Real-Life Efficacy of Palbociclib and Ribociclib in Advanced Breast Cancer

<b>Background:</b> Clinical trials in metastatic hormone receptor-positive (HR+) human epidermal growth factor receptor-2 (HER2)-negative patients have shown that cyclin-dependent kinase 4/6 (CDK 4/6) inhibitors both increase response rates and provide survival benefits. The efficacy of these therapies needs to be supported by real-life data. In this study, we aimed to evaluate treatment response, survival and affecting factors in patients with HR+/HER2- metastatic breast cancer (MBC) who were followed up with CDK 4/6 inhibitors in our center. <b>Materials and methods:</b> A retrospective analysis of 120 patients with HR+/HER2- MBC treated with ribociclib or palbociclib in combination with letrozole or fulvestrant was performed. <b>Results:</b> Median progression-free survival (mPFS) was 24 months in the general population, 27 months in the ribociclib arm and 20 months in the palbociclib arm, with no significant difference in progression-free survival (PFS) in both arms (<i>p</i> = 0.25). The mPFS was longer in the ribociclib + letrozole arm compared to palbociclib + letrozole (27 vs. 20 months, respectively). PFS was also longer in patients receiving ribociclib + fulvestrant compared to palbociclib + fulvestrant but not statistically significant (33 vs. 21 months, respectively). Median overall survival (mOS) was not reached, but 3-year overall survival (OS) was statistically significantly longer in the ribociclib arm (87% vs. 55.5%, respectively, <i>p</i> = 0.03). <b>Conclusion</b>: Palbociclib and ribociclib are first-line treatment options for metastatic HR+/HER2- disease and have similar efficacy. In our study, while the mPFS was not statistically significant in both arms, the 3-year OS rate was higher in the ribociclib arm and statistically significant. Our findings were confirmed in randomized studies comparing both agents head-to-head.