Loibl, S.

Abstract Background Central nervous system (CNS) metastases are a major clinical concern in patients with HER2-positive metastatic breast cancer (MBC). The PATINA phase III study showed improved PFS with palbociclib added to maintenance anti-HER2/endocrine therapy (44.3 vs. 29.1 months; hazard ratio 0.75; 95% CI 0.59-0.96; 1-sided P=0.0109). Preclinical and early clinical data suggest palbociclib may have CNS activity. We conducted a pre-specified secondary analysis to evaluate the incidence and timing of CNS progression. Methods CNS progression was defined as the emergence of new intracranial lesions per investigator assessment. Baseline CNS imaging was not mandated but recommended if symptomatic. Competing risks analyses evaluated time from registration to first CNS progression or death (CNS PFS), treating non-CNS progression as a competing event. All randomized patients were included in the primary analysis, regardless of known baseline CNS involvement. Patients with baseline CNS metastases were not censored if progression involved the CNS. A secondary analysis excluded patients with CNS metastases present at baseline. Cumulative incidence functions were estimated and compared using 2-sided Gray’s test. Results Data cutoff for this analysis was on October 15, 2024. 518 participants were enrolled between June 2017 and July 2021, 261 to the palbociclib arm and 257 to the control arm. CNS metastases at baseline were present in 11 patients in the palbociclib arm and 9 patients in the control arm. Among all randomized patients, CNS progression occurred in 35 of 261 (13.4%; 95% CI: 9.5-18.2) in the palbociclib arm and 50 of 257 (19.5%; 95% CI: 14.8-24.8) in the control arm. When excluding patients with CNS metastases at baseline, CNS progression was observed in 32 of 250 (12.8%; 95% CI: 8.9-17.6) in the palbociclib arm and 47 of 248 (19.0%; 95% CI: 14.3-24.4) in the control arm. The median follow-up for patients who were alive and progression-free was 53.5 months. At 36 months, the cumulative incidence of CNS PFS was 13.4% vs. 19.9% for palbociclib versus control (Gray’s test p=0.0386). Importantly, when the analysis was restricted to pts without evidence of CNS disease at baseline (Table 1), —the CNS benefit with palbociclib was maintained (13.0% vs. 19.2% at 36 months, Gray’s test p = 0.0378). Conclusion The addition of palbociclib to anti-HER2 and endocrine therapy was associated with a lower incidence of CNS metastases. Among patients without known baseline CNS involvement, the absolute reduction in CNS progression at 36 months was 6.2% in favor of palbociclib. This benefit was observed at 12 months and sustained over 3 years (Table 1). These findings suggest that palbociclib may help delay or prevent CNS involvement in HR+/HER2+ MBC, warranting confirmation in future studies. Citation Format: O. Metzger, S. Mandrekar, T. Dockter, L. Gianni, E. Ciruelos, A. De Michele, J. Gligorov, E. Lim, S. Loibl, X. Gonzàlez Farré, F. Lynce, J. Lanzillotti, L. Carey, S. Goel, A. Partridge, E. Winer. Central Nervous System Outcomes from the Phase III PATINA Trial (AFT-38) [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 RF4-01.

Abstract Background: Invasive lobular carcinoma (ILC) accounts for up to 15% of breast cancer (BC) cases and has distinct clinical and biological features compared to invasive carcinoma of no special type (IBC-NST). Prior studies, including PENELOPE-B, suggested greater benefit from adjuvant palbociclib (palbo) in ILC. We assessed the prognostic and predictive value of histology in patients (pts) treated with endocrine therapy (ET) ± palbo. Methods: PALLAS is a global, phase III trial in which pts with stage II-III, HR+/HER2− BC were randomized to 2 years (y) of adjuvant palbo + ET or ET alone. Pts with IBC-NST or pure ILC per local pathology assessment were included. Endpoints included invasive disease-free (IDFS; primary), distant recurrence-free (DRFS), locoregional recurrence-free (LRRFS), and overall survival (OS). Prognostic value was assessed using univariable and multivariate Cox models adjusted for age, T and N stage, grade, and progesterone receptor. Predictive value was assessed using histology-by-treatment interactions. Results: Among 5,748 pts in the ITT population, 4,982 (87%) were included (3,838 IBC-NST [67%], 1,144 ILC [20%]); median follow-up 6.9y. Compared with IBC-NST, ILC was associated with older age (median 54 vs 51 y), higher T stage (T3/T4: 41.7% vs 20.4%), fewer grade 3 tumors (12.7% vs 33.5%), higher mastectomy rates (74.5% vs 58.4%), less (neo)adjuvant chemotherapy (72.2% vs 86.2%), and less tamoxifen as initial ET (28.8% vs 34.5%) (all p<0.001). ILC prevalence varied by country: lower in Asia (Japan 10.5%, Korea 0%, Taiwan 2.8%) and higher in Europe (23.1%) and North America (24.0%). When evaluating the predictive value, palbo appeared to confer greater benefit in ILC for all endpoints (HRs ranging from 0.41 to 0.86) and not in IBC-NST (HRs ranging from 0.97 to 1.01), although the histology-by-treatment interaction was significant only for LRRFS (Table). In the overall population, 7-year IDFS was 79.3% for IBC-NST and 75.8% for ILC. While the difference was significant in univariable analysis (HR 1.17, 95% CI 1.01-1.35), it was not in multivariable analysis (HR 1.06, 0.91-1.24). Notably, the proportional hazards assumption was violated, and time-dependent modeling demonstrated that worse outcomes for ILC on IDFS, DRFS, and other endpoints emerged predominantly beyond 3 years. Conclusions: In this large, randomized dataset of pts with early HR+/HER2− breast cancer with long-term follow-up, ILC demonstrated distinct clinicopathologic features and geographic distribution. While ILC was not an independent prognostic factor over the entire follow-up, time-dependent modeling revealed worse long-term outcomes which only emerged after 3 years. Palbo appeared to be associated with more favorable outcomes in ILC than in IBC-NST across endpoints. These findings suggest a potential benefit of palbo in ILC that warrants further investigation in other adjuvant trials evaluating CDK4/6 inhibitors. Citation Format: G. Nader-Marta, C. Desmedt, D. Hlauschek, A. DeMichele, G. Zoppoli, E. Blondeaux, E. de Azambuja, M. Bellet-Ezquerra, O. Metzger-Filho, J. M. Suga, G. Pfeiler, M. P. Goetz, M. Ruiz-Borrego, S. Loibl, J. Meisel, A. Ring, K. Van Baelen, E. P. Mamounas, N. Zdenkowski, E. Agostinetto, M. Lambertini, E. Gauthier, L. Soelkner, A. C. Dueck, M. Gnant, E. L. Mayer. Long-term prognostic and predictive value of lobular histology in the PALLAS trial [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 PS3-11-06.