CCNE1 mRNA and cyclin E1 protein expression as predictive biomarkers for efficacy of palbociclib plus fulvestrant versus capecitabine in the phase III PEARL study.
Authors: Pascual Javier , Gil-Gil Miguel , Zielinski Christoph , Hills Margaret , Ruiz-Borrego Manuel , Ciruelos Eva M. , Garcia-Murillas Isaac , Muñoz Montserrat , Bermejo Begoña , Swift Claire , Vila Mireia Margeli , Antón Torres Antonio , Nissenbaum Bella , Murillo Laura , Liu Yuan , Herranz Jesus , Caballero Rosalia , Guerrero-Zotano Angel , Turner Nicholas C. , Martin Miguel 
Published: June 2021

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Background: The randomized PEARL trial found no superiority of palbociclib plus endocrine therapy over capecitabine in patients (pts) with metastatic HR-positive, HER2-negative breast cancer resistant to prior aromatase inhibitors (Martin M, Ann Oncol 2020). Gene expression analysis showed high CCNE1 mRNA ( CCNE1) conferring relative resistance to palbociclib in the PALOMA-3 trial (Turner N, JCO 2019), but further validation is needed. Cyclin E1 protein (cyclin E1) expression in this context has not been studied in randomized trials. We explored CCNE1 and cyclin E1 as predictive biomarkers in tumor samples from the PEARL study. Methods: Formalin-fixed paraffin-embeded tumor samples were retrieved from pts enrolled in PEARL cohort 2 (palbociclib (PAL) + fulvestrant (FUL) vs capecitabine (CAPE)). We measured CCNE1 using the HTG EdgeSeq Oncology Biomarker Panel (HTG Molecular Diagnostics). Cyclin E1 immunohistochemistry (IHC) staining was performed using specific mouse monoclonal antibody HE12 (Abcam) and scored as percentage of invasive nuclei stained (0-100%). CCNE1 and cyclin E1 correlations were explored using Pearson coefficients. Cox regression models were used for progression free-survival (PFS) analyses using expression levels split by median, to define high ( > median values) vs. low expression. Site of disease and prior chemotherapy were used as confounders in multivariate models. Results: Analyses were conducted in 219 pts (47% receiving PAL + FUL and 53% CAPE) with available tumors, with the analysed patients representative of the overall study. Most samples were from the archival primary (72%), obtained > 5 years before this analysisº (74%). CCNE1 and cyclin E1 were only moderately correlated (r = 0.5). Median CCNE1 was higher in metastatic vs primary (7.37 vs 6.94, p < 0.01), and in luminal B and non-luminal subtypes compared to luminal A (p < 0.001). In patients with high CCNE1 expression, median PFS on CAPE was 10.35 and PAL + FUL was 5.68 (HR = 1.63, 95% CI 1.02-2.59, p = 0.042). In patients with low CCNE1 expression, median PFS on CAPE was 9.43 and PAL + FUL was 8.97 (adjusted HR = 0.93, 95% CI 0.59-1.48, p = 0.762, interaction p = 0.072). Median cyclin E1 protein was higher in luminal B and non-luminal subtypes compared to luminal A (p < 0.01). Cyclin E1 protein expression was not predictive of treatment effect (high cyclin E1 expression CAPE vs PAL + FUL HR = 1.17, low cyclin E1 expression CAPE vs PAL + FUL HR = 1.21, interaction p = 0.977). Conclusions: High tumor CCNE1 mRNA expression identified patients with relative resistance to palbociclib plus fulvestrant, validating prior observations although without statistical significance for interaction. Assessment of Cyclin E1 protein expression did not show predictive value. Investigation treatments to enhance CDK4/6 inhibitor efficacy in tumors with high CCNE1 expression is warranted. Clinical trial information: NCT02028507 .