Inclusion Criteria:
1. Provide written informed consent prior to any study procedures and agree to adhere to all protocol requirements
2. Aged at least 18 years.
3. Has histopathological or cytologic diagnosis of unresectable, locally advanced or metastatic BTC.
4. Has measurable disease as assessed by RECIST v1.1.
5. Must have failed on a gemcitabine + cisplatin-based chemotherapy, regardless of whether immune checkpoint inhibitor, such as durvalumab or pembrolizumab, or S-1 (tegafur, gimeracil, and oteracil potassium), was also administered. Oxaliplatin or carboplatin may be substituted for cisplatin when renal or auditory function is of concern. Participants also have failed (disease progression or intolerance) on, or refused FOLFOX chemotherapy, including modified FOLFOX variants, or failed on or irinotecan + fluorouracil chemotherapy.
6. Participants with tumors expressing the following biomarkers can be enrolled if they have not previously received FOLFOX but have received appropriate targeted therapies until disease progression or intolerance: fibroblast growth factor receptors aberrations, microsatellite instability biomarker/deficient DNA mismatch repair, or isocitrate dehydrogenase mutations.
7. ECOG PS of 0-2.
8. Life expectancy is ≥12 weeks.
9. Has adequate bone marrow function, demonstrated by: (a) ANC ≥1500 cell/mm3; (b) Platelet count ≥85,000 cells/mm3; (c) Hemoglobin ≥9 g/dL.
10. Has adequate liver function, demonstrated by: (a) AST and ALT ≤2.5 × ULN, or ≤5.0 × ULN in the case of liver lesions; (b) Total bilirubin ≤1.5 × ULN; (c) Albumin ≥3.0 g/dL; (d) INR \<1.5.
11. Has adequate renal function, demonstrated by CCR ≥ 45 mL/min or eGFR ≥ 45 mL/min/1.73 m2
12. No clinically significant abnormalities in coagulation results.
13. Participant is eligible to participate if not pregnant (as demonstrated by serum pregnancy testing at screening), not breastfeeding, and at least 1 of the following conditions applies:
1. Not of childbearing potential (CBP).
2. A participant of CBP who is sexually active with a partner who could impregnate them agrees to use a highly effective form of contraception during the study and for at least 30 days after the end of study intervention.
14. With partners of childbearing potential whom they could impregnate must agree to use contraception during the study and for 90 days after the end of study intervention.
15. Participants who are able to donate sperm must refrain from sperm donation during the study and for 90 days after the end of study intervention.
16. Participant is willing to comply with the protocol-required visit schedule and visit requirements.
17. More than 14 days have elapsed between the participant completing a prior line of chemotherapy or target therapy, and enrollment. More than 28 days have elapsed between the participant receiving concurrent radiotherapy (CCRT) and enrollment.
Exclusion Criteria:
1. Has a diagnosis of active malignancy other than BTC within the past 2 years, except nonmelanoma skin carcinoma and carcinoma-in-situ of uterine cervix treated with curative intent.
2. Participant discontinued prior gemcitabine due to pulmonary or hepatic toxicity or hemolytic uremic syndrome, hypersensitivity, allergic reaction, or intolerance.
3. Participant had a prior unanticipated severe reaction to capecitabine or metabolites or to fluoropyrimidine therapy.
4. Participant received treatment with brivudine, sorivudine, or its chemically related analogs ≤28 days prior to the date of enrollment.
5. Participant is currently receiving flucytosine treatment.
6. Participant has residual toxicity from prior chemotherapy or CCRT that is Grade ≥2 (residual Grade 2 neuropathy and alopecia are permitted).
7. Participant has any gastrointestinal disorder or prior gastrointestinal surgery that would significantly impede absorption of an oral agent, such as gastrectomy, Crohn's disease, ulcerative colitis, or short gut syndrome.
8. Participant has known brain or leptomeningeal metastases.
9. Participant had major surgery or definitive ablation-intent (excluding palliative radiotherapy for bone metastasis) radiation therapy within the past 28 days.
10. Participant has any active disease or condition that would not permit compliance with the protocol.
11. Participant has clinically significant cardiovascular disease (e.g., uncontrolled hypertension, unstable angina, congestive heart failure, New York Heart Association Grade 2 or greater), or uncontrolled serious cardiac arrhythmia.
12. Participant has documented cerebrovascular disease.
13. Participant has a seizure disorder not controlled with medication (based on Investigator's decision).
14. Participant has received an investigational agent within 28 days of enrollment.
15. Participant has an uncontrolled active viral, bacterial, or systemic fungal infection.
16. Participant has positive hepatitis B surface antigen with positive hepatitis B virus DNA ≥2000 copies/mL and/or anti-hepatitis C virus antibody.
17. Participant has received yellow fever vaccine or other live attenuated vaccine(s) within the 4 weeks before screening.
18. Participant has a history of drug or alcohol abuse within the year before signing the informed consent form.
19. Participant has any other serious medical condition that, in the Investigator's medical opinion, would preclude safe participation in or compliance with the clinical trial.
