Pretargeted adjuvant radioimmunotherapy with yttrium-90-biotin in malignant glioma patients: a pilot study

Tenascin-C targeting strategies in cancer

Tenascin-C (TNC) is a matricellular and multimodular glycoprotein highly expressed under pathological conditions, especially in cancer and chronic inflammatory diseases. Since a long time TNC is considered as a promising target for diagnostic and therapeutic approaches in anti-cancer treatments and was already extensively targeted in clinical trials on cancer patients. This review provides an overview of the current most advanced strategies used for TNC detection and anti-TNC theranostic approaches including some advanced clinical strategies. We also discuss novel treatment protocols, where targeting immune modulating functions of TNC could be center stage.

Optimizing the Safety and Efficacy of Bio-Radiopharmaceuticals for Cancer Therapy

The precise delivery of cytotoxic radiation to cancer cells through the combination of a specific targeting vector with a radionuclide for targeted radionuclide therapy (TRT) has proven valuable for cancer care. TRT is increasingly being considered a relevant treatment method in fighting micro-metastases in the case of relapsed and disseminated disease. While antibodies were the first vectors applied in TRT, increasing research data has cited antibody fragments and peptides with superior properties and thus a growing interest in application. As further studies are completed and the need for novel radiopharmaceuticals nurtures, rigorous considerations in the design, laboratory analysis, pre-clinical evaluation, and clinical translation must be considered to ensure improved safety and effectiveness. Here, we assess the status and recent development of biological-based radiopharmaceuticals, with a focus on peptides and antibody fragments. Challenges in radiopharmaceutical design range from target selection, vector design, choice of radionuclides and associated radiochemistry. Dosimetry estimation, and the assessment of mechanisms to increase tumor uptake while reducing off-target exposure are discussed.

Radionuclide Delivery Strategies in Tumor Treatment: A Systematic Review

The aim of this review was to assess recent progress in targeted radionuclide tumor therapy, focusing on the best delivery strategies. A literature search was conducted in PubMed, Web of Science, and Scopus using the terms "radionuclides", "liposomes", "avidin-biotin interaction", "theranostic", and "molecular docking". The 10 year filter was applied, except for the avidin-biotin interaction. Data were retrieved from both preclinical and clinical settings. Three targeting strategies were considered: pretargeting, liposomes, and ligands. Pretargeting can be achieved by exploiting the avidin-biotin interaction. This strategy seems very promising, although it has been investigated mainly in resectable tumors. Radiolabeled liposomes have attracted new interest as probes to identify the most suitable patients for treatment with liposomal formulations of common chemotherapeutics. The use of ligands for the delivery of radiotherapeutics to a specific target is still the most appealing strategy for treating tumors. The most appropriate ligand can be identified by virtually simulating its interaction with the receptor. All strategies showed great potential for use in targeted radionuclide therapy, but they also have numerous drawbacks. The most promising option is probably the one based on the use of new ligands.

Reducing the renal retention of low- to moderate-molecular-weight radiopharmaceuticals

The field of nuclear imaging and therapy is rapidly progressing with the development of targeted radiopharmaceuticals that show rapid targeting and rapid clearance with minimal background. Unfortunately, they are often reabsorbed in the kidneys, leading to possible nephrotoxicity, limiting the therapeutic dose, and/or reducing imaging quality. The blocking of endocytic receptors has been extensively used as a strategy to reduce kidney radiation. Alternatively, the physicochemical properties of radiotracers can be modulated to either prevent their reuptake or promote the excretion of radiometabolites. Other interesting strategies focus on the insertion of a cleavable linker between the radiolabel and the targeting moiety or pretargeting approaches in which the targeting moiety and radiolabel are administered separately. In the context of this review, we will discuss the latest advances and insights on strategies used to reduce renal retention of low- to moderate-molecular-weight radiopharmaceuticals.

Radioimmunotherapy of glioblastoma multiforme - Current status and future prospects

Glioblastoma multiforme (GBM) or grade IV astrocytoma is the most diagnosed form of primary brain tumours in adults. Radioimmunotherapy (RIT), mostly in combination with conventional therapies, is presented in the current review as a therapeutic strategy of high potential in the management of GBM. A systematic literature search was performed following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) to identify clinical studies that employed a form of radioimmunotherapy using alpha- or beta-emitting radioisotopes. The available literature on RIT in GBM and high-grade gliomas is presented and discussed. The results suggest that this promising treatment approach merits further investigation in future clinical studies.

Current Landscape in Clinical Pretargeted Radioimmunoimaging and Therapy

The principle of pretargeted radioimmunoimaging and therapy has been investigated over the past 30 y in preclinical and clinical settings with the aim of reducing the radiation burden of healthy tissue for antibody-based nuclear medicine techniques. In the past few decades, 4 pretargeting methodologies have been proposed, and 2 of them-the bispecific antibody-hapten and the streptavidin-biotin platforms-have been evaluated in humans in phase 1 and 2 studies. With this review article, we aim to survey clinical pretargeting studies in order to understand the challenges that these platforms have faced in human studies and to provide an overview of how the clinical approval of the pretargeting system has proceeded in the past several decades. Additionally, we will discuss the successes of the pretargeting human studies and compare and highlight the pretargeting approaches and conditions that will advance clinical translation of the pretargeting platform in the future.

Harnessing molecular recognition for localized drug delivery

A grand challenge in drug delivery is providing the right dose, at the right anatomic location, for the right duration of time to maximize therapeutic efficacy while minimizing off-target toxicity and other deleterious side-effects. Two general modalities are receiving broad attention for localized drug delivery. In the first, referred to as "targeted accumulation", drugs or drug carriers are engineered to have targeting moieties that promote their accumulation at a specific tissue site from circulation. In the second, referred to as "local anchoring", drugs or drug carriers are inserted directly into the tissue site of interest where they persist for a specified duration of time. This review surveys recent advances in harnessing molecular recognition between proteins, peptides, nucleic acids, lipids, and carbohydrates to mediate targeted accumulation and local anchoring of drugs and drug carriers.

Therapy of Myeloid Leukemia using Novel Bispecific Fusion Proteins Targeting CD45 and 90Y-DOTA

Pretargeted radioimmunotherapy (PRIT) has been investigated as a multi-step approach to decrease relapse and toxicity for high-risk acute myeloid leukemia (AML). Relevant factors including endogenous biotin and immunogenicity, however, have limited the use of PRIT with an anti-CD45 antibody streptavidin conjugate and radiolabeled DOTA-biotin. To overcome these limitations we designed anti-murine and anti-human CD45 bispecific antibody constructs using 30F11 and BC8 antibodies, respectively, combined with an anti-yttrium (Y)-DOTA single-chain variable fragment (C825) to capture a radiolabeled ligand. The bispecific construct targeting human CD45 (BC8-Fc-C825) had high uptake in leukemia HEL xenografts [7.8 ± 0.02% percent injected dose/gram of tissue (% ID/g)]. Therapy studies showed that 70% of mice with HEL human xenografts treated with BC8-Fc-C825 followed by 44.4 MBq (1,200 μCi) of ⁹⁰Y-DOTA-biotin survived at least 170 days after therapy, while all nontreated controls required euthanasia because of tumor progression by day 32. High uptake at sites of leukemia (spleen and bone marrow) was also seen with 30F11-IgG1-C825 in a syngeneic disseminated SJL murine leukemia model (spleen, 9.0 ± 1.5% ID/g and bone marrow, 8.1 ± 1.2% ID/g), with minimal uptake in all other normal organs (<0.5% ID/g) at 24 hours after ⁹⁰Y-DOTA injections. SJL leukemia mice treated with the bispecific 30F11-IgG1-C825 and 29.6 MBq (800 μCi) of ⁹⁰Y-DOTA-biotin had a survival advantage compared with untreated leukemic mice (median, 43 vs. 30 days, respectively; P < 0.0001). These data suggest bispecific antibody-mediated PRIT may be highly effective for leukemia therapy and translation to human studies.

Review: PET imaging with macro- and middle-sized molecular probes

Recent progress in radiolabeling of macro- and middle-sized molecular probes has been extending possibilities to use PET molecular imaging for dynamic application to drug development and therapeutic evaluation. Theranostics concept also accelerated the use of macro- and middle-sized molecular probes for sharpening the contrast of proper target recognition even the cellular types/subtypes and proper selection of the patients who should be treated by the same molecules recognition. Here, brief summary of the present status of immuno-PET, and then further development of advanced technologies related to immuno-PET, peptidic PET probes, and nucleic acids PET probes are described.

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.

Enhancement of HIFU ablation by sonosensitizer-loading liquid fluorocarbon nanoparticles with pre-targeting in a mouse model

High intensity focused ultrasound (HIFU) is a noninvasive thermal ablation technique for the treatment of benign and malignant solid masses. To improve the efficacy of HIFU ablation, we developed poly (lactide-co-glycolide) (PLGA) nanoparticles encapsulating perfluoropentane (PFP) and hematoporphyrin monomethyl ether (HMME) as synergistic agents (HMME+PFP/PLGA). Two-step biotin-avidin pre-targeting technique was applied for the HIFU ablation. We further modified the nanoparticles with streptavidin (HMME+PFP/PLGA-SA). HMME+PFP/PLGA-SA were highly dispersed with spherical morphology (477.8 ± 81.8 nm in diameter). The encapsulation efficiency of HMME and PFP were 46.6 ± 3.3% and 40.1 ± 2.6%, respectively. The binding efficiency of nanoparticles to streptavidin was 95.5 ± 2.5%. The targeting ability of the HMME+PFP/PLGA-SA nanoparticles was tested by parallel plate flow chamber in vitro. In the pre-targeting group (HMME+PFP/PLGA-SA), a large number of nanoparticles bound to the peripheral and surface of the cell. In the HIFU ablation experiment in vivo, compared with the other groups, the largest gray-scale changes and coagulation necrosis areas were observed in the pre-targeting (HMME+PFP/PLGA-SA) group, with the lowest energy efficiency factor value. Moreover, the microvessel density and proliferation index declined, while the apoptotic index increased, in the tumor tissue surrounding the coagulation necrosis area in the pre-targeting group. Meanwhile, the survival time of the tumor-bearing nude mice in the pre-targeting group was significantly longer than that in the HIFU treatment group. These results suggest that HMME+PFP/PLGA-SA have high potential to act as synergistic agents in HIFU ablation.

Chemically triggered drug release from an antibody-drug conjugate leads to potent antitumour activity in mice

Current antibody-drug conjugates (ADCs) target internalising receptors on cancer cells leading to intracellular drug release. Typically, only a subset of patients with solid tumours has sufficient expression of such a receptor, while there are suitable non-internalising receptors and stroma targets. Here, we demonstrate potent therapy in murine tumour models using a non-internalising ADC that releases its drugs upon a click reaction with a chemical activator, which is administered in a second step. This was enabled by the development of a diabody-based ADC with a high tumour uptake and very low retention in healthy tissues, allowing systemic administration of the activator 2 days later, leading to efficient and selective activation throughout the tumour. In contrast, the analogous ADC comprising the protease-cleavable linker used in the FDA approved ADC Adcetris is not effective in these tumour models. This first-in-class ADC holds promise for a broader applicability of ADCs across patient populations.

Current antibody-drug conjugates (ADCs) target internalising receptors on cancer cells. Here, the authors report the development and in vivo validation of a non-internalising ADC with the capacity to target cancer cells and release its therapeutic cargo extracellularly via a chemical trigger.

Small Molecule and Monoclonal Antibody Therapies in Neurooncology

Background:

The prognosis for most patients with primary brain tumors remains poor. Recent advances in molecular and cell biology have led to a greater understanding of molecular alterations in brain tumors. These advances are being translated into new therapies that will hopefully improve the prognosis for patients with brain tumors.

Methods:

We reviewed the literature on small molecule targeted agents and monoclonal antibodies used in brain tumor research and brain tumor clinical trials for the past 20 years.

Results:

Brain tumors commonly express molecular abnormalities. These alterations can lead to the activation of cell pathways involved in cell proliferation. This knowledge has led to interest in novel anti-brain-tumor therapies targeting key components of these pathways. Many drugs and monoclonal antibodies have been developed that modulate these pathways and are in various stages of testing.

Conclusions:

The use of targeted therapies against brain tumors promises to improve the prognosis for patients with brain tumors. However, as the molecular pathogenesis of brain tumors has not been linked to a single genetic defect or target, molecular agents may need to be used in combinations or in tandem with cytotoxic agents. Further study of these agents in well-designed cooperative clinical trials is needed.

Pretargeted Imaging and Therapy

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

Triggered Drug Release from an Antibody-Drug Conjugate Using Fast "Click-to-Release" Chemistry in Mice

The use of a bioorthogonal reaction for the selective cleavage of tumor-bound antibody-drug conjugates (ADCs) would represent a powerful new tool for ADC therapy, as it would not rely on the currently used intracellular biological activation mechanisms, thereby expanding the scope to noninternalizing cancer targets. Here we report that the recently developed inverse-electron-demand Diels-Alder pyridazine elimination reaction can provoke rapid and self-immolative release of doxorubicin from an ADC in vitro and in tumor-bearing mice.

Cancer Therapy with Radiolabeled and Drug/Toxin-conjugated Antibodies

Radioimmunotherapy and antibody-directed chemotherapy have emerged as cancer treatment modalities with the regulatory approval of products for non-Hodgkin's lymphoma and acute myeloid leukemia. Antibody-toxin therapy is likewise on the verge of clinical fruition. Accumulating evidence suggests that radioimmunotherapy may have the best impact in minimal-disease and adjuvant settings, especially with radioresistant solid tumors. For the latter, ongoing efforts in ‘pretargeting’ to increase deliverable tumor radiation dose, combination therapies, and locoregional applications are also of importance. Antibody-drug conjugates have the potential to increase the therapeutic index of chemotherapy by minimizing systemic toxicity and improving tumor targeting. The design of optimal drug conjugates in this regard is predicated upon the proper choice of the target antigen, the cleavable-linker, and the drug. In respect of antibody-toxin conjugates, considerable progress has been made in chemical and recombinant immunotoxin designs, and in the advancement of many products to clinical trials. Continued development of antibody-directed therapies should expand the options available for the management of cancer.

Targeted Cancer Therapy Using Radiolabeled Monoclonal Antibodies

Radioimmunotherapy (RIT) combines the advantages of targeted radiation therapy and specific immunotherapy using monoclonal antibodies. RIT can be used either to target tumor cells or to specifically suppress immunocompetent host cells in the setting of allogeneic transplantation. The choice of radionuclide used for RIT depends on its distinct radiation characteristics and the type of malignancy or cells targeted. Beta-emitters with their lower energy and longer path length are more suitable to target bulky, solid tumors whereas α-emitters with their high linear energy transfer and short path length are better suited to target hematopoietic cells (normal or malignant). Different approaches of RIT such as the use of stable radioimmunoconjugates or of pretargeting strategies are available. Encouraging results have been obtained with RIT in patients with hematologic malignancies. The results in solid tumors are somewhat less favorable but new strategies for patients with minimal residual disease using adjuvant and locoregional treatment are evolving. This report outlines basic principles of RIT, gives an overview of available radionuclides and radioimmunoconjugates, and discusses clinical results with special emphasis on their use in hematologic malignancies including use in conditioning regimens for bone marrow transplantation.

Bioorthogonal two-component drug delivery in HER2(+) breast cancer mouse models

The HER2 receptor is overexpressed in approximately 20% of breast cancers and is associated with tumorigenesis, metastasis, and a poor prognosis. Trastuzumab is a first-line targeted drug used against HER2(+) breast cancers; however, at least 50% of HER2(+) tumors develop resistance to trastuzumab. To treat these patients, trastuzumab-based antibody-drug conjugates (ACDs) have been developed and are currently used in the clinic. Despite their high efficacy, the long circulation half-life and non-specific binding of cytotoxic ADCs can result in systemic toxicity. In addition, standard ADCs do not provide an image-guided mode of administration. Here, we have developed a two-component, two-step, pre-targeting drug delivery system integrated with image guidance to circumvent these issues. In this strategy, HER2 receptors are pre-labeled with a functionalized trastuzumab antibody followed by the delivery of drug-loaded nanocarriers. Both components are cross-linked by multiple bioorthogonal click reactions in situ on the surface of the target cell and internalized as nanoclusters. We have explored the efficacy of this delivery strategy in HER2(+) human breast cancer models. Our therapeutic study confirms the high therapeutic efficacy of the new delivery system, with no significant toxicity.

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.

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).

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.

Radioimmunotherapy of human tumours

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

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.

Radiolabeling strategies for tumor-targeting proteinaceous drugs

Owing to their large size proteinaceous drugs offer higher operative information content compared to the small molecules that correspond to the traditional understanding of druglikeness. As a consequence these drugs allow developing patient-specific therapies that provide the means to go beyond the possibilities of current drug therapy. However, the efficacy of these strategies, in particular "personalized medicine", depends on precise information about individual target expression rates. Molecular imaging combines non-invasive imaging methods with tools of molecular and cellular biology and thus bridges current knowledge to the clinical use. Moreover, nuclear medicine techniques provide therapeutic applications with tracers that behave like the diagnostic tracer. The advantages of radioiodination, still the most versatile radiolabeling strategy, and other labeled compounds comprising covalently attached radioisotopes are compared to the use of chelator-protein conjugates that are complexed with metallic radioisotopes. With the techniques using radioactive isotopes as a reporting unit or even the therapeutic principle, care has to be taken to avoid cleavage of the radionuclide from the protein it is linked to. The tracers used in molecular imaging require labeling techniques that provide site specific conjugation and metabolic stability. Appropriate choice of the radionuclide allows tailoring the properties of the labeled protein to the application required. Until the event of positron emission tomography the spectrum of nuclides used to visualize cellular and biochemical processes was largely restricted to iodine isotopes and 99m-technetium. Today, several nuclides such as 18-fluorine, 68-gallium and 86-yttrium have fundamentally extended the possibilities of tracer design and in turn caused the need for the development of chemical methods for their conjugation.

General overview of radioimmunotherapy of solid tumors

Radioimmunotherapy (RIT) represents an attractive tool for the treatment of local and/or diffuse tumors with radiation. In RIT, cytotoxic radionuclides are delivered by monoclonal antibodies that specifically target tumor-associated antigens or the tumor microenvironment. While RIT has been successfully employed for the treatment of lymphoma, mostly with radiolabeled antibodies against CD20 (Bexxar(®); Corixa Corp., WA, USA and Zevalin(®); Biogen Idec Inc., CA, USA and Schering AG, Berlin, Germany), its use in solid tumors is more challenging and, so far, few trials have progressed beyond Phase II. This review provides an update on antibody-radionuclide conjugates and their use in RIT. It also discusses possible optimization strategies to improve the clinical response by considering biological, radiobiological and physical features.

Improvement of radioimmunotherapy using pretargeting

During the past two decades, considerable research has been devoted to radionuclide therapy using radiolabeled monoclonal antibodies and receptor binding agents. Conventional radioimmunotherapy (RIT) is now an established and important tool in the treatment of hematologic malignancies such as Non-Hodgkin lymphoma. For solid malignancies, the efficacy of RIT has not been as successful due to lower radiosensitivity, difficult penetration of the antibody into the tumor, and potential excessive radiation to normal tissues. Innovative approaches have been developed in order to enhance tumor absorbed dose while limiting toxicity to overcome the different limitations due to the tumor and host characteristics. Pretargeting techniques (pRIT) are a promising approach that consists of decoupling the delivery of a tumor monoclonal antibody (mAb) from the delivery of the radionuclide. This results in a much higher tumor-to-normal tissue ratio and is favorable for therapy as well and imaging. This includes various strategies based on avidin/streptavidin-biotin, DNA-complementary DNA, and bispecific antibody-hapten bindings. pRIT continuously evolves with the investigation of new molecular constructs and the development of radiochemistry. Pharmacokinetics improve dosimetry depending on the radionuclides used (alpha, beta, and Auger emitters) with prediction of tumor response and host toxicities. New constructs such as the Dock and Lock technology allow production of a variety of mABs directed against tumor-associated antigens. Survival benefit has already been shown in medullary thyroid carcinoma. Improvement in delivery of radioactivity to tumors with these pretargeting procedures associated with reduced hematologic toxicity will become the next generation of RIT. The following review addresses actual technical and clinical considerations and future development of pRIT.

Targeted radioimmunotherapy: the role of ¹³¹I-chTNT-1/B mAb (Cotara) for treatment of high-grade gliomas

The prognosis for patients with malignant gliomas remains poor, and novel treatment paradigms are needed. Radioimmunotherapeutic drugs have been studied in clinical trials as adjuncts to treatment for these tumors. One such agent is (131)I-chTNT-1/B mAb (Cotara(®)), a compound locally delivered to the tumor site through convection-enhanced delivery. It is a genetically engineered chimeric monoclonal antibody that binds to the DNA-histone H1 complex, and carries (131)I, which locally delivers its radioactive payload to kill adjacent tumor cells. Clinical experience with Cotara is emerging; completed Phase I and II trials with a total of 51 patients helped to define dosing regimens for the drug. A recent Phase II dose-confirmation trial with Cotara for patients with glioblastoma multiforme at first relapse has demonstrated promising overall survival results of 41 weeks. This review explores the clinical experience of radioimmunotherapy and describes the role of Cotara for treatment of patients with malignant gliomas.

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.

Poly(ADP-ribose) polymerase inhibitors combined with external beam and radioimmunotherapy to treat aggressive lymphoma

PURPOSE

To assess the possible radiosensitizing capabilities of two different poly(ADP-ribose) polymerase (PARP) inhibitors in combination with external beam and I-tositumomab in a non-Hodgkin's lymphoma cell line.

METHODS AND MATERIALS

Epstein-Barr virus-infected human Raji lymphoma cells with lentivirally transfected green fluorescent protein and luciferase in log-phase growth were incubated with various doses of AZD-2281 and ABT-888 24 h before external beam radiation exposure. A 500 nmol/l concentration of AZD-2281 and ABT-888 was used to assess the growth curve of Raji lymphoma cells over 5 days. The number of double-stranded breaks was visually assessed using a H2AX antibody and confocal microscopy. Intracellular PARP activity was measured 2 h after incubation with AZD-2281 (500 nmol/l) and ABT-888 using a colorimetric PARP assay kit. The radiosensitizing effect of AZD-2281 (500 nmol/l) with various doses of I-tositumomab was assessed after 24 h.

RESULTS

A volume of 500 nmol/l of AZD-2281 and 500 nmol/l of ABT-888, in combination with 0, 4, 8, and 12 Gy external beam radiation, showed a 5.2, 7.1, 10.1, and 33.1% radiosensitization. A measure of 500 nmol/l AZD-2281 and ABT-888 significantly reduced the percentage of viable cells on days 3-5 compared with controls. The maximal relative reduction in viable cells was 78.5%, and this occurred with AZD-2281 (500 nmol/l) on day 5. AZD-2281 revealed a higher number of double-stranded breaks with confocal microscopy than did ABT-888. Two hours after incubation of Raji cells with 500 nmol/l of AZD-2281 or ABT-888, the colorimetric PARP activity assay showed a reduction of 30.36% with ABT-888 and of 47.8% with AZD-2281. Combining AZD-2281 (500 nmol/l) with 0, 5 μCi (0.185 MBq), 10 μCi (0.37 MBq) and 20 μCi (0.74 MBq) ¹³¹I-tositumomab revealed a significant reduction in cell viability after 24 h with 5 μCi (0.185 MBq) (P<0.01) and 10 μCi (0.37 MBq) (P<0.01) radiation dose.

CONCLUSION

PARP inhibitors AZD-2281 and ABT-888 are highly radiosensitizing agents when used before external beam radiation and ¹³¹I-tositumomab.

Clinical radioimmunotherapy--the role of radiobiology

Conventional external-beam radiation therapy is dedicated to the treatment of localized disease, whereas radioimmunotherapy represents an innovative tool for the treatment of local or diffuse tumors. Radioimmunotherapy involves the administration of radiolabeled monoclonal antibodies that are directed specifically against tumor-associated antigens or against the tumor microenvironment. Although many tumor-associated antigens have been identified as possible targets for radioimmunotherapy of patients with hematological or solid tumors, clinical success has so far been achieved mostly with radiolabeled antibodies against CD20 ((131)I-tositumomab and (90)Y-ibritumomab tiuxetan) for the treatment of lymphoma. In this Review, we provide an update on the current challenges aimed to improve the efficacy of radioimmunotherapy and discuss the main radiobiological issues associated with clinical radioimmunotherapy.

90Y labeled phosphorodiamidate morpholino oligomer for pretargeting radiotherapy

While (188)Re has been used successfully in mice for tumor radiotherapy by MORF/cMORF pretargeting, previous radiolabeling of the amine-derivatized cMORF with (90)Y, a longer physical half-life nuclide, was not very successful. After developing a method involving a prepurification heating step during conjugation that increases labeling efficiency and label stability, the biodistribution of (90)Y-DOTA-Bn-SCN-cMORF ((90)Y-DOTA-cMORF) was measured in normal mice and in MORF-CC49 pretargeted mice that bear LS174T tumors. Absorbed radiation doses were then estimated and compared to those estimated for (188)Re. The pharmacokinetics of the (90)Y-DOTA-cMORF in normal mice and in the pretargeted nude mice was similar to that observed previously with (99m)Tc- and (188)Re-MAG(3)-cMORFs. While the (90)Y-DOTA-cMORF cleared rapidly from normal tissues, tumor clearance was very slow and tumor radioactivity accumulation was constant for at least 7 days such that the tumor/blood (T/B) ratio increased linearly from 6 to 25 over this period. Therefore, by extrapolation, normal tissue toxicities following administration of therapeutic doses of (90)Y may be comparable to that observed for (188)Re in which the T/B increased from 5 to 20. In conclusion, radiolabeling of DOTA-cMORF with (90)Y was improved by introducing a prepurification heating step during conjugation. The (90)Y-DOTA-cMORF provided a similar T/B ratio and biodistribution to that of (188)Re-MAG(3)-cMORF and was retained well in the tumor pretargeted with MORF-CC49. Because of the longer physical half-life, the T/NT absorbed radiation dose ratios were improved in most organs and especially in blood.

Preclinical pharmacology and safety of a novel avidin derivative for tissue-targeted delivery of radiolabelled biotin

We recently described an oxidized avidin variant, named AvidinOX(®) , which is a product that chemically links to tissue proteins while maintaining the capacity to uptake intravenously administered biotin. Such product proved to be successful in targeting radionuclide therapy in a mouse model of inoperable breast cancer. Here, we show that the uptake of a single or multiple doses of biotin (up to five times), by the tissue-bound AvidinOX(®) , is stable for 2 weeks. Taking into account that oxidized avidin is the first chemically reactive protein to be proposed for clinical use, we evaluated its tolerability, immunogenicity and mutagenicity. Present in vitro data indicate that AvidinOX(®) (up to 10 μg/5 × 10(5) cells) does not affect cell viability or proliferation of PC3 human prostate cancer or 3T3 mouse fibroblast cell lines as well as primary mouse spleen cells. Safety pharmacology and toxicology studies were conducted using AvidinOX(®) up to the highest concentration compatible with its solubility (about 12 mg/mL), representing four times the product concentration intended for human use, and in the maximum administrable volume compatible with each study system. The intramuscular administration in rat and monkey induced a moderate to strong inflammatory response particularly after a second administration and consistently with the induction of an immune response. Interestingly, the intramuscular administration of AvidinOX(®) to rodents and monkeys exhibiting very high anti-avidin antibody titres was well tolerated with no systemic symptoms of any kind. Intravenous administration of AvidinOX(®) , performed to mimic an accidental injection of the dose intended for a local administration (15 μL of 3.3 mg/mL solution), showed significant localization of the product into the spleen not associated with uptake of the radiolabelled biotin intravenously injected after 24 hr, thus suggesting rapid inactivation. No mutagenic activity was induced by oxidized avidin in prokaryotic and eukaryotic cells. Overall, the present data indicate that AvidinOX(®) is well tolerated in rodents and non-human primates, thus supporting its clinical use within protocols of radionuclide therapy of inoperable tumour lesions.

Radiation-induced tumor neoantigens: imaging and therapeutic implications

Exposure of tumor cells to ionizing radiation (IR) is widely known to induce a number of cellular changes. One way that IR can affect tumor cells is through the development of neoantigens which are new molecules that tumor cells express at the cell membrane following some insult or change to the cell. There have been numerous reports in the literature of changes in both tumor and tumor vasculature cell surface molecule expression following treatment with IR. The usefulness of neoantigens for imaging and therapeutic applications lies in the fact that they are differentially expressed on the surface of irradiated tumor cells to a greater extent than on normal tissues. This differential expression provides a mechanism by which tumor cells can be "marked" by radiation for further targeting. Drug delivery vehicles or imaging agents conjugated to ligands that recognize and interact with the neoantigens can help to improve tumor-specific targeting and reduce systemic toxicity with cancer drugs. This article provides a review of the molecules that have been reported to be expressed on the surface of tumor cells in response to IR either in vivo or in vitro. Additionally, we provide a discussion of some of the methods used in the identification of these antigens and applications for their use in drug delivery and imaging.

Experimental approaches for the treatment of malignant gliomas

Malignant gliomas, which include glioblastomas and anaplastic astrocytomas, are the most common primary tumors of the brain. Over the past 30 years, the standard treatment for these tumors has evolved to include maximal safe surgical resection, radiation therapy and temozolomide chemotherapy. While the median survival of patients with glioblastomas has improved from 6 months to 14.6 months, these tumors continue to be lethal for the vast majority of patients. There has, however, been recent substantial progress in our mechanistic understanding of tumor development and growth. The translation of these genetic, epigenetic and biochemical findings into therapies that have been tested in clinical trials is the subject of this review.

Avidin-biotin technology in targeted therapy

IMPORTANCE OF THE FIELD

The goal of drug targeting is to increase the concentration of the drug in the vicinity of the cells responsible for disease without affecting healthy cells. Many approaches in cancer treatment are limited because of their broad range of unwanted side effects on healthy cells. Targeting can reduce side effects and increase efficacy of drugs in the patient.

AREAS COVERED IN THIS REVIEW

Avidin, originally isolated from chicken eggs, and its bacterial analogue, streptavidin, from Streptomyces avidinii, have extremely high affinity for biotin. This unique feature is the basis of avidin-biotin technology. This article reviews the current status of avidin-biotin systems and their use for pretargeted drug delivery and vector targeting.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the following approaches using the avidin-biotin system: i) targeting antibodies and therapeutic molecules are administered separately leading to a reduction of drug dose in normal tissues compared with conventional (radio)immunotherapies; ii) introducing avidin gene into specific tissues by local gene transfer, which subsequently can sequester and concentrate considerable amounts of therapeutic ligands; and iii) enabling transductional targeting of gene therapy vectors.

TAKE HOME MESSAGE

Avidin and biotin technology has proved to be an extremely versatile tool with broad applications, such as pretargeting, delivering avidin gene into cells enabling targeting of biotinylated compounds and targeting of viral vectors.

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.

Preliminary studies on the selective accumulation of vitamin-targeted polymers within tumors

Many different cancer types have previously been found to show increased uptake of the vitamins folate, vitamin B12, and biotin; however, it is not known whether these tumor lines show increased uptake of one or more of the vitamins. The current study was designed to examine the relative uptake of the three vitamins in 10 different types of cell lines. Rhodamine-labeled hydroxypropyl-methacrylamide (HPMA) was targeted with vitamin B(12), folate, or biotin, and the uptake of the labeled polymer was compared both in in vitro cell cultures and in mice-bearing tumors from a variety of tumor cell lines. Fluorescent microscopy of cell cultures and histological examination of tumor sections showed greatly increased uptake of the fluorescently labeled polymer in many tumors when the polymer was targeted with folate, biotin, or vitamin B(12). Tumors with enhanced uptake of vitamin B(12)- or folate-targeted rhodamine-HPMA also showed increased uptake of biotin-Rho-HPMA. In contrast, tumors with increased uptake of folate-Rho-HPMA did not show increased uptake of vitamin B12 (VB(12))-HPMA and vice versa. These findings suggest that vitamin-targeted polymers may greatly increase the uptake of drug-polymer complexes in certain tumors, which may result in an increased efficacy of antitumor agents, and which may allow for easier imaging of both the primary and metastatic tumors.

Avidin fusion protein-expressing lentiviral vector for targeted drug delivery

One of the main objectives of cancer therapy is to enhance the effectiveness of the drug by concentrating it at the target site and to minimize the undesired side effects to nontarget cells. We have previously constructed a fusion protein, Lodavin, consisting of avidin and the endocytotic part of the low-density lipoprotein receptor, and demonstrated its applicability to transient drug targeting in vivo. In this study we produced a lentiviral vector expressing this fusion protein and evaluated its safety and efficacy. The results showed that lentivirus-mediated gene transfer led to long-term avidin fusion protein expression on glioma cells and that the receptor was able to bind biotinylated compounds. Repeated administration was proven feasible and the optimal time frame(s) for administration of biotinylated therapeutic and/or imaging compounds was elucidated. Intravenous or intracranial injection of the virus into BDIX rats led to the production of antibodies against transgene (avidin), but repeated administration of the vector was unable to boost this effect. Neutralizing antibodies against the lentivirus were also detected. Furthermore, we showed that the anti-avidin antibodies did not significantly affect the ligand-binding capacity of the avidin fusion protein. The therapeutic efficacy of avidin fusion protein in tumor treatment was tested in vitro with biotinylated and nonbiotinylated nanoparticles loaded with paclitaxel. In vivo applicability of lentivirus was studied in the BDIX rat glioma model, in which high receptor expression was detected in the tumor area. The lentivirus-mediated delivery of the avidin fusion protein thus represents a potential approach for the repeated targeting of cytotoxic compounds to cancer cells.

Advances in Radioimmunotherapy in the Age of Molecular Engineering and Pretargeting

Now that radioimmunotherapy is an approved method for the treatment of certain types of non-Hodgkin's lymphoma, investigators are turning to new approaches to further improve radionuclide targeting in hopes of expanding the use of this technology. A number of innovative recombinant proteins have been developed with more favorable pharmacokinetic and targeting properties than standard whole IgG, which conceivably could improve the therapeutic index for cancer treatment. Pretargeting methods also are coming of age, with preclinical and early clinical studies in a variety of cancers illustrating how this alternative approach can enhance the therapeutic window several-fold of what has been possible with directly radiolabeled IgG. This review will discuss some of these promising new developments.

Expression of HMP/AN2, a melanoma associated antigen, in murine cerebral gliomas: potential for radioimmunotargeting

Human melanoma proteoglycan (HMP), a melanoma-associated antigen, is expressed in both human melanomas and gliomas. We used HMP-specific monoclonal antibody (mAb) VT68.2 to investigate whether murine GL261 cerebral gliomas express the HMP homologue AN2 and to determine whether AN2 could be targeted for selective delivery of radiation in vivo. HMP-specific mAb VT68.2 stained murine GL261 glioma cells grown in culture and intracerebrally in syngeneic C57BL/6 mice. Positron emission tomography with radiolabeled mAb VT68.2 showed high-contrast, positive images of gliomas against a negative background. At 96 h after injection, glioma uptake of radiolabeled mAb VT68.2 was 10x greater than that of the isotype control mAb and 20x greater than that detected in normal cerebral tissue. Our results show murine GL261 cerebral gliomas express AN2 and HMP-specific mAb VT68.2 accumulates selectively and specifically at high concentration and is retained within murine cerebral gliomas. Thus, HMP is a potential target for antibody-mediated selective delivery of radiation to cerebral gliomas in vivo.

Immunotherapeutic approaches for glioma

The development of effective immunotherapy strategies for glioma requires adequate understanding of the unique immunological microenvironment in the central nervous system (CNS) and CNS tumors. Although the CNS is often considered to be an immunologically privileged site and poses unique challenges for the delivery of effector cells and molecules, recent advances in technology and discoveries in CNS immunology suggest novel mechanisms that may significantly improve the efficacy of immunotherapy against gliomas. In this review, we first summarize recent advances in the CNS and CNS tumor immunology. We address factors that may promote immune escape of gliomas. We also review advances in passive and active immunotherapy strategies for glioma, with an emphasis on lessons learned from recent early-phase clinical trials. We also discuss novel immunotherapy strategies that have been recently tested in non-CNS tumors and show great potential for application to gliomas. Finally, we discuss how each of these promising strategies can be combined to achieve clinical benefit for patients with gliomas.