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Potentiation of a weakly active ricin A chain immunotoxin recognizing the neural cell adhesion molecule
E J Derbyshire1, R A Stahel, E J Wawrzynczak
1Drug Targeting Laboratory, Section of Immunology, Institute of Cancer Research, Sutton, UK.
Abstract:
A ricin A chain immunotoxin, SEN36-ricin A chain, directed against the neural cell adhesion molecule (N-CAM) had no selective cytotoxic activity against three different small cell lung cancer (SCLC) cell lines in tissue culture despite expression of the target antigen on more than 98% of cells in each line detected by indirect immunofluorescence. Treatment of the SW2 SCLC cell line with suramin and interferons alpha and gamma increased the level of N-CAM expression only slightly and had no significant effect on the cytotoxic activity of the SEN36 immunotoxin. In the presence of the carboxylic ionophore monensin at a concentration of 0.1 microM, the toxicity of SEN36-ricin A chain to the SW2 cell line was enhanced by 12,000-fold. In contrast, lysosomotropic amines showed little or no potentiation of activity, suggesting that lysosomal degradation was not the major factor limiting the action of the anti-N-CAM immunotoxin. The findings of this study indicate that ricin A chain immunotoxins directed against N-CAM on SCLC are unlikely to have sufficient activity to be useful therapeutic agents in the absence of potentiating agents such as monensin, which can interfere with the normal intracellular pathways of antigen routing.
Insights
This study found that SEN36-ricin A chain immunotoxins targeting neural cell adhesion molecule (N-CAM) showed limited effectiveness against small cell lung cancer (SCLC) cells. However, monensin significantly enhanced their toxicity, suggesting a potential therapeutic strategy.
Area of Science:
- Oncology
- Immunology
- Molecular Biology
Background:
- Neural cell adhesion molecule (N-CAM) is expressed on small cell lung cancer (SCLC) cells.
- Ricin A chain immunotoxins are designed to target cancer cells expressing specific antigens.
Purpose of the Study:
- To evaluate the efficacy of a novel N-CAM-directed ricin A chain immunotoxin (SEN36) against SCLC.
- To investigate methods for enhancing the cytotoxic activity of SEN36 immunotoxin.
Main Methods:
- Testing SEN36-ricin A chain immunotoxin cytotoxicity against SCLC cell lines in vitro.
- Assessing the effect of suramin, interferons, and monensin on N-CAM expression and immunotoxin activity.
- Evaluating the role of lysosomotropic amines in potentiating immunotoxin toxicity.
Main Results:
- SEN36-ricin A chain showed no significant selective cytotoxicity against SCLC cell lines despite high N-CAM expression.
- Suramin and interferons had minimal impact on N-CAM levels and immunotoxin efficacy.
- Monensin (0.1 microM) dramatically enhanced SEN36-ricin A chain toxicity by 12,000-fold.
- Lysosomotropic amines did not significantly potentiate the immunotoxin's activity.
Conclusions:
- N-CAM-directed ricin A chain immunotoxins alone are unlikely to be effective SCLC therapeutics.
- Monensin can significantly potentiate immunotoxin activity, potentially by interfering with intracellular antigen routing pathways.
- Further research into potentiating agents like monensin is warranted for developing targeted immunotoxin therapies for SCLC.