Loading is not delivering: Intracellular drug retention and migration capacity govern macrophage-based drug delivery.

Living cells are increasingly explored as drug carriers for their active targeting capacity, yet robust intracellular loading in vitro often fails to translate into efficacy in vivo. Here, we address this disconnect by loading macrophages with nano- and microdrugs with equal intracellular mass and matched release kinetics. Quantitative tracking revealed that in vitro retention overestimates in vivo stability: Nanodrug-loaded macrophages lost up to 83% of their payload after reinfusion, nearly double the loss observed in vitro. Nanodrug loading also elevated oxidative stress in macrophages, thereby impairing chemotactic migration. By contrast, microdrugs persisted intracellularly for ≥7 days and preserved macrophage migration toward inflamed sites. Using intravital imaging, we provided previously unavailable spatiotemporal evidence of vascular transmigration while retaining cargo. These properties translated into superior efficacy in systemic and localized inflammation models. Our findings underscore that effective cell-based drug delivery requires looking beyond uptake to evaluate in vivo cargo persistence and its impact on cell migration.