Site-specific polysialylation of an antitumor single-chain Fv fragment

Protein pharmacokinetic modulation is becoming an important tool in the development of biotherapeutics. Proteins can be chemically or recombinantly modified to alter their half-lives and bioavailability to suit particular applications as well as improve side effect profiles. The most successful and clinically used approach to date is chemical conjugation with poly(ethylene glycol) polymers (PEGylation). Here, therapeutic protein half-life can be increased significantly while retaining biological function, reducing immunogenicity and cross-reaction. Naturally occurring alternatives to such synthetic polymers could have major advantages such as lower side effects due to biodegradability and metabolism. Polysialic acid (PSA) has been investigated as a pharmacokinetic modulatory biopolymer with many successful examples in preclinical and clinical development. Single-chain Fvs (scFvs) are a choice antibody format for human therapeutic antibody discovery. Because of their small size, they are rapidly eliminated from the circulation and often are rebuilt into larger proteins for drug development and a longer half-life. Here we show that chemical polysialylation can increase the half-life of an antiplacental alkaline (PLAP) and anticarcinoembryonic antigen (CEA) scFv (F1 and MFE-23, respectively) 3.4-4.9-fold, resulting in a 10.6-15.2-fold increase in blood exposure. Amine-directed coupling of the MFE-23 scFv reduced its immunoreactivity 20-fold which was resolved by site-specific polysialylation through an engineered C-terminal thiol residue. The site-specifically polysialylated MFE-23 scFv demonstrated up to 30-fold improved tumor uptake while displaying favorable tumor:normal tissue specificity. This suggests that engineering antibody fragments for site-specific polysialylation could be a useful approach to increase the half-life for a variety of therapeutic applications.

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.

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.

Catalytic activity of an in vivo tumor targeted anti-CEA scFv::carboxypeptidase G2 fusion protein

Antibody-directed enzyme prodrug therapy (ADEPT) targets an enzyme selectively to a tumor where it converts a relatively non-toxic prodrug to a potent cytotoxic drug. Previous clinical work using antibody-enzyme chemical conjugates has been limited by the moderate efficiency of tumor targeting of these molecules. To address this a recombinant fusion protein composed of MFE-23, an anti-carcinoembryonic antigen (CEA) single chain Fv (scFv) antibody, fused to the amino-terminus of the enzyme carboxypeptidase G2 (CPG2) has been constructed to achieve ADEPT in CEA-producing tumors. MFE-23::CPG2 fusion protein was overexpressed in Escherichia coli and purified using CEA affinity chromatography. Efficacy of MFE-23::CPG2 delivery to tumors in vivo was assessed by measuring catalytic activity after intravenous injection of purified MFE-23::CPG2 into nude mice bearing CEA-positive LS174T human colon adenocarcinoma xenografts. Recombinant MFE-23::CPG2 cleared rapidly from circulation and catalytic activity in extracted tissues showed tumor to plasma ratios of 1.5:1 (6 hr), 10:1 (24 hr), 19:1 (48 hr) and 12:1 (72 hr). (125)I-MFE-23::CPG2 was retained in kidney, liver and spleen but MFE-23::CPG2 catalytic activity was not, resulting in excellent tumor to normal tissue enzyme ratios 48 hr after injection. These were 371:1 (tumor to liver), 450:1 (tumor to lung), 562:1 (tumor to kidney), 1,477:1 (tumor to colon) and 1,618:1 (tumor to spleen). Favorable tumor : normal tissue ratios occurred at early time points when there was still 21% (24 hr) and 9.5% (48 hr) of the injected activity present per gram of tumor tissue. The high tumor concentrations and selective tumor retention of active enzyme delivered by MFE-23::CPG2 establish that this recombinant fusion protein has potential to give improved clinical efficiency for ADEPT.

Taking engineered anti-CEA antibodies to the clinic

There is a need to improve on existing targeting technologies in order to develop effective cancer therapy. We have investigated this for colorectal cancer using antibodies directed against carcinoembryonic antigen (CEA). Chemical and molecular protein engineering has been used to produce antibody molecules which differ in molecular weight, affinity, valency and specificity. These have been characterised and tested in animal tumour models and clinical trials to test the parameters important for optimising tumour penetration, increasing residence time in viable areas of the tumour, accelerating clearance from normal tissues and improving therapeutic efficacy.

In vitro characterization of a recombinant 32P-phosphorylated anti-(carcinoembryonic antigen) single-chain antibody

The major limitations of monoclonal antibody conjugates as therapeutic agents have been their poor tumour targeting, inadequate tumour penetration and immunogenicity. More even and deeper tissue penetration has been demonstrated with smaller antibody fragments. The smaller size and absence of an Fc segment may contribute to a lowered immunogenicity with single-chain antibodies (scFv) and also permit their recombinant engineering and bacterial expression. We describe the successful engineering, expression and pre-clinical characterisation of a phosphorylatable "kemptide" (Leu-Arg-Arg-Ala-Ser-Gly) anti-carcinoembryonic antigen (anti-CEA) scFv (PKS-scFv), for use as a radioimmunotherapeutic agent. Specifically, a yield of 6 mg/l induced culture was obtained. Site-specific phosphorylation was demonstrated without loss of specificity. In vitro assays revealed a selective cytotoxicity of 32P-PKS-scFv for high-CEA-expressing LS-174T cells compared to the low-CEA-expressing HT-29 cells, with a rapid internalisation rate.

Exemplifying guidelines for preparation of recombinant DNA products in phase I trials in cancer: preparation of a genetically engineered anti-CEA single chain Fv antibody

Products of recombinant DNA technology have potential for the diagnosis or treatment of cancer. There is a need to investigate whether they function by the intended mechanism in small phase I clinical trials before their suitability for more extensive studies can be assessed. Quality and safety of these products should be assured prior to their use in humans in a way which is appropriate to the preliminary nature of the trials but not inhibitory to progress. The Cancer Research Campaign control recommendations for products derived from recombinant DNA technology (Begent RHJ and associates. Eur J Cancer 1993, 29A, 13, 1907-1910) provide guidelines for the production of new biotechnology products in academic research units within a relatively short time, while ensuring appropriate quality and safety. The practical application of the guidelines requires that solutions are found for the quality and safety issues during the production of recombinant products. We describe an approach to the relevant quality and safety issues during and after the production and purification of a genetically engineered anti-carcinoembryonic antigen (CEA) single chain Fv (scFv) antibody for a phase I trial of radioimmunoguided surgery with the intention of providing a model for other products.

Targeting strategy for gene delivery to carcinoembryonic antigen-producing cancer cells by retrovirus displaying a single-chain variable fragment antibody

Cancer-specific antigens are promising targets for the specific delivery of certain drugs or genes to cancer cells in cancer therapy. Carcinoembryonic antigen (CEA) is one of the cancer-associated antigens predominantly detected in the gastrointestinal cancer of the colon and stomach. Targeting strategies for CEA-producing cancer cells have been thoroughly developed mainly by the production of monoclonal antibodies to CEA and further single-chain variable fragment (scFv) antibodies. Here, we have generated Moloney murine leukemia virus-derived retroviral vectors co-displaying an anti-CEA scFv-envelope chimeric protein and an unmodified envelope protein to deliver a gene for herpes simplex virus thymidine kinase (HSV-tk) or Escherichia coli beta-galactosidase. The harvested viruses successfully incorporated the chimeric envelope protein as well as the unmodified envelope into the viral particles, and specifically bound to and infected human CEA-producing cancer cells via recognition of CEA, depending on the CEA-producing phenotype of the target cells. These results may have significant implications for the use of scFv directed against tumor-specific antigens for targeting specific antigen-producing cancer cells, a potential step toward in vivo cancer therapy.

Preclinical characterization and in vivo imaging studies of an engineered recombinant technetium-99m-labeled metallothionein-containing anti-carcinoembryonic antigen single-chain antibody

We describe the engineering of a novel single-chain fragment (scFv) metallothionein (MET) containing anti-carcinoembryonic antigen (CEA) antibody (referred to as MET-scFv) for use as a diagnostic imaging agent in colorectal cancer.

METHODS

Site-directed cloning of annealed oligonucleotides, containing both the MET and a c-myc tag sequence, into a pUC19-based expression vector enabled soluble secreted protein expression from Escherichia coli. Affinity purification was used to purify the protein using an anti-c-myc affinity column. The specificity of both the unlabeled and labeled MET-scFv for CEA was demonstrated by solid-phase enzyme-linked immunosorbent assay and radioimmunoassay and by fluorescence-activated cell sorting analysis on CEA-expressing human colorectal LS-174T cells. Technetium-99m labeling was achieved using a Zn2+ transchelation step, enabling direct 99mTc transfer without separate reduction of MET. In vitro stability was demonstrated by fast protein liquid chromatography analysis of labeled MET-scFv, incubated with bovine serum albumin (BSA), transferrin and mouse serum. Last, in vivo pharmacokinetics, biodistribution and imaging were performed.

RESULTS

Yields of 6 mg/liter induced culture purified protein were achieved. Successful site-specific labeling was demonstrated using a Zn2+ transchelation modification of a pretinning method, which also enabled lower amounts of the reducing agent to be used. The specificity for CEA was retained after labeling. Despite a rapid serum clearance (t(1/2alpha) = 2.8 min), adequate localization to tumor of 5.37% injected dose/g at 4 hr was demonstrated. Moreover, the short-lived t(1/2alpha) of scFv, its early tumor targeting and rapid blood-pool clearance gave tumor-to-blood ratios of 2.07 by 4 hr, enabling early gamma camera imaging. Successful and specific imaging was achieved using LS-174T xenografts in nude mice by 3-6 hr.

CONCLUSION

A recombinant MET containing scFv was successfully expressed, purified and labeled with 99Tc. The stable site-specific labeling of 99Tc, combined with the rapid plasma clearance of the scFv, led to successful early in vivo imaging of xenografted mice.

Clinical evidence of efficient tumor targeting based on single-chain Fv antibody selected from a combinatorial library

We present a system for cancer targeting based on single-chain Fv (scFv) antibodies selected from combinatorial libraries, produced in bacteria and purified by using an engineered tag. Combinatorial libraries of scFv genes contain great diversity, and scFv antibodies with characteristics optimized for a particular task can be selected from them using filamentous bacteriophage. We illustrate the benefits of this system by imaging patients with carcinoembryonic antigen (CEA)-producing cancers using an iodine-123 labeled scFv anti-CEA selected for high affinity. All known tumor deposits were located, and advantages over current imaging technology are illustrated. ScFvs are produced in a cloned form and can be readily engineered to have localizing and therapeutic functions that will be applicable in cancer and other diseases.

Technetium-99m radiolabeling using a phage-derived single-chain Fv with a C-terminal cysteine

Single-chain Fv (scFv) antibody fragments have potential for clinical imaging because of their rapid tumor penetration and high tumor-to-tissue ratios at early time points. ScFvs clear rapidly from the circulation so radiolabels such as 99mTc which have short half-lives are desirable, but the free thiol groups necessary for labeling with 99mTc are not normally found on these molecules.

METHODS

We constructed a vector which enabled a free cysteine to be linked to the C-terminus of scFvs. MFE-23, a scFv directed against carcinoembryonic antigen (CEA), was cloned into this vector and cys-tagged MFE-23 was labeled with 99mTc using a D-glucarate transfer method.

RESULTS

The radiolabeled product was stable in vivo and in vitro and showed favorable tumor-to-blood ratios in vivo at early time points (4:1 at 24 hr and 8:1 at 48 hr), although high kidney levels were also detected.

CONCLUSION

Our study demonstrates an effective method to enable scFvs radiolabeling with 99mTc and also shows the potential of using a 99mTc-labeled scFv for clinical imaging studies.

In vitro and in vivo characterisation of a recombinant carboxypeptidase G2::anti-CEA scFv fusion protein

BACKGROUND

There is considerable interest in the specific targeting of therapeutic agents to cancer cells. Of particular promise is a technique known as Antibody-Directed Enzyme Prodrug Therapy (ADEPT). In this approach an enzyme is targeted to the tumour by its conjugation to a tumour specific-antibody tumour. After allowing sufficient time for the conjugate to localise at the tumour and clear from the circulatory system, a relatively non-toxic prodrug is administered. This prodrug is converted to a highly cytotoxic drug by the action of the targeted enzyme localised at the tumour site.

OBJECTIVES

To construct gene fusions between the pseudomonad carboxypeptidase G2 (CPG2) gene and DNA encoding MFE-23 (an anti-carcinoembryonic antigen (CEA) single-chain Fv (scFv) molecule), derived from a phage display library. To overexpress the resultant gene fusions in Escherichia coli, and assess the in vitro and in vivo properties of the purified fusion proteins.

STUDY DESIGN

To introduce unique cloning restriction sites into the 5'-end of the CPG2 gene by site-directed mutagenesis to facilitate fusion to the 3'-end of the gene encoding MFE-23 (constructs with or without a flexible (Gly4Ser)3 linker-encoding sequence were designed). To overexpress the resultant gene fusions under transcriptional control of the lac promoter and to direct the fusion proteins produced to the periplasmic space of E. coli through translational coupling to the pelB signal peptide.

RESULTS

Biologically active recombinant CPG2::MFE-23 scFv fusion proteins were produced in E. coli and shown to possess enzyme and anti-CEA activity. Affinity chromatography followed by size exclusion gel filtration yielded approximately 0.7-1.4 mg/l from shake flask culture. The fusion protein in which the enzyme and antibody moieties were joined by a linker peptide was shown to be effectively localised in nude mice bearing human colon tumour xenografts, giving favourable tumour to blood ratios.

CONCLUSION

MFE-23 scFv serves as an ideal candidate for the antibody arm of a bacterially expressed fusion protein with CPG2. The biological properties of this recombinant protein suggest that it may be employed for tumour specific prodrug activation. However, further assessment of its stability and pharmokinetics is required if genetic fusion is to be considered as an alternative to chemical conjugation.

Enhanced tumour specificity of an anti-carcinoembrionic antigen Fab' fragment by poly(ethylene glycol) (PEG) modification

Polyethylene glycol (PEG) modification of a chimeric Fab' fragment (F9) of A5B7 (alpha-CEA), using an improved coupling method, increases its specificity for subcutaneous LS174T tumours. PEGylation increased the area under the concentration-time curve (AUC0-144) in all tissues but there were significant differences (variance ratio test, F = 27.95, P < 0.001) between the proportional increases in AUC0-144, with the tumour showing the greatest increase. The increase in AUCtumour from F9 to PEG-F9 was similar to the reported increase from Fab' to F(ab')2 while the increase in AUCblood by PEGylation of F9 was only 21% of the reported increase from Fab' to whole IgG. A two sample t-test showed no significant differences between maximal tumour/tissue ratios for PEG-F9 and F9 while the tumour/tissue ratios for PEG-F9 remained high over a longer period, with tumour levels at least double those for F9. PEG-F9 emerges as a new generation antibody with potential advantages for both radioimmunotherapy and tumour imaging. Since there was a reduction in antigen binding, optimisation of PEGylation might further improve tumour specificity. The latter resulted from complex effects on both the entry into and exit rates from tumour and normal tissues in a tissue-specific fashion.

Clinical issues in antibody design

Antibody genes can now be cloned and expressed in various ways to give new versions of antibodies that possess reduced immunogenicity, improved affinity, altered size, increased avidity and novel effector functions. The task for any clinical application is, first, to define a relevant target, and then to design the optimal antibody-based therapeutic molecule to react with that target. This article reviews these improved antibody-based molecules and examines their role in cancer therapy.

A single chain Fv derived from a filamentous phage library has distinct tumor targeting advantages over one derived from a hybridoma

Single-chain antibodies (scFvs) can be derived from a monoclonal antibody (MAb) or produced directly using filamentous phage technology, where antibodies with desired binding and purification characteristics can be readily selected from libraries. To test the hypothesis that the latter approach is more useful, we compared 2 anti-carcinoembryonic antigen (CEA) scFvs produced by these 2 different approaches. Our study showed that, both in the purification process and in the biodistribution pattern, MFE-23, produced by filamentous phage technology, gave favourable results compared to A5-SC, which is derived from the A5B7 MAb. This indicates the value of the filamentous phage approach for obtaining tumour-targeting scFvs.

Purification of bacterially expressed single chain Fv antibodies for clinical applications using metal chelate chromatography

A new procedure is described for the purification of an anti-carcinoembryonic antigen (CEA) single chain Fv (scFv), referred to as MFE-23, from bacterial supernatant. A simple insertion of a hexa-histidine tail fused at the C-terminus (MFE-23 His) provides an affinity tag which selectively binds to transition metal ions immobilised on an iminodiacetic acid (IDA) derivitised solid phase matrix. This method proved to be superior to standard CEA antigen affinity chromatography in the following ways. (1) A higher yield was produced (10 mg/l as opposed to 2.2 mg/l of bacterial supernatant). The latter figure was largely affected by the limited availability (size of the column) of immobilised CEA antigen. (2) Scale-up was relatively simple and less costly. (3) The risk of tumour derived antigen leaching from the column is eliminated. Results showed that immobilised Cu2+ ions were more effective than Ni2+ and Zn2+ ions in retaining the His tagged product giving a 90% pure product on elution. Clinical grade material was generated using size exclusion chromatography to remove aggregated material, and Detoxi gel to remove bacterial endotoxins. Validation assays to measure DNA, copper and endotoxins were performed to assess the levels of contaminants. MFE-23 His retained 84% antigen binding after 6 months storage at 4 degrees C and > 75% after radiolabelling. Further experiments confirmed that the His tail did not affect biodistribution and tumour localisation in nude mice bearing human colorectal tumour xenografts.

Production and tumour-binding characterization of a chimeric anti-CEA Fab expressed in Escherichia coli

A recombinant chimeric Fab (rcFab), with Fv derived from the monoclonal A5B7 antibody to carcinoembryonic antigen (CEA), and with human CHI and C kappa was cloned into pUC 19 and expressed in Escherichia coli. rcFab (10 to 12 mg per litre) was produced in bacterial culture fluid, and functional purified rcFab was isolated by affinity chromatography (using antibody to human C kappa) and size-exclusion gel filtration. The rcFab did not show reduced affinity for CEA, and reacted with human colorectal tumours showing a typical anti-CEA pattern by immunocytochemistry; it was also stable after iodination. Biodistribution studies in nude mice bearing human tumour xenografts showed no toxicity and good tumour localization. Therapeutic ratios at early time points were better than those obtained with whole murine antibody. The results demonstrate that bacterially produced anti-CEA Fab is of use for tumour targeting.

Phage libraries for generation of clinically useful antibodies

Insertion of antibody genes into filamentous bacteriophage makes it possible to generate and screen libraries of 10(7) or more antibodies. Each phage expresses an antibody on its surface and contains the corresponding antibody gene. Genes that encode antibodies with desired characteristics are readily selected and their antibodies expressed as soluble proteins in Escherichia coli. We used this system to produce an antibody to carcinoembryonic antigen with higher affinity and better tumour specificity than antibodies currently in use.

CD44 is associated with proliferation in normal and neoplastic human colorectal epithelial cells

Flash-frozen biopsies obtained from surgical specimens of three adenomatous polyps and 22 colorectal adenocarcinomas (19 primary and three metastatic) were tested by immunohistochemistry for CD44 expression using F10-44-2 monoclonal antibody. CD44 positivity was correlated with proliferative status defined by Ki-67 monoclonal antibody reactivity. In normal colonic mucosa, CD44 was expressed in the proliferative zone of crypts. In tumours, CD44 expression was associated with proliferative areas irrespective of tumour stage or differentiation. Non-proliferating areas of the carcinomatous epithelium did not express CD44 although non-proliferating stromal lymphoid tissue did. There was no apparent association with tumour progression. F10-44-2-defined CD44 is consistently expressed during proliferation by normal colorectal epithelial cells and by both benign and malignant colorectal tumour cells.

Localization of CEA messenger RNA by in situ hybridization in normal colonic mucosa and colorectal adenocarcinomas

Carcinoembryonic antigen (CEA) mRNA expression was studied in 14 cases of normal colorectal mucosa and colonic adenocarcinomas using in situ hybridization with a 32P-labelled cDNA probe to the unique 3'-untranslated region of CEA. This approach has the advantage that the target mRNA remains in the cell of origin, whereas there is considerable ambiguity in immunocytochemistry data for CEA because the protein is secreted. Furthermore, the specific cDNA probe overcomes potential problems of immunological cross-reactivity with other members of the CEA family. The results demonstrated that abundant, heterogeneously distributed CEA mRNA was present in colorectal adenocarcinomas, with the highest levels in cells lining glandular structures. Parallel immunohistochemistry with anti-CEA monoclonal antibody A5B7 showed that the regions of tumours with the highest levels of CEA mRNA also had the highest CEA protein levels, suggesting that the heterogeneous distribution reflects CEA expression rather than differential secretion of the protein. In the normal colonic mucosa, CEA mRNA expression was observed in surface epithelial cells and goblet cells of the upper crypts, with very low hybridization in the mid crypt and at the base. This crypt-surface distribution was identical to that observed for CEA protein. In situ hybridization therefore confirms that high levels of CEA mRNA are expressed in differentiated surface epithelial cells of the normal colon.

Mitochondrial gene expression in the human gastrointestinal tract

In the human gastrointestinal epithelium, in situ hybridisation demonstrates that 12 S and 16 S mitochondrial ribosomal RNAs show maximal steady-state levels on the surface epithelial cells of the normal small intestine and colon. The mitochondrial mRNAs, cytochrome b and NADH dehydrogenase (IV) have a uniform distribution throughout the crypt and surface (villus) epithelial cells of the small intestine and colon. Histochemical stains for the activity of the mitochondrial respiratory chain enzymes succinate dehydrogenase and cytochrome oxidase also show almost uniform activities throughout the crypt-surface epithelial cell axis in the small and large intestines. In sections of normal human oesophagus the levels of mitochondrial ribosomal RNAs, mitochondrial mRNAs and the activities of mitochondrial respiratory chain enzymes are maximal over the basal cells of the stratified squamous epithelium. These results show a relative increase in mitochondrial ribosomal RNA expression compared with mitochondrial mRNAs in surface cells of simple intestinal epithelia.

Increased expression of c-myc proto-oncogene in biopsies of ulcerative colitis and Crohn's colitis

The steady state levels of c-myc mRNA have been measured in RNA samples extracted from colonoscopic biopsies of inflammatory bowel disease patients obtained at routine endoscopy sessions. Biopsies were immediately frozen in liquid nitrogen limiting the ischaemic time to less than 15 seconds, and can be stored for up to 96 hours before separation of RNA. Yields of RNA using biopsies were 0.137 (0.041)% wet wt (mean (SD), n = 68), these are significantly better than those obtained from surgical material (0.064 (0.063)% wet wt (mean (SD), n = 21) where the tissue ischaemic time was 45 minutes to one hour 40 minutes. Functional activity of RNA extracted was demonstrated by the ability to direct in vitro protein translation in the rabbit reticulocyte system. We have used this technique to show that there is an increased ratio of steady state c-myc proto-oncogene expression in inflamed tissue from 18 patients with left sided ulcerative colitis and five patients with segmental Crohn's colitis, compared with an uninvolved region of the colon in each case. No difference in c-myc expression was seen in biopsies at least 30 cm apart in 11 control patients with no macroscopic or histological abnormalities. Increased expression of c-myc in inflammatory bowel disease is consistent with the activation of this proto-oncogene during altered cell cycle control resulting from the inflammatory process.

In situ and slot hybridization analysis of RNA in colorectal tumours and normal colon shows distinct distributions of mitochondrial sequences

cDNA clones of mRNAs with an abnormal abundance in familial adenomatous polyposis were used to examine the levels and distribution of the mRNAs in tissues from 15 patients with colorectal cancer. Of 12 cloned sequences studied by slot hybridization, one was substantially reduced in tumours compared with normal tissue. Sequence analysis showed this to code for IgA. In situ hybridization was consistent with slot hybridization and immunohistochemistry. Two mitochondrially encoded sequences had distinct distributions detected by in situ hybridization but did not have detectable quantitative differences in whole tumour or mucosa extracts.

Identification of a human ribosomal protein mRNA with increased expression in colorectal tumours

A human ribosomal protein cDNA was selected from a normal colon cDNA library on the basis of overexpression in familial adenomatous polyposis. Nucleotide sequence analysis was used to identify this cDNA as corresponding to the human equivalent of the rat ribosomal protein L31 (HL31). We have quantified the expression of HL31 mRNA in colorectal tumours and found overexpression in 23 out of 23 cases. Our results indicate that HL31 is associated with a malfunction of normal growth regulatory mechanisms in these tumours, and suggest a role for HL31 in proliferation and neoplasia.

Second antibody for improvement of antibody imaging: liposome-entrapped and free preparations in animal and human studies

When anti-tumour antibodies are given systematically for tumour imaging or therapy, second antibody directed against the first (anti-tumour) antibody can be used to accelerate clearance of first antibody, thus improving discrimination between tumour and normal tissues. Liposome-entrapped, and free second antibodies (LESA and FSA, respectively) have been compared in an animal tumour model system and in patients with cancer. Nude mice bearing xenografts of human colon carcinoma were given goat antibody to carcino-embryonic antigen (CEA) as first antibody and horse anti-goat second antibody. Patients with gastrointestinal carcinomas received i.v. 131I-labelled goat anti-CEA or mouse monoclonal 17-1A first antibody and unlabelled horse angi-goat or rabbit anti-mouse second antibody, respectively. Antibody distribution was studied by serial gamma camera imaging. The effectiveness of LESA and FSA depended on dose. Tumour-to-blood ratios were increased up to eight-fold by either method in animals. Tumour imaging was enhanced among 15 patients with gastrointestinal cancer and tumour was correctly identified at five sites where it was not seen by a background subtraction method. No significant toxicity occurred in patients nor in rabbits studied for evidence of immune complex mediated disease. LESA and FSA appear to be equally effective.

Lambert-Eaton syndrome antibodies: reaction with membranes from a small cell lung cancer xenograft

Lambert-Eaton myasthenic syndrome (LEMS) is a paraneoplastic autoimmune disorder caused by an IgG-mediated reduction in number of presynaptic voltage-gated calcium channels (VGCC) at the neuromuscular junction. In at least 50% of cases, the stimulus for antibody production may be VGCC on small cell lung cancer (SCLC). In this study membranes isolated from a human small cell lung cancer xenograft (Mar), that bound [3H]PN200-110, a VGCC antagonist, were subjected to Western blotting using plasma from 12 LEMS patients and eight controls. Although one band recognised by 3/12 LEMS IgGs might be associated with the VGCC, a number of other proteins were recognised both by LEMS plasma, and by plasma from patients with other disorders. The results illustrate the difficulties found using Western blotting with autoimmune plasma to identify specific polypeptides in a crude antigen preparation.

Circulating immune complexes (CIC), carcinoembryonic antigen (CEA) and CIC containing CEA as markers for colorectal cancer

It has been suggested that circulating immune complexes (CIC) would provide a useful tumour marker system and that carcinoembryonic antigen (CEA) may form an antigen component of CIC found in patients with colorectal cancer. In this study the clinical usefulness of CIC and CIC containing CEA (CEA-IC) was investigated. Concentrations of CIC were measured in 30 patients with colorectal cancer. Fourteen patients were studied sequentially at approximately 1 month intervals after apparent curative resection of the primary tumour. Results were correlated with those obtained from serum CEA and compared to clinical status. CEA-IC were measured using a novel assay and compared with CIC and CEA values in 29 patients. CIC concentrations were elevated in patients with known disease and predicted clinical relapse in four of 14 patients. In two patients CIC remained elevated despite sustained remission. CEA-IC were not detectable in any of the patients studied. CIC estimations may augment CEA measurements as indicators of disease recurrence but lack of specificity makes them of little practical value as tumour markers in colorectal cancer. No evidence was found to support previous reports that CEA was an antigen component of CIC in this disease.

Circulating immune complexes as markers of response to chemotherapy in malignant teratomas and gestational trophoblastic tumours

Concentrations of circulating immune complexes (CIC) were measured serially during chemotherapy of 22 patients with gestational trophoblastic tumours (GTT) and 11 patients with malignant teratoma (MT) by the polyethylene glycol precipitation and CIq solid-phase assays. Results were correlated with tumour response as measured by serum concentrations of human chorionic gonadotrophin (hCG) and alpha-foetoprotein (AFP). CIC concentrations correlated with disease status in the early stages of treatment in 4/22 patients with GTT and 5/11 with MT. CIC assays were less sensitive than hCG and AFP as a monitor of disease, and also less specific, in that 8 patients with GTT and 5 with MT developed raised CIC concentrations during chemotherapy in spite of sustained complete remission. Measurements of CIC concentrations by present methods are neither sufficiently sensitive nor specific to be of clinical value as a tumour marker in GTT and MT, and this casts doubt on their potential value in other malignancies. Attention should be directed to identification of the components of CIC, some of which may be more cancer-specific.