Abstract PO1-01-08: The impact of drug-drug interactions between palbociclib and proton pump inhibitors on clinical outcomes of patients with hormone receptor positive, HER2-negative, early breast cancer: an exploratory analysis of the PALLAS study

Abstract Background: Data suggest that concomitant intake of proton pump inhibitors (PPI) may create drug-drug interactions, potentially impacting efficacy of agents including palbociclib, which is widely used in the treatment of metastatic hormone-receptor-positive/HER2-negative (HR+/HER2-) breast cancer. In the open label, randomized, phase 3 PALLAS trial, the addition of palbociclib for two years to standard adjuvant endocrine therapy (ET) in patients with HR+/HER2- early breast cancer (EBC) did not improve invasive disease-free survival (IDFS) compared to adjuvant ET alone. We explored whether concomitant use of PPIs could have affected clinical outcomes in patients treated with palbociclib in PALLAS. Methods: This is an exploratory, unplanned analysis of the PALLAS trial (AFT-05/ABCSG-42/PrE0109/BIG-14-13, NCT02513394), which enrolled 5796 patients with stage II-III HR+/HER2- EBC. Patients were randomized to receive either 2 years of palbociclib (125 mg oral capsules once daily on days 1-21 of a 28-day cycle) with adjuvant ET or ET alone. Primary endpoint was IDFS in the intention-to-treat population. The present exploratory analysis included patients who received at least one dose of palbociclib. Definitions of IDFS, distant relapse-free survival (DRFS) and overall survival (OS) were according to the PALLAS protocol (STEEP criteria), using first day of palbociclib intake as start date. Only PPI concomitant intake during palbociclib treatment was considered. To determine the association of concomitant PPI use with IDFS, DRFS, and OS, uni- and multivariable Cox models with time-dependent PPI (the latter adjusted for age, prior chemotherapy, progesterone receptor, grade, tumor stage, nodal stage, body mass index (BMI)) were used. Also, the association between PPI use and neutropenia was investigated in patients initiating PPI prior to randomization vs never. Odds ratios (OR) unadjusted and adjusted for age and BMI estimated with logistic regression models are reported. Results: Of 2840 patients treated with palbociclib + ET, 533 (18.8%) had concomitant PPI and palbociclib intake. Median duration of PPI intake was 10.8 months (interquartile range 2.3-22.3). Most patients received omeprazole as PPI (n=265, 49.7%). PPI intake was significantly associated with older age, post-menopausal status, use of aromatase inhibitors as ET, higher BMI, and worse ECOG status (all p 0.001). Concomitant PPI intake was not significantly associated with survival outcomes (Table 1). Unadjusted all grade neutropenia rates were slightly lower in patients who initiated PPI prior study start vs patients never initiating (79% vs 85%, unadjusted OR 0.66, 95%CI 0.50-0.88). After adjustment for BMI and age, the difference between the groups decreased (adjusted OR = 0.77, 95%CI 0.57-1.04). Similar effect sizes were found for grade 3/4 neutropenia rates (54% vs 64%, unadjusted OR 0.66, 95%CI 0.52-0.83, adjusted OR 0.81, 95%CI 0.64-1.03). Conclusions: Our exploratory analysis did not demonstrate a statistically significant relationship between concomitant use of PPIs and palbociclib and survival outcomes of patients in the PALLAS trial. Nonetheless, careful consideration of concomitant medications and of drug-drug interactions is important when studying novel agents in the adjuvant breast cancer setting. Support: AFT, Pfizer; https://acknowledgments.alliancefound.org Table 1. Association between PPI intake and survival outcomes. Citation Format: Elisa Agostinetto, Georg Pfeiler, Dominik Hlauschek, Erica Mayer, Matteo Lambertini, Evandro de Azambuja, Meritxell Bellet- Ezquerra, Jane Meisel, Gabor Rubovszky, Nicholas Zdenkowski, Yelena Novik, Manuel Ruíz - Borrego, Karen Gelmon, Eleftherios Mamounas, Hiroji Iwata, Dongrui Lu, Lidija Sölkner, Christian Fesl, Michael Gnant, Angela DeMichele. The impact of drug-drug interactions between palbociclib and proton pump inhibitors on clinical outcomes of patients with hormone receptor positive, HER2-negative, early breast cancer: an exploratory analysis of the PALLAS study 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 PO1-01-08.