Abstract P3-01-05: Laboratory monitoring in patients with hormone receptor–positive/human epidermal growth factor receptor 2–negative (HR+/HER2–) advanced breast cancer treated with palbociclib in a real-world setting: Results from POLARIS

Abstract Background: Palbociclib (PAL) is a cyclin-dependent kinase 4/6 inhibitor indicated 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). Neutropenia was the most commonly reported adverse event in clinical trials. Per the prescribing information for PAL, complete blood counts (CBCs) should be monitored before initiation of PAL (Cycle 1, Day 1 [C1D1]), at the beginning of each subsequent cycle, on Day 15 of the first 2 cycles, and as clinically indicated for at least the first 6 cycles. Methods: POLARIS is a prospective, observational, multicenter, real-world study of patients with HR+/HER2‒ ABC who received PAL as deemed appropriate by the treating physician. CBC collected at baseline, anytime during the first 3 cycles, then on Day 1 (or before the start of a new cycle) of subsequent cycles as per standard of care were recorded. We analyzed the frequency of laboratory monitoring for patients with HR+/HER2– ABC receiving PAL in routine clinical practice. Lab collections outside these windows were also obtained (data not shown). Results: At data cutoff (March 30, 2022), 1242 patients were treated with PAL + ET; 902 patients as first-line (1L) and 340 as second-line or later (≥2L) therapies for ABC; 90% of all patients received 125 mg of PAL in the first cycle. The frequency of laboratory collection, overall and by site category (academic vs. community), within the first 6 months of treatment is reported in the Table. CBC collection reported by investigators and analyzed within the defined window of day 1 of each cycle (C1–C6). Day 1 CBC collection was 87% at C1D1, 69.4% at C2D1, 66.5% at C3D1, 63.2% at C4D1, 61.4% at C5D1, to 59.9% at C6D1. Reports of CBC collection on Day 15 for the first two cycles was 37.3% (C1D15) and 31.2% (C2D15). CBC collection pattern was comparable across the community and academic sites. Conclusions: In this real-world CBC monitoring data set analyzed in a defined cycle window, most sites reported CBC collection prior to PAL initiation, and over two thirds of sites in the subsequent cycles. Day 15 of Cycles 1 and 2 monitoring was conducted in one third of the patients. These results suggest that CBC collections in real-world clinical practice are performed less frequently than in clinical trials, which may reflect the sites’ desire to reduce patient and/or clinic procedural burden, and treating physician’s familiarity with PAL safety profile and their comfort level of managing patients in clinical practice. Clinical trial identification: Pfizer; NCT03280303 Table 1 Citation Format: Gabrielle B. Rocque, Joanne L. Blum, Jennifer M. Specht, Carrie Dul, Steven Corso, Daniel Cuevas, Eric Gauthier, Monica Z. Montelongo, Zhe Zhang, Yao Wang, Debu Tripathy. Laboratory monitoring in patients with hormone receptor–positive/human epidermal growth factor receptor 2–negative (HR+/HER2–) advanced breast cancer treated with 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 P3-01-05.