Abstract CT156: A Phase I study evaluating AC676, an innovative BTK chimeric degrader, in patients with relapsed and refractory B-cell malignancies

Abstract Study Background Bruton tyrosine kinase (BTK) is a key signaling molecule that plays a central role in B-cell receptor transduction. The development of drugs that inhibit BTK has transformed the management of patients with B-cell malignancies. However, therapy using covalent BTK inhibitors such as ibrutinib, and non-covalent inhibitors such as pirtobrutinib, can still result in resistance, primarily due to the development of mutations in BTK including residues C481 and L528. AC676 was designed as a chimeric degrader to target and degrade BTK using Accutar's proprietary Protein-Protein Interaction Targeting Chimeras (PPI-TAC) platform. By effectively linking a BTK ligand to the cereblon E3-ligase recruiting ligand, AC676 brings BTK in proximity to cereblon, thereby inducing subsequent ubiquitination and degradation of BTK. AC676 degrades BTK proteins irrespective of mutations; including C481, kinase dead L528 and others, and thus may be effective for the treatment of patients who progress on both covalent and non-covalent BTK inhibitors. Notably, it is also effective in cell lines expressing gain of function PLCG2 mutants, suggesting that it removes BTK’s scaffolding function. AC676 does not degrade cereblon neo-substrates, so neutropenia is not expected to be an on-target effect. This abstract describes an ongoing first in human Phase 1 trial of AC676. Study Description AC676-001 is a Phase 1 dose-escalation study of AC676 administered orally once daily as monotherapy in patients with relapsed and refractory B-cell malignancies. Approximately 60 patients may be enrolled. Eligible patients must be ≥ 18 years, and have one of the following histologically confirmed relapsed or refractory disease types: Chronic lymphocytic leukemia and small lymphocytic lymphoma, Diffuse large B cell lymphoma - non-GCB subtype, Follicular lymphoma, Mantle cell lymphoma, Marginal zone lymphoma, Lymphoplasmacytic lymphoma including Waldenstrom macroglobulinemia. Patients must have received at least two prior systemic therapies or have no other standard of care therapies to provide significant clinical benefit. Patients must have measurable disease per disease-specific response criteria and Eastern Cooperative Oncology Group performance status≤ 1. Dose-escalation will begin with an accelerated titration phase, followed by a standard 3+3 phase. AC676 is administered orally once daily on a 28 day per cycle schedule at doses ranging from 50mg to 600mg. The primary objective of the study is to evaluate the safety and tolerability of AC676. Secondary objectives include the evaluation of anti-tumor activity and the pharmacokinetic profile following single and multiple doses. Study enrollment began in April 2023 with three sites currently open in the United States (NCT05780034). Citation Format: Jennifer Woyach, Michael Tees, Nancy Mota, Gladys Brown, Su Young Kim, Manish R. Patel. A Phase I study evaluating AC676, an innovative BTK chimeric degrader, in patients with relapsed and refractory B-cell malignancies abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (7Suppl): Abstract nr CT156.