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

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. 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. 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. 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.