Lessons learned: Studies targeting the androgen receptor (AR) signaling pathway in aromatase inhibitor (AI)-resistant breast cancer are limited. Bicalutamide, one of the commonly used AR inhibitors in prostate cancer, in combination with AI, did not show synergistic activity in patients with estrogen receptor-positive and AI-resistant disease in this phase II, single-arm study. The clinical benefit rate and objective response rate at 6 months were 16.7% and 0%, respectively, and the study was terminated after the first stage.
Background: Endocrine resistance is a major problem in clinical practice. Studies have shown that androgen receptor (AR) signaling activation may be one of the mechanisms, and targeting AR showed some promising results in AR-positive triple-negative breast cancer. The aim of this study was to assess the efficacy and safety of bicalutamide plus another aromatase inhibitor in patients with nonsteroidal aromatase inhibitor (AI) or steroidal AI resistance and estrogen receptor (ER)-positive and AR-positive advanced breast cancer.
Methods: A Simon's two-stage, phase II, single-arm study was conducted. We assumed the clinical benefit rate (CBR) of 40% would be significant in clinical practice. In this case, if ≥4 patients of the 19 patients in the first stage benefited from treatment, the CBR would achieve the assumed endpoint. If fewer than four patients benefited from treatment in the first stage, the trial would be terminated. All patients received bicalutamide 50 mg per day orally plus another aromatase inhibitor. The primary outcome was CBR; secondary outcomes included objective response rate (ORR), progression-free survival (PFS), and tolerability.
Results: A total of 19 patients enrolled in the first stage, and 18 patients met all criteria for analysis. The trial terminated according to protocol after the first stage. After a median follow-up of 14 months, the CBR at 6 months was 16.7% (3/18); no patients with partial or complete response were observed. The median PFS was 2.7 months. Bicalutamide in combination with AI was well tolerated.
Conclusion: Bicalutamide in combination with another AI did not show synergistic activity in patients with ER-positive breast cancer and AI resistance. Results suggest that no more large-sample clinical trials should be conducted in this population for overcoming endocrine resistance.
Trial registration: ClinicalTrials.gov NCT02910050.
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