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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.

Importance Many cancer subtypes, including KIT-mutant gastrointestinal stromal tumors (GISTs), are driven by activating mutations in tyrosine kinases and may initially respond to kinase inhibitors but frequently relapse owing to outgrowth of heterogeneous subclones with resistance mutations. KIT inhibitors commonly used to treat GIST (eg, imatinib and sunitinib) are inactive-state (type II) inhibitors. Objective To assess whether combining a type II KIT inhibitor with a conformation-complementary, active-state (type I) KIT inhibitor is associated with broad mutation coverage and global disease control. Design, setting, and participants A highly selective type I inhibitor of KIT, PLX9486, was tested in a 2-part phase 1b/2a trial. Part 1 (dose escalation) evaluated PLX9486 monotherapy in patients with solid tumors. Part 2e (extension) evaluated PLX9486-sunitinib combination in patients with GIST. Patients were enrolled from March 2015 through February 2019; data analysis was performed from May 2020 through July 2020. Interventions Participants received 250, 350, 500, and 1000 mg of PLX9486 alone (part 1) or 500 and 1000 mg of PLX9486 together with 25 or 37.5 mg of sunitinib (part 2e) continuously in 28-day dosing cycles until disease progression, treatment discontinuation, or withdrawal. Main outcomes and measures Pharmacokinetics, safety, and tumor responses were assessed. Clinical efficacy end points (progression-free survival and clinical benefit rate) were supplemented with longitudinal monitoring of KIT mutations in circulating tumor DNA. Results A total of 39 PLX9486-naive patients (median age, 57 years [range, 39-79 years]; 22 men [56.4%]; 35 [89.7%] with refractory GIST) were enrolled in the dose escalation and extension parts. The recommended phase 2 dose of PLX9486 was 1000 mg daily. At this dose, PLX9486 could be safely combined with 25 or 37.5 mg daily of sunitinib continuously. Patients with GIST who received PLX9486 at a dose of 500 mg or less, at the recommended phase 2 dose, and with sunitinib had median (95% CI) progression-free survivals of 1.74 (1.54-1.84), 5.75 (0.99-11.0), and 12.1 (1.34-NA) months and clinical benefit rates (95% CI) of 14% (0%-58%), 50% (21%-79%), and 80% (52%-96%), respectively. Conclusions and relevance In this phase 1b/2a nonrandomized clinical trial, type I and type II KIT inhibitors PLX9486 and sunitinib were safely coadministered at the recommended dose of both single agents in patients with refractory GIST. Results suggest that cotargeting 2 complementary conformational states of the same kinase was associated with clinical benefit with an acceptable safety profile. Trial registration ClinicalTrials.gov Identifier: NCT02401815.

3018 Background: PLX2853 is a potent, orally active small molecule BET inhibitor. Its unique pharmacokinetic (PK) profile is associated with less thrombocytopenia and improved tolerability by allowing transient target engagement with a prolonged pharmacodynamic (PD) response and time for recovery after dosing. Methods: We conducted a first-in-human 3+3 Ph1b/2a study of PLX2853 in adults with relapsed or refractory solid tumors and lymphoma to determine the safety, PK and recommended phase II dose (RP2D) (NCT03297424). Secondary endpoints included efficacy and PD. Results: As of 2 February 2021, 44 subjects (median age 65, range 39 - 84) received PLX2853 in escalating doses from 5mg to 120mg QD and 40mg to 60mg BID. Ovarian cancer (n = 11), uveal melanoma, colorectal, and prostate (n = 5 each) were the most represented tumor types. Adverse events (AE) occurring in ≥15% of subjects included nausea (41%), decrease appetite (39%), fatigue (27%), vomiting (25%), diarrhea (25%), dysgeusia (25%), dehydration (23%), anemia (20%), dry mouth (18%), dizziness (16%), abdominal pain (16%), and pyrexia (16%). Thrombocytopenia occurred in 11% of subjects. Most AEs ( > 85%) were grades (G) 1-2. Of all AEs, 40% were related. There were 5 treatment-related serious AEs in 2 subjects (n = 1 G4 thrombocytopenia, G4 ischemic stroke, G3 subarachnoid hemorrhage [SAH], and G3 thromboembolic event; n = 1 G3 vomiting). Dose-limiting toxicities were observed at 120mg QD (G4 thrombocytopenia, G4 ischemic stroke, G3 thromboembolic event, and G3 SAH; asymptomatic G4 thrombocytopenia), 60mg BID (G3 thrombocytopenia with recovery > 7 days), and 40mg BID (dose reduction for transient G3 fatigue). PLX2853 systemic exposure was dose-proportional up to 120mg with a short terminal half-life ( < 3.5 hr). Plasma concentrations were above the IC90 in the MYC-responsive reporter assay for 9 hr at 80mg or higher doses. RNA-seq analyses of peripheral blood mononuclear cells showed a dose-dependent modulation of BET target gene expression. One complete response (ongoing 9+ months) was seen in a patient with DLBCL, two patients had partial responses (1 uveal melanoma, 1 primary peritoneal cancer), and 14 patients had stable disease. The median PFS was 82.5 days (range: 51 – 209 days). Conclusions: PLX2853 shows encouraging signs of clinical activity and is well tolerated at the anticipated RP2D of 80 mg/day. PLX2853 is being evaluated as monotherapy and in combination. Clinical trial information: NCT03297424.

Background: PLX2853 is an orally available, non-benzodiazepine BET (bromodomain and extraterminal domain) inhibitor that exhibits low nanomolar potency and a modest preference for binding to the second bromodomain (BD2) of the BET proteins. By regulating genes (e.g. MYC and BCL2) critical to leukemic cell growth and survival, PLX2853 demonstrated broad anti-leukemic activity both as a single agent and in combination with other therapeutic agents in preclinical models. The pharmacokinetic (PK) profile in solid tumor patients revealed a short half-life (< 3 hour) enabling high peak plasma concentrations and nearly complete elimination from the plasma 9 hour post dose. Since strong and prolonged suppression of BET proteins have often untoward effects in normal tissues, the PLX2853 PK profile is hypothesized to be associated with improved tolerability by allowing transient target engagement followed by time for recovery after daily dosing. Methods: We are conducting an open-label, Phase 1b (Ph1b) study of PLX2853 as a single oral agent administered daily in adult patients with relapsed or refractory acute myeloid leukemia (AML) or high risk myelodysplastic syndrome (MDS) using a modified continuous reassessment model (mCRM) with escalation with overdose control (EWOC) to determine the recommended phase 2 dose (RP2D). Up to 36 patients are expected to enroll. The dosing cycle and dose limiting toxicity window (DLT) is 21 days. Primary objectives include safety and PK. Secondary objectives include measures of preliminary efficacy, and exploratory objectives include pharmacodynamics (PD) biomarker assessments in various tissues. Enrollment through Cohort 2 (40 mg QD) is ongoing as of July 2019. Results: Five subjects with relapsed or refractory AML (median age 65 years) have received PLX2853 in escalating doses from 20 to 40 mg QD. Among these first 5 patients treated, the most common treatment emergent adverse events (AEs) regardless of causality in > 1 patient: decreased appetite (n=3), nausea (n=2), diarrhea (n=2), peripheral edema (n=2), cough (n=2), oropharyngeal pain (n=2), blood bilirubin increase (n=2), anemia (n=2), febrile neutropenia (n=2), fatigue (n=2), bacteremia (n=2), headache (n=2), dyspnea (n=2), and hypertension (n=2). Most were grade (G) 1-2. Treatment emergent AEs > G2 in > 1 patient included: anemia (n=2), febrile neutropenia (n=2) and hypertension (n=2). No treatment-related serious AEs or DLTs have been observed. Following a 20 mg daily dose of PLX2853, median time to reach maximal plasma concentrations (Tmax) is 1 hour and the absorption half-life (T1/2) is < 3 hours. Conclusions: In an ongoing Ph1b study, PLX2853 has now completed its first dosing cohort for patients with relapsed or refractory AML or high risk MDS, and no DLT has been observed yet. As dose escalation continues, PK, PD, preliminary safety and efficacy data will be assessed further to determine the clinical significance of target engagement. This clinical trial is registered at clinicaltrials.gov: NCT03787498. Disclosures Pemmaraju: mustangbio: Consultancy, Research Funding; abbvie: Consultancy, Honoraria, Research Funding; samus: Research Funding; celgene: Consultancy, Honoraria; cellectis: Research Funding; Stemline Therapeutics: Consultancy, Honoraria, Research Funding; novartis: Consultancy, Research Funding; plexxikon: Research Funding; Daiichi-Sankyo: Research Funding; sagerstrong: Research Funding; affymetrix: Research Funding; incyte: Consultancy, Research Funding. Borate:Novartis: Consultancy; Takeda: Consultancy; Pfizer: Consultancy; Daiichi Sankyo: Consultancy; AbbVie: Consultancy. Solh:ADC Therapeutics: Research Funding; Amgen: Speakers Bureau; Celgene: Speakers Bureau. Borthakur:Polaris: Research Funding; Arvinas: Research Funding; Agensys: Research Funding; Tetralogic Pharmaceuticals: Research Funding; Cantargia AB: Research Funding; Argenx: Membership on an entity's Board of Directors or advisory committees; BioLine Rx: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Oncoceutics, Inc.: Research Funding; Eli Lilly and Co.: Research Funding; BMS: Research Funding; AstraZeneca: Research Funding; Bayer Healthcare AG: Research Funding; Novartis: Research Funding; FTC Therapeutics: Membership on an entity's Board of Directors or advisory committees; AbbVie: Research Funding; Strategia Therapeutics: Research Funding; Cyclacel: Research Funding; Xbiotech USA: Research Funding; Eisai: Research Funding; Merck: Research Funding; BioTheryX: Membership on an entity's Board of Directors or advisory committees; Oncoceutics: Research Funding; NKarta: Consultancy; Incyte: Research Funding; Janssen: Research Funding; GSK: Research Funding; PTC Therapeutics: Consultancy. DeZern:Astex Pharmaceuticals, Inc.: Consultancy; Celgene: Consultancy. Zhang:Plexxikon Inc.: Employment. Powell:Plexxikon Inc.: Employment. Severson:Plexxikon Inc.: Employment. Inokuchi:Plexxikon Inc.: Employment. Matusow:Plexxikon Inc.: Employment. Halladay:Plexxikon Inc.: Employment. Hsu:Daiichi Sankyo, Inc.: Employment. Watkins:Plexxikon Inc.: Employment. Walling:Myovant Sciences: Consultancy; Nurix: Consultancy; Aduro Biotech: Consultancy; Plexxikon: Consultancy; CytomyX: Consultancy; Flag Therapeutics: Consultancy; Aminex: Consultancy; Immunext: Consultancy; SensenBio: Consultancy; Harpoon Therapeutics: Consultancy. Tsiatis:Plexxikon Inc.: Employment. Mims:PTC Therapeutics: Membership on an entity's Board of Directors or advisory committees; Astellas Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees.

1546Background: There is considerable uncertainty on cancer risk and recommendations for genetic testing in various populations. The purpose of this study is to test types and frequencies of cancer...