Real-world palbociclib dosing patterns and potential wastage associated with dose modifications among post-menopausal women with HR-positive/HER2-negative metastatic breast cancer (mBC)

17 Background: CDK4/6 inhibitor palbociclib is recently approved for the treatment of HR+/HER2- mBC and its treatment patterns have not been thoroughly understood. This study described real-world palbociclib dosing patterns in the US. Methods: Postmenopausal women with HR+/HER2- mBC who initiated palbociclib-based therapy (date of initiation was defined as index date) between 02/03/2015 (palbociclib approval date) and 02/29/2016 (end of data) were identified from Symphony Source Lx database. Patients were required to have continuous quarterly medical/pharmacy activity for 1 year prior to and 6 months after the index date. Dose decrease or increase (collectively, dose modification) was defined as a change of daily dose of >=25 mg compared to the last dose. Dose interruption was defined as a treatment gap within 60 days before restarting the treatment as the last dose. Kaplan-Meier analyses were conducted to assess dose decrease rate and dose decrease/interruption rate within one year from the index date. The proportion of patients with and days of overlap in fills during dose modification were assessed. The costs of potential palbociclib wastage related to dose modification were estimated by applying the wholesale acquisition costs (as of Oct 2016) to the number of overlapping days of supply between two adjacent fills. Results: A total of 1,242 patients initiated palbociclib-based therapy in line 1 through 4 for mBC (mean age=62.7 years, median follow-up time=8.7 months). Across line 1 to 4, dose decrease rates were 31.9%-33.7%, and dose decrease/interruption rates were 63.5%-80.9%. A total of 411 (33.1%) patients had dose modification, among whom 203 patients (49.4%) experienced an overlap in prescription fills during dose modification with an average overlap of 12.3 (SD=10.6) days per patient, resulting in costs of potential wastage for $4,556 (SD=$3,920) per patient. Conclusions: This real-world study showed most patients initiating palbociclib-based therapy had dose modification or interruption over a median follow-up of 8.7 months. Half of the patients experiencing dose modifications were associated with potential wastage.