Abstract 6044: EPriL: A platform for the rapid identification of degraders of inhibitor-resistant kinases
Authors: Muller Michelle , de Man Jos , Uitdehaag Joost C. , van Cauter Freek , van Gemert Sander , Hoffmann Milan , van Mil Yvonne G. , Mulder Winfried R. , Prinsen Martine B. , Sterrenburg Jan Gerard , Vu Diep , de Wit Joeri , Ensing Erik , Buijsman Rogier C. 
Published: March 2024

Abstract
Heterobifunctional kinase degraders (HKDs) are an important new class of therapeutic modalities, of which the first have recently entered clinical trials. HKDs are small molecules that comprise a kinase-of-interest binding moiety and an E3 ligase binding moiety connected via a spacer. HKDs are excellently suited to target kinases that have a scaffolding function, or that are resistant against conventional inhibitors. A HKD brings a kinase and an E3 ligase in proximity forming a ternary complex leading to ubiquitin-dependent catalytic degradation of the kinase of interest. This ternary complex formation is unique for each kinase which can make HKDs highly specific.
To discover novel HKDs, we have developed a platform called Energetically Privileged Ligands (EPriLs), which is based on a macrocycle scaffold that binds non-covalently in the kinase ATP pocket with near-covalent affinity, and allows binding to kinases that have acquired known inhibitor resistance mutations. The macrocyclic structure allows attachment of E3 ligase ligands with multiple exit vectors. With the EPriL platform, we have developed novel HKDs for various clinically relevant kinase targets, including Bruton’s Tyrosine Kinase (BTK).
BTK inhibitor drugs are used as first line treatment for chronic lymphocytic leukemia (CLL), Waldenström macroglobulinemia (WM) and as second line treatment for mantle cell lymphoma (MCL). Despite their great success in the clinic, many patients on BTK inhibitors relapse eventually, mostly because of the emergence of resistance mutations in the BTK protein. These include BTKC481S, which abrogates binding of covalent drugs, and BTKT474I, BTKL528W and BTKV416L which preclude binding of non-covalent inhibitors like pirtobrutinib. HKDs can combat resistance by degrading such BTK variants, and by removing the scaffold function of BTK. BTK degraders such as NX-5948 and NX-2127 are currently in clinical Phase 1.
To show the applicability of the EPriL platform for the discovery of novel BTK degraders, we screened a library of more than 100 EPriL HKDs for biochemical binding to BTK, and tested their propensity to induce degradation of HiBiT-tagged BTKWT, BTKC481S and BTKT474I in HEK293 cell lines. The degrading activity of hits was verified through western blot on the BTK-dependent DLBCL cell line TMD8. The best degraders - based on potency and efficacy - were tested in cell proliferation assays on a panel of TMD8 BTK mutant cell lines. In these lines, endogenous BTK was mutated by CRISPR to BTKC481S, BTKT474I, BTKL528W or BTKV416L. Several BTK degraders had potent (&lt; 20 nM IC50) antiproliferative activity on most TMD8 lines, including difficult-to-target variants such as BTKV416L and BTKL528W.
The unique binding mode of the EPriL scaffold thus translates to broad activity on resistance variants, while its selectivity ensures minimal degradation of off-target tyrosine kinases as observed for other degraders.
Citation Format: Michelle Muller, Jos de Man, Joost C. Uitdehaag, Freek van Cauter, Sander van Gemert, Milan Hoffmann, Yvonne G. van Mil, Winfried R. Mulder, Martine B. Prinsen, Jan Gerard Sterrenburg, Diep Vu, Joeri de Wit, Erik Ensing, Rogier C. Buijsman. EPriL: A platform for the rapid identification of degraders of inhibitor-resistant kinases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6044.