Phase II Study of Avelumab in Metastatic Gastronetro-pancreatic （GEP） Neuroendocrine Carcinoma （NEC, WHO Grade 3） as Second-line Treatment After Failing to Etoposide+Cisplatin: Integration of Genomic Analysis to Identify Predictive Molecular Subtypes

Inclusion Criteria: 1. Be willing and able to provide written informed consent/assent for the trial. The subject may also provide consent for Biomedical Research. However, the subject may participate in the main trial without participating in Biomedical Research. 2. Be20 years of age on day of signing informed consent 3. Have histologically or cytologically-confirmed diagnosis of GEP-NECs including all gastrointestinal tracts 4. Have metastatic disease or locally advanced, unresectable disease with feasible biopsy sites (baseline and follow up) 5. Has experienced documented objective radiographic or clinical disease progression during or after first-line therapy containing any platinum/etoposide 6. Have measurable disease based on mRECIST as determined by investigator. Tumor lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions. Exclusion Criteria: 1. Is currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of treatment. 2. Has grade 1 or 2 neuroendocrine tumor in GI tracts. 3. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. 4. Has a known history of active TB 5. Hypersensitivity to avelumab or any of its excipients. 6. Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier. 7. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to a previously administered agent.