Abstract PO4-02-03: Distribution of MammaPrint, BluePrint, and Response Predictive Subtypes based on ImPrint and Reprint in ER+/HER2- Invasive Lobular Carcinoma – A FLEX sub study

Abstract Background: Invasive lobular carcinoma (ILC), the second most common histologic subtype of breast cancer, is increasingly recognized as a distinct tumor type compared to the more common invasive ductal carcinoma (IDC). While ILC is known to have lower rates of pathologic complete response to neoadjuvant chemotherapy, ILC tumors are biologically heterogenous. As such, genomic signatures might identify patients with ILC who might benefit from tailored treatment options. The gene expression signature MammaPrint (MP) classifies tumors as having a Low Risk or High Risk of distant recurrence. MP combined with BluePrint (BP), a molecular subtyping signature, categorizes tumors as Luminal A (MP Low Risk), Luminal B (MP High Risk), Basal or HER-2 type. ImPrint is a 53-gene signature that identifies HR+HER2- patients predicted to benefit from immune checkpoint inhibitors. RePrint is a 60-gene DNA repair deficiency (DRD) signature that identifies HR+HER2- patients who may benefit from PARP inhibitors with platinum agents. Response Predictive Subtypes (RPS), used in the I-SPY2 trial, combine clinical subtype and these genomic signatures to personalize treatment planning and improve outcomes. In this study, we determined the distribution of 3 RPS in HR+HER2-: (1) ImPrint-/RePrint-, (2) ImPrint-/RePrint+ (3) ImPrint+, in patients ILC compared to IDC enrolled in the FLEX trial. Methods: This study includes 1078 HR+HER2-women with ILC and 6078 with IDC enrolled in FLEX registry. FLEX (NCT03053193) is a prospective, observational trial that includes stage I-III breast cancer patients who undergo MammaPrint testing (with or without BluePrint) as standard of care, and consent to full transcriptome and clinical data collection. MP High Risk tumors were further stratified into High 1 and High 2. ImPrint and Reprint results were used to determine RPS. A two-tailed proportional z-test was used to assess differences between ILC and IDC as well as between RPS. Results: ILC patients had a significantly lower percentage of MP High Risk tumors in comparison to IDC. Among MP High Risk tumors, those with ILC had significantly more MP High 1 than patients with IDC. BP subtyping in ILC tumors showed significantly lower percentage of Basal, and Luminal B, and higher percentage of Luminal A compared to IDC tumors. For RPS, a higher percentage of ILC tumors were ImPrint-/RePrint- compared to IDC, whereas lower frequencies of ImPrint-/RePrint+ and ImPrint+ were found. Within the ILC ImPrint+ subgroup (n=14), 1 (7.1%) was classified as High 2 and 10 (71.4%) as High 1 and 3 (21.4%) as Low Risk. All 14 (100%) ImPrint+ patients were BP Luminal. In contrast, within the IDC MP High Risk ImPrint+ subgroup (n=328), 233 (71.0%) were High 2, 88 (26.8%) High 1 and 7 Low Risk (2.1%). Furthermore 193 (60.7%) were classified as BP Basal, 2 HER2 (0.6%) and 123 (38.7%) Luminal within this IDC RPS. Conclusions: This is the first study to investigate the distribution of Response Predictive Subtypes in ILC, which will be beneficial to optimize the treatment selection for patients with early-stage HR+/HER2- ILC. Though the percentage of ImPrint+ is lower in ILC, this study revealed a small subset of patients in ILC with potential response to Immunotherapy. Furthermore, these results underscore the heterogeneity of ILC tumors and generate further hypotheses to investigate the immune cell abundance in ILC compared to IDC and correlate immune cell abundance to ImPrint status. Table: Genomic and Tumor Characteristics of HR+HER2- ILC and IDC tumors Citation Format: Rita Mukhtar, Christina Yau, Denise Wolf, Adam Brufsky, Hannah Linden, Natasha Hunter, Reshma Mahtani, Abirami Sivapiragasam, Trevor Feinstein, Fengting Yan, Ian Grady, Priscilla McAuliffe, Michaela Tsai, Joyce O'Shaughnessy, Blanche Mavromatis, Sasha Davis, Josien Haan, William Audeh, Lavanya Samraj, FLEX Investigators' Group. Distribution of MammaPrint, BluePrint, and Response Predictive Subtypes based on ImPrint and Reprint in ER+/HER2- Invasive Lobular Carcinoma – A FLEX sub study 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 PO4-02-03.