Effects of l,3-Bis(2-chloroethyl)-l-nitrosourea and Related Compoundson Nuclear RNA Metabolism

SUMMARY This study examines the effects of 1,3-bis(2-chloroethyl)-l-nitrosourea and certain related compounds on the synthesis and processing of nucleolar and nucleoplasmic RNA in cultured L1210 cells. Treatment with 0.25 mM 1,3-bis(2-chloroethyl)-lnitrosourea has little effect on either quantity or size distribution of RNA synthesized during the first 10 min after drug addition. By 30 min, however, synthesis of both RNA fractions is markedly inhibited. By contrast, the processing of 45 S nucleolar (ribosomal precursor) RNA is inhibited immediately. The inhibition of nucleolar RNA processing is due to an action at some step after the normal synthesis and methylation of 45 S species. The processing or exit of high-molecular-weight RNA from the nucleoplasmic fraction also is inhibited. Structure-activity studies of some nitrosoureas indicated that inhibition of RNA processing requires a substitution at the N-3 nitrogen of the urea moiety. The isocyanate decomposition products of 1,3-bis(2-chloroethyl)-lnitrosourea and of l-(2-chloroethyl)-3-cyclohexyl-lnitrosourea were found to be more effective than the parent compounds, suggesting that it is these isocyanate products that are responsible for the inhibition of RNA processing. In accord with this interpretation, l-(2-chloroethyl)-lnitrosourea, which lacks an N-3 nitrogen substituent and therefore cannot form an isocyanate product, was inactive. Since this nitrosourea is known to be highly active against experimental neoplasms, it is concluded that inhibition of RNA processing or other effects of isocyanate products are probably not directly responsible for the antitumor activity. These effects may instead be side effects, potentially related to toxicity.