Evaluating the efficacy of neoadjuvant endocrine therapy in HER2 low vs. HER2 negative breast cancer: An NCBD analysis.

599 Background: Neoadjuvant endocrine therapy (NET) is an important treatment option for hormone receptor (HR)-positive breast cancer (BC). Innovative ADCs targeting HER2 low tumors has sparked debate regarding the distinct nature of HER2 low tumors and whether it merits a unique treatment approach. With favorable outcomes in metastatic disease, there is interest in adopting similar strategies for early-stage treatment approaches. Methods: We examined the National Cancer Database for stage I-III ER-positive BC in women who underwent neoadjuvant endocrine therapy (NET) from 2010 to 2017, for at least 90 days to a maximum of 270 days. Patients with prior neoadjuvant chemotherapy, other malignancies, HER2-positive disease by either IHC or FISH, metastatic disease, or unknown stage were excluded. HER2 status was grouped as negative or low per 2023 ASCO-CAP guidelines. Patients’ response to therapy included pathologic complete response (CR), partial pathologic response (PR), stable pathologic response, or progression in primary tumor at time of resection. We employed multivariable binomial logistic regression to examine how clinicopathologic factors influence the probability of any response (combining CR and PR) to neoadjuvant hormonal therapy. Results: 3,808 patients met the inclusion criteria. 1,055 (27.8%) patients had HER2 negative disease, and 2,753 (72.2%) patients were HER2-low. 48 (1.2%) patients had primary tumor CR, and 1,484 (39%) had PR. Among HER2-low patients, 37 (1.3%) had pathologic CR, and 1,134 (41.1%) had PR. Among HER2-negative patients, 11 (1.0%) had pathologic CR, and 350 (33.1%) had PR. Multivariable logistic models revealed that age, duration of NET, and HER2 status were significant predictors of any response to NET. HER2-low tumors were more likely to have any response compared to HER2-negative (aOR=1.16, 95% CI 1.01 - 1.34, p=0.04). Patients who received >150 days of hormonal therapy were more likely to respond (aOR=1.33, 95% CI 1.16 - 1.52, p=50 years old were more likely to have any response (aOR=1.41, 95% CI 1.07 - 1.90, p=0.02). Clinical node positivity and tumor grade did not have significant effect on response to NET at time of resection. Conclusions: Our study offers a real-world estimate of outcomes associated with NET in localized BC. HER2 low tumors had greater significant clinical responses to NET when compared to HER2 negative, suggesting that HER2 low tumors may benefit from tailored treatment in the neoadjuvant setting. Table: see text