Ansell, Stephen M.

Abstract Introduction and Purpose: Immune checkpoint signaling represents a major barrier to effective antitumor immunity by suppressing cytotoxic T-cell activity. Cytokine-inducible SH2-containing protein (CISH) is a genetically validated intracellular immune checkpoint that constrains cytotoxic T-cell and natural killer cell signaling. Emerging evidence supports a critical, T cell-intrinsic role for CISH in lymphoma. Despite strong biological validation, CISH targeting has thus far been limited to genetic manipulation in adoptive cell therapies. We sought to establish CISH as a drug-tractable target by discovering a first-in-class, cell-permeable small-molecule CISH inhibitor/degrader suitable for scalable immunotherapy development. Methods: CISH biology was characterized using datasets of patients with B-cell lymphoma and isogenic cellular models to define target-dependence benchmarks. Structure-guided small-molecule discovery was enabled through homology modeling and AlphaFold-based predictions of CISH structure. A focused library of covalent CISH binders was synthesized using systematic variation of SH2-engaging motifs and electrophilic warheads. Compounds were evaluated using biophysical binding assays, mass spectrometry–based covalent engagement, cellular degradation, and signaling readouts. Results: Cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) analysis identified CISH as one of the most highly expressed genes in intratumoral T cells associated with a poor prognosis in patients with non-Hodgkin lymphoma, supporting disease relevance and target enrichment. Genetic ablation or knockdown of CISH in a T cell model (KARPAS299) induced reproducible, target-dependent phenotypes, including enhanced S-phase entry and augmented STAT5 phosphorylation following IL-2 stimulation, establishing rigorous cellular benchmarks for pharmacologic modulation. We designed and synthesized over 50 candidate CISH binders incorporating covalent SH2-domain engagement. Multiple compounds showed direct binding to CISH by thermal shift analysis and selective covalent modification at Cys144 by mass spectrometry. Introduction of a masked phosphate motif significantly improved intracellular exposure, confirmed using a BODIPY-labeled analog. Lead compounds demonstrated target engagement by cellular thermal shift assay and induced rapid, concentration-dependent degradation of endogenous CISH within 4 hours in cells, while maintaining high selectivity over related SOCS family members, demonstrating both target engagement and specificity. Conclusions: This work establishes CISH as a chemically tractable intracellular immune checkpoint and reports the discovery of the first selective, cell-permeable small-molecule CISH degrader. These data provide a strong foundation for further structure activity relationship optimization and position CISH degradation as a promising drug discovery strategy to modulate T cell function and enhance antitumor immunity. Citation Format: Hao Xie, Xinyi Tang, Sujeewa Ranatunga, Stephen M. Ansell. Discovery of a small-molecule degrader targeting the intracellular immune checkpoint CISH [abstract]. In: Proceedings of AACR Drug Discovery and Development (AACR D3) Conference; 2026 Jul 21-24; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(14_Suppl):Abstract nr B049.