Inclusion Criteria:
* Patients with metastatic advanced esophageal squamous cell carcinoma (stage IV) confirmed by histology or cytology who are ready to receive PD-1 monoclonal antibody combined with TP chemotherapy regimen.
* Eastern Cooperative Oncology Group (ECOG) performance status score between 0 and 2.
* Have measurable lesions.
* Expected survival \> 3 months.
* Subjects who have received anti-tumor therapy in the past should be enrolled after the toxicity of the previous treatment has returned to the baseline level (except for residual hair loss effects) or CTCAE v4.03 scale score ≤ 1.
* Female or male subjects of reproductive age and their partners should agree to use effective contraception from the time of signing the ICF until 6 months after the last dose of study drug.
* Absolute neutrophil count (ANC) ≥1.5×109/L, platelets ≥100×109/L, hemoglobin ≥9 g/dL. Liver: Bilirubin ≤1.5 times the upper limit of normal, alkaline phosphatase (AP) , Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤3.0 times the upper limit of normal (AP, AST, ALT ≤5 times the upper limit of normal are allowed if there is liver metastases) Renal: Calculated creatinine clearance ≥45 mL/min.
Exclusion Criteria:
* Have active, or have had an autoimmune disease that is likely to recur (eg, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, autoimmune thyroid disease, vasculitis, psoriasis, etc.) or at risk (e.g., organ transplant recipients requiring immunosuppressive therapy).
* Subjects who need to receive glucocorticoid (prednisone \> 10 mg/day or equivalent dose of other similar drugs) or other immunosuppressive therapy due to certain conditions within 14 days prior to study drug administration.
* Major surgery, or radical radiation therapy, or palliative radiation therapy within the previous 14 days, or radiopharmaceuticals (strontium, samarium, etc.) within 56 days before starting study treatment.
* Received systemic anti-tumor therapy, including immunotherapy, biological therapy (tumor vaccines, cytokines, or growth factors to control cancer), etc. 14 days before starting study treatment.
* Have suffered from interstitial lung disease, chemical pneumonia, hypersensitivity pneumonitis, connective tissue disease pneumonia, pulmonary fibrosis, acute lung disease, etc. (except for local interstitial pneumonia induced by radiotherapy), or uncontrolled systemic disease, including diabetes and hypertension.
* Patients with human immunodeficiency virus (HIV) infection.
* patients with active pulmonary tuberculosis.
* Any active infection requiring systemic therapy by intravenous infusion within 2 weeks prior to the first dose of study drug.
* People who have received a solid organ transplant.
* Patients who have received any antibody/drug (including anti-programmed death-1 (PD-1), PD-L1, etc.) targeting T-cell co-regulatory proteins (immune checkpoints).
