Real-world dosing and CBC monitoring in patients with metastatic breast cancer during palbociclib plus letrozole therapy

Background: Palbociclib in combination with letrozole (P+L) was the first cyclin-dependent-kinase 4/6 inhibitor approved in the U.S for treatment of metastatic breast cancer (mBC). Per the U.S. label, the recommended starting dose for Palbociclib (P) is 125 mg and patients should have complete blood count (CBC) monitoring prior to the start of treatment, at the beginning of each cycle, on day 15 of the first 2 cycles, and as clinically indicated. We sought to evaluate adherence to these FDA label in patients receiving P+L as initial endocrine-based therapy. Methods: Adult post-menopausal women with metastatic HR+/HER2- breast cancer who initiated P+L on or after 02/03/2015 were randomly identified by providers in the Cardinal Health Oncology Provider Extended Network (OPEN), which includes over 7,000 US oncologists/hematologists. Providers who were part of OPEN who responded to an initial feasibility request (survey) indicating an interest to participate in the study and treating the patients of interest were invited to participate. Providers were asked to randomly select eligible patients treated with P+L who were initiated on first-line P+L at least 3 months following their first treatment of any patient with P+L. Providers were asked to indicate that the patient had been randomly selected from among all eligible patients and were able to enter up to 10 total patients. All data were abstracted by the patient9s treating provider in an electronic case report form (eCRF). Providers abstracted data related to patient characteristics, dosing, and frequency of CBC monitoring in the 30 days prior to and during the first 2 cycles of therapy of randomly selected patients from the time of initiation of P+L through Feb 2019 (or end of follow up/death). Providers completed data validation by re-entering select data from randomly selected patients (10% of the total sample) and for patients which were flagged for quality control review by Cardinal Health clinical research staff and data analytics team members when the results were inconsistent or deviated from the population averages. Results: Thirty-one providers submitted 202 eCRFs, of which 193 were eligible (9 patients removed with non-verifiable data). Demographics: mean age was 65.0 y/o (SD = 10.5), 74.6% white, 38.6% commercially insured. Clinical characteristics: 65.8% de novo metastatic, 51.3% visceral disease at initiation of P+L, 25.4% bone only disease, 10.4% ECOG-PS ≥2. Median follow-up from P+L initiation was 15.4 months, 45.6% of patients had discontinued P+L at data cut-off. Overall, 86.0%, 13.5%, and 0.5% of patients initiated treatment of P at the 125 mg, 100 mg, and 75mg dose, respectively. Dose reductions were reported in 17.1% of patients. CBC testing was conducted prior to P+L initiation in 99.0% of patients; median number of CBC tests during cycle 1 was 2.0; 37% of patients had only one CBC test. In cycle 2, median number of CBC tests was 1.5 and 46.5% of patients had only one CBC test. Conclusions: Physicians were generally compliant with the Palbociclib package insert recommendations for dosing and monitoring during the first cycle, less so for monitoring during the second cycle in women with HR+/HER2- mBC. Funding: Pfizer Inc. Citation Format: Jonathan Kish, Talia Miller, Damion Nero, Djibril Liassou, Xianchen Liu, Lynn McRoy, Bruce Feinberg, Lin Zhan, Jeffrey Trocio. Real-world dosing and CBC monitoring in patients with metastatic breast cancer during palbociclib plus letrozole therapy [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 P5-14-13.