Background: Biliary tract cancer (BTC) is one of the most lethal cancers with limited treatment options. Early clinical trials showed a hint of activity of HER2 blockade for HER2 positive BTC, the prevalence of which was reported to be from 5% to 20%. Trastuzumab deruxtecan (DS-8201) is an antibody-drug conjugate composed of an anti-HER2 antibody, cleavable terapeptide-based linker, and a topoisomerase I inhibitor, which showed durable response in HER2 positive breast cancer as well as in a wide spectrum of cancer subtypes in a phase I study. In addition, preclinical research demonstrated the effectiveness of trastuzumab deruxtecan for HER2 positive BTC patient derived xenograft model. This phase II study is being conducted to evaluate the efficacy and safety of trastuzumab deruxtecan for HER2 positive BTC. Methods: The main inclusion criteria are unresectable or recurrent BTC, histologically diagnosed as adenocarcinoma or adenosquamous carcinoma, confirmed HER2-expressing status by central pathological examination, refractory or intolerant to treatment including gemcitabine, and adequate organ function. Patients are registered and receive 5.4 mg/kg trastuzumab deruxtecan every 3 weeks until disease progression or unacceptable toxicities. Primary endpoint is the overall response rate (ORR) in HER2 positive (defined as IHC3+, or IHC2+/ISH+; ISH+ defined as HER2/CEP17≥2.0) patients by central imaging review. The ORR in all HER2-expressing patients (including HER2 low expressing defined as IHC/ISH status of 0/+, 1+/-, 1+/+, or 2+/-), progression-free survival, overall survival, and incidence of adverse events are assessed as secondary endpoints. Thirty-two patients will be enrolled, including 24 with HER2 positive BTC as primary cohort and 8 with HER2 low expressing BTC. The study has 80% power for primary endpoint in HER2 positive BTC patients, with one-sided alpha error of 5%; threshold ORR of 15% and expected ORR of 40%. Pharmacokinetics and circulating tumor DNA analyses serially are performed. The study was initiated in May 2019 with enrollment ongoing. A total of 15 patients were enrolled as of January 2020. Funding: Japan Agency for Medical Research and Development, and Daiichi Sankyo. Clinical trial information: JMA-IIA00423.