Overcoming the blood‒brain barrier: nanomedicine strategies for targeted delivery and multimodal therapy in Alzheimer's disease.

Alzheimer's disease (AD) remains a significant therapeutic challenge, primarily because the formidable blood‒brain barrier (BBB), which drastically limits the brain bioavailability of most drugs. Nanoparticle-based drug delivery systems offer a promising strategy to overcome this central obstacle. This review systematically examines the design, mechanisms, and applications of nanomedicine in AD therapy. We analyze key strategies for enhancing BBB penetration through surface engineering and the utilization of various nanocarriers, including liposomes, exosomes, dendrimers, and carbon dots. Furthermore, we discuss how stimuli-responsive release mechanisms (e.g. responsive to pH, enzymes, reactive oxygen species, light, or ultrasound) enable targeted and precise drug delivery. A critical focus is placed on how these multifunctional nanoplatforms can address multiple AD pathogenic pathways simultaneously, such as amyloid-β and tau aggregation, cholinergic dysfunction, oxidative stress, neuroinflammation, and gut‒brain axis dysregulation. Although preclinical evidence is compelling, the clinical translation of these nanotherapies is hindered by challenges related to long-term biocompatibility, scalable manufacturing, patient heterogeneity, and regulatory frameworks. This review highlights the translational potential of nanomedicine in AD treatment while outlining the key hurdles that must be addressed for its successful implementation.