Abstract 3761: HSK26784: An oral PROTAC-BTK degrader for multiple B lymphocyte derived malignancies

Abstract PROteolysis TArgeting Chimeras (PROTACs) have emerged as a new and promising modality utilizing an event-driven MOA, whereby protein levels could be modulated by PROTAC-induced degradation by hijacking the ubiquitin-proteasome system. Starting from 2001, many target proteins have been studied, such as BET and kinase families as well as nuclear receptors. Bruton's tyrosine kinase (BTK) is an essential component of BCR pathway, which plays an important role in development, activation, proliferation, and survival of B lymphocytes. Ibrutinib is an irreversible inhibitor of BTK, and shows promising activity against multiple B lymphocyte derived malignancies. However, there remains a big concern about the development of Ibrutinib induced resistance by BTK C481S mutant. In our study, HSK26784 was found as a novel orally available BTK-PROTAC molecule, which could selectively degrade the BTK proteins. HSK26784 selectively degraded BTK proteins in Mino cell lines, with a half degradation concentration (DC50) of 22.9 nM. HSK26784 also effectively induced BTK degradation in mouse spleen, with a DC50 of 3.8 mg/kg. In addition, HSK26784 significantly inhibit the expression of CD69 on human PBMC cells after ex vivo BCR stimulation. HSK26784 could inhibit cell proliferation in many malignant B cell lines. In Ibrutinib sensitive cell lines, the inhibitory activity of HSK26784 is similar to that of Ibrutinib. While in the Ibrutinib non-sensitive cell lines, HSK26784 was more potent than Ibrutinib. Importantly, in the xenograft models, HSK26784 could also inhibit the tumor growth in a dose dependent manner. And HSK26784 achieved much better therapeutic effects than Ibrutinib. HSK26784 could inhibit the kinase activity of BTK wt and BTK C481S mutant, with IC50 of 11 nM and 14 nM respectively. It indicated that the efficacy of HSK26784 in BTK wt and BTK C481S mutant was comparable. HSK26784 showed good orally bioavailability in different species. The oral bio availabilities in mice, rats and dogs were 15.4 %, 11.8 % and 12.8 % respectively. In conclusion, HSK26784 is a promising BTK PROTAC degrader with better efficacy than Ibrutinib in multiple B lymphocytes. Citation Format: Meilin Qian, Fei Ye, Chen Zhang, Jianming Wang, Yuanli Zhang, Yangyang Cui, Linli Li, Xiaoli Gou, Jia Ni. HSK26784: An oral PROTAC-BTK degrader for multiple B lymphocyte derived malignancies [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3761.