title: 3-aminobenzamide and/or O6-benzylguanine evaluated as an adjuvant to temozolomide or BCNU treatment in cell lines of variable mismatch repair status and O6-alkylguanine-DNA alkyltransferase activity.
abstract: O6-benzylguanine (O6-BG) and 3-aminobenzamide (3-AB) inhibit the DNA repair proteins O6-alkylguanine-DNA alkyltransferase (AGT) and poly(ADP-ribose) polymerase (PARP) respectively. The effect of O6-BG and/or 3-AB on temozolomide and 1,3-bis(2-chloroethyl)-nitrosourea (BCNU) cytotoxicity, was assessed in seven human tumour cell lines: six with an AGT activity of > 80 fmol mg-1 protein (Mer+) and one with an AGT activity of < 3 fmol mg-1 protein (Mer-). Three of the Mer+ cell lines (LS174T, DLD1 and HCT116) were considered to exhibit resistance to methylation by a mismatch repair deficiency (MMR-), each being known to exhibit microsatellite instability, and DLD1 and HCT116 having well-characterised defects in DNA mismatch binding. Potentiation was defined as the ratio between an IC50 achieved without and with a particular inhibitor treatment. Temozolomide or BCNU cytotoxicity was not potentiated by either inhibitor in the Mer- cell line. Preincubation with O6-BG (100 microM for 1 h) was found to potentiate the cytotoxicity of temozolomide by 1.35- to 1.57-old in Mer+/MMR+ cells, but had no significant effect in Mer+/MMR- cells. In comparison, O6-BG pretreatment enhanced BCNU cytotoxicity by 1.94- to 2.57-fold in all Mer+ cell lines. Post-incubation with 3-AB (2 mM, 48 h) potentiated temozolomide by 1.35- to 1.59-fold in Mer+/MMR+ cells, and when combined with O6-BG pretreatment produced an effect which was at least additive, enhancing cytotoxicity by 1.97- to 2.16-fold. 3-AB treatment also produced marked potentiation (2.20- to 3.12-fold) of temozolomide cytotoxicity in Mer+/MMR- cells. In contrast, 3-AB produced marginal potentiation of BCNU cytotoxicity in only three cell lines (1.19- to 1.35-fold), and did not enhance the cytotoxicity of BCNU with O6-BG treatment in any cell line. These data suggest that the combination of an AGT and PARP inhibitor may have a therapeutic role in potentiating temozolomide activity, but that the inhibition of poly(ADP-ribosyl)ation has little effect on the cytotoxicity of BCNU.
authors.affiliation: Department of Medical Oncology, Charing Cross Hospital, London, UK.
authors.forename: S R
authors.initials: SR
authors.lastname: Wedge
authors.forename: J K
authors.initials: JK
authors.lastname: Porteous
authors.forename: E S
authors.initials: ES
authors.lastname: Newlands
mesh_terms: D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D001549:Benzamides / Q000008:administration & dosage / Q000494:pharmacology; D002330:Carmustine / Q000008:administration & dosage / Q000494:pharmacology; D004260:DNA Repair; D003606:Dacarbazine / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000494:pharmacology; D004354:Drug Screening Assays, Antitumor; D004357:Drug Synergism; D004791:Enzyme Inhibitors / Q000008:administration & dosage / Q000494:pharmacology; D006147:Guanine / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D009369:Neoplasms / Q000188:drug therapy* / Q000201:enzymology / Q000235:genetics; D000077204:Temozolomide; D014407:Tumor Cells, Cultured
chemical_list: D018906:Antineoplastic Agents, Alkylating; D001549:Benzamides; D004791:Enzyme Inhibitors; C064976:O(6)-benzylguanine; D006147:Guanine; D003606:Dacarbazine; C025160:3-aminobenzamide; D002330:Carmustine; D000077204:Temozolomide