Efficacy of Curcumin in Neurodegenerative Diseases: From Pharmacokinetic Barriers to Advanced Delivery Systems

Background and Objectives: The main neurodegenerative diseases (NDs)-Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), and amyotrophic lateral sclerosis (ALS)-represent a growing global health burden with no available disease-modifying therapies. Curcumin, a polyphenol from Curcuma longa, is a promising candidate owing to its pleiotropic antioxidant, anti-inflammatory, and neuroprotective profile observed mainly in preclinical models, but the poor oral bioavailability (<1%) and negligible BBB penetration (<0.1%) have substantially limited curcumin's clinical translation. The objective of this work was to critically examine the therapeutic potential of curcumin in NDs, focusing on advanced drug delivery systems (DDSs) designed to overcome its pharmacokinetic barriers. Methods: This is a narrative, non-systematic review of PubMed/MEDLINE, Scopus, and Web of Science. The review is organized around five complementary thematic areas selected to span the full translational pipeline of curcumin in neurodegeneration, from mechanistic rationale to clinical applicability: (1) molecular mechanisms, addressing the pleiotropic activities that justify therapeutic interest; (2) pharmacokinetic barriers, the principal obstacle to clinical translation; (3) the evolution of drug delivery systems (DDSs), documenting the technological strategies developed to overcome these barriers; (4) disease-specific applications, evaluating the available evidence across the four main NDs; and (5) translational limitations, identifying the methodological and regulatory gaps that must be closed to enable clinical implementation. Results: Curcumin exhibits neuroprotective activity in preclinical models of the four NDs analysed, acting on six interconnected mechanisms and the gut-brain axis. Four generations of DDSs have been developed, from phytosomes and clinically used lipid dispersions (Meriva^®, BCM-95^®, Longvida^®, and Theracurmin^®) to fourth-generation systems (biomimetic nanoparticles, MOFs, microneedles, 3D scaffolds, hydrogels, and carbon dots) that substantially increase the bioavailability in preclinical studies. Combination strategies, such as curcumin with resveratrol and dutasteride, show preliminary clinical signals in ALS. However, clinical translation remains limited: over 80% of positive animal findings have not been replicated in humans, formulation characterization is frequently incomplete, and most trials lack CNS-exposure biomarkers. Importantly, most of the reported bioavailability claims are based on total curcumin measurements (parent aglycone plus its inactive Phase II conjugates) rather than the active aglycone alone, a methodological limitation that should be considered when interpreting the magnitude of the bioavailability improvements reported for novel formulations. Conclusions: Curcumin exhibits pleiotropic neuroprotective activity in preclinical models of AD, PD, MS, and ALS, mediated by interconnected antioxidant, anti-inflammatory, anti-amyloidogenic, mitochondrial, and gut-brain axis mechanisms. However, its poor systemic bioavailability (<1%), minimal blood-brain barrier penetration, and extensive first-pass metabolism have limited clinical translation. Advanced drug delivery systems (including lipid-based carriers (liposomes, solid lipid nanoparticles, and nanostructured lipid carriers), polymeric nanoparticles (PLGA and chitosan), and bioinspired vesicles (exosomes)) are essential in order to overcome these barriers. Nevertheless, the formulation heterogeneity, limited long-term safety data, and reliance on preclinical models remain major obstacles; a definitive clinical translation will therefore require well-characterized formulations validated in phase II/III trials with cerebrospinal fluid exposure biomarkers, the pharmacokinetic monitoring of active aglycone (rather than total curcumin including inactive conjugates), and adaptive trial designs in neurological populations.