Tetrazepinones are equally cytotoxic to Mer+ and Mer- human tumor cell lines

Human brain and colon tumor cell lines SF-188 (Mer+) and WiDR (Mer+), which express the DNA repair protein O-6-methylguanine-DNA methyl transferase (MGMT), were 3- to 30-fold less sensitive to temozolomide, mitozolomide, and N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) than the MGMT-deficient tumor cells SF-126 (Mer-) and BE (Mer-). This differential sensitivity was not observed when these cells were exposed to the novel tetrazepinones PYRZ, NIME, QUINCL, and PYRCL, which contain, like temozolomide and mitozolomide, a ureidotriazene moiety. Flow cytometric studies revealed that temozolomide induced G(2)-M arrest in the Mer- cells, but exerted a minor effect on the cycle of the Mer+ cells. Similarly, mitozolomide (25-100 mu M) induced a stronger S-phase arrest in the SF-126 cells than in the SF-188 cells. In the same dose range (25-100) BCNU induced a significant cell cycle accumulation in G(2)2-M in the SF-126 cells but little in the SF-188 cell line. In contrast, the cell cycle effects of the tetrazepinones were independent of the cell phenotypes. When O-6-benzylguanine (O-6-BG) was used to deplete MGMT activity in the SF brain tumor cell lines, significant potentiation of temozolomide (67-fold), mitozolomide (7-fold), and BCNU (3-fold) was observed in the SF-188 cell line. By contrast, O-6-BG did not potentiate PYRZ, PYRCL, QUINCL, and NIME. Moreover, an MGMT inhibitory assay showed that all the tetrazepinones were capable of inactivating MGMT in the SF-188 cell line, the strongest inhibitor being PYRCL. The results suggest that, unlike temozolomide, mitozolomide, and BCNU, the cytotoxicity of the tetrazepinones does not correlate with the alkylation of the O-6 position of guanine and that the mechanism of MGMT inactivation by tetrazepinones may differ from that of hitherto known inhibitors.