scFv multimers of the anti-neuraminidase antibody NC10: length of the linker between VH and VL domains dictates precisely the transition between diabodies and triabodies

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.

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.

Comparison of recombinant derivatives of chimeric TNT-3 antibody for the radioimaging of solid tumors

Although intact monoclonal antibodies (MAbs) are well suited as therapeutic reagents, their relatively slow clearance rates render them less useful for imaging applications. Over the last several years, our laboratory has developed a unique targeting approach to solid tumors that utilizes MAbs directed against DNA and its components to bind to degenerating cells and necrotic regions of tumors in a specific manner. Because these MAbs have considerable potential for the early diagnosis of cancer and for the monitoring of cytoreductive therapies, the availability of an effective imaging agent is highly desirable. To accomplish this goal, a series of genetically engineered derivatives of MAb chTNT-3 including the single-chain Fv, diabody, triabody, Fab, and F(ab')(2) were generated and expressed in NS0 myeloma cells using the Glutamine Synthetase Amplification System. Initial in vitro studies demonstrated that each of the antibody derivatives maintained its antigen binding in a stable manner. In vivo analyses after radiolabeling were then performed to evaluate their pharmacokinetic, biodistribution, and tumor-imaging properties in solid tumor-bearing mice. The results of these studies showed that compared with intact parental chTNT-3, which has a half-life of 134.2 h, the smaller derivatives were eliminated more rapidly (4.9-8.1 h). Importantly, the smaller derivatives were found to have significantly higher tumor-to-organ ratios, but lower overall uptake levels compared with parental (125)I-chTNT-3 in two different tumor models. A comparison of the five derivatives showed that the F(ab')(2) reagent consistently gave the best results in imaging and biodistribution studies. Based upon these results, further studies are warranted to demonstrate the potential of this reagent for the diagnosis and monitoring of solid tumors using noninvasive imaging techniques such as immunoscintigraphy and positron emission tomography (PET).

The production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi

In this review we will focus on the current status and views concerning the production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi. We will focus on single-chain antibody fragment production (scFv and VHH) by these lower eukaryotes and the possible applications of these proteins. Also the coupling of fragments to relevant enzymes or other components will be discussed. As an example of the fusion protein strategy, the 'magic bullet' approach for industrial applications, will be highlighted.

Single-chain Fv multimers of the anti-neuraminidase antibody NC10: the residue at position 15 in the V(L) domain of the scFv-0 (V(L)-V(H)) molecule is primarily responsible for formation of a tetramer-trimer equilibrium

Single-chain variable fragment of the murine monoclonal antibody NC10 specific to influenza virus N9 neuraminidase, joined directly in the V(L) to V(H) orientation (scFv-0), forms an equilibrium mixture of tetramer and trimer with the tetramer as the preferred multimeric species. In contrast, the V(H)-V(L) isomer was previously shown to exist exclusively as a trimer. Computer-generated trimeric and tetrameric scFv models, based on the refined crystal structure for NC10 Fv domain, were constructed and used to evaluate factors influencing the transition between V(L)-V(H) trimer and tetramer. These model structures indicated that steric restrictions between loops spanning amino acid residues L55-L59 and L13-L17 from the two adjacent V(L) domains within the V(L)-V(H) trimer were responsible for four scFv-0 molecules assembling to form a tetramer. In particular, leucine at position L15 and glutamate at position L57 appeared to interfere significantly with each other. To minimize this steric interference, the site-directed mutagenesis technique was used to construct several NC10 scFv-0 clones with mutations at these positions. Size-exclusion chromatographic analyses revealed that several of these mutations resulted in the production of NC10 scFv-0 proteins with significantly altered tetramer-trimer equilibrium ratios. In particular, introduction of a polar residue, such as asparagine or threonine, at position L15 generated a highly stable NC10 scFv-0 trimer.

Phage display technology: clinical applications and recent innovations

Phage display is a molecular diversity technology that allows the presentation of large peptide and protein libraries on the surface of filamentous phage. Phage display libraries permit the selection of peptides and proteins, including antibodies, with high affinity and specificity for almost any target. A crucial advantage of this technology is the direct link that exists between the experimental phenotype and its encapsulated genotype, which allows the evolution of the selected binders into optimized molecules. Phage display facilitates engineering of antibodies with regard to their size, valency, affinity, and effector functions. The selection of antibodies and peptides from libraries displayed on the surface of filamentous phage has proven significant for routine isolation of peptides and antibodies for diagnostic and therapeutic applications. This review serves as an introduction to phage display, antibody engineering, the development of phage-displayed peptides and antibody fragments into viable diagnostic reagents, and recent trends in display technology.

Modulation of the CD40-CD40 ligand interaction using human anti-CD40 single-chain antibody fragments obtained from the n-CoDeR phage display library

CD40 plays a central regulatory role in the immune system and antibodies able to modulate CD40 signalling may consequently have a potential in immunotherapy, in particular for treatment of lymphomas and autoimmune disease like multiple sclerosis. As a first step to achieve this goal, we describe the selection and characterization of a novel set of fully human anti-CD40 antibody fragments (scFv) from a phage display library (n-CoDeR). In order to determine their biological potential, these antibody fragments have been analysed for their ability to promote B-cell activation, rescue from apoptosis and to block the CD40-CD40 ligand (CD40L) interaction. The selected cohort of human scFv could be subcategorized, each expressing a distinct functional signature. Thus scFv were generated that induced B-cell proliferation, rescued B cells from apoptosis and blocked the CD40-CD40L interaction to different extents. In particular, one of the scFv clones (F33) had the ability to abrogate completely this interaction. The epitope recognition patterns as well as individual rate constants were also determined and the affinity was shown to vary from low to high nanomolar range. In conclusion, this panel of human anti-CD40 scFv fragments displays a number of distinct properties, which may constitute a valuable source when evaluating candidates for in vivo trials.

Functional production and characterization of a fibrin-specific single-chain antibody fragment from Bacillus subtilis: effects of molecular chaperones and a wall-bound protease on antibody fragment production

To develop an ideal blood clot imaging and targeting agent, a single-chain antibody (SCA) fragment based on a fibrin-specific monoclonal antibody, MH-1, was constructed and produced via secretion from Bacillus subtilis. Through a systematic study involving a series of B. subtilis strains, insufficient intracellular and extracytoplasmic molecular chaperones and high sensitivity to wall-bound protease (WprA) were believed to be the major factors that lead to poor production of MH-1 SCA. Intracellular and extracytoplasmic molecular chaperones apparently act in a sequential manner. The combination of enhanced coproduction of both molecular chaperones and wprA inactivation leads to the development of an engineered B. subtilis strain, WB800HM[pEPP]. This strain allows secretory production of MH-1 SCA at a level of 10 to 15 mg/liter. In contrast, with WB700N (a seven-extracellular-protease-deficient strain) as the host, no MH-1 SCA could be detected in both secreted and cellular fractions. Secreted MH-1 SCA from WB800HM[pMH1, pEPP] could be affinity purified using a protein L matrix. It retains comparable affinity and specificity as the parental MH-1 monoclonal antibody. This expression system can potentially be applied to produce other single-chain antibody fragments, especially those with folding and protease sensitivity problems.

Reversible dimer formation and stability of the anti-tumour single-chain Fv antibody MFE-23 by neutron scattering, analytical ultracentrifugation, and NMR and FT-IR spectroscopy

MFE-23 is a single chain Fv (scFv) antibody molecule used to target colorectal cancer through its high affinity for the tumour marker carcinoembryonic antigen (CEA). ScFv molecules are formed from peptide-linked antibody V(H) and V(L) domains, and many of these form dimers. Our recent crystal structure for MFE-23 showed that this formed an unusual symmetric back-to-back association of two monomers that is consistent with a domain-swapped diabody structure. Neutron scattering and modelling fits showed that MFE-23 existed as compact V(H)-V(L)-linked monomers at therapeutically relevant concentrations below 1 mg/ml. Size-exclusion gel chromatography showed that the monomeric and dimeric forms of MFE-23 could be separated, and that the proportions of these two forms depended on the starting MFE-23 concentration. Sedimentation equilibrium experiments by analytical ultracentrifugation at nine concentrations of MFE-23 indicated a reversible monomer-dimer self-association equilibrium with an association constant of 1.9x10(3)-2.2x10(3) M(-1). Sedimentation velocity experiments using the time derivative g(s(*)) method showed that MFE-23-His has a concentration-dependent weight average sedimentation coefficient that increased from 1.8 S for the monomer to about 3-6 S for the dimer. Both values agreed with those calculated from the MFE-23 crystal structure. In relation to the thermal stability of MFE-23, denaturation experiments by (1)H NMR and FT-IR spectroscopy showed that the molecule is stable up to 47 degrees C, after which denaturation was irreversible. MFE-23 dimerisation is discussed in terms of a new model for diabody structures, in which the V(H) and V(L) domains in the monomer are able to dissociate and reassociate to form a dimer, or diabody, but in which symmetric back-to-back contacts between the two monomers are formed. This dimerisation in solution is attributed to the complementary nature of the C-terminal surface of the MFE-23 monomer. Crystal structures for seven other scFv molecules have shown that, while the contact residues for symmetric back-to-back dimer formation in MFE-23 are not fully conserved, in principle, back-to-back contacts can be formed in these too. This offers possibilities for the creation of other forms of scFv molecules.

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.

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.

RASA, a recombinant single-chain variable fragment (scFv) antibody directed against the human sperm surface: implications for novel contraceptives

BACKGROUND

A recombinant single-chain variable fragment (scFv) antibody was engineered to a tissue-specific carbohydrate epitope located on human sperm agglutination antigen-1 (SAGA-1), a sperm glycoform of CD52.

METHODS AND RESULTS

cDNAs encoding the variable regions of the S19 [IgG(1)kappa] monoclonal antibody (mAb) were identified, linked, and cloned into the pCANTAB 5E vector. The recombinant anti-sperm antibody (RASA) was expressed in E. coli HB2151 cells as a 29 kDa monomer and, remarkably, also formed multimers of approximately 60 and 90 kDa. RASA reacted with the endogenous SAGA-1 antigen by Western blot analysis, labelled the entire human sperm surface by indirect immunofluorescence, and aggregated human spermatozoa in a tangled (head-to-head, head-to-tail, tail-to-tail) pattern of agglutination, as was also observed with the native S19 mAb.

CONCLUSIONS

These results demonstrate that active recombinant antibodies can be produced to a tissue-specific carbohydrate epitope on the human sperm surface, thereby opening opportunities for novel contraceptive agents.

Design and application of diabodies, triabodies and tetrabodies for cancer targeting

Multivalent recombinant antibody fragments provide high binding avidity and unique specificity to a wide range of target antigens and haptens. This review describes the design and expression of diabodies, triabodies and tetrabodies using examples of scFv molecules that target viruses (influenza neuraminidase) and cancer (Ep-CAM; epithelial cell adhesion molecule). We discuss the preferred choice of linker length between V-domains to direct the formation of either diabodies (60 kDa), triabodies (90 kDa) or tetrabodies (120 kDa), each with size, flexibility and valency suited to different applications for in vivo imaging and therapy. The increased binding valency of these scFv multimers results in high avidity (low off-rates). A particular advantage for tumour targeting is that molecules of 60-100 kDa have increased tumour penetration and fast clearance rates compared to the parent Ig (150 kDa). We highlight a number of cancer-targeting scFv multimers that have recently successfully undergone pre-clinical trials for in vivo stability and efficacy. We also review the design of multi-specific Fv modules suited to cross-link two or more different target antigens. These bi- and tri-specific multimers can be formed by association of different scFv molecules and, in the first examples, 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).

Increased binding affinity enhances targeting of glioma xenografts by EGFRvIII-specific scFv

Combinatorial variation of CDR3 of V(H) and V(L), followed by phage display, was used to select affinity mutants of the parental anti-epidermal growth factor receptor-vIII (EGFRvIII) scFv MR1. One mutant, MR1-1(scFv), had increased specific binding affinity for EGFRvIII. It was produced and radiolabeled, and its biodistribution was evaluated in human glioma-bearing athymic mice. MR1-1 targeted the same EGFRvIII epitope as MR1 with an approximately 15-fold higher affinity (K(d) = 1.5 x 10(-9) M) measured by surface resonance analysis. Labeling with (131)I or (125)I was performed, and the immunoreactive fraction of the labeled MR1-1(scFv) was 50% to 55%. After incubation at 37 degrees C for 4 days, the binding affinity was maintained at 60% of initial levels. The specificity of MR1-1 for EGFRvIII was demonstrated in vitro by flow cytometry and incubation of FITC-labeled scFv with the EGFRvIII-expressing U87MG. DeltaEGFR cell line or with the EGFRvIII-negative U87MG cell line in the presence or absence of competing unlabeled MR1-1(scFv). We also investigated the internalization and processing of MR1-1 compared with MR1; MR1-1 exhibited levels of both cell surface retention and internalization up to 5 times higher than those by MR1. In biodistribution studies performed in athymic mice bearing s.c. U87MG. DeltaEGFR tumor xenografts, animals received paired-label intratumoral infusions of (131)I-labeled MR1-1(scFv) and (125)I-labeled MR1(scFv). Our results showed an up to 244% +/- 77% increase in tumor uptake for MR1-1 compared with that for MR1. The improved tumor retention of MR1-1(scFv) combined with its rapid clearance from normal tissues also resulted in sustained higher tumor:normal organ ratios. These results suggest that the improved affinity of MR1-1 can significantly impact in vivo glioma-specific targeting and immunotherapy.

Expression of a bispecific dsFv-dsFv' antibody fragment in Escherichia coli

A bispecific disulfide-stabilized Fv antibody fragment (dsFv-dsFv') consisting of two different disulfide-stabilized Fv antibody fragments connected by flexible linker peptides was produced by secretion of three polypeptide chains into the periplasm of Escherichia coli. The dsFv-dsFv' molecules were enriched by immobilized metal affinity chromatography and further purified by anion-exchange chromatography. The recombinant antibody constructs retained the two parental antigen binding specificities and were able to cross-link the two different antigens. The described dsFv-dsFv' design might be of particular value for therapeutic in vivo applications since improved stability is expected to be combined with minimal immunogenicity.

Synthesis of high avidity antibody fragments (scFv multimers) for cancer imaging

Multivalent antibody fragments (scFv dimers, trimers and tetramers) provide high avidity and ideal pharmacokinetics for tumour targeting applications. This protocol describes our optimised protocol for high-level bacterial synthesis of soluble antibody scFv fragments, as either monomers or multimers, using the heat-inducible bacterial expression vector pPOW3. Our protocol is rapid, which minimizes protein degradation, and utilises inexpensive reagents for cost-effective product synthesis. The strong, temperature-regulated promoters in pPOW3 provide efficient production of either monomeric or multimeric single-chain antibody fragments as dictated by the gene construct design.

ScFv multimers of the anti-neuraminidase antibody NC10: shortening of the linker in single-chain Fv fragment assembled in V(L) to V(H) orientation drives the formation of dimers, trimers, tetramers and higher molecular mass multimers

Synthetic genes encoding single-chain variable fragments (scFvs) of NC10 anti-neuraminidase antibody were constructed by joining the V(L) and V(H) domains with linkers of fifteen, five, four, three, two, one and zero residues. These V(L)-V(H) constructs were expressed in Escherichia coli and the resulting proteins were characterized and compared with the previously characterized NC10 scFv proteins assembled in V(H)-V(L) orientation. Size-exclusion chromatography and electron microscope images of complexes formed between various NC10 scFvs and anti-idiotype Fab' were used to analyse the oligomeric status of these scFvs. The result showed that as the linker length between V(L) and V(H) was reduced, different patterns of oligomerization were observed compared with those with V(H)-V(L) isomers. As was the case for V(H)-V(L) orientation, the scFv-15 V(L)-V(H) protein existed mainly as a monomer whereas dimer (diabody) was a predominant conformation for the scFv-5, scFv-4 and scFv-3 V(L)-V(H) proteins. In contrast to the V(H)-V(L) isomer, direct ligation of V(L) to V(H) led to the formation of predominantly a tetramer (tetrabody) rather than to an expected trimer (triabody). Furthermore, the transition between dimers and higher order oligomers was not as distinct as for V(H)-V(L). Thus reducing the linker length in V(L)-V(H) from three to two residues did not precisely dictate a transition between dimers and tetramers. Instead, two-residue as well as one-residue linked scFvs formed a mixture of dimers, trimers and tetramers.

Monoclonal antibodies and therapy of human cancers

This survey is an overview of the applications of murine, humanized and recombinant monoclonal antibodies for in vivo diagnostic and therapeutic applications. Monoclonal antibodies (mAb) have been applied to the diagnosis and therapy of an array of human diseases. The initial failures of early clinical trials have been overcome through the production of a new generation of mAb which features reduced immunogenicity and improved targeting abilities. The early models of mAb therapy were focused on enhancing the cytolytic mechanisms against the tumor cells. More recently, successful mAb-based therapies were targeted to molecules involved in the regulation of growth of cancer cells. This has highlighted the relevance of understanding receptor-mediated signaling events, and may provide new opportunities for anti-tumor antibody targeting. Despite all the difficulties, clinical data is outlining an increasingly significant role for antibody-mediated cancer therapy as a versatile and powerful instrument in cancer treatment. One reasonable expectation is that treatment at an earlier stage in the disease process or in minimal residual disease may be more advantageous.

High avidity scFv multimers; diabodies and triabodies

Multivalent recombinant antibody fragments provide high binding avidity and unique specificity to a wide range of target antigens and haptens. This review describes how careful choice of linker length between V-domains creates new types of Fv modules with size, flexibility and valency suited to in vivo imaging and therapy. Further, we review the design of multi-specific Fv modules suited to cross-linking target antigens for cell-recruitment, viral delivery and immunodiagnostics. Single chain Fv antibody fragments (scFvs) are predominantly monomeric when the V(H) and V(L) domains are joined by polypeptide linkers of at least 12 residues. An scFv molecule with a linker of 3 to 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. The increased binding valency in these scFv multimers results in high avidity (long off-rates). A particular advantage for tumor targeting is that molecules of approximately 60-100 kDa have increased tumor penetration and fast clearance rates compared to the parent Ig. A number of cancer-targeting scFv multimers have recently undergone pre-clinical evaluation for in vivo stability and efficacy. Bi- and tri-specific multimers can be formed by association of different scFv molecules and, in the first examples, have been designed as cross-linking reagents for T-cell recruitment into tumors (immunotherapy) and as red blood cell agglutination reagents (immunodiagnostics).

Recombinant antibody constructs in cancer therapy

Recombinant antibodies and their fragments now represent over 30% of all biological proteins undergoing clinical trials for diagnosis and therapy. The focus on antibodies as the ideal cancer-targeting reagents recently culminated in approval by the Food and Drugs Administration for the first engineered therapeutic antibodies. In the past year, important advances have been made in the design, selection and production of new types of engineered antibodies. Innovative selection methods have enabled the isolation of high-affinity cancer-targeting and antiviral antibodies, the latter capable of redirecting viruses for gene therapy applications. In other strategies for cancer diagnosis and therapy, recombinant antibody fragments have been fused to radioisotopes, drugs, toxins, enzymes and biosensor surfaces. Bispecific antibodies and related fusion proteins have been produced for cancer immunotherapy, effectively enhancing the human immune response in anticancer vaccines and T cell recruitment strategies.