Toward <i>In Vivo</i> CAR T Cell Therapy: Perfecting CAR and Lipid Nanoparticle Design

CAR T cell therapy represents a leading therapeutic modality in the treatment of hematological cancers. However, conventional ex vivo manufacturing processes are extremely complex, costly, and time-consuming. Hence, <i>in vivo</i> CAR-T cell therapy represents a promising alternative, where T cells are genetically modified directly within the patient. This novel approach requires the careful design of three modular and interdependent nanoscale parameters: CAR architecture, targeted lipid nanoparticle delivery systems, and nucleic acid cargo format. Each component is highly modular, therefore systematic optimization can enhance the safety and efficacy of <i>in vivo</i> CAR T cell generation. Here, we review the molecular and mechanistic principles underlying this therapeutic strategy, emphasizing how these nanoscale design features govern CAR T cell production and functional performance in vivo. Advancing our understanding of these parameters is critical to developing efficient, safe, and clinically translatable in vivo CAR T cell cancer therapies.