Abstract PS10-34: Clinical outcomes in de novo metastatic HER2-positive inflammatory breast cancer

Background: Advances in HER2-directed therapy have significantly improved survival in HER2+ metastatic breast cancer. Approximately 30% of de novo metastatic inflammatory breast cancer (mIBC) is HER2+, though there has been limited IBC representation in clinical trials evaluating the role of HER2-directed agents as first-line metastatic therapy. Elevated rates of locoregional progression or recurrence (LRPR) with systemic therapy alone also raise the question of whether trimodality therapy (TMT) may improve outcomes in IBC patients (pts) with limited metastatic disease, particularly in the setting of increasingly effective anti-HER2 therapy. Methods: Pts diagnosed with de novo HER2+ mIBC were identified from an IRB-approved IBC registry at Dana-Farber Cancer Institute. Clinical, pathology and treatment data were manually abstracted by chart review. Progression-free survival (PFS) was defined as time from IBC diagnosis to LRPR, distant progression/relapse or death (in the absence of event, censored at date of last follow-up). Overall survival (OS) was defined as time from IBC diagnosis to death from any cause or censored at date last known alive. For pts who underwent surgery of the primary tumor, pathologic complete response (pCR) was defined as no residual invasive carcinoma in the breast and axilla and survival was estimated from date of surgery. Median PFS and OS were estimated by Kaplan-Meier method; cumulative incidence of CNS metastasis and LRPR were estimated with death as competing risk. Results: 78 pts diagnosed between 1998-2019 with de novo HER2+ mIBC (41 hormone receptor (HR)-positive; 37 HR-negative) were identified. Median age at diagnosis was 53 years (yr; range: 24-91). Sites of metastatic disease at presentation included bone only (n=12), lymph node/contralateral chest wall only (n=17), bone and lymph node (n=5), visceral (n=40) and CNS with extracranial disease (n=4). As initial HER2-directed therapy, 37 pts received trastuzumab (H), 40 H plus pertuzumab and 1 T-DM1. At a median follow-up of 2.7 yr in the overall cohort, median PFS was 1.0 yr (IQR: 0.5-2.8 yr; 60 events) and median OS was 4.6 yr (IQR: 2.9-8.7 yr; 39 deaths). 34 pts had CNS metastasis with a cumulative incidence of 20% and 28% at 1 and 2 yr, respectively. LRPR developed in 26 pts, of which 16 occurred within 12 months (mo) of diagnosis. Cumulative incidence of LRPR at 1 and 2 yr was 21% and 29%, respectively. In 41 pts (53%), mastectomy was performed after receipt of systemic therapy. Median time from IBC diagnosis to surgery was 7.5 mo (IQR: 6.0-9.9 mo). Radiation was administered in 33 pts, 3 pre- and 30 post-mastectomy. Median OS from surgery was 5.2 yr (IQR: 3.1-8.4 yr). 9/41 pts (22%) achieved pCR; all pCR pts were alive at 1.3-8.9 yr since surgery. To investigate the value of therapy for locoregional control, a landmark analysis was performed in the subset of pts alive and LRPR free at 12 mo from diagnosis (n=56). 27 had surgery within 12 mo from diagnosis and 29 did not. LRPR occurred in 10/56; 9 were among pts who did not undergo surgery. Cumulative LRPR incidence at 1 and 2 yr since the 12 mo landmark was 21% and 29%, respectively, in pts who did not undergo surgery, and 0% at both time points in pts who had surgery (1 LRPR at 8 yr). Conclusion: Long-term outcomes in de novo HER2+ mIBC are overall similar to those reported in metastatic HER2+ non-IBC. More than half of pts underwent systemic and local therapy with good locoregional control and prolonged survival, suggesting a potential role for aggressive local therapy in this mIBC subset with favorable prognosis and effective systemic therapy. The high incidence of early CNS involvement in de novo HER2+ mIBC prompted us to explore this group in detail (Warren SABCS 2020 abstract). Larger studies are needed to better understand the effectiveness of TMT in HER2+ mIBC, highlighting the importance of collaborative research efforts in this rare subset. Citation Format: Ana C. Garrido-Castro, Samuel M. Niman, Marie Claire Remolano, Jennifer M. Rosenbluth, Caroline Block, Laura E. Warren, Jennifer Bellon, Beth T. Harrison, Faina Nakhlis, Meredith Regan, Beth Overmoyer. Clinical outcomes in de novo metastatic HER2-positive inflammatory breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-34.