Concordance analysis between tumor tissue HER2 status by immunohistochemistry (IHC) and in situ hybridization (ISH) and a translational analysis of plasma ctDNA in patients (pts) with biliary tract cancer (BTC): An exploratory analysis from the phase

4102 Background: HER2 is a target for precision oncology. HER2 protein overexpression and/or gene amplification is observed in a subset of pts with BTC. Zanidatamab (zani), a dual HER2-targeted bispecific antibody, received accelerated approval as a treatment for adults with previously treated, unresectable, or metastatic HER2-positive (IHC 3+) BTC based on the phase 2b HERIZON-BTC-01 trial. In this exploratory analysis, we evaluated concordance between tissue-based HER2 IHC and ISH in screened pts, and HER2 gene amplification between tissue-based HER2 ISH and plasma ctDNA by NGS in zani-treated pts. Methods: In HERIZON-BTC-01 (NCT04466891) pts were screened for HER2 gene-amplified tumors by ISH (VENTANA HER2 Dual ISH DNA Probe Cocktail assay) at a central laboratory. Pts with HER2 gene amplification were prospectively assigned to cohorts based on centrally determined HER2 IHC score (Ventana PATHWAY [4B5] IHC assay); cohort 1: IHC 2+ or 3+; cohort 2: IHC 0 or 1+. HER2 status was assessed in a fresh biopsy or an archived sample per ASCO/CAP guidelines, with gastric cancer algorithm. Enrolled pts received zani 20 mg/kg IV Q2W in 28-day cycles. Plasma ctDNA samples were collected prior to the first cycle of zani (baseline) and on-treatment at cycle 2 day 28 for testing with NGS Guardant360 (Guardant Health). Guardant360 Molecular Response (MR) scores were calculated based on changes in plasma ctDNA levels from baseline. Results: Overall, 756 screened pts had central results for both HER2 IHC and ISH. Nearly all pts (94%) with HER2 IHC 3+ BTC had HER2 -amplified tumors per ISH (Table). Among all 87 pts treated with zani across both cohorts, 48 samples from 25 pts were available for testing with NGS. The concordance between HER2 gene amplification by ISH and ctDNA NGS was 59%. Co-mutations at baseline in plasma ctDNA occurring in > 10% of pts included KRAS (n = 1 [3%]), HER2 S310F (n = 3 [9%]), and PIK3CA (n = 6 [18%]). Overall, 18/25 (72%) pts had a decrease in ctDNA levels from baseline at cycle 2 day 28; decreases > 90% were observed in pts with a best response of partial or stable disease. MR scores correlated with tumor response (ANOVA, P = 0.0189). Conclusions: In this analysis, there was a high concordance (94%) observed between tumor tissue HER2 IHC 3+ status and HER2 gene amplification among pts screened in HERIZON-BTC-01. Exploratory translational analysis shows that treatment with zani after 2 cycles was associated with a decrease in plasma ctDNA levels in the majority of pts. Clinical trial information: NCT04466891 . HER2 IHC and ISH status among screened patients. IHC Status ISH Status Amplification n/N (%) Amplified Non-Amplified Total 0 12 330 342 12/342 (3.5) 1+ 8 94 102 8/102 (7.8) 2+ 34 167 201 34/201 (16.9) 3+ 104 7 111 104/111 (93.7) Total 158 598 756 158/756 (20.9)