Triabodies: single chain Fv fragments without a linker form trivalent trimers

Multivalent protein-drug conjugates - An emerging strategy for the upgraded precision and efficiency of drug delivery to cancer cells

With almost 20 million new cases per year, cancer constitutes one of the most important challenges for public health systems. Unlike traditional chemotherapy, targeted anti-cancer strategies employ sophisticated therapeutics to precisely identify and attack cancer cells, limiting the impact of drugs on healthy cells and thereby minimizing the unwanted side effects of therapy. Protein drug conjugates (PDCs) are a rapidly growing group of targeted therapeutics, composed of a cancer-recognition factor covalently coupled to a cytotoxic drug. Several PDCs, mainly in the form of antibody-drug conjugates (ADCs) that employ monoclonal antibodies as cancer-recognition molecules, are used in the clinic and many PDCs are currently in clinical trials. Highly selective, strong and stable interaction of the PDC with the tumor marker, combined with efficient, rapid endocytosis of the receptor/PDC complex and its subsequent effective delivery to lysosomes, is critical for the efficacy of targeted cancer therapy with PDCs. However, the bivalent architecture of contemporary clinical PDCs is not optimal for tumor receptor recognition or PDCs internalization. In this review, we focus on multivalent PDCs, which represent a rapidly evolving and highly promising therapeutics that overcome most of the limitations of current bivalent PDCs, enhancing the precision and efficiency of drug delivery to cancer cells. We present an expanding set of protein scaffolds used to generate multivalent PDCs that, in addition to folding into well-defined multivalent molecular structures, enable site-specific conjugation of the cytotoxic drug to ensure PDC homogeneity. We provide an overview of the architectures of multivalent PDCs developed to date, emphasizing their efficacy in the targeted treatment of various cancers.

Emerging new therapeutic antibody derivatives for cancer treatment

Monoclonal antibodies constitute a promising class of targeted anticancer agents that enhance natural immune system functions to suppress cancer cell activity and eliminate cancer cells. The successful application of IgG monoclonal antibodies has inspired the development of various types of therapeutic antibodies, such as antibody fragments, bispecific antibodies, and antibody derivatives (e.g., antibody–drug conjugates and immunocytokines). The miniaturization and multifunctionalization of antibodies are flexible and viable strategies for diagnosing or treating malignant tumors in a complex tumor environment. In this review, we summarize antibodies of various molecular types, antibody applications in cancer therapy, and details of clinical study advances. We also discuss the rationale and mechanism of action of various antibody formats, including antibody–drug conjugates, antibody–oligonucleotide conjugates, bispecific/multispecific antibodies, immunocytokines, antibody fragments, and scaffold proteins. With advances in modern biotechnology, well-designed novel antibodies are finally paving the way for successful treatments of various cancers, including precise tumor immunotherapy, in the clinic.

The mosaic puzzle of the therapeutic monoclonal antibodies and antibody fragments - A modular transition from full-length immunoglobulins to antibody mimetics

The use of monoclonal antibodies represents an important and efficient diagnostic and therapeutic tool in disease management and modern science but remains limited by several factors including the uneven distribution in diseased tissues as well as undesired activation of side immune reactions. Major scientific advancements including Recombinant DNA Technology, Hybridoma Technology, and Polymerase Chain Reaction have considerably impacted the use of monoclonal antibodies providing technical and effective solutions to overcome the shortcomings encountered with conventional antibodies. Initially, the introduction of antibody fragments allowed a more uniform and deeper penetration of the targeted tissue and reduced unwanted activation of Fc-mediated immune reactions. On another level, the immunogenicity of murine-derived antibodies was overcome by humanizing their encoding genes with specific sequences of human origin andtransgenic mice able to synthesize fully human antibodies were successfully created. Moreover, the advancement of genetic engineering techniques supported by the modular structure of antibody coding genes paved the way for the development of a new generation of antibody fragments with a wide spectrum of monospecific and bispecific agents. These later could be monovalent, bivalent, or multivalent, and either expressed as a single chain, assembled in multimeric forms or stringed in tandem. This has conferred improved affinity, stability, and solubility to antibody targetting. Lately, a new array of monoclonal antibody fragments was introduced with the engineering of nanobody and antibody mimetics as non-immunoglobulin-derived fragments with promising diagnostic and therapeutic applications. In this review, we decipher the molecular basis of monoclonal antibody engineering with a detailed screening of the antibody derivatives that provides new perspectives to expand the use of monoclonal fragments into previously unexplored fields.

Evolution of Escherichia coli Expression System in Producing Antibody Recombinant Fragments

Antibodies and antibody-derived molecules are continuously developed as both therapeutic agents and key reagents for advanced diagnostic investigations. Their application in these fields has indeed greatly expanded the demand of these molecules and the need for their production in high yield and purity. While full-length antibodies require mammalian expression systems due to the occurrence of functionally and structurally important glycosylations, most antibody fragments and antibody-like molecules are non-glycosylated and can be more conveniently prepared in E. coli-based expression platforms. We propose here an updated survey of the most effective and appropriate methods of preparation of antibody fragments that exploit E. coli as an expression background and review the pros and cons of the different platforms available today. Around 250 references accompany and complete the review together with some lists of the most important new antibody-like molecules that are on the market or are being developed as new biotherapeutics or diagnostic agents.

Modulating Antibody Structure and Function through Directed Mutations and Chemical Rescue

Monoclonal antibody therapeutics have revolutionized the treatment of diseases such as cancer and autoimmune disorders, and also serve as research reagents for diverse and unparalleled applications. To extend their utility in both contexts, we have begun development of tunable antibodies, whose activity can be controlled by addition of a small molecule. Conceptually, we envision that incorporating cavity-forming mutations into an antibody can disrupt its structure, thereby reducing its affinity for antigen; addition of a small molecule may then restore the active structure, and thus rescue antigen binding. As a first proof of concept toward implementing this strategy, we have incorporated individual tryptophan to glycine mutations into FITC-E2, an anti-fluorescein single-chain variable fragment (scFv). We find that these can disrupt the protein structure and diminish antigen binding, and further that both structure and function can be rescued by addition of indole to complement the deleted side chain. While the magnitude of the affinity difference triggered by indole is modest in this first model system, it nonetheless provides a framework for future mutation/ligand pairs that may induce more dramatic responses. Disrupting and subsequently rescuing antibody activity, as exemplified by this first example, may represent a new approach to "design in" fine-tuned control of antibody activity for a variety of future applications.

Antibody engineering of a cytotoxic monoclonal antibody 84 against human embryonic stem cells: Investigating the effects of multivalency on cytotoxicity

Antibody fragments have shown targeted specificity to their antigens, but only modest tissue retention times in vivo and in vitro. Multimerization has been used as a protein engineering tool to increase the number of binding units and thereby enhance the efficacy and retention time of antibody fragments. In this work, we explored the effects of valency using a series of self-assembling polypeptides based on the GCN4 leucine zipper multimerization domain fused to a single-chain variable fragment via an antibody upper hinge sequence. Four engineered antibody fragments with a valency from one to four antigen-binding units of a cytotoxic monoclonal antibody 84 against human embryonic stem cells (hESC) were constructed. We hypothesized that higher cytotoxicity would be observed for fragments with increased valency. Flow cytometry analysis revealed that the trimeric and tetrameric engineered antibody fragments resulted in the highest degree of cytotoxicity to the undifferentiated hESC, while the engineered antibody fragments were observed to have improved tissue penetration into cell clusters. Thus, a trade off was made for the trimeric versus tetrameric fragment due to improved tissue penetration. These results have direct implications for antibody-mediated removal of undifferentiated hESC during regenerative medicine and cell therapy.

Insertion of single-chain variable fragment (scFv) peptide linker improves surface display of influenza hemagglutinin (HA1) on non-recombinant Lactococcus lactis

Heterologous protein displayed on the surface of Lactococcus lactis using the binding domain of N-acetylmuramidase (AcmA) has a potential application in vaccine delivery. In this study, we developed a non-recombinant L. lactis surface displaying the influenza A (H1N1) 2009 hemagglutinin (HA1). Three recombinant proteins, HA1/L/AcmA, HA1/AcmA, and HA1 were overexpressed in Escherichia coli, and purified. In the binding study using flow cytometry, the HA1/L/AcmA, which contained the single-chain variable fragment (scFv) peptide linker showed significantly higher percentage of binding counts and mean fluorescence binding intensity (MFI) (51.7 ± 1.4% and 3,594.0 ± 675.9, respectively) in comparison to the HA1/AcmA without the scFv peptide linker (41.1 ± 1.5% and 1,652.0 ± 34.1, respectively). Higher amount of HA1/L/AcmA (∼2.9 × 10⁴ molecules per cell) was displayed on L. lactis when compared to HA1/AcmA (∼1.1 × 10⁴ molecules per cell) in the immunoblotting analysis. The HA1/L/AcmA completely agglutinated RBCs at comparable amount of protein to that of HA1/AcmA and HA1. Computational modeling of protein structures suggested that scFv peptide linker in HA1/L/AcmA kept the HA1 and the AcmA domain separated at a much longer distance in comparison to HA1/AcmA. These findings suggest that insertion of the scFv peptide linker between HA1 and AcmA improved binding of recombinant proteins to L. lactis. Hence, insertion of scFv peptide linker can be further investigated as a potential approach for improvement of heterologous proteins displayed on the surface of L. lactis using the AcmA binding domain. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 33:154-162, 2017.

Engineering tandem single-chain Fv as cell surface reporters with enhanced properties of fluorescence detection

A recently described fluorescence biosensor platform utilizes single-chain Fv (scFvs) that selectively bind and activate fluorogen molecules. In this report we investigated the display of tandem scFv biosensors at the surface of mammalian cells with the aim of advancing current fluorescence detection strategies. We initially screened different peptide linkers to separate each scFv unit, and discovered that tandem proteins joined by either flexible or α-helical linkers properly fold and display at the surface of mammalian cells. Accordingly, we performed a combinatorial scFv-dimer study and identified that fluorescence activation correlated with the cellular location (membrane distal versus proximal) and selections of the different scFvs. Furthermore, in vitro measurements showed that the stability of each scFv monomer unit influenced the folding and cell surface activities of tandem scFvs. Additionally, we investigated the absence or poor signals from some scFv-dimer combinations and discovered that intramolecular and intermolecular scFv chain mispairings led to protein misfolding and/or secretory-pathway-mediated degradation. Furthermore, when tandem scFvs were utilized as fluorescence reporter tags with surface receptors, the biosensor unit and target protein showed independent activities. Thus, the live cell application of tandem scFvs permitted advanced detection of target proteins via fluorescence signal amplification, Förster resonance energy transfer resulting in the increase of Stokes shift and multi-color vesicular traffic of surface receptors.

The coming of age of engineered multivalent antibodies

The development of monoclonal antibody (mAb) technology has had a profound impact on medicine. The therapeutic use of first-generation mAb achieved considerable success in the treatment of major diseases, including cancer, inflammation, autoimmune, cardiovascular, and infectious diseases. Next-generation antibodies have been engineered to further increase potency, improve the safety profile and acquire non-natural properties, and constitute a thriving area of mAb research and development. Currently, a variety of alternative antibody formats with modified architectures have been generated and are moving fast into the clinic. In fact, the bispecific antibody blinatumomab was the first in its class to be approved by the US Food and Drug Administration (FDA) as recently as December 2014. Here, we outline the fundamental strategies used for designing the next generation of therapeutic antibodies, as well as the most relevant results obtained in preclinical studies and clinical trials.

Engineering antibodies for cancer therapy

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

Engineering aggregation-resistant antibodies

The ability of antibodies to bind to target molecules with high affinity and specificity has led to their widespread use in diagnostic and therapeutic applications. Nevertheless, a limitation of antibodies is their propensity to self-associate and aggregate at high concentrations and elevated temperatures. The large size and multidomain architecture of full-length monoclonal antibodies have frustrated systematic analysis of how antibody sequence and structure regulate antibody solubility. In contrast, analysis of single and multidomain antibody fragments that retain the binding activity of mono-clonal antibodies has provided valuable insights into the determinants of antibody aggregation. Here we review advances in engineering antibody frameworks, domain interfaces, and antigen-binding loops to prevent aggregation of natively and nonnatively folded antibody fragments. We also highlight advances and unmet challenges in developing robust strategies for engineering large, multidomain antibodies to resist aggregation.

Cloning single-chain antibody fragments (ScFv) from hyrbidoma cells

Despite the rising impact of the generation of antibodies by phage display and other technologies, hybridoma technology still provides a valuable tool for the generation of high-affinity binders against different targets. But there exist several limitations of using hybridoma-derived antibodies. The source of the hybridoma clones are mostly rat or mouse B-lymphocytes. Therefore a human-anti-mouse or human-anti-rat antibody response may result in immunogenicity of these antibodies. This leads to the necessity of humanization of these antibodies where the knowledge of the amino acid sequence of the proteins is inalienable. Furthermore, additional in vitro modifications, e.g., affinity maturation or fusion to other proteins, are dependent on cloning of the antigen-binding domains.Here we describe the isolation of RNA from hybridoma cells and the primers that can be used for the amplification of VL and VH as well as the cloning of the antibody in scFv format and its expression in Escherichia coli.

Continuing pursuit for ideal systemic anticancer radiotherapeutics

Cancer is one of the major causes of death for non-transmissible chronic diseases worldwide. Conventional treatments including surgery, chemotherapy and external beam radiotherapy are generally far from curative. Complementary therapies are attempted for achieving more successful treatment response. Systemic targeted radiotherapy (STR) is a radiotherapeutic modality based on systemic administration of radioactive agents for selectively delivering high doses of energy to destroy cancer cells. For this purpose, diverse tumour-target specific agents including monoclonal antibodies (MoAb), MoAb fragments and peptides have been tested and some of them have already got FDA approval for clinical use. However, MoAbs and their tailored analogues have shown non-homogeneous tumour distribution, limited diffusion, insufficient intratumoral accumulation and retention, unwanted uptake in normal tissues and scarcity of identified cancer antigens for generating new MoAbs. Similarly, peptides have also exhibited retention in normal organs, lacks of favourable membrane permeability or drug cell internalization and short-term residence in cancer cells. Recently, a new category of target-specific agent with strong affinity for necrosis has emerged as an excellent option for developing targeted radiotherapeutic agents to be used after necrosis-inducing treatments (NITs). The combination of their high, specific and long-term accumulation and retention at necrotic sites with the crossfire effect of ionizing particle-emitters allows irradiating adjacent residual viable tumour cells during a prolonged period of time. It may considerably enhance the therapeutic response and open a new horizon for improved cancer treatability or curability.

Improving the characteristics of a mycobacterial 16 kDa-specific chicken scFv

Recombinant antibodies can be engineered to improve their binding or other characteristics. A chicken single chain variable fragment (scFv) phage display library was panned against the mycobacterial 16 kDa antigen. Three fusion phages which bound specifically to the antigen were selected, each of which produced low signals in ELISA when secreted as a soluble scFv. One scFv was therefore chosen to be modified in an attempt to improve its binding. Firstly, a mutant sublibrary was created by random mutagenesis. High stringency panning of this sublibrary yielded binders which produced ELISA signals up to eleven times higher than the parent scFv. An increase in the intrinsic affinity was confirmed by surface plasmon resonance. Secondly, the flexible linker between the heavy and light chains of the parent scFv was either shortened to one glycine residue or deleted entirely. No ELISA signal was obtained when the linker was absent, but the glycine-linked scFv showed enhanced binding. Size exclusion chromatography revealed that the enhanced binder had aggregated to form tetramers. This study confirms that the strategies used to improve the binding of human and mouse scFvs can also enhance chicken scFvs.

Chemically self-assembled antibody nanorings (CSANs): design and characterization of an anti-CD3 IgM biomimetic

A number of clever recombinant methodologies have been developed that recapitulate the valencies of IgG's (bivalent) and IgA's (tetravalent). Although higher synthetic valencies have been achieved by conjugation of either monoclonal antibodies or single-chain antibodies to nanoparticles and liposomes, a method for the preparation of recombinant antibodies with valencies similar to IgM's (decavalent) but considerably less than what is generally found after antibody particle conjugation has yet to be devised. Recently, we have developed a methodology for the design of bivalent Chemically Self-Assembled Antibody Nanorings (CSANs). We now report the crystal structure of the nanoring subunit composed of the E. coli DHFR dimer and a methotrexate dimerizer (MTX2-C9) containing a visible nine methylene linker and a protocol for the preparation of CSANs from this subunit with valencies similar to IgM's, ranging from 8-10 single chain antibodies (scFvs). The multivalent CSANs were reversibly assembled from a fusion protein dihydrofolate reductase (DHFR)-DHFR-antiCD3 scFv containing a single glycine linker between the two DHFR scaffolding proteins. We also demonstrate that, similar to the parental bivalent anti-CD3 monoclonal antibody (mAB), anti-CD3 CSANs selectively bind to CD3+ leukemia cells and undergo rapid internalization through a caveolin-independent pathway that requires cholesterol, actin polymerization, and protein tyrosine kinase activation. While treatment with the monoclonal antibody leads to T-cell activation and nearly complete loss (i.e., 90%) of the surface displayed T-cell receptor (TCR), only 25-30% of the TCR down regulate and no significant T-cell proliferation is observed after treatment of peripheral blood mononuclear cells (PBMCs) with anti-CD3 CSANs. Consistent with the proliferation findings, 15-25% less CD25 (IL-2 receptor) was found on the surface of PBMCs treated with either the polyvalent or bivalent anti-CD3 CSANs, respectively, than on PBMCs treated with the parental mAB. Comparative experiments with F(ab')2 derived from the mAB confirm that the activation of the T-cells by the mAB is dependent on the Fc domain, and thus interactions of the PBMC T-cells with accessory cells, such as macrophages. Taken together, our results demonstrate that anti-CD3 CSANs with valencies ranging from 2 to 8 could be employed for radionuclide, drug, or potentially oligonucleotide delivery to T-cells without, as has been observed for other antibody conjugated nanoparticles, the deleterious effects of activation observed for mAB. Further the CSAN construct may be adapted for the preparation of other multivalent scFvs.

PH1-derived bivalent bibodies and trivalent tribodies bind differentially to shed and tumour cell-associated MUC1

Most adenocarcinomas express altered MUC1 as a tumour-associated antigen. Due to suboptimal glycosylation in tumour-associated MUC1, the apomucin core is exposed, revealing new epitopes for antibody-directed immunotherapy. The human PH1 Fab binds specifically to this MUC1 apomucin. We describe the engineering and functional characterization of bi- and trivalent recombinant antibody derivatives from the PH1 Fab. Bi- and tribodies were made using the disulfide-stabilized Fab fragment as a heterodimerization scaffold with PH1 single-chain variable fragments fused to either one or both Fab-chain C-termini. Immunoassays revealed 27- and 165-fold improved dissociation constants (K(D) = 30 and 5 nM) of the PH1 bi- and tribodies compared with the parental Fab (K(D) = 820 nM). Unexpectedly, major differences were seen in the ability of the antibody constructs to bind shed and tumour cell-tethered MUC1. While the tribody did not discriminate between both MUC1 forms, the bibody demonstrated preferential interaction with membrane-bound MUC1 compared with shed MUC1. This preferential recognition of membrane-bound MUC1, along with the high serum stability of the bibody, its intermediate size and efficient internalization by MUC1(+) cells, makes the human PH1-derived bibody a valuable candidate as a cancer-targeting therapeutic.

Multivalent antibodies: when design surpasses evolution

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

Oligomeric forms of single chain immunoglobulin (scIgG)

Assembly of immunoglobulin G (IgG) molecules from two heavy and two light chains can be facilitated by connecting the light chain to the heavy chain by an oligopeptide linker. Production of the anti-lysozyme D1.3-single chain (sc) IgG1 in HEK293T cells yielded up to 8 mg/L functional scIgG polypeptide. Size exclusion chromatography of material purified by protein-A affinity chromatography revealed that the majority of the D1.3-scIgG1 molecules were 150 kDa monomers, with a K(D) of 1.8 x 10(-10) M measured by surface plasmon resonance; however, significant fractions of scIgG dimers and oligomers with molecular masses of 300 kDa and >600 kDa, respectively, were identified. The oligomerization resulted in an increased avidity. The observed oligomerization capability may allow new approaches for the generation of bispecific/multivalent antibodies.

The design and characterization of oligospecific antibodies for simultaneous targeting of multiple disease mediators

Monoclonal antibodies are traditionally used to block the function of a specific target in a given disease. However, some diseases are the consequence of multiple components or pathways and not the result of a single mediator; thus, blocking at a single point may not optimally control disease. Antibodies that simultaneously block the functions of two or more disease-associated targets are now being developed. Herein, we describe the design, expression, and characterization of several oligospecific antibody formats that are capable of binding simultaneously to two or three different antigens. These constructs were generated by genetically linking single-chain Fv fragments to the N-terminus of the antibody heavy and light chains and to the C-terminus of the antibody C(H)3 domain. The oligospecific antibodies were expressed in mammalian cells, purified to homogeneity, and characterized for binding to antigens, Fcgamma receptors, FcRn, and C1q. In addition, the oligospecific antibodies were assayed for effector function, protease susceptibility, thermal stability, and size distribution. We demonstrate that these oligospecific antibody formats maintain high expression level, thermostability, and protease resistance. The in vivo half-life, antibody-dependent cellular cytotoxicity function, and binding ability to Fcgamma receptors and C1q of the test oligospecific antibodies remain similar to the corresponding properties of their parental IgG antibodies. The excellent expression, biophysical stability, and potential manufacturing feasibility of these multispecific antibody formats suggest that they will provide a scaffold template for the construction of similar molecules to target multiple antigens in complex diseases.

Multimerization domains for antibody phage display and antibody production

High-throughput generation of antibodies for proteome research has become feasible by using antibody gene libraries and in vitro selection methods like phage display. Typically monovalent antibody fragments like scFv, Fab or scFab are obtained by this technology. To mimic the IgG molecule and gain avidity, resulting in stronger binding, multimerization domains can be fused to antibody fragments. Here we systematically analyzed different multimerization domains in respect to three key parameters, crucial for the high-throughput generation of binders. (i) The compatibility to be displayed on phage (assessed for at least three different antibody formats, scFv, Fab and scFab) in combination with five different multimerization domains; (ii) production yields and (iii) oligomerization properties were analyzed for three different scFv fragments. We found that the use of a biotin acceptor domain in combination with an in vivo biotinylation system performed best concerning the key parameters and thus would be a useful tool to generate multimeric antibody complexes on demand from phage display selected antibody fragments with the least effort.

Applications of single-chain variable fragment antibodies in therapeutics and diagnostics

Antibodies (Abs) are some of the most powerful tools in therapy and diagnostics and are currently one of the fastest growing classes of therapeutic molecules. Recombinant antibody (rAb) fragments are becoming popular therapeutic alternatives to full length monoclonal Abs since they are smaller, possess different properties that are advantageous in certain medical applications, can be produced more economically and are easily amendable to genetic manipulation. Single-chain variable fragment (scFv) Abs are one of the most popular rAb format as they have been engineered into larger, multivalent, bi-specific and conjugated forms for many clinical applications. This review will show the tremendous versatility and importance of scFv fragments as they provide the basic antigen binding unit for a multitude of engineered Abs for use as human therapeutics and diagnostics.

Multivalency: the hallmark of antibodies used for optimization of tumor targeting by design

High-precision tumor targeting with conventional therapeutics is based on the concept of the ideal drug as a "magic bullet"; this became possible after techniques were developed for production of monoclonal antibodies (mAbs). Innovative DNA technologies have revolutionized this area and enhanced clinical efficiency of mAbs. The experience of applying small-size recombinant antibodies (monovalent binding fragments and their derivatives) to cancer targeting showed that even high-affinity monovalent interactions provide fast blood clearance but only modest retention time on the target antigen. Conversion of recombinant antibodies into multivalent format increases their functional affinity, decreases dissociation rates for cell-surface and optimizes biodistribution. In addition, it allows the creation of bispecific antibody molecules that can target two different antigens simultaneously and do not exist in nature. Different multimerization strategies used now in antibody engineering make it possible to optimize biodistribution and tumor targeting of recombinant antibody constructs for cancer diagnostics and therapy.

Engineering peptide linkers for scFv immunosensors

Using A10B single-chain fragment variable (scFv) as a model system, we demonstrated that the flexibility of scFv linker engineering can be combined with the inherent quick and adaptable characters of surface coupling chemistry (e.g., electrostatic, hydrogen bonding, or covalent attachment) to attach scFv to preformed functionalized self-assembled monolayers (SAMs). Six arginines, which were separated by glycine or serine as spacer, were incorporated in the peptide linker to form a 15-mer peptide linker (RGRGRGRGRSRGGGS). The polycationic arginine peptide was engineered into the A10B scFv-RG3 to favor its adsorption at anionic charged template surface (11-mercaptoundecanoic acid (MUA) and poly(sodium 4-styrenesulfonate (PSS))). This new approach was compared with the other engineered scFv constructs. Our results demonstrated that the anionic charged SAM template facilitated the oriented immobilization of scFvs on the SAM template surface as well as reduced the possibility of protein denaturation when directly immobilized on the solid surface. A 42-fold improvement of detection limits using MUA/A10B scFv-RG3 (less than 0.2 nM experimentally determined) was achieved compared to A10B Fab antibody and a 5-fold improvement was observed compared to A10B scFv that was engineered with a cysteine in the linker sequence. Using protein A-coated gold nanoparticles, a picomolar experimental detection limit was achieved. With 20 amino acids to choose from, engineered recombinant scFv in combination with SAM technology and nanoparticle mass amplification provide an emerging strategy for the development of highly sensitive and specific scFv immunosensors.

A physiologically based pharmacokinetic (PBPK) model to characterize and predict the disposition of monoclonal antibody CC49 and its single chain Fv constructs

Optimization of the use of monoclonal antibodies (MAbs) as diagnostic tools and therapeutic agents in the treatment of cancer is aided by quantitative characterization of the transport and tissue disposition of these agents in whole animals. This characterization may be effectively achieved by the application of physiologically based pharmacokinetic (PBPK) models. The purpose of this study was to develop a PBPK model to characterize the biodistribution of the pancarcinoma MAb CC49 IgG in normal and neoplastic tissues of nude mice, and to further apply the model to predict the disposition of multivalent single chain Fv (scFv) constructs in mice. Since MAbs are macromolecules, their transport is membrane-limited and a two-pore formalism is employed to describe their extravasation. The influence of binding of IgG to the protective neonatal Fc receptor (FcRn) on its disposition is also accounted for in the model. The model successfully described (131)I-CC49 IgG concentrations in blood, tumor and various organs/tissues in mice. Sensitivity analysis revealed the rate of transcapillary transport to be a critical determinant of antibody penetration and localization in the tumor. The applicability of the model was tested by predicting the disposition of di- and tetravalent scFv constructs of CC49 in mice. The model gave reasonably good predictions of the disposition of the scFv constructs. Since the model employs physiological parameters, it can be used to scale-up mouse biodistribution data to predict antibody distribution in humans. Therefore, the clinical utility of the model was tested with data for (131)I-CC49 obtained in patients, by scaling up murine parameter values according to known empirical relationships. The model gave satisfactory predictions of CC49 disposition and tumor uptake in man.

Single chain Fab (scFab) fragment

Background

The connection of the variable part of the heavy chain (VH) and and the variable part of the light chain (VL) by a peptide linker to form a consecutive polypeptide chain (single chain antibody, scFv) was a breakthrough for the functional production of antibody fragments in Escherichia coli. Being double the size of fragment variable (Fv) fragments and requiring assembly of two independent polypeptide chains, functional Fab fragments are usually produced with significantly lower yields in E. coli. An antibody design combining stability and assay compatibility of the fragment antigen binding (Fab) with high level bacterial expression of single chain Fv fragments would be desirable. The desired antibody fragment should be both suitable for expression as soluble antibody in E. coli and antibody phage display.

Results

Here, we demonstrate that the introduction of a polypeptide linker between the fragment difficult (Fd) and the light chain (LC), resulting in the formation of a single chain Fab fragment (scFab), can lead to improved production of functional molecules. We tested the impact of various linker designs and modifications of the constant regions on both phage display efficiency and the yield of soluble antibody fragments. A scFab variant without cysteins (scFabΔC) connecting the constant part 1 of the heavy chain (CH1) and the constant part of the light chain (CL) were best suited for phage display and production of soluble antibody fragments. Beside the expression system E. coli, the new antibody format was also expressed in Pichia pastoris. Monovalent and divalent fragments (DiFabodies) as well as multimers were characterised.

Conclusion

A new antibody design offers the generation of bivalent Fab derivates for antibody phage display and production of soluble antibody fragments. This antibody format is of particular value for high throughput proteome binder generation projects, due to the avidity effect and the possible use of common standard sera for detection.

Therapeutic Expression of an Anti-Death Receptor 5 Single-Chain Fixed-Variable Region Prevents Tumor Growth in Mice

The clinical use of the single-chain fixed-variable (scFv) fragments of recombinant monoclonal antibodies as credible alternatives for classic therapeutic antibodies has two limitations: rapid blood clearance and inefficient local expression of functional molecules. In attempt to address these issues, we have developed a novel gene therapy protocol in which the anti-death receptor 5 (DR5) scFv fragments were either in vitro expressed in several tumor cell lines, or in vivo expressed in mice, using recombinant adeno-associated virus (rAAV)-mediated gene transfer. Viral transduction using the rAAV-S3C construct, which encodes a scFv molecule (S3C scFv) specific to DR5, led to stable expression in tumor cell lines and showed apoptosis-inducing activity in vitro, which could be inhibited by recombinant DR5 but not by DR4. A single i.m. injection of rAAV-S3C virus in nude mice resulted in stable expression of DR5-binding S3C scFv proteins in mouse sera for at least 240 days. Moreover, the expression of S3C scFv was associated with significant suppression of tumor growth and the increase of tumor cell apoptosis in previously established s.c. human lung LTEP-sml and liver Hep3B tumor xenografts.

Basic immunology of antibody targeted radiotherapy

Antibody targeted radiotherapy brings an important new treatment modality to the radiation oncology clinic. Radiation dose to tumor and normal tissues are determined by a complex interplay of antibody, antigen, tumor, radionuclide, and host-related factors. A basic understanding of these immunologic and physiologic factors is important to optimally utilize this therapy in the clinic. Preclinical and clinical studies need to be continued to broaden our understanding and to develop new strategies to further improve the efficacy of this promising form of targeted therapy.

Systemic targeted radionuclide therapy: potential new areas

Radiation oncology is entering an exciting new era with therapies being delivered in a targeted fashion through an increasing number of novel approaches. External beam radiotherapy now integrates functional and anatomic tumor imaging to guide delivery of conformal radiation to the tumor target. Systemic targeted radionuclide therapy (STaRT) adds an important new dimension by making available to the radiation oncologist biologically targeted radiation therapy. Impressive clinical results with antibody-targeted radiotherapy, leading to the Food and Drug Administration's approval of two anti-CD20 radiolabeled antibodies, highlight the potential of STaRT. Optimization strategies will further improve the efficacy of STaRT by improving delivery systems, modifying the tumor microenvironment to increase targeted dose, and maximizing dose effect. Ultimately, the greatest potential for STaRT will not be as monotherapy, but as therapy integrated into established multimodality regimens and used as adjuvant or consolidative therapy in patients with minimal or micrometastatic disease.

Construction and Purification of a Covalently Linked Divalent Tandem Single-Chain Fv Antibody Against Placental Alkaline Phosphatase

Multivalency is a recognized means to increase the functional affinity of single-chain antibody fragments (scFvs) for optimizing tumor uptake at radioimmunotargeting. A unique divalent tandem single-chain Fv antibody [sc(Fv)2], based on the variable regions of the monoclonal antibody (MAb) H7, has now been generated. The antibody differs from other dimeric single-chain constructs in that a linker sequence (L) is introduced between the repeats of VL and VH domains (VL-L-VH-L-VL-L-VH). This construct was expressed as a His-tagged TrxA fusion protein in the Escherichia coli strain Origami B. Following cleavage with AcTev protease, the antibody was obtained in soluble and active form in E. coli and could be purified by Ni-NTA and cation-exchange chromatography. Purity and immunochemical properties were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), enzyme-linked immunosorbent assay (ELISA), Western blot, and Biacore analyses. The [sc(Fv)2] displayed proper stability and could be purified to homogeneity. This antibody also maintained immunoreactivity at 42 degrees C with only slight decrease at 52 degrees C. The high affinity displayed by the original antibody H7, 6.7 x 10(9) M(-1), was only slightly decreased to 1.2 x 10(9) M(-1) as determined by Biacore. The generation of such a divalent single-chain Fv with a molecular weight around 60 kd would be of value for clinical applications such as radioimmunolocalization and radioimmunotherapy.

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.

Engineered antibody fragments and the rise of single domains

With 18 monoclonal antibody (mAb) products currently on the market and more than 100 in clinical trials, it is clear that engineered antibodies have come of age as biopharmaceuticals. In fact, by 2008, engineered antibodies are predicted to account for >30% of all revenues in the biotechnology market. Smaller recombinant antibody fragments (for example, classic monovalent antibody fragments (Fab, scFv)) and engineered variants (diabodies, triabodies, minibodies and single-domain antibodies) are now emerging as credible alternatives. These fragments retain the targeting specificity of whole mAbs but can be produced more economically and possess other unique and superior properties for a range of diagnostic and therapeutic applications. Antibody fragments have been forged into multivalent and multi-specific reagents, linked to therapeutic payloads (such as radionuclides, toxins, enzymes, liposomes and viruses) and engineered for enhanced therapeutic efficacy. Recently, single antibody domains have been engineered and selected as targeting reagents against hitherto immunosilent cavities in enzymes, receptors and infectious agents. Single-domain antibodies are anticipated to significantly expand the repertoire of antibody-based reagents against the vast range of novel biomarkers being discovered through proteomics. As this review aims to show, there is tremendous potential for all antibody fragments either as robust diagnostic reagents (for example in biosensors), or as nonimmunogenic in vivo biopharmaceuticals with superior biodistribution and blood clearance properties.

Controlled-release and local delivery of therapeutic antibodies

Human and humanised antibodies are now poised to become a major new class of protein-based therapeutic agents. A significant fraction of new drugs in clinical testing (approximately 20% in 2002) are antibody classes. Monoclonal antibodies (mAbs) with high affinities against newly discovered disease targets, both cellularly and extracellularly, are now clinically proven to elicit high bioactivities against numerous diseases, including tumours, infections, asthma, inflammation, arthritis and osteoporosis. Clinical humanised antibody delivery is typically intravenous, with large multiple doses (grams) required for systemic volumes of distribution. Due to the relatively high costs of both this drug type, and its common mode of administration, alternatives are sought where doses might be reduced and the bioavailability and efficacy enhanced. Local, controlled-release methods that deliver antibodies locally to site of disease, offer new possibilities with these potential advantages. However, protein drugs frequently exhibit formulation challenges when packaged in delivery vehicles, and as globular proteins, antibodies are no exception. Several examples of mAb controlled-release and local delivery strategies against several disease targets are reviewed. Importantly, several antibody delivery methods work in tandem with existing clinically-accepted therapeutics, sometimes exhibiting potentiating or synergistic effects in animal models with small molecule, systemically administered drugs.

Antigen binding and stability properties of non-covalently linked anti-CD22 single-chain Fv dimers

By varying linker length and domain orientation three multivalent derivatives of a monovalent anti-CD22 single-chain fragment variable (scFv) antibody were generated. Shortening the linker of the V(H)-V(L) oriented scFv to 5 or 0 residues resulted in the formation of diabodies or a mixture of tetramers and trimers, respectively. Unexpectedly, a V(L)-0-V(H) scFv assembled to homogenous dimers, remained substantially more stable than the V(H)-5-V(L) diabody when incubated in human serum at 37 degrees C, and retained its dimeric state when concentrated up to 4 mg/ml. These properties suggest the V(L)-0-V(H) scFv could become an attractive vehicle for the selective delivery of multiple effector molecules to CD22(+) tumor cells.

Suppression of human cytochrome P450 aromatase activity by monoclonal and recombinant antibody fragments and identification of a stable antigenic complex

Human cytochrome P450 aromatase (P450arom) is responsible for biosynthesis of estrogens from androgens. Monoclonal antibody MAb3-2C2 to P450arom specifically binds to a conformational epitope and suppresses the enzyme activity in a dose-dependent manner. The crystal structure of the Fab fragment of MAb3-2C2 has been used to engineer a recombinant single chain antibody fragment (scFv) and a homodimeric variable domain of the light chain (VL(2)). These recombinant antibody fragments have been expressed in Escherichia coli and purified. Here, we show that the recombinant scFv suppresses P450arom activity with an IC(50) value similar to that of natural MAb3-2C2 F(ab')(2). The recombinant VL(2) also exhibits dose-dependent suppression of the P450arom activity, but at a reduced level, demonstrating that the homodimer is unable to fully mimic the complementarity determining region (CDR) of a variable heavy chain (VH)-VL heterodimer. We prepare and purify a stable complex of P450arom with MAb3-2C2 F(ab')(2) and show that the complex migrates and precipitates as a single molecular assembly. Efforts to crystallize P450arom for structure-function studies have yielded small single crystals. Our results suggest that formation of stable complexes with fragments of the monoclonal antibody could provide an alternative method for crystallization of P450arom.

Structure of an Influenza Neuraminidase–Diabody Complex by Electron Cryomicroscopy and Image Analysis

The structure of a complex between a bivalent diabody and its antigen, influenza neuraminidase, has been determined by electron cryomicroscopy of single particles and image analysis. A three-dimensional reconstruction has been interpreted in terms of high-resolution X-ray models of the component proteins. The complex consists of two neuraminidase tetramers cross-linked by four diabodies with 422 point symmetry. The structure and symmetry of the complex is determined uniquely by packing constraints consistent with the maximum possible number of diabody cross-links. Diabodies may provide a useful approach to the structure determination of small proteins by incorporating the proteins into large symmetric complexes followed by single-particle electron microscopy.

Isolation and expression of recombinant antibody fragments to the biological warfare pathogen Brucella melitensis

Brucella melitensis is a highly infectious animal pathogen able to cause a recurring debilitating disease in humans and is therefore high on the list of biological warfare agents. Immunoglobulin genes from mice immunized with gamma-irradiated B. melitensis strain 16M were used to construct a library that was screened by phage display against similarly prepared bacteria. The selected phage particles afforded a strong enzyme-linked immunosorbent assay (ELISA) signal against gamma-irradiated B. melitensis cells. However, extensive efforts to express the respective single chain antibody variable region fragment (scFv) in soluble form failed due to: (i) poor solubility and (ii) in vivo degradation of the c-myc tag used for the detection of the recombinant antibodies. Both problems could be addressed by: (i) fusing a human kappa light chain constant domain (Ck) chain to the scFv to generate single chain antibody fragment (scAb) antibody fragments and (ii) by co-expression of the periplasmic chaperone Skp. While soluble, functional antibodies could be produced in this manner, phage-displaying scFvs or scAbs were still found to be superior ELISA reagents for immunoassays, due to the large signal amplification afforded by anti-phage antibodies. The isolated phage antibodies were shown to be highly specific to B. melitensis and did not recognize Yersinia pseudotuberculosis in contrast to the existing diagnostic monoclonal YST 9.2.1.

Noncovalent scFv multimers of tumor-targeting anti-Lewis(y) hu3S193 humanized antibody

Single-chain variable fragments (scFvs) of anti-Lewis(y) hu3S193 humanized antibody were constructed by joining the V(H) and V(L) domains with either +2 residues, +1 residue, or by directly linking the domains. In addition two constructs were synthesized in which one or two C-terminal residues of the V(H) domain were removed (-1 residue, -2 residue) and then joined directly to the V(L) domain. An scFv construct in the reverse orientation with the V(L) joined directly to the V(H) domain was also synthesized. Upon transformation into Escherichia coli all scFv constructs expressed active protein. Binding activity, multimeric status, and multivalent properties were assessed by flow cytometry, size exclusion chromatography, and biosensor analysis. The results for hu3S193 scFvs are consistent with the paradigm that scFvs with a linker of +3 residues or more associate to form a non-covalent dimer, and those with a shorter linker or directly linked associate predominantly to form a non-covalent trimer and tetramer that are in equilibrium. While the association of V domains to form either a dimer or trimer/tetramer is governed by the length of the linker, the stability of the trimer/tetramer in the equilibrium mixture is dependent on the affinity of the interaction of the individual V domains to associate to form the larger Fv module.

Therapeutic antibodies for human diseases at the dawn of the twenty-first century

Antibodies are highly specific, naturally evolved molecules that recognize and eliminate pathogenic and disease antigens. The past 30 years of antibody research have hinted at the promise of new versatile therapeutic agents to fight cancer, autoimmune diseases and infection. Technology development and the testing of new generations of antibody reagents have altered our view of how they might be used for prophylactic and therapeutic purposes. The therapeutic antibodies of today are genetically engineered molecules that are designed to ensure high specificity and functionality. Some antibodies are loaded with toxic modules, whereas others are designed to function naturally, depending on the therapeutic application. In this review, we discuss various aspects of antibodies that are relevant to their use as as therapeutic agents.

Biochips beyond DNA: technologies and applications

Technological advances in miniaturization have found a niche in biology and signal the beginning of a new revolution. Most of the attention and advances have been made with DNA chips yet a lot of progress is being made in the use of other biomolecules and cells. A variety of reviews have covered only different aspects and technologies but leading to the shared terminology of "biochips." This review provides a basic introduction and an in-depth survey of the different technologies and applications involving the use of non-DNA molecules such as proteins and cells. The review focuses on microarrays and microfluidics, but also describes some cellular systems (studies involving patterning and sensor chips) and nanotechnology. The principles of each technology including parameters involved in biochip design and operation are outlined. A discussion of the different biological and biomedical applications illustrates the significance of biochips in biotechnology.

Biomedical applications of protein chips

The development of microchips involving proteins has accelerated within the past few years. Although DNA chip technologies formed the precedent, many different strategies and technologies have been used because proteins are inherently a more complex type of molecule. This review covers the various biomedical applications of protein chips in diagnostics, drug screening and testing, disease monitoring, drug discovery (proteomics), and medical research. The proteomics and drug discovery section is further subdivided to cover drug discovery tools (on-chip separations, expression profiling, and antibody arrays), molecular interactions and signaling pathways, the identification of protein function, and the identification of novel therapeutic compounds. Although largely focused on protein chips, this review includes chips involving cells and tissues as a logical extension of the type of data that can be generated from these microchips.

Large-scale bacterial fermentation and isolation of scFv multimers using a heat-inducible bacterial expression vector

This protocol describes optimised large-scale bacterial fermentation conditions for recombinant single-chain Fv molecule (scFv) monomers and multimers (diabodies and triabodies). The heat-inducible bacterial secretion vector, pPOW3, utilising the temperature-regulated tandem lambda promoters is particularly suited to the large-scale fermentation of single-chain antibodies, providing low-cost recombinant protein synthesis. The protein expressed by this vector is secreted into the periplasm where it is found as both the soluble and insoluble protein that is associated with the cell membranes. A protein fractionation method for the rapid extraction and affinity purification of the soluble protein fraction and the urea solubilization and refolding of the insoluble protein fraction expressed from single-chain antibody (Ab) fragment gene constructs is described. This method is simple to perform and utilises inexpensive reagents to provide cost-effective protein synthesis.

Antibody engineering

Antibodies are unique in their high affinity and specificity for a binding partner, a quality that has made them one of the most useful molecules for biotechnology and biomedical applications. The field of antibody engineering has changed rapidly in the past 10 years, fueled by novel technologies for the in vitro isolation of antibodies from combinatorial libraries and their functional expression in bacteria. This review presents an overview of the methods available for the de novo generation of human antibodies, for engineering antibodies with increased antigen affinity, and for the production of antibody fragments. Select applications of recombinant antibodies are also presented.

Multimerization of a chimeric anti-CD20 single-chain Fv-Fc fusion protein is mediated through variable domain exchange

A series of single-chain anti-CD20 antibodies was produced by fusing single-chain Fv (scFv) with human IgG1 hinge and Fc regions, designated scFv-Fc. The initial scFv-Fc construct was assembled using an 18 amino acid (aa) linker between the antibody light- and heavy-chain variable regions, with the Cys residue in the upper hinge region (Kabat 233) mutagenized to Ser. Anti-CD20 scFv-Fc retained specific binding to CD20-positive cells and was active in mediating complement-dependent cytolysis. Size-exclusion HPLC analysis revealed that the purified scFv-Fc included multimeric as well as monomeric components. Variant scFv-Fcs were constructed incorporating four different hinges between the scFv and Fc regions, or three different linkers in the scFv domain. All formed multimers, with the highest level of multimerization found in the scFv-Fc with the shortest linker (8 aa). Elimination of an unusual salt bridge between residues L38 and H89 in the V(L)-V(H) domain interface failed to reduce the formation of higher order forms. Structural analysis of the scFv-Fc constructed with 18 or 8 aa linkers by pepsin or papain cleavage suggested the proteins contained a form in which scFv units had cross-paired to form a 'diabody'. Thus, domain exchange or cross-pairing appears to be the basis of the observed multimerization.

Dimeric and trimeric antibodies: high avidity scFvs for cancer targeting

Recombinant antibody fragments can be engineered to assemble into stable multimeric oligomers of high binding avidity and specificity to a wide range of target antigens and haptens. This review describes the design and expression of diabodies (dimers), triabodies (trimers) and tetrabodies (tetramers). In particular we discuss the role of linker length between V-domains and the orientation of the V-domains to direct the formation of either diabodies (60 kDa), triabodies (90 kDa) or tetrabodies (120 kDa), and how the size, flexibility and valency of each molecules is suited to different applications for in vivo imaging and therapy. Single chain Fv antibody fragments joined by polypeptide linkers of at least 12 residues irrespective of V-domains orientation predominantly form monomers with varying amounts of dimer and higher molecular mass oligomers in equilibrium. A scFv molecule with a linker of 3-12 residues cannot fold into a functional Fv domain and instead associates with a second scFv molecule to form a bivalent dimer (diabody, approximately 60 kDa). Reducing the linker length below three residues can force scFv association into trimers (triabodies, approximately 90 kDa) or tetramers ( approximately 120 kDa) depending on linker length, composition and V-domain orientation. A particular advantage for tumour targeting is that molecules of 60-100 kDa have increased tumour penetration and fast clearance rates compared with the parent Ig (150 kDa). We highlight a number of cancer-targeting scFv diabodies that have undergone successful pre-clinical trials for in vivo stability and efficacy. We also briefly review the design of multi-specific Fv modules suited to cross-link two or more different target antigens. Bi-specific diabodies formed by association of different scFv molecules have been designed as cross-linking reagents for T-cell recruitment into tumours (immunotherapy), viral retargeting (gene therapy) and as red blood cell agglutination reagents (immunodiagnostics). The more challenging trispecific multimers (triabodies) remain to be described.

Optimized linker sequences for the expression of monomeric and dimeric bispecific single-chain diabodies

Bispecific single-chain diabodies (scDb) consist of the variable heavy and light chain domains of two antibodies connected by three linkers. The structure of an scDb in the V(H)-V(L) orientation is V(H)A-linkerA-V(L)B-linkerM-V(H)B-linkerB-V(L)A, with linkers A and B routinely chosen to be 5-6 residues and linker M 15-20 residues. Here, we applied display of scDb on filamentous phage to analyse the composition of optimal linker sequences. The three linkers were randomized in length and sequence using degenerated triplets coding for only six hydrophilic or aliphatic amino acids (Thr, Ser, Asp, Asn, Gly, Ala). Antigen-binding clones were then isolated by one to two rounds of selection on the two different antigens recognized by the bispecific scDb. Using an scDb directed against carcinoembryonic antigen (CEA) and beta-galactosidase (Gal), we found that monomeric scDb had a preferred length of 15 or more amino acid residues for the middle linker M and of 3-6 residues for the linkers A and B. No obvious bias towards a preferred linker sequence was observed. Reduction of the middle linker below 13 residues led to the formation of dimeric scDb, which most likely results from interchain pairing between all the V(H) and V(L) domains. Dimeric scDb were also formed by fragments possessing a long linker M and linkers A and B of 0 or 1 residue. We assume that these dimeric scDb are formed by intrachain pairing of the central variable domains and interchain pairing of the flanking variable domains. Thus, the latter molecules represent a novel format of bispecific and tetravalent molecules. The described strategy allows for the isolation of both optimized and minimal linker sequences for the assembly of monomeric or dimeric single-chain diabodies.

Antibody constructs for radioimmunodiagnosis and treatment of human pancreatic cancer

Pancreatic cancer (PC) is a common disease that is seldom cured. Current approaches to the treatment of PC are not effective because the non-specific nature of both chemotherapy and external beam radiation results in toxicity to normal tissue. Monoclonal antibodies (MAbs) can be used as selective carriers for delivering radionuclides, toxins, or cytotoxic drugs to malignant cell populations. Therefore, MAb-technology has led to a significant amount of research in targeted therapy. Targeted therapy would generally allow the concentration of cytotoxic agents in tumors and would markedly lessen the toxicity to normal tissues, which limits the dosage and effectiveness of systemically administered drugs. A variety of MAbs are being pre-clinically evaluated for the diagnosis and treatment of PC. Novel recombinant antibody constructs hold a promising future in both the diagnosis and treatment of cancer. By genetic-engineering methods, several high affinity antibody fragments with optimum tumor targeting properties, such as higher functional affinity (divalent and multivalent scFvs) and blood residence time (good tumor localization with high radiolocalization index), have been generated. Animal models have permitted the in vivo assessment of these antibody-based reagents, therapeutic/diagnostic radionuclide, radiolabeling conditions, and efficacy of administration regimes. For PC, immunoscintigraphy using MAbs has taken new strides. The use of MAbs and their fragments for radioimmunoguided surgery and therapy of PC has shown encouraging results at preclinical levels and warrants further attention.