HER3 expression status following systemic chemotherapy treatment in biliary tract cancers

e16153 Background: Erb-b2 receptor tyrosine kinase 3 (HER3) is frequently overexpressed in a variety of cancers. Activation of HER3 signaling serves an important role in promoting cell proliferation and is associated with chemoresistance. Thus, there is interest in exploring HER3 status of tumors as a potential prognostic and/or therapeutic target. Prior studies have reported HER3 status in various cancers after treatment with chemotherapy (CT). However, HER3 expression profile in biliary tract cancer (BTC) remains unknown. Here, we evaluated HER3 status of patients with BTC post CT. Methods: The medical records of patients (pts) who had been initially diagnosed with BTC and treated at the National Cancer Center Hospital (Tokyo, Japan), between January 2010 and June 2020, were retrospectively analyzed. Pts whose post CT specimens were available were included in this study. Immunohistochemical staining for HER3 was performed using HER3/ErbB3 (D22C5) XP Rabbit mAb (Cell Signaling Technology) as a primary antibody. IHC scoring (0, 1+, 2+, 3+) of membranous staining intensity was performed according to HER2 IHC gastric scoring guideline. Results: A total of 19 pts with BTC, with a median age of 59 (range 44-79) years, had specimens available after post CT and were evaluated for HER3 expression status. HER3 status was 3+ in 10 pts, 2+ in 3 pts, 1+ in 4 pts, and 0 in 2 pts. The specimens were obtained from liver in 17 pts, bone in 1 pt, and omental lymph node in 1 pt. The histological type was adenocarcinoma in all pts, and HER3 status according to primary tumor type is shown in the table below. Fourteen pts were treated with gemcitabine (GEM) plus cisplatin (CDDP), 1 pt with GEM plus CDDP plus S-1, 1 pt with GEM plus S-1 as first-line CT for advanced BTC, and 3 pts with S-1 as an adjuvant treatment. Fifteen pts were biopsied after CT to participate in clinical trial. The remaining 4 obtained specimens after CT during primary surgery or surgery for metastases or for cholangitis. The median overall survival of pts with advanced BTC was 23.6 months in HER3 3+, 17.1 months in HER3 2+, 27.7 months in HER3 1+, and 24.6 months in HER3 0. Four pts had specimens available from pre and post CT, all of which were intrahepatic cholangiocarcinoma with the treatment of GEM plus CDDP. HER3 status was upregulated in 3 pts (1 pt 1+ to 2+, 2 pts 2+ to 3+) and downregulated in 1 pt (1+ to 0) at post CT. Conclusions: HER3 expression (1+, 2+, 3+) was observed in 89.5% of previously treated pts with BTC, and HER3 status was upregulated after CT in some pts. These results suggest that HER3 might be a potential therapeutic target in BTC post CT. [Table: see text]