Abstract P2-13-07: Zanidatamab (ZW25), a HER2-targeted bispecific antibody, in combination with chemotherapy (chemo) for HER2-positive breast cancer (BC): Results from a phase 1 study

Background: Despite the availability of multiple HER2-targeted therapies, most patients (pts) with advanced HER2-positive BC experience disease progression, and an unmet medical need remains. Zanidatamab (zani) binds to HER2 across a range of expression levels and induces formation of receptor clusters, resulting in receptor internalization and downregulation that affect signal transduction. Zani potently activates antibody-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis, and complement-dependent cytotoxicity. In this ongoing phase 1 study (NCT02892123), data from BC pts treated with zani monotherapy have been previously reported. Here, we evaluated the safety and antitumor activity of zani in combination with chemo in HER2-positive BC pts. Methods: Pts with locally advanced and/or metastatic HER2-positive (immunohistochemistry [IHC] 3+ or IHC 2+/fluorescence in situ hybridization+) BC who received prior trastuzumab (tras), pertuzumab (pert), and T-DM1 were enrolled. Eligibility criteria included Eastern Cooperative Oncology Group performance status ≤1 and adequate organ function. Prior history of treated stable brain metastases was allowed. Zani (20 mg/kg Q2W or 30 mg/kg Q3W) was administered in combination with paclitaxel (pac), capecitabine (cap), or vinorelbine (vin). Zani + chemo was continued until disease progression or toxicity; if chemo was discontinued due to toxicity, zani treatment could be continued. Primary endpoints were safety and tolerability assessments; secondary efficacy endpoints included objective response rate (ORR) and disease control rate (DCR; complete response [CR] + partial response [PR] + stable disease [SD]) per RECIST v1.1, and progression-free survival (PFS). Clinical benefit rate (CBR) was defined as SD ≥24 weeks or best overall response of CR or PR. Results: As of 3 May 2021, 20 HER2-positive BC pts had enrolled (zani + pac, n=4; zani + cap, n=9; zani + vin, n=7). Ten pts were hormone receptor positive (data available for 19 pts), and 7 pts had a prior history of brain metastases. Median age was 53 years (range, 38-72), with median prior systemic therapies of 4.5 (range, 2-7) and median of 2 regimens in the metastatic setting. Pts received a median of 3 prior HER2 regimens; all pts received tras and T-DM1, 17 received pert, and 6 received tyrosine kinase inhibitors. In the 16 response evaluable (measurable disease and either post-baseline assessments or earlier death/progression) pts, confirmed ORR was 37.5% (with 1 additional response pending confirmation), CBR was 50%, and DCR was 81.3%. Median duration of response was not reached (range, 1.8+ to 16.8+ mo), with 4 of 6 confirmed responses ongoing. Among all pts (n=20), the most common (≥30% of pts) zani- and/or chemo-related adverse events (AEs) were diarrhea (65%), nausea (45%), peripheral neuropathy (35%), and fatigue (30%). One pt experienced grade 3 diarrhea leading to dose reduction of zani, and 1 pt discontinued zani due to AEs (abdominal pain and nausea, both grade ≤2). Four grade 4 AEs were observed in 4 (20%) pts: neutrophil count decreased (2 pts) and neutropenia (1 pt), all related to chemo, and hyponatremia (1 pt, not related to study treatment). Serious AEs were observed in 2 (10%) pts (pleural effusion, 1 pt; pneumonitis and upper respiratory tract infection, 1 pt), none related to zani. No grade 5 AEs were reported. Conclusions: Zani in combination with chemo is well tolerated, with encouraging and durable antitumor activity in heavily pretreated (including prior tras, pert, and T-DM1) pts with HER2-positive BC. These data support further investigation of zani as a novel therapeutic for these pts. The cap and vin cohorts continue to enroll, and additional cohorts are evaluating zani ± chemo in combination with tucatinib. Citation Format: Philippe L Bedard, Seock-Ah Im, Elena Elimova, Sun Young Rha, Rachel Goodwin, Cristiano Ferrario, Keun-Wook Lee, Diana Hanna, Funda Meric-Bernstam, Jose Mayordomo, Murali Beeram, Erika Hamilton, Jorge Chaves, Melody Cobleigh, Tony Mwatha, Joseph Woolery, Do-Youn Oh. Zanidatamab (ZW25), a HER2-targeted bispecific antibody, in combination with chemotherapy (chemo) for HER2-positive breast cancer (BC): Results from a phase 1 study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-07.