A Phase II Open Label Study of Everolimus in Combination With Endocrine Therapy in Resistant Hormone Receptor-Positive HER2-Negative Advanced Breast Cancer

Abstract

Background: Therapies targeting estrogen receptor signaling are standard for patients with hormone receptor (HR)-positive (HR⁺) metastatic breast cancer (MBC). Dysregulation of the phosphoinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway is associated with treatment resistance. Addition of the mTOR inhibitor, everolimus, to exemestane doubled progression-free survival (PFS) in HR⁺/HER2^- MBC patients whose disease had previously progressed during endocrine therapy. In this phase II study, we used everolimus in addition to the most recent endocrine therapy during which a patient's disease progressed, in an attempt to restore and extend the benefit of the antiestrogen therapy in patients with HR⁺/HER2^- MBC.

Patients and methods: Patients with HR⁺ MBC who progressed on antiestrogen therapy received everolimus (10 mg orally daily) in combination with the antiestrogen therapy most recently administered. Treatment was administered in 4-week cycles and continued until disease progression or unacceptable toxicity. Blood and archival tumor specimens were collected for VeriStrat (Biodesix, Inc) and Foundation One (Foundation Medicine) assays, respectively. Accrual of 42 evaluable patients allowed detection of improvement in median PFS from 2.8 months (expected with hormonal treatment alone) to 5 months (power 80%, α = 5%).

Results: Forty-seven patients were enrolled and treated. After a median follow-up of 22.2 months, median PFS was 6.6 months. Secondary efficacy end points included: overall response rate, 6%; clinical benefit rate, 40%; and median overall survival, 21.1 months. No unexpected toxicity was observed. Efficacy could not be correlated with PI3K/AKT/mTOR alterations or VeriStrat (Biodesix, Inc) prognostic signatures.

Conclusion: After progression during antiestrogen therapy, the addition of everolimus, without changing the hormonal therapy, resulted in a median PFS of 6.6 months, suggesting efficacy in patients with HR⁺/HER2^- MBC.

Keywords: Antiestrogen therapy; Endocrine therapy-resistant breast cancer; HER2-negative breast cancer; Hormone receptor-positive breast cancer; Mammalian target of rapamycin (mTOR) inhibitor.