Acquisition of resistance to antitumor alkylating agent ACNU: a possible target of positron emission tomography monitoring.

Early detection of tumor response to chemotherapy is of great importance for appropriate treatment of tumors. In this study, characteristics of two positron emission tomography (PET) tracers, [(18)F]2-fluoro-2-deoxy-D-glucose (FDG) and[(18)F]3'-fluoro-3'-deoxy-thymidine (FLT), in the early detection of tumor cell response as well as tolerance development to chemotherapy was compared using rat C6 glioma cells and 1-(4-amino-2-methyl-5-pyrimidinyl)-methyl-3-(2-chloroethyl)-3-nitrosoureahydrochloride (ACNU). ACNU is an alkylating agent known to induce drug resistance through expression of O(6)-methylguanine-deoxyribonucleic acid methyl transferase (O(6)-MGMT). We established an ACNU-resistant C6 glioma cell line (C6/ACNU) and investigated the effect of ACNU on the uptake of FLT and FDG. In C6 cells, DNA synthesis presented as [(3)H]thymidine ([(3)H]Thd) incorporation into DNA was quickly suppressed by ACNU. In C6/ACNU cells, the suppression was recovered promptly, indicating that DNA alkylation occurs initially but highly expressed O(6)-MGMT repairs DNA, leading to the recovery of DNA synthesis. The patterns of FLT uptake in C6 and C6/ACNU were difficult to distinguish in the very early stage of the treatment, though it was reported that FLT uptake well correlated with proliferation in certain conditions. FDG uptake showed different patterns between the resistant and control cells, with significantly decreased uptake in C6 cells and unchanged uptake in C6/ACNU cells at 18-24 h after the treatment. Though difficult to be directly translated into clinical situation, the present study will provide a base to develop an appropriate protocol to assess tumor response to treatment by PET and to design effective treatment plans.