Abstract OT2-16-01: The SMILE study: A phase 2 trial of onapristone in combination with fulvestrant for patients with ER+ and HER2- metastatic breast cancer after progression on endocrine therapy and CDK4/6 inhibitors

Abstract

Abstract Background Antiprogestins, including selective progesterone receptor (PgR) modulators (SPRMs) that act as PgR antagonists, are a promising class of therapeutics for overcoming endocrine resistance including in patients who develop activating estrogen receptor 1 (ESR1) mutations after prior endocrine therapy (ET). The SMILE study (NCT04738292) is a multi-institutional phase II clinical trial to determine the efficacy and safety of an antiprogestin, onapristone in combination with fulvestrant as second-line therapy for patients with ER+,PgR+, HER2 -metastatic breast cancer (MBC). Methods Patients with locally advanced or MBC who progress on ≥2 lines of prior therapy with ET as a single agent or in combination with CDK4/6 inhibitors, mTOR inhibitors and one line of chemotherapy in the metastatic setting are eligible. Other criteria include ECOG performance status ≤ 2, measurable disease per RECIST 1.1 criteria, adequate organ function, availability of an archived tumor biopsy block confirming the diagnosis of MBC, optional biopsy at disease progression on trial, and optional 18F-fluorofuranylnorprogesterone (18F-FFNP) PET/CT imaging. Ovarian suppression is allowed. Prior ET with tamoxifen or aromatase inhibitor therapy in the adjuvant or metastatic setting is allowed. Patients with prior exposure to fulvestrant, antiprogestins, or CDK inhibitors in the adjuvant setting are ineligible. Following consent, all subjects will receive fulvestrant at the recommended dosing schedule plus onapristone 50 mg orally, twice daily, until disease progression, unacceptable toxicity, or discontinuation for any other reason. Using the Simon Two-Stage design, a total of 39 patients will be enrolled over two years from 6-8 sites within Wisconsin Oncology Network (WON). In the first stage of the trial, 21 subjects will be enrolled; if ≥2 responses are observed, then 18 additional subjects will be enrolled in the second stage. This design yields a one-sided type I error rate of 5% if the ORR is 7% and power of 80% when the combination's true response rate is 20%. The primary objective is to evaluate the objective response rate; secondary objectives include safety and tolerability, progression-free survival, disease control rate, and duration of response. Other correlates include optional functional imaging of PgR binding with 18F-FFNP PET/CT and biomarker analysis (ER, total PgR, HER2, Ki67, CD24, CD44, LDH1, KLF4, CK 5/6, PhosphoSer294-PgR on tissue samples, ESR1 mutations, and circulating tumor DNA analysis). The study was approved in January 2021, and enrollment is active at the time of this submission. Citation Format: Sailaja Kamaraju, Amy Fowler, Chung Yee Cheng, Lubna N Chaudhary, John Burfeind, Janet Retseck, Amye J Tevaarwerk, Mark Burkard, Elisavet Paplomata, Amanda M Parkes, Julie M Jorns, Sergey Tarima, Douglas Yee, Hallgeir Rui, Carol Lange, Tarek Sahmoud, Kari B Wisinski. The SMILE study: A phase 2 trial of onapristone in combination with fulvestrant for patients with ER+ and HER2- metastatic breast cancer after progression on endocrine therapy and CDK4/6 inhibitors [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-16-01.