Time to treatment failure (TTF) and overall survival (OS) with palbociclib (PAL) plus endocrine therapy (ET) versus ET in HR+/HER2-advanced breast cancer (ABC) patients: A real-world multicenter study from China

e13048 Background: Randomized controlled trials (RCTs) found that PAL plus ET had a considerable survival advantage in patients with HR+/HER2- ABC compared to ET. This real-world study aimed to complement the existing RCTs in addition to accounting for patient characteristics, adherence to treatment, and socioeconomic factors. Methods: Data of patients who had HR+HER2- ABC and received PAL plus ET (N=216) and ET (N=215) from September 2007 to May 2020 were retrospectively retrieved from the electronic medical record system of seven cancer centers across China. Stabilized inverse probability of treatment weighting (SIPTW) was applied to strictly balance the variables between the two cohorts. Results: A total of 400 patients were eligible for the study analysis. PAL plus ET was administered to 204 (51%) patients, and 196 (49%) patients received ET only. At the end of the observation, treatment failure occurred in 162 (79%) patients and 183 (93%) patients in both cohorts, due to disease progression (93.2% vs. 96.8%), financial inability (4.9% vs. 0%), intolerable adverse reactions (1.9% vs. 0.5%), and unknown reasons (0% vs. 2.7%). Most patients were postmenopausal (71.8%) and had visceral metastasis (65.2%). De-novo metastatic disease was seen in 13.0% of patients, and 45.2% of patients received first-line ET. More than half of the patients (57.8%) received aromatase inhibitors, and 38.5% received estrogen receptor degrader fulvestrant. With a median follow-up period of 39.5 and 40.5 months, the median TTF in PAL plus ET vs. ET was 11.8 vs. 9.47 months (HR=0.69, 95% CI: 0.56-0.86, p=0.00057), and OS analysis showed a median OS not reached (NR) vs. 40.7 months (HR=0.84, 95% CI: 0.62-1.13, p=0.25). Upon SIPTW, the standardized differences (-0.117 to 0.096) showed a good balance between the cohorts. Weighted median TTF was significantly longer in the PAL plus ET group than the ET group (11.8 vs. 7.07 months, HR=0.63, 95% CI: 0.49-0.80, p=0.000139). However, the results of weighted median OS were not statistically significant (NR vs. 38 months, HR=0.80, 95% CI: 0.53-1.21, p=0.29). In the balanced subgroup analysis, TTF showed a similar trend to that in the overall population, and in patients with de-novo metastatic disease and non-central nervous system (CNS) metastasis, OS was significantly longer in the PAL plus ET group than ET (HR=0.256, 95% CI: 0.112-0.586, p for interaction=0.013; HR=0.657, 95% CI: 0.443-0.973, p for interaction=0.013, respectively). Conclusions: Comparatively, PAL plus ET had significantly longer TTF than ET, supporting the use of PAL plus ET in HR+/HER2– ABC patients in real-world settings. In patients with de-novo metastatic disease and non-CNS metastasis, PAL plus ET showed significant OS benefit compared to ET, however, follow-up time needs to be increased in further studies.