Abstract P3-08-24: Racial disparities in neutrophil counts among patients with metastatic breast cancer during treatment with CDK4/6 inhibitors

Background: Addition of cyclin-dependent kinase (CDK) 4/6 inhibitors to aromatase inhibitors or fulvestrant prolongs progression-free survival (PFS) in hormone-receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) metastatic breast cancer (MBC). Grade 3/4 neutropenia occurs in up to 65% of patients treated with CDK 4/6 inhibitors, often leading to dose delay and/or reduction. Black (B) race has been associated with neutropenia. We aimed to compare changes in neutrophil counts among patients with HR+/HER2- MBC treated with CDK4/6 inhibitors by racial groups. Methods: Patients with HR+/HER2- MBC prescribed CDK4/6 inhibitors at Montefiore Medical Center/Albert Einstein Cancer Center were identified. Cases with unknown race were excluded. Data regarding clinical, pathological, and treatment characteristics were collected through manual chart review. Race was defined as Black (B) or Non-Black (NB). PFS was defined as the time from treatment initiation to progression of disease or death. Laboratory parameters evaluated included absolute neutrophil count (ANC) at baseline (ANC-0), at 14 days (C1D14) and during each month of treatment (ANC-2, ANC-3, etc.). Changes in ANC (Delta-ANC) from baseline were estimated at specific time-points. Results: A total of 130 patients with a mean age of 65 years were included, of whom 60 (46.2%) were B and 70 (53.8%) were NB. There were 38 (29.2%) Hispanics. CDK4/6 inhibitors were given in combination with first-line endocrine therapy in 59 patients (45.4%), and palbociclib was the most commonly used agent (117; 90%). There were no differences in the baseline characteristics among B vs. NB. The median ANC-0 was lower in B vs. NB (3.0 vs. 3.9, p=0.02); however, there were no differences in the median ANC at C1D14 (1.3 vs. 1.3, p=0.85), C2 (1.4 vs. 1.8, p=0.07), C3 (1.5 vs. 1.6, p=0.13), C4 (1.8 vs. 1.6, p=0.36) and all other time-points when comparing B vs. NB, respectively. Rate of CDK 4/6 inhibitor dose reductions for B vs. NB was similar (19% vs. 18%, p=0.47), as well as all other side-effects including grade 3/4 neutropenia (62.7% vs 45.6%, p=0.052), grade 3/4 infection (7% vs. 3.1%, p=0.32), any-grade anemia (55.9% vs. 58%, p=0.82), and grade 3/4 thrombocytopenia (5.2% vs. 5.9%, p=0.86). Delta-ANC was significantly lower for B compared to NB at C1D14 (-1.7 vs. -2.6, p=0.02), C2 (-1.5 vs. -2.3, p=0.05) and C3 (-1.8 vs. -2.6, p=0.006). There was no difference in PFS among B vs. NB receiving CDK4/6 inhibitors plus endocrine therapy in the overall cohort (10.4 vs. 11 months, p=0.68) or as first-line treatment (15 months vs. 12.4 months, p=0.28). Conclusions: Although blacks who received CDK4/6 inhibitors had lower baseline neutrophil counts than non-black patients, they experienced less declines in their neutrophil counts from baseline, had similar rates of grade 3/4 neutropenia and dose reductions, and had similar clinical outcomes. Citation Format: Ashley Weiner, Ana Acuna-Villaorduna, Joseph Sparano, Jesus Anampa. Racial disparities in neutrophil counts among patients with metastatic breast cancer during treatment with CDK4/6 inhibitors [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-08-24.