Evaluation of efficacy and safety of MGN1703 administered twice weekly subcutaneously (SC) as maintenance treatment in patients with extensive disease small cell lung cancer (SCLC) who achieved at least a partial response (PR) following platinum-based first-line therapy.

1. Male and female patients with extensive disease SCLC ≥ 18 years of age receiving first line treatment;
2. Histology of SCLC or mixed histology of SCLC if SCLC histology is at least 80%;
3. Completion of 4 cycles of first-line therapy with a platinum-based regimen and no other prior chemotherapy;
4. Documented evidence of tumor response as assessed by investigators after the first 2 cycles of platinum-based chemotherapy followed by a confirmed PR or CR at the end of fourth cycle by CT or MRI scan;
5. Brain metastases are allowed only after cranial irradiation, if not requiring continuous treatment with steroids or anticonvulsants;
6. ECOG performance status 0 or 1;
7. Adequate organ function with total bilirubin, lactate dehydrogenase [LDH], alkaline phosphatase [AP], gamma glutamyltransferase [GGT], albumin, creatinine, urea, electrolytes, and coagulation parameters ≤ 1.5 × upper limit of normal (ULN), and with aspartate aminotransferase [AST] and ALT ≤ 2.5 × ULN in the absence of liver metastases or ≤ 5.0 × ULN in the presence of liver metastases;
8. Adequate hematological parameters: absolute neutrophil count ≥ 1.5 × 109/L; platelet count ≥ 100 × 109/L; leukocyte count ≥ 3.0 × 109/L; lymphocytes ≥ 1.0 × 109/L; hemoglobin ≥ 9.0 g/dL or 5.59 mmol/L;
9. Patients with reproductive potential agree to use effective contraceptive measures throughout the study;
10. Negative pregnancy test in women of childbearing potential;
11. Signed informed consent form (ICF).