Purpose: This work assessed the antitumor activity of selective small molecule IRAK4 degraders in human ABC DLBCL cell lines in vitro and in tumor xenograft models in vivo, alone and in combination with BTK inhibition.

Introduction: ABC DLBCL comprises approximately 45% of DLBCL and has a worse outcome with R-CHOP chemotherapy compared to GCB DLBCL. Activating mutations in MYD88 occur in 30-40% of ABC DLBCL; L265P, the most prevalent MYD88 mutation, causes constitutive assembly and activation of the Myddosome. IRAK4 kinase and scaffolding functions are essential for full signaling through the Myddosome to NFκB and MAPK pathways. Kymera Therapeutics is using a chemical knockdown strategy to develop heterobifunctional small molecule IRAK4 degraders, exemplified by KYM-001, for the treatment of MYD88-driven lymphomas.

Methods: IRAK4 in human PBMC, ABC DLBCL cell lines and xenografts was quantified by immunoassays or targeted MS/MS. Myddosome signaling was monitored by mRNA and phosphoprotein endpoints. Cell viability and cell cycle were monitored by flow cytometry. Tumor xenograft studies were conducted by implanting human ABC DLBCL lines into immunocompromised mouse strains and assessing tumor volume.

Key data: KYM-001 led to potent E3 ligase-dependent degradation of IRAK4. Notably, KYM-001 more effectively inhibited TLR-activated Myddosome signaling compared to IRAK4 kinase inhibitors in human PBMC. Degradation was highly selective for IRAK4 vs >10,000 other detected proteins in the MYD88 L265P mutant ABC DLBCL line OCI-LY10. IRAK4 degradation by KYM-001 resulted in cell cycle inhibition and apoptosis within 48-72 h in ABC DLBCL, with preferential activity in MYD88-mutant vs MYD88-WT cell lines. Oral dosing of KYM-001 showed dose-dependent antitumor activity in several mouse xenograft models of human MYD88-mutant ABC DLBCL at tolerated doses and schedules. In the OCI-LY10 model, tumor regression was associated with >80% degradation of IRAK4, establishing the pharmacodynamic effect required for maximal efficacy. Since alterations in BCR signaling and MYD88 frequently co-occur in B-cell malignancies, we investigated the potential for combined activity of IRAK4 degradation and BTK inhibition. In the OCI-LY10 xenograft model, which has activating mutations in both CD79B and MYD88, BTK inhibition with ibrutinib had an additive effect on KYM-001 antitumor activity.

Conclusions: KYM-001 is a first-in-class, potent, selective and orally active IRAK4 degrader that causes tumor regression in ABC-DLBCL models. Degradation of IRAK4 removes both the kinase and scaffolding functions of IRAK4, and may be superior to kinase inhibition alone. These data support IRAK4 degraders as a promising new therapeutic opportunity for MYD88-driven lymphoma, both alone and in combination with other targeted approaches such as BTK inhibition.

Citation Format: Joseph F. Kelleher, Veronica Campbell, Jesse Chen, Jared Gollob, Nan Ji, Hari Kamadurai, Christine Klaus, Henry Li, Christine Loh, Alice McDonald, Haojing Rong, Scott Rusin, Kirti Sharma, Dominico Vigil, Duncan Walker, Matt Weiss, Karen Yuan, Yi Zhang, Laurent Audoly, Nello Mainolfi. KYM-001, a first-in-class oral IRAK4 protein degrader, induces tumor regression in xenograft models of MYD88-mutant ABC DLBCL alone and in combination with BTK inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-272.