Characterization of the colorectal antigen IOR-C2

Construction of an immunotoxin with the pore forming protein StI and ior C5, a monoclonal antibody against a colon cancer cell line

Sticholysin I (StI), a potent cytolysin isolated from the sea anemone Stichodactyla helianthus, was linked to the monoclonal antibody (mAb) ior C5. StI acts by forming hydrophilic pores in the membrane of the attacked cells leading to osmotic lysis. ior C5 is a murine IgG1, which recognizes the tumor associated antigen (TAA) ior C2. The cytolysin and the mAb were coupled by using the heterobifunctional cross-linking reagent sulfosuccinimidyl 4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC). Two hybrid molecules composed by one ior C5 and one or two StI molecules were obtained (named conjugated I and II, respectively). The purified conjugates were evaluated by a binding affinity assay against an ior C2-positive colon cancer cell line (SW948). Both molecules were able to recognize the antigen (Ag) in the same way that unconjugated ior C5 does. The activity of both conjugates against human erythrocytes and SW948 cells was assessed. They lost most of their hemolytic activity but their residual activity was very similar. Nevertheless, when their cytotoxicity was studied on the SW948 cell line, only conjugate II killed efficiently the cells, indicating a specific mAb-Ag interaction. In this chimeric molecule the ratio between the cytotoxic and the hemolytic activity was larger than that of the free cytolysin. This fact indicates an increase of the specificity of the toxic effect toward the SW948 cell line and consequently an increase of the difference between its hemolytic and cytotoxic doses. The results herein support the feasibility of directing StI to the surface of cancer cells expressing ior C2 Ag via the mAb ior C5.

Humanization of predicted T-cell epitopes reduces the immunogenicity of chimeric antibodies: new evidence supporting a simple method

Genetic engineering has provided several approaches to reduce immunogenicity of murine antibodies. We described previously a new method based on the humanization of the linear epitopes presented to T cells. In brief, potential immunogenic epitopes in the variable region were identified and subjected to point mutations to make them human and/or to modify amphipatic motifs. The resulting recombinant antibody retained its antigen binding affinity and was less immunogenic in monkeys than their murine or chimeric predecessors are. The present study provides two new examples of this T-cell epitope humanization approach: ior-t1A murine monoclonal antibody (mMAb), which recognizes the human-CD6 molecule, and ior-C5 mMAb, which recognizes a novel glycoprotein expressed on the surface of malignant colorectal cells. Seven amino acids were substituted in ior-C5 and eleven residues in ior-t1A, by the corresponding residues from the highest homologous human sequences. Surprisingly, the homology between re-shaped chimeric antibody variable region frameworks and human sequences was 80-90%. Experiments in monkeys showed that T1AhT and C5hT "detopes" antibodies were less immunogenic than their chimeric analogues while they retained 30-50% of antigen binding affinities. The proposed method might be of general applicability to reduce immunogenicity of chimeric antibodies with therapeutic potential.

[Pilot study with the monoclonal antibody IOR-C5 as a potential agent of radioimmunoscintigraphy in colorectal cancer]

IOR C-5 is a G1 immunoglobulin type intact murine monoclonal antibody (MAb) that was developed in the Center of Molecular Immunology in Havana City, Cuba. In immunohistochemical studies, this demonstrated a significant affinity for the epithelial tissues so that it was used in a pilot clinical study to perform a radioimmunoscintigraphy of the colorectal primary tumors and their locoregional recurrences. It was labeled with 99mTc using the Schwarz method, with a > 95% performance. Planar images of the chest, abdomen and pelvis were performed at 10 minutes, 4-6 hours and 18-24 hours post-injection in the anterior and posterior projections and the SPECT was performed 4-6 hours and 18-24 hours post-injection of 1.85 GBq 99mTC. This study has aimed to verify in vivo the capacity of ior-C5 MAb to accumulate in the malignant colorectal lesions. ior-C5 accumulated in 5 out of the 7 patients who were studied and who were suffering from colorectal cancer or in whom there was suspicion of recurrence. There was a negative case of primary tumors, which was an adenocarcinoma in situ in a tubular-papillary adenoma. The second case with a negative radioimmunoscintigraphy was a true negative case.

CONCLUSIONS

It can be concluded that even though the number of patients is quite low, ior-C5 fulfilled the expectations of recognizing the epitope expressed in colorectal tumors in an in vivo human environment.

Humanized versus murine anti-human epidermal growth factor receptor monoclonal antibodies for immunoscintigraphic studies

The anti-human epidermal growth factor receptor (EGF-R) humanized antibody h-R3 (IgG(1)), which binds to an extracellular domain of EGF-R, was used to evaluate the biodistribution on nude mice xenografted with A431 epidermoid carcinoma cell line. Results are compared with its murine version ior egf/r3 monoclonal antibody (mAb). Twenty-one athymic female 4NMRI nu/nu mice were injected intravenously with 10 microg/100 microCi of (99m)Tc-labeled mAbs. The mAb ior C5 that recognizes an antigen expressed preferentially on the surface of malignant and cytoplasm of normal colorectal cells was used as negative control. Immunoreactivity of (99m)Tc-labeled mAbs was measured by enzyme linked immunosorbent assay on A431 cell line and the immunoreactive fractions determined by Lindmo method. Among all organs significative accumulation was found in tumor (6.14 +/- 2.50 %ID/g, 5.06 +/- 2.61 %ID/g for murine and humanized mAbs, respectively) 4 h after injection. The immunoreactive fractions were found to be 0.88 and 0.81 for murine and humanized mAb, respectively. Thus, we expect better results using the humanized mAb h-R3 for diagnostic immunoscintigraphy.

188Re-direct labeling of monoclonal antibodies for radioimmunotherapy of solid tumors: biodistribution, normal organ dosimetry, and toxicology

The increased interest in the availability of radionuclides for therapy has resulted from the recent success and potential importance of radiolabeled antibodies for both diagnosis and therapy. There is a widespread interest in the availability of 188Re for various therapeutic applications, particularly for attachment to tumor-specific monoclonal antibodies for radioimmunotherapy. This review provides a perspective of 188Re-direct labeled MAbs for radioimmunotherapy of solid tumors, normal organ biodistribution, absorbed radiation doses to normal organs and tumors, and the toxicity to bone marrow and normal tissues. Methods for calculation of mean absorbed radiation doses to the whole body, various normal organs, and tumors have been developed using source-organ residence times and the methods developed by the Medical Internal Radiation Dose (MIRD) committee. The toxicity for 188Re-labeled antibodies is predominantly hematopoietic, with platelets and white blood cells being most sensitive to the effects of radiation. Rhenium-188 would be the isotope of choice for radioimmunotherapeutic applications because of cost, availability, and favorable radiation characteristics. Rhenium-188 has a half-life of 16.9 h and maximum beta energy of 2.118 MeV. This isotope is particularly attractive because it can be supplied conveniently from 188W/188Re-radionuclide generator system.