Abstract P3-01-03: Characterization of neutropenia in patients with hormone receptor–positive/human epidermal growth factor receptor 2–negative (HR+/HER2–) advanced breast cancer on palbociclib in a real-world setting: Results from POLARIS
Authors: Tripathy Debu , Blum Joanne L. , Sleckman Bethany , Patel Kamal , Nakhoul Ibrahim , Gauthier Eric , Montelongo Monica Z. , Zhang Zhe , Wang Yao , Rocque Gabrielle B. 
Published: March 2023

Abstract
Background: The cyclin-dependent kinase 4/6 inhibitor palbociclib (PAL) is approved for the treatment of HR+/HER2– advanced breast cancer (ABC) in combination with an aromatase inhibitor (AI) as initial endocrine-based therapy or with fulvestrant (FUL) in patients whose disease progressed on prior endocrine therapy (ET). In clinical trials, the most common any-grade adverse event (AE) associated with PAL treatment was neutropenia. Here we evaluated the incidence of and risk factors for neutropenia by treatment cycle and line of therapy (LOT), as well as incidence of dose modification due to neutropenia, in patients with HR+/HER2– ABC receiving PAL in a real-world setting. Methods: POLARIS is a prospective, noninterventional, multicenter, real-world study of patients with HR+/HER2‒ ABC receiving PAL in routine clinical practice in the United States and Canada. Per protocol, sites were instructed to report absolute neutrophil count at baseline, anytime during the first 3 cycles, then at Day 1 (or before the start of a new cycle) of subsequent cycles per standard of care or as needed up to 6 months. The safety analysis set included all patients enrolled in the study who received at least one dose of study medication.Results: As of March 30, 2022, 1242 patients had enrolled in the study (median age, 64.0 years; 98.8% female; 94.8% stage IV; 39.1% visceral disease). Of these, 902 (72.6%) patients received PAL as first LOT and 340 (27.4%) as second LOT or greater. Among all patients, 722 received PAL + AI, 487 PAL + FUL, and 33 PAL + none or other ET (other than letrozole, anastrozole, fulvestrant, or exemestane); 90% of all patients started with 125 mg of PAL in the first cycle. Incidence of neutropenia among patients receiving PAL by cycle and LOT is shown in the Table. Percentages of patients experiencing on-treatment neutropenia were similar regardless of LOT. Over the course of treatment grade ≥3 neutropenia and febrile neutropenia were reported in 16.1% and 0.8% patients, respectively, and 19.2% of patients experienced a decreased neutrophil count. A total of 337 (27.1%) had a dose modification due to neutropenia. Of these 337 patients, 190 (56.4%) had a dose reduction associated with neutropenia. Conclusions: In this real-world study, incidences of treatment-related neutropenia associated with PAL were consistent with those reported in randomized clinical trials and were manageable by dose modification. Febrile neutropenia was uncommon. Incidences of neutropenia reported within the first 6 months were comparable among patients regardless of LOT. Further research is needed to understand the impact of the incidence of and risk factors for treatment-related neutropenia on therapy.Clinical trial identification: Pfizer; NCT03280303
Citation Format: Debu Tripathy, Joanne L. Blum, Bethany Sleckman, Kamal Patel, Ibrahim Nakhoul, Eric Gauthier, Monica Z. Montelongo, Zhe Zhang, Yao Wang, Gabrielle B. Rocque. Characterization of neutropenia in patients with hormone receptor–positive/human epidermal growth factor receptor 2–negative (HR+/HER2–) advanced breast cancer on palbociclib in a real-world setting: Results from POLARIS [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 P3-01-03.