Tolaney, S. M.

Abstract Background PARP inhibitors (PARPi) are active in patients (pts) with germline BRCA1/2 or PALB2 mutated (gBRCA1/2; gPALB2m) breast cancer (BC). Preclinical data suggest PARPi leads to intratumoral activation of the proinflammatory cGAS/STING pathway, recruiting CD8+ T cells, and sensitizing gBRCAm cancers to immune-targeted therapy. Prolonged preoperative treatment with PARPi monotherapy has demonstrated notable pathologic complete response (pCR) rates. The randomized phase II TBCRC 056 study evaluates the PARPi niraparib (N) with the anti-PD-1 antibody dostarlimab (D) as neoadjuvant treatment of gBRCAm or gPALB2mHER2-negative BC, with cohorts for both triple negative BC (TNBC, Arms A and B) and estrogen receptor positive (ER+) BC (Arm C). Results from Arms A and B are being presented. Methods Eligible pts (gBRCA1/2m/gPALB2m BC, T >1.0 cm, ER- (<10%), HER2-) were randomized to either Arm A: upfront combination N 200 mg orally QD and D 500 mg IV every 3 weeks (wks) for 18 wks (1 cycle = 3 wks), or Arm B: N monotherapy for 3 wks, followed by combination N plus D for 15 wks, with baseline (BL) and wk 3 tumor biopsies. After 18 wks, pts either underwent surgery or received additional preoperative systemic therapy (per MD choice) after an end of treatment biopsy if residual disease present. The primary study objectives are to evaluate pCR in Arms A and B combined, as well as change in stromal tumor infiltrating lymphocytes (sTILs) from BL to 3 wks in each Arm separately; 46 pts and a target pCR rate of 50% provides 85% power to reject the null hypothesis of pCR < 30%. Results A total of 46 pts with TNBC enrolled to Arms A and B (38 gBRCA1m (82.6%), 8 gBRCA2m (17.4%), 0 gPALB2m). Median age was 39.3 years (range 24.8-72.8); 84.8% self-reported White, 6.5% Black, 4.3% Asian, and 8.7% Hispanic. Clinical stage distribution was 37.0% stage I, 45.7% stage II, and 17.4% stage III; most had ER- BC (42 ER <1%, and 4 ER 1-10%). Among all pts, 38 (82.6%) completed the target number of cycles of D, with mean cycles received 5.1. Five pts discontinued D early (3 inadequate response/progression, 2 unacceptable AE: LFTs, rash). A total of 38 pts (82.6%) completed 6 cycles of N, with mean cycles received 5.7. Seven pts discontinued N early (3 inadequate response/progression, 4 unacceptable AE: LFTs, anemia, rash (2)). Most common AEs (> grade 2) with D+N were anemia (26.1%), fatigue (21.7%), hypertension (15.2%), hypothyroidism (15.2%), and neutropenia (15.2%). Most common Grade >3 AEs with D+N were anemia (17%) and neutropenia (6.5%). Among all pts, 23 (50%) had a pCR at surgery (90% CI 37.1% - 62.9%), 12 (26.1%) had residual disease (4 RCB-I, 5 RCB-II, 2 RCB-III, 1 RCB not calculable), and 11 (23.9%) crossed over to additional preoperative therapy. The pCR rate exceeded the protocol efficacy criterion of >43%, a significant increase over null hypothesis. pCR rates were the same on Arms A (50%, 90% CI 31.1% - 68.9%) and B (50%, 90% CI 31.9% - 68.1%). Among 15 pts on Arm A with evaluable sTILs at BL and 3 wks, mean sTILs at BL was 16% (range 1% - 40%), and at 3 wks was 27.4% (range 0.5% - 60%), for an average increase of 11.4% (p=0.009). Among 22 pts on Arm B with evaluable sTILs at BL and 3 wks, mean sTILs at BL was 19.5% (range 1% - 75%), and at 3 wks was 42.1% (range 1% - 85%), for an average increase of 22.7% (p=0.0003). Among all pts, BL sTILs (continuous) were significantly higher among those who achieved pCR than those who did not (median 15% vs. 5%, p=0.03). There was no relationship between pCR and BL PD-L1 score, or pCR and ER-0 vs. ER-low. Conclusions In pts with gBRCAm early TNBC, 18 wks of targeted therapy using PARPi and anti-PD1 agents, with or without 3-week PARPi lead-in, resulted in a pCR rate of 50%, and a statistically significant increase in sTILs from BL to 3 wks. Further ongoing correlative work may identify best candidates for this non-chemotherapy-based approach. Citation Format: E. L. Mayer, N. Graham, R. A. Leon-Ferre, M. Rozenblit, C. A. Santa-Maria, S. Isakoff, J. Specht, N. Tung, V. Abramson, J. Desrosiers, A. Schubert, B. Koca, S. M. Tolaney, E. P. Winer, I. E. Krop, A. C. Wolff, G. I. Shapiro, N. Tayob, J. L. Guerriero. Tbcrc 056: a phase 2 study of neoadjuvant niraparib with dostarlimab for patients with BRCA- or PALB2-mutated breast cancer: results from the TNBC cohorts [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 RF5-02.

Abstract Background: Antibody-drug conjugates (ADCs) are increasingly used in the treatment of breast cancer due to their superior efficacy compared to conventional chemotherapy. The management of male breast cancer is largely extrapolated from clinical data in females and evidence regarding the efficacy of ADCs in men remains limited. This study aimed to compare survival outcomes between male and female breast cancer patients treated with ADCs across the spectrum of human epidermal growth factor receptor 2 (HER2) expression. Methods: We included total of 2,954 (Male: 34, Female: 2,920 breast cancer samples) from patients who underwent treatment with ADCs. We excluded patients with triple-negative breast cancer status due to the rarity of this subtype in males and the worse prognosis of triple-negative breast cancer, so that the cohorts would be more homogeneous without overrepresentation of triple-negative breast cancer in females. We categorized patients by HER2 status as: HER2-null (if there was 0% stain on HER2 IHC), HER2-ultra low (if HER2 IHC was 0 but with stain 1-10%), HER2-low (if HER2 IHC was 1+ or 2+ with a negative chromogenic in situ hybridization [CISH] assay), and HER2-positive (if HER2 IHC was 3+ or 2+ with a positive CISH assay). Real-world median overall survival was derived from insurance claims and calculated from the start of first ADC with either trastuzumab deruxtecan (T-DXd) or trastuzumab emtansine (T-DM1) or sacituzumab govitecan (SG) to last contact using Kaplan-Meier. Statistical significance was assessed using chi-square and Mann-Whitney U. Results: The distribution of sex by HER2 expression group was: HER2-null: 8 males vs 612 females; HER2-ultra low 6 males vs 467 females; HER2-low 11 males vs 903 females; HER2-positive 9 males vs 938 females. Median survival from treatment initiation with either T-DXd or SG in the HER2-null cohort was 21.31 months in males vs 15.72 months in females (HR 0.82, 95% CI 0.33 - 2.01, p=0.67); in the HER2-ultra low cohort was 17.73 months in males vs 14.47 months in females (HR 0.84, 95% CI 0.26 - 2.63, p=0.76); in the HER2-low cohort was 11.08 months in males vs 18.62 months in females (HR 1.53, 95% CI 0.63 - 3.7; p=0.34). In the HER2-positive cohort, 5/9 males (55.5%) and 544/938 females (58%) were treated with T-DXd. Median survival from treatment initiation with either T-DXd or T-DM1 in the HER2-positive cohort was 36.55 months in males vs 46.52 months in females (HR 1.77, 95% CI 0.84 - 3.74, p=0.12). Conclusions: In this study, survival appears to be comparable between males and females with breast cancer across the spectrum of HER2 expression when both are treated with ADCs, although the survival in the HER2-positive cohort appears numerically lower in males, consistent with data on HER2-positive MaBC. An important limitation is the small sample size of the male breast cancer cohorts, and the lack of a categorization of HER2 in HR+ vs HR-, for the small MaBC sample size. However, to our knowledge this is the first analysis evaluating outcomes of men treated with ADCs across the spectrum of HER2 expression. Citation Format: D. Trapani, S. Deshmukh, S. Wu, J. Xiu, N. Lin, G. Curigliano, P. Spanheimer, S. Gandhi, S. Chumsri, S. Graff, M. Lustberg, G. Sledge Jr, S. Tolaney, J. Leone. Comparative efficacy of antibody drug conjugates (ADCs) in male versus female breast cancer across the spectrum of HER2 expression [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 PS4-01-13.

Abstract Background: Invasive lobular carcinoma (ILC) is the second most common breast cancer (BC) subtype, accounting for up to 15% of cases. Although several biologic differences between ILC and invasive carcinoma of no special type (NST) have been described, most data are derived from primary tumors. In contrast, the biology of metastatic ILC remains poorly defined. This study aimed to investigate the unique genomic, transcriptomic, and immune landscapes of metastatic ILC using a multi-omic approach. Methods: This real-world study included patients with confirmed NST or pure ILC who had a biopsy of a metastatic lesion or a breast lesion in the setting of known metastatic disease. Tumors underwent whole exome sequencing (NGS 592, NextSeq; WES, NovaSeq), whole transcriptome sequencing (WTS, NovaSeq) (Caris Life Sciences, Phoenix, AZ). Demographic and baseline molecular features were compared across histologic subtypes. Tumor mutational burden (TMB; high ≥10 mut/MB) was assessed by NGS. Immune cell populations were estimated by RNAseq deconvolution (quanTIseq). Pathway enrichment analysis was determined by GSEA. Overall survival (OS) was defined from the date of biopsy to death from any cause using Kaplan-Meier estimates. Statistical tests included chi-square and Mann-Whitney U with multiple testing correction (q<0.05). Results: Among 2,651 metastatic BC samples (ILC n = 608; NST n = 2,043), patients with ILC were older (median 67 vs 63 yrs) and more likely white (81.5% ILC vs 73.1% NST), all p<0.05. By IHC, ILC were more frequently HR+/HER2- (83.6% vs 55.9%), less commonly triple-negative (11.9% vs 29.8%) or HER2+ (4.6% vs 14.3%), all p<0.05. PAM50 subtypes differed between ILC and NST: luminal A (Lum A) (31.1% vs 13.7%) and luminal B (Lum B) (44.9% vs 43.8%) intrinsic subtypes were enriched in ILC, while NST was more frequently basal (1.2% vs 22.4%) and HER2-enriched (19.8% vs 22.2%), all p<0.05. Within luminal BC, Lum B was more common than Lum A in both histologies, but Lum A was proportionally more frequent in ILC (ILC: 40.3% Lum A vs 59.7% Lum B; NST: 23.4% Lum A vs 76.6% Lum B), p<0.0001. Comparative multi-omic analysis focused on the 922 HR+/HER2- tumors (ILC n = 275; NST n = 647). Compared to NST, ILC had higher frequency of CDH1 (87.3% vs 6.1%), PIK3CA (52.7% vs 38.5%), FOXA1 (9.0% vs 3.4%), ERBB2 (7.1% vs 2.0%), ARID1A, and NF1 (both q < 0.05) mutations and TMB high (17.7% vs 9.9%), but lower frequency of TP53 (15.7% vs 33.3%), ESR1 (10.9% vs 18.1%) and GATA3 (2.3% vs 14.9%) mutations along with FGFR1 amplifications (15.3% vs 6.7%), all q<0.05. ILC had higher expression of androgen receptor (91.2% vs 83.5%, q<0.05) and higher MAPK activation score (-0.81 vs -1.19, q<0.05), consistent with a luminal, hormone-driven phenotype. NST tumors demonstrated enrichment of pathways involved in the cell cycle and metabolic activity, including E2F targets, G2M checkpoint, MYC targets, mTORC1 signaling, glycolysis, and unfolded protein response (NES: 1.5-2.7, FDR<0.25). ILC tumors had higher infiltration of B cells, dendritic cells, neutrophils, NK cells and M2 macrophages, but lower M1 macrophages, all q<0.05. ILC had differential expression of immune checkpoint genes (upregulation: CD274 (PD-L1), PDCD1 (PD-1), TNFSF14, CEACAM1, CD160, fold change (FC): 1.2-1.5; down: HAVCR2 (TIM-3): FC: 1.2), all q<0.05. Median OS (mOS) was similar between luminal ILC and NST (33.8 vs 35.4 months (mo); HR 1.0, 95% CI 0.91-1.3). No mOS difference was seen between Lum A ILC and NST (47.4 vs 50.5 mo; HR 1.0, 0.84-1.43), while Lum B ILC had worse mOS than NST (27.4 vs 35.3 mo; HR 1.3, 1.1-1.6). Conclusions: This study represents the largest cohort to date with multi-omic characterization of metastatic ILC, revealing distinct genomic, transcriptomic, and immune features compared to NST. These findings highlight the unique biology of ILC and may inform future subtype-specific therapeutic strategies. Citation Format: G. Nader-Marta, S. Kumar Deshmukh, K. Fanucci, P. Tarantino, S. Schnitt, A. Lee, S. Oesterreich, S. Wu, J. Xiu, P. Advani, M. Lustberg, N. Lin, S. Tolaney, E. Mayer, O. Metzger, G. Sledge Jr., R. Jeselsohn. The Genomic, Transcriptomic, and Immune Hallmarks of Metastatic Lobular Breast Cancer [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 PD9-07.