Abstract 6099: STEAP1 targeting CAR-T cells equipped with collagen-binding IL-12 for advanced prostate cancer

Abstract Metastatic castration-resistant prostate cancer (mCRPC) is a largely incurable, immunologically cold solid tumor. Six-transmembrane epithelial antigen of the prostate 1 (STEAP1) is a promising target specifically expressed in >85% of mCRPC patients. Chimeric antigen receptor (CAR)-T cells transformed the clinical landscape of blood cancers. However, CAR-T therapy is still not very effective against solid cancers including prostate cancer, in part due to immunosuppressive tumor microenvironment (TME) and antigen heterogeneity. Interleukin-12 (IL-12) is a promising cytokine to address these issues through activation and recruitment of immune cells to the tumor. IL-12 can induce epitope spreading to fight antigen heterogeneity. Previously, armored CAR-T cells that express wild-type (wt) IL-12 were developed, but there is still room for improvement in tuning potency and safety. Adoptive transfer of autologous IL-12-expressing T cells showed objective responses in melanoma patients; however, the trial was limited by toxicity. The result suggests that IL-12 secreted from CAR-T cells are locally produced but still toxic due to the cytokine’s diffusion into systemic circulation. Here, we report STEAP1-targeting CAR-T cells that conditionally express collagen-binding domain fused IL-12 (CBD-IL-12) upon recognizing STEAP1 protein. We show that CBD fusion to IL-12 significantly increased the concentration of IL-12 in mouse prostate tumors and suppressed its systemic spread following secretion from the CAR-T cells. This resulted in equivalently high intra-tumoral levels of interferon-gamma (IFN-γ), C-X-C motif chemokine ligand 9 (CXCL9) and granulocyte-macrophage colony-stimulating factor (GM-CSF) without increase of serum alanine aminotransferase (ALT) and T cell infiltration into non-target organs compared to wt IL-12. Flow cytometric analysis showed increase of T cells, NK cells and cross-presenting dendritic cells and decrease of monocytic myeloid-derived suppressor cells (M-MDSCs) in the tumor following CBD-IL-12 expressing STEAP1 CAR-T cell therapy. CBD-IL-12 expressing CAR-T cells combined with anti-PD-1 and anti-CTLA-4 antibodies eradicated an established mouse prostate cancer model without preconditioning and generated anti-tumor immune memory. CBD fusion to potent yet toxic immunotherapies may make a breakthrough in the development of CAR-T therapy that can safely overcome solid tumors. Citation Format: Koichi Sasaki, Vipul Bhatia, John K. Lee, Jun Ishihara. STEAP1 targeting CAR-T cells equipped with collagen-binding IL-12 for advanced prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6099.