unique radiolabeled hapten-peptide system that can be modified to bind numerous therapeutic and imaging radionuclides. Together with a specialized recombinant humanized bsMAb prepared with by a technique known as the Dock-and-Lock method, this pretargeting procedure has been examined in many different animal models, showing a high level of sensitivity and specificity for localizing tumors, and improved efficacy with less hematologic toxicity associated with directly radiolabeled IgG. The bsMAb is a tri-Fab structure, having 2 binding arms for the tumor antigen and 1 capable of binding a hapten-peptide. Preclinical studies were preformed to support the clinical use of a bsMAb and a hapten-peptide bearing a single DOTA moiety (IMP-288). A phase 0 trial found an (131)I-tri-Fab bsMAb, TF2, that targets carcinoembryonic antigen was stable in vivo, quickly clears from the blood, and localizes known tumors. The first-in-patient pretargeting experience with the (111)In-IMP-288 also observed rapid clearance and low tissue (kidney) retention, as well as localization of tumors, providing initial promising evidence for developing these materials for radioimmunotherapy.

Bispecific antibodies for cancer therapy: the light at the end of the tunnel?

With 23 approvals in the US and other countries and four approvals outside US, antibodies are now widely recognized as therapeutic molecules. The therapeutic and commercial successes met by rituximab, trastuzumab, cetuximab and other mAbs have inspired antibody engineers to improve the efficacy of these molecules. Consequently, a new wave of antibodies with engineered Fc leading to much higher effector functions such as antibody-dependent cell-mediated cytotoxicity or complement-dependent cytotoxicity is being evaluated in the clinic, and several approvals are expected soon. In addition, research on a different class of antibody therapeutics, bispecific antibodies, has recently led to outstanding clinical results, and the first approval of the bispecific antibody catumaxomab, a T cell retargeting agent that was approved in the European Union in April 2009. This review describes the most recent advances and clinical study results in the field of bispecific antibodies, a new class of molecules that might outshine conventional mAbs as cancer immunotherapeutics in a near future.

Multivalent antibodies: when design surpasses evolution

Evolutionary pressure has selected antibodies as key immune molecules acting against foreign pathogens. The development of monoclonal antibody technology has allowed their widespread use in research, real-time diagnosis and treatment of multiple diseases, including cancer. However, compared with hematologic malignancies, solid tumors have often proven to be relatively resistant to antibody-based therapies. In an attempt to improve the tumor-targeting efficacy of antibodies, new formats with modified, multivalent properties have been generated. Initially, these formats imitated the structure of native IgG, creating mostly monospecific, bivalent antibodies. Recently, novel trivalent antibodies have been developed to maximize tumor targeting capabilities through enhanced biodistribution and functional affinity. We review recent advances in the engineering of multivalent antibodies and further discuss their promise as agents for in vivo diagnostics and therapy.

Rapid detection of hypoxia-inducible factor-1-active tumours: pretargeted imaging with a protein degrading in a mechanism similar to hypoxia-inducible factor-1alpha

PURPOSE

Hypoxia-inducible factor-1 (HIF-1) plays an important role in malignant tumour progression. For the imaging of HIF-1-active tumours, we previously developed a protein, POS, which is effectively delivered to and selectively stabilized in HIF-1-active cells, and a radioiodinated biotin derivative, (3-(123)I-iodobenzoyl)norbiotinamide ((123)I-IBB), which can bind to the streptavidin moiety of POS. In this study, we aimed to investigate the feasibility of the pretargeting method using POS and (123)I-IBB for rapid imaging of HIF-1-active tumours.

METHODS

Tumour-implanted mice were pretargeted with POS. After 24 h, (125)I-IBB was administered and subsequently, the biodistribution of radioactivity was investigated at several time points. In vivo planar imaging, comparison between (125)I-IBB accumulation and HIF-1 transcriptional activity, and autoradiography were performed at 6 h after the administration of (125)I-IBB. The same sections that were used in autoradiographic analysis were subjected to HIF-1alpha immunohistochemistry.

RESULTS

(125)I-IBB accumulation was observed in tumours of mice pretargeted with POS (1.6%ID/g at 6 h). This result is comparable to the data derived from (125)I-IBB-conjugated POS-treated mice (1.4%ID/g at 24 h). In vivo planar imaging provided clear tumour images. The tumoral accumulation of (125)I-IBB significantly correlated with HIF-1-dependent luciferase bioluminescence (R=0.84, p<0.01). The intratumoral distribution of (125)I-IBB was heterogeneous and was significantly correlated with HIF-1alpha-positive regions (R=0.58, p<0.0001).

CONCLUSION

POS pretargeting with (123)I-IBB is a useful technique in the rapid imaging and detection of HIF-1-active regions in tumours.

Improving the treatment of non-Hodgkin lymphoma with antibody-targeted radionuclides

Radioimmunotherapy of non-Hodgkin lymphoma comprises a (90)Y- or (131)I-labeled murine anti-CD20 IgG, but both agents also include a substantial dose of unlabeled anti-CD20 IgG given immediately before the radioconjugate to reduce its uptake in the spleen (primary normal B-cell antigen sink); this extends its plasma half-life and improves tumor visualization. Thus, these treatments combine an effective anti-CD20 radioconjugate with an unconjugated anti-CD20 antibody that is also therapeutically active, but the large anti-CD20 IgG predose ( approximately 900 mg) may diminish the tumor localization of the radioimmunoconjugate (eg, 10-35 mg). We have examined alternative approaches that enhance radionuclide targeting and improve antitumor responses. One uses a (90)Y-labeled anti-CD22 IgG (epratuzumab) combined with an antibody therapy regimen of a humanized anti-CD20 IgG (veltuzumab). Pretargeted radionuclide therapy using a trivalent, humanized, recombinant bispecific anti-CD20 antibody with a (90)Y-hapten-peptide is another highly effective method that is also less toxic than directly radiolabeled IgG. Finally, all approaches benefit from the addition of a consolidation-dosing regimen of the anti-CD20 IgG antibody. This article reviews these various options and discusses how some fundamental changes could potentially enhance the response and duration from radionuclide-targeted therapy.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Pretargeted radioimmunotherapy of pancreatic cancer xenografts: TF10-90Y-IMP-288 alone and combined with gemcitabine

Pancreatic cancer is a silent disease that most commonly presents in an already metastatic form. Current treatment options provide little survival benefit. Radiolabeled PAM4 IgG, a monoclonal antibody that recognizes a unique epitope associated with a mucin found almost exclusively in pancreatic cancer, has shown encouraging therapeutic effects in animal models and in early clinical testing ((90)Y-humanized PAM4 IgG, (90)Y-clivatuzumab tetraxetan). The studies reported herein examine a new pretargeting procedure for delivering therapeutic radionuclides.

METHODS

We prepared a humanized, recombinant tri-Fab bispecific monoclonal antibody (bsmAb) (TF10) using specificity for targeting pancreatic cancer of PAM4 and another Fab binding to a hapten (histamine-succinyl-glycine [HSG]) and tested this in a pretargeting setting with a (90)Y-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-di-HSG-peptide (pretargeted radioimmunotherapy [PT-RAIT]). Nude mice bearing established Capan-1 human pancreatic cancer xenografts were given TF10 and then received the (90)Y peptide as a single bolus dose 19 h later, or the therapy cycle was fractionated weekly. Other studies examined different combinations with gemcitabine.

RESULTS

PT-RAIT of 18.5 MBq ( approximately 50% of its maximum tolerated dose [MTD]) was as effective as the MTD of (90)Y-PAM4 IgG (5.55 MBq). Three monthly doses of 9.25 MBq of PT-RAIT combined with a monthly cycle of gemcitabine (3 weekly, 6-mg doses) significantly enhanced survival, compared with PT-RAIT alone. Adding gemcitabine as a radiosensitizer to 9.25 MBq of PT-RAIT enhanced objective responses. Weekly fractionation of the PT-RAIT, as compared with a single treatment, improved responses.

CONCLUSION

PAM4-based PT-RAIT with (90)Y hapten peptide is an effective treatment for pancreatic cancer, with less toxicity than (90)Y-PAM4 IgG, in this model. Combinations with gemcitabine and dose fractionation of the PT-RAIT enhanced therapeutic responses.

A modular IgG-scFv bispecific antibody topology

Here we present a bispecific antibody (bsAb) format in which a disulfide-stabilized scFv is fused to the C-terminus of the light chain of an IgG to create an IgG-scFv bifunctional antibody. When expressed in mammalian cells and purified by one-step protein A chromatography, the bsAb retains parental affinities of each binding domain, exhibits IgG-like stability and demonstrates in vivo IgG-like tumor targeting and blood clearance. The extension of the C-terminus of the light chain of an IgG with an scFv or even a smaller peptide does appear to disrupt disulfide bond formation between the light and heavy chains; however, this does not appear to affect binding, stability or in vivo properties of the IgG. Thus, we demonstrate here that the light chain of an IgG can be extended with an scFv without affecting IgG function and stability. This format serves as a standardized platform for the construction of functional bsAbs.