The aim of this study is to investigate whether IMA910 plus GM-CSF (1st cohort) is an effective maintenance therapy with a favorable toxicity profile. A single dose of low-dose cyclophosphamide is applied as an immune modulator 3 days before the start of vaccination. Effectiveness is measured in terms of the disease control rate (DCR) at Visit 14 (= 27 weeks of vaccination) according to RECIST. The baseline tumour response assessment is performed after a first-line oxaliplatin-based standard chemotherapy for 12 weeks. The DCR is compared to a no effect level of 21% derived from the PFS curve of the chemotherapy-free interval cohort (arm B) of the OPTIMOX 2 study. Tumour response assessments will be performed every 9 weeks according to RECIST. Further a 2nd cohort of patients will be treated with IMA910 plus GM-CSF in combination with imiquimod and a single dose of lowdose cyclophosphamide. The aim of the 2nd cohort is to investigate whether the addition of imiquimod is safe, enhances the immune response to IMA910 and shows effectiveness.