Abstract PD13-10: PD13-10 Impact of Proton Pump Inhibitors (PPI) on Palbociclib (PAL) Outcomes in Hormone Receptor-Positive, HER2-Negative Advanced Breast Cancer (HR+/HER2- ABC): Exploratory Analysis of the PARSIFAL Trial

Background The use of PPI among cancer patients (pts) is quite frequent. PAL is an oral, cyclin-dependent kinase 4 and 6 inhibitor recommended to be taken under fed conditions. PAL showed a reduced solubility when gastric pH is >4.5, a level commonly achieved by PPI. Observational, retrospective studies on concomitant PPI with PAL or ribociclib showed a shorter progression-free survival (PFS) among PPI users than nonusers. In the randomized, phase 2 PARSIFAL trial, PAL plus fulvestrant demonstrated no improvement in PFS and overall survival (OS) versus PAL plus letrozole as frontline treatment in HR+/HER2- ABC pts (Llombart-Cussac et al, JAMA Oncol 2021). Here we assessed the impact of PPI on PAL efficacy and safety in pts included in the PARSIFAL study. Methods Pts with endocrine-sensitive HR+/HER2- ABC and no prior therapy in advanced setting were randomly assigned to receive PAL (hard capsule formulation) plus either fulvestrant or letrozole. Pts with ≥1 PPI received over the entire PAL-based regimen were defined as PPI users, or PPI naïve (N-PPI) if no PPI was administered over the whole study treatment. We carried out two analyses considering early PPI users (E-PPI) –composed by pts who were receiving PPI since the PAL-based regimen initiation– and long-term PPI users (LT-PPI) –composed by pts who received PPI over the entire or ≥⅔ of the PAL-based regimen. PPI users defined as neither E-PPI nor LT-PPI were excluded from the analysis to avoid biases due to the PPI limited exposition. PFS, OS, and safety were compared among groups. Landmark analysis at 3, 6, 12, 18, 24, and 30 months (mo) was used for survival estimates conditional on surviving to certain time points and adjust for immortality bias in comparison between N-PPI and PPI users. Analyses were adjusted by stratification factors and patient characteristics. Results Of 486 pts included in the study, 325 (66.9%) were N-PPI. Among 161 (33.1%) PPI users, 64 (13.2%) were E-PPI and 91 (18.7%) were LT-PPI. Omeprazole was the most prescribed PPI in 80.7% (130 of 161) of PPI users. Median exposition to PPI for PPI users, E-PPI, and LT-PPI was 13.6, 15.9, and 19.4 mo, respectively. Compared with N-PPI, E-PPI and LT-PPI were older (median age, 60.5 vs 66.5 vs 67.0 years, respectively; P< 0.001) and had a worse functional status (ECOG PS of 0, 60.0% vs 34.0% vs 43.0%, respectively; P=0.002). Median follow-up for the whole population was 32 mo. Median PFS was 28.7 mo in N-PPI compared with 23.0 mo in E-PPI (HR 1.5; 95%CI 1.1–2.2; P=0.024) and 23.0 mo in LT-PPI (HR 1.4; 95%CI 1.0–1.9; P=0.035). Both PPI groups had poorer median PFS than N-PPI by landmark analysis at 3 and 12 mo. Subgroup analysis showed a consistent trend regardless of endocrine partner. Three-year OS rate was 81.1% in N-PPI compared with 63.5% in E-PPI (HR 2.2; 95%CI 1.3–3.7; P=0.003) and 62.0% in LT-PPI (HR 2.1; 95%CI 1.4–3.4; P=0.001). Both PPI groups had poorer 3-year OS rate than N-PPI by landmark analysis at 3, 6, 12, and 18 mo. Grade ≥3 hematological adverse events (AEs) occurred in 71.7% (233 of 325 pts) of N-PPI compared with 57.8% (37 of 64 pts; P=0.021) of E-PPI and 54.9% (50 of 91 pts; P=0.003) of LT-PPI. Dose reductions and delays due to hematological AEs were reported in 70.8% (230 of 325 pts) of N-PPI compared with 56.3% (36 of 64 pts; P=0.018) of E-PPI and 52.7% (48 of 91 pts; P=0.002) of LT-PPI. At 3 mo, 45.8% (149 of 325 pts) of N-PPI required a dose reduction or delay due to hematological AEs compared with 39.1% (25 of 64 pts; P=0.42) of E-PPI. Conclusions Early and sustained coadministration of PPI with PAL and endocrine therapy were associated with lower efficacy, hematological toxicities, and dose modifications. Despite the post-hoc nature of the study, these findings suggest pharmacokinetic interactions between PPI and PAL capsules. Further confirmatory studies including the tablet formulation of PAL, which is expected to assure its optimal absorption, are needed. Citation Format: Serena Di Cosimo, José Manuel Pérez-García, Meritxell Bellet Ezquerra, Florence Dalenc, Miguel Gil Gil, Manuel Ruiz Borrego, Joaquín Gavilá, Miguel Sampayo-Cordero, Elena Aguirre, Peter Schmid, Frederik Marmé, Joseph Gligorov, Andreas Schneeweiss, Joan Albanell, Pilar Zamora, Duncan Wheatley, Eduardo Martínez De Dueñas, Vicente Carañana, Kepa Amillano, Andrea Malfettone, Javier Cortés, Antonio Llombart-Cussac. PD13-10 Impact of Proton Pump Inhibitors (PPI) on Palbociclib (PAL) Outcomes in Hormone Receptor-Positive, HER2-Negative Advanced Breast Cancer (HR+/HER2- ABC): Exploratory Analysis of the PARSIFAL Trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD13-10.