Study of PD-1 Antibody and PARP Inhibitor in Extensive Stage Small Cell Lung Cancer

Inclusion Criteria: 1. All subjects were required to sign the informed consent before starting the study; 2. Histologically documented Extensive stage small cell lung cancer； 3. ECOG PS 0\~1 ； 4. 18-75 years； 5. According to recist1.1, CR or PR was achieved after 4-6 cycles of first-line etoposide and platinum chemotherapy; 6. .Life expectancy ≥ 3 months； Exclusion Criteria: 1. Recurrence or distant metastasis of localized small cell lung cancer after chemotherapy or concurrent chemoradiotherapy； 2. Active infection including tuberculosis, HIV, hepatitis B and C； 3. Interstitial pneumonia (ILD), drug-induced pneumonia, radiation-induced pneumonia requiring steroid therapy or active pneumonia with clinical symptoms, or other moderate to severe lung diseases that seriously affect lung function; 4. Severe infection existed, including but not limited to infection complications, bacteremia, severe pneumonia, etc; 5. Symptomatic brain metastasis and cancerous meningitis;