Abstract CT160: Phase 1b/2a clinical trial of the oral BET inhibitor PLX2853 as monotherapy for ARID1A mutated gynecologic cancers and in combination with carboplatin for platinum resistant ovarian cance

Abstract The Bromodomain and Extra-Terminal (BET) Domain proteins facilitate the development of many human cancers via epigenetic regulation. BET inhibitors may be effective in reversing platinum resistance in ovarian cancer (OC) and may generate synthetic lethality with ARID1A loss. PLX2853 is an orally active, small molecule inhibitor of BET bromodomain-mediated interactions that exhibits low nanomolar potency in blocking all 4 BET family members (BRD2, BRD3, BRD4, and BRDT). PLX2853 development is continuing at Opna Bio as OPN-2853. Clinical experience with PLX2853 monotherapy in subjects with heavily pretreated solid tumors and lymphoma showed signs of activity. The current study (NCT04493619) was designed as a multicenter, open-label trial with two parallel arms: (1) a phase 2a study of PLX2853 monotherapy in advanced gynecological malignancies with a known ARID1A mutation and (2) a phase Ib/2a combination study of PLX2853 plus carboplatin in platinum resistant OC. The primary objective of the Ib portion of the study was safety and tolerability, with the primary objective of both phase 2a portions being efficacy. In the monotherapy arm, up to 6 patients were treated at 80 mg PLX2853 daily in a safety lead-in, with progression to phase 2a using a Simon 2-stage design if dose limiting toxicities (DLTs) were observed in fewer than 33% subjects. In Stage 1, 6 additional subjects (N=12 total) were planned, with progression to stage 2 if two or more patients responded in stage 1. The combination arm included an escalation phase Ib. Three to six evaluable subjects were planned for each group, with dose escalation pending less than 33% DLT rate. The combination arm defined by the phase 1b portion of the study continued to a planned phase 2a Simon 2 stage design similar to that described for the monotherapy arm. 34 of 37 enrolled patients were evaluable with data from at least 1 post-baseline response (14 monotherapy, 20 combination therapy). Of the 14 evaluable patients on the monotherapy arm, 1 (7.1%) achieved a partial response (PR) with progression-free survival of 278 days, 5 (35.7%) had stable disease (SD) and 8 (57.1%) had progressive disease (PD). Of the 20 evaluable OC patients on the PLX2843 + carboplatin combination, 1 (5.0%) had PR, 9 (45.0%) had SD, and 10 (50%) had PD. This study in a larger cohort of gynecologic cancer patients confirmed the safety profile of the agent and demonstrated the feasibility of combination with carboplatin. While these results did not meet the pre-specified response criteria, evidence of clinical activity nevertheless highlights the rationale for further exploration of BRD4 inhibitors in patients with ARID1A-mutated gynecologic malignancies, possibly in combination with agents targeting potential feedback mechanisms such as the PI3K pathway, frequently activated in these cancers. Citation Format: Elizabeth M. Swisher, Linda R. Duska, Erika P. Hamilton, Amit M. Oza, Gini Fleming, Oladapo O. Yeku, Alexander I. Spira, Debra L. Richardson, Robin Guo, Jackie Walling, Kerry Inokuchi, Dmitriy Zamarin. Phase 1b/2a clinical trial of the oral BET inhibitor PLX2853 as monotherapy for ARID1A mutated gynecologic cancers and in combination with carboplatin for platinum resistant ovarian cance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT160.