Abstract 7037: Comprehensive molecular and immune profiling of triple negative invasive lobular carcinoma

Abstract Background: Triple-negative invasive lobular carcinoma (TN-ILC) is a rare (0. 1-1. 4%) breast cancer with prognosis worse than ER positive ILC. Currently there are no targeted therapies or clinical trials specifically for TN-ILC. A comprehensive analysis of the molecular and immune landscape can help identify novel targets and pathways for TN-ILC to improve patient outcomes. Here, we characterized the molecular and immune signature of TN-ILC. Methods: 11, 760 breast cancer samples (Invasive ductal (ID) TNBC, n=364; TN-ILC, n=31) were analyzed (592, NextSeq; WES, NovaSeq), (WTS; NovaSeq) (Caris Life Sciences, Phoenix, AZ). Tumor mutational burden (TMB) totaled somatic mutations per tumor (high ≥10 mt/MB) was tested by NGS. Microsatellite-instability (MSI) was tested by NGS. Immune cell fractions were calculated by deconvolution of WTS. Statistical significance was determined using chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q 0. 05). Results: TN-ILC had higher frequency of CDH1 (0. 83% vs. 66. 67%), ERBB2 (0. 84% vs. 28. 57%), ARID1A (1. 65% vs. 23. 33%), CBFB (0. 55% vs. 16. 67%), AKT1 (2. 22% vs. 16. 13%) and KMT2C (3. 83% vs. 13. 79%), but lower frequency of TP53 (89. 7%5 vs. 38. 71%) mutations (all p0. 05) compared to ID-TNBC. TN-ILC had higher frequency of TMB high (23. 3% vs. 5. 2%, p0. 05) but there was no difference in dMMR/MSI-H (0% vs 1. 1%, p=1). TN-ILC had higher AR RNA (FC: 14. 2), protein expression (80. 6% vs. 24. 8%) and higher frequency of fusion variant-AR (16. 1% vs 4. 9%) (all p 0. 05) compared to ID-TNBC. Analysis of inferred immune cell infiltrates showed that TN-ILC had higher infiltration of M2 macrophages (5. 34% vs. 2. 94%) and neutrophils (4. 83% vs. 2. 65%) but lower infiltration of M1 macrophages (3. 46% vs 2. 17%) and CD8 T cells (0. 58% vs. 0. 11%) (all p0. 05). TN-ILC had lower T cell inflamed signature (16. 1% vs 35. 7%, p0. 05), decreased immune checkpoint genes (CD274, CTLA4, FOXP3, LAG3, IDO, FC: 1. 2-2. 7, all p 0. 05), PD-L1 protein expression (SP142: 30. 4% vs. 50. 2%, p=0. 06; 22c3: 15. 4% vs. 42. 6%, p = 0. 05) and differential expression of BCL2 family genes (upregulation: BIK, FC: 2. 2; downregulation: BAX, BAK1, BID, PMAIP1, MCL1, BCL2A1, BCL2L10, FC: 1. 0-6. 3, all p 0. 05) compared to ID-TNBC. Conclusion: These data suggest that TN-ILC had higher frequency of CDH1, ERBB2, AKT1, ARID1A mutations, higher M2 macrophages and neutrophils and lower M1 macrophages and CD8 T cells infiltration and, lower T cell inflamed signature. High TMB and AR expression can translate into use of immunotherapy (ICI) and AR antagonists in these patients. Additonal analysis to determine the optimal biomarker for ICI response in TN-ILC is needed. Results of our study need to be validated in larger studies with survival correlation. Citation Format: Pooja Advani, Sachin Deshmukh, Sharon Wu, Jacob Andring, Joanne Xiu, Jose Leone, Priya Jayachandran, Stephanie Graff, Mathew Oberley, George Sledge, Asher Chanan-Khan. Comprehensive molecular and immune profiling of triple negative invasive lobular carcinoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 7037.