Safety and Efficacy of Bezuclastinib (CGT9486), a Novel, Highly Selective, Potent KIT D816V Tyrosine Kinase Inhibitor, in Patients with Advanced Systemic Mastocytosis (AdvSM): Results from Part 1 of the Phase 2 Apex Trial
Authors: Vachhani Pankit , Tashi Tsewang , Schiller Gary J. , Lee Stephanie , Piris Villaespesa Miguel , Pomares Helena , Livideanu Cristina Bulai , Lambert Jonathan , Hunter Anthony M. , George Tracy I. , Papayannidis Cristina , Shoumariyeh Khalid , Petroro Rita , Zhang Jenna , Pilla Amanda , Jolin Hina , Easton Rachael , Pullarkat Vinod 
Published: December 2023

Background: Systemic mastocytosis (SM) is a rare disease characterized by the aberrant proliferation of mast cells (MC). In about 95% of adult patients, SM is driven by a gain-of-function mutation (D816V) in exon 17 of KIT. Advanced SM (AdvSM) is a life-threatening form of SM and includes three subtypes: aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and Mast Cell leukemia (MCL). Bezuclastinib is an oral, potent, and selective tyrosine kinase inhibitor (TKI) with activity against KIT D816V. Preliminary results from the on-going Apex study in patients with AdvSM (NCT04996875) were presented previously (DeAngelo et al. [abstract] In: Blood (ASH) 2022; 140 (Supplement 1): 1512-13).

Methods: Apex is a randomized Phase 2, open-label, multicenter trial in adult patients diagnosed with AdvSM per 2022 WHO criteria with a baseline serum tryptase of ≥20 ng/mL. In Part 1, patients were randomized 1:1:1:1 to bezuclastinib 50mg BID, 100mg BID, 200mg BID, or 400mg QD. Patients with a history of prior TKI therapy (e.g., avapritinib, midostaurin) are permitted in the trial. Data from Part 1 informed dose selection for Part 2 of the trial. The primary efficacy endpoint for this trial is overall response rate (complete response [CR], CR with incomplete hematologic recovery [CRh], partial response [PR] and clinical improvement [CI]) as assessed by a central review committee based on mIWG-MRT-ECNM response criteria.

Results: As of April 2023, Part 1 was fully enrolled with 33 AdvSM patients. Patients were randomized to 50mg BID (n=8), 100mg BID (n=8), 200mg BID (n=8), or 400mg QD (n=9). Disease subtypes included ASM (n=7), MCL (n=2), and SM-AHN (n=24); AHN subtypes included CMML (n=18), MDS (n=3), MPN (n=1), and MDS/MPN-U (n=2). The majority of patients were male (64%), ECOG PS 0-1 (82%) and KITD816V positive at baseline (91%). At baseline, 67% of patients were TKI-naïve; 33% had prior midostaurin, and 15% had prior avapritinib. Median (range) BM MC burden, serum tryptase, and KIT D816V VAF at baseline were 30% (5-90), 163 ng/mL (35-1578), and 7% (0-47), respectively. Of the 33 patients enrolled, 25 patients (76%) had C-findings evaluable per mIWG-MRT-ECNM criteria; 20 patients (61%) were positive for SRSF2/ASXL1/RUNX1 mutation which are associated with poor survival in SM.

Conclusion: Enrollment in Part 1 of the Apex trial is complete. Patients enrolled in Part 1 of the Apex trial are generally representative of the population of patients with AdvSM based on patient characteristics and markers of disease. Part 1 includes a small subset of patients with prior use of TKIs. Safety, clinical activity, and clinical objective response for all patients in Part 1 will be presented.