Bispecific antibody pretargeting of radionuclides for immuno single-photon emission computed tomography and immuno positron emission tomography molecular imaging: an update

Cell-based Relay Delivery Strategy in Biomedical Applications

The relay delivery strategy is a two-step targeting approach based on two distinct modules in which the first step with an initiator is to artificially create a target/environment which can be targeted by the follow-up effector. This relay delivery concept creates opportunities to amplify existing or create new targeted signals through deploying initiators to enhance the accumulation efficiency of the following effector at the disease site. As the "live" medicines, cell-based therapeutics possess inherent tissue/cell homing abilities and favorable feasibility of biological and chemical modifications, endowing them the great potential in specifically interacting with diverse biological environments. All these unique capabilities make cellular products great candidates that can serve as either initiators or effectors for relay delivery strategies. In this review, we survey recent advances in relay delivery strategies with a specific focus on the roles of various cells in developing relay delivery systems.

PET Imaging of Lung Cancers in Precision Medicine: Current Landscape and Future Perspective

Despite the recent advances in cancer treatment, lung cancer remains the leading cause of cancer mortality worldwide. Immunotherapies using immune checkpoint inhibitors (ICIs) achieved substantial efficacy in nonsmall cell lung cancer (NSCLC). Currently, most ICIs are still a monoclonal antibody (mAb). Using mAbs or antibody derivatives labeled with radionuclide as the tracers, immunopositron emission tomography (immunoPET) possesses multiple advantages over traditional ¹⁸F-FDG PET in imaging lung cancers. ImmunoPET presents excellent potential in detecting, diagnosing, staging, risk stratification, treatment guidance, and recurrence monitoring of lung cancers. By using radiolabeled mAbs, immunoPET can visualize the biodistribution and uptake of ICIs, providing a noninvasive modality for patient stratification and response evaluation. Some novel targets and associated tracers for immunoPET have been discovered and investigated. This Review introduces the value of immunoPET in imaging lung cancers by summarizing both preclinical and clinical evidence. We also emphasize the value of immunoPET in optimizing immunotherapy in NSCLC. Lastly, immunoPET probes developed for imaging small cell lung cancer (SCLC) will also be discussed. Although the major focus is to summarize the immunoPET tracers for lung cancers, we also highlighted several small-molecule PET tracers to give readers a balanced view of the development status.

ImmunoPET imaging of hematological malignancies: From preclinical promise to clinical reality

Immuno-positron emission tomography (immunoPET) imaging is a paradigm-shifting imaging technique for whole-body and all-lesion tumor detection, based on the combined specificity of tumor-targeting vectors [e.g., monoclonal antibodies (mAbs), nanobodies, and bispecific antibodies] and the sensitivity of PET imaging. By noninvasively, comprehensively, and serially revealing heterogeneous tumor antigen expression, immunoPET imaging is gradually improving the theranostic prospects for hematological malignancies. In this review, we summarize the available literature regarding immunoPET in imaging hematological malignancies. We also highlight the pros and cons of current conjugation strategies, and modular chemistry that can be leveraged to develop novel immunoPET probes for hematological malignancies. Lastly, we discuss the use of immunoPET imaging in guiding antibody drug development.

Comparative Evaluation of Novel 177Lu-Labeled PNA Probes for Affibody-Mediated PNA-Based Pretargeting

Simple Summary

Affibody molecules are small, engineered affinity proteins based on a nonimmunoglobulin scaffold. Affibody-based radionuclide imaging probes have demonstrated excellent tumor targeting. However, the renal clearance of affibody molecules is accompanied by high reabsorption and retention of activity in the kidney, which prevents their use for radionuclide therapy. We have previously shown the feasibility of overcoming the high renal uptake using a pretargeting approach for affibody-mediated therapy based on peptide nucleic acid (PNA) hybridization. In this study, we test the hypothesis that shortening the PNA pretargeting probes would further increase the difference between the accumulation of radiometals in tumor xenografts and in kidneys. A series of novel PNA probes has been designed and evaluated in vitro and in vivo. We have found that a variant containing 9 nucleobases enables a two-fold increase of the tumor-to-kidney dose ratio compared with a variant containing 15 nucleobases. This creates preconditions for more efficient therapy of cancer.

Abstract

Affibody-mediated PNA-based pretargeting is a promising approach to radionuclide therapy of HER2-expressing tumors. In this study, we test the hypothesis that shortening the PNA pretargeting probes would increase the tumor-to-kidney dose ratio. The primary probe Z_HER2:342-SR-HP15 and the complementary secondary probes HP16, HP17, and HP18, containing 9, 12, and 15 nucleobases, respectively, and carrying a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator were designed, synthesized, characterized in vitro, and labeled with ¹⁷⁷Lu. In vitro pretargeting was studied in HER2-expressing SKOV3 and BT474 cell lines. The biodistribution of these novel probes was evaluated in immunodeficient mice bearing SKOV3 xenografts and compared to the previously studied [¹⁷⁷Lu]Lu-HP2. Characterization confirmed the formation of high-affinity duplexes between HP15 and the secondary probes, with the affinity correlating with the length of the complementary PNA sequences. All the PNA-based probes were bound specifically to HER2-expressing cells in vitro. In vivo studies demonstrated HER2-specific uptake of all ¹⁷⁷Lu-labeled probes in xenografts in a pretargeting setting. The ratio of cumulated radioactivity in the tumor to the radioactivity in kidneys was dependent on the secondary probe’s size and decreased with an increased number of nucleobases. The shortest PNA probe, [¹⁷⁷Lu]Lu-HP16, showed the highest tumor-to-kidney ratio. [¹⁷⁷Lu]Lu-HP16 is the most promising secondary probe for affibody-mediated tumor pretargeting.

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.

Proximity proteomics identifies cancer cell membrane cis-molecular complex as a potential cancer target

Cancer‐specific antigens expressed in the cell membrane have been used as targets for several molecular targeted strategies in the last 20 years with remarkable success. To develop more effective cancer treatments, novel targets and strategies for targeted therapies are needed. Here, we examined the cancer cell membrane‐resident “cis‐bimolecular complex” as a possible cancer target (cis‐bimolecular cancer target: BiCAT) using proximity proteomics, a technique that has attracted attention in the last 10 years. BiCAT were detected using a previously developed method termed the enzyme‐mediated activation of radical source (EMARS), to label the components proximal to a given cell membrane molecule. EMARS analysis identified some BiCAT, such as close homolog of L1 (CHL1), fibroblast growth factor 3 (FGFR3) and α2 integrin, which are commonly expressed in mouse primary lung cancer cells and human lung squamous cell carcinoma cells. Analysis of cancer specimens from 55 lung cancer patients revealed that CHL1 and α2 integrin were highly co–expressed in almost all cancer tissues compared with normal lung tissues. As an example of BiCAT application, in vitro simulation of effective drug combinations used for multiple drug treatment strategies was performed using reagents targeted to BiCAT molecules. The combination treatment based on BiCAT information moderately suppressed cancer cell proliferation compared with single administration, suggesting that the information about BiCAT in cancer cells is useful for the appropriate selection of the combination among molecular targeted reagents. Thus, BiCAT has the potential to contribute to several molecular targeted strategies in future.

A Revisit to the Pretargeting Concept-A Target Conversion

Pretargeting is often used as a tumor targeting strategy that provides much higher tumor to non-tumor ratios than direct-targeting using radiolabeled antibody. Due to the multiple injections, pretargeting is investigated less than direct targeting, but the high T/NT ratios have rendered it more useful for therapy. While the progress in using this strategy for tumor therapy has been regularly reviewed in the literature, this review focuses on the nature and quantitative understanding of the pretargeting concept. By doing so, it is the goal of this review to accelerate pretargeting development and translation to the clinic and to prepare the researchers who are not familiar with the pretargeting concept but are interested in applying it. The quantitative understanding is presented in a way understandable to the average researchers in the areas of drug development and clinical translation who have the basic concept of calculus and general chemistry.

Prorenin receptor acts as a potential molecular target for pancreatic ductal adenocarcinoma diagnosis

Recent studies have implicated the prorenin receptor (PRR) is associated with pancreatic tumorigenesis. We therefore investigated the role of PRR in pancreatic tumorigenesis and assessed whether PRR can serve as a target for imaging diagnosis at early stages of PDAC. Here we show that aberrant expression of PRR in premalignant PanIN lesions, and human PDAC samples, and PDAC cell lines, particularly in Panc-1 cells. Interestingly, PRR expression was positively associated with PDAC progression. Moreover, overexpression of human PRR resulted in increased cell proliferation and decreased apoptosis, while knockdown of human PRR caused decreased cell proliferation and enhanced apoptosis in pancreatic cancer cells. We also observed that overexpression of human PRR enhanced MAPK and PI3K/Akt signaling pathways in PDAC cells, while knockdown of human PRR suppressed both of pathways. The confocal imaging analysis showed that human PRR was highly expressed in Panc-1, ASPC, and Miapaca cells, whereas BXPC-3, and HPAC cells had a significantly lower fluorescent signals. Consistently, the single-photon emission computed tomography (SPET/CT) showed that the uptake of anti-PRR labelled with ¹²⁵I was higher in Panc-1 and ASPC tumors-bearing mice after 96 hours injection. Importantly, tumors in pancreas of Pdx1-cre; LSL-Kras^G12D mice had a significant increased PRR expression and accumulation of radioactivity at 96 h after injection. These data suggest that ¹²⁵I-anti-PRR can detect the orthotopic tumors in Pdx1-cre; LSL-Kras^G12D mice. Therefore, anti-PRR labelled with ¹²⁵I is a promising radiotracer for imaging diagnosis at early stages of pancreatic cancer.

SPECT/NIRF Dual Modality Imaging for Detection of Intraperitoneal Colon Tumor with an Avidin/Biotin Pretargeting System

We describe herein dual-modality imaging of intraperitoneal colon tumor using an avidin/biotin pretargeting system. A novel dual-modality probe, ^99mTc-HYNIC-lys(Cy5.5)-PEG₄-biotin, was designed, synthesized and characterized. Single-photon emission computed tomography/ computed tomography (SPECT/CT) imaging and near infrared fluorescence (NIRF) imaging were developed using intraperitoneal LS180 human colon adenocarcinoma xenografts. Following avidin preinjection for 4 hours, ^99mTc-HYNIC-lys(Cy5.5)-PEG₄-biotin could successfully detect colon tumors of different sizes inside the abdominal region using both modalities, and the imaging results showed no differences. Biodistribution studies demonstrated that the tumors had a very high uptake of the probe ^99mTc-HYNIC-lys(Cy5.5)-PEG₄-biotin (12.74 ± 1.89% ID/g at 2 h p.i.), and the clearance from blood and other normal tissues occured very fast. The low tumor uptake in the non-pretargeted mice (1.63 ± 0.50% ID/g at 2 h p.i.) and tumor cell staining results showed excellent tumor binding specificity of the pretargeting system. The ability of the novel probe to show excellent imaging quality with high tumor-to-background contrast, a high degree of binding specificity with tumors and excellent in vivo biodistribution pharmacokinetics should prove that the avidin/biotin based dual-modality pretargeting probe is a promising imaging tool during the entire period of tumor diagnosis and treatment.

In vivo demonstration of an active tumor pretargeting approach with peptide nucleic acid bioconjugates as complementary system

A novel, promising strategy for cancer diagnosis and therapy is the use of a pretargeting approach. For this purpose, the non-natural DNA/RNA analogues Peptide Nucleic Acids (PNAs) are ideal candidates as in vivo recognition units due to their high metabolic stability and lack of unspecific accumulation. In the pretargeting approach, an unlabeled, highly specific antibody-PNA conjugate has sufficient time to target a tumor before administration of a small fast-clearing radiolabeled complementary PNA that hybridizes with the antibody-PNA conjugate at the tumor site. Herein, we report the first successful application of this multistep process using a PNA-modified epidermal growth factor receptor (EGFR) specific antibody (cetuximab) and a complementary 99mTc-labeled PNA. In vivo studies on tumor bearing mice demonstrated a rapid and efficient in vivo hybridization of the radiolabeled PNA with the antibody-PNA conjugate. Decisively, a high specific tumor accumulation was observed with a tumor-to-muscle ratio of >8, resulting in a clear visualization of the tumor by single photon emission computed tomography (SPECT).

An innovative pre-targeting strategy for tumor cell specific imaging and therapy

A programmed pre-targeting system for tumor cell imaging and targeting therapy was established based on the "biotin-avidin" interaction. In this programmed functional system, transferrin-biotin can be actively captured by tumor cells with the overexpression of transferrin receptors, thus achieving the pre-targeting modality. Depending upon avidin-biotin recognition, the attachment of multivalent FITC-avidin to biotinylated tumor cells not only offered the rapid fluorescence labelling, but also endowed the pre-targeted cells with targeting sites for the specifically designed biotinylated peptide nano-drug. Owing to the successful pre-targeting, tumorous HepG2 and HeLa cells were effectively distinguished from the normal 3T3 cells via fluorescence imaging. In addition, the self-assembled peptide nano-drug resulted in enhanced cell apoptosis in the observed HepG2 cells. The tumor cell specific pre-targeting strategy is applicable for a variety of different imaging and therapeutic agents for tumor treatments.

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

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

A tale of two specificities: bispecific antibodies for therapeutic and diagnostic applications

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Recombinant DNA technologies are leading the rapid expansion of bispecific antibody formats.

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The therapeutic potential of bispecific antibodies is being realized through creative design.

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Bispecific antibodies are potentially underutilized reagents for diagnostics.

Artificial manipulation of antibody genes has facilitated the production of several unique recombinant antibody formats, which have highly important therapeutic and biotechnological applications. Although bispecific antibodies (bsAbs) are not new, they are coming to the forefront as our knowledge of the potential efficacy of antibody-based therapeutics expands. The next generation of bsAbs is developing due to significant improvements in recombinant antibody technologies. This review focuses on recent advances with a particular focus on improvements in format and design that are contributing to the resurgence of bsAbs, and in particular, on innovative structures applicable to next generation point-of-care (POC) devices with applicability to low resource environments.

Mapping biological behaviors by application of longer-lived positron emitting radionuclides

With the technological development of positron emission tomography (PET) and the advent of novel antibody-directed drug delivery systems, longer-lived positron-emitting radionuclides are moving to the forefront to take important roles in tracking the distribution of biotherapeutics such as antibodies, and for monitoring biological processes and responses. Longer half-life radionuclides possess advantages of convenient on-site preparation procedures for both clinical and non-clinical applications. The suitability of the long half-life radionuclides for imaging intact monoclonal antibodies (mAbs) and their respective fragments, which have inherently long biological half-lives, has attracted increased interest in recent years. In this review, we provide a survey of the recent literature as it applies to the development of nine-selected longer-lived positron emitters with half-lives of 9-140h (e.g., (124)I, (64)Cu, (86)Y and (89)Zr), and describe the biological behaviors of radionuclide-labeled mAbs with respect to distribution and targeting characteristics, potential toxicities, biological applications, and clinical translation potentials.

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.

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.

Bispecific Antibodies for Diagnostic Applications

Bispecific monoclonal antibodies (BsMAb) are unique engineered macromolecules that have two different pre-determined binding specificities. Their ability to simultaneously bind to a specific antigen and a given detection moiety enables them to function as excellent bifunctional immunoprobes in diagnostic assays. BsMAb are being exploited for the development of simple, rapid, and highly sensitive immunoassays for diagnosis of bacterial and viral infectious diseases. This chapter describes the use of BsMAb for the detection of Mycobacterium tuberculosis, Escherichia coli O157:H7, Bordetella pertussis, Severe Acute Respiratory Syndrome coronavirus, and Dengue virus. Further, BsMAb have been utilized for diagnosis of various types of cancers. The use of BsMAb in detection of prostate cancer and in cancer diagnostic imaging is also discussed.

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.

A semiempirical model of tumor pretargeting

This article provides an overview of a semiempirical pretargeting model now under development. After a brief review of the pretargeting concept, the strategies available, and the complexities of optimizing the dosage and timing, a semiempirical model is described that is not only capable of optimizing dosage and timing but also capable of predicting the results of pretargeting as a function of most pretargeting variables. The model requires knowledge of the pharmacokinetics of both the pretargeting agent (usually an antibody) and the effector, the accessibility of the pretargeting antibody for the effector, and their quantitative relationships in vivo. Several misconceptions that often surround pretargeting are also clarified.

Antibodies and antimatter: the resurgence of immuno-PET

The completion of the human genome, coupled with parallel major research efforts in proteomics and systems biology, has led to a flood of information on the roles of individual genes and proteins in normal physiologic processes and their disruptions in disease. In practical terms, this information has opened the door to increasingly targeted therapies as specific molecular markers are identified and validated. The ongoing transition from empiric to molecular medicine has engendered a need for corresponding molecular diagnostics, including noninvasive molecular imaging. Convergence of knowledge regarding key biomarkers that define normal biologic processes and disease with protein and imaging technology makes this an opportune time to revisit the combination of antibodies and PET, or immuno-PET.

Antibodies for molecular imaging of cancer

Antibodies have attained a central role as targeted therapeutics, with several significant drugs on the market and many more in clinical development for oncological applications. Expansion of the role of antibodies in cancer imaging has been accelerated by a number of factors, including the recognition that antibodies can provide a powerful class of molecular imaging probes for interrogating cell surfaces in vivo. Identification of relevant cell surface biomarkers as imaging targets, coupled with advances in antibody technology, facilitate the generation of antibodies optimized for noninvasive imaging. Developments in imaging instrumentation and radionuclide availability have paved the way for broader evaluation and implementation of radioimmunoscintigraphy and immunoPET. Antibody imaging can provide a sensitive, noninvasive means for molecular characterization of cell surface phenotype in vivo, which can in turn guide diagnosis, prognosis, therapy selection, and monitoring of treatment in cancer.

Radionuclide-Based Cancer Imaging Targeting the Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA), highly expressed in many cancer types, is an important target for cancer diagnosis and therapy. Radionuclide-based imaging techniques (gamma camera, single photon emission computed tomography [SPECT] and positron emission tomography [PET]) have been extensively explored for CEA-targeted cancer imaging both preclinically and clinically. Briefly, these studies can be divided into three major categories: antibody-based, antibody fragment-based and pretargeted imaging. Radiolabeled anti-CEA antibodies, reported the earliest among the three categories, typically gave suboptimal tumor contrast due to the prolonged circulation life time of intact antibodies. Subsequently, a number of engineered anti-CEA antibody fragments (e.g. Fab’, scFv, minibody, diabody and scFv-Fc) have been labeled with a variety of radioisotopes for CEA imaging, many of which have entered clinical investigation. CEA-Scan (a ^99mTc-labeled anti-CEA Fab’ fragment) has already been approved by the United States Food and Drug Administration for cancer imaging. Meanwhile, pretargeting strategies have also been developed for CEA imaging which can give much better tumor contrast than the other two methods, if the system is designed properly. In this review article, we will summarize the current state-of-the-art of radionuclide-based cancer imaging targeting CEA. Generally, isotopes with short half-lives (e.g. ¹⁸F and ^99mTc) are more suitable for labeling small engineered antibody fragments while the isotopes with longer half-lives (e.g. ¹²³I and ¹¹¹In) are needed for antibody labeling to match its relatively long circulation half-life. With further improvement in tumor targeting efficacy and radiolabeling strategies, novel CEA-targeted agents may play an important role in cancer patient management, paving the way to “personalized medicine”.

A novel bispecific, trivalent antibody construct for targeting pancreatic carcinoma

Preclinical and clinical studies have demonstrated the application of radiolabeled mAb-PAM4 for nuclear imaging and radioimmunotherapy of pancreatic carcinoma. We have now examined the ability of a novel PAM4-based, bispecific monoclonal antibody (mAb) construct, TF10, to pretarget a radiolabeled peptide for improved imaging and therapy. TF10 is a humanized, bispecific mAb, divalent for mAb-PAM4 and monovalent for mAb-679, reactive against the histamine-succinyl-glycine hapten. Biodistribution studies and nuclear imaging of the radiolabeled TF10 and/or TF10-pretargeted hapten-peptide (IMP-288) were conducted in nude mice bearing CaPan1 human pancreatic cancer xenografts. (125)I-TF10 cleared rapidly from the blood, with levels decreasing to <1% injected dose per gram (ID/g) by 16 hours. Tumor uptake was 3.47 +/- 0.66% ID/g at this time point with no accumulation in any normal tissue. To show the utility of the pretargeting approach, (111)In-IMP-288 was administered 16 hours after TF10. At 3 hours postadministration of radiolabeled peptide, imaging showed intense uptake within the tumors and no evidence of accretion in any normal tissue. No targeting was observed in animals given only the (111)In-peptide. Tumor uptake of the TF10-pretargeted (111)In-IMP-288 was 24.3 +/- 1.7% ID/g, whereas for (111)In-IMP-288 alone it was only 0.12 +/- 0.002% ID/g at 16 hours. Tumor/blood ratios were significantly greater for the pretargeting group ( approximately 1,000:1 at 3 hours) compared with (111)In-PAM4-IgG ( approximately 5:1 at 24 hours; P < 0.0003). Radiation dose estimates suggested that TF10/(90)Y-peptide pretargeting would provide a greater antitumor effect than (90)Y-PAM4-IgG. Thus, the results suggest that TF10 pretargeting may provide improved imaging for early detection, diagnosis, and treatment of pancreatic cancer as compared with directly radiolabeled PAM4-IgG.

Novel radiolabeled antibody conjugates

This article reviews the development of radioimmunoconjugates as a new class of cancer therapeutics. Numerous conjugates involving different antigen targets, antibody forms, radionuclides and methods of radiochemistry have been studied in the half-century since radioactive antibodies were first used in model systems to selectively target radiation to tumors. Whereas directly conjugated antibodies, fragments and subfragments have shown promise preclinically, the same approaches have not gained success in patients except in radiosensitive hematological neoplasms, or in settings involving minimal or locoregional disease. The separation of tumor targeting from the delivery of the therapeutic radionuclide in a multistep process called pretargeting has the potential to overcome many of the limitations of conventional, or one-step, radioimmunotherapy, with initial preclinical and clinical data showing increased sensitivity, specificity and higher radiation doses delivered. Our particular focus in pretargeting is the use of bispecific, trimeric (three Fab's) constructs made by a new antibody engineering method termed 'dock-and-lock.