Linkage between O6-methylguanine-DNA methyltransferase (O6-MT) activity and cellular resistance to antitumour nitrosoureas in cultured rat brain tumour cell strains.

We have examined O6-methylguanine-DNA methyltransferase (O6-MT) activity of rat brain tumour cell strains with reference to cellular resistance to antitumour nitrosoureas, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (nimustine, ACNU) and methyl-6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyrano side (ramustine, MCNU). The values of O6-MT activity were 52 and 160 fmol/mg protein extract in 9L and C6 rat brain tumour cells, respectively; while HeLa S3 cells, as a methyl excision repair positive (Mer+) cell strain, revealed a rather high value of 488 fmol/mg. 9L cells indicative of a low O6-MT activity showed 13 microM for ACNU and 18 microM for MCNU at a 10% survival dose (SD10), determined by a clonogenic cell assay as an index of cellular resistance. In contrast to this, C6 cells revealed a SD10 value of 67 microM and 36 microM for ACNU and MCNU, respectively, indicating higher resistance than 9L cells. HeLa S3 cells showed the highest SD10 value as follows: 84 microM for ACNU and 73 microM for MCNU. The relationship between the O6-MT activity and the cellular resistance was almost linear, with relatively resistant cell lines exhibiting the higher levels of the O6-MT activity. This correlation between the O6-MT activity and the cellular resistance to nitrosoureas as ACNU and MCNU was not observed among other antitumour drugs, which included bleomycin (BLM), neocarzinostatin (NCS), cis-diamminedichloroplatinum (II) (CDDP), and etoposide (VP-16) in clinical use for brain tumour chemotherapy. This indicates that O6-MT activity can be an indicator of cellular resistance to antitumour nitrosoureas in the chemotherapy of brain tumours.