The Combination of Sintilimab and AI (Doxorubicin, ADM/Ifosfamide, IFO) for the First Line Treatment of Select Type of Metastatic/Unresectable Soft Tissue Sarcoma

Inclusion Criteria: * Men and women aged 18-75 years; * Provide written informed consent; * Local advanced or metastatic unresectable sarcoma; * Histologically confirmed undifferentiated pleomorphic sarcoma, synovial sarcoma, de-differentiated liposarcoma and myxoid liposarcoma; * Performance status of 0 to 1 on the Eastern Cooperative Oncology Group (ECOG) performance scale; * Have measurable disease based on Response Evaluation Criteria in Solid Tumors (RECIST) 1.1; * Life span expectation over 3 months * Absolute neutrophil count (ANC) ≥1,500/mcL (within 7 days of treatment initiation) ； * Hemoglobin ≥9 g/dL (within 7 days of treatment initiation) ； * Platelets ≥ 90,000/mcL (within 7 days of treatment initiation) ； * Serum creatinine ≤ 1.5 X upper limit of normal (ULN) or creatinine clearance \[CrCl\]) ≥ 50 mL/min for subject with creatinine levels (within 7 days of treatment initiation) ； * Serum total bilirubin ≤ 1.5 X ULN (within 7 days of treatment initiation) ； * Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) ≤ 2.5 X ULN or =\< 5 X ULN for subjects with liver metastases (within 7 days of treatment initiation)； Exclusion Criteria: * Prior systemic therapy for advanced and metastatic disease, except adjuvant treatment（not received anthracycline）replase over 6 months; * Received any testing anti-cancer drugs within four weeks of treatment initiation; * Prior immune related therapy including but not limited to PD-1, PD-L1, CD137, CTLA4, T cell stimulation, check point inhibitor etc; * Symptomatic, untreated, or uncontrolled brain metastases present * clinical meaningful active bleeding; * Other malignant cancer history rather than soft tissue sarcoma within five year prior to treatment initiation; * Have active infections requiring therapy; * Have active autoimmune disease or a documented history of autoimmune disease or syndrome that requires systemic steroids or immunosuppressive agents. Patients that require inhaled steroids or local steroid injections would not be excluded from the study. Patients with hypothyroidism not from autoimmune disease that is stable on hormone replacement will not be excluded from the study. * Pregnant or breast-feeding； * Any serious or unstable medical condition or mental illness； * Serious systemic diseases such as heart disease, history of (non-infectious) pneumonitis; * Active HBV （\>10000 copy/ml） and HCV （RNA\> 1000copy/ml） infection； * Positive human immunodeficiency virus (HIV）or any acquired immunodeficiency syndrome, organ implantation;