Vol. 288, Issue 2, 484-489, February 1999

Tetrazepinones Are Equally Cytotoxic to Mer+ and Mer Human Tumor Cell Lines

Bertrand J. Jean-Claude , Amir Mustafa, Amanda J. Watson, Zoe Damian, Daniela Vasilescu, Tak Hang Chan and Brian Leyland-Jones

Department of Oncology, McGill University, Montreal, Quebec, Canada (B.J.J.-C., A.M., Z.D., D.V., B.L.-J.); Department of Chemistry, McGill University, Montreal, Quebec, Canada (B.J.J.-C., T.H.C.); and Paterson Institute for Cancer Research, Christie Hospital National Health Service Trust, Manchester, United Kingdom (A.J.W.)

Human brain and colon tumor cell lines SF-188 (Mer+) and WiDR (Mer+), which express the DNA repair protein O⁶-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 ureido-triazene moiety. Flow cytometric studies revealed that temozolomide induced G₂-M arrest in the Mer cells, but exerted a minor effect on the cycle of the Mer+ cells. Similarly, mitozolomide (25-100 µM) 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 G₂2-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 O⁶-benzylguanine (O⁶-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, O⁶-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 O⁶ position of guanine and that the mechanism of MGMT inactivation by tetrazepinones may differ from that of hitherto known inhibitors.