Abstract 148: <i>In vivo</i> CAR-T T-LNP system (GT801) drives potent B cell depletion and clinical feasibility in hematologic and autoimmune conditions.

Abstract Background: CAR-T therapies have revolutionized hematologic malignancies treatment but remain constrained by complex autologous manufacturing, high cost, and lymphodepletion toxicity. In vivo CAR-T approaches aim to overcome these barriers by engineering T cells directly in patients, which requires targeted delivery, durable CAR expression, potent cytotoxicity, and re-dosing potential. Toward this goal, we present the development, preclinical data, and preliminary clinical results for GT801, a novel anti-CD19 in vivo CAR-T candidate. Methods: GT801 uses T cell-targeted lipid nanoparticles (T-LNPs) encapsulating chemically modified linear mRNA encoding an anti-CD19 CAR. T-LNPs were surface-engineered with a VHH antibody using the CLAMP platform, enabling site-specific antibody attachment, controlled ligand density, and efficient, selective T-cell uptake. Formulation and mRNA design were optimized for targeting specificity, robust CAR expression, and functionality. B-cell depletion, PK, and preliminary toxicology of GT801 were assessed in vitro and in humanized NOG mouse models. Results: With optimized mRNA chemistry, the T-LNP platform achieves robust and durable CAR expression in human PBMCs for &amp;gt;14 days in vitro. Targeted delivery at 0.1 mpk reached receptor-saturating levels across multiple lymphoid tissues, while off-target uptake remained &amp;lt;1%. In human PBMC-engrafted NOG mice, a single i.v. dose as low as 0.01 mpk achieved &amp;gt;95% B-cell depletion, and 0.1 mpk achieved near-complete clearance (&amp;lt;0.1%) across multiple lymphoid tissues. CDX models demonstrated potent antitumor activity and enhanced CAR-T expansion upon repeat dosing, suggesting good in vivo fitness post-transfection and expansion driven by B cell depletion. Primary B-cell killing was robust in PBMCs from healthy donors and autoimmune patients at only 0.1 μg within 24 hours. Serial-dose toxicology elicited minimal cytokine release (IL-6, TNF-α) and no organ toxicity in any tested organs, supporting the platform's good safety profile. Preliminary clinical data with B-cell hematological malignancies and autoimmune diseases demonstrated high in vivo CAR expression and confirmed repeat-dosing feasibility. Conclusion: These findings demonstrate that our T-LNP platform enables efficient, targeted, and sustained in vivo CAR expression with a favorable safety profile and scalable manufacturing. The platform's robust preclinical B cell depletion efficacy, validated by preliminary clinical data confirming high in vivo CAR expression and repeat dosing feasibility, successfully validates the system for clinical use. Ongoing patient accrual will further inform clinical outcomes. Citation Format: Jingwei Sun, Xi Zhu, Jiahui Jin, Yiyang Tan, Liang Lin, Zhao Xu, Jingman Wang, Dalang Li, Hong Chen, Jiaming Ren, Jun Cui, Jing Yu, Pin Wang, Yarong Liu. In vivo CAR-T T-LNP system (GT801) drives potent B cell depletion and clinical feasibility in hematologic and autoimmune conditions [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 148.