Cascading attrition of in vivo CAR-T therapy: From systemic delivery failure to functional collapse.

Ex vivo chimeric antigen receptor (CAR)-T cell therapy demonstrates strong efficacy but remains limited by high costs and lengthy manufacturing. In vivo in situ reprogramming provides a scalable "off-the-shelf" alternative by directly engineering endogenous T cells; however, clinical translation is hindered by an efficacy-attrition paradox. Here we review the cascading loss of functional vector dose across multi-scale physiological barriers-from opsonin-driven sequestration and protein-corona formation in circulation, to metabolic checkpoints and nuclear transport restrictions within resting T cells. By comparing lentiviral vectors and lipid nanoparticles, we examine how each platform navigates these distinct hurdles. A key insight is that overcoming such stochastic failure cascades requires deterministic rather than incremental design: rational integration of synthetic biology tools-immune-cloaking surfaces, logic-gated circuits-enables precise navigation through complex in vivo microenvironments. This progress shifts the field from passive dose escalation toward engineered resilience, transforming in vivo CAR-T generation into a clinically viable platform for hematologic malignancies, solid tumors, and autoimmune diseases.