Antitumour imidazotetrazines--XVIII. Modification of the level of 5-methylcytosine in DNA by 3-substituted imidazotetrazinones.

The effect of 3-methyl(temozolomide) and 3-ethyl (CCRG 82019) substituted imidazotetrazinones on cytosine methylation has been studied in the human lymphoblastoid cell line GM892. There was a decrease in the 5-methylcytosine content of newly synthesized DNA in cells treated with the 3-methyl and a small increase in cells treated with the 3-ethyl analogue, which was maximal 4 days after drug treatment. There was a progressive decrease in nuclear DNA methyltransferase after treatment with temozolomide with complete inhibition at 11-12 hr after drug addition, followed by a re-establishment of enzyme levels towards control values. While the free drugs had no effect on DNA methyltransferase activity in vitro, DNA isolated from GM892 cells previously treated with temozolomide inhibited the transfer of methyl groups from S-adenosyl-L-methionine to M. lysodeiktious DNA. The maximum effect was observed at 6 hr after drug addition and was proportional to the concentration of temozolomide to which the cells had previously been exposed. These results suggest that temozolomide may induce a block in cellular replication as a result of an indirect inhibition of DNA methylation and cells which escape this block progress with hypomethylated DNA.