Phage libraries for generation of clinically useful antibodies

A cell-based phenotypic library selection and screening approach for the de novo discovery of novel functional chimeric antigen receptors

Anti-tumor therapies that seek to exploit and redirect the cytotoxic killing and effector potential of autologous or syngeneic T cells have shown extraordinary promise and efficacy in certain clinical settings. Such cells, when engineered to express synthetic chimeric antigen receptors (CARs) acquire novel targeting and activation properties which are governed and orchestrated by, typically, antibody fragments specific for a tumor antigen of interest. However, it is becoming increasingly apparent that not all antibodies are equal in this regard, with a growing appreciation that ‘optimal’ CAR performance requires a consideration of multiple structural and contextual parameters. Thus, antibodies raised by classical approaches and intended for other applications often perform poorly or not at all when repurposed as CARs. With this in mind, we have explored the potential of an in vitro phenotypic CAR library discovery approach that tightly associates antibody-driven bridging of tumor and effector T cells with an informative and functionally relevant CAR activation reporter signal. Critically, we demonstrate the utility of this enrichment methodology for ‘real world’ de novo discovery by isolating several novel anti-mesothelin CAR-active scFv candidates.

Nanoparticles and trained immunity: Glimpse into the future

Emerging evidences show that innate immune cells can display changes in their functional programs after infection or vaccination, which lead to immunomodulation (increased or reduced responsiveness) upon secondary activation to the same stimuli or even to a different one. Innate cells acquire features of immunological memory, nowadays using the new term of "trained immunity" or "innate immune memory", which is different from the specific memory immune response elicited by B and T lymphocytes. The review focused on the concept of trained immunity, mostly on myeloid cells. Special attention is dedicated to the pathogen recognition along the evolution (bacteria, plants, invertebrate and vertebrate animals), and to techniques used to study epigenetic reprogramming and metabolic rewiring. Nanomaterials can be recognized by immune cells offering a very promising way to learn about trained immunity. Nanomaterials could be modified in order to immunomodulate the responses ad hoc. Many therapeutic possibilities are opened, and they should be explored.

Deletion of Cbl-b inhibits CD8+ T-cell exhaustion and promotes CAR T-cell function

Background

Chimeric antigen receptor (CAR) T-cell therapy is an emerging option for cancer treatment, but its efficacy is limited, especially in solid tumors. This is partly because the CAR T cells become dysfunctional and exhausted in the tumor microenvironment. However, the key pathways responsible for impaired function of exhausted cells remain unclear, which is essential to overcome CAR T-cell exhaustion.

Methods

Analysis of RNA-sequencing data from CD8⁺ tumor-infiltrating lymphocytes (TILs) led to identification of Cbl-b as a potential target. The sequencing data were validated using a syngeneic MC38 colon cancer model. To analyze the in vivo role of Cbl-b in T-cell exhaustion, tumor growth, % PD1⁺Tim3⁺ cells, and expression of effector cytokines were analyzed in cbl-b^+/+ and cbl-b^–/– mice. To evaluate the therapeutic potential of Cbl-b depletion, we generated a new CAR construct, hCEAscFv-CD28-CD3ζ.GFP, that recognizes human carcinoembryonic antigen (CEA). cbl-b^+/+ and cbl-b^–/– CEA-CAR T cells were generated by retroviral transduction. Rag^–/– mice bearing MC38-CEA cells were injected with cbl-b^+/+ and cbl-b^–/–; CEA-CAR T cells, tumor growth, % PD1⁺Tim3⁺ cells and expression of effector cytokines were analyzed.

Results

Our results show that the E3 ubiquitin ligase Cbl-b is upregulated in exhausted (PD1⁺Tim3⁺) CD8⁺ TILs. CRISPR-Cas9-mediated inhibition of Cbl-b restores the effector function of exhausted CD8⁺ TILs. Importantly, the reduced growth of syngeneic MC38 tumors in cbl-b^–/– mice was associated with a marked reduction of PD1⁺Tim3⁺ CD8⁺ TILs. Depletion of Cbl-b inhibited CAR T-cell exhaustion, resulting in reduced MC38-CEA tumor growth, reduced PD1⁺Tim3⁺ cells and increased expression of interferon gamma, tumor necrosis factor alpha, and increased tumor cell killing.

Conclusion

Our studies demonstrate that deficiency of Cbl-b overcomes endogenous CD8⁺ T-cell exhaustion, and deletion of Cbl-b in CAR T cells renders them resistant to exhaustion. Our results could facilitate the development of efficient CAR T-cell therapy for solid tumors by targeting Cbl-b.

Human immunology and immunotherapy: main achievements and challenges

The immune system is a fascinating world of cells, soluble factors, interacting cells, and tissues, all of which are interconnected. The highly complex nature of the immune system makes it difficult to view it as a whole, but researchers are now trying to put all the pieces of the puzzle together to obtain a more complete picture. The development of new specialized equipment and immunological techniques, genetic approaches, animal models, and a long list of monoclonal antibodies, among many other factors, are improving our knowledge of this sophisticated system. The different types of cell subsets, soluble factors, membrane molecules, and cell functionalities are some aspects that we are starting to understand, together with their roles in health, aging, and illness. This knowledge is filling many of the gaps, and in some cases, it has led to changes in our previous assumptions; e.g., adaptive immune cells were previously thought to be unique memory cells until trained innate immunity was observed, and several innate immune cells with features similar to those of cytokine-secreting T cells have been discovered. Moreover, we have improved our knowledge not only regarding immune-mediated illnesses and how the immune system works and interacts with other systems and components (such as the microbiome) but also in terms of ways to manipulate this system through immunotherapy. The development of different types of immunotherapies, including vaccines (prophylactic and therapeutic), and the use of pathogens, monoclonal antibodies, recombinant proteins, cytokines, and cellular immunotherapies, are changing the way in which we approach many diseases, especially cancer.

Phage Display Technology and its Applications in Cancer Immunotherapy

Background:

Phage display is an effective technology for generation and selection targeting protein for a variety of purpose, which is based on a direct linkage between the displayed protein and the DNA sequence encoding it and utilized in selecting peptides, improving peptides affinity and indicating protein-protein interactions. Phage particles displaying peptide have the potential to apply in the identification of cell-specific targeting molecules, identification of cancer cell surface biomarkers, identification anti-cancer peptide, and the design of peptide-based anticancer therapy.

Method/Results:

Literature searches, reviews and assessments about Phage were performed in this review from PubMed and Medline databases.

Conclusion:

The phage display technology is an inexpensive method for expressing exogenous peptides, generating unique peptides that bind any given target and investigating protein-protein interactions. Due to the powerful ability to insert exogenous gene and display exogenous peptides on the surface, phages may represent a powerful peptide delivery system that can be utilized to develop rapid, efficient, safe and inexpensive cancer therapy methods.

IL15-Based Trifunctional Antibody-Fusion Proteins with Costimulatory TNF-Superfamily Ligands in the Single-Chain Format for Cancer Immunotherapy

IL15 and costimulatory receptors of the tumor necrosis superfamily (TNFRSF) have shown great potential to support and drive an antitumor immune response. However, their efficacy as monotherapy is limited. Here, we present the development of a novel format for a trifunctional antibody-fusion protein that combines and focuses the activity of IL15/TNFSF-ligand in a targeting-mediated manner to the tumor site. The previously reported format consisted of a tumor-directed antibody (scFv), IL15 linked to an IL15Rα-fragment (RD), and the extracellular domain of 4-1BBL, where noncovalent trimerization of 4-1BBL into its functional unit led to a homotrimeric molecule with 3 antibody and 3 IL15-RD units. To reduce the size and complexity of the molecule, we have now designed a second format, where 4-1BBL is introduced as single-chain (sc), that is 3 consecutively linked 4-1BBL ectodomains. Thus, a monomeric trifunctional fusion protein presenting only 1 functional unit of each component was generated. Interestingly, the in vitro activity on T-cell stimulation was conserved or even enhanced for the soluble and target-bound molecule, respectively. Also, in a lung tumor mouse model, comparable antitumor effects were observed. Furthermore, corroborating the concept, OX40L and GITRL were also successfully incorporated into the novel single-chain format and the advantage of target-bound trifunctional versus corresponding combined bifunctional fusion proteins demonstrated by measuring T-cell proliferation and cytotoxic potential in vitro and antitumor effects of RD_IL15_scFv_scGITRL in a lung tumor mouse model in vivo Thus, the trifunctional antibody-fusion protein single-chain format constitutes a promising innovative platform for further therapeutic developments.

Antigen-Specific Human Monoclonal Antibodies from Transgenic Mice

Due to the difficulties found when generating fully human monoclonal antibodies (mAbs) by the traditional method, several efforts have attempted to overcome these problems, with varying levels of success. One approach has been the development of transgenic mice carrying immunoglobulin (Ig) genes in germline configuration. The engineered mouse genome can undergo productive rearrangement in the B-cell population, with the generation of mouse B lymphocytes expressing human Ig (hIg) chains. To avoid the expression of mouse heavy or light chains, the endogenous mouse Ig (mIg) loci must be silenced by gene-targeting techniques. Subsequently, to obtain antigen-specific mAbs, conventional immunization protocols can be followed and the mAb technique used (fusion of activated B cells with mouse myeloma cells, screening, cloning, freezing, and testing) with these animThis chapter summarizes the most common chromatographic mAb andals expressing human Ig genes. This chapter describes the type of transgenic-knockout mice generated for various research groups, provides examples of human mAbs developed by research groups and companies, and includes protocols of immunization, generation, production, and purification of human mAbs from such mice. In addition, it also addresses the problems detected, and includes some of the methods that can be used to analyze functional activities with human mAbs.

Rapid de novo generation of antigen specific human B cells with expression of Blimp-1 and AID by in vitro immunization

In vitro immunization with antigens and cytokines triggers specific human B-cell response in short periods and is therefore superior to conventional in vivo immunization for antibody development. However, this new technology is limited by low efficiency, poor reproducibility, and requirement of pre-immunized lymphocytes. In this study, we demonstrate a novel method for de novo inducing antigen-specific human B cells in vitro. Unlike previous in vitro immunization of unfractionated PBMCs, we firstly optimized the conditions for inducing monocyte-derived dendritic cells (DCs) to efficiently capture, process, and present antigens. Instead of using the conventional method to activate Th2 cells for in vitro immunization, we succeeded to differentiate naïve CD4⁺ T cells into T follicular helper (Tfh) cells using antigen-sensitized DCs and cytokine cocktail. We discovered the differentiated T cells expressed ICOS, PD-1, BCL-6, and IL-21 at high levels. After 12 days of T-B co-culture, we observed induced T cells efficiently promoted naïve B cells to differentiate into plasmablasts secreting antigen-specific antibodies, with expression of Blimp-1 and AID related to affinity maturation and class switching. Thus, we established a new co-culture system with naïve lymphocyte populations for de novo acquisition of specifically in vitro immunized B cells potentially for development of therapeutic antibodies, which also provides novel insights into understanding the complex interactions among immune cells in lymph nodes.

Pharmacokinetic properties of IgG and various Fc fusion proteins in mice

Fusion to an IgG Fc region is an established strategy to extend the half-life of therapeutic proteins. Most Fc fusion proteins, however, do not achieve the long half-life of IgGs. Based on findings that scFv-Fc fusion proteins exhibit a shorter half-life than the corresponding IgG molecules, we performed a comparative study of different antibody-derived Fc fusion proteins. We could confirm that fusion of single-chain Fv (scFv) and single-chain diabody (scDb) molecules to an Fc region yields in fusion proteins with substantially extended half-lives compared with the single-chain versions. However, even fusion proteins with a size similar to that of IgG, e.g., scDb-Fc, did not have a half-life as long as an IgG molecule. Binding to the neonatal Fc receptor (FcRn) under acidic and neutral conditions was similar for IgG and all Fc fusion proteins. However, we observed differences between IgG and the Fc fusion proteins for dissociation of FcRn-bound proteins induced by shifting from acidic to neutral pH, reflecting the physiological release mechanism, further supporting a contribution of the kinetics of pH-dependent release from FcRn to the pharmacokinetic properties of IgG and Fc fusion proteins.

In vitro Fab display: a cell-free system for IgG discovery

Selection technologies such as ribosome display enable the rapid discovery of novel antibody fragments entirely in vitro. It has been assumed that the open nature of the cell-free reactions used in these technologies limits selections to single-chain protein fragments. We present a simple approach for the selection of multi-chain proteins, such as antibody Fab fragments, using ribosome display. Specifically, we show that a two-chain trastuzumab (Herceptin) Fab domain can be displayed in a format which tethers either the heavy or light chain to the ribosome while retaining functional antigen binding. Then, we constructed synthetic Fab HC and LC libraries and performed test selections against carcinoembryonic antigen (CEA) and vascular endothelial growth factor (VEGF). The Fab selection output was reformatted into full-length immunoglobulin Gs (IgGs) and directly expressed at high levels in an optimized cell-free system for immediate screening, purification and characterization. Several novel IgGs were identified using this cell-free platform that bind to purified CEA, CEA positive cells and VEGF.

Antigen-specific human monoclonal antibodies from transgenic mice

Due to the difficulties found when generating fully human monoclonal antibodies (mAbs) by the traditional method, several efforts have attempted to overcome these problems, with varying levels of success. One approach has been the development of transgenic mice carrying immunoglobulin (Ig) genes in germ line configuration. The engineered mouse genome can undergo productive rearrangement in the B cell population, with the generation of mouse B lymphocytes expressing human Ig (hIg) chains. To avoid the expression of mouse heavy or light chains, the endogenous mouse Ig (mIg) loci must be silenced by gene-targeting techniques. Subsequently, to obtain antigen-specific mAbs, conventional immunization protocols can be followed and the mAb technique used (fusion of activated B cells with mouse myeloma cells, screening, cloning, freezing, and testing) with these animals expressing human Ig genes. This chapter describes the type of transgenic knockout mice generated for various research groups, provides examples of human mAbs developed by research groups and companies, and includes protocols of immunization, generation, production, and purification of human mAbs from such mice. In addition, it also addresses the problems detected, and includes some of the methods that can be used to analyze functional activities with human mAbs.

Phage display--a powerful technique for immunotherapy: 1. Introduction and potential of therapeutic applications

One of the most effective molecular diversity techniques is phage display. This technology is based on a direct linkage between phage phenotype and its encapsulated genotype, which leads to presentation of molecule libraries on the phage surface. Phage display is utilized in studying protein-ligand interactions, receptor binding sites and in improving or modifying the affinity of proteins for their binding partners. Generating monoclonal antibodies and improving their affinity, cloning antibodies from unstable hybridoma cells and identifying epitopes, mimotopes and functional or accessible sites from antigens are also important advantages of this technology. Techniques originating from phage display have been applied to transfusion medicine, neurological disorders, mapping vascular addresses and tissue homing of peptides. Phages have been applicable to immunization therapies, which may lead to development of new tools used for treating autoimmune and cancer diseases. This review describes the phage display technology and presents the recent advancements in therapeutic applications of phage display.

Plasma half-life extension of small recombinant antibodies by fusion to immunoglobulin-binding domains

Many therapeutic proteins possessing a small size are rapidly cleared from circulation. Half-life extension strategies have therefore become increasingly important to improve the pharmacokinetic and pharmacodynamic properties of protein therapeutics. Here, we performed a comparative analysis of the half-life extension properties of various bacterial immunoglobulin-binding domains (IgBDs) derived from Staphylococcus protein A (SpA), Streptococcus protein G (SpG), and Finegoldia (formerly Peptostreptococcus) protein L (PpL). These domains, composed of 50-60 amino acid residues, were fused to the C terminus of a single-chain Fv and a bispecific single-chain diabody, respectively. All fusion proteins were produced in mammalian cells and retained their antigen-binding properties. The half-lives of the antibody molecules were prolonged to varying extents for the different IgBDs. The strongest effects in mice were observed for domain C3 of SpG (SpG(C3)) followed by domains B and D of SpA, suggesting that SpG(C3) is particularly useful to extend the plasma half-life of small proteins.

Exploiting bias in a non-immune human antibody library to predict antigenicity

Non-immune human antibody fragment libraries have generated antigen-binding proteins useful as prospective research, imaging, diagnostic and therapeutic agents. However, because the generation of such libraries relies on cloning antibody sequences from the circulating immune repertoire rather than truly naïve, germline sequences, their composition may reflect the deletion of autoreactive sequences, making them less suited for isolating binding clones to human antigens, but perhaps useful in applications where an in vitro handle on representative circulating antibody diversity is desired. Here we demonstrate that a large non-immune human scFv library is relatively depleted of sequences capable of recognizing human antigens as compared with orthologs antigens. Additionally, because this non-naïve, non-immune library may capture a representative section of antibody diversity, we explore its possible utility in conducting early pre-screens to predict the antigenicity of prospective therapeutics and find a correlation between the clinical immunogenicity of a small panel of protein therapeutics with their propensity for interacting with the library.

Establishing a knowledge trail from molecular experiments to clinical trials

During the development cycle of a new antibody therapy, the therapeutic agent will be tested on subsequently more biologically complex models. New experiments' designs are based upon data gathered from prior models. New researchers who inherit the data and researchers from groups with different cultures or expertise are often called upon to interpret these data. Experiments which are not recorded consistently or employ ambiguous terminology can make interpreting these results difficult. The researcher who had originally collected the data may not be at hand to correct any misunderstanding or offer clarification and data can be unknowingly misused. This introduces an element of risk into the therapy development process. We have developed a reporting guideline for recording therapy experiments. This guideline consists of a checklist of data to be recorded from antibody therapy experiments performed in molecular, cellular, animal and clinical model.

Convergent potency of internalized gelonin immunotoxins across varied cell lines, antigens, and targeting moieties

Gelonin-based immunotoxins vary widely in their cytotoxic potency as a function of antigen density, target cell internalization and trafficking kinetics, and conjugate properties. We have synthesized novel gelonin immunotoxins using two different binding scaffold types (single-chain antibody variable fragments and fibronectin domains) targeting two different tumor antigens (carcinoembryonic antigen and EGF receptor). Constructs were characterized using an antigen-negative cell line (HT-1080), cell lines positive for each antigen (HT-1080(CEA) for carcinoembryonic antigen and A431 for EGF receptor), and a cell line positive for both antigens (HT-29). Immunotoxins exhibited K(d) values between 8 and 15 nm and showed 20-2000-fold enhanced cytotoxicity compared with gelonin (IC(50) ∼ 0.25-30 nM versus 500 nM). Using quantitative fluorescence flow cytometry, we measured internalization of gelonin (via pinocytosis) and gelonin-based immunotoxins (via antigen-dependent, receptor-mediated endocytosis). Results were matched with cytotoxicity measurements made at equivalent concentration and exposures. Unexpectedly, when matched internalization and cytotoxicity data were combined, a conserved internalized cytotoxicity curve was generated that was common across experimental conditions. Considerable variations in antigen expression, trafficking kinetics, extracellular immunotoxin concentration, and exposure time were all found to collapse to a single potency curve on the basis of internalized immunotoxin. Fifty percent cytotoxicity occurred when ∼ 5 × 10(6) toxin molecules were internalized regardless of the mechanism of uptake. Cytotoxicity observed at a threshold internalization was consistent with the hypothesis that endosomal escape is a common, highly inefficient, rate-limiting step following internalization by any means tested. Methods designed to enhance endosomal escape might be utilized to improve the potency of gelonin-based immunotoxins.

Site-specific polysialylation of an antitumor single-chain Fv fragment

Protein pharmacokinetic modulation is becoming an important tool in the development of biotherapeutics. Proteins can be chemically or recombinantly modified to alter their half-lives and bioavailability to suit particular applications as well as improve side effect profiles. The most successful and clinically used approach to date is chemical conjugation with poly(ethylene glycol) polymers (PEGylation). Here, therapeutic protein half-life can be increased significantly while retaining biological function, reducing immunogenicity and cross-reaction. Naturally occurring alternatives to such synthetic polymers could have major advantages such as lower side effects due to biodegradability and metabolism. Polysialic acid (PSA) has been investigated as a pharmacokinetic modulatory biopolymer with many successful examples in preclinical and clinical development. Single-chain Fvs (scFvs) are a choice antibody format for human therapeutic antibody discovery. Because of their small size, they are rapidly eliminated from the circulation and often are rebuilt into larger proteins for drug development and a longer half-life. Here we show that chemical polysialylation can increase the half-life of an antiplacental alkaline (PLAP) and anticarcinoembryonic antigen (CEA) scFv (F1 and MFE-23, respectively) 3.4-4.9-fold, resulting in a 10.6-15.2-fold increase in blood exposure. Amine-directed coupling of the MFE-23 scFv reduced its immunoreactivity 20-fold which was resolved by site-specific polysialylation through an engineered C-terminal thiol residue. The site-specifically polysialylated MFE-23 scFv demonstrated up to 30-fold improved tumor uptake while displaying favorable tumor:normal tissue specificity. This suggests that engineering antibody fragments for site-specific polysialylation could be a useful approach to increase the half-life for a variety of therapeutic applications.

The effects of affinity and valency of an albumin-binding domain (ABD) on the half-life of a single-chain diabody-ABD fusion protein

Fusion of small recombinant antibody fragments to an albumin-binding domain (ABD) from streptococcal protein G strongly extends their plasma half-life. This ABD binds with nanomolar affinity to human (HSA) and mouse serum albumin (MSA). It was speculated that an increase in albumin-binding affinity should lead to a further increase in half-life. In the present study, we analyzed the effects of affinity and valency of the ABD on the pharmacokinetic properties of a bispecific single-chain diabody (scDb), applied previously to investigate various half-life extension strategies. The scDb is directed against carcinoembryonic antigen (CEA) and CD3 capable of mediating T cell retargeting to tumor cells. Two scDb derivatives with increased (scDb-ABD-H) and decreased (scDb-ABD-L) affinity as well as an scDb molecule fused to two ABD (scDb-ABD(2)) were generated and produced in mammalian cells. The altered binding of these constructs to HSA and MSA was confirmed by ELISA and quartz crystal microbalance measurements. All constructs bound efficiently to CEA and CD3-positive cells and were able to activate T cells in a target cell-dependent manner, although T cell activation was reduced in the presence of serum albumin. All three derivatives showed a strongly increased half-life in mice as compared with scDb. Compared with the wild-type scDb-ABD, the half-life of scDb-ABD-H exhibited a prolonged half-life and scDb-ABD-L a reduced half-life, while the half-life scDb-ABD(2) was almost identical to that of scDb-ABD. However, these changes were only moderate, indicating that the half-life-extending property of the ABD in mice is only weakly influenced by affinity for serum albumin or valency of albumin binding.

Imaging in targeted delivery of therapy to cancer

We review the current status of imaging as applied to targeted therapy with particular focus on antibody-based therapeutics. Antibodies have high tumor specificity and can be engineered to optimize delivery to, and retention within, the tumor. Whole antibodies can activate natural immune effector mechanisms and can be conjugated to beta- and alpha-emitting radionuclides, toxins, enzymes, and nanoparticles for enhanced therapeutic effect. Imaging is central to the development of these agents and is used for patient selection, performing dosimetry and assessment of response. gamma- and positron-emitting radionuclides may be used to image the distribution of antibody-targeted therapeutics While some radionuclides such as iodine-131 emit both beta and gamma radiation and are therefore suitable for both imaging and therapy, others are more suited to imaging or therapy alone. Hence for radionuclide therapy of neuroendocrine tumors, patients can be selected for therapy on the basis of gamma-emitting indium-111-octreotide imaging and treated with beta-emitting yttrium-90-octreotate. Positron-emitting radionuclides can give greater sensitivity that gamma-emitters but only a single radionuclide can be imaged at one time and the range of radionuclides is more limited. The multiple options for antibody-based therapeutic molecules, imaging technologies and therapeutic scenarios mean that very large amounts of diverse data are being acquired. This can be most effectively shared and progress accelerated by use of common data standards for imaging, biological, and clinical data.

Lymphocyte display: a novel antibody selection platform based on T cell activation

Since their onset, display technologies have proven useful for the selection of antibodies against a variety of targets; however, most of the antibodies selected with the currently available platforms need to be further modified for their use in humans, and are restricted to accessible antigens. Furthermore, these platforms are not well suited for in vivo selections. We present here a novel cell based antibody display platform, which takes advantage of the functional capabilities of T lymphocytes. The display of antibodies on the surface of T lymphocytes, as a part of a chimeric-immune receptor (CIR) mediating signaling, may ideally link the antigen-antibody interaction to a demonstrable change in T cell phenotype, due to subsequent expression of the early T cell activation marker CD69. In this proof-of-concept, an in vitro selection was carried out using a human T cell line lentiviral-transduced to express a tumor-specific CIR on the surface, against a human tumor cell line expressing the carcinoembryonic antigen. Based on an effective interaction between the CIR and the tumor antigen, we demonstrated that combining CIR-mediated activation with FACS sorting of CD69⁺ T cells, it is possible to isolate binders to tumor specific cell surface antigen, with an enrichment factor of at least 10³-fold after two rounds, resulting in a homogeneous population of T cells expressing tumor-specific CIRs.

In vivo tumor targeting and imaging with engineered trivalent antibody fragments containing collagen-derived sequences

There is an urgent need to develop new and effective agents for cancer targeting. In this work, a multivalent antibody is characterized in vivo in living animals. The antibody, termed "trimerbody", comprises a single-chain antibody (scFv) fragment connected to the N-terminal trimerization subdomain of collagen XVIII NC1 by a flexible linker. As indicated by computer graphic modeling, the trimerbody has a tripod-shaped structure with three highly flexible scFv heads radially outward oriented. Trimerbodies are trimeric in solution and exhibited multivalent binding, which provides them with at least a 100-fold increase in functional affinity than the monovalent scFv. Our results also demonstrate the feasibility of producing functional bispecific trimerbodies, which concurrently bind two different ligands. A trimerbody specific for the carcinoembryonic antigen (CEA), a classic tumor-associated antigen, showed efficient tumor targeting after systemic administration in mice bearing CEA-positive tumors. Importantly, a trimerbody that recognizes an angiogenesis-associated laminin epitope, showed excellent tumor localization in several cancer types, including fibrosarcomas and carcinomas. These results illustrate the potential of this new antibody format for imaging and therapeutic applications, and suggest that some laminin epitopes might be universal targets for cancer targeting.

Molecular imaging with nanoparticles: giant roles for dwarf actors

Molecular imaging, first developed to localise antigens in light microscopy, now encompasses all imaging modalities including those used in clinical care: optical imaging, nuclear medical imaging, ultrasound imaging, CT, MRI, and photoacoustic imaging. Molecular imaging always requires accumulation of contrast agent in the target site, often achieved most efficiently by steering nanoparticles containing contrast agent into the target. This entails accessing target molecules hidden behind tissue barriers, necessitating the use of targeting groups. For imaging modalities with low sensitivity, nanoparticles bearing multiple contrast groups provide signal amplification. The same nanoparticles can in principle deliver both contrast medium and drug, allowing monitoring of biodistribution and therapeutic activity simultaneously (theranostics). Nanoparticles with multiple bioadhesive sites for target recognition and binding will be larger than 20 nm diameter. They share functionalities with many subcellular organelles (ribosomes, proteasomes, ion channels, and transport vesicles) and are of similar sizes. The materials used to synthesise nanoparticles include natural proteins and polymers, artificial polymers, dendrimers, fullerenes and other carbon-based structures, lipid-water micelles, viral capsids, metals, metal oxides, and ceramics. Signal generators incorporated into nanoparticles include iron oxide, gadolinium, fluorine, iodine, bismuth, radionuclides, quantum dots, and metal nanoclusters. Diagnostic imaging applications, now appearing, include sentinal node localisation and stem cell tracking.

Engineering a single-chain Fv antibody to alpha v beta 6 integrin using the specificity-determining loop of a foot-and-mouth disease virus

The alpha v beta 6 integrin is a promising target for cancer therapy. Its expression is up-regulated de novo on many types of carcinoma where it may activate transforming growth factor-beta1 and transforming growth factor-beta 3, interact with the specific extracellular matrix proteins and promote migration and invasion of tumor cells. The viral protein 1 (VP1) coat protein of the O(1) British field strain serotype of foot-and-mouth disease virus is a high-affinity ligand for alpha v beta 6, and we recently reported that a peptide derived from VP1 exhibited alpha v beta 6-specific binding in vitro and in vivo. We hypothesized that this peptide could confer binding specificity of an antibody to alpha v beta 6. A 17-mer peptide of VP1 was inserted into the complementarity-determining region H3 loop of MFE-23, a murine single-chain Fv (scFv) antibody reactive with carcinoembryonic antigen (CEA). The resultant scFv (B6-1) bound to alpha v beta 6 but retained residual reactivity with CEA. This was eliminated by point mutation (Y100bP) in the variable heavy-chain domain to create an scFv (B6-2) that was as structurally stable as MFE-23 and reacted specifically with alpha v beta 6 but not with alpha 5 beta 1, alpha v beta 3, alpha v beta 5, alpha v beta 8 or CEA. B6-2 was internalized into alpha v beta 6-expressing cells and inhibited alpha v beta 6-dependent migration of carcinoma cells. B6-2 was subsequently humanized. The humanized form (B6-3) was obtained as a non-covalent dimer from secretion in Pichia pastoris (115 mg/l) and was a potent inhibitor of alpha v beta 6-mediated cell adhesion. Thus, we have used a rational stepwise approach to create a humanized scFv with therapeutic potential to block alpha v beta 6-mediated cancer cell invasion or to deliver and internalize toxins specifically to alpha v beta 6-expressing tumors.

N-glycosylation as novel strategy to improve pharmacokinetic properties of bispecific single-chain diabodies

The therapeutic efficacy of recombinant antibodies such as single-chain Fv fragments and small bispecific or bifunctional molecules is often limited by rapid elimination from the circulation because of their small size. Here, we have investigated the effects of N-glycosylation on the activity and pharmacokinetics of a small bispecific single-chain diabody (scDb CEACD3) developed for the retargeting of cytotoxic T cells to CEA-expressing tumor cells. We could show that the introduction of N-glycosylation sequons into the flanking linker and a C-terminal extension results in the production of N-glycosylated molecules after expression in transfected HEK293 cells. N-Glycosylated scDb variants possessing 3, 6, or 9 N-glycosylation sites, respectively, retained antigen binding activity and bispecificity for target and effector cells as shown in a target cell-dependent IL-2 release assay, although activity was reduced approximately 3-5-fold compared with the unmodified scDb. All N-glycosylated scDb variants exhibited a prolonged circulation time compared with scDb, leading to a 2-3-fold increase of the area under curve (AUC). In comparison, conjugation of a branched 40-kDa PEG chain increased AUC by a factor of 10.6, while a chimeric anti-CEA IgG1 molecule had the longest circulation time with a 17-fold increase in AUC. Thus, N-glycosylation complements the repertoire of strategies to modulate pharmacokinetics of small recombinant antibody molecules by an approach that moderately prolongs circulation time.

Intracellular antibodies (intrabodies) and their therapeutic potential

Combining exquisite specificity and high antigen-binding affinity, intrabodies have been used as a biotechnological tool to interrupt, modulate, or define the functions of a wide range of target antigens at the posttranslational level. An intrabody is an antibody that has been designed to be expressed intracellularly and can be directed to a specific target antigen present in various subcellular locations including the cytosol, nucleus, endoplasmic reticulum (ER), mitochondria, peroxisomes, plasma membrane and trans-Golgi network (TGN) through in frame fusion with intracellular trafficking/localization peptide sequences. Although intrabodies can be expressed in different forms, the most commonly used format is a singlechain antibody (scFv Ab) created by joining the antigen-binding variable domains of heavy and light chain with an interchain linker (ICL), most often the 15 amino acid linker (GGGGS)(3) between the variable heavy (VH) and variable light (VL) chains. Intrabodies have been used in research of cancer, HIV, autoimmune disease, neurodegenerative disease, and transplantation. Clinical application of intrabodies has mainly been hindered by the availability of robust gene delivery system(s) including target cell directed gene delivery. This review will discuss several methods of intrabody selection, different strategies of cellular targeting, and recent successful examples of intrabody applications. Taking advantage of the high specificity and affinity of an antibody for its antigen, and of the virtually unlimited diversity of antigen-binding variable domains available for molecular targeting, intrabody techniques are emerging as promising tools to generate phenotypic knockouts, to manipulate biological processes, and to obtain a more thorough understanding of functional genomics.

Improved pharmacokinetics of recombinant bispecific antibody molecules by fusion to human serum albumin

Recombinant bispecific antibodies such as tandem scFv molecules (taFv), diabodies (Db), or single chain diabodies (scDb) have shown to be able to retarget T lymphocytes to tumor cells, leading to their destruction. However, therapeutic efficacy is hampered by a short serum half-life of these small molecules having molecule masses of 50-60 kDa. Thus, improvement of the pharmacokinetic properties of small bispecific antibody formats is required to enhance efficacy in vivo. In this study, we generated several recombinant bispecific antibody-albumin fusion proteins and analyzed these molecules for biological activity and pharmacokinetic properties. Three recombinant antibody formats were produced by fusing two different scFv molecules, bispecific scDb or taFv molecules, respectively, to human serum albumin (HSA). These constructs (scFv(2)-HSA, scDb-HSA, taFv-HSA), directed against the tumor antigen carcinoembryonic antigen (CEA) and the T cell receptor complex molecule CD3, retained full binding capacity to both antigens compared with unfused scFv, scDb, and taFv molecules. Tumor antigen-specific retargeting and activation of T cells as monitored by interleukin-2 release was observed for scDb, scDb-HSA, taFv-HSA, and to a lesser extent for scFv(2)-HSA. T cell activation could be further enhanced by a target cell-specific costimulatory signal provided by a B7-DbCEA fusion protein. Furthermore, we could demonstrate that fusion to serum albumin strongly increases circulation time of recombinant bispecific antibodies. In addition, our comparative study indicates that single chain diabody-albumin fusion proteins seem to be the most promising format for further studying cytotoxic activities in vitro and in vivo.

A phase I study of single administration of antibody-directed enzyme prodrug therapy with the recombinant anti-carcinoembryonic antigen antibody-enzyme fusion protein MFECP1 and a bis-iodo phenol mustard prodrug

PURPOSE

Antibody-directed enzyme prodrug therapy is a two-stage treatment whereby a tumor-targeted antibody-enzyme complex localizes in tumor for selective conversion of prodrug. The purpose of this study was to establish optimal variables for single administration of MFECP1, a recombinant antibody-enzyme fusion protein of an anti-carcinoembryonic antigen single-chain Fv antibody and the bacterial enzyme carboxypeptidase G2 followed by a bis-iodo phenol mustard prodrug. MFECP1 is manufactured in mannosylated form to facilitate normal tissue elimination.

EXPERIMENTAL DESIGN

Pharmacokinetic, biodistribution, and tumor localization studies were used to test the hypothesis that MFECP1 localizes in tumor and clears from normal tissue via the liver. Firstly, safety of MFECP1 and a blood concentration of MFECP1 that would avoid systemic prodrug activation were tested. Secondly, dose escalation of prodrug was done. Thirdly, the dose of MFECP1 and timing of prodrug administration were optimized.

RESULTS

MFECP1 was safe and well tolerated, cleared rapidly via the liver, and was less immunogenic than previously used products. Eighty-fold dose escalation from the starting dose of prodrug was carried out before dose-limiting toxicity occurred. Confirmation of the presence of enzyme in tumor and DNA interstrand cross-links indicating prodrug activation were obtained for the optimal dose and time point. A total of 28 of 31 patients was evaluable for response, the best response being a 10% reduction of tumor diameter, and 11 of 28 patients had stable disease.

CONCLUSIONS

Optimal conditions for effective therapy were established. A study testing repeat treatment is currently being undertaken.

Antibody-directed enzyme prodrug therapy (ADEPT) for cancer

Antibody-directed enzyme prodrug therapy was conceived as a means of restricting the action of cytotoxic drugs to tumor sites. Since antigenic targets were a central component of the approach, colonic cancer, with its virtually universal expression of carcinoembryonic antigen at the cellular level, presented an obvious starting point. The principle of antibody-directed enzyme prodrug therapy is to use an antibody directed at a tumor-associated antigen to vector an enzyme to tumor sites. The enzyme should be retained at tumor sites after it has cleared from blood and normal tissues. A nontoxic prodrug, a substrate for the enzyme, is then given and, by cleaving an inactivating component from the prodrug, a potent cytotoxic agent is generated. One of the potential advantages of such a system is that a small cytotoxic agent, generated within a tumor site, is much more diffusible than a large antibody molecule. Moreover, failure to express the target antigen by cancer cells does not protect them from the bystander action of the cytotoxic agent. This review will primarily consider the studies of the London group since this is the only group that has so far reported clinical trials and it is only through clinical trials that the requirements of a successful antibody-directed enzyme prodrug therapy system can be identified.

Development of antibodies for cancer therapy

Antibody therapies have become an important component in the management of malignant disease. Recombinant technology offers enormous opportunities to tailor antibodies to meet clinical requirements. This includes the reduction of immunogenicity and the development of smaller antibody fragments that can be incorporated into fusion proteins. Antibodies can block tumour growth factors or their receptors, activate immunological attack on the tumour, or be used to deliver payloads such as radioisotopes, cytotoxic drugs or toxins. Pretargeting includes streptavidin/biotin systems and antibody-directed enzyme prodrug therapy (ADEPT). ADEPT uses an antibody-enzyme complex to deliver a prodrug-activating enzyme to tumours for selective prodrug conversion at the tumour site. New antibody targets, refined antibodies, antibody fusion proteins, combination therapies and the use of antibodies as adjuvant therapy are important topics in the development of antibody therapy against cancer.

Deglycosylation to obtain stable and homogeneous Pichia pastoris-expressed N–A1 domains of carcinoembryonic antigen

Carcinoembryonic antigen (CEA) is a seven domain membrane glycoprotein widely used as a tumour marker for adenocarcinomas and as a target for antibody-directed therapies. Structural models have proposed that the first two domains of CEA (the N terminal and adjoining A1 domains) bind MFE-23, a single chain Fv antibody in experimental clinical use. We aimed to produce recombinant N-A1 to test this hypothesis. The N-A1 domains were expressed as soluble protein with a C-terminal hexahistidine tag (His6-tag) in the yeast Pichia pastoris. His6-tagged N-A1 was captured from the supernatant by batch purification with copper-loaded Streamline Chelating, an immobilised metal affinity chromatography (IMAC) matrix usually utilised in expanded bed techniques. Purified N-A1 was heterogeneous with a molecular weight range from 38 to 188 kDa. Deglycosylation with endoglycosidase H (Endo H) resulted in three discrete molecular weight forms of N-A1, one partially mannosylated, one fully Endo H-digested and one fully Endo H-digested but lacking the His6-tag. These were separated by concanavalin A chromatography followed by HiTrap IMAC. The procedure resulted in single-band-purity, mannose-free N-A1. The binding interaction of MFE-23 to N-A1 was analysed by surface plasmon resonance. The affinity constants retrieved were KD = 4.49 x 10(-9)M for the P. pastoris expressed, native N-A1, and 5.33 x 10(-9) M for the Endo H-treated N-A1. To our knowledge this is the first time that two consecutive domains of CEA have been stably expressed and purified from P. pastoris. This work confirms that the CEA epitope recognised by MFE-23 resides in N-A1.

Human monocytes expressing a CEA-specific chimeric CD64 receptor specifically target CEA-expressing tumour cells in vitro and in vivo

Antibody-dependent cellular cytotoxicity (ADCC) is one means by which macrophages (as well as natural killer cells and granulocytes) elicit a cytotoxic response. This is achieved via interaction of the Fc-gamma-receptor (CD64) with the Fc portion of antibody bound to target cells. We have created a chimeric CD64 molecule that incorporates a single chain Fv molecule, targeted against human carcinoembryonic antigen (CEA), fused to the membrane spanning and cytosolic domains of human CD64. Following adenoviral transfer to primary human monocytes, this chimeric CD64 receptor induced antigen-specific cytokine secretion during culture on immobilised CEA protein or on CEA-expressing tumour cells. Moreover, CEA targeted, but not control, monocytes effectively retarded CEA-positive tumour cell growth in vitro. Importantly, targeted monocyte cultures significantly reduced in vivo tumour growth rates in xenograft studies resulting in improved survival rates over that of control monocyte cultures. These data suggest that genetically directing monocytes against tumour antigens may be a useful means of achieving an immunotherapeutic response.

Tailoring antibodies for radionuclide delivery

Therapeutic antibodies are well established as an important class of drugs in modern medicine. The exquisite specificity and affinity for a specific target offered by antibodies has also encouraged their development as delivery vehicles for agents such as radionuclides to target tissues, for radioimmunoimaging and radioimmunotherapy. Specifically, in nuclear medicine, radionuclide-conjugated antibody molecules make it possible to image diseased loci with greater sensitivity than other imaging modalities such as magnetic resonance imaging. Furthermore, two radionuclide-conjugated antibodies have recently been approved for the therapy of non-Hodgkin's lymphoma. However, optimal implementation of antibodies has been limited by the extended circulation persistence that is characteristic of native antibodies, which is responsible for increased background activity in radioimmunoimaging applications and dose-related normal organ toxicities in radioimmunotherapy. In this article the current status of radiolabelled intact antibodies is reviewed, focusing on strategies to improve their pharmacokinetic properties to suit a desired application. Examples from the literature that represent different approaches to accomplishing this task in terms of their successes as well as limitations, and perspectives for the future are discussed.

Antibody-directed enzyme prodrug therapy (ADEPT) for cancer

Antibody-directed enzyme prodrug therapy (ADEPT) aims to restrict the cytotoxic action to tumour sites. The obstacles to achieve this were recognised at the outset, but time and experience have given these better definition. The development of fusion proteins has provided the means of making consistent antibody-enzyme constructs on an adequate scale, and glycosylation has provided the means to control the clearance of enzyme from non-tumour sites. Human enzymes have yet to be tested in a clinical setting, and there are pointers indicating that the immunological response to foreign enzymes can be overcome. The relatively small number of purpose-designed prodrugs tested so far leaves this an area ripe for further development. The ongoing iterative process between preclinical and clinical studies is critical to achieving the objective.

The role of extracellular spacer regions in the optimal design of chimeric immune receptors: evaluation of four different scFvs and antigens

Human peripheral blood lymphocytes can be transduced to express antigen-dependent CD3zeta chimeric immune receptors (CIRs), which function independently of the T-cell receptor (TCR). Although the exact function of these domains is unclear, previous studies imply that an extracellular spacer region is required for optimal CIR activity. In this study, four scFvs (in the context of CIRs with or without extracellular spacer regions) were used to target the human tumor-associated antigens carcinoembryonic antigen (CEA), neural cell adhesion molecule (NCAM), the oncofetal antigen 5T4, and the B-cell antigen CD19. In all cases human T-cell populations expressing the CIRs were functionally active against their respective targets, but the anti-5T4 and anti-NCAM CIRs showed enhanced specific cytokine release and cytotoxicity only when possessing an extracellular spacer region. In contrast, the anti-CEA and anti-CD19 CIRs displayed optimal cytokine release activity only in the absence of an extracellular spacer. Interestingly, mapping of the scFv epitopes has revealed that the anti-CEA scFv binds close to the amino-terminal of CEA, which is easily accessible to the CIR. In contrast, CIRs enhanced by a spacer domain appear to bind to epitopes residing closer to the cell membrane, suggesting that a more flexible extracellular domain may be required to permit the efficient binding of such epitopes. These results show that a spacer is not necessary for optimal activity of CIRs but that the optimal design varies.

Envelope targeting: hemagglutinin attachment specificity rather than fusion protein cleavage-activation restricts Tupaia paramyxovirus tropism

To engineer a targeting envelope for gene and oncolytic vector delivery, we characterized and modified the envelope proteins of Tupaia paramyxovirus (TPMV), a relative of the morbilli- and henipaviruses that neither infects humans nor has cross-reactive relatives that infect humans. We completed the TPMV genomic sequence and noted that the predicted fusion (F) protein cleavage-activation site is not preceded by a canonical furin cleavage sequence. Coexpression of the TPMV F and hemagglutinin (H) proteins induced fusion of Tupaia baby fibroblasts but not of human cells, a finding consistent with the restricted TPMV host range. To identify the factors restricting fusion of non-Tupaia cells, we initially analyzed F protein cleavage. Even without an oligo- or monobasic protease cleavage sequence, TPMV F was cleaved in F1 and F2 subunits in human cells. Edman degradation of the F1 subunit yielded the sequence IFWGAIIA, placing the conserved phenylalanine in position 2, a novelty for paramyxoviruses but not the cause of fusion restriction. We then verified whether the lack of a TPMV H receptor limits fusion. Toward this end, we displayed a single-chain antibody (scFv) specific for the designated receptor human carcinoembryonic antigen on the TPMV H ectodomain. The H-scFv hybrid protein coexpressed with TPMV F mediated fusion of cells expressing the designated receptor, proving that the lack of a receptor limits fusion and that TPMV H can be retargeted. Targeting competence and the absence of antibodies in humans define the TPMV envelope as a module to be adapted for ferrying ribonucleocapsids of oncolytic viruses and gene delivery vectors.

Anticorps monoclonaux à usage thérapeutique : un peu d'histoire, beaucoup d'ingénierie, et … quelques succès cliniques

Thirty years after their discovery by Milstein and Köhler, monoclonal antibodies have now come of age as therapeutics. Nineteen monoclonal antibodies are on the market and/or have got authorization to be used for the treatment of severe diseases. Many technical efforts have been devoted over the last two decades to the generation of second generation mAbs with better affinities, decreased immunogenicity and optimized effector functions. The development of molecular engineering techniques applied to antibody molecules has also made it possible to design bi-specific antibodies and fusion molecules exhibiting different modules with bi-functional activities. The use of proteomics and genomics combined with phage display allows now the rapid selection of antibodies directed against new targets at a high rate. Many efforts are currently focused on the selection of high-responder patients, the optimization of antibody delivery, schemes of infusion, antibody pharmaco-kinetics and bio-distribution, as well as on a better control of the severe side-effects generated by some antibody treatments.

Killing of non-Hodgkin lymphoma cells by autologous CD19 engineered T cells

Adoptive immunotherapy with tumour-specific T cells is an emerging technology that may be applicable to a broad range of cancers. However, tumours can avoid T cell-mediated attack through multiple mechanisms including downregulation of major histocompatability complex (MHC). Consequently, engineering T cells to target intact protein antigen directly, thus bypassing the need for MHC presentation, can facilitate T cell targeting of tumour cells. Peripheral blood lymphocytes from nine of nine patients with non-Hodgkin lymphoma (NHL) were successfully gene-modified to express a receptor consisting of a CD19 single chain variable fragment (scFv) fused to the T cell CD3zeta signalling molecule. These T cells were functionally active against the CD19(+) Raji Burkitt's lymphoma cell line. Importantly, engineered T cells from seven of nine NHL patients efficiently lysed autologous lymph node tumour biopsy cells. There was a clear correlation between levels of CD19 expression on the tumour and effective killing by the engineered T cells. For two patients with a low or absent CD19(+) cells within the biopsy, no significant killing was observed. These results demonstrate that patients with CD19(+) NHL would be suitable candidates for this form of therapy in the setting of a phase I clinical trial.

Nonuniform absorbed dose distribution in the kidney: the influence of organ architecture

The development of novel, systemically administered radionuclide therapies (such as radioimmunotherapy) relies on the ability to predict dose-limiting toxicity to normal tissue. Where the kidney is the principal route of excretion of the radionuclide preparation and/or breakdown of products, nephrotoxicity may be the dose-limiting factor. Until recently, conventional (MIRD) dosimetry assumed the distribution in the kidney to be uniform. A new MIRD phantom of the kidney models it as a set of uniform suborgans. In the work described here, the assumption of uniformity of distribution and of heterogeneity of dose rate (and, thus, absorbed dose) was tested in the mouse model. In this paper, we examine the nonuniformity of distribution and the subsequent dose rate for 4 antibody preparations (IgG (150 kD), F(ab)'(2) (100 kD), Fab (50 kD) and sFv (27 kD)) labeled with 4 radionuclides ((125)I, (131)I, (186)Re, and (90)Y) of interest in radioimmunotherapy (RIT). The kidney was considered as a whole and as two suborgans (cortex and medulla), and the nonuniformity of the dose-rate distribution was measured by a correlation of modeled dose-rate distribution with the dose-rate distribution obtained for an equivalent uniform radionuclide distribution. The heterogeneity of distribution, the inter- and intra-suborgan, was seen to increase as the molecular weight of the antibodies decreased. The assumption of uniform activity distribution for the whole kidney gives a poor estimation of the distribution of the dose rate. In the cortex, the longer-range emitters smooth out the effect of heterogeneous distribution and, in mice, an assumption of uniform cortex self-dose distribution may be sufficient for simple calculations. It is unclear how much this smoothing would be relevant in the human kidney.

The Evolution of Antibodies into Versatile Tumor-Targeting Agents

In recent years, monoclonal antibodies have become important weapons in the arsenal of anticancer drugs, and in select cases are now the drugs of choice due to their favorable toxicity profiles. Originally developed to confer passive immunity against tumor-specific antigens, clinical uses of monoclonal antibodies are expanding to include growth factor sequestration, signal transduction modulation, and tumor-specific drug delivery. In this review, we shall present the origins of antibody therapeutics within the field of immunotherapy and their evolution into effective anticancer agents, then discuss their multiple mechanisms of action, the basis of their tumor selectivity, and their therapeutic properties compared with traditional therapies. Antibodies are complex molecules whose efficacy and toxicity depend on the antigen, the antibody, any conjugated groups, and even the patient. Finally, we shall present new technologies being developed to increase the efficacy and selectivity of antibody-based therapeutics. Interestingly, many of the new approaches straddle the middle ground between immunotherapy and the traditional modalities of chemotherapy and radiotherapy, and can be seen as ways of combining the selectivity of the former with the efficacy of the latter.

Demystified...recombinant antibodies

Recombinant antibodies are important tools for biomedical research and are increasingly being used as clinical diagnostic/therapeutic reagents. In this article, a background to humanized antibodies is given, together with details of the generation of antibody fragments--for example, single chain Fv fragments. Phage antibody fragments are fast becoming popular and can be generated by simple established methods of affinity enrichment from libraries derived from immune cells. Phage display methodology can also be used for the affinity enrichment of existing antibody fragments to provide a reagent with a higher affinity. Here, phage antibodies are demystified to provide a greater understanding of the potential of these reagents and to engage clinicians and biomedical scientists alike to think about potential applications in pathology and clinical settings.

Modifying an immunogenic epitope on a therapeutic protein: a step towards an improved system for antibody-directed enzyme prodrug therapy (ADEPT)

Carboxypeptidase G2 (CP) is a bacterial enzyme, which is targeted to tumours by an antitumour antibody for local prodrug activation in antibody-directed enzyme prodrug therapy (ADEPT). Repeated cycles of ADEPT are desirable but are hampered by human antibody response to CP (HACA). To address this, we aimed to identify and modify clinically important immunogenic sites on MFECP, a recombinant fusion protein of CP with MFE-23, a single chain Fv (scFv) antibody. A discontinuous conformational epitope at the C-terminus of the CP previously identified by the CM79 scFv antibody (CM79-identified epitope) was chosen for study. Modification of MFECP was achieved by mutations of the CM79-identified epitope or by addition of a hexahistidine tag (His-tag) to the C-terminus of MFECP, which forms part of the epitope. Murine immunisation experiments with modified MFECP showed no significant antibody response to the CM79-identified epitope compared to A5CP, an unmodified version of CP chemically conjugated to an F(ab)(2) antibody. Success of modification was also demonstrated in humans because patients treated with His-tagged MFECP had a significantly reduced antibody response to the CM79-identified epitope, compared to patients given A5CP. Moreover, the polyclonal antibody response to CP was delayed in both mice and patients given modified MFECP. This increases the prospect of repeated treatment with ADEPT for effective cancer treatment.

Recombinant antibodies for the diagnosis and treatment of cancer

The advent of recombinant antibody technology led to an enormous revival in the use of antibodies as diagnostic and therapeutic tools for fighting cancer. This review provides a brief historical sketch of the development of recombinant antibodies for the diagnosis and immunotherapy of cancer and summarizes the most significant clinical data for the best established reagents to date. It also discusses clinically relevant aspects of the use of recombinant antibodies in cancer patients.

Efficient Cancer Therapy with a Nanobody-Based Conjugate

Nanobodies are the smallest fragments of naturally occurring single-domain antibodies that have evolved to be fully functional in the absence of a light chain. Nanobodies are strictly monomeric, very stable, and highly soluble entities. We identified a nanobody with subnanomolar affinity for the human tumor-associated carcinoembryonic antigen. This nanobody was conjugated to Enterobacter cloacae beta-lactamase, and its site-selective anticancer prodrug activation capacity was evaluated. The conjugate was readily purified in high yields without aggregation or loss of functionality of the constituents. In vitro experiments showed that the nanobody-enzyme conjugate effectively activated the release of phenylenediamine mustard from the cephalosporin nitrogen mustard prodrug 7-(4-carboxybutanamido) cephalosporin mustard at the surface of carcinoembryonic antigen-expressing LS174T cancer cells. In vivo studies demonstrated that the conjugate had an excellent biodistribution profile and induced regressions and cures of established tumor xenografts. The easy generation and manufacturing yield of nanobody-based conjugates together with their potent antitumor activity make nanobodies promising vehicles for new generation cancer therapeutics.

Antibody-targeted cell fusion

Membrane fusion has many potential applications in biotechnology. Here we show that antibody-targeted cell fusion can be achieved by engineering a fusogenic viral membrane glycoprotein complex. Three different single-chain antibodies were displayed at the extracellular C terminus of the measles hemagglutinin (H) protein, and combinations of point mutations were introduced to ablate its ability to trigger fusion through the native viral receptors CD46 and SLAM. When coexpressed with the measles fusion (F) protein, using plasmid cotransfection or bicistronic adenoviral vectors, the retargeted H proteins could mediate antibody-targeted cell fusion of receptor-negative or receptor-positive index cells with receptor-positive target cells. Adenoviral expression vectors mediating human epidermal growth factor receptor (EGFR)-targeted cell fusion were potently cytotoxic against EGFR-positive tumor cell lines and showed superior antitumor potency against EGFR-positive tumor xenografts as compared with control adenoviruses expressing native (untargeted) or CD38-targeted H proteins.

Efficient Retroviral Vector Targeting of Carcinoembryonic Antigen-Positive Tumors

Many gene therapy approaches require specific, efficient gene delivery to cells in vivo. To target colorectal tumors we fused a single-chain variable fragment (scFv) directed against carcinoembryonic antigen (CEA) to the amphotropic murine leukemia virus envelope. A proline-rich hinge and matrix metalloprotease (MMP) cleavage site linked the two proteins. Following attachment to CEA, MMP cleavage of the envelope at the cell surface removed the scFv and proline-rich hinge, allowing transduction. This allowed selective targeting of CEA-positive cells in vivo after injection of producer cells at the site of the tumor, with up to 10% of cells within a CEA-positive tumor xenograft becoming transduced. Intraperitoneal injection of amphotropic producer cells resulted in transduction of cells in spleen, liver, and kidney, which was not detected when CEA-targeted producer cells were used. These results demonstrate the feasibility of using targeted retroviral vectors for in vivo gene delivery to tumors. Furthermore, the lack of transduction of host cells eliminates the risk of insertional mutagenesis leading to transformation of host hematopoietic cells.