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Systemic mastocytosis (SM) is characterized by mast cell infiltration of ≥ 1 extracutaneous organs and encompasses a spectrum of diagnoses that can range from a non-advanced to advanced disease (Shomali et al, 2018). There are two nonadvanced variants of SM: indolent systemic mastocytosis (ISM), which accounts for approximately 85% and smoldering systemic mastocytosis (SSM), which includes about 5% of the general SM population (Cohen et al, 2014, Jennings et al, 2014). ISM is characterized by 0 or 1 B-findings and SSM by 2 or more B findings and absence of organ damage or an associated hematologic neoplasm (Gotlib et al, 2018). There are currently no approved therapies to treat the underlying disease of ISM or SSM. Although anti-mediator therapies (e.g. anti-H1 and H2 antihistamines, leukotriene receptor antagonists, cromolyn sodium, corticosteroids) are used to control symptoms such as anaphylaxis, GI intolerance, and flushing to improve quality of life, their effectiveness and tolerability are variable. Many patients experience a persistently high symptom burden despite maximized anti-mediator therapies. Because the molecular pathogenesis of SM is driven by KIT D816V mutations in 95% of patients (Garcia-Montero et al, 2006, Jara-Acevedo et al, 2015, Vaes et al, 2017), other agents targeting this mutated kinase have been used to treat the spectrum of SM variants; however, toxicities such as cognitive impairment, GI effects, intracranial hemorrhage, and edema may limit dosing and thus efficacy. In addition to targeting KIT D816V, bezuclastinib was designed to avoid other closely related kinases with known liabilities, such as PDGFRα, PDGFRβ, wild-type KIT, VEGFR2 (KDR), and CSF1R (FMS). Furthermore, bezuclastinib has demonstrated minimal brain penetration and no CNS toxicities have been identified in preclinical studies. Preliminary clinical activity with bezuclastinib has been observed in patients with advanced solid tumors or locally advanced, unresectable, or metastatic gastrointestinal stromal tumor (GIST). A reduction in KIT exon 17 mutational burden was observed in patients treated with bezuclastinib. This reduction was temporally associated with a reduction in tumor burden supporting bezuclastinib as an active therapy in KIT-driven diseases (Wagner et al, 2018). This is a multi-center, Phase 2, double blind, placebo-controlled, 3-part clinical study to evaluate the safety, efficacy, and biomarker correlates (e.g. bone marrow mast cell percentage, serum tryptase level, and KIT D816V mutation burden) of the KIT inhibitor bezuclastinib in patients with ISM and SSM. This study will enroll patients with SSM and moderate-to-severe ISM who have inadequate control of their symptoms despite at least 2 anti-mediator treatments. Part 1 of the study is intended to determine the recommended dose of bezuclastinib in Part 2. Subjects will be randomized to placebo or 1 of 3 doses of bezuclastinib which will be administered in combination with a baseline regimen of best supportive care (BSC). Part 2 will evaluate the efficacy of bezuclastinib at the selected dose as compared with placebo. Efficacy will be measured by the reduction of symptom burden as assessed by the mastocytosis activity score (MAS), a disease specific patient reported outcome tool (Siebenhaar et al, 2018). In Part 3, patients who have completed treatment in Part 1 or Part 2 of the study, including those initially randomized to placebo, may participate in a long-term extension and receive open-label bezuclastinib in combination with BSC. The study will enroll approximately 138 subjects. Data from this study will support further development of bezuclastinib in SM. Disclosures Gotlib: Cogent Biosciences: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Chair for the Eligibility and Central Response Review Committee, Research Funding; PharmaEssentia: Honoraria, Membership on an entity's Board of Directors or advisory committees; Kartos: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Research Funding; Abbvie: Membership on an entity's Board of Directors or advisory committees, Research Funding; Deciphera: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Allakos: Consultancy; Incyte: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Blueprint Medicines: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Deininger: Novartis: Consultancy, Research Funding; SPARC, DisperSol, Leukemia & Lymphoma Society: Research Funding; Incyte: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Fusion Pharma, Medscape, DisperSol: Consultancy; Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees. DeAngelo: Incyte: Consultancy; Jazz: Consultancy; Novartis: Consultancy, Research Funding; Pfizer: Consultancy; Servier: Consultancy; Takeda: Consultancy; Abbvie: Research Funding; Forty-Seven: Consultancy; Autolus: Consultancy; Amgen: Consultancy; Agios: Consultancy; Blueprint: Research Funding; Glycomimetrics: Research Funding. Payumo: Cogent Biosciences, Inc: Current Employment. Mensing: Cogent Biosciences, Inc.: Current Employment. Jolin: Cogent Biosciences: Current Employment. Sachs: Cogent Biosciences: Current Employment. George: Bristol Meyers Squibb: Consultancy; Blueprint Medicines: Consultancy; Celgene: Consultancy; Incyte Corporation: Consultancy. OffLabel Disclosure: study of investigational agent

The molecular pathogenesis of Systemic Mastocytosis (SM) is driven by mutations in the KIT gene, with 95% of patients having a mutation in exon 17, D816V, leading to constant proliferation of mast cells (Garcia-Montero et al, 2006; Jara-Acevedo et al, 2015; Vaes et al, 2017). Targeted therapeutics have revealed clinical activity in these patients, but toxicities such as cognitive effects, intracranial hemorrhage, hypertension, and edema may limit dosing and availability of these therapies. While the exact cause of these effects is difficult to determine, numerous closely related kinases, such as wild type PDGFRα, PDGFRβ, KIT, VEGFR2 (KDR), and CSF1R (FMS), are considered to be anti-targets, with previous evidence of their inhibition linked to observed clinical toxicities (Liu & Kurzrock, 2015; Giles et al., 2009; Jayson et al., 2005). Bezuclastinib (CGT9486) was designed to selectively inhibit KIT D816V versus these other closely related kinase anti-targets. Additionally, we demonstrate that bezuclastinib has minimal brain penetration, together with no observed CNS-related toxicities in nonclinical studies. Herein, we present results from cell-based kinase profiling assays, which demonstrate that bezuclastinib has a significant and unique selectivity to KIT D816V relative to the aforementioned kinases when tested head-to-head against other clinically relevant compounds in SM. Additionally, a similar selectivity profile was observed for a broader panel of kinases, ion channels, transporters, and enzymes, which will be presented here, including drug concentrations and target engagement achieved with recent in vivo studies. Importantly, we also show that bezuclastinib has minimal brain penetration, a preferred feature of an anti-Kit molecule due to CNS-related adverse events observed in these indications. In a tissue distribution study performed in rats, bezuclastinib shows a brain:plasma ratio Disclosures Guarnieri: Cogent Biosciences: Current Employment. Cable: Cogent Biosciences: Current Employment. Bouhana: Cogent Biosciences: Current Employment. Sullivan: Cogent Biosciences: Current Employment. Ball: Cogent Biosciences: Current Employment. Sachs: Cogent Biosciences: Current Employment. Winski: Cogent Biosciences: Current Employment. Robinson: Cogent Biosciences: Current Employment.

Systemic mastocytosis (SM) is characterized by excessive accumulation of mast cells in bone marrow and other extracutaneous tissues encompassing a spectrum of variants ranging from non-advanced to advanced disease (Shomali et al, 2018). Advanced systemic mastocytosis (AdvSM) is an aggressive and life-threatening form of SM with an overall survival from The molecular pathogenesis of SM is driven by mutations in the KIT gene leading to ligand-independent proliferation of mast cells, with 95% of patients carrying the D816V mutation in exon 17 (Garcia-Montero et al, 2006; Jara-Acevedo et al, 2015; Vaes et al, 2017). Bezuclastinib, an oral, highly selective tyrosine kinase inhibitor with potent activity against KIT D816V has shown preliminary clinical activity and a tolerable safety profile in a Ph 1/2 study of patients with advanced solid tumors including gastrointestinal stromal tumor (GIST). In that study, a reduction in KIT exon 17 mutational burden was observed in subjects treated with bezuclastinib. This reduction was temporally associated with a reduction in tumor burden supporting bezuclastinib as an active therapy in KIT-driven diseases (Wagner et al, 2018). Other agents targeting the KIT D816V mutations have shown activity in the treatment of AdvSM; however, toxicities such as cognitive impairment, intracranial hemorrhage, and edema may limit dosing and appropriateness of these therapies. Besides targeting KIT D816V, bezuclastinib was designed to avoid other closely related kinases with known liabilities, such as PDGFRα, PDGFRβ, wild-type KIT, VEGFR2 (KDR), and CSF1R (FMS). Additionally, minimal brain penetration has been observed with bezuclastinib, and no CNS toxicities have been identified in preclinical studies. This is a multi-center, Phase 2, open-label, 2-part clinical study to evaluate the safety, efficacy, pharmacokinetics (PK), and pharmacodynamics (PD) of bezuclastinib in subjects with AdvSM. This study will enroll subjects who meet WHO diagnostic criteria for AdvSM, have measurable SM-related organ damage according to modified IWG-MRT-ECNM consensus eligibility and response criteria, and have a baseline serum tryptase of ≥20 ng/mL. The diagnosis of AdvSM and evidence of measurable disease will be confirmed by an Eligibility Committee prior to enrollment. This study will enroll approximately 140 patients. Part 1 will consist of a dose optimization period to determine the optimal dose of bezuclastinib for subjects with AdvSM. Subjects will be randomized into one of four dose cohorts (50, 100, or 200 mg twice daily, or 400 mg once daily) in a 1:1:1:1 manner. A planned interim analysis to assess safety, efficacy, and biomarker endpoints will occur when approximately half the planned number of Part 1 patients have been evaluated through at least 2 cycles of treatment. After all subjects in Part 1 complete at least two cycles, the optimal dose will be selected. Part 2 will enroll subjects at the selected dose level to determine efficacy of bezuclastinib and to further characterize safety, PK, and PD. The primary assessment of efficacy is overall response rate, defined as the percentage of subjects classified as confirmed responders (CR, CR with incomplete hematologic recovery [CRh], partial response [PR], and clinical improvement [CI]) according to modified IWG-MRT-ECNM response criteria as assessed by a Central Response Review Committee (CRRC). Data from this study will support continued development of bezuclastinib in SM including Non-Advanced SM. This study opened in June 2021. Disclosures Gotlib: BMS: Research Funding; Kartos: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; PharmaEssentia: Honoraria, Membership on an entity's Board of Directors or advisory committees; Cogent Biosciences: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Chair for the Eligibility and Central Response Review Committee, Research Funding; Allakos: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Deciphera: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Incyte: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Blueprint Medicines: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. George: Celgene: Consultancy; Blueprint Medicines: Consultancy; Bristol Meyers Squibb: Consultancy; Incyte Corporation: Consultancy. Deininger: Sangamo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding; Novartis: Consultancy, Research Funding; Incyte: Consultancy, Honoraria, Research Funding; SPARC, DisperSol, Leukemia & Lymphoma Society: Research Funding; Fusion Pharma, Medscape, DisperSol: Consultancy; Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Part of a Study Management Committee, Research Funding. Tashi: PharmaEssentia: Membership on an entity's Board of Directors or advisory committees, Other: Advisory Board. Pullarkat: Amgen, Dova, and Novartis: Consultancy, Honoraria; AbbVie, Amgen, Genentech, Jazz Pharmaceuticals, Novartis, Pfizer, and Servier: Membership on an entity's Board of Directors or advisory committees. Bose: Celgene Corporation: Honoraria, Research Funding; Astellas: Research Funding; NS Pharma: Research Funding; Novartis: Honoraria; Promedior: Research Funding; BMS: Honoraria, Research Funding; CTI BioPharma: Honoraria, Research Funding; Sierra Oncology: Honoraria; Incyte Corporation: Honoraria, Research Funding; Blueprint Medicines: Honoraria, Research Funding; Pfizer: Research Funding; Kartos Therapeutics: Honoraria, Research Funding; Constellation Pharmaceuticals: Research Funding. Payumo: Cogent Biosciences, Inc: Current Employment. Pilla: Cogent Biosciences: Current Employment. Jolin: Cogent Biosciences: Current Employment. DeAngelo: Abbvie: Research Funding; Takeda: Consultancy; Servier: Consultancy; Pfizer: Consultancy; Novartis: Consultancy, Research Funding; Jazz: Consultancy; Incyte: Consultancy; Forty-Seven: Consultancy; Autolus: Consultancy; Amgen: Consultancy; Agios: Consultancy; Blueprint: Research Funding; Glycomimetrics: Research Funding. OffLabel Disclosure: Study is for investigational agent