Abstract LB118: Mechanisms underlying synergistic activity in MYD88MTDLBCL of KT-413, a targeted degrader of IRAK4 and IMiD substrate

Relapsed/refractory DLBCL remains an incurable disease, and single-agent therapies typically show low response rates and/or transient clinical responses. Oncogenic MYD88 mutations occur in ~25% of DLBCL and drive constitutive NFkB activation, promoting proliferation and survival. Despite its role in tumor biology, targeting MYD88MT by inhibiting or degrading IRAK4 alone, a key component of the MYD88 complex, does not drive significant antitumor activity in preclinical models. One potential reason is the frequent redundant NFkB pathway activation by co-mutations, highlighting the need for combination therapies to effectively target the NFkB pathway in DLBCL. To address this challenge, we have developed IRAKIMiDs, heterobifunctional degraders that simultaneously degrade both IRAK4 and IMiD substrates, as a rational therapeutic combination in a single molecule. IRAKIMiDs show increased antitumor activity in vitro and in vivo in MYD88MT cells as compared to IMiD or IRAK4-targeting alone, highlighting suggesting their potential as single agents in R/R DLBCL.Our lead IRAKIMiD, KT-413, is a potent degrader of IRAK4 (DC50 6nM) and IMiD substrates (Ikaros/Aiolos DC50 2nM), inducing rapid and potent cell killing in vitro and complete and sustained tumor regressions in vivo in MYD88MT models of DLBCL. This activity is superior to the IMiD CC-220, which has similar activity against IMiD substrates (Ikaros/Aiolos DC50 1 nM), supporting the synergistic role of IRAK4 degradation in the context of IMiD biology. We show here that the combined activity of these 2 mechanisms drives a synergistic effect on NFkB and IRF4 signaling with greater downstream effect on NFkB and type 1 interferon (IFN) signaling and cell cycle gene expression than either mechanism alone. In THP1 cells engineered with NFkB and IRF4 reporters, KT-413 but not CC-220 inhibits TLR-stimulated NFkB and IRF4 transcription, supporting a role for IRAK4 but not IMiDs in MYD88-driven survival and proliferation signals. IMiDs have previously been shown to modulate type1 IFN signaling through downregulation of IRF4. We propose that simultaneous targeting of both NFkB and type 1 IFN signaling with KT-413 drives synergistic cell killing in MYD88MT cells. In MYD88MT OCI-Ly10 cells, KT-413 leads to greater IRF4 downregulation, increased type 1 IFN signaling, and preferential downregulation of NFkB pathway and cell cycle transcripts when compared to CC-220. These data support the hypothesis that the synergistic activity of targeting IRAK4 and IMiD substrates by KT-413 in MYD88MT DLBCL is a result of dual targeting of NFkB and IRF4/Type1 IFN through degradation of both IRAK4 and IMiD substrates,driving significantly greater cell killing as compared to either mechanism alone, supporting the potential for the first single agent targeted therapy in MYD88MT DLBCL. KT-413 is on track for initiation of a Phase 1 trial in B cell lymphoma in 2H 2021. Citation Format: Christine R. Klaus, Scott F. Rusin, Kirti Sharma, Samyabrata Bhaduri, Matthew M. Weiss, Alice A. McDonald, Michele F. Mayo, Duncan Walker, Rahul Karnik. Mechanisms underlying synergistic activity in MYD88MTDLBCL of KT-413, a targeted degrader of IRAK4 and IMiD substrate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB118.