The purpose of this this trial is to prove the efficacy and safety of MGN1703 as a maintenance therapy after first-line chemotherapeutic treatment of metastatic colorectal cancer.

Due to the toxicity of chemotherapeutic medications treatment breaks are common practice in the treatment of metastatic colorectal cancer. During these treatment breaks a variety of maintenance treatments are available.

MGN1703 is developed not only to help the Patient to recover from chemotherapy but also to boost the patient's immune System and thus help him to effectively fight the Tumor himself.

After the concept has already been proved in a previous study this Trial is conducted to further evaluate the safety and efficacy of MGN1703 Treatment.

Inclusion Criteria:

• Signed written informed consent
• Male or female patient 18 years or older
• Histologically confirmed diagnosis of colorectal cancer presenting with unresectable stage IV (UICC) disease (primary tumor may be present)
• Complete or partial response, as assessed by local investigators according to RECIST 1.1, within 12-30 weeks from start of induction treatment with standard first-line chemotherapy with or without biological agents
• ECOG PS 0-1
• Haematology and biochemistry laboratory results within the limits normally expected for the patient population recovering after receiving induction treatment
• Male and female patients of childbearing potential (i.e. did not undergo surgical sterilization - hysterectomy, bilateral tubal ligation, or bilateral oophorectomy - and are not post-menopausal for at least 24 consecutive months) must use an effective method of contraception. Females of child bearing potential must have a negative serum pregnancy test

Exclusion Criteria:

• History of other malignant tumors within the last 5 years, except basal cell carcinoma or curatively excised cervical carcinoma in situ
• Known brain metastases (present or treated)
• Contraindication to receiving MGN1703 as per current investigator brochure
• Known hypersensitivity to any component of the study product
• Prior allogeneic stem cell transplantation or organ transplantation
• Active or uncontrolled infections or undiagnosed febrile condition
• Severe anemia requiring repeated blood cell transfusion
• Pre-existing autoimmune or antibody-mediated diseases or immune deficiency
• Chronic systemic immune therapy or immunosuppressant medication other than steroids within the last 6 weeks; continuous systemic steroid treatment within the last 2 weeks prior to start of study treatment
• Use of systemic antibiotic therapy within the last 2 weeks prior to start of study treatment
• Inadequate pulmonary function according to the Investigator's judgment, history of interstitial lung disease e.g. pneumonitis or pulmonary fibrosis or evidence of interstitial lung disease on baseline chest CT scan
• HIV seropositivity or active HBV/HCV infection
• Serious concomitant disease or medical condition that in the judgment of the investigator renders the patient at high risk of treatment complications
• Female patient who is pregnant or breast feeding
• Contraindication to receive the planned standard maintenance treatment according to applicable SmPC
• Treatment with any anti-cancer investigational drug within 12 months prior to study treatment or participation in another clinical study with other investigational drugs within 28 days prior to study treatment
• Vaccination within 1 months prior to start of study treatment
• Any medical, mental, psychological or psychiatric condition that in the opinion of the investigator would not permit the patient to complete the study or understand the patient information