236P Survival outcomes of patients (pts) with metastatic lobular breast cancer (mLBC) receiving cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) and endocrine therapy (ET): A retrospective analysis

First-line treatment of metastatic breast cancer (mBC) consists of the combination of CDK4/6i & ET, regardless of histoype. Most pts enrolled in clinical trials had ductal mBC, with limited data on CDK4/6i in mLBC. This study aims to analyze the survival outcomes of pts with mLBC treated with CDK4/6i, compared to pts with non-lobular mBC. In this retrospective study, we included pts with hormone receptor-positive/HER2-negative mBC treated with CDK4/6i (Jan 2016 to Jan 2024). We estimated the median progression-free survival (mPFS) and overall survival (mOS) by Kaplan-Meier and compared by log-rank test and Cox models (significance at p-value<0.05). 210 pts were identified, 42 had mLBC. Baseline characteristics are reported in the table. At a median follow-up of 53.6 months (95% Confidence Interval [CI], 45.1-62.1), mPFS was 16.7 months (95% CI, 12.9-20.4) and mOS 46.8 months (95% CI, 38.8-55.2), with no differences for pts with mLBC compared to those with non-lobular mBC (PFS, p=0.45; OS, p=0.83), even after adjusting for all baseline characteristics (Hazard Ratio [HR] PFS: 1.1 [95% CI, 0.7-16]; HR OS: 0.9 [95% CI, 0.5-1.5]).Table: 236PCharacteristicAll (N=210)Lobular (N=42)Non-lobular§ (N=168)PAge, median (IQR)57 (49-66)62 (55-67)56 (49-65).01Luminal A-like*, n (%)62 (30)16 (38)46 (27).17De novo M1, n (%)55 (26)9 (21)46 (27).43(Neo)adjuvant CT, n (%)78 (37)12 (28)66 (39).19Previous CT for M1, n (%)56 (26)5 (12)51 (30).02ET Sensitive | Resistant**, n (%)96 (45) | 114 (55)16 (28) | 26 (61)80 (48) | 88 (52).26Line of CDK4/6i, median (IQR)1 (1-2)1 (1-2)1 (1-2).06Palbociclib | Abemaciclib| Ribociclib, n (%)150 (72) | 18 (8) | 42 (20)33 (78) | 2 (5) | 7 (17)117 (70) | 16 (10) | 35 (21).45ORR, % (95% CI)31 (25-38)38 (21-52)30 (23-37).45mPFS, month16.717.116.1.45mOS, month46.845.547.6.83§ Including ductal and other histotype *PgR³20%, Ki67<20% and grade 1-2. **ESMO-ABC criteria.Legend: CT, chemotherapy; IQR, interquartile range; M1, metastatic; n, number; ORR, objective response rate. Open table in a new tab § Including ductal and other histotype *PgR³20%, Ki67<20% and grade 1-2. **ESMO-ABC criteria. Legend: CT, chemotherapy; IQR, interquartile range; M1, metastatic; n, number; ORR, objective response rate. In this study, we report that pts with mLBC derive no dissimilar benefits from CDK4/6i compared to those with non-lobular mBC.