O6-methylguanine and temozolomide can reverse the resistance to chloroethylnitrosoureas of a mouse L1210 leukemia.

Using L1210 and a subline resistance to chloroethylnitrosoureas (L1210/BCNU), we found that the resistance to 1-(2-chloroethyl)-1-nitrosourea (CNU) or to diethyl-1-3-(2-chloroethyl)-3-nitrosoureido ethyl phosphonate (fotemustine) can be reversed by a pretreatment with O6-methyl Guanine (O6-mGua) or temozolomide. In L1210/BCNU but not in L1210 the pretreatment with O6mGua caused an increased peak level of CNU-induced DNA-interstrand crosslinks. We then evaluated whether the resistance to BCNU could be counteracted in vivo by i.p. O6mGua treatment of L1210/BCNU bearing mice. The results were negative due to the fact that O6mGua, which was not toxic when given alone, caused a high toxicity when associated with BCNU.