Tables S5-8 showing results from univariate analysis for ORR, CBR, PFS and OS

patient-level raw gene expression data of FFPE tumor samples and best overall response rate

PURPOSE:Hormone receptor-positive (HR+), HER2-negative (HER2-) metastatic breast cancer (mBC) is biologically distinct from early-stage disease, with a higher prevalence of genomically defined nonluminal subtypes, particularly the HER2-enriched (HER2-E) and basal-like subtypes. These tumors are highly proliferative, less dependent on hormone signaling, and associated with poor outcomes and early resistance to endocrine therapy and CDK4/6 inhibition (CDK4/6i). This biological shift highlights the need for biomarker-driven strategies in the CDK4/6i-resistant setting. PATIENTS AND METHODS:The SOLTI-1716 TATEN trial (NCT04251169) is a phase II, single-arm study evaluating the combination of pembrolizumab and paclitaxel in patients with HR+/HER2- mBC classified as HER2-E or basal-like by PAM50 following progression on CDK4/6i. A total of 126 patients were screened using the PAM50 genomic assay; 20 with HER2-E or basal-like subtypes were enrolled and received pembrolizumab (200 mg every 3 weeks) and paclitaxel (80 mg/m2 weekly). RESULTS:The primary endpoint, overall response rate (ORR), was 61.1% [95% confidence interval (CI), 35.7-82.7], and the clinical benefit rate was 94.4% (95% CI, 72.7-99.9). The median progression-free survival was 8.1 months (95% CI, 5.9-10.4), and the median overall survival was 26 months [95% CI, 18-not reached (NR)]. Three patients achieved durable responses lasting ≥24 months. Gene expression analyses revealed that high expression of proliferation- and immune-related genes predicted improved ORR and survival, whereas luminal-related gene expression was associated with lower clinical benefit. CONCLUSIONS:These results suggest that chemo-immunotherapy may be an effective strategy in genomically defined nonluminal subtypes of HR+/HER2- mBC following CDK4/6i resistance and highlight the value of molecular subtyping to guide post-endocrine treatment decisions.

Tables S9-S17 showing mutivariate and bivariate analysis for ORR, CBR, PFS and OS

The anti-HER2 monoclonal antibody trastuzumab and new derivative formulations are the standard treatment for HER2-positive breast cancer. However, after 1 to 5 years of treatment, some patients acquire resistance to therapy, leading to relapse. The microRNA-449 family members were downregulated in HER2-positive breast cancer cell lines and low levels were associated with patients’ worse prognosis. Moreover, trastuzumab-resistant HER2-positive breast cancer cell lines showed lower microRNAs-449 and higher Fatty Acid Synthase (FASN) expression, compared to sensitive cell lines. The direct regulation of FASN by microRNA-449a and microRNA-449b-5p was demonstrated. Moreover, microRNAs-449 overexpression and FASN inhibition decreased cell proliferation and sensitized cells to trastuzumab treatment by inhibiting the PI3K/AKT signaling pathway. Together, these results suggest the microRNAs-449/FASN axis as a potential therapeutic target in combination with anti-HER2 agents to overcome trastuzumab resistance and to improve treatment response in HER2-positive breast cancer patients.

BACKGROUND:Several endocrine-based strategies have been evaluated following CDK4/6 inhibition (CDK4/6i) in hormone receptor-positive advanced breast cancer. However, the absence of head-to-head comparisons leave uncertainty regarding the optimal treatment selection. METHODS:A systematic literature search was performed to identify randomized controlled trials (RCTs) evaluating endocrine-based strategies after CDK4/6i for hormone receptor-positive advanced breast cancer. A network meta-analysis was used to compare overall treatment strategies, rather than individual treatments. In addition, an extracted individual patient data meta-analysis was conducted. The primary endpoint was progression-free survival (PFS) evaluated independently in tumors with PI3K-AKT-PTEN alterations, ESR1-mutated and wild-type tumors. RESULTS:A total of 20 RCTs including 4,716 patients were included. In ESR1-mutated tumors, oral SERD/SERM/PROTAC showed numerically better PFS compared with switching CDK4/6i plus fulvestrant (HR = 0.67, 95 % CI 0.45-1.00). In this population, the addition of i) CDK4/6i or ii) mTORi to oral SERDs improved PFS compared with oral SERD/SERM/PROTAC alone (HR = 0.44, 95 % CI 0.27-0.72 and HR = 0.45, 95 % CI 0.23-0.89, respectively). In PI3K-AKT-PTEN altered tumors, the greatest benefit was observed with PI3K/AKT/mTORi plus fulvestrant and oral SERDs plus CDK4/6i (P-scores > 0.75). In ESR1 and PI3K/AKT/PTEN wild-type tumors, several treatment combinations outperformed fulvestrant. The use of PI3K/AKT/mTORi plus fulvestrant was associated with the highest incidence of grade ≥ 3 adverse events (66.0 %). CONCLUSION:This meta-analysis reinforces the importance of molecular stratification in treatment decisions after CDK4/6i progression, highlighting the need for efficacy and safety assessments across biomarker-selected subgroups.

Introduction: Breast cancer (BC) in very young women (VYBC), specifically those under 35 years old, presents biological and clinical differences compared to older patients with breast cancer (OBC). Previous studies have shown discordance in tumor characteristics and treatment responses between younger and older patients, notably within HR+/HER2- subtype. This study aims to elucidate the transcriptomic landscape of HR+/HER2- in VYBC compared to OBC, focusing on identifying age-associated molecular differences that could decipher personalized treatment strategies for these patients. Methods: This study included 49 BC patients treated at Hospital Clínico Universitario de Valencia. RNA sequencing was performed on formalin-fixed samples from 22 VYBC (age ≤35 years) and 27 OBC (≥50 years) patients. Differential expression analysis and gene set enrichment analysis (GSEA) were performed to identify differentially expressed genes. Deconvolution analysis using MCP-counter and quanTIseq algorithms was employed to evaluate tumor cellular composition. Histological examination described tumor infiltrating lymphocytes (TILs) and tertiary lymphoid structures (TLS). Statistical analyses were assessed using R (version 4.0.2). Results: The BCVY cohort included 12 (55%) luminal, and 10 (45%) non-luminal BCs while the OBC cohort were 17 (63%) luminal and 10 (37%) non-luminal patients. Transcriptomic analysis revealed significant age-associated differences particularly in HR+/HER2- subtype (133 and 106 genes down- and up-regulated (log2 fold change < ± 2, adjusted p value < 0.05). HR+/HER2- VYBC tumors exhibited greater proliferation signatures (p = 5.17e-05) and ki67 (IHC, p = 0.021), increased chromosomal instability (CIN70, p = 1e-06), and higher expression of immune-related gene signatures compared to OBC tumors. Interestingly, despite no differences in estrogen and progesterone receptor by IHC, lower expression was found at RNA level in HR+/HER2- VYBC patients (ESR1: p= 0.0027, PGR: p= 0.0476). Deconvolution analysis confirmed an enhanced infiltration of immune cells in HR+/HER2- VYBC tumors, identifying them as immunoreactive tumors, characterized by high infiltration of cytotoxic T lymphocytes and overexpression of immune checkpoint molecules such as PD-1 and its ligand PD-L1. Histological validation confirmed increased TILs and TLS in VYBC tumors. Finally, PAM50-based chemoendocrine score (CES) was also determined for HR+/HER2- samples showing higher levels of CES, which are predictive of better response to chemotherapy in HR+/HER2- VYBC patients. Conclusions: Our study provides a comprehensive understanding of the molecular landscape of HR+/HER2- BC in very young women. We found age-related gene expression changes not only in cancer cells but also in the tumor microenvironment. These data suggest that HR+/HER2- tumors from VYBC patients could be more endocrine-resistant and immunoreactive than those from OBC patients. Accordingly, personalized therapeutic strategies in this subgroup are needed. Citation Format: Marta Tapia, Iris Garrido-Cano, Juan Carbonell, Carlos Peña, Sandra Torres-Ruiz, Anna Ágreda-Roca, Cristina Tébar, Octavio Burgués, Cristina Hernando, Ana Lluch, Begoña Bermejo, María Teresa Martínez, Juan Miguel Cejalvo. Deciphering the transcriptomic landscape of early HR+/HER2- breast cancer in very young women [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P5-02-25.

Background: SERENA-1 (NCT03616587) is a Phase 1, multi-part, open-label study of camizestrant in women with ER+/HER2− advanced breast cancer. Study parts examining camizestrant as monotherapy and in combination with palbociclib, abemaciclib, and capivasertib have been presented previously. Here, we present the data from Parts K and L, which examined camizestrant in combination with ribociclib.MethodsThe primary objective was to determine the safety and tolerability of camizestrant 75 mg once daily (QD) in combination with intermittent ribociclib 400 mg or 600 mg QD (21 days on, 7 days off). Objectives included assessment of the anti-tumor response and pharmacokinetics (PK). Patients were women with advanced breast cancer, and premenopausal women were required to receive ovarian function suppression. Prior treatment with ≤2 lines of chemotherapy in the advanced setting was permitted, with no limit on prior endocrine treatment lines; previous treatment with CDK4/6 inhibitors (CDK4/6i) and/or fulvestrant was also permitted.Results: As of April 26, 2024, 58 patients (median age 57 (range [31–80]) were enrolled; 26 and 32 patients received camizestrant 75 mg in combination with ribociclib 400 or 600 mg, respectively. Overall, patients were heavily pre-treated in the advanced setting; 17/58 (29%) patients had received prior chemotherapy, 40/58 (69%) prior CDK4/6i, and 28/58 (48%) prior fulvestrant. 35/58 (60%) patients had visceral metastases.Steady state ribociclib exposure in combination with camizestrant 75 mg was comparable to simulations from a published ribociclib monotherapy population PK (popPK) model. Exposure to camizestrant 75 mg in combination with ribociclib 400 or 600 mg is comparable to a popPK simulation exposure of monotherapy camizestrant 150 mg.Visual effects and bradycardia, causally related to camizestrant (per investigator’s opinion), were reported in 20/58 (35%) and 15/58 (26%) patients, respectively; all grade 1. The most common ribociclib-related adverse events (AEs) were neutropenia (24/58; 41%), nausea (15/58; 26%), and fatigue (14/58; 24%). 17/58 (29%) patients reported ribociclib-related AEs that led to ribociclib-only interruptions (most commonly neutropenia [n=13] and QT prolongation [n=3]); 6/58 (10%) patients reported AEs related to both camizestrant and ribociclib leading to interruption of both treatments (most commonly QT prolongation [n=2]). One patient discontinued both camizestrant 75 mg and ribociclib 600 mg due to grade 4 QTc prolongation, which resolved 7 days after cessation of dosing. There were no patients with AEs leading to camizestrant-only interruptions.At Cycle 1, Day 1, 20/58 (34%) patients had a detectable ESR1 mutation (ESR1m) by ctDNA and an evaluable Cycle 2 Day 1 (C2D1) result; 85% of these patients experienced ESR1m reduction of >50% at C2D1, including 50% of patients where ESR1m was cleared.While the median duration of exposure to camizestrant and ribociclib at this data cut was 4.5 months, median progression-free survival has not yet been reached, with 40% (23/58) experiencing a progression event to date. Updated data will be presented at the meeting.Conclusion: Camizestrant 75 mg in combination with ribociclib 400 or 600 mg was well tolerated, consistent with data for each drug individually, and resulted in ESR1m ctDNA suppression in 85% of patients with ESR1m at baseline. This combination is included in the ongoing Phase 3 SERENA-6 trial (NCT04964934) of camizestrant combined with CDK4/6i versus an aromatase inhibitor plus CDK4/6i in patients with detectable ESR1m during first-line therapy, which will further clarify the role of camizestrant in the treatment of patients with ER+/HER2− advanced breast cancer. Citation Format: Richard Baird, Manuel Ruiz-Borrego, Ivan Victoria Ruiz, Christos Vaklavas, Anne Armstrong, Begoña Bermejo de las Heras, Mafalda Oliveira, Nicholas Turner, Irene Moreno, Peter Kabos, Chris Twelves, Jason Incorvati, Maria Borrell, Alberto Indacochea Cusirramos, Jennifer Diamond, Gabriel Funingana, Alejandro Falcón González, Cristina Hernando, Ciara O’Brien, Jorge Ramon-Patino, Mei Wei, Maxine Ajimi, Carmela Ciardullo, Teresa Klinowska, Justin P O Lindemann, Patricia Martin Romano, Alastair Mathewson, Christopher J Morrow, Andy Sykes, Ewa Warwick, Jincheng Yang, Bairu Zhang. Results from SERENA-1 Parts K/L: A Phase 1 study of the next-generation oral selective estrogen receptor degrader (SERD) camizestrant (AZD9833) in combination with ribociclib in women with ER-positive, HER2 negative advanced breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr PS7-08.

Introduction The prognostic value of PAM50 intrinsic subtypes (IS), cell cycle, and immune-related gene expression in HR+/HER2- advanced breast cancer (BC) treated with CDK4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) in a first-line metastatic setting is unclear. This study evaluates these biomarkers in metastatic biopsies from patients diagnosed with HR+/HER2- advanced BC. Methods CDK-PREDICT study is a multicentric, ambispective observational cohort study conducted in six Spanish hospitals. It included patients diagnosed with HR+/HER2- advanced BC treated in the first-line setting with CDK4/6i and ET. Baseline biopsies were obtained prior to treatment to determine research-based PAM50 IS, cell cycle and immune-related gene expression. The primary objective was to evaluate progression-free survival (PFS) differences among PAM50 IS using uni- and multivariable Cox regression models. Secondary objectives included overall survival (OS), overall response rate (ORR), and correlating cell cycle and immune response gene expression with PFS. Results A total of 185 patients were included, with a median follow-up of 38.5 months. PAM50 luminal subtypes were predominant (82.7%). Non-luminal subtypes showed significantly shorter median PFS (10.2 vs. 25.7 months; HR, 2.50; p<0.001) and OS (32.3 vs. 58.1 months; HR, 2.54; p<0.001) than luminal subtypes. Higher cell cycle and immune-related genes expression, such as CCNE1 and PDCD1, as well as tumor infiltrating lymphocytes were associated with poorer outcomes. Conclusions This study confirms the independent prognostic value of PAM50 IS in HR+/HER2- advanced BC treated with CDK4/6i and ET. Non-luminal subtypes exhibited the worst prognosis, underscoring the need for novel therapeutic strategies in this population.

Abstract Background: Male breast cancer (MBC) accounts for approximately 0.25% of all male cancers and < 1% of all BCs. Its incidence is increasing by 1.1% per year. Most population studies indicate that 10-15% of MBC patients (pts) have a germline BRCA2 mutation (BRCA2 mut). Around 5-10% of BRCA2mut male carriers will develop BC at their lives. Preliminary evidence suggests that BRCA2mut MBC pts tend to have a more aggressive disease. Here we present comparative series of MBC with and without BRCA to gain insight on differences between both groups. Methods: GEICAM/2016-04 (NCT03800355) is a retrospective, observational study which includes data from MBC pts diagnosed between 2000 and 2019 throughout Spain. This analysis includes the 1st unselected 186 pts included in the study, who had BRCA1/2 genes mutational status assessed within clinical practice. BC clinical subtypes are defined: luminal (HR+, HER2-), Triple Negative (TN) (HR-, HER2-), and HER2+ (HR+ or HR-); subtype is unknown in 6% pts. Results: 186 validated pts with BRCA1/2 testing were identified, representing the 24% of pts available in the database at the cut-off date (6-Mar-2023). 36 (19%) pts had germline BRCA1/2 mut (4 pts in BRCA1, 31 pts in BRCA 2 and 1 pt in both BRCA1 and BRCA2), and 150 (81%) pts had BRCA1/2 wild type (wt) or a variant of uncertain significance (VUS)/unknown (UK). Median age at BC diagnosis was 62 (34-87) years, 98% pts were Caucasian, and the median body mass index was 27 (18-46) Kg/m2 (overweight). Prior cancer history was reported in 6% BRCA1/2mut BC pts and 11% BRCA1/2wt or vus/uk, and family history included: 56% BC, 8% ovarian cancer, and 14% prostate cancer (PC). PC as part of prior medical history, cancer family history, and 2nd non-breast primary malignancy, was most frequent in BRCA1/2wt or vus/uk pts, with no statistical differences. At first diagnosis, stages III-IV were higher in BRCA1/2mut pts (28% vs. 17%, p=0.0093). Morphologically, invasive carcinoma of no special type (NST) was the most common pattern (92% in BRCA1/2mut vs. 86% in BRCA1/2wt or vus/uk), and lobular invasive carcinoma was not reported up-to-date. Histological grade 2 was the most frequent (54%), but grade 3 was present in 17/36 BRCA1/2mut pts. 88% pts received adjuvant treatment, 7% (neo-)adjuvant, 2% neoadjuvant and 3% did not receive any systemic therapy. Breast conserving surgery was performed in 4% (5/140) BRCA1/2wt or vus/uk EBC pts, and mastectomy was performed in 55% (n=6/11) de novo metastatic pts. In the advanced setting (n=38), visceral lesions were more frequent in BRCA1/2mut pts. Additionally, 92% BRCA1/2mut pts had ≤ 3 organs involved while one third of BRCA1/2wt or vus/uk pts had ≥ 4 metastatic locations. With a median follow-up of 64 months (mo.), median iDFS and DDFS were higher, but not statistically significant, in BRCA1/2wt or vus/uk pts vs. BRCA1/2mut pts. In advanced setting, median PFS to 1L-3L do not show statistically significant differences between both groups. Regarding OS, no differences were also observed, but the median values were not reached. Further detailed information according to BRCA1/2 mutational status and BC subtypes is included in the table below. Conclusions: In this subset analysis of GEICAM/2016-04, BRCA1/2mut pts had some features of worse prognosis, with a more prevalent de novo metastatic disease. No statistically significant differences were observed in outcomes in both early-stage and advanced setting vs. BRCA1/2wt or vus/uk pts. Citation Format: Noelia Martínez-Jáñez, Purificación Martínez, Cristina Hernando, Sabela Recalde, Alfonso Sánchez, David Morales, Marta Santisteban, Isaura Fernández, Sonia Del Barco, Esther Zamora, Vega Iranzo, Tamara Martos, Eduardo Martínez-de Dueñas, Silvia Antolin Novoa, Severina Domínguez, Paula Domínguez, Mª José Echarri, Ana Santaballa Bertrán, Xavier Mira, Andrea Blasco, Susana Bezares, Ander Urriticoechea. BRCA mutated Male Breast Cancer, hallmarks of a distinct disease. Data from the Spanish Male Breast Cancer Registry (GEICAM/2016-04) [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 PO1-16-10.

In this study, we performed genomic analyses of cell cycle and tumor microenvironment changes during and after ribociclib and letrozole or chemotherapy in the CORALLEEN trial. 106 women with untreated PAM50-defined Luminal B early breast cancers were randomly assigned to receive neoadjuvant ribociclib and letrozole or standard-of-care chemotherapy. Ki67 immunohistochemistry, tumor-infiltrating lymphocytes quantification, and RNA sequencing were obtained from tissue biopsies pre-treatment, on day 14 of treatment, and tumor specimens from surgical resection. Results showed that at surgery, Ki67 and the PAM50 proliferation scores were lower after ribociclib compared to chemotherapy. However, consistent reactivation of tumor cell proliferation from day 14 to surgery was only observed in the ribociclib arm. In tumors with complete cell cycle arrest (CCCA) at surgery, PAM50 proliferation scores were lower in the ribociclib arm compared to chemotherapy ( p < 0.001), whereas the opposite was observed with tumor cellularity ( p = 0.002). Gene expression signatures (GES) associated with antigen-presenting cells (APCs) and innate immune system activity showed increased expression post-chemotherapy but decreased expression post-ribociclib. Interferon-associated GES had decreased expression with CCCA and increased expression with non-CCCA. Our findings suggest that while both treatment strategies decreased proliferation, the depth and the patterns over time differed by treatment arm. Immunologically, ribociclib was associated with downregulated GES associated with APCs and the innate immune system in Luminal B tumors, contrary to existing preclinical data. Further studies are needed to understand the effect of CDK4/6 inhibition on the tumor cells and microenvironment, an effect which may vary according to tumor subtypes.

Abstract Background. Premenopausal women diagnosed with HR+/HER2- breast cancer (BC) often have a different biology and worse prognosis. The SOFT and TEXT studies demonstrated an increase diseasefree survival (DFS) with ovarian function suppression (OFS) plus tamoxifen or exemestane compared to tamoxifen alone. Of note, high-risk clinicopathologic features, adjuvant chemotherapy, or aged ≤35 years correlated with greater OFS benefit. However, adding OFS in this context has intrinsic issues as higher toxicity, which led to treatment discontinuation (20% in SOFT trial), and suboptimal OFS was found in around 25% of patients (pts) with exemestane plus monthly triptorelin (SOFT-EST sub-study). Thus, new effective ET options without OFS are needed for premenopausal patients (pts). Elacestrant is the first oral, non-steroidal, selective estrogen receptor degrader (SERD) to demonstrate improved efficacy to SOC treatments and specifically compared to fulvestrant in postmenopausal pts with HR+/HER2- metastatic BC at the phase III EMERALD trial. In menopausal patients with HR+/HER2- early BC, the window of opportunity SOLTI-1905 ELIPSE trial (NCT04797728), showed that elacestrant was associated with a 27.3% rate of Complete Cell Cycle Arrest (CCCA) and a statistically significant suppression of Ki-67. Among Luminal A tumors, the CCCA rate was 45% and the average decrease in Ki-67 was 64.6%, while no CCCA was reported and the suppression of Ki-67 was less pronounced (31.7%) in the Luminal B population. There is still a need for investigating elacestrant without OFS in the premenopausal scenario. We hypothesize that elacestrant as a single agent or in combination with triptorelin is an effective and safe treatment regimen in premenopausal pts with HR+/HER2-negative early BC capable of achieving an equivalent CCCA rate as a subrogate of effectiveness regardless the use of OFS Study design. PREMIERE is a parallel, non-comparative, two-arm, randomized 1:1, open-label, multicenter, exploratory study in premenopausal women with primary operable HR+/HER2-negative BC. The study aims to evaluate the biological effects of elacestrant with or without triptorelin. Participants must have histologically confirmed HR+ (≥ 10%) and HER2- operable early BC stage I to stage IIB >1 cm with a Ki-67 between 10-35%. The primary objective is to assess the ability of each treatment arm to induce CCCA determined by central assessment of Ki-67 (% Ki-67 ≤ 2.7%). No formal comparison between treatment arms is intended. Secondary objectives include evaluating the biological activity of elacestrant with or without OFS, antiproliferative activity, changes in gene expression including PAM50 subtype changes, and the effect of optimal vs suboptimal suppression on CCCA . Serum E2 and FSH levels will also be evaluated. Safety and tolerability will be assessed based on adverse events and clinical laboratory test results. Pts will undergo screening and randomization, with stratification by PAM50 subtype (Luminal A vs Non-luminal A). Treatment will be elacestrant 400 mg once daily or elacestrant 400 mg once daily plus triptorelin 3.75 mg days +1 and +29 for 30 (+7) days. Surgery or biopsy will be performed after treatment completion, and a post-surgery visit will mark the end of the active follow-up period. Patients will receive SOC treatment after surgery. 48 patients will be recruited in 9 sites within SOLTI Spanish network in 9 months period. This study is financially supported by Menarini-Stemline. Citation Format: Meritxell Bellet- Ezquerra, Cristina Hernando, Pablo Tolosa, Maria Vidal, Yolanda Fernández, Santiago González-Santiago, Pilar Sanchez, Susana De La Cruz, Vanesa Ortega, Xavier Gonzalez-Farré, Milana Bergamino, Alejandra Espinosa, Tomás Pascual. A phase 2 randomized pre-operative,window of opportunity trial investigating the effect of elacestrant with/without triptorelin in premenopausal patients with HR+/HER2- breast cancer – SOLTI-2104-PremiÈRe trial [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 PO3-19-08.

Abstract Background Within HR+/HER2-negative breast cancer, PAM50 non-luminal tumors (HER2-enriched [HER2-E] and Basal-like) have higher expression of proliferation and immune-related genes and tumor infiltrating lymphocytes and might benefit from immunotherapy. Here, we report the efficacy, safety, and correlative analysis data of the TATEN trial, the first study designed to evaluate pembrolizumab and paclitaxel in HR+/HER2-negative, PAM50 non-luminal, ABC. Methods TATEN trial (NCT04251169) is a single-arm, multicenter, phase II study evaluating pembrolizumab in combination with paclitaxel in patients with HR+/HER2-, PAM50 non-luminal ABC. Key inclusion criteria include progression to prior CDK4/6 inhibitors (CDK4/6i), presence of measurable disease, no prior chemotherapy for ABC, ECOG 0-1, and non-luminal metastatic disease by PAM50. Included patients received pembrolizumab at 200 mg every 3 weeks (beginning at cycle 1) in combination with weekly paclitaxel at 80 mg/m2 beginning at cycle 2. The primary endpoint was overall response rate (ORR) according to RECIST V1.1. in patients who received at least one dose of combination treatment and had a first, post-baseline tumor assessment (evaluable population). Secondary endpoints included progression-free survival (PFS), clinical benefit rate (CBR), safety, and predictive biomarkers. The study was based on a Simon two-stage design. Stage I of the trial would be considered successful if at least 6 of 15 patients achieved a partial response and/or complete response. In that case, the trial would recruit up to 46 evaluable patients for a target ORR ≥ 41%. Metastatic biopsies from patients enrolled in pre-screening were also evaluated for tumor infiltrating lymphocytes (TILs) and were further analyzed with an expression panel of 192 genes, including PDL1 and PD1. Results From July 2020 to December 2021, 132 tumors were screened, and 27 PAM50 non-luminal tumors were identified (20%). Non-luminal tumors trended to have higher PDL1 expression (p=0.090) and TILs (p=0.084) compared to luminal tumors, while no difference was observed for PD1 (p=0.850). Of 20 recruited patients in the study (stage I+II), 18 were evaluable for the primary endpoint. Baseline characteristics were as follows: median age 55 years, ECOG 0 55%, de novo MBC 22%, and visceral disease 72.2%. Eleven patients had received paclitaxel treatment in the adjuvant setting. Regarding PAM50 subtype, 2 patients had basal-like and 16 HER2-E tumors. At the time of data cut-off (June 2023), 13 patients (72.2%) had stopped their treatment because of progressive disease and 3 (16.6%) due to toxicity. Two patients (11.1%) were still on treatment. The ORR was 61.1 % (11 of 18, 95% CI 35.7-82.7). CBR was 88.9% (16 of 18, 95% CI 65.3-98.6), and median PFS was 8.3 months (95% CI 7.3 – 14.1). Treatment-related adverse events (TRAEs) of any grade (G) occurred in 19 patients (95%), while 45% of patients experienced G3 TRAEs. No G4 or G5 TRAEs were reported in the evaluable population. Gene expression analysis was successful for all recruited patients (n=20). High expression of the PAM50 luminal A signature (p=0.049), and the luminal genes PGR (p=0.028) and RRAGA (p=0.038) were associated with worse PFS (univariate analyses). The pan-leucocyte receptor CD84 (p=0.028) was associated with better PFS (univariate analysis). Conclusions Pembrolizumab in combination with paclitaxel is safe and exhibits promising efficacy outcomes in patients with CDK4/6i resistant HR+/HER2- ABC with a PAM50 non-luminal subtype. Although meeting the criteria for stage II, the enrollment was stopped prematurely due to the lack of funding and pembrolizumab supply. More correlative analyses will be presented at the conference. This study was funded in part by MSD. Citation Format: Aleix Prat, Benedetta Conte, Fara Brasó-Maristany, Nuria Chic, Montserrat Muñoz, Cristina Hernando, Manuel Alva, Silvia Vazquez, Salvador Blanch, Mafalda Oliveira, Esther Fernández, Oleguer Castillo, Patricia Galván, Ángela Aguirre, Esther Sanfeliu, Lorea Villanueva, Tomás Pascual, Eva Ciruelos. Solti-1716 TATEN phase II trial: Targeting non-Luminal disease by PAM50 with pembrolizumab and paclitaxel in Hormone Receptor-positive/HER2-negative advanced breast cancer (ABC) [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 PO1-06-02.

556 Background: Male BC management is based, by default, on evidence produced from female BC. There is a high need to validate gender specific performance of established prognostic biomarkers. We present here descriptive results from a large, country specific, registry of Male BC. Methods: GEICAM/2016-04 (NCT03800355) is a retrospective, observational study including Male BC pts diagnosed from 2000 to 2019 in 51 Spanish sites. Data from BC diagnosis until 10-Mar-2020 was collected from medical charts; biological specimens were obtained. Pts and tumors characteristics, therapies, and outcomes were analyzed. Molecular subtypes were categorized as hormonal receptor positive (HR+)/HER2 negative (HER2−), triple negative (TN) (HR−/HER2−), and HER2+ (any HR). Results: 773 pts were analyzed, at first diagnosis, 721 (93%) had early BC (EBC) (stages I [28%], II [41%], III [21%]), and 52 (7%) de novo metastatic BC; median age was 66 (23-96) years; median body mass index was 28 (18-50) Kg/m2 (overweight), with obesity in 4% as prior medical history. Previous history of other cancers: 20 (3%) pts were diagnosed with prostate cancer (PC) and 20 (3%) pts with skin cancer (melanoma and non-melanoma). BC family history was reported in 212 (35%) pts, 56 (9%) PC, and 35 (6%) ovarian cancer. Germline genetic testing for hereditary risk was performed in 238 (31%) pts, with BRCA1/2 mutations present in 46 (19%) pts (BRCA1 in 6, BRCA2 in 39, and both in 1); BRCA1/2 mutations were observed in 9 (33%) HER2+ pts and 32 (18%) HR+/HER2− pts. Of EBC pts, 322 (45%) were node-positive, 274 (38%) had T2 tumors; 4% had breast conserving surgery, and 42% sentinel lymph node biopsy; 42% received adjuvant radiation therapy; 336 (47%) adjuvant and 44 (6%) neoadjuvant chemotherapy, 609 (84%) adjuvant endocrine therapy, mainly tamoxifen (72%); and 6% pts did not receive any systemic therapy. Morphologically, invasive carcinoma of no special type was reported in 89% pts, and 52% were grade 2. Per local pathological assessment, 97% estrogen receptor positive (ER+), 90% progesterone receptor positive (PgR+), 84% androgen receptor positive; 51% presented Ki67 index expression ≥20%, and 11% HER2+. Frequency according to molecular subtype: 599 (77%) HR+/HER2−, 75 (10%) HER2+, 6 (1%) TN, and 93 (12%) unknown. With a median follow-up of 64 months, medians of invasive disease-free survival (iDFS) and distant DFS (dDFS) were not statistically different in HR+/HER2− vs HER2+ pts and according to levels of Ki67 index expression (<20% vs. ≥20%). In a Cox multivariate model, stage I-II vs III and age were statistically significant (p<0.05) for both iDFS and dDFS. Conclusions: HR+/HER2− is the most common Male BC subtype, with ER and PgR highly positive, and 20% as median Ki67 index. No statistically significant differences were observed in terms of iDFS and dDFS based on level of Ki67 index expression. Clinical trial information: NCT03800355 .