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Effect of Chemotherapy on IRC 741 Leukemia in Fischer Rats

II. Sublines Developed by Treatment with Dimethylbenzimidazole^*

( Cancer Research Laboratory, University of Miami, Coral Gables, Florida)

This is a report showing that attenuation of malignancy induced in IRC 741 leukemia of Fischer rats, following early treatment with dimethylbenzimidazole (DMBZ), persisted for more than 40 transfer generations without treatment. The survival of rats bearing the attenuated tumor, IRC 741/A546, ranged from 24 to 43 days compared with 11 to 18 days' survival of rats bearing implants of IRC 741 tumor tissue. Resistance to DMBZ was developed in IRC 741 by initiating treatment 10 days after transplantation and in the attenuated subline, IRC 741/A546R, by initiating treatment of the host 20 days after tumor transplantation. Rats bearing implants of IRC 741R (DMBZ) lived 13–14 days after tumor transplantation, and A546R lived 21–39 days. Daily treatment of the sensitive subline proved to be twice as effective as treatment on alternate days. Daily treatment of the resistant subline was no more effective than treatment on alternate days. It was possible to cure some of the isologous hosts of IRC 741/A546 leukemia by initiating treatment 10–22 days after transplantation; but none of the rats given inoculations of IRC 741 were cured when treatment was initiated later than 4 days after transplantation. Rats cured of established tumors were resistant or immune to reinoculation with the altered and/or the parent tumor.

^* This investigation was supported by research grant CY-2498 from the National Cancer Institute, National Institutes of Health, Public Health Service.

Received 12/14/62.