Gem+Nab-P+LEN+TIS for Advanced Unresectable BTC (GALENT-BT)

Inclusion Criteria: 1. Aged 18-75 years, regardless of gender. 2. Histologically or cytologically confirmed, untreated primary advanced unresectable biliary tract malignancies (BTC), including intrahepatic cholangiocarcinoma (ICC), extrahepatic chololiocarcinoma (ECC), and gallbladder cancer (GBC); or untreated recurrent BTC (prior adjuvant/neoadjuvant chemotherapy allowed if completed ≥3 months before recurrence, excluding regimens containing PD-1/L1 inhibitors, gemcitabine, nab-paclitaxel, or lenvatinib). 3. ECOG performance status score 0-1. 4. Expected survival ≥3 months. 5. At least one measurable target lesion per RECIST v1.1 criteria. 6. Adequate organ function: Hematologic: Hemoglobin ≥90 g/L; WBC ≥lower limit of normal (LLN); ANC ≥1.5×10⁹/L; platelets ≥100×10⁹/L. Renal: Serum creatinine ≤1.5×ULN; endogenous creatinine clearance rate ≥55 mL/min. Hepatic: Total bilirubin ≤1.5×ULN; ALT/AST ≤2.5×ULN (≤3×ULN for intrahepatic BTC or liver metastases; ALT/AST ≤5×ULN for liver metastases). Coagulation: INR ≤1.5×ULN; APTT within normal range. 7. No prior systemic therapy for advanced BTC (chemotherapy, radiotherapy, targeted therapy, immunotherapy, or hormonal therapy). Patients with post-R2 resection are eligible. 8. Negative serum/urine pregnancy test (for women of childbearing potential) and agreement to use contraception during the study and for 6 months post-treatment. 9. Willing and able to provide written informed consent. Exclusion Criteria: 1. Severe systemic infection or uncontrolled comorbidities (e.g., heart failure, thyroid disorders, psychiatric conditions). 2. Known hypersensitivity or intolerance to study drugs or their excipients. 3. Pregnancy, lactation, or refusal to use effective contraception. 4. Participation in other clinical trials within 30 days prior to enrollment. 5. Inability to understand or unwillingness to sign informed consent. 6. Any condition that, in the investigator's judgment, may compromise patient safety or compliance (e.g., severe concurrent illness, abnormal lab results, psychosocial factors). 7. Prior use of PD-1/L1 inhibitors, gemcitabine, nab-paclitaxel, or lenvatinib in adjuvant/neoadjuvant settings.