• Participant provides written informed consent.

• Male/female Participants who are at least 18 years of age on the day of signing informed consent.

• Participant is, in the investigator's judgement, willing and able to comply with the study protocol.

• Participant has histologically confirmed diagnosis of cholangiocarcinoma or gallbladder cancer.

• Participant is not eligible for surgery.

• Participants must have HER2-positive disease defined as either IHC 3+ or IHC 2+, the latter in combination with FISH+, as assessed locally on primary tumor OR positively confirmed by NGS-analysis OR positively confirmed by mRNA

• Participant has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. Evaluation of ECOG is to be performed within 7 days prior to the first dose of study intervention.

• Male Participants must agree to use a contraception as detailed in Appendix 3 of this protocol during the treatment period and for at least 7 months after the last dose of study treatment and refrain from donating sperm during this period.

• Female Participants are eligible to participate if they are not pregnant (see Appendix 3), not breastfeeding, and at least one of the following conditions applies:

⁃ Not a woman of childbearing potential (WOCBP) as defined in Appendix 3 OR

⁃ A WOCBP who agrees to follow the contraceptive guidance as given in Appendix 3 during the treatment period and for at least 7 months after the last dose of study intervention

• Participant has measurable disease based on RECIST v1.1. Lesions situated in a previously irradiated area are considered measurable if progression has been demonstrated in such lesions.

⁃ Have adequate organ function as defined in the following table (Table 2).

⁃ Criteria for known Hepatitis B and C positive subjects Hepatitis B and C screening tests are not required unless there is a known history of HBV or HCV infection and/or as mandated by local health authority

∙ Hepatitis B positive subjects

⁃ Participants who are HBsAg positive are eligible if they have received HBV antiviral therapy for at least 4 weeks and have undetectable HBV viral load (\< 100 IU/mL) prior to enrollment

• Participants should remain on anti-viral therapy throughout study intervention and follow local guidelines for HBV anti-viral therapy post completion of study intervention.

‣ Participants with history of HCV infection are eligible if HCV viral load is undetectable at screening.

⁃ Participants must have completed curative anti-viral therapy at least 4 weeks prior to enrollment.