A Phase II Study of Fulvestrant plus Abemaciclib in Hormone Receptor-Positive Advanced or Recurrent Endometrial Cancer

Purpose: Inhibition of the cyclin D-cyclin dependent kinase (CDK)4/6-INK4-retinoblastoma pathway can overcome acquired or de novo treatment resistance to endocrine monotherapy. Responses to endocrine monotherapy in advanced endometrial cancer (EC) are suboptimal, perhaps due to genomic alterations that promote estrogen receptor (ER)-independent cyclin D1-CDK4/6 activation. We hypothesized that addition of abemaciclib, a CDK4/6 kinase inhibitor, to antiestrogen therapy with fulvestrant will be an effective therapeutic strategy in patients with advanced or recurrent EC. Methods: In this phase II study, patients with advanced or recurrent EC received 150 mg of abemaciclib orally twice daily with 500 mg of fulvestrant intramuscularly monthly with a 2-week loading dose. Eligibility included ER or progesterone receptor expression ³1% by immunohistochemistry, measurable disease, £2 prior lines of chemotherapy, and £1 prior line of hormonal therapy. The primary endpoint was objective response rate (ORR) by RECIST v1.1. Results: Twenty-seven patients initiated therapy and 25 were evaluable for efficacy. Eleven patients achieved partial response; 10 responses (91%) were in copy number-low/no specific molecular profile tumors, 1 (9%) was in a microsatellite instability-high tumor, and no responses were observed in copy number-high/TP53abnormal tumors. The ORR was 44% (90% CI, 27.0%-62.1%). Median duration of response was 15.6 months. Median progression-free survival was 9.0 months (90% CI: 1.8-20.4). The most common grade ³3 treatment-related adverse events were neutropenia (26%) and anemia (19%); no new safety signals were identified. Conclusions: The combination of abemaciclib and fulvestrant has promising activity with durable responses in advanced or recurrent EC; a randomized trial is planned.