Abstract P5-14-08: Predictors of relative dose intensity and early dose reduction in patients with metastatic breast cancer treated with palbociclib and endocrine therapy

Background: The combination of endocrine therapy with the cyclin-dependent kinases 4/6 inhibitor palbociclib was proven to be effective for the treatment of hormone receptor (HR)-positive/HER2-negative metastatic breast cancer (MBC). Although generally well tolerated, treatment delays and dose reductions are frequently observed with palbociclib, mainly due to severe neutropenia. Predictors of palbociclib-related toxicities are still unknown, as well as the clinical relevance of its relative dose intensity (RDI). Henceforth, this study aimed to identify baseline clinicopathological features associated with a RDI Methods: We reviewed data of 150 consecutive patients with HR-positive/HER2-negative MBC patients treated with palbociclib at two Italian cancer centers from 2017 to 2019. Eligible patients must have received at least 3 cycles of treatment. Those who experienced early suspension due to unacceptable toxicities were still eligible. RDI was calculated as the ratio of actual dose intensity (cumulative administered dose/treatment duration) and planned dose intensity (cumulative planned dose/planned treatment duration). The association of both RDI Results: Overall, 142 patients were deemed eligible. Of these patients, 98 (69.0%) were treated with palbociclib plus fulvestrant, 44 (31.0%) with palbociclib plus aromatase inhibitors, and 73 (51.4%) in the first-line setting. The median number of administered palbociclib cycles was 8 (range: 1-24) and 61 patients (43.0%) required at least a first-level dose reduction (29 within 3 months). Furthermore, the median time to first dose reduction was 3.22 months, with neutropenia being responsible for 85.24% of first-level dose reductions. In the whole cohort, median RDI was 90.5% (95.1% for patients without dose reduction and 80% for those who had received a dose reduction). Notably, 28 patients (19.7%) experienced a RDI Conclusions: In our analysis, baseline weight and WBC were statistically associated with a RDI Citation Format: Giacomo Pelizzari, Elisa Bertoli, Marco Giavarra, Carla Corvaja, Lorenzo Gerratana, Debora Basile, Michele Bartoletti, Camilla Lisanti, Lucia Bortot, Silvia Buriolla, Mattia Garutti, Debora Avoledo, Marta Bonotto, Lucia Da Ros, Silvia Bolzonello, Mauro Mansutti, Paola Di Nardo, Gianpiero Fasola, Simon Spazzapan, Alessandro M Minisini, Fabio Puglisi. Predictors of relative dose intensity and early dose reduction in patients with metastatic breast cancer treated with palbociclib and endocrine 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 P5-14-08.