Production of recombinant protein in Pichia pastoris by fermentation

This protocol is applicable to recombinant protein expression by small-scale fermentation using the Pichia pastoris expression system. P. pastoris has the capacity to produce large quantities of protein with eukaryotic processing. Expression is controlled by a methanol-inducible promoter, which allows a biomass-generation phase before protein production is initiated. The target protein is secreted directly into a protein-free mineral salt medium, and is relatively easy to purify. The protocol is readily interfaced with expanded bed adsorption for immediate capture and purification of recombinant protein. The setting up of the bioreactor plus the fermentation itself takes 1 wk. Making the master and user seed lots takes approximately 2 wk for each individual clone.

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

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

Clearance mechanism of a mannosylated antibody-enzyme fusion protein used in experimental cancer therapy

MFECP1 is a mannosylated antibody-enzyme fusion protein used in antibody-directed enzyme prodrug therapy (ADEPT). The antibody selectively targets tumor cells and the targeted enzyme converts a prodrug into a toxic drug. MFECP1 is obtained from expression in the yeast Pichia pastoris and produced to clinical grade. The P. pastoris-derived mannosylation of the fusion protein aids rapid normal tissue clearance required for successful ADEPT. The work presented provides evidence that MFECP1 is cleared by the endocytic and phagocytic mannose receptor (MR), which is known to bind to mannose-terminating glycans. MR-transfected fibroblast cells internalize MFECP1 as revealed by flow cytometry and confocal microscopy. Immunofluorescence microscopy shows that in vivo clearance in mice occurs predominantly by MR on liver sinusoidal endothelial cells, although MR is also expressed on adjacent Kupffer cells. In the spleen, MFECP1 is taken up by MR-expressing macrophages residing in the red pulp and not by dendritic cells which are found in the marginal zone and white pulp. Clearance can be inhibited in vivo by the MR inhibitor mannan as shown by increased enzyme activities in blood. The work improves understanding of interactions of MFECP1 with normal tissue, shows that glycosylation can be exploited in the design of recombinant anticancer therapeutics and opens the ways for optimizing pharmacokinetics of mannosylated recombinant therapeutics.

Expanded-bed adsorption immobilized-metal affinity chromatography

The protocol describes a method for capture of secreted hexahistidine-tagged proteins using expanded-bed adsorption immobilized-metal affinity chromatography. The starting material for the procedure is any crude feedstock that contains a histidine (His)-tagged target protein. The protocol is exemplified using unclarified broth from Pichia pastoris fermentation as feedstock. The protocol can be used for laboratory studies or as part of a process for production of recombinant biotherapeutics to standards of good manufacturing practice. It takes approximately 5 h to purify proteins from 10 liters of feedstock and a further 5-6 h to sterilize and regenerate the column.

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

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

Sustained tumor regression of human colorectal cancer xenografts using a multifunctional mannosylated fusion protein in antibody-directed enzyme prodrug therapy

PURPOSE

Antibody-directed enzyme prodrug therapy (ADEPT) requires highly selective antibody-mediated delivery of enzyme to tumor. MFE-CP, a multifunctional genetic fusion protein of antibody and enzyme, was designed to achieve this by two mechanisms. First by using a high affinity and high specificity single chain Fv antibody directed to carcinoembryonic antigen. Second by rapid removal of antibody-enzyme from normal tissues by virtue of post-translational mannosylation. The purpose of this paper is to investigate these dual functions in an animal model of pharmacokinetics, pharmacodynamics, toxicity, and efficacy.

EXPERIMENTAL DESIGN

MFE-CP was expressed in the yeast Pichia pastoris and purified via an engineered hexahistidine tag. Biodistribution and therapeutic effect of a single ADEPT cycle (1,000 units/kg MFE-CP followed by 70 mg/kg ZD2767P prodrug at 6, 7, and 8 hours) and multiple ADEPT cycles (9-10 cycles within 21-24 days) was studied in established human colon carcinoma xenografts, LS174T, and SW1222.

RESULTS

Selective localization of functional enzyme in tumors and rapid clearance from plasma was observed within 6 hours, resulting in tumor to plasma ratios of 1,400:1 and 339:1, respectively for the LS174T and SW1222 models. A single ADEPT cycle produced reproducible tumor growth delay in both models. Multiple ADEPT cycles significantly enhanced the therapeutic effect of a single cycle in the LS174T xenografts (P = 0.001) and produced regressions in the SW1222 xenografts (P = 0.0001), with minimal toxicity.

CONCLUSIONS

MFE-CP fusion protein, in combination with ZD2767P, provides a new and successful ADEPT system, which offers the potential for multiple cycles and antitumor efficacy. These results provide a basis for the next stage in clinical development of ADEPT.

Sustained Tumor Regression of Human Colorectal Cancer Xenografts Using a Multifunctional Mannosylated Fusion Protein in Antibody-Directed Enzyme Prodrug Therapy

Purpose: Antibody-directed enzyme prodrug therapy (ADEPT) requires highly selective antibody-mediated delivery of enzyme to tumor. MFE-CP, a multifunctional genetic fusion protein of antibody and enzyme, was designed to achieve this by two mechanisms. First by using a high affinity and high specificity single chain Fv antibody directed to carcinoembryonic antigen. Second by rapid removal of antibody-enzyme from normal tissues by virtue of post-translational mannosylation. The purpose of this paper is to investigate these dual functions in an animal model of pharmacokinetics, pharmacodynamics, toxicity, and efficacy.

Experimental Design: MFE-CP was expressed in the yeast Pichia pastoris and purified via an engineered hexahistidine tag. Biodistribution and therapeutic effect of a single ADEPT cycle (1,000 units/kg MFE-CP followed by 70 mg/kg ZD2767P prodrug at 6, 7, and 8 hours) and multiple ADEPT cycles (9-10 cycles within 21-24 days) was studied in established human colon carcinoma xenografts, LS174T, and SW1222.

Results: Selective localization of functional enzyme in tumors and rapid clearance from plasma was observed within 6 hours, resulting in tumor to plasma ratios of 1,400:1 and 339:1, respectively for the LS174T and SW1222 models. A single ADEPT cycle produced reproducible tumor growth delay in both models. Multiple ADEPT cycles significantly enhanced the therapeutic effect of a single cycle in the LS174T xenografts (P = 0.001) and produced regressions in the SW1222 xenografts (P = 0.0001), with minimal toxicity.

Conclusions: MFE-CP fusion protein, in combination with ZD2767P, provides a new and successful ADEPT system, which offers the potential for multiple cycles and antitumor efficacy. These results provide a basis for the next stage in clinical development of ADEPT.

Engineered single chain antibody fragments for radioimmunotherapy

An ideal molecule to deliver radioimmunotherapy (RIT) would be target specific and have prolonged residence time at high concentrations in the tumour with rapid clearance from normal tissues. It would also be non-immunogenic. These features can be rationally introduced into recombinant antibody-based proteins using antibody engineering techniques. This review focuses on the use of antibody engineering in the design and development of RIT molecules which have single chain Fv (scFv) antibody fragments as building blocks.

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

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

Efficient Retroviral Vector Targeting of Carcinoembryonic Antigen-Positive Tumors

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

Glycoforms obtained by expression in Pichia pastoris improve cancer targeting potential of a recombinant antibody-enzyme fusion protein

MFE-CP is a recombinant antibody-enzyme fusion protein used for antibody-mediated delivery of an enzyme to cancer deposits. After clearance from normal tissues, the tumor-targeted enzyme is used to activate a subsequently administered prodrug to give a potent cytotoxic in the tumor. MFE-CP localizes to cancer deposits in vivo, but we propose that its therapeutic potential could be improved by N-glycosylation, obtained by expression in Pichia pastoris. Glycosylation could enhance clearance from healthy tissue and result in better tumor:normal tissue ratios. To test this, glycosylated MFE-CP was expressed and purified from P. pastoris. The resultant MFE-CP fusion protein was enzymatically active and showed enhanced clearance from normal tissues in vivo. Furthermore, it showed effective tumor localization. This favorable glycosylation pattern was analyzed by tandem mass spectrometry. High-resolution, high-detection sensitivity collision-induced dissociation experiments proved essential for this task. Results showed that of the three potential N-glycosylation sites only two were consistently occupied with oligomannose structures. Asn-442 appeared the most heterogeneously populated with oligomannose carbohydrates extending from 5 to 13 units in length. Asn-484 was found only in its nonglycosylated form. There was less heterogeneity at Asn-492, which was glycosylated with oligosaccharide structures ranging from 8 to 10 mannose units. Nonglycosylated forms of Asn-442 and Asn-492 were not observed.

Targeted retroviral infection of tumor cells by receptor cooperation

Retroviruses expressing two different receptor-binding domains linked by proline-rich spacers infect only cells expressing both retroviral receptors (Valsesia-Wittman et al., EMBO J. 6:1214-1223, 1997). Here we apply this receptor cooperation strategy to target human tumor cells by linking single-chain antibodies recognizing tumor antigens via proline-rich spacers to the 4070A murine leukemia virus surface protein.

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.

A strategy for mapping and neutralizing conformational immunogenic sites on protein therapeutics

Antibodies are highly specific recognition molecules which are increasingly being applied to target therapy in patients. One type of developmental antibody-based therapy is antibody directed enzyme prodrug therapy (ADEPT) for the treatment of cancer. In ADEPT, an antibody specific to a tumor marker protein delivers a drug-activating enzyme to the cancer. Subsequent intravenous administration of an inactive prodrug results in drug activation and cytotoxicity only within the locale of the tumor. Pilot clinical trials with chemical conjugates of the prodrug activating enzyme carboxypeptidase G2 (CPG2) chemically conjugated with an antibody to and carcinoembryonic antigen (CEA), have shown that CPG2-mediated ADEPT is effective but limited by formation of human antibodies to CPG2 (HACA). We have developed a recombinant fusion protein (termed MFE-CP) of CPG2 with an anti-CEA single chain Fv antibody fragment and we have developed methods to address the immunogenicity of this therapeutic. A HACA-reactive discontinuous epitope on MFE-CP was identified using the crystal structure of CPG2, filamentous phage technology and surface enhanced laser desorption/ionization affinity mass spectrometry. This information was used to create a functional mutant of MFE-CP with a significant reduction (range 19.2 to 62.5%, median 38.5%) in reactivity with the sera of 11 patients with post-therapy HACA. The techniques described here are valuable tools for identifying and adapting undesirable immunogenic sites on protein therapeutics.

In vivo tumor delivery of a recombinant single chain Fv::tumor necrosis factor-alpha fusion [correction of factor: a fusion] protein

Locoregional and intratumoral administration of tumor necrosis factor alpha (TNF alpha) has been successful in obtaining inhibition or regression of tumor growth in the clinic. This potent antitumor activity of TNF alpha has not yet been exploited as a systemic agent in cancer therapy, mainly due to high levels of toxicity to normal tissues before a therapeutic dose of TNF alpha in the tumor has been achieved. To address this, we have targeted TNF alpha using antitumor antibodies. We have used a genetic fusion of human recombinant TNF alpha with MFE-23, a single-chain Fv antibody fragment directed against carcinoembryonic antigen. MFE-23::TNF alpha fusion protein is isolated in high yields (28 mg/L) from bacterial inclusion bodies and purified to homogeneity by affinity chromatography. It is a 144 kDa trimer in native form and possesses the antigen-binding activity of the sFv and the cytotoxicity to both WEHI 164 and a human adenocarcinoma cell line (LoVo) of rhTNF alpha. Radiolabeled MFE-23::TNF alpha binds both human and mouse TNF receptor 1 in vitro and is able to localize effectively in nude (nu/nu) mice bearing human LS174T xenografts; tumor/tissue ratios of 21:1 and 60:1 are achieved 24 and 48 h after intravenous injection. These studies indicate that MFE-23::TNF alpha will provide an effective means for systemically administered cancer therapy with TNF alpha.

Clinical applications of phage-derived sFvs and sFv fusion proteins

Single chain Fv antibodies (sFvs) have been produced from filamentous bacteriophage libraries obtained from immunised mice. MFE-23, the most characterised of these sFvs, is reactive with carcinoembryonic antigen (CEA), a glycoprotein that is highly expressed in colorectal adenocarcinomas. MFE-23 has been expressed in bacteria and purified in our laboratory for two clinical trials; a gamma camera imaging trial using ¹²³I-MFE-23 and a radioimmunoguided surgery trial using ¹²⁵I-MFE-23, where tumour deposits are detected by a hand-held probe during surgery. Both these trials show MFE-23 is safe and effective in localising tumour deposits in patients with cancer. We are now developing fusion proteins which use MFE-23 to deliver a therapeutic moiety; MFE-23::CPG2 targets the enzyme carboxypeptidase G2 (CPG2) for use in the ADEPT (antibody directed enzyme prodrug therapy) system and MFE::TNFα aims to reduce sequestration and increase tumor concentrations of systemically administered TNFα.

A strategy for antitumor vascular therapy by targeting the vascular endothelial growth factor: receptor complex

Vascular endothelial growth factor (VEGF) is produced by cancer cells in response to hypoxia and is the primary stimulant of vascularization in solid tumors. Endothelial cells lining the blood vessels of these tumors have a high concentration of receptor-bound VEGF on their surface, providing a target for antibody- directed cancer therapy. To obtain a cloned antibody to this target when bound to its receptor on tumor endothelium, we used phage display technology to create a single-chain Fv (sFv) antibody library from mice immunized with the 165-amino acid isoform of human VEGF-A. We selected, purified, and characterized LL4, an anti-VEGF sFv that was shown to react with receptor-bound VEGF. LL4 bound selectively to blood vessel endothelium, as shown by immunohistochemistry on tissue sections of human tumors. Furthermore, using autoradiography and grain counting of histological sections, systemically administered LL4 was shown to localize selectively to the endothelial lining of tumor blood vessels in human colorectal carcinoma xenografts in vivo. This study demonstrates the feasibility of targeting tumor vasculature using recombinant antibodies to the VEGF:receptor complex.

Recombinant anti-carcinoembryonic antigen antibodies for targeting cancer

Antibodies can be used to target cancer therapies to malignant tissue; the approach is attractive because conventional treatments such as chemo- and radiotherapy are dose limited due to toxicity in normal tissues. Effective targeting relies on appropriate pharmacokinetics of antibody-based therapeutics, ideally showing maximum uptake and retention in tumor and rapid clearance from normal tissue. We have studied the factors influencing these dynamics for antibodies against carcinoembryonic antigen (CEA). Protein engineering of anti-CEA antibodies, in vivo biodistribution models, and mathematical models have been employed to improve understanding of targeting parameters, define optimal characteristics for the antibody-based molecules employed, and develop new therapies for the clinic. Engineering antibodies to obtain the desired therapeutic characteristics is most readily achieved using recombinant antibody technology, and we have taken the approach of immunizing mice to provide high-affinity anti-CEA single-chain Fv antibodies (sFvs) from filamentous bacteriophage libraries. MFE-23, the most characterized of these sFvs, has been expressed in bacteria and purified in our laboratory for two clinical trials: a gamma camera imaging trial using 123I-MFE-23 and a radioimmunoguided surgery trial using 125I-MFE-23, where tumor deposits are detected by a hand-held probe during surgery. Both these trials showed that MFE-23 is safe and effective in localizing tumor deposits in patients with cancer. We are now developing fusion proteins that use the MFE-23 antibody to deliver a therapeutic moiety; MFE-23:: carboxypeptidase G2 (CPG2) targets the enzyme CPG2 for use in the antibody-directed enzyme prodrug therapy system and MFE::tumor necrosis factor alpha (TNFalpha) aims to reduce sequestration and increase tumor concentrations of systemically administered TNFalpha.

Radioimmunoguided surgery in colorectal cancer using a genetically engineered anti-CEA single-chain Fv antibody

In radioimmunoguided surgery (RIGS), a radiolabeled antibody is given i.v. before surgery and a hand-held gamma-detecting probe is used to locate tumor in the operative field. The rapid blood clearance and good tumor penetration of single-chain Fv antibodies (scFv) offer potential advantages over larger antibody molecules used previously for RIGS. A Phase I clinical trial is reported on RIGS with scFv (MFE-23-his) to carcinoembryonic antigen (CEA). Thirty-four patients undergoing surgery for colorectal carcinoma (17 primary tumors, 16 liver metastases, and 1 anastomotic recurrence) and 1 patient with liver metastases of pancreatic carcinoma received 125I-labeled MFE-23-his scFv (125I-MFE-23-his) 24, 48, 72, or 96 h before operation. 125I-MFE-23-his showed biexponential blood clearance with alpha and beta half-lives of 0.32 and 10.95 h, respectively. The abdomen was scanned during surgery with a hand-held gamma detecting probe (Neoprobe Corp.). 125I-MFE-23-his showed good tumor localization; comparison with histology showed overall accuracy of 84%. Highest median ratios for tumor:normal tissue and tumor:blood were recorded 72 or 96 h after scFv injection for patients undergoing resection of liver metastases. High levels of radioactivity were found in the kidneys. Five patients had grade 1 fever, and three had a grade 1 rise in blood pressure according to the Common Toxicity Criteria. There was a significant correlation between these ratios and those measured in excised tissues using a laboratory gamma counter (P < 0.001). MFE-23-his scFv antibody localizes in CEA-producing carcinomas. The short interval between injection and operation, the lack of significant toxicity, and the relatively simple production in bacteria make MFE-23-his scFv suitable for RIGS.

Crystal structure of the anti-(carcinoembryonic antigen) single-chain Fv antibody MFE-23 and a model for antigen binding based on intermolecular contacts

MFE-23 is the first single-chain Fv antibody molecule to be used in patients and is used to target colorectal cancer through its high affinity for carcinoembryonic antigen (CEA), a cell-surface member of the immunoglobulin superfamily. MFE-23 contains an N-terminal variable heavy-chain domain joined by a (Gly(4)Ser)(3) linker to a variable light-chain (V(L)) domain (kappa chain) with an 11-residue C-terminal Myc-tag. Its crystal structure was determined at 2.4 A resolution by molecular replacement with an R(cryst) of 19.0%. Five of the six antigen-binding loops, L1, L2, L3, H1 and H2, conformed to known canonical structures. The sixth loop, H3, displayed a unique structure, with a beta-hairpin loop and a bifurcated apex characterized by a buried Thr residue. In the crystal lattice, two MFE-23 molecules were associated back-to-back in a manner not seen before. The antigen-binding site displayed a large acidic region located mainly within the H2 loop and a large hydrophobic region within the H3 loop. Even though this structure is unliganded within the crystal, there is an unusually large region of contact between the H1, H2 and H3 loops and the beta-sheet of the V(L) domain of an adjacent molecule (strands DEBA) as a result of intermolecular packing. These interactions exhibited remarkably high surface and electrostatic complementarity. Of seven MFE-23 residues predicted to make contact with antigen, five participated in these lattice contacts, and this model for antigen binding is consistent with previously reported site-specific mutagenesis of MFE-23 and its effect on CEA binding.