Messenger RNA (mRNA) therapeutics formulated in lipid nanoparticles (LNPs) have transformed the field of nucleic acid medicine by enabling transient, programmable protein expression for vaccination, protein replacement, immunotherapy, and genome engineering. Extending their clinical success beyond vaccines requires a mechanistic understanding of where LNP components and mRNA travel, which cells they enter, where cargo is translated, and how each component is cleared. Such biodistribution outcomes are governed by a complex interplay between nanoparticle physicochemical properties, administration route, protein corona formation, host physiology, intracellular trafficking, and degradation. Importantly, the biodistribution of lipid components, intact mRNA, degraded RNA fragments, and translated proteins represents distinct yet interconnected biological processes that should not be interpreted interchangeably. Advances in molecular imaging, quantitative nucleic acid analysis, mass spectrometry, spatial and single-cell technologies, and pharmacokinetic modeling are improving their resolution across tissues and time. In parallel, rational lipid engineering, selective organ targeting, biodegradable ionizable lipids, ligand-directed delivery, and artificial intelligence-assisted design are expanding opportunities for tissue-specific delivery beyond the liver and lymphoid organs. However, interpretation remains constrained by assay heterogeneity, incomplete understanding of protein corona dynamics and endosomal escape, species-dependent differences, and the absence of harmonized regulatory standards. This review examines the mechanisms governing mRNA-LNP disposition, evaluates what current analytical methods detect and miss, synthesizes preclinical and clinical evidence, and considers the strategies needed to achieve predictable, tissue-selective, and clinically translatable mRNA delivery.
