Abstract PO4-02-04: Overall survival and disease recurrence rates in patients with invasive lobular breast cancer of the PenelopeB cohort

Background: In the PENELOPEB trial, the addition of 1-year of cyclin-dependent kinase 4/6 (CDK4/6) inhibitor palbociclib to standard endocrine therapy (ET) for women with hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) early breast cancer at high risk of recurrence after neoadjuvant chemotherapy (NACT) did not improve invasive disease-free survival (iDFS) or overall survival (OS) in the main study analysis. Invasive lobular breast cancer (ILC) accounts for ~15% of all breast cancers (BC) and represents an underinvestigated subtype of BC with typically less sensitivity to (neo)adjuvant chemotherapy, and poor distant disease-free survival (DDFS) irrespective of response to NACT. Here, we report the post-hoc analysis results of iDFS, DDFS and OS in pre- and postmenopausal women enrolled on PENELOPEB and with ILC. Methods: Patients with HR+/HER2-negative BC without pathological complete response (pCR) after taxane-containing NACT and at high risk of relapse (CPS-EG score ≥3, or 2 and ypN+) were randomized (1:1) to receive 13 cycles of palbociclib 125mg daily or placebo on days 1-21 of a 28-day cycle in addition to standard ET with tamoxifen (TAM) +/- gonadotropin-releasing hormone analogue (GnRHa) or aromatase inhibitor (AI) +/- GnRH. Randomization was stratified by nodal status at surgery, age at first diagnosis (< 50 vs. ≥50 years), Ki-67, region, and CPS-EG score. ILC diagnosis was locally assessed and reported by the investigators on the pathology case report form. The primary objective of this post-hoc analysis was to evaluate iDFS, DDFS and OS by treatment arm in patients with high risk ILC. Results: A total of 1,250 patients were randomized, of whom 110 had ILC and were nearly uniformly distributed between both treatment arms (palbociclib n=58 vs. placebo n=52), with a higher proportion of postmenopausal women (58.6% in the palbociclib arm vs. 53.8% in the placebo arm) compared to premenopausal women (41.4% in the palbociclib arm vs. 46.2% in placebo arm). There was no difference in the distribution of AI/TAM use between the treatment arms. An estimated absolute 3-year-iDFS difference of 18.3% with palbociclib compared to placebo (iDFS 88.4% [95% CI 76.0-94.6] vs. 70.1% [95% CI 55.3-80.7]) with a HR of 0.66 [95% CI 0.27 – 1.61, log-rank p=0.354]) was observed. A comparable 3-year-DDFS difference of 16.3% was observed. An estimated 3-year-OS difference of 16.4% (98.0% [95% CI 86.6 – 99.7] vs. 81.6% [95% CI 67.5 – 90.0]) with a hazard ratio (HR) of 0.27 (95% CI 0.05 – 1.43, log-rank p=0.108) was observed. Out of 12 observed deaths (n=2 palbociclib vs. n=10 placebo), 11 (in placebo arm) were related to metastasic BC. Conclusions: In this post-hoc analysis, a trend towards improvement in OS and a trend in favor of iDFS and DDFS for the addition of palbociclib to ET was observed among women with HR+/HER2- ILC at high risk of recurrence after NACT, but these differences were not statistically significant. This could represent a valuable treatment option for patients with high risk ILC. Due to the small sample size of the ILC subgroup, further follow-up evaluation is necessary. Moreover, analyses in the ILC subgroup from other adjuvant CDK4/6 inhibitor trials could substantiate these findings. Table. Overall survival and disease recurrence rates in patients with invasive lobular breast cancer of the PenelopeB cohort Results of iDFS, DDFS and OS. Citation Format: Hervé Bonnefoi, Frederik Marmé, Miguel Martín, Michael Untch, Sung-Bae Kim, Harry Bear, Nicole McCarthy, Karen Gelmon, José Ángel García-Sáenz, Catherine M. Kelly, Toralf Reimer, Zhe Zhang, Masakazu Toi, Hope Rugo, Michael Gnant, Andreas Makris, Nader Hirmas, Valentina Nekljudova, Johannes Holtschmidt, Sibylle Loibl. Overall survival and disease recurrence rates in patients with invasive lobular breast cancer of the PenelopeB cohort [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 PO4-02-04.