PARP inhibition suppresses the emergence of temozolomide resistance in a model system.

Introduction: Temozolomide (TMZ) is a life prolonging DNA alkylating agent active against glioblastomas (GBM) in which the O6-methylguanine-DNA methyltransferase (MGMT) gene is silenced by promoter methylation. Unfortunately acquired TMZ resistance severely undermines its clinical efficacy. Using an in vitro model, we tested whether poly (ADP-ribose) polymerase-1 and -2 (PARP) inhibition could suppress the emergence of resistance to enhance the effectiveness of TMZ.

Methods: Using the MGMT-methylated GBM line U251N, in which TMZ resistance can be induced, we developed a method to rapidly recreate mechanisms of TMZ resistance seen in GBMs, including MMR mutations and MGMT re-expression. We then assessed whether TMZ resistant U251N sub-clones could be re-sensitized to TMZ by co-treatment with the PARP inhibitor ABT-888, and also whether the emergence of resistance could be suppressed by PARP inhibition.

Results: U251N cultures chronically exposed to TMZ developed discrete colonies that expanded during TMZ treatment. These colonies were isolated, expanded further as sub-clones, and assessed for mechanisms of TMZ resistance. Most resistant sub-clones had detectable mutations in one or more mismatch repair (MMR) genes, frequently MSH6, and displayed infrequent re-expression of MGMT. TMZ resistance was associated with isolated poly(ADP-ribose) (pADPr) up-regulation in one sub-clone and was unexplained in several others. TMZ resistant sub-clones regressed during co-treatment with TMZ and ABT-888, and early co-treatment of U251N parental cultures suppressed the emergence of TMZ resistant colonies.

Conclusion: In a model of acquired resistance, co-treatment with TMZ and a PARP inhibitor had two important benefits: re-sensitization of TMZ resistant cells and suppression of TMZ resistance.