Abstract P3-07-15: Preliminary data from a Phase 1b dose escalation study of OP-1250, an oral CERAN, in combination with palbociclib in patients with advanced and/or metastatic estrogen receptor (ER)-positive, HER2-negative breast cancer
Authors: Chan Arlene , Day Daphne , Hui Rina , McCarthy Nicole , Wilson Rosalind , Faltaos Demiana , Shaw Morena , Murphy Caitlin 
Published: March 2023

Abstract
Background: OP-1250 is a small molecule oral complete estrogen receptor antagonist (CERAN) that binds the ligand binding domain of the ER and completely blocks ER-driven transcriptional activity. CDK4/6 inhibitors in combination with endocrine therapy have improved progression free survival and overall survival for patients (pts) with metastatic breast cancer (MBC) in the first- and second-line settings. However, most patients will progress and newer combinations such as with OP-1250 may provide improved clinical outcome. OP-1250 potently inactivates both wild-type ER and mutant forms of ER. The latter confers ligand independent activity and is a mechanism of resistance to standard of care endocrine therapies. In preclinical studies, OP-1250 in combination with palbociclib demonstrated synergistic activity in models of wild-type ER and those containing ESR1 activating mutations, and in models of brain metastasis. A Phase 1/2 monotherapy study of OP-1250 is ongoing in MBC subjects who have received 1 or more prior endocrine therapies (NCT04505826). Monotherapy is well tolerated and the recommended phase 2 dose is 120 mg QD. The aim of this combination trial is to define the maximum tolerated dose (MTD), safety, tolerability, and pharmacokinetics (PK) of OP-1250 in combination with palbociclib. Methods: Eligibility criteria include pts with MBC or locally advanced breast cancer who have received no more than 1 prior line of endocrine therapy (prior CDK4/6 inhibitors and one line of chemotherapy are permitted) and measurable or non-measurable disease. Using a 3+3 design, cohorts are sequentially enrolled, and pts receive escalating doses of OP-1250 orally QD continuously in combination with 125 mg of palbociclib orally for 21 of 28 days. Pts are evaluable for dose limiting toxicities (DLTs) if &gt;75% of both treatments were administered within the first 28-day treatment cycle. Blood is collected for PK on cycle 1 days 1, 2 and cycle 2 days 15, 16 for OP-1250 and cycle 1 day 15 for palbociclib. PK profiles, exposure parameters, and drug-drug interactions (DDIs) are assessed. Pts are monitored for adverse events (AE) and tumor assessments (RECIST 1.1) are conducted every two cycles. Results: As of July 7, 2022, 9 pts were evaluable for DLTs after 28 days of treatment in dose levels 30 mg, 60 mg, and 90 mg of OP-1250 in combination with palbociclib. No DLTs occurred at any of the dose levels. As of May 13, 2022, of the 7 pt safety data set, the most common Grade 1/2 treatment emergent adverse events (TEAE), which occurred in 2 patients, were nausea, gastroesophageal reflux, vomiting, and fatigue. Grade 3 neutropenia occurred in 4 pts and all were attributed to palbociclib by the investigator. No Grade 4 events were observed. OP-1250 was highly bioavailable and showed dose proportional exposure in combination with palbociclib. The single and multiple dose exposure of OP-1250 was consistent with that observed in the monotherapy study, indicating an absence of effect of palbociclib on OP-1250 PK. 90 mg steady-state evaluation is ongoing. Palbociclib concentrations at steady state did not demonstrate a meaningful difference from published exposure parameters for the three dose levels evaluated, indicating an absence of DDI. Conclusions: In the first three cohorts of OP-1250 and palbociclib, the combination was well tolerated and no DLTs occurred. There were no clinically significant DDIs observed between OP-1250 and palbociclib at the doses evaluated and exposure of each drug was consistent with observed monotherapy exposure levels. Dose escalation of OP-1250 continues to 120 mg, to be followed by dose expansion. Updated data will be presented. (NCT05266105)
Citation Format: Arlene Chan, Daphne Day, Rina Hui, Nicole McCarthy, Rosalind Wilson, Demiana Faltaos, Morena Shaw, Caitlin Murphy. Preliminary data from a Phase 1b dose escalation study of OP-1250, an oral CERAN, in combination with palbociclib in patients with advanced and/or metastatic estrogen receptor (ER)-positive, HER2-negative breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P3-07-15.