Background: The double-blind placebo-controlled phase 2 IMPACT trial aimed to assess the clinical efficacy, safety, and immunological effects of the potent TLR9 agonist MGN1703, given at the dose of 60 mg subcutaneously twice weekly as switch maintenance after disease control due to first-line induction chemotherapy +/- bevacizumab in patients with metastatic colorectal cancer (mCRC). Methods: After randomization of 59 patients the trial was prematurely closed and final analysis showed a superior effect of MGN1703 compared to placebo with hazard ratios (HR) for the primary endpoint PFS on maintenance of 0.55 (p = 0.041) and 0.56 (p = 0.070) by local investigator assessment or independent radiological review, respectively. Exploratory PFS analyses of pretreatment characteristics identified patients with normalized CEA, objective response, and the presence of activated NKT cells at the end of induction chemotherapy to benefit the most from maintenance with MGN1703. The impact of these factors on the secondary endpoint overall survival (OS) is presented here. Results: OS data were not mature at time of final study analysis, since only 35% of MGN1703 patients and 50% of placebo patients had an event. The HR for OS of the whole study population was 0.63 (median 22.6 vs. 15.1 months). The subgroup of patients randomized into the study with confirmed RECIST response had a HR of 0.40 (median 24.5 vs. 15.1 months), suggesting this may be the population with greater benefit. HR for patients with normalized CEA or with activated NKT cells were 0.69 and 0.43, respectively. Based on this evidence, patients with mCRC and objective response after standard induction therapy are randomized in the phase 3 IMPALA study to standard treatment or switch maintenance with MGN1703. CEA and activated NKT cells are stratification factors for the study and will be prospectively assessed. Conclusions: The pretreatment characteristics predictive of a PFS benefit in the IMPACT study seem to retain their value also in exploratory analyses for OS. This information has been used to design the phase 3 IMPALA study, currently recruiting patients. Clinical trial information: NCT01208194