Abstract OT1-05-02: A phase II clinical trial of the combination of pembrolizumab and selective androgen receptor modulator GTx-024 in patients with advanced androgen receptor positive triple negative breast cancer

Background: Androgen receptor (AR) targeted therapy and immunotherapy represent one of the most promising strategies for metastatic triple negative breast cancer (mTNBC), which accounts for 15-20% of all breast cancers. As a nonsteroidal selective androgen receptor modulator (SARM), GTx-024 demonstrated preclinical activity in AR+ TNBC PDX model. Pembrolizumab is a highly selective humanized monoclonal antibody of the programmed cell death 1 receptor (PD-1). The complementary modes of action and low potential for overlapping toxicity make the combination promising in patients with AR+ mTNBC. Trial Design: This is an open-label Phase 2 study for AR+ mTNBC. Eligible participants receive pembrolizumab 200mg IV every 3 weeks in combination with GTx-024 18mg po daily. Eligibility Criteria: Eligible patients must have AR+ (>10%, 1+ by IHC) TNBC; failed up to 2 lines of therapy in metastatic setting; and have measurable disease per RECIST1.1. Patients are excluded if they have had prior checkpoint inhibitors or AR targeted agents. Patients with current or prior use of testosterone, testosterone-like agents, androgenic compounds, or anti-androgens (including systemic steroids and immunosuppressive medications)are excluded, as well as current or prior history of noninfectious pneumonitis requiring systemic steroid therapy. Specific Aims: The primary objective is to evaluate the safety/tolerability of GTx-024 and pembrolizumab and determine the response rate (CR or PR via RECIST 1.1) in patients with advanced AR+ TNBC. We will use clinical benefit rate (CBR), duration of response (DOR), PFS, and OS to test the efficacy of this novel drug combination. Statistical Design: A Simon's MiniMax two-stage Phase 2 design will be utilized. Based on the previously reported response rate associated with single agent pembrolizumab (19%), we consider a response rate of 19% for the combination as discouraging, and a 39% response rate as encouraging. As a result, we will initially accrue 15 patients (including 6 patients from safety lead-in treated at the tolerable dose). If 2 or fewer patients respond, we will stop accrual for futility. Otherwise, the study will accrue an additional 14 patients for a total of 29 patients. With 29 patients, if only 8 or fewer respond (≤27.6%), the study will be considered discouraging unless secondary evidence of clinical benefit is substantial. With more than 8 patients responding out of the 29 patients, the combination would be considered promising. This design has 85% power to declare a true response rate of 39% as promising (power), and a 10% probability of declaring a true 19% response rate as encouraging (type I error). The probability of early termination if the true response rate is 19% is 44%. Target Accrual: 29 Study Contact: Yuan Yuan MD PhD, City of Hope Comprehensive Cancer Center; Duarte, CA 91030; Email: yuyuan@coh.org Citation Format: Yuan Y, Frankel P, Synold T, Lee P, Yost S, Martinez N, Tang A, Mendez B, Schmolze D, Apple S, Hurria A, Waisman J, Somlo G, Tank N, Sedrak M, Mortimer J. A phase II clinical trial of the combination of pembrolizumab and selective androgen receptor modulator GTx-024 in patients with advanced androgen receptor positive triple negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr OT1-05-02.