Redefining intranasal brain delivery: from nasal entry to meaningful parenchymal and cellular exposure

Nose-to-brain (N2B) delivery is a promising noninvasive strategy to circumvent the blood‒brain barrier (BBB). However, the clinical translation of N2B platforms remains hindered by a conceptual oversimplification that equates crude central nervous system (CNS) exposure with effective therapeutic delivery. This review reconceptualizes the N2B pathway as a stage-resolved sequential transport cascade extending from nasal entry to meaningful parenchymal and cellular access. We identify a critical imbalance in current formulation strategies, which have advanced two extreme poles, namely initial mucosal retention and epithelial permeation at one end and final target-cell engagement at the other, while leaving the intermediate post-epithelial gateways, including cerebrospinal fluid (CSF) and perivascular space (PVS)-mediated redistribution and deep intraparenchymal diffusion, as structural and interpretive blind spots. To bridge this gap, we propose an integrated engineering paradigm with three core design principles. First, multistage-aware design harmonizes sequential spatiotemporal functionalities within a single vehicle architecture. Second, disease-informed design integrates pathologically remodeled barrier states and glymphatic hydrodynamics into the formulation rationale. Third, cargo-informed design establishes the distinct biophysical identity and inherent liabilities of each payload, from small molecules to viral vectors and CRISPR machinery, as the primary engineering starting point. Finally, we advocate for a methodological standard demanding stage-specific validation of vehicle integrity, cross-boundary penetration, and functional target engagement, beyond compartment-blind whole-brain readouts. We further underscore the biofate and clearance of delivery systems as an emerging axis of safety and regulatory evaluation. Collectively, this framework provides a systematic basis for transitioning N2B engineering into a viable clinical modality.