Abstract B37: PLX-4107, a selective IKZF2 degrader, reprograms suppressive regulatory T cells and demonstrates anti-tumor activity
Authors: Thompson Peggy A , Yang Pengyu , Yang Linette , Song Susan , Huang Yujun , Li Xiaoming , Dearie Alejandro , Chien Stephen , Spalding Mary E , Blanco Gabrielle , Daniele Elizabeth , Toth Julia , Jamborcic Aleksandar , Parker Gregory , Bailey Simon 
Published: December 2022

Abstract
Immune checkpoint therapy has demonstrated durable clinical responses in multiple solid tumor types. Reduced clinical response to checkpoint therapy has been linked to the presence of potent immunosuppressive regulatory T cells (Tregs) within the tumor microenvironment that contribute to tumor immune evasion. The transcription factor Helios (IKZF2) is a marker of highly suppressive Tregs and is required to maintain a stable, suppressive Treg cell phenotype in the inflammatory tumor microenvironment. Depletion of IKZF2 in Tregs results in both loss of suppressive activity and conversion of Tregs into effector-like T cells, leading to anti-tumor immunity. Targeted protein degradation using the endogenous Ubiquitin Proteasome System (UPS) has enabled targeting undruggable proteins, such as IKZF2, that have no known small molecule binding pocket. We have designed small molecules that promote a novel interaction between IKZF2 with the E3 ubiquitin ligase substrate receptor, Cereblon, leading to proximity induced protein degradation. PLX-4107 is a novel molecular glue that is a highly selective, potent, and rapid degrader of IKZF2 via the redirection of the E3 substrate receptor, Cereblon. Degradation of IKZF2 by PLX-4107 is blocked in the presence of proteasome and neddylation inhibitors as well as a Cereblon knock-out cell line, confirming that degradation is mediated by the UPS and specifically through the involvement of Cereblon. Proteome-wide analysis demonstrated that PLX-4107 selectively depletes IKZF2 protein levels without degrading other known Cereblon neo-substrates. In vitro, PLX-4107 mediated degradation of IKZF2 resulted in conversion of suppressive Tregs into CD4+ effector-like T cells, coupled with an increased production of the effector cytokines IL2 and IFNg. Oral administration of PLX-4107 to cynomolgus monkeys demonstrated sustained pharmacodynamic response, persistent depletion of IKZF2, and reprogramming of Tregs, consistent with the catalytic mechanism of protein degradation. In vivo, T cell expansion studies showed that administration of PLX-4107 decreased both Treg CD25 expression and proliferation, along with increased activation of CD8+ T effector cells. PLX-4107 was evaluated in in vivo xenograft efficacy studies and demonstrated dose dependent single agent anti-tumor activity that was dependent on the presence of T cells. In addition, co-administration of PLX-4107 and anti-PD-1 antibody Pembrolizumab resulted in tumor growth inhibition and significant combination benefit. PLX-4107 is a novel molecular glue that selectively degrades the undruggable transcription factor, IKZF2. PLX-4107 mediated IKZF2 degradation results in conversion of Tregs to an effector-like T cell phenotype, single agent antitumor activity and the ability to enhance the efficacy of immune checkpoint therapy in vivo.
Citation Format: Peggy A Thompson, Pengyu Yang, Linette Yang, Susan Song, Yujun Huang, Xiaoming Li, Alejandro Dearie, Stephen Chien, Mary E Spalding, Gabrielle Blanco, Elizabeth Daniele, Julia Toth, Aleksandar Jamborcic, Gregory Parker, Simon Bailey. PLX-4107, a selective IKZF2 degrader, reprograms suppressive regulatory T cells and demonstrates anti-tumor activity [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr B37.