Tim Kong, Alex Mabry, Maureen Highkin, Anthony Z. Wang, Jeremy Hoog, Zhanfang Guo, Adrian Gonzales-Gonzales, Shana Thomas, Yingduo Song, Feng Gao, Mateusz Opyrchal, Lindsay Peterson, Foluso Ademuyiwa, Julie Margenthaler, Rebecca Aft, Katherine Glover-Collins, Leslie Nehring, Yu Tao, Souzan Sanati, Ian S. Hagemann, Fouad Boulos, Matthew Holt, Li Ding, Wenge Zhu, Stephen T. Oh, Jianxin Wang, Agnieszka K. Witkiewicz, Erik S. Knudsen, Ron Bose, Jason D. Weber, Matthew Goetz, Donald Northfelt, Jingqin Luo, Cynthia X. Ma
Abstract CDK4/6 inhibitors (CDK4/6i) improve outcomes for estrogen receptor (ER) positive/HER2-negative breast cancers (BCs), yet intrinsic and acquired resistance exist. Here, we evaluated anastrozole in combination with palbociclib (ANA/PAL) in the NeoPalAna Endocrine-Resistant cohort (NCT01723774). Thirty-four patients with clinical stage II/III ER + /HER2– BCs resistant to standard neoadjuvant endocrine therapy (on-treatment Ki67 > 10%) received neoadjuvant ANA/PAL, with serial biopsies analyzed. The primary endpoint, complete cell cycle arrest (CCCA; Ki67 C1D15 ≤ 2.7% at cycle 1, day 15), was achieved in 57.6% of patients (95%CI: 39.2–74.5%). Resistance to ANA/PAL (Ki67 C1D15 > 10%) was associated with higher pre-treatment tumor grade, Ki67, and specific PAM50 subtypes. Resistant tumors demonstrated reduced ER signaling and upregulation of cell cycle, mTOR, interferon, JAK/STAT, and immune checkpoints. Additionally, a 33-gene signature that predicted neoadjuvant Ki67 response to ANA/PAL was prognostic in a metastatic validation cohort. These findings underscore dysregulated oncogenic pathways as potential resistance mechanisms and biomarkers of response to CDK4/6i.