Abstract:
A complex comprising barnase bound with high affinity to barstar, and comprising a therapeutic and/or diagnostic agent bound to barnase and/or barstar.

Description:
This application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/539,976 filed Jan. 30, 2004. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Dec. 1, 2011, is named SCH2025.txt and is 54,128 bytes in size. 
     FIELD OF THE INVENTION 
     The invention relates to multivalent complexes, their production and method of use. 
     BACKGROUND OF THE INVENTION 
     Abnormal cellular proliferation, notably hyperproliferation, is the source of numerous diseases, the most severe one being cancer. In the United States alone approximately 1.5 million people are diagnosed with cancer and 0.5 million die from it each year. The fight against cancer has seen some success but also numerous set-backs. There is a great need for innovative therapeutics. 
     Targeted drug delivery, as opposed to systemic delivery, can dramatically increase drug efficacy while decreasing side effects. Targeted delivery requires a target (antigen, receptor), a delivery vehicle (cytokines, antibodies and fragments thereof) and a drug. Currently, several strategies are used for delivering drugs to targets: they are based either on direct conjugates to the targeting protein or on derivatized carriers that interact with specific adapters that are conjugated to the targeting protein. Recently, heterobifunctional recombinant antibodies recognizing one epitope on the cell surface and another epitope on the drug carrier have been proposed for targeted drug delivery. However, these constructs only make contact with one binding site and therefore show high dissociation rates. The strong interaction of streptavidin with biotin has been particularly widely explored for a targeting approach. Unfortunately, streptavidin is highly immunogenic and furthermore shows an inherent high kidney accumulation. 
     Over the last few years, a burst of reports on the construction of multivalent recombinant antibody fragments has appeared (Pluckthun, A. and Pack, P. Immunotechnology 3, 83-105, 1997. Todovroska, A. et all., J. Immunol. Methods 248, 47-66, 2001). Multivalency not only enhances the strength of binding, but it also amplifies binding selectivity. These designs have included a variety of recombinant fusions using adhesive protein domains, peptides or specially designed linkers for formation of single chain antibody fragment multimers. For medical applications such as targeting to tumor-associated antigens, efficient tissue penetration must be combined with high functional affinity, and fragments must be stable against denaturation or proteolysis until they have reached the tumor site. Most of these published constructs will not meet all of these prerequisites: multimeric constructs assembled by domains or peptides show dissociation at dilution, multimeric constructs assembled by specific linkers are non-homogeneous products and the production of all of these constructs is hampered by aggregation and subsequent precipitation of the proteins at high protein concentration. 
     The invention presents a novel method for the design of multimers based on the ribonuclease barnase and its inhibitor, barstar. The complex between barnase and barstar is extremely tight with a K d ˜10 −14  M and forms very rapidly, comparable in affinity with the streptavidin/biotin system (Hnatowich et al., J. Nucl. Med. 28: 1294-1302, 1987). 
     The barnase and barstar proteins are small (110-residues for barnase and 89-residues for barstar), stable, very soluble and resistant to proteases—features commensurate with bacterial expression. Moreover, the three-dimensional structure of the complex is known both from X-ray crystallography 3  and NMR spectroscopy 4,5  and shows that the N-terminal as well C-terminal parts of both proteins localize outside of the barnase:barstar interface. They are therefore accessible for fusion with targeting proteins and then form extremely stable multimers due to the practically irreversible pairing of these ligands. 
     In addition barnase and barstar have been used in genetically engineered plants. It has been shown that male fertility can be restored to the plant with a chimeric fertility-restorer gene comprising another DNA sequence (or fertility-restorer DNA) that codes, for example, for a protein that inhibits the activity of the cytotoxic product or otherwise prevents the cytotoxic product to be active in the plant cells (European patent publication “EP” 0,412,911). For example the barnase gene of  Bacillus amyloliquefaciens  codes for an RNase, the barnase, which can be inhibited by a protein, the barstar, that is encoded by the barstar gene of  B. amyloliquefaciens . The barnase gene can be used for the construction of a sterility gene while the barstar gene can be used for the construction of a fertility-restorer gene. Experiments in different plant species, e.g. oilseed rape, have shown that a chimeric barstar gene can fully restore the male fertility of male sterile lines in which the male sterility was due to the presence of a chimeric barnase gene (EP 0,412,911, Mariani et al., Proceedings of the CCIRC Rapeseed Congress, Jul. 9-11, 1991, Saskatoon, Saskatchewan, Canada; Mariani et al., Nature 357: 384-387, 1992,). By coupling a marker gene, such as a dominant herbicide resistance gene (for example the bar gene coding for phosphinothricin acetyl transferase (PAT) that converts the herbicidal phosphinothricin to a non-toxic compound (De Block et al., EMBO J. 6:2513, 1987), to the chimeric male-sterility and/or fertility-restorer gene, breeding systems can be implemented to select for uniform populations of male sterile plants (EP 0,344,029; EP 0,412,911). 
     Barnase and barstar have been used in a new approach for effective positive selection during gene manipulation. Several plasmid vectors for molecular cloning were constructed. They are based on the expression plasmid for a bacterial ribonuclease, barnase. In addition to the barnase gene under control of a synthetic tac promoter, these plasmids carry the gene for the barnase inhibitor, barstar, the constitutive expression of which protects the bacterium from the detrimental effects of moderate barnase production. Full expression of the barnase gene overcomes protection by barstar and becomes lethal. The entire pUC polylinker was inserted into the barnase gene for convenient cloning of genes of interest. Uncut or religated vectors will preclude growth while plasmids with inserts in the barnase gene will let the cells survive. The resulting plasmids are generally useful selective cloning vectors representing the &lt;&lt;kill-the-rest&gt;&gt; approach for studies in molecular biology and biotechnology (RU 2105064 C1, 1996). 
     SUMMARY OF THE INVENTION 
     The present invention is based on the observation that the ribonuclease barnase and its inhibitor barstar form a very tight complex in which all N and C termini are accessible for fusion. 
     The invention relates to barnase and barstar used in a modular approach
     (a) when attached to scFv fragment via a hinge region they serve as building blocks for bivalent miniantibodies.   (b) By fusing more than one barnase (or barstar) in series, complexes of higher valency can be created.   (c) By fusing different scFv fragments to barnase and barstar, homo- as well as heteromultimeric proteins complexes can be designed.   

     The invention relates to heteromultimeric proteins carrying antibody fragments or peptide homing fragments and fused to the second partner carrying any type of diagnostic or therapeutic protein or peptide or vice versa. 
     In particular the invention relates to barnase: barstar complexes of exact stoichiometric ratio of partners. 
     The invention further relates to production of barnase fusions by barstar coexpression and a method of removing barstar fixed on solid support. 
     The invention also relates to the use of a barnase and barstar derivative according to the invention in a method of diagnosis of a neoplastic disease or in a method of treatment of a mammal suffering from a neoplastic disease. 
     The invention also relates to the use of barnase and barstar derivatives as a method of visualization, said visualization units being directly bonded together through chemical groups or backbone moieties of adjacent said visualization units, or linked by a coupling agent covalently bonded to chemical groups or backbone moieties of adjacent said visualization units. 
     Said visualization unit being an enzyme, wherein the enzymatic site is the visualization site, a tagged natural or synthetic polypeptide, a tagged polyol, a tagged polyolefin or a tagged polycarbohydrate wherein the tag of the tagged polypeptide, tagged polyol, tagged polyolefin or tagged polycarbohydrate is the visualization site thereof; and the tag of said tagged polypeptide, tagged polyol, tagged polyolefin or tagged carbohydrate is a fluorescent group, a dye, a radioactive group, a photon emitter or an electron dense group. 
     Said visualization unit is combinatorially labeled with a fluorophore selected from the group consisting of the fluorophores FITC, Cy3, Cy3.5, Cy5, Cy5.5 and Cy7. 
     Said visualization unit being a radioactive isotope. Radioactive isotopes considered are radiometalls such as  94m Tc,  99m Tc,  188 Re,  186 Re,  111 In,  90 Y,  64 Cu,  67 Cu and Lu, in particular  99m Tc,  88 Re,  186 Re and  111 In. 
     The invention further relates to pharmaceutical compositions comprising barnase and barstar derivatives of the invention, in particular pharmaceutical compositions suitable for diagnostic applications and pharmaceutical compositions suitable for therapeutic applications, and to the use of such pharmaceutical compositions in a method of diagnosis and in a method of therapeutic treatment, respectively. 
     The invention also relates to intermediates for the preparation of compounds useful in a diagnostic or therapeutic treatment according the invention, in particular to compounds substituted with radioactive isotopes. Radioactive metals considered are radioisotopes such as  94m Tc,  99m Tc,  188 Re,  186 Re,  111 In,  90 Y,  64 Cu,  67 Cu and  177 Lu, in particular  99m Tc,  188 Re,  18 Re and  111 In. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1 . The concept of creating multimeric miniantibodies using heterodimeric barnase:barstar module and scFv fragments. Engineering of scFv fusion proteins with barstar and barnase or dibarnase yields dimeric and trimeric complexes due to ligand pairings. 
         FIG. 2 . Schematic representation of homogeneous preparation of mono- and multivalent proteins. Expression of scFv-barnase fusion proteins requires co-expression of the barnase inhibitor, barstar; denaturation of the His-tag-immobilized scFv-barnase:barstar complex removes the inhibitor and, following refolding of the Ni 2+ -NTA adsorbed fusion protein, results in its functional preparation. The affinity chromatography based on the His-tag-immobilized scFv-barnase fusions and saturation of the column with scFv-barstar protein (without His-tag) results in one-step preparation of oligomeric complexes with an exact stoichiometric ratio of subunits. 
         FIG. 3 . Overview of the gene constructs for expression of the 4D5 scFv-barstar (I), 4D5 scFv-barstar-His 5  (II) (His 5  tag disclosed as SEQ ID NO: 10), 4D5 scFv-barnase-His 5  (III) (His 5  tag disclosed as SEQ ID NO: 10) and 4D5 scFv-dibarnase-His 5  (IV) (His 5  tag disclosed as SEQ ID NO: 10) fusion proteins. The triangle in construct IV indicates a deletion of the five N-terminal amino acids of the second barnase. Inducible expression of the gene fusions is under transcriptional control of the lac promoter and the ompA signal peptide is used to direct secretion of the recombinant proteins produced to the periplasmic space of  E. coli . Barstar coexpression under the control of constitutive promoter (p) is required to suppress cytotoxicity of barnase fusions 
         FIG. 4 . 12% SDS-PAGE documenting the purification of monovalent fusion proteins and multimeric complexes. (I) Coomassie stained gel; (II, III) Western blot analysis screened with anti-barstar and anti-barnase sera, respectively. The same blot was used for both Western probing after stripping the first detecting antiserum. Lanes are: 1-4D5 scFv fragment, 2-4D5 scFv-barstar-His 5  (His 5  to disclosed as SEQ ID NO: 10), 3-4D5 scFv-barnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10), 4-4D5 scFv-dibarnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10), 5-dimeric complex, 6-trimeric complex, 7-markers 
         FIG. 5 . Assembly of multimeric proteins and their antigen binding. Association of 4D5 scFv-barnase-His 5  (his 5  tag disclosed as SEQ ID NO: 10) and 4D5-scFv-dibarnase-His 5  (His 5  to disclosed as SEQ ID NO: 10) with their partner 4D5 scFv-barstar fusion. The preparations were separated on a gel filtration column. In this assay the elution peak of nonlabeled 4D5 scFv-barstar (solid line) was shifted from an apparent molecular weight of 39.9 kDa to 81 kDa when associated with radiolabeled 4D5 scFv-dibarnase-His 5  (dotted line) (His 5  tag disclosed as SEQ ID NO: 10). 
         FIG. 6 . Antigen binding of radiolabeled mono-, di- and trimeric 4D5 scFv-barnase-barstar constructs. The shift in molecular weight was followed on a gelfiltration column when the antibodies were mixed with soluble recombinant p185-HER2-ECD antigen. Gelfiltration analysis of the derivative mixed with TCl (shift of the peak from 1.5 kDa to 70 kDa) 
         FIG. 7 . Dissociation kinetics of monomeric, dimeric and trimeric 4D5 scFv-constructs measured by surface plasmon resonance. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A barnase as used herein is any protein which is capable of degrading single-stranded RNA and which comprises the amino acid sequence of barnase (secreted barnase) as secreted by  Bacillus amyloliquefaciens  (SEQ ID NO: 11) (Hartley, 1988, J. Mol. Biol. 202:913) or an amino acid sequence having at least 80%, preferably at least 85% sequence identity with this sequence. Barnases, as used herein are capable of degrading RNA by a reaction which involves the initial cleaving of the phosphodiester bond between the 5′ ribose of one nucleotide and the phosphate group attached to the neighbouring 3′ nucleotide. The initial product of this reaction is a 2′,3′-cyclic phosphate intermediate which is subsequently hydrolyzed to the corresponding 3′ nucleoside phosphate. Barnases are also capable of hydrolyzing polyethenoadenosine phosphate to yield a highly fluorogenic nucleotide analogue 1,N-ethenoadenosine (Fitzgerald and Hartley, Anal. Biochem. 214:544-547, 1993,) and have at least 10% of the activity of secreted barnase as measured under standard conditions (Fitzgerald and Hartley, Anal. Biochem. 214:544-547, 1993; Hartley, Biochemistry 32:5978-5984, 1993,). Barnases are further capable of specific binding to wild-type barstar (see below) with a dissociation constant of 10 −12  M or less, preferably with a dissociation constant of the order 10 −13  M to 10 −14 M (Schreiber and Fersht, Biochemistry 32:5145-5150, 1993; Hartley, Biochemistry 32:5978-5984, 1993). 
     Binase is the extracellular ribonuclease secreted by  Bacillus intermedius  (Schulga et al, NAR 20:2375, 1992) and is also considered to be a barnase as used in this invention. 
     For convenience barnase, as used in the description or in the Examples below, will designate a protein having the amino acid sequence of the barnase encoded by pVE108 (SEQ ID NO: 36) (WO 92/09696). 
     A barstar is any protein that is capable of specific binding to secreted barnase with a dissociation constant of 10 −12  M or less, preferably with a dissociation constant of the order of 10 −13  M to 10 −14  M (Schreiber and Fersht, Biochemistry 32:5145-5150, 1993; Hartley, Biochemistry 32:5978-5984, 1993). Barstars are capable of inhibiting at least 50%, particularly at least 75%, more particularly at least 90% of the activity of secreted barnase in an equimolar mixture of barstar and secreted barnase in standard conditions (Hartley, Biochemistry 32:5978-5984, 1993). A barstar is a protein comprising the amino acid sequence of (SEQ ID NO: 12) (Hartley, J. Mol. Biol. 202:913, 1988) or an amino acid sequence having at least 80%, preferably at least 85% sequence identity with this sequence. Wild type barstar is the barstar produced by  Bacillus amyloliquefaciens  and having the amino acid sequence of (SEQ ID NO: 12) (Hartley, J. Mol. Biol. 202:913, 1988). It goes without saying that barstars as used herein include for example the biologically active barstar mutants described by Hartley (SEQ ID NOs: 13-35) (Hartley, Biochemistry 32:5978-5984, 1993). 
     A barnase DNA (or barnase coding sequence) as used herein is any DNA fragment having a nucleotide sequence coding for a barnase. A particularly preferred barnase DNA is the barnase DNA as present in pVE108 (SEQ ID NO: 36) (WO 92/09696). 
     A barstar DNA (or barstar coding sequence) as used herein is any DNA fragment having a nucleotide sequence coding for a barstar. A wild type barstar DNA is the DNA which codes for wild-type barstar (SEQ ID NO: 12) and which has the nucleotide sequence as described (Hartley, J. Mol. Biol. 202:913, 1988). 
     Compounds of fusions of barnase to antibody scFv fragments or antibody VL domains (Martsey, S. P., et all., Protein Eng. 17, 85-93, 2004), but not restricted to these examples. 
     Compounds of fusion of barstar to antibody scFv fragments, but not restricted to these examples. 
     Compounds of formula mentioned useful in a method of diagnostic and/or therapeutic treatment according to the invention. 
     Radioactive metals considered are radioisotopes such as  94m Tc,  99m Tc,  188 Re,  186 Re,  111 In,  90 Y,  64 Cu,  67 Cu and  177 Lu, in particular  99m Tc,  188 Re,  186 Re and  111 In. 
     Compounds of the invention carrying an antiproliferative agent are useful for transporting the agent in an inactive form in to the hyperproliferative cells where it can exert its action after intracellular amidolysis. 
     In a method of treatment of a neoplastic and/or infectious disease, a compound of the invention carrying a suitable therapeutic agent can be administered alone or in combination with one or more other therapeutic agents, possible combination therapy taking the form of fixed combinations, or the administration of a compound of the invention and one or more other therapeutic agents being staggered or given independently of one another, or the combined administration of fixed combinations and one or more other therapeutic agents. A compound of the invention can, besides or in addition, be administered especially for tumor therapy in combination with chemotherapy, immunotherapy, surgical intervention, or a combination of these. Long-term therapy is equally possible as is adjuvant therapy in the context of other treatment strategies. 
     The invention further relates to pharmaceutical compositions comprising barnase/barstar derivatives of the invention, in particular pharmaceutical compositions suitable for diagnostic applications and pharmaceutical compositions suitable for therapeutic applications. 
     Preferred are pharmaceutical compositions for parenteral administration, such as intravenous, intramuscular or subcutaneous administration. The compositions comprise the active ingredient alone or together with a pharmaceutically acceptable carrier. The dosage of the active ingredient depends upon the disease to be treated and upon the species, its age, weight, and individual condition, the individual pharmacokinetic data, and the mode of administration. 
     METHODS OF MANUFACTURE 
     Compounds of the invention are prepared by standard methods known in the art. The following Examples serve to illustrate the invention without limiting the invention in its scope. 
     EXAMPLES 
     Reagent grade chemicals were from Sigma or Fluka, Buchs (CH) and were used without further purification. Rabbit anti-barnase and rabbit anti-barstar sera was provided by R. W. Hartley, Bethesda, NIH, USA 
     FPLC analyses were performed on a BioLogic DuoFlow system equipped with a EG&amp;G Berthold LB 508 radiometric detector, using Superdex 75 and Superdex 200 columns at a flow rate of 1 ml/min. Chromatograms were recorded at 280 nm. Solvent were predominantly aqueous buffers. 
     Example 1 
     Construction of Expression Plasmids 
     The plasmids for the periplasmic expression of scFv-barnase and barstar fusion proteins are based on the vector pIG64D5 (Knappik, A. &amp; Plückthun, A. Protein Eng. 8, 81-89, 1995). 
     The barstar and barnase genes were amplified out of the plasmid pMT413 (Hartley, R. W. Biochemistry 32,5978-5984, 1993) using the primers AscI_bs-fwd: 5′-TGGCGCGCCGAAAAAAGCAGTCATTAACGGG-3′ (SEQ ID NO: 1), bs_AscI-rev: 5′-CG GCGCGCCAGAAAGTATGATGGTGATGTC-3′ (SEQ ID NO: 2), AscI_bn-fwd: 5′-GTGGCGCGCCTGCACAGGTTATCAACACGTTTG-3′ (SEQ ID NO: 3) and bn_AscI-rev: 5′-GTGCGGCGCGCCTCTGATTTTTGTAAAGGTCTG-3′(SEQ ID NO: 4). The AscI fragments were ligated into the vector pIG64D5. The plasmid encoded fusion protein: VL-linker-VH-hinge-barstar-His 5 -tag (His 5  tag disclosed as SEQ ID NO: 10) was easily obtained. In contrast, no clones with the correct barnase gene were found. To obtain 4D5 scFv-barstar derivative without a His-tag, the BspEI/HindIII fragment was cut out from the 4D5 scFv-barstar-His 5  (His 5  tag disclosed as SEQ ID NO: 10) encoding plasmid and replaced by BspEI/HindIII part from the plasmid pMT413. To introduce the barstar gene for bicistronic transcription followed by the scFv-barnase encoding gene barstar gene was amplified out of the plasmid pMT413 using primers HindIII_bs-fwd: 5′-CGTCTAAGCTTGATGAAAAAAGCAGTCATTAACG-3′ fSEQ ID NO: 5) and bs_HindIII-rev: 5′-AACAGCTATGACCATGATTACG-3′ (SEQ ID NO: 6) and ligated into the HindIII-digested plasmid pIG64D5. The resulting plasmid was used for cloning of barnase gene flanked by AscI sites. To introduce in pIG64D5 vector the barstar gene under a control of its own promotor the fragment was amplified out of the plasmid pMT413 using primers HindIII_pbs-fwd: 5′-ATCAGACCTTTACAAAAAGCTTATAAC-3′ (SEQ ID NO: 7) and bs_HindIII-rev and cloned via HindIII site. The obtained vector was used to attach barnase gene to 4D5scFv fragment using AscI site. The resulting plasmid had no mutations and encoded the fusion protein: VL-linker-VH-hinge-barnase-His 5 -tag (His 5  tag disclosed as SEQ ID NO: 10). The plasmid carrying the dimeric barnase fragment instead of barnase was derived analogously using overlap extension PCR and primers dibn-fwd: 5′-CAGACCTTTACAAAAATCAGAGACACGTTTGACGGGGTTGC-3′ (SEQ ID NO: 8), dibn-rev: 5′-GCAACCCCGTCAAACGTGTCTCTGATTTTTGTAAAGGTCTG-3′ (SEQ ID NO: 9), AscI_bn-fwd and bn_AscI-rev. 
     Example 2 
     Molecular Modeling 
     The models are based-on the X-ray structures of humanized anti-p185 HER2-ECD  antibody 4D5 version 8 (PDB entry 1FVC, 2.2 Å resolution) 14  and the structure of the barnase:barstar complex (PDB entry 1BGS, 2.6 A resolution) 15 . The flexible tags, linkers and hinge residues were modeled as beta strands and shaped by simulated melting and annealing using the Discover module of the InsightII molecular modeling suite (MSI/Biosym, San Diego). 
     Example 3 
     Freshly transformed  E. coli  K12 strain SB536 (Bass, S. et al., J. Bacteriol. 178, 1154-61, 1996) (F-, WG1, ΔfhuA (ton Δ), ΔhhoAB (SaclI), shh) cells, grown in SB medium containing 0.1 g/L ampicillin; lac promoter was induced with 1 mM IPTG at an OD550=0.7. Expression was allowed to continue for 16 h at 27° C. Cells were harvested by centrifugation at 6000 rpm for 15 min at 4° C. Cell extracts were prepared by French press lysis at 10,000 psi and centrifugation at 20,000 rpm for 1 h. The supernatants were applied to a Ni2+-NTA column (Qiagen, Germany) according to the manufacturer&#39;s instructions. The immobilized proteins were denaturated with 6 M GuHCl in PBS, pH 8.0, refolded using linear gradient 6-0 M GuHCl in the same buffer, washed with 25 mM imidazole and eluted with 200 mM imidazole. For final purification of 4D5 scFv-barnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10) and 4D5 scFv-dibarnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10) elution fractions were directly applied onto a Protein-A Sepharose column (Pharmacia) and eluted with 0.1 M sodium citrate, pH 3.5; the eluate was immediately neutralized with 1 M Tris. For preparation of dimeric and trimeric complexes 4D5 scFv-barnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10) or 4D5 scFv-dibarnase-His 5  (His 5  tag to disclosed as SEQ ID NO: 10), respectively, were immobilized onto Ni2+-NTA column and saturated with diluted 1:100 with PBS cleared cell lysate containing 4D5 scFv-barstar. Alternatively, the isoluble periplasmic protein was solubilized with 6 M GuHCl in PBS, pH 8, centrifugated 20.000 rpm for 30 minutes and diluted 1:200 with PBS, pH 8. After extensive washing with 25 mM imidazole, 1 M NaCl in PBS the resulting complexes were eluted with 200 mM imidazole, 300 mM NaCl in PBS and dialyzed against 300 mM NaCl, 10% glycerol in PBS. MW (MS, MALDI data) 4D5 scFv-barstar, 38830 (39109 theor.), 4D5 scFv-barstar-His 5  (His 5  tag disclosed as SEQ ID NO: 10) 39880 (39885 theor.), 4D5 scFv-barnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10), 42110 (42120 theor.), and 4D5 scFv-dibarnase-His 5  (His 5  tag disclosed as SEQ ID NO: 10), 54060 (54074 theor.). 
     Example 4 
     Surface Plasmon Resonance Biosensor 
     Measurements were performed with a BIAcore instrument (BIACORE 3000, Sweden). Recombinant p185 HER2-EcD  was coupled onto a CM5 chip at a density of 4500 RU by standard amine coupling chemistry. All proteins were used at the same concentration, 1.60 nM in PBS, pH7.4, with 0.005% of Tween-20. The sensograms were obtained at a flow rate of 30 μl/min at 15° C. and the dissociation phase was followed for 120 min. 
     Example 5 
     SDS-PAGE and Western Blot Analysis 
     SDS-PAGE analyses were performed under reducing conditions according to standard protocols using 12 and 18% polyacrylamide gels. Immunoblots on Immobilon-P transfer membrane (Millipore, USA) were carried out according to the manufacturers instructions using rabbit anti-barstar serum followed by a goat anti-rabbit IgG peroxidase conjugate (Sigma, USA) for detection. The blots were visualized with chemiluminescent Pierce Supersignal ECL reagent (Pierce, USA). The same membrane was reprobed after stripping for immunodetection with the rabbit anti-barnase serum. Stripping was performed in 100 mM 2-mercaptoethanol, 2% SDS, 65 mM Tris-HCl, pH 7.0 at 60° C. for 1 h. 
     Example 6 
       99m Tc Radiolabeling Conditions and Stability Studies 
     All constructs were concentrated to 5 to 10 μM by centrifugation with a 10 kDa cutoff membrane (Ultrafree-MC, Millipore, Bedford, Mass.). Constructs were mixed 1:1 with freshly synthesized  99m Tc-tricarbonyl trihydrate, as described (Waibel, R. et al., Nat. Biotechnol 17, 897-901, 1999). Labeling efficiency was &gt;95%. Labeled constructs were incubated with human sera 1:10 at 37° C. and analyzed on a gel filtration column (Superdex200), connected to a UV-monitor and a HPLC radioactivity monitor (LB 508, Berthold, Bad Wildbach, Germany). The elution profiles were compared to those of the initial constructs. 
     Example 7 
     Immunoreactivity 
     For immunoreactivity studies, recombinant p185 HER2-ECD  antigen (10 μg) was mixed with 5×10 4  Bq (30 ng) of labeled construct for 30 min at room temperature and separated on a Superdex200 gel filtration column. 
     Example 8 
     Bioreactivity 
     The bioreactivity was defined as the counts of fraction eluting with the higher molecular weight peaks (analytical gel-shift assay). 
     Example 9 
     Blood Clearance 
     Blood clearance studies were performed in xenografted nu/nu mice. They were injected with 10 μg (22 MBq) of 99mTc-labeled constructs in 100 μl PBS. Blood samples were taken at 7.5, 15, 30, 60, 120, and 240 min after injection. 
     Example 10 
     Organ Distribution 
     For organ distribution, animals were sacrificed at 24 h and 48 h after injection and tissues were collected and measured in a gamma counter. For the tumor targeting study, groups of 3 mice each were xenografted with 10 7  SK-OV-3 cells (#HTB77, ATCC, Rockville, Md.) on the left side and with 10 7  SK-BR-3 cells (3HTB30, ATCC) on the right side and were injected with the radiolabeled constructs two weeks later, when the tumors have reached a size of about 40 mg. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Biodistribution of  99m Tc-labeled mono-, di- and trimeric 
               
               
                 4D5-barnase-barstar constructs in nu/nu mice after 24 h 
               
               
                 Percentage of injected dose per gram of organ 
               
             
          
           
               
                   
                 monomeric 
                 dimeric 
                 trimeric 
               
               
                 Organ 
                 24 h 
                 24 h 
                 24 h 
               
               
                   
               
               
                 Blood 
                 0.09 ± 0.01 
                 0.57 ± 0.17 
                 0.88 ± 0.15 
               
               
                 Heart 
                 0.20 ± 0.07 
                 1.84 ± 0.34 
                 2.47 ± 0.62 
               
               
                 Lung 
                 0.31 ± 0.07 
                 1.25 ± 0.28 
                 2.54 ± 0.30 
               
               
                 Spleen 
                 0.24 ± 0.21 
                 2.90 ± 0.58 
                 4.91 ± 1.14 
               
               
                 Kidney 
                 140.63 ± 2.83  
                 56.43 ± 7.64  
                 21.16 ± 0.72  
               
               
                 Stomach 
                 0.31 ± 0.11 
                 0.80 ± 0.17 
                 0.91 ± 0.26 
               
               
                 Intestine 
                 0.31 ± 0.01 
                 1.15 ± 0.03 
                 1.73 ± 0.38 
               
               
                 Liver 
                 1.61 ± 0.21 
                 4.56 ± 0.42 
                 14.29 ± 0.51  
               
               
                 Muscle 
                 0.18 ± 0.05 
                 0.57 ± 0.09 
                 0.72 ± 0.15 
               
               
                 Bone 
                 0.16 ± 0.11 
                 0.67 ± 0.49 
                 1.71 ± 0.59 
               
               
                 SKOV3 
                 0.97 ± 0.32 
                 3.43 ± 1.01 
                 7.04 ± 2.35 
               
               
                 SKBR3 
                 1.91 ± 0.79 
                 5.91 ± 2.30 
                 9.80 ± 0.82 
               
               
                 T/B 
                 10.80 ± 4.70  
                 6.01 ± 3.56 
                 8.00 ± 4.03 
               
               
                 SKOV3 
               
               
                 T/B 
                 21.20 ± 11.05 
                 10.37 ± 7.12  
                 11.10 ± 2.83  
               
               
                 SKBR3 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Biodistribution of  99m Tc-labeled mono-, di- and trimeric 
               
               
                 4D5-barnase-barstar constructs in nu/nu mice after 48 h 
               
             
          
           
               
                   
                 monomeric 
                 dimeric 
                 trimeric 
               
               
                 Organ 
                 48 h 
                 48 h 
                 48 h 
               
               
                   
               
               
                 Blood 
                 0.05 ± 0.01 
                 0.20 ± 0.04 
                 0.30 ± 0.04 
               
               
                 Heart 
                 0.14 ± 0.01 
                 1.16 ± 0.25 
                 1.39 ± 0.24 
               
               
                 Lung 
                 0.17 ± 0.01 
                 0.81 ± 0.03 
                 1.19 ± 0.12 
               
               
                 Spleen 
                 0.46 ± 0.07 
                 3.64 ± 1.72 
                 5.00 ± 1.28 
               
               
                 Kidney 
                 106.00 ± 6.04  
                 40.49 ± 6.47  
                 15.95 ± 0.98  
               
               
                 Stomach 
                 0.18 ± 0.05 
                 0.48 ± 0.15 
                 0.33 ± 0.13 
               
               
                 Intestine 
                 0.15 ± 0.01 
                 0.97 ± 0.25 
                 1.00 ± 0.10 
               
               
                 Liver 
                 1.04 ± 0.06 
                 3.44 ± 0.43 
                 12.92 ± 2.80  
               
               
                 Muscle 
                 0.08 ± 0.01 
                 0.26 ± 0.07 
                 0.46 ± 0.11 
               
               
                 Bone 
                 0.12 ± 0.10 
                 0.54 ± 0.58 
                 1.06 ± 0.18 
               
               
                 SKOV3 
                 0.47 ± 0.16 
                 3.34 ± 0.44 
                 4.56 ± 1.00 
               
               
                 SKBR3 
                 0.65 ± 0.20 
                 5.38 ± 1.39 
                 8.11 ± 0.41 
               
               
                 T/B 
                 9.41 ± 5.08 
                 16.70 ± 5.54  
                 15.20 ± 5.32  
               
               
                 SKOV3 
               
               
                 T/B 
                 13.00 ± 6.60  
                 26.9 ± 12.33 
                 27.03 ± 4.96  
               
               
                 SKBR3 
               
               
                   
               
             
          
         
       
     
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
     In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 
     The entire disclosure of all applications, patents and publications, cited herein and of corresponding U.S. Provisional Application Ser. No. 60/539,976, filed Jan. 30, 2004, is incorporated by reference herein. 
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.