Abstract PO3-03-05: Clinical outcomes in early-stage TNBC according to HER2-low status

Abstract Background: Clinical outcomes according to HER2 immunohistochemistry (IHC) in patients with early-stage HER2-negative breast cancer have differed across studies. For early-stage TNBC (eTNBC), less representation of this subtype in cohorts studied to date has limited interpretation of results. We sought to evaluate outcomes according to HER2 IHC in a multi-institutional cohort of patients (pts) with eTNBC who received neoadjuvant therapy (NAT). Methods: Pts diagnosed with stage I-III TNBC (including HR-low; ER and PR 10%) who received NAT and underwent surgery between 1/1/16-6/30/19 were identified across three institutional prospective databases. HER2 was defined as low (1+ or 2+/ISH non-amplified) or HER2-0 according to local testing at diagnosis. Pathological complete response (pCR) was defined as no residual invasive disease in breast and axilla. Multivariable logistic regression was used to compare pCR rates (HER2-low vs HER2-0) adjusting for age at diagnosis, race, anatomic clinical stage, germline BRCA1/2 (gBRCA), histology, HR status and receipt of anthracycline and taxane NAT. Recurrence-free (RFS), distant recurrence-free (DRFS), and overall survival (OS) were estimated using the Kaplan-Meier method. Multivariable Cox proportional hazards model was used to estimate adjusted hazard ratios. Results: A total of 978 pts were identified of which 388 (39.7%) had HER2-low and 590 (60.3%) had HER2-0 tumors at diagnosis. Median age was 50.3 (range: 21.0-83.4) yrs. 174 (17.8%) pts had HR-low tumors. 142 (14.5%) pts had a known gBRCA mutation. 790 pts (80.8%) received anthracycline- and taxane NAT. No significant differences were observed in age, race, gBRCA, histology, HR status (low vs negative), clinical tumor size, or type of NAT between HER2-low and HER2-0 groups. Clinical nodal positivity was higher in HER2-low (55.2%) vs HER2-0 (46.6%), p=0.011. No significant difference in pCR was observed between HER2-low (32.0%) and HER2-0 (32.7%) groups, adjusted p=0.928. Among pts with residual disease (RD) post-NAT with HER2 IHC also available in the RD sample, 244/363 (67.2%) had concordant HER2 status. 70/225 (31.1%) of HER2-0 tumors at diagnosis had IHC expression post-NAT (66 HER2-low, 4 HER2-positive); 48/138 (34.8%) of HER2-low tumors at diagnosis were HER2-0 post-NAT. At a median follow-up of 3.1 yrs, RFS did not significantly differ between HER2-low vs HER2-0 pts with pCR (p=0.368) or in those with RD post-NAT (p=0.573). Similarly, DRFS did not differ according to HER2 category for pts with pCR (p=0.509) or RD (p=0.812), nor did OS for pts with pCR (p=0.514) or RD (p=0.285) (Table 1). Conclusion: In a large cohort of pts with eTNBC treated with NAT, HER2-low status was not associated with pCR or survival after adjusting for clinical factors. High rate of discordance in HER2 IHC between diagnostic and RD post-NAT was observed, highlighting the importance of repeat IHC testing in serial samples given the development of antibody drug conjugates for HER2-low breast cancer. Table 1. RFS, DRFS and OS by HER2 expression Citation Format: Ana Garrido-Castro, Danielle Brandes Zakon, Qingchun Jin, Michael Grimm, Akshara Singareeka Raghavendra, Melissa Hughes, Mathew Cherian, Julie Vincuilla, Tonia Parker, Paolo Tarantino, Elizabeth Mittendorf, Tari King, Vicente Valero, Debu Tripathy, Bhuvaneswari Ramaswamy, Sara Tolaney, Nabihah Tayob, Nancy Lin, Daniel Stover, Carlos Barcenas. Clinical outcomes in early-stage TNBC according to HER2-low status abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-03-05.