Characterizing of Breast Cancer translational PEARL clinical trial

Introduction In HR+/HER2- metastatic breast cancer (MBC) is imperative to identify patients who respond poorly to CDK4/6i and to discover therapeutic targets to reverse this resistance. Non-luminal breast cancer subtype and high levels of CCNE1 are candidate biomarkers in this setting but further validation is needed. Methods We performed mRNA gene expression profiling and correlation with progression-free-survival (PFS) on 455 tumor samples included in the phase III PEARL study, that assigned HR+/HER2- MBC patients to receive palbociclib+ET vs capecitabine. ER+/HER2- breast cancer cell lines were used to generate and characterize resistance to palbociclib+ET. Results Non-luminal subtype was more prevalent in metastatic (14%) than in primary tumor samples (4%). Patients with non-luminal tumors had median PFS of 2.4months (m) with palbociclib+ET and 9.3m with capecitabine; HR:4.16, adjusted p-value<0.0001. Tumors with high CCNE1 expression (above median) had also worse median PFS with palbociclib+ET (6.2m) than with capecitabine (9.3m); HR:1.55, adjusted p-value=0.0036. In patients refractory to palbociclib+ET (PFS in the lower quartile) we found higher levels of Polo Like Kinase 1 (PLK1). In an independent data set (PALOMA3), tumors with high PLK1 show worse median PFS than those with low PLK1 expression under palbociclib+ET treatment. In ER+/HER2- cell line models we show that PLK1 inhibition reverses resistance to palbociclib+ET. Conclusion We confirm the association of non-luminal subtype and CCNE1 with resistance to CDK4/6i+ET in HR+ MBC. High levels of PLK1 mRNA identify patients with poor response to palbociclib, suggesting PLK1 could also play a role in the setting of resistance to CDK4/6i. Overall design: Breast Cancer samples were included