Abstract PS4-03-21: Neoadjuvant chemotherapy impacts her2 status as defined by sataloff classification and consequent follow up in a lower middle-income country

Abstract Introduction: Neoadjuvant chemotherapy (NACT) followed by surgery remains a common therapeutic strategy for the management of breast cancer (BC) in Algeria. NACT can induce molecular heterogeneity of BC, which presents a challenge in achieving a complete response and impacts patient follow-up, particularly when it affects tumor HER2 expression and modifies patient eligibility for targeted therapies. This evidence can be notably demanding in Algerian public hospitals, where it presents a consequential issue in the face of a shortage of health products, especially for targeted therapies such as HER2 therapy. We therefore assessed post-NCAT HER2 status change in response to different protocols proposed to patients with BC. Methods: This study aimed to assess the change in HER2 status before and after NCAT, as determined by immunohistochemistry, for 61 patients managed at our single institution. According to guidelines, all patients received NACT, with or without anti-HER2 (Trastuzumab with or without Pertuzumab). Pathological response (pR) was evaluated according to Sataloff classification. All patients underwent post-NCAT surgery. Descriptive statistics and the Chi-square test were used to determine statistical differences of pR. Results: The mean age of patients was 50,01 ± 13.39 years (range 24-80). Among the 61 BC, 41 tumors were invasive carcinoma of no specific type (67%), 20 carcinomas (33%) were 10 ductal (16%), five lobular (8%), two mixed (3%), and three mucinous (5%). Hormone receptors (HR) were positive in 43 patients (70%) and negative in 12 patients (20%). Six patients were HER2-enriched. Of all evaluable tumors, the HER2 score was 0 in 18 patients (30%), low in 33 patients (54%), and 3+ in 10 patients (16%). There were 4 complete pR (TA) out of 61 (6,55%). Indeed, there were 7 grade TA (11%), 27 (44%) TB, 17 (28%) TC, and 10 (16%) TD. Among post-NACT-evolved HER2 tumors, 33 (54%) maintained their HER2 level, whereas 28 (46%) exhibited a modified HER2 level. Among 18 HER2-0 tumors (30%), 10 (16%) remained HER2-0, 7 (11%) transitioned to HER2-Low expression, and one to HER2-3+ expression. Among 33 HER2-low tumors (54%), 14 tumors (22%) remained HER2-low, 15 tumors (24%) transitioned to HER2-0, and 3 (9%) transitioned to HER2-3+. Among the 10 HER2-3+ tumors, 8 (80%) remained HER2-3+, while 2 (20%) transitioned to HER2-0 and Low status. Interestingly, there was a statistically significant difference in the percentage of tumors with better pR that maintained HER2 status grade TA (10%), TB (30%), TC (11%), TD (3%) in comparison to tumors that exhibited a changed HER2 status with no grade TA, TB (16%), TC (16%) and TD (13%) (p < 0,0001). Conclusion: Our results support the well-established post-NACT heterogeneity. This reflects a real issue for achieving a pR for patients and managing BC in our country. Indeed, we found for the first time that concomitant analysis of Sataloff response and HER2 changes can represent an endpoint surrogate for treatment achievement after NACT in BC, and a favorable prognosis is associated with HER2 upregulation. The assessment of HER2 after NCAT in combination with Sataloff response is a prerequisite for managing the follow-up of patients. Further studies are needed to characterize this finding. Citation Format: R. BOUCHELAGHEMI. BoulouhW. BechairiaM. GhassoulS. BouachaS. BouharoudS. Mohammed BenaliR. HachichiA. KitouniN. NécheH. Djedi. Neoadjuvant chemotherapy impacts her2 status as defined by sataloff classification and consequent follow up in a lower middle-income country [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-03-21.