Antitumour activity of a sterically blocked ricin immunotoxin on a human colorectal adenocarcinoma grafted subcutaneously in nude mice

Use of antibodies and immunoconjugates for the therapy of more accessible cancers

There are currently 6 unconjugated antibodies and 3 immunoconjugates approved for use in the United States in a variety of cancers, with a considerable number of new agents in clinical testing and preclinical development. Unconjugated antibodies alone can be effective, but more often, antibodies need to be combined with chemotherapy, which enhances the efficacy of the standard treatment. Immunoconjugates tend to be more effective than their unconjugated counterparts, but their increased toxicity often restricts when and how they are used. In order to improve efficacy, a number of immunoconjugates are being examined in settings where the disease is more easily accessible, such as leukemias, or within compartments that allow easier and more direct access to the tumor, such as in the peritoneal cavity or brain, or both locally and systemically, in adjuvant situations, where the disease burden has been reduced by some other means, and with the main goal of these treatments being to kill residual disease.

Growth suppression of human colorectal carcinoma in nude mice by monoclonal antibody C27-abrin A chain conjugate

PURPOSE

The aim of this study was to assess an immunotoxin, monoclonal antibody C27-abrin A chain conjugate (MAAAC), that might be effective in the treatment of colorectal carcinoma.

METHODS

The immunotoxin was prepared by a specific monoclonal antibody against carcinoembryonic antigen (CEA), monoclonal antibody C27, linked to N-succinimidyl-3-(2-pyridyldithio)propionate and then coupled covalently to the toxic abrin-A chain to synthesize MAAC. The therapeutic role of this immunotoxin in suppressing the in vitro and in vivo growth of CEA-secreting human colorectal cancer cells (LS174T) was assayed by methods of protein biosynthesis inhibition, cell colony proliferation, and treatment of tumor cells before and after inoculation in nude mice.

RESULTS

We found that MAAC effectively suppressed the growth of LS174T in culture medium and completely eradicated cells in inoculated nude mice. In contrast, irrelevant immunotoxin antiferritin-abrin A chain conjugate and isotype-matched monoclonal immunoglobin (MOPC21IgG1)-abrin A chain conjugate did not cause such effects. The in vitro toxicity was highly specific because the conjugate (MAAC) inhibited de novo protein biosynthesis, impeded growth, and caused death of cells possessing surface CEA determinants. The 50 percent inhibition dose values of the conjugate for colonogenic survival and for protein biosynthesis in LS174T cells were 0.09 microgram/ml and 0.06 microgram/ml, respectively. Colon survival was inhibited 96.3 percent after prolonged MAAC treatment. MAAC showed selective cytotoxicity; the inhibitory effect of MAAC to the CEA-secreting LS174T cells over the CEA-nonsecreting human embryonic kidney cells was 16-fold.

CONCLUSION

These results indicate that MAAC may be of benefit in therapy during or soon after resection of colorectal carcinoma or in patients who have micrometastasis.

Pharmacokinetics of an antibody-ricin conjugate administered intraperitoneally to mice

Immunotoxins have been extensively studied for the treatment of neoplasias; their intracavitary administration could be useful for the therapy of tumors confined to the pleural or peritoneum spaces. To study the feasibility of this "locoregional" treatment, a pharmacokinetic study of immunotoxins delivery is necessary. Ricin, a plant toxin extracted from the seeds of Ricinus communis, has often been used in immunoconjugates for its high activity; nevertheless, appropriate strategies have been necessary to limit the aspecific toxicity. We previously prepared a AR-3-ricin immunotoxin lacking the ability to bind galactosidic cell surface residues, a so-called sterically blocked immunotoxin. The monoclonal antibody AR-3, an IgG1 specific to the CAR-3 antigen, was able to recognize human colorectal adenocarcinomas. Preclinical trials in nude mice, intraperitoneally grafted with the target neoplasia, showed that this immunotoxin suppressed tumor growth without showing any undesirable ricin toxicity. In the present work we report the pharmacokinetic properties of this immunotoxin, showing the in vivo stability and a relatively long blood survival. With a biodistribution study in tumor-bearing mice, we demonstrate that in tumor-invaded tissues, the concentration of the specific AR-3-ricin immunotoxin was higher and progressively increased in a multiple-dose regimen. In contrast, an irrelevant immunotoxin behaved differently because it did not show specific tumor uptake. Moreover the pharmacokinetic data reported in this work improve the potential for "locoregional" treatment of malignancy with blocked immunotoxins.

Immunotoxins: is there a clinical value?

Drug targeting is an attractive new approach to killing malignant cells, thereby leaving normal tissue unharmed. A decisive breakthrough was the advent of hybridoma technology, making monoclonal antibodies (MoAb) available in limitless supply. To construct reagents with selectivity for certain tumor cells, MoAbs or Fab' fragments were chemically linked to ribosome-damaging toxins derived from plants or bacteria like ricin, abrin, saporin, Pseudomonas exotoxin (PE), and diphtheria toxin (DT) to form immunotoxins, which combined the selectivity of the carrier moiety with the potency of the toxin moiety. The first generation of these immunotoxins showed impressive results in vitro but in most cases disappointing antitumour effects in animals or humans. By contrast, the second generation of immunotoxins, consisting of either A chain immunotoxins with a greatly improved stability in vivo or so-called 'blocked' ricin immunotoxins, have been demonstrated to be extremely effective in several animal models. Preliminary results of the current clinical trials suggest a possible clinical use of immunotoxins in leukemia and lymphoma patients. Genetically engineered fusion toxins have become available, which consist of a growth factor or a cytokine fused to a toxin moiety. In this paper, we will review the features of the three groups of immunotoxins which are most frequently used, i.e., ricin A chain and similar immunotoxins, blocked ricin immunotoxins, and recombinant toxins constructed with Pseudomonas exotoxin or diphtheria toxin.