Abstract PO1-17-04: Influence of regulatory agency approval of trastuzumab-deruxtecan on the diagnosis of HER2-low status at a reference cancer center in Brazil
Authors: Vianna Paula Martinez , Alves Venancio Avancini Ferreira , Exman Pedro 
Published: May 2024

Abstract
BACKGROUND HER-2 low status is defined by immunohistochemistry (IHC) expression for human epidermal growth factor receptor 2 (HER-2) 1+ or IHC 2+ without gene ERBB2 amplification. HER2-low breast cancers (BC) accounted for approximately 55% of all cases, and among hormone receptor positive (HR+) tumors it represents up to 60% of patients (pts), while 10-20% in triple negative tumors (TNBC). Trastuzumab-deruxtecan (T-dxd) has been FDA approved for HER-2 low breast cancer after Destiny-Breast 04 trial (DB04) demonstrated a robust overall survival benefit either in HR+ or TNBC heavily pre-treated pts. The Brazilian Health Regulatory Agency (ANVISA) has later approved T-dxd for HER-2 low BC. However, due to its dynamic nature and subjectivity, the definition of HER-2 low status still lacks better criteria. This study aims to evaluate whether HER-2 low status diagnosis was impacted according to the time of the diagnosis - before DB04 publication and after ANVISA drug approval - in a reference cancer center in Brazil. METHODS This retrospective study was conducted in a single institution in Sao Paulo, Brazil. Pts diagnosed with BC between October 1st 2021 to June 1st 2023 were classified in 2 groups according to the time of diagnosis. Group 1 (g.01) included those who were diagnosed between October 1st 2021 until DB04 publication in July 7th 2022; and Group 2 (g.02) included pts diagnosed after ANVISA T-dxd approval on October 31st, 2022 until June 1st 2023. All pathological analysis of the primary breast tumor was performed by 2 experienced breast pathologists. HER-2 IHQ Score was defined according to ASCO/CAP guideline 2018. The primary endpoint was to evaluate the proportion of patients who were defined as HER-2 low according to the time of diagnosis. Secondary endpoint consists in evaluate change in pathologists practice to distinguish IHC 1+ results from 0 after the definition of HER-2 low. To compare the categorical variables between the groups, the Chi-square test was used. P values below 0.05 were considered statistically significant. Statistical analyses were carried out with the R program (R Foundation, Vienna, Austria). RESULTS A total of 713 pts with diagnosis BC were included in the study, 401 (56%) in g.01 and 312 (44%) in g.02. Overall, before DB04 publication, the diagnosis HER “Negative-NOS” without subdivision in score 0 or 1 was found in 127 (31.7%), but only in 27 (8.7%) reports in g.02 (p=0.007-13). HER-2 low status was observed in 12.2% in g01 and in 33.3% of the pts in g.02 (p &lt; 0.00001). Specifically, negative score 0 was found in 45.6% of pts in g.01 and 44.6% in g.02 (p= 0.014). A significant difference was also observed among pts with negative Score 1+ between groups (6.0% vs 21.2%, p=0.009-3), as shown in Table 1. No significant difference was seen in the frequency of HER2 score 2+ and score 3+ between g.01 and g.02. CONCLUSION The recent impact of a highly effective treatment derived from definition of HER-2 low BC led to a greater relevance in differentiating IHC 1+ from IHC 0, both considered HER negative by ASCO/CAP protocols. Distinguishing between HER2 0 and 1+ is subject to considerable interobserver variability , and the time of regulatory approval demonstrated a direct influence on the HER-low diagnosis.
Table 1
Distribution of HER2 IHQ results in Group 1 and Group 2
Citation Format: Paula Martinez Vianna, Venancio Avancini Ferreira Alves, Pedro Exman. Influence of regulatory agency approval of trastuzumab-deruxtecan on the diagnosis of HER2-low status at a reference cancer center in Brazil [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 PO1-17-04.