Zymeworks Inc. announced new and updated clinical data for the HER2 `targeted bispecific antibody zanidatamab, in both HER2-expressing biliary tract cancer (BTC) and gastroesophageal adenocarcinoma (GEA). The data are being presented January 15, at the American Society of Clinical Oncology (ASCO) Gastrointestinal Cancers Symposium, taking place virtually January 15 " 17, 2021. HER2 is overexpressed in approximately 20% of GEA patients. For these patients, trastuzumab in combination with chemotherapy is the only approved HER2 `targeted therapy. Treatment options are currently limited if disease progression occurs after trastuzumab in combination with chemotherapy. The data being reported are from HER2-expressing GEA patients who received zanidatamab either as monotherapy (n=35) or in combination with chemotherapies (n=28). The groups had a median of two to three (range 0-7) prior therapies, with a high percentage (88-91%) having received prior HER2-targeted therapies. The data continue to demonstrate that zanidatamab is well tolerated with the majority of treatment-related AEs considered mild to moderate in severity (Grade 1 or 2) and manageable in the outpatient setting. In 33 response-evaluable patients who received zanidatamab as monotherapy (10 mg/kg weekly or 20 mg/kg every two weeks), the objective response rate (ORR) was 39% (13/33), 11 (33%) of which were confirmed by a subsequent scan. The disease control rate (DCR) was 61% (20/33) and median duration of response (DOR) was six months. In 10 response-evaluable patients who received zanidatamab (20 mg/kg every two weeks) plus paclitaxel, the ORR was 60% (6/10), five (50%) of which were confirmed by a subsequent scan including one patient who experienced a complete response. The DCR was 90% (9/10) for this group. In 14 response-evaluable patients who received zanidatamab (20 mg/kg every two weeks or 30 mg/kg every three weeks) plus capecitabine, the confirmed ORR was 57% (8/14) and the DCR was 71% (10/14). Overall the median DOR for zanidatamab plus chemotherapy was 8.9 months and the median progression-free survival (PFS) was 5.6 months with eight (29%) patients still on study at the time of data cut-off. Zanidatamab was well tolerated and demonstrated durable antitumor activity in these patients. All zanidatamab-related adverse events (AEs) were mild or moderate in severity (Grade 1 or 2). The confirmed objective response rate (ORR) in trastuzumab-naïve patients was 47% (7/15) and overall ORR was 40% (8/20). The overall disease control rate (DCR) was 65% (13/20), and median duration of response (DOR) was 7.4 months with several patients still on study at the time of data cut-off. Based on these results, in July 2020Zymeworks initiated a global pivotal Phase 2b trial (HERIZON-BTC-01; ZW25-203) [NCT04466891 [4]] of zanidatamab monotherapy in patients with HER2-amplified BTC that has been previously treated with at least one gemcitabine-containing systemic chemotherapy regimen. The US Food and Drug Administration (FDA) recently granted Breakthrough Therapy designation for zanidatamab in patients with previously treated HER2 gene-amplified BTC, based on evaluation of the Phase 1 data. This global pivotal Phase 2 study of zanidatamab in BTC may enable the submission of a Biologics License Application by Zymeworks in the United States as early as 2022.