583 Background: O(6)-methylguanine-DNA-methyltransferase (MGMT) is a DNA repair protein. About 40% of mCRC display MGMT deficiency due to promoter hypermethylation, leading to susceptibility to treatment with cytotoxic alkylating agents. We previously reported that objective clinical response to dacarbazine is confined to tumors harboring MGMT promoter hypermethylation (Amatu et al. Clin Cancer Res 2013). Based on these findings, we conducted a phase II study with TMZ in MGMT-deficient mCRCs after failure of standard therapies (EudraCT 2012-003338-17). Methods: We screened mCRC patients for MGMT promoter hypermethylation on archival FFPE specimens by methylation-specific PCR (MSP). All eligible patients underwent tumor biopsy prior to start of treatment. Patients received TMZ 200 mg/m2 po qd days 1-5 q28 until progression or intolerable toxicity. We used a Fleming single-stage design to determine whether PFS rate at 12 week would be ≥35% (H0≤15%, type I error=0.059 (1-sided) and power=0.849). Secondary en...