Abstract OT2-01-07: A phase 2 open label, multi-center, multinational study investigating the efficacy and safety of GTx-024 on advanced, androgen receptor-positive triple negative breast cancer (AR+ TNBC)
Authors: Rugo H , Overmoyer B , Schwartzberg L , Palmieri C , Taylor R , Hancock M , Small S , Johnston MA 
Published: May 2016

Background: The androgen receptor (AR) is the most highly expressed steroid receptor in breast cancer (BC) with expression seen in up to 95% of estrogen receptor positive (ER+) and up to 50% of ER negative disease. Historically, steroidal androgens exhibited virilizing side effects, thus limiting clinical use. In TNBC, the expression of AR and androgen synthesizing enzymes is associated with lower proliferation, lower tumor grade, better overall survival, and more favorable clinical outcomes as compared to those patients with TNBC not expressing AR. Data from two trials targeting AR in TNBC indicates low level but encouraging clinical activity. A non-steroidal, tissue-selective, AR modulator (SARM), such as GTx-024, offers a targeted approach of AR activation in AR+ TNBC without virilization or estrogenic effects.

Trial Design: Open label, multicenter, multinational, Phase 2 study for the treatment of advanced or metastatic TNBC. Subjects will receive GTx-024, 18 mg orally (PO) daily, continued until evidence of disease progression or toxicity. Subjects whose tumors demonstrate Clinical Benefit (CB) will be treated on study drug until progression.

Eligibility Criteria:

Inclusion: Female, ≥18 years old, confirmed AR+ (≥10% staining) TNBC (confirmed by medical history), up to 1-2 prior chemotherapy regimens, available archived tumor tissue, measurable or bone-only disease, ECOG 0 or 1, with prior toxicities from chemotherapy resolved.

Exclusion: Life expectancy &lt;4 months, uncontrolled CNS metastases, radiotherapy ≤14 days prior to enrollment, active hepatitis, HIV positive, prior treatment with anti-androgens, testosterone or testosterone-like agents, or estrogens or megesterol acetate, or prior treatment for a different cancer (other than BC or non-melanoma carcinoma of the skin) within the past 2 year

Specific Aims: The primary aim is to measure the proportion of AR+ subjects with CB at 16 weeks; defined as subjects with a complete response (CR), partial response (PR), or stable disease (SD); per modified RECIST 1.1. Secondary endpoints include: objective response rate, progression free survival, and time to progression.

Statistical Methods: Simon's two-stage (optimal) design will be used to assess primary efficacy, requiring up to 41 evaluable subjects; i.e., subjects with centrally confirmed AR+ who receive at least one dose of study drug. The first stage will be assessed among the first 21 evaluable subjects. If at least 2/21 achieve CB per modified RECIST 1.1 at 16 weeks, then the trial will proceed to the second stage of recruitment of up to a total of 41 subjects in the evaluable subset of the Full Analysis Set. Otherwise, the trial will be discontinued for lack of efficacy. The trial will test for an unacceptably low CBR of ≤5% versus a CBR more consistent with ≥20%.

Target Accrual: Up to 55 subjects will be enrolled.

Trial Information: www.gtxinc.com

Citation Format: Rugo H, Overmoyer B, Schwartzberg L, Palmieri C, Taylor R, Hancock M, Small S, Johnston MA. A phase 2 open label, multi-center, multinational study investigating the efficacy and safety of GTx-024 on advanced, androgen receptor-positive triple negative breast cancer (AR+ TNBC). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr OT2-01-07.