Gnant, M.

Abstract PURPOSE: CDK4/6 inhibitors (CDK4/6i), used in combination with endocrine therapy (ET), are standard of care for advanced and higher risk early stage hormone receptor-positive (HR+), HER2 negative (HER2-) breast cancer. PALLAS investigated the addition of 2 years (y) of palbociclib to adjuvant ET (ET/P) compared to ET alone in patients (pts) with stage II-III HR+/HER2- breast cancer. We now present overall survival (OS) with 7-y outcomes and impact of post-recurrence therapy. PATIENTS AND METHODS: This global phase III trial randomized pts to ET for at least 5 y with or without 2 y of P. Primary endpoint was iDFS; distant recurrence-free survival (DRFS) and OS were preplanned endpoints. Kaplan Meier estimates and hazard ratios (HR) from Cox and Fine & Gray models were performed. Post-recurrence OS was analyzed as time from first distant recurrence (DR) until death. The post-recurrence OS model adjusted for both clin/path factors (including DR-free y, DR site) and time-dependent post-DR therapy (chemo, CDK4/6i). Results: 5,796 pts enrolled at 406 centers in 21 countries worldwide over 3 y. In the intent-to-treat (ITT) population, with a median follow-up of 82.7 months (mo), 7-y outcomes for iDFS, DRFS and OS are shown in the table. DR occurred in 781 pts (377 ET/P, 404 ET alone) with balanced pt/disease characteristics between arms. ET/P had more visceral (46% vs. 36%) and fewer non-visceral (61% vs. 68%) DR compared to ET alone (p=0.03). Unadjusted post-recurrence OS was significantly worse for ET/P vs. ET (median 22.6 vs. 27.9 mo, HR 1.21 (95% CI 1.01-1.46, p=0.04). However, receipt of any post-recurrence CDK4/6i was significantly higher on ET vs. ET/P arms (41% vs. 66%) and time to post-progression CDK4/6i initiation was significantly shorter for those in the ET vs. ET/P arms (HR 0.50, 95% CI 0.41-0.61, p<0.001) adjusting for region, bone/visceral DR type, DR-free time and age. Conversely, time to post-progression chemotherapy initiation was significantly longer for ET alone vs. ET/P arms (HR 1.31, 95% CI 1.07-1.61, p=0.01). However, after adjustment for clinical factors and post-DR treatment, the OS difference was no longer seen (HR 1.05, 95% CI 0.85-1.28, p=0.65). 3-mo landmark analysis showed similar effect on OS after adjusting for CDK4/6i initiation within 3 mo from DR (unadjusted HR 1.24, p=0.03 vs. adjusted HR 1.15, p=0.22). CONCLUSION: The PALLAS trial revealed significant differences in treatment post-DR that impact OS. Reduced/delayed use of metastatic CDK4/6i after adjuvant P was associated with significantly poorer OS, suggesting that patients who relapse after adjuvant CDK4/6i may still benefit from metastatic CDK4/6i. This has implications for optimal treatment in those who relapse after adjuvant CDK4/6i and warrants further study to determine if this is therapy-specific or an overarching biological effect. SUPPORT: AFT, ABCSG, NRG, PrECOG; Pfizer Citation Format: A. DeMichele, A. Dueck, M. Gnant, D. Hlauschek, M. Martin, A. Wolff, G. Rubovsky, F. Henao, O. Hahn, A. Chan, A. Brusky, P. Morris, H. Burstein, G. Huber, D. Anderson, L. Garcia-Estevez, G. Pfeiler, H. Rugo, N. Zdenkowski, S. Gampenrieder, N. Wolmark, D. Sabanathan, K. Miller, D. Cameron, E. Winer, C. Brunner, E. Gauthier, P. O’Brien, C. Fesl, E. Mayer. Adjuvant Palbociclib for ER+ Breast Cancer in the PALLAS Trial (ABCSG-42/AFT-05/PrE0109/BIG-14-13): Post-Recurrence Treatment and Overall Survival [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr RF7-07.

Abstract Background PALLAS trial compared palbociclib plus endocrine therapy (ET) vs. ET alone in stage II-III HR+/HER2- breast cancer. SETER/PR index quantifies transcriptional activity related to estrogen and progesterone receptors and serves as a measure of endocrine sensitivity. In CALGB 9741, low SETER/PR (< 0.75) - reflecting endocrine resistance - predicted survival benefit from adjuvant anthracycline/taxane-based treatment. In PALLAS low SETER/PR was associated with worse prognosis and early recurrence. Early recurrences driven by endocrine-resistant tumors may obscure the benefit of palbociclib by disproportionately contributing events from a more resistant population. We hypothesized that palbociclib efficacy might vary by endocrine sensitivity and prior chemotherapy regimen, with greatest benefit in patients with intermediate SETER/PR index. Methods Invasive disease-free survival (iDFS) was summarized with Kaplan-Meier plots and the treatment arms were compared with a log-rank test. Cox models estimated the hazard ratio (HR) with 95% confidence interval (95%CI) (significance level used was a two-sided 0.05). SETER/PR index was successfully obtained for 3,090 PALLAS patients (TransPALLAS ITT cohort, Metzger et al. ASCO 2024). SETER/PR index cut points (0.75 and 1.50) were proposed from previous studies comparing different adjuvant chemotherapy regimens. Results Table 1 summarizes the efficacy of adjuvant palbociclib by prior (neo)adjuvant chemotherapy regimen. Palbociclib was associated with longer iDFS in the group who received prior anthracycline-paclitaxel chemotherapy (AC-P), though the chemo regimen x palbociclib interaction term was not significant (Table 1). In the 1,132 patients from the TransPALLAS cohort who received AC-P chemotherapy, the 518 (46%) patients with intermediate SETER/PR (0.75≤ SETER/PR <1.50) had longer iDFS with palbociclib (HR 0.58; 95% CI: 0.40-0.84; p=0.004; 7-yr iDFS 80.2% vs. 68.4%), but this subset did not predict palbociclib benefit relative to all others, i.e., 206 (18%) patients with low SETER/PR (<0.75) and 408 (36%) high SETER/PR (≥1.50); P interaction=0.11. Exploratory analysis of SETER/PR index as a continuous variable in patients who received prior AC-P chemotherapy revealed a non-linear relationship with the hazard ratio favoring palbociclib observed in the subset with SETER/PR values between 1.1 and 1.8, compared to all others (P interaction=0.01). Conclusions In PALLAS, we observed a numerical benefit favoring palbociclib among pts treated with adjuvant AC-P; in this subset a moderate level of endocrine transcriptional activity (SETER/PR index between 1.1 and 1.8) predicted palbociclib benefit. These results are hypothesis-generating. Citation Format: O. Metzger, P. O'Brien, K. Ballman, M. Gnant, M. Watson, E. Chen, K. Tran, D. Hlauschek, M. Martin, J. Balko, Z. Nowecki, O. Hahn, C. Denkert, C. Curtis, F. Liu, A. Dueck, C. Fesl, E. Mayer, A. De Michele, W. Symmans. Exploratory analysis of palbociclib benefit in the PALLAS trial by SETERPR index and prior chemotherapy regimens (ABCSG-42/AFT-05/BIG-14-13) [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-08-13.