Abstract P2-17-05: Evaluation of safety, pharmacokinetics and pharmacodynamics of proxalutamide (GT0918), a potent androgen receptor (AR) blocker, in patients with metastatic breast cancer (mBC): Phase I dose escalation trial

Background: Androgen receptor (AR) blocker has become an increased interest in the treatment of BC, in which about 60-80% patients showed AR positive. However, currently no AR blocker has been approved in mBC. GT0918 is a new chemical entity of AR blocker with possible AR down-regulation. This study is an open-label, single-center, dose escalation phase I trial to assess GT0918 in mBC female patients who have progressed after systemic treatments in China. The primary objectives are to identify the maximum tolerated dose (MTD) and dose-limiting toxicities (DLTs). The secondary objectives are to assess pharmacokinetics and pharmacodynamics of GT0918 with single and multiple dosage applications. (CTR20170757) Methods: Patients (pts) with historically confirmed mBC who had progressed after either chemotherapy, hormonal or targeted therapy, or could not tolerate currently standard therapies were eligible. With the starting dose at 100 mg of GT0918, the decision of dose escalation in the 3+3 design was based on the safety and tolerability assessments. GT0918 was administered orally once, followed by a 7-day off-treatment period for single dose PK analysis of drug elimination. Then GT0918 oral administration was resumed once daily for 28 consecutive days and multiple dose PK analysis was assessed at the end of first cycle (28 days). The first 28-days on treatment (cycle 1) was defined as DLT period. Pts manifesting an objective response or stable disease and likely to have clinical benefit from continued treatment were continued on GT0918 thereafter until they experienced one of following events of intolerable toxicities, disease progression or withdrew consent. Results: 18 pts were enrolled and treated with GT0918 since 9/6/2017 as defined in protocol at five dose levels: 100 mg (n = 3), 200 mg (n = 4), 300 mg (n = 4), 400 mg (n = 4) and 500 mg (n = 3) (as to 7/2/2019). All pts progressed more than two lines of therapies with 83.3% (15/18) pts were treated ≥3rd lines. Out of 6 confirmed AR positive pts, two (33.3%) at 300 mg cohort had finished 17 and 19 cycles individually and continue treatment (as 7/2/19). No DLT was observed and MTD has not been reached. GT0918 related adverse events (AEs) were grade 1 or 2 as per CTCAE v4.03, including fatigue, hypertriglyceridemia, anemia, hypercholesterolemia, increased LDL, nausea, loss of appetite, increased ALT, increase of weight loss, constipation and thrombocytopenia. PK profile analysis showed that in the single-dose study, GT0918 showed a fast absorption profile. In the multiple-dose study, the steady-state serum concentration level of GT0918 and its main metabolite were attained at 21 days. Conclusions: Proxalutamide (GT0918) administrated orally once a day is well tolerated in late-stage pts. No DLT has occurred at maximum dose 500 mg. Pts with AR positive biomarker could have better clinical outcomes with GT0918 treatment. GT0918 and its main metabolite exhibited a nonlinear pharmacokinetic profile over the dose range from 100mg to 500 mg. An expanded/phase Ib in AR positive mBC pts has launched in China to evaluate the anti-tumor activity and safety of GT0918. 200 mg and 300 mg were selected for dose expansion. Citation Format: Huiping Li, Ran Ran, Guohong Song, Hanfang Jiang, Ruyan Zhang, Yaxin Liu, Luping Meng, Phoebe Zhang, Ke Chen, Qiaoxia Zhou, Karl Zhou, Youzhi Tong. Evaluation of safety, pharmacokinetics and pharmacodynamics of proxalutamide (GT0918), a potent androgen receptor (AR) blocker, in patients with metastatic breast cancer (mBC): Phase I dose escalation trial [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-17-05.