Barbara E. Power

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.

A new generation of protein display scaffolds for molecular recognition

Engineered antibodies and their fragments are invaluable tools for a vast range of biotechnological and pharmaceutical applications. However, they are facing increasing competition from a new generation of protein display scaffolds, specifically selected for binding virtually any target. Some of them have already entered clinical trials. Most of these nonimmunoglobulin proteins are involved in natural binding events and have amazingly diverse origins, frameworks, and functions, including even intrinsic enzyme activity. In many respects, they are superior over antibody‐derived affinity molecules and offer an ever‐extending arsenal of tools for, e.g., affinity purification, protein microarray technology, bioimaging, enzyme inhibition, and potential drug delivery. As excellent supporting frameworks for the presentation of polypeptide libraries, they can be subjected to powerful in vitro or in vivo selection and evolution strategies, enabling the isolation of high‐affinity binding reagents. This article reviews the generation of these novel binding reagents, describing validated and advanced alternative scaffolds as well as the most recent nonimmunoglobulin libraries. Characteristics of these protein scaffolds in terms of structural stability, tolerance to multiple substitutions, ease of expression, and subsequent applications as specific targeting molecules are discussed. Furthermore, this review shows the close linkage between these novel protein tools and the constantly developing display, selection, and evolution strategies using phage display, ribosome display, mRNA display, cell surface display, or IVC (in vitro compartmentalization). Here, we predict the important role of these novel binding reagents as a toolkit for biotechnological and biomedical applications.

In vitro display technologies reveal novel biopharmaceutics

Display technologies are fundamental to the isolation of specific high‐affinity binding proteins for diagnostic and therapeutic applications in cancer, neurodegenerative, and infectious diseases as well as autoimmune and inflammatory disorders. Applications extend into the broad field of antibody (Ab) engineering, synthetic enzymes, proteomics, and cell‐free protein synthesis. Recently, in vitro display technologies have come to prominence due to the isolation of high‐affinity human antibodies by phage display, the development of novel scaffolds for ribosome display, and the discovery of novel protein‐protein interactions. In vitro display represents an emerging and innovative technology for the rapid isolation and evolution of high‐affinity peptides and proteins. So far, only one clinical drug candidate produced by in vitro display technology has been approved by the FDA for use in humans, but several are in clinical or preclinical testing. This review highlights recent advances in various engineered biopharmaceutical products isolated by in vitro display with a focus on the commercial developments.—Rothe, A., Hosse, R. J., Power, B. E. In vitro display technologies reveal novel biopharmaceutics. FASEB J. 20, 1599–1610 (2006)

High-level temperature-induced synthesis of an antibody VH-domain in Escherichia coli using the PelB secretion signal

We have constructed a temperature-inducible Escherichia coli expression vector (pPOW) for enhanced secretion of antibody (Ab) domains and other foreign proteins. The vector contains the lambda pRpL promoters in tandem, and the cI857 gene encoding the temperature-sensitive repressor which provide tight control over protein production. The PelB secretion signal directs the synthesized foreign protein through the cytoplasmic membrane. A mouse Ab fragment (the variable heavy (VH) domain of NC41) was synthesized efficiently by this vector and accumulated with the cell membranes (not as inclusion bodies) at levels of 30 mg/l. This represents the highest yields reported to date for Ab fragments with a native N terminus. An octapeptide (FLAG) tail was fused to the C terminus of the VH domain to aid in purification, and remained intact throughout the protein purification process. The optimum conditions for protein production were controlled by the type of culture medium used, the age of the bacterial population at the time of induction, and the period of synthesis of the protein product. The purified Ab VH fragment showed binding affinity (Ka less than 10(4)/M) to its target antigen (neuraminidase).

The Crystal Structure of an Anti-CEA scFv Diabody Assembled from T84.66 scFvs in VL-to-VH Orientation: Implications for Diabody Flexibility

Diabodies (scFv dimers) are small, bivalent antibody mimetics of approximately 55kDa in size that possess rapid in vivo targeting pharmacokinetics compared to the intact parent antibody, and may prove highly suitable for imaging and therapeutic applications. Here, we describe T84.66Di, the first diabody crystal structure in which the scFvs comprise V domains linked in the V(L)-to-V(H) orientation. The structure was determined by X-ray diffraction analysis to 2.6 A resolution. The T84.66Di scFv was constructed from the anti-carcinoembryonic antigen (anti-CEA) antibody T84.66 variable domains connected by an eight residue peptide linker to provide flexibility between Fv modules and promote dimer formation with bivalent affinity to the cell-surface target, CEA. Therefore, it was surprising to observe a close association of some Fv module complementarity-determining regions in the T84.66 diabody crystal, especially compared to other diabody structures all of which are linked in the opposite V(H)-to-V(L) orientation. The differences between the arrangement of Fv modules in the T84.66Di V(L)-to-V(H) linked diabody structure compared to the crystal structure of L5MK16 and other proposed V(H)-to-V(L) linked diabodies has been investigated and their potential for flexibility discussed. The comparison between V(H)-to-V(L) and V(L)-to-V(H) linked diabodies revealed in this study represents a limited repertoire of possible diabody Fv orientations, but one that reveals the potential flexibility of these molecules. This analysis therefore provides some signposts that may impact on future molecular designs for these therapeutic molecules with respect to diabody flexibility and avidity.

Ribosome display for improved biotherapeutic molecules

Ribosome display presents an innovative in vitro technology for the rapid isolation and evolution of high-affinity peptides or proteins. Displayed proteins are bound to and recovered from target molecules in multiple rounds of selection in order to enrich for specific binding proteins. No transformation step is necessary, which could lead to a loss of library diversity. A cycle of display and selection can be performed in one day, enabling the existing gene repertoire to be rapidly scanned. Proteins isolated from the panning rounds can be further modified through random or directed molecular evolution for affinity maturation, as well as selected for characteristics such as protein stability, folding and functional activity. Recently, the field of display technologies has become more prominent due to the generation of new scaffolds for ribosome display, isolation of high-affinity human antibodies by phage display, and their implementation in the discovery of novel protein–protein interactions. Applications for this technology extend into the broad field of antibody engineering, proteomics, and synthetic enzymes for diagnostics and therapeutics in cancer, autoimmune and infectious diseases, neurodegenerative diseases and inflammatory disorders. This review highlights the role of ribosome display in drug discovery, discusses advantages and disadvantages of the system, and attempts to predict the future impact of ribosome display technology on the development of novel engineered biopharmaceutical products for biological therapies.

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.

Construction, expression and characterisation of a single-chain diabody derived from a humanised anti-Lewis Y cancer targeting antibody using a heat-inducible bacterial secretion vector

Abstract A single-chain antibody fragment (scFv) of the humanised monoclonal antibody, hu3S193, that reacts specifically with Ley antigen expressed in numerous human epithelial carcinomas was constructed. A five-residue linker joined the C-terminus of the VH and the N-terminus of the VL, which prevented V-domain association into a monomeric scFv and instead directed non-covalent association of two scFvs into a dimer or diabody. The diabody was secreted into the E. coli periplasm using a heat-inducible vector, pPOW3, and recovered as a soluble, correctly processed protein, following osmotic shock or solubilised with 4M urea from the insoluble fraction. The diabody from both fractions was isolated by a rapid batch affinity chromatography procedure, using the FLAG affinity tag to minimise degradation and aggregation. The purified diabody has an Mr of ˜54 kDa, was stable and demonstrated similar binding activity as the parent monoclonal antibody, as measured by FACS and BIAcore analyses. The radiolabelled diabody showed a rapid tumour uptake, with fast blood clearance, proving it to be an excellent potential candidate as a tumour-imaging agent.

Solution properties ofEscherichia coli-expressed VH domain of anti-neuraminidase antibody NC41

The VH domain of anti-influenza neuraminidase antibody NC41, with and without a C-terminal hydrophilic marker peptide (FLAGTM), has been expressed in high yield (15–27 mg/L) inEscherichia coli. Both forms were secreted into the periplasm where they formed insoluble aggregates which were solubilized quantitatively with 2 M guanidine hydrochloride and purified to homogeneity by ion-exchange chromatography. The VH-FLAG was composed of three isoforms (pI values of ∼4.6, 4.9, and 5.3) and the VH molecule was composed of two isoforms with pI values of 5.1 and 6.7; the difference between the VH isoforms was shown to be due to cyclization of the N-terminal glutamine residue in the pI 5.1 isoform. At 20°C and concentrations of 5–10mg/ml the VH domain dimerized in solution and then partly precipitated, resulting in the broadening of resonances in its1H NMR spectrum. Reagents such as CHAPS,n-octylglucoside, and ethylene glycol, which presumably mask the exposed hydrophobic interface of the VH molecule, prevented dimerization of the VH and permitted good-quality NMR spectra on isotope-labeled protein to be obtained.

Noncovalent scFv multimers of tumor‐targeting anti‐Lewisy hu3S193 humanized antibody

Single‐chain variable fragments (scFvs) of anti‐Lewisy hu3S193 humanized antibody were constructed by joining the VH and VL 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 VH domain were removed (−1 residue, −2 residue) and then joined directly to the VL domain. An scFv construct in the reverse orientation with the VL joined directly to the VH 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.