Inclusion Criteria:
* Pathologically (histologically or cytologically) confirmed pancreatic ductal adenocarcinoma (PDAC).
* Recurrent disease or metastatic disease evaluated by abdominal contrast-enhanced CT, MRI, and chest CT. PET/CT or other imaging examinations would be used if necessary.
* Fail or could not tolerate standard chemotherapy.
* ECOG score 0 or 1.
* Have measurable target lesion.
* Serum total bilirubin level is less than 1.5 x ULN. ALT and AST are less than 2 x ULN. Hemoglobin≥9g/dL, platelet≥75×10\*9/L, white blood count ≥3.0×10\*9/L, neutrophil ≥1.5×10\*9/L. Serum creatinine ≤ 1.5 × ULN or creatinine clearance rate \> 60 mL/min.
* Signed informed consent.
Exclusion Criteria:
* History of participation of other clinical trials within 4 weeks
* Known or suspected allergy to Sintilimab or tetrahydrobiopterin.
* Female patients during pregnancy or lactation.
* Patients who lack the ability to provide informed consent due to psychological, family, social, or other factors.
* Patients with a history of other malignant tumors besides pancreatic cancer before enrollment, except for non-melanoma skin cancer, carcinoma in situ of the cervix, or cured early-stage prostate cancer.
* Patients with severe cardiac, pulmonary, hepatic, or renal dysfunction, hematopoietic system diseases, cachexia, or other conditions that are intolerable to radiotherapy, chemotherapy, or surgery.
* Patients with autoimmune diseases, a history of autoimmune diseases (such as colitis, hepatitis, hyperthyroidism, including but not limited to these diseases or syndromes), as well as a history of immune deficiency, including HIV-positive test results, or other acquired or congenital immune deficiency diseases, or a history of organ transplantation and allogeneic bone marrow transplantation.
* Patients who require systemic corticosteroid therapy (\> 10 mg/day prednisone equivalent dose) or other immunosuppressive drugs within 14 days before the first administration or during the study. However, the following situations are allowed for enrollment: In the absence of active autoimmune diseases, patients are allowed to use topical or inhaled steroids, or adrenal hormone replacement therapy at a dose ≤ 10 mg/day prednisone equivalent dose.
* Patients with a history of interstitial lung disease or non-infectious pneumonia.
* Patients with active pulmonary tuberculosis infection detected through medical history or CT examination, or a history of active pulmonary tuberculosis infection within 1 year before enrollment, or patients with a history of active pulmonary tuberculosis infection more than 1 year ago but without proper treatment.
* Subjects with active hepatitis B (HBV DNA ≥ 2000 IU/mL or 10\*4 copies/mL), hepatitis C (positive hepatitis C antibody and HCV-RNA higher than the lower limit of detection of the analytical method).
