Enzalutamide Synergizes With Trastuzumab and Everolimus to Inhibit Breast Cancer Growth

The oral androgen receptor inhibitor, enzalutamide, blocks several steps in the androgen receptor signaling pathway and prevents the entry of androgen receptor into the cell nucleus.¹

Enzalutamide prolongs survival in men with metastatic castration-resistant prostate cancer (CRPC) in whom the disease has progressed after chemotherapy. More recently, enzalutamide has been shown to significantly improve both radiographic progression-free survival and overall survival versus placebo in the treatment of chemotherapy-naïve men with metastatic CRPC.²

Currently, enzalutamide is being evaluated in clinical trials for the treatment of breast cancer, because the androgen receptor is widely expressed in breast cancers. In fact, approximately 60% of human epidermal growth factor receptor-2 (HER2)—positive breast cancer and one third of triple negative breast cancer (TNBC) tumors express the androgen receptor.

“If the androgen receptor is outside the nucleus, both estrogen and androgen-driven tumor growth is inhibited. Therefore, our research suggests that the androgen receptor may serve as a therapeutic target in both estrogen receptor—positive and –negative breast cancers,” said Michael A. Gordon, PhD, of the Department of Pathology at the University of Colorado Anschutz Medical Campus in Aurora, on March 7, 2015, at the 97^thAnnual Meeting of the Endocrine Society.

Furthermore, evidence suggests that some breast cancer cells are critically dependent on androgen receptor signaling in numerous cellular function and signaling cascades, including the HER2/phosphoinositide-3 kinase (PI3K)/ mammalian target of rapamycin (mTOR) pathways. Conversely, the HER2/PI3K/mTOR pathway regulates steroid hormone receptor activity and expression, including the androgen receptor.

Therapeutics targeting this pathway, such as the anti-HER2 agent, trastuzumab, and the anti-mTOR drug, everolimus, have demonstrated significant clinical benefit for the treatment of breast cancer.

However, new and acquired resistance to these agents continues to pose a significant challenge. Given that androgen receptor signaling intersects with that of the HER2/3 and mTOR signaling pathways, targeting androgen receptor in combination with growth factor targeted therapies may produce a synergistic effect and, perhaps, reduce the risk of recurrence in subsets of breast cancer.

In this study, HER2+ and TNBC cell lines were treated with enzalutamide, trastuzumab, and everolimus, alone or in combination, at 3 clinically relevant doses per drug. Cell proliferation was measured on an IncuCyte live cell imager. Drug synergy and dose reduction index were calculated from proliferation data using Calcusyn software by comparing combinations of multiple drug concentrations. Pathway component proteins were measured by western blot, and gene expression was measured using quantitative real-time polymerase chain reaction (qRT-PCR).

Dihydrotestosterone-mediated nuclear localization of AR was inhibited by enzalutamide in 3 HER2+ BC cell lines tested (MDAMB453, BT474, SKBR3). Enzalutamide inhibited the proliferation of HER2+ breast cancer cells, and a combination of enzalutamide with trastuzumab inhibited proliferation more effectively than either agent alone. Furthermore, this inhibitory effect was synergistic in HER2+/ER- cell lines and additive in HER2+/ER+ cells and trastuzumab-resistant cell lines.

Enzalutamide attenuated the increased expression of pHER3 and total HER3 protein that occurred in response to HER2+/ER+ and HER2+ non-amplified/ER- cell lines that were exposed to dihydrotestosterone. Treatment with a combination of enzalutamide and everolimus synergistically inhibited proliferation in three HER2+ cell lines (SKBR3, SKBR3-Pool2, BT474) and one TNBC cell line tested (BT549). Treatment with everolimus caused a compensatory increase in androgen receptor protein in some cell lines. However, this increase was attenuated with combination enzalutamide plus everolimus treatment.

In the in-vitro setting, the effects of enzalutamide synergized with that of FDA-approved breast cancer therapies, trastuzumab and everolimus, through distinct mechanisms. Consequently, combination therapies containing enzalutamide may provide benefit for breast cancer subtypes that include HER2+ and TNBC.

References

Gordon MA, D’Amato N, Gu H, et al. The anti-androgen enzalutamide synergizes with trastuzumab and everolimus to inhibit breast cancer growth via distinct mechanisms. Presented at the 2015 Endocrine Society Annual Meeting; March 5-8, 2015; San Diego, CA. Poster Board SAT-312.

1. Keating GM. Enzalutamide: a review of its use in chemotherapy-naïve metastatic castration-resistant prostate cancer.Drugs Aging. 2015 Feb 25. [Epub ahead of print]
2. Beer TM, Armstrong AJ, Rathkopf DE, et al. Enzalutamide in metastatic prostate cancer before chemotherapy.N Engl J Med. 2014;371:424-433.

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