Efficacy and safety of fulvestrant combined with abemaciclib in patients with hormone receptor-positive advanced breast cancer

This study evaluated the efficacy and safety of fulvestrant combined with abemaciclib in patients with hormone receptor-positive advanced breast cancer and compared the outcomes with those reported in the MONARCH 2 trial. Forty-six patients with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer received fulvestrant 500 mg administered intramuscularly (days 1, 15, and 28, then every 28 days) combined with abemaciclib 150 mg taken orally twice daily until disease progression or unacceptable toxicity. The primary endpoint was progression-free survival (PFS). Secondary endpoints included objective response rate (ORR), disease control rate (DCR), and safety. All 46 patients were evaluable for efficacy. No complete response was observed; 21 patients (45.7%) achieved partial response, 8 (17.4%) had stable disease, and 17 (37.0%) experienced progressive disease (PD). The ORR was 45.7%, and the DCR was 63.0%. Median PFS was 19.1 months (95% confidence interval: 16.0-NR). Compared with the MONARCH 2 trial, ORR was slightly higher (45.7% vs. 35.2%), whereas the DCR was lower (63.0% vs. 83.0%), accompanied by a markedly higher PD rate (37.0% vs. 9.0%, P < 0.001). The most frequently reported adverse events were leukopenia (30.4%), nausea/vomiting (47.8%), and fatigue (32.6%), consistent with findings from MONARCH 2, with no new safety signals identified. Fulvestrant combined with abemaciclib demonstrated antineoplastic activity and a manageable safety profile in patients with hormone receptor-positive advanced breast cancer. Relative to the MONARCH 2 trial, this real-world cohort exhibited a higher ORR but a lower DCR, potentially reflecting differences in prior treatments and baseline characteristics. Additional large-scale, multicenter studies are warranted to validate these findings.