Beyond BBB permeability: spatial and dynamic post-BBB barriers governing CNS drug action

Central nervous system (CNS) drug delivery has traditionally focused on overcoming the blood-brain barrier (BBB). For small-molecules, CNS penetration is largely governed by physicochemical properties and endothelial influx/efflux mechanisms, whereas macromolecular, biologic, and nanoparticulate therapeutics face additional spatial and dynamic barriers after BBB crossing. Successful BBB penetration therefore does not necessarily ensure effective therapeutic action within the CNS. Even after entering the brain, therapeutic agents must navigate perivascular compartments, extracellular diffusion barriers, non-target cellular uptake, and intracellular trafficking processes that regulate spatial distribution, retention, target-cell access, and intracellular availability. In this Mini Review, we discuss CNS delivery as a spatial and dynamic pharmacokinetic process rather than a single-step event of BBB penetration. We outline how perivascular spaces, glymphatic clearance pathways, glial and extracellular structures, cell-type-specific uptake, and intracellular sequestration can limit effective target-site exposure despite apparent brain entry. We also discuss why conventional measurements such as cerebrospinal fluid concentration or whole-brain homogenate analysis may overestimate successful delivery, and highlight the value of imaging-based approaches, including intravital imaging and mass spectrometry imaging, for resolving drug localization, molecular identity, and biological effect. Finally, we consider how temporal modifiers, including circadian regulation and aging-related barrier changes, may alter CNS drug retention and therapeutic response. By shifting the focus from BBB permeability alone to post-BBB spatial localization, temporal retention, and functional target engagement, this Review provides a conceptual framework for evaluating and designing next-generation CNS therapeutics.