Inclusion Criteria:

          -  Histologically- or cytologically-confirmed BTC, including ICC, ECC or GBC.

          -  Locally advanced or metastatic BTC and not eligible for curative resection,
             transplantation, or ablative therapies.

          -  Received at least 1 prior gemcitabine-containing systemic chemotherapy regimen for
             advanced disease, and experienced disease progression after or developed intolerance
             to the most recent prior therapy. For subjects who received gemcitabine in prior
             adjuvant or neoadjuvant treatment, if progression occurred < 6 months from the latter
             of primary surgical resection or completion of gemcitabine-containing adjuvant
             therapy, they will be considered as having received 1 prior line of therapy for
             advanced disease.

          -  Subjects must test positive for HER2 amplification by ISH-assay at a central
             laboratory on a new biopsy or archival tissue. Note that fine needle aspirates (FNAs;
             cytology samples) and biopsies from sites of bone metastases are not acceptable.
             Testing may occur at any time after diagnosis of advanced or metastatic disease and
             before study enrollment.

          -  Male or female, ≥18 years of age (or the legal age of adulthood per country-specific
             regulations).

          -  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1.

          -  Adequate organ function.

          -  Adequate cardiac function, as defined by left ventricular ejection fraction ≥ 50%.

        Exclusion Criteria:

          -  Received systemic anti-cancer therapy within 3 weeks of the first dose of ZW25.
             Received radiotherapy within 2 weeks of the first dose of ZW25.

          -  Prior treatment with HER2-targeted agents.

          -  Untreated central nervous system (CNS) metastases, symptomatic CNS metastases, or
             radiation treatment for CNS metastases within 4 weeks of start of study treatment.
             Stable, treated brain metastases are allowed (defined as subjects who are off steroids
             and anticonvulsants and are neurologically stable with no evidence of radiographic
             progression for at least 4 weeks at the time of screening).

          -  Known leptomeningeal disease (LMD). If LMD has been reported radiographically on
             baseline MRI, but is not suspected clinically by the investigator, the subject must be
             free of neurological symptoms of LMD.

          -  Concurrent uncontrolled or active hepatobiliary disorders or untreated or ongoing
             complications after laparoscopic procedures or stent placement, including but not
             limited to active cholangitis, unresolved biliary obstruction, infected biloma or
             abscess. Any complications must be resolved more than 2 weeks prior to the first dose
             of ZW25.

          -  Prior or concurrent malignancy whose natural history or treatment has, in the opinion
             of the investigator or medical monitor, the potential to interfere with the safety or
             efficacy assessment of the investigational regimen.

          -  Active hepatitis

          -  Infection with human immunodeficiency virus (HIV)-1 or HIV-2

          -  QTc Fridericia (QTcF) > 470 ms.

          -  History of myocardial infarction or unstable angina within 6 months prior to
             enrollment, troponin levels consistent with myocardial infarction, or clinically
             significant cardiac disease.

          -  Acute or chronic uncontrolled pancreatitis or Child-Pugh Class C liver disease.