Abstract PO1-15-10: Lower pre-treatment B-cell gene expression signatures correspond with improved overall survival with palbociclib + endocrine therapy in HR+/HER2- metastatic breast cancer: a biomarker analysis from the GEICAM/2013-02 PEARL trial

Abstract Introduction: CDK4/6 inhibitors (CDK4/6i) with endocrine therapy (ET) is standard-of-care for HR+/HER2- metastatic breast cancer (MBC), but little is known about which gene expression and microenvironment features are associated with benefit from CDK4/6i or differentiate response compared to chemotherapy. We examined the relationship of gene expression signatures (GES) to progression-free (PFS) and overall survival (OS) in the GEICAM/2013-02 PEARL phase III randomized study of palbociclib + ET vs. capecitabine in aromatase inhibitor-resistant HR+/HER2- MBC (NCT02028507). Methods: RNA sequencing (RNAseq) was performed on baseline tumor samples from 328 patients. Intrinsic subtype was derived by the PAM50 predictor after HER2/estrogen receptor subgroup-specific normalization. Correlations to intrinsic subtype signatures, PAM50 proliferation signature, CCNE1 expression, and 10 GES were selected for analysis. After excluding 15 Normal-like samples, Cox models were fitted for PFS and OS endpoints on each selected variable for the combined study population as well as for each treatment arm, adjusted for disease site, prior ET sensitivity, prior chemotherapy for MBC, number of involved sites, treatment arm, and intrinsic subtype. P-values were adjusted based on the Bonferroni method, with an adjusted p-value < 0.05 defined as significant. Results: PAM50 intrinsic subtype (54.95% Luminal A, 38.98% Luminal B and 6.07% non-luminal (4.79% Her2-enriched and 1.28% Basal-like)) was associated with worse PFS with palbociclib + ET for non-luminal vs. Luminal A tumors (HR 3.19, 95% CI 1.65-6.16) and for Luminal B vs. Luminal A tumors (HR 1.90, 95% CI 1.28-2.82). PAM50 luminal status (Luminal A vs. Luminal B vs. non-luminal) was significantly associated with PFS with palbociclib + ET (p=0.0003) but not capecitabine (p=0.89). In the palbociclib + ET arm, lower expression of two B-cell-associated signatures (BCAS) correlated with improved OS - the Immune1 TCGA BRCA GES (PMID: 32573490) (HR 1.52, 95% CI 1.20-1.92, p=0.009) and the B-cell/T-cell cooperativity GES (PMID: 31730857) (HR 1.42, 95% CI 1.14-1.79, p=0.04). Interaction analysis for OS showed that when stratified by low and high expression using the median, the Immune1 TCGA BRCA GES had a significant interaction with treatment arm (p=0.014). On comparison of outcomes between treatment arms, low Immune1 TCGA BRCA GES expression was significantly associated with improved OS with palbociclib + ET compared to capecitabine (HR 0.44, 95% CI 0.26-0.74, p=0.03), whereas high expression was not associated with a significant OS difference (HR 1.73, 95% CI 1.07-2.79, p=0.40). Further exploratory analysis of 164 immune GES found that, after adjustment for confounders, the top eight GES associated with OS with palbociclib + ET were BCAS (unadjusted p-values < 0.005), including our pre-selected Immune1 TCGA BRCA and B-cell/T-cell cooperativity GES, and all eight negatively correlated with OS. In the capecitabine arm, this exploratory analysis failed to demonstrate a similar association between the immune GES and OS. Conclusion: PAM50 intrinsic subtype was significantly associated with PFS with second-line palbociclib + ET. Lower expression of BCAS was associated with improved OS outcomes with palbociclib + ET independent of intrinsic subtype, and for the Immune1 TCGA BRCA GES, lower expression corresponded with improved OS with palbociclib + ET compared to capecitabine. This informs recent findings that in HR+/HER2- breast cancer, tumor immune activity may be a negative predictor of CDK4/6i benefit. Citation Format: Yash Agrawal, Aranzazu Fernandez-Martinez, Miguel Gil-Gil, Christoph Zielinski, Manuel Ruíz- Borrego, Eva Ciruelos, Montserrat Muñoz, Mireia Margelí, Begoña Bermejo, Antonio Antón, Zsuzsanna Kahan, Tibor Csöszi, José L Alonso-Romero, Jose Angel García-Sáenz, Pedro Sánchez-Rovira, Elena Álvarez, J. Ignacio Chacón, Santiago González-Santiago, César A Rodríguez, Sonia Servitja, Adam D. Pfefferle, Jesús Herranz, Yuan Liu, Lisa A. Carey, Isabel Romero - Camarero, Rosalía Caballero, Charles M. Perou, Miguel Martín. Lower pre-treatment B-cell gene expression signatures correspond with improved overall survival with palbociclib + endocrine therapy in HR+/HER2- metastatic breast cancer: a biomarker analysis from the GEICAM/2013-02 PEARL 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 PO1-15-10.