Abstract P1-13-16: Dissecting the biological activity of different CDK4/6 inhibitors (CDK4/6i) in hormone receptor-positive/HER2-negative (HR+/HER2-) breast cancer (BC)
Authors: Lorman-Carbó Natàlia , Martínez-Sáez Olga , Fernandez-Martinez Aranzazu , Galván Patricia , Chic Nuria , Adamo Barbara , Vidal Maria , Muñoz Montserrat , Perou Charles M. , Gavilá Joaquín , Pascual Tomás , Prat Aleix , Brasó-Maristany Fara 
Published: March 2023

Abstract
Background: Palbociclib might be less effective than ribociclib in HR+/HER2- and HER2-enriched BC. This hypothesis is currently being tested in the HARMONIA prospective phase III clinical trial (NCT05207709). Here, we evaluated the downstream biological effects of both drugs using cell lines and patient tumor samples. Methods: Three HR+ BC cell lines (i.e., MCF7, T47D and BT474) were treated at 2 different dose levels of palbociclib or ribociclib (i.e., 100 nM and 500 nM) +/- fulvestrant (1 nM) for 72 hours (h). PAM50 gene signatures were determined on the nCounter, as well as phosphorylation of RB (p-RB) by Western Blot and senescence-associated β-galactosidase activity by FACS. In vitro experiments were performed in triplicates. PAM50 gene signatures were obtained from 49 paired baseline versus week-2 samples and 49 paired baseline versus surgery samples of the CORALLEEN phase II study (Prat, Lancet Oncol. 2020), which treated 49 women with PAM50 Luminal B HR+/HER2- early BC with neoadjuvant ribociclib (600 mg daily) plus endocrine therapy (ET). PAM50 signature scores were also evaluated in publicly available data from 23 paired baseline versus week-2 samples and 16 paired baseline versus surgery samples of the NEOPALANA phase II trial (Ma, Clin Cancer Res. 2017) which treated 50 patients with HR+/HER2- early BC with palbociclib (125 mg daily) plus anastrozole. Changes in PAM50 signatures upon CDK4/6i were determined by paired t-tests and significant analysis of microarray (SAM). Results: Across all cell lines, both palbociclib and ribociclib statistically significantly reduced p-RB at 72h with both doses (i.e., 100 and 500 nM) compared to non-treated cells (p&lt; 0.001). Senescence was also observed at 72h with both doses. Both drugs +/- ET significantly increased the Luminal A signature and decreased Luminal B and proliferation signatures with both doses. However, the HER2-enriched signature was only significantly reduced when both CDK4/6 inhibitors were given at 500 nM. In tumor samples from the CORALLEEN and NEOPALANA phase II studies, a similar change in PAM50 biology was observed with both drugs, namely an increase in Luminal A signature and a decrease in Luminal B and proliferation signatures after 2 weeks of treatment and at surgery. At 2 weeks of treatment, the HER2-enriched signature was significantly decreased in both studies with ribociclib (p&lt; 0.001) and palbociclib (p&lt; 0.001). However, the decrease in the HER2-enriched signature was only observed in surgical samples of patients treated with ribociclib (p&lt; 0.001), but not palbociclib (p=0.194). A difference in sample size could explain this result. Nevertheless, in CORALLEEN, the median number of days between the last dose of ribociclib and surgery was 13.1 days (range: 1-78). In NEOPALANA, the median number of days between the last dose of palbociclib and surgery was 29 days (range: 8-49), except for 8 patients who received additional 10-12 days of palbociclib immediately before surgery (Ma, Clin Cancer Res. 2017). In patients who underwent surgery at 8 days or before, the HER2-enriched signature was significantly decreased for both ribociclib (p&lt; 0.001) and palbociclib (p=0.013). Interestingly, in patients that underwent surgery after &gt;8 days from the last dose, a significant reduction of the HER2-enriched signature was only observed with ribociclib (p&lt; 0.001), but not with palbociclib (p=0.500). Conclusions: Both palbociclib and ribociclib have similar effects on PAM50 biology when given at the same dose. However, in clinical practice, palbociclib is given at a lower dose than ribociclib, and although HER2-enriched signature is significantly decreased in tumors after 2 weeks of CDK4/6i+ET, this effect is only maintained at later time points with ribociclib, indicating a dose-dependent efficacy of CDK4/6i in this biologically aggressive subtype.
Citation Format: Natàlia Lorman-Carbó, Olga Martínez-Sáez, Aranzazu Fernandez-Martinez, Patricia Galván, Nuria Chic, Barbara Adamo, Maria Vidal, Montserrat Muñoz, Charles M. Perou, Joaquín Gavilá, Tomás Pascual, Aleix Prat, Fara Brasó-Maristany. Dissecting the biological activity of different CDK4/6 inhibitors (CDK4/6i) in hormone receptor-positive/HER2-negative (HR+/HER2-) breast cancer (BC) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-13-16.