Abstract PO5-06-14: Genomic Landscape and Clinical Outcomes of Triple-Negative Invasive Lobular Carcinoma

Abstract Background: Triple-negative (TN) invasive lobular carcinoma (ILC) of the breast is a rare entity, comprising roughly 2% of primary ILC. Clinicopathologic studies have shown that TN ILC is associated with more pleomorphism, higher nuclear grade, older patient age, and worse disease-related outcomes compared to estrogen receptor-positive (ER+) ILC. Intrinsic subtyping indicates that TN ILC less frequently expresses basal markers than TN invasive ductal carcinoma (IDC). This prompted investigation of the genomic landscape and clinical management of TN ILC to provide insight into potential therapeutic approaches. Methods: Twenty patients with TN ILC were included in this study. Three patients were treated at a single academic center and underwent comprehensive genomic profiling (CGP) of breast or metastatic tissue; clinicopathologic details were obtained from their electronic health records. Seventeen patients with TN ILC who underwent CGP of breast or metastatic biopsies were included from the Foundation Medicine, Inc. database. PD-L1 testing for all patients was determined by immunohistochemistry (IHC) using the VENTANA SP142 or Dako 22C3 commercial assays. HER2 expression was determined by IHC. Negative HER2 expression was defined as an IHC score of 0, or IHC 1+ or 2+ with non-amplified fluorescence in situ hybridization (FISH). Results: In the Foundation Medicine database of patients with TN ILC, the most frequent genomic alterations were in CDH1 (15/17 patients 88.2%), TP53 (10/17 patients 58.8%), and PIK3CA (10/17 patients 58.8%). Short variant mutations in ERBB2 were found in 3/17 patients (17.6%). Fourteen patients were HER2-low (HER2 IHC 1+ or 2+ with non-amplified FISH). No cases were MSI high or expressed PD-L1 (8 of 17 cases underwent PD-L1 assessment by SP142). Median tumor mutational burden (TMB) was 5 muts/Mb. Among the three patients with TN ILC treated at a single center, the most common somatic mutations were in CDH1 and TP53, and no cases were MSI high or expressed PD-L1 (by SP142 or 22C3). All three patients had pleomorphic ILC, histologic grade 2 or 3, and HER2 IHC 0. One patient with mpT1cN1a(sn) ER+/HER2- ILC of the right breast in 2011 received adjuvant TAC (docetaxel, doxorubicin, cyclophosphamide), breast/axillary radiation, and ten years of adjuvant endocrine therapy. She was diagnosed with TN ILC of the left supraclavicular nodes in 2022; she did not exhibit a response to docetaxel and carboplatin with pembrolizumab, and she subsequently received palliative radiation followed by capecitabine. Another patient with locally advanced TN ILC progressed on the KEYNOTE-522 regimen, immediately developed metastases, and is currently receiving palliative sacituzumab govitecan. The third patient with cT3N1 TN ILC progressed on neoadjuvant therapy (through the I-SPY 2 trial) with oral paclitaxel, encequidar, and dostarlimab, followed by dostarlimab with dose-dense doxorubicin and cyclophosphamide. Her surgical pathology demonstrated ypT3N3a disease with 10 × 7 cm of tumor, and she developed skin metastases on the chest wall within weeks post-operatively. She received comprehensive chest wall and nodal radiation and is currently receiving palliative capecitabine with neratinib given a high variant allele frequency driver somatic ERBB2 mutation. Conclusions: The most common somatic mutations among TN ILC patients included in this study were in CDH1, TP53, and PIK3CA. Currently the management of TN ILC is extrapolated from that of TN IDC, however our patients did not respond to chemoimmunotherapy. The presence of HER2-low status or somatic ERBB2 mutations may provide opportunities for treatment with an anti-HER2 antibody-drug conjugate or tyrosine kinase inhibitor. Studies with larger sample sizes of TN ILC are needed for molecular profiling and assessment of outcomes with specific therapeutic approaches. Citation Format: Hemali Batra-Sharma, Smruthy Sivakumar, Prashanth Ashok Kumar, Ethan Sokol, Rebecca Shatsky, Jeffrey Ross. Genomic Landscape and Clinical Outcomes of Triple-Negative Invasive Lobular Carcinoma 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 PO5-06-14.