[Study on potentiation of nitrosourea-cytotoxicity by DNA repair enzyme inhibitors in human brain tumor cells].
Nitrosoureas are antitumor alkylating agents widely used in the chemotherapy of malignant brain tumors. However, the effectiveness of adjuvant nitrosourea chemotherapy has proved inadequate, failing to provide any significant prolongation of survival time. One of the reasons for the poor results is a drug resistance system in the form of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). O6-alkylguanine derivatives are well known to be inhibitors of MGMT, and inactivation of MGMT by these derivatives leads to increased tumor cell sensitivity to nitrosoureas. In this study, the authors tested the ability of O6-benzylguanine, O6-(4-, 3- and 2-fluorobenzyl) guanines, O6-(4-, 3- and 2-trifluoromethylbenzyl) guanines, O6-(4-, 3- and 2-pyridylmethyl) guanines and O6-(2- and 1-naphthylmethyl) guanines to reduce MGMT activity in SF-188 cell-free extract by using [3H] methylated substrate DNA and analyzed their enhancing effect on the cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl) -3-nitrosourea hydrochloride (ACNU) by using a calorimetric cytotoxicity assay. The MGMT activity in the SF-188 cell-free extract was 944 +/- 43 fmol/mg protein (Mean +/- SD, n = 5). O6-(4- and 3-fluorobenzyl) guanines were found to be more effective in inactivating MGMT than O6-benzylguanine. O6-(4-trifluoromethylbenzyl) guanine considerably reduced MGMT activity as did O6-benzylguanine. O6-(3-trifluoromethylbenzyl) guanine, O6-(4- and 3-pyridylmethyl) guanines, and O6-(2-naphthylmethyl) guanine were intermediately effective, but O6-(2-fluorobenzyl) guanine, O6-(2-trifluoromethylbenzyl) guanine and O6-(1-naphthylmethyl) guanine were less effective. ACNU cytotoxicity in SF-188 cells was strongly enhanced by pretreatment with O6-(4- and 3-fluorobenzyl) guanines and O6-(4-trifluoromethylbenzyl) guanine and moderately enhanced by O6-(3- trifluoromethylbenzyl) guanine and O6-(4- and 3-pyridylmethyl) guanines, but not enhanced by O6-(2-fluorobenzyl) guanine, O6-(2-trifluoromethylbenzyl) guanine and O6-(1-naphthylmethyl) guanine. The test compounds were not cytotoxic at concentrations between 0.5 and 5.0 microM. The enhancing effects on ACNU cytotoxicity were consistent with the inhibition of MGMT activity after two-hour pretreatment with O6-arylmethylguanine derivatives. These results indicate that the 2-position of the O6-benzyl group plays an important role in the inactivation of the MGMT activity and the potentiation of ACNU cytotoxicity.
Fukuchi
M
M
Neurosurgical Service, Akita University Hospital, Japan.
Mineura
K
K
Kowada
M
M
Terashima
I
I
Kohda
K
K
D000970:Antineoplastic Agents / Q000737:chemistry / Q000494:pharmacology*; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D001932:Brain Neoplasms / Q000473:pathology*; D002470:Cell Survival / Q000187:drug effects; D019008:Drug Resistance, Neoplasm; D004357:Drug Synergism; D005910:Glioma / Q000473:pathology*; D006147:Guanine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000494:pharmacology; D006801:Humans; D008780:Methyltransferases / Q000037:antagonists & inhibitors*; D015376:Nimustine / Q000494:pharmacology*; D019853:O(6)-Methylguanine-DNA Methyltransferase; D013329:Structure-Activity Relationship; D014407:Tumor Cells, Cultured
D000970:Antineoplastic Agents; D018906:Antineoplastic Agents, Alkylating; C064976:O(6)-benzylguanine; D015376:Nimustine; D006147:Guanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase