The cytotoxicity of the antitumour nitrosoureas
BCNU and
CCNU and the
isocyanates which they liberate (chloroethylisocyanate and
cyclohexylisocyanate respectively) has been measured utilising an in vitro-in vivo bioassay. Lines of the TLX5
lymphoma and L1210 leukaemia were used which were either sensitive or resistant to nitrosoureas in vivo. An estimated logarithmic cell kill produced by each compound in vitro (before injecting the cells into animals) was calculated by reference to assays of the survival time of animals given from 2 X 10(5) to 2 X 10(0) cells of each line. Resistance to both
BCNU and
CCNU was observed in vitro in the cell lines of the TLX5
lymphoma made resistant to either
BCNU or a dimethyltriazene in vivo. The latter tumour was cross-resistant in vivo to nitrosoureas. Resistance in vitro to nitrosoureas was also observed in a line of L1210 leukaemia which had had resistance to
BCNU induced in vivo. The nitrosourea resistant TLX5
lymphomas were cross-resistant in vitro to both
cyclohexylisocyanate and chloroethylisocyanate whereas the nitrosourea resistant L1210 line showed no cross-resistance to
cyclohexylisocyanate and marginal cross-resistance to chloroethylisocyanate. The results suggest that the TLX5
lymphoma, which is naturally resistant to
alkylating agents of the
2-chloroethylamine type, may be sensitive to in vivo to nitrosoureas as a consequence of the intracellular release of
isocyanates. This hypothesis was supported by the finding that the resistant TLX5
lymphoma showed no cross-resistance to other electrophilic agents, for example
formaldehyde, monomethyltriazene or HN2. The transport of nitrosoureas into the sensitive and resistant cell lines was similar in profile and there was no difference in the concentration of non-
protein thiols.