Abemaciclib plus endocrine therapy versus chemotherapy after progression on prior palbociclib in HR+/HER2- metastatic breast cancer: A single center real-world study in China.

Background: Cyclin-dependent kinase (CDK) 4/6 inhibitor plus endocrine therapy (ET) become standard-of-care for patients with hormone receptor-positive, human epidermal growth factor receptor-2 negative (HR+/HER2-) metastatic breast cancer (MBC). However, the optimal therapeutic paradigm after progression on CDK4/6 inhibitor remains unclear. This study aimed to evaluate the efficacy and safety of abemaciclib with switching ET versus chemotherapy after progression on prior palbociclib-based ET in Chinese patients with HR+/HER2- MBC.

Methods: From 414 consecutive patients with HR+/HER2- MBC who had been treated with palbociclib plus ET from September 2018 to May 2022 in Peking University Cancer Hospital, we identified 80 patients who received abemaciclib plus switching ET or chemotherapy after progression on palbociclib, matched for age, original stage at diagnosis, disease-free interval, and tumor burden at 1:1 ratio. The primary endpoint was progression-free survival (PFS) compared using the Kaplan-Meier method. A Cox proportional hazard model was performed to identify clinical factors associated with PFS in the abemaciclib group.

Results: The median PFS was 6.0 months (95% confidence interval [CI]: 3.94-8.06) in abemaciclib group and 4.0 months (95% CI, 2.52-5.49) in chemotherapy group (p = 0.667). And, there was no difference in median PFS between the sequential and nonsequential arm (6.0 vs. 6.0 months) in the abemaciclib group though fewer lines of prior systemic therapy and longer PFS from prior palbociclib in the sequential arm. However, patients with prior palbociclib as the first-line therapy had a significantly longer median PFS versus prior palbociclib as ≥2nd-line therapy (11.0 vs. 5.0 months, p = 0.043). Based on multivariable analysis, ER+/PR+ was an independent factor associated with longer PFS. There was no significant difference in overall survival between the abemaciclib and chemotherapy groups (p = 0.069).

Conclusion: Our findings indicate that abemaciclib plus switching ET might be one of feasible treatment options for Chinese patients with HR+/HER2- MBC after progression on prior palbociclib-based therapy in addition to chemotherapy.