CDK4/6 inhibitors combined with endocrine therapy are now first-line of treatment in metastatic hormone positive Her2 negative breast cancer – unless visceral crisis is present. Palbociclib, ribociclib and abemaciclib all showed significant PFS benefit. Our research focused on real life survival data of these drugs.
A total of 97 metastatic HR+/Her2- breast cancer female patients were selected from our database who received CDK 4/6 inhibitors plus letrozole first-line treatment. Treatment was initiated between 2018-2021. Due to low number of patients receiving abemaciclib at that time, only palbociclib and ribociclib was included in this interim analysis. Median age of patients was 60 years. Patients were stratified according to CDK 4/6 partner (palbociclib or ribociclib), level of estrogen receptor expression (0-50 vs 50-80 vs 80-100%) and MIB1 labeling index (0-20% or above 20%). Location of metastases was also considered (bone-only or visceral metastases). Primary end point was PFS, with a total of 53 occurred events until first primary analysis.
PFS was 23.71 months, similar to those of PALOMA-2 and MONALEESA-2. No statistical difference was found between the mPFS of palbociclib and ribociclib subgroups. ER expression was found to be correlated with mPFS, patients with lower levels of ER expression had significantly shorter mPFS (16.54 vs 23.71 vs 53.54 months). mPFS was better in patients with bone-only disease (53.54 vs 23.71 months). However, in the subgroup of patients with high ER expression, mPFS was found to be very similar among patient with bone-only and visceral disease (53.54 vs 66.29 months). MIB1 labeling index seems to affect mPFS as well, as patients with a higher initial MIB1 index have better mPFS (21.68 vs 28.82. months). This difference seems to be even more profound in patients with visceral disease (20.0 vs 39.61 months).
Real-world data of patients receiving palbociclib or ribociclib in combination with letrozole show that these CDK4/6 inhibitors are more effective in patients with higher ER expression. MIB1 expression also seem to be correlated with mPFS, patients with higher MIB1 have better prognosis, although more research is needed.
University of Debrecen Faculty of Medicine Department on Oncology.
Has not received any funding.
All authors have declared no conflicts of interest.