The role of isocyanates in the toxicity of antitumour haloalkylnitrosoureas

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 × 10⁵ to 2 × 10⁰ 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 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.