Nanoparticle-Mediated Lutein Delivery for Diabetic Retinopathy: From Blood-Retinal Barrier Crossing to Neurovascular Unit Protection

Abstract

Diabetic retinopathy (DR) is increasingly understood as an integrated neurovascular unit (NVU) pathology driven by chronic hyperglycemia, rather than solely a microvascular complication. Anti-vascular endothelial growth factor (anti-VEGF) therapy remains limited by repeated intravitreal injections, treatment resistance, and failure to reverse early neurodegeneration. Lutein, a natural carotenoid, may counter these refractory dimensions by activating the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant pathway and suppressing neuroinflammation across NVU components. However, its poor aqueous solubility, chemical instability, and restricted blood–retinal barrier (BRB) permeation limit retinal bioavailability and clinical translation. Nanoscale targeted delivery can improve lutein solubility and stability and enable trans-BRB transport via the enhanced permeability and retention effect or receptor-mediated transcytosis, thereby enhancing precise NVU protection. This review integrates lutein’s mechanisms against DR-related NVU injury, design strategies for nanoscale delivery systems and trans-BRB transport, and preclinical efficacy with translational bottlenecks, to support advancement of lutein from dietary supplement to precision adjuvant therapy for DR. Unlike reviews treating lutein as an isolated nutraceutical or materials target, it connects nanomaterial engineering with clinical translation and regulatory science. Preclinical studies show that nanoformulated lutein improves oral bioavailability, retinal accumulation, and oxidative/inflammatory injury in diabetic models. Translation remains hindered by limited long-term safety data, scalable manufacturing, and unclear regulatory classification. Future priorities include stage-adaptive nanoplatforms, standardized preclinical evaluation, and rigorously designed multicenter clinical trials to establish efficacy, safety, and regulatory readiness.

Introduction

Diabetic retinopathy (DR) is the leading cause of blindness in the working-age population. Among individuals with diabetes, the global prevalence of DR is estimated at 22.27%, with vision-threatening DR affecting 6.17%.¹ Notably, DR-related vision impairment in the working-age population reached 2.85 million prevalent cases in 2021, representing a 2.8-fold increase since 1990, with the burden disproportionately concentrated in South Asia and middle-income countries.² These statistics underscore the escalating public health challenge posed by DR and the urgent need for innovative therapeutic strategies that address its early pathological drivers.

DR represents an integrated pathological alteration of the neurovascular unit (NVU) driven by chronic hyperglycemia, encompassing neurodegeneration, blood-retinal barrier (BRB) breakdown, and inflammatory activation.^3–5 The NVU comprises vascular, glial, and neuronal components.⁶ In early DR, neurodegeneration frequently precedes clinically evident microvascular damage. Dysfunction across NVU components is intricately interdependent, involving impaired neurovascular coupling, BRB breakdown, and inflammatory activation, culminating in a complex pathological network.⁷ Although anti-VEGF agents and laser photocoagulation effectively target neovascularization and vascular leakage in advanced disease, these modalities are limited by the requirement for repeated intravitreal injections, treatment refractoriness in a subset of patients, and failure to reverse early neurodegeneration.^8–10 Consequently, noninvasive nutritional interventions that provide holistic NVU protection through multi-target mechanisms have emerged as an active research frontier.¹¹

Lutein, a xanthophyll carotenoid, is selectively concentrated in the retinal macula. It protects the retina through filtration of blue light, quenching of reactive oxygen species (ROS), and activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant pathway.^12–15 Current anti-VEGF monotherapy is limited by its inability to control inflammation and fibrosis.¹⁶ In Ins2Akita/+ mice, long-term oral lutein preserves retinal function by suppressing inflammation and vascular permeability. It thereby targets the inflammation unaddressed by anti-VEGF monotherapy, supporting lutein as an adjunct rather than an alternative.¹⁷ However, lutein exhibits poor aqueous solubility and marked photothermal and oxidative instability, and its limited permeation across the BRB severely compromises retinal bioavailability, markedly hampering clinical translation.^18,19 Nanocarrier-based strategies enhance lutein solubility and stability and facilitate surface functionalization with targeting ligands, thereby improving delivery efficiency and targeted retinal accumulation.²⁰ In recent years, lutein nano-delivery strategies have primarily been developed for age-related macular degeneration (AMD) and related disorders, whereas NVU-targeted approaches in DR remain scarce.²¹ Critically, nano-lutein systems should be positioned as an adjuvant to anti-VEGF therapy, primarily intended for early NVU protection, delaying disease progression, and reducing injection frequency.

This review integrates NVU disruption in DR, the multitarget protection conferred by lutein across NVU constituents, and nanoscale delivery strategies that overcome its pharmacokinetic limitations. Its distinctive contribution is to bridge materials characterization with clinical translation and regulatory perspectives, dimensions largely overlooked by technology-centric reviews of formulation physics alone. Although nano-lutein has been reviewed in age-related macular degeneration and blue-light injury,^20,21 a synthesis focused on DR-related NVU pathology is still lacking. Existing studies treat lutein either as a standalone nutraceutical or as a delivery target. They rarely examine its adjuvant potential alongside anti-VEGF therapy, and they do not confront the manufacturing scalability, quality-control standardization, and regulatory classification ambiguities that govern clinical viability. We therefore show how nano-targeted lutein can fill the neuroprotective and anti-inflammatory voids left by VEGF inhibition, offering a disease-modifying adjunct for early- to mid-stage DR. By coupling preclinical evidence with translational analysis, namely regulatory science, pharmacoeconomic considerations, and stage-adaptive implementation, we aim to advance lutein from a dietary supplement to a precision adjuvant therapy, yielding a clinically actionable framework rather than a technical compendium.

Neurovascular Unit Impairment in Diabetic Retinopathy

Structural and Functional Fundamentals of the Neurovascular Unit

The NVU forms the structural and functional basis of retinal homeostasis, supporting highly coordinated metabolic coupling and bidirectional signaling among neurons, glial cells, and vascular cells.^22,23 Photoreceptors, among the most energy-demanding retinal neurons, depend on the retinal pigment epithelium (RPE) and choriocapillaris for nutrient and oxygen supply.²⁴ Müller glial cells span the entire retinal thickness and provide metabolic support to neurons through glutamate uptake, K⁺ spatial buffering, and the lactate shuttle.^25,26

Beyond these supportive functions, Müller cells contribute to neurovascular coupling through functional interactions with retinal vascular endothelial cells and pericytes, thereby maintaining vascular integrity and neuronal homeostasis. Under chronic hyperglycemia, however, Müller cells undergo reactive gliosis, leading to reduced neurotrophic support, increased pro-inflammatory cytokine release, and disrupted intercellular signaling, which collectively drive NVU dysfunction and structural deterioration.^27,28

Hyperglycemia-Driven Progressive Damage to the Neurovascular Unit

Figure 1 schematically illustrates the progressive, stage-dependent disruption of the NVU under chronic hyperglycemia, encompassing neurodegeneration, BRB breakdown, and pathological neovascularization, thereby highlighting the rationale for stage-specific therapeutic interventions.

Figure 1.

Overview of hyperglycemia-driven progressive damage to the NVU in diabetic retinopathy. The NVU disintegration encompasses three sequential phases: early neurodegeneration; mid-stage BRB breakdown and late proliferative retinopathy. Gray arrows indicate the direction of pathological progression and crosstalk between disease stages. The upward arrow (↑) denotes upregulation or increased accumulation, whereas the downward arrow (↓) indicates downregulation or decreased expression. Colored stage headers (blue, Orange, red) correspond to early, middle, and late disease phases, respectively.

Early Stage: Neurodegeneration Preceding Microvascular Changes

Early-stage DR is characterized by prominent neurodegeneration preceding clinically evident microvascular lesions. Diabetic patients exhibit progressive thinning of the retinal nerve fiber layer, visual dysfunction, and ganglion cell loss before overt microvascular changes emerge.²⁹ Chronic hyperglycemia impairs mitochondrial electron transport chain function, leading to excessive superoxide generation, while concurrently activating the polyol pathway, the protein kinase C (PKC) signaling cascade, and the formation of advanced glycation end products (AGEs).³⁰ The hyperglycemic milieu triggers retinal neuronal apoptosis, aberrant glial activation, and neuroinflammatory responses.^31,32 Microglia activation contribute to early vascular functional perturbations through aberrant microglia-vascular crosstalk.^33,34 Concomitantly, reduced neurotrophic factor expression compromises neuroprotective capacity, further aggravating primordial NVU injury.³⁵

At this stage, neuroinflammation predominates in the absence of overt microvascular structural alterations, representing an optimal therapeutic window for pharmacological intervention through antioxidant and neuroprotective mechanisms. However, current clinical diagnostic modalities remain insufficient for identifying high-risk patients during the early neurodegenerative phase, which constitutes a major challenge to the clinical translation of nutritional intervention strategies.

Mid-Stage: Blood-Retinal Barrier Breakdown and Inflammatory Amplification

In the mid-stage, sustained neuroinflammation and ischemia drive progressive BRB breakdown, culminating in vascular hyperpermeability, retinal edema, and capillary non-perfusion.^36,37 Vascular inflammation accelerates pericyte dropout and endothelial injury, resulting in microvascular degeneration and focal ischemia.³⁸ IL-33 activates the NOX1-PKCδ signaling pathway, inducing phosphorylation of the tight junction (TJ) protein ZO-1, thereby compromising its structural and functional integrity.³⁹ TGF-β1 upregulation promotes basement membrane thickening, extracellular matrix remodeling, and increased vascular permeability.⁴⁰ Concurrently, hyperglycemia-induced impairment of mitophagy disrupts mitochondrial homeostasis and further aggravates BRB disruption.⁴¹ Sustained Müller cell activation further amplifies vascular leakage and neural damage.⁴²

The markedly increased BRB permeability at this stage provides a pathological basis for passive targeting via the enhanced permeability and retention (EPR) effect. However, BRB disruption exhibits substantial spatial heterogeneity across retinal quadrants and vascular hierarchies, which may result in heterogeneous nanomedicine distribution and remains a fundamental limitation of passive targeting in clinical translation.

Late Stage: Proliferative Retinopathy and NVU Collapse

The late proliferative stage is characterized by pathological neovascularization driven by chronic ischemia and inflammatory stimuli, marking extensive disruption of NVU structure and function.⁴³ In proliferative DR, aberrant overexpression of pro-angiogenic factors, notably VEGF, drives pathological neovascularization, accompanied by activation of the Netrin-1/CD146 signaling axis and ceramide metabolic dysregulation.^44–46 These neovessels lack adequate pericyte coverage, are structurally immature and highly fragile, and are prone to leakage, predisposing to severe complications including vitreous hemorrhage and tractional retinal detachment.^47,48 Although anti-VEGF therapies can partially control vascular leakage and neovascularization, they fail to reverse fibrovascular membrane formation and the associated structural damage.^49,50

At this advanced stage, neurodegeneration and vascular damage are largely irreversible; the therapeutic value of neuroprotective agents such as lutein therefore lies mainly in delaying progression in the fellow eye and reducing injection frequency when used adjunctively with anti-VEGF therapy, rather than in serving as a standalone disease-modifying treatment. This underscores the critical importance of early intervention.

Given the limitations of current anti-VEGF therapies and laser photocoagulation, DR management strategies must shift from control of late-stage vascular complication toward early preservation of the NVU, and from single-target blockade toward multi-target synergistic modulation. We therefore position lutein not as a replacement for anti-VEGF agents but as a disease-modifying adjuvant that protects neurovascular components beyond VEGF blockade. Its multitarget profile may thus complement vascular-targeted therapy and reduce treatment burden, provided that nanodelivery overcomes its bioavailability limitations.

Multi-Target Protective Mechanisms of Lutein in Diabetic Retinopathy-Associated Neurovascular Unit Impairment

Antioxidant Defense and Preservation of Cellular Integrity

Sustained hyperglycemia-induced oxidative stress represents a central mechanism underlying NVU damage in DR.⁵¹ With potent antioxidant activity, lutein attenuates hyperglycemia-induced oxidative damage by scavenging free radicals and suppressing excessive ROS generation.^52–54 Photoreceptor outer segment disc membranes are enriched in polyunsaturated fatty acids, rendering them highly susceptible to oxidative stress.⁵⁵ As a core constituent of macular pigment, lutein protects the retina against photochemical damage by selectively filtering short-wavelength blue light and exerting antioxidant effects.^53,56

In ARPE-19 cells, lutein induces Nrf2 nuclear translocation and upregulates heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase 1 (NQO1). Under hyperglycemic conditions, lutein reverses high-glucose-induced impairment of Nrf2 nuclear translocation, indicating that its antioxidant effects are mediated, at least in part, through activation of the Nrf2 pathway.^57,58 However, studies on lutein-mediated Nrf2 activation have been limited to RPE cell lines and require validation in co-culture models of the NVU. Furthermore, in vitro hyperglycemic models fail to recapitulate the complex microenvironment of the diabetic retina, in which sustained oxidative stress, inflammatory mediators, and AGEs coexist and interact. Consequently, Nrf2 pathway activation currently remains a mechanistically plausible hypothesis supported by in vitro data rather than a clinically validated therapeutic target.

Neuroprotection and Signaling Convergence

In early-stage DR, retinal ganglion cell (RGC) apoptosis frequently precedes clinically evident microvascular lesions.^59,60 As a macula-enriched carotenoid, lutein exerts neuroprotective effects that extend beyond antioxidant scavenging alone.⁶¹ In vitro studies indicate that lutein modulates pro-survival signaling pathways, including PI3K/AKT and MAPK/ERK, and upregulates neuroplasticity-related molecules, including brain-derived neurotrophic factor (BDNF) and growth-associated protein 43 (GAP-43), thereby potentially counteracting hyperglycemia-induced RGC apoptosis and axonal degeneration.⁶²

Moreover, in DR, the hyperglycemic milieu and pro-inflammatory cytokines synergistically disrupt glutamate homeostasis by impairing glutamine synthetase-mediated glutamate recycling. Concomitantly, reduced activity of the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) exacerbates energy metabolic disturbance. This metabolic dysregulation elevates extracellular glutamate concentrations, leading to excessive activation of N-methyl-D-aspartate (NMDA) receptors, calcium influx, mitochondrial dysfunction, and ultimately neuronal apoptosis.⁶³ Lutein effectively attenuates glutamate-induced oxidative stress, neuroinflammation, and dysregulated iron metabolism.⁶⁴ However, its direct regulatory effects on glutamate transporter expression and function in diabetic NVU co-culture systems and in vivo models remain to be elucidated.

Regulation of Vascular Structure and Microcirculatory Homeostasis

Functional coupling within the NVU among neurons, glial cells, and vascular endothelial cells depends on coordinated intercellular signaling.⁶⁵ As a critical NVU constituent, the functional status of Müller cells profoundly influences neurovascular coupling efficiency.²⁸ Under hyperglycemic conditions, the antioxidant defense capacity of Müller cell is compromised, establishing a retinal glial-vascular-neuronal vicious cycle. Although lutein has been shown to attenuate glial reactivity and exert anti-inflammatory effects in Müller cell cultures and in a retinitis pigmentosa mouse model,^66,67 direct evidence that lutein ameliorates DR-associated vascular lesions in diabetic animals remains limited.

Furthermore, in blue light-induced retinal degeneration models, lutein nanoparticles (NPs) have shown vascular normalization effects, including reduced abnormal vascular area, shortened mean vessel length, and decreased pathological vascular junctions. These findings suggest that nanocarrier technology not only enhances lutein bioavailability but also improves its spatial distribution profile, thereby enhancing its structural reparative effects on the retinal microvascular network.⁵⁶ Nevertheless, direct evidence for lutein-mediated improvement of DR-associated vascular lesions remains limited, and barrier function indices, including TJ protein expression and transendothelial electrical resistance (TEER), have been insufficiently assessed.

Integrative Perspective: Lutein as a Multitarget NVU Modulator

Collectively, the foregoing mechanistic analyses demonstrate that lutein-mediated protection against DR-associated NVU impairment is characterized by pronounced multitarget features (Figure 2). At the molecular level, lutein integrates regulation of redox homeostasis and inflammatory response through the Nrf2/ARE, PI3K/Akt, MAPK/ERK, and NF-κB pathways. At the cellular level, it concurrently targets core NVU constituents, including the RPE, photoreceptors, RGCs, and Müller cells. At the systems level, it preserves overall retinal function by enhancing neurovascular coupling and metabolic crosstalk. However, the effective dose windows for these distinct targets may not converge, precluding simultaneous optimization of all pathways with a single dose. Moreover, the optimal intervention window for lutein varies with disease stage. Consequently, effective lutein intervention as an adjunct to standard anti-VEGF therapy requires not only enhanced bioavailability but also spatiotemporally controlled release to achieve on-demand therapeutic modulation that fills the neuroprotective gaps left by VEGF inhibition alone.

Figure 2.

Overview of lutein as a multitarget modulator of NVU impairment in diabetic retinopathy. Lutein exerts antioxidant defense, neuroprotection, and vascular homeostasis. It targets redox/inflammatory pathways, core NVU constituents and neurovascular coupling. Blue curved arrows indicate the directional flow of lutein-mediated protective effects toward target pathways. Black straight arrows indicate activation or upregulation of signaling pathways or molecules. Red perpendicular bars (┴) indicate inhibition or suppression of pathological processes. The upward arrow (↑) denotes increased expression or activity of the indicated molecules (HO-1, NQO1, BDNF, GAP-43).

Lutein Nano-Delivery Platforms and Their Suitability for Diabetic Retinopathy

Polymer-Based Nanoplatforms

Poly(Lactic-Co-Glycolic Acid) (PLGA) Nanoparticles

PLGA is an FDA-approved biodegradable polymer that degrades into lactic and glycolic acids and has an excellent biosafety profile.⁶⁸ Lutein-loaded PLGA NPs demonstrate favorable biocompatibility in human lens epithelial cells in vitro and, following topical administration in vivo, significantly attenuate ROS production penetrating the ocular surface to reach the anterior chamber and lens.⁶⁹ These NPs are detectable in ocular tissues, including the cornea and choroid, after topical administration in non-diabetic animals. However, inner retinal accumulation is limited by rapid clearance, indicating an insufficient capacity to traverse the inner blood-retinal barrier (iBRB).⁷⁰ Consequently, PLGA NPs are better suited to ocular surface diseases or to sustained-release systems for systemic administration than to direct retinal NVU-targeted delivery. It is important to note that the antioxidant evidence derives from in vitro monoculture, whereas the distribution data derive from non-diabetic in vivo models; neither directly demonstrates NVU protection in diabetic retinopathy.

Chitosan-Based Nanoparticles

Chitosan, a natural cationic polysaccharide, exhibits mucoadhesive properties and biodegradability. Chitosan NPs complexed with phosphatidylcholine enhance the sustained-release properties and transmembrane transport of lutein.⁷¹ Sodium alginate Ca²⁺-crosslinked hydrogels are pH-sensitive and show differential swelling across pH environments, with lutein-loaded hydrogels undergoing enzymatic degradation to release the drug.⁷²

Chitosan-oleic acid-sodium alginate hybrid nanocarriers exploit the mucoadhesive properties of chitosan and the pH sensitivity of sodium alginate to improve intestinal permeability in diabetes, significantly enhancing lutein bioavailability in diabetic rats and protecting ARPE-19 cells from H₂O₂-induced oxidative stress in vitro, thereby providing direct preclinical evidence relevant to DR treatment.⁷³ However, delayed gastric emptying and intestinal pH alterations in diabetic patients may compromise the predictability of release kinetics, necessitating careful evaluation of systemic administration safety.

Protein-Based Nanoparticles

Protein-based carriers enable retinal targeting through ligand-receptor interactions, and their tunable particle size makes them suitable for BRB traversal after systemic administration. Lutein-loaded glycosylated casein nanocarriers effectively protect RPE cells from oxidative damage, potentially through interactions between casein phosphopeptides and RPE cell surface receptors.⁷⁴ Casein-xanthan gum co-assembled nanocarriers exhibit pH-responsive release profiles.⁷⁵ Chitooligosaccharide-modified corn peptide-lutein NPs significantly improve physicochemical stability and bioactivity.⁷⁶ Heat-treated soy protein isolate nanocarriers show markedly enhanced encapsulation efficiency and storage stability.⁷⁷ However, under diabetic conditions, non-enzymatic glycation of endogenous proteins may generate glycated species that compete with exogenous carriers for RPE receptor binding, thereby diminishing targeting efficiency. Furthermore, repeated systemic administration carries the risk of inducing anti-protein antibodies and accelerating carrier clearance.

From a translational perspective, ligand-receptor targeting offers a viable route for systemic BRB traversal, yet clinical advancement depends on resolving glycation-mediated competition and immunogenicity risks in diabetic patients. The current lack of in vivo BRB-crossing data in diabetic models confines this platform to early-stage exploration.

Lipid-Based Nanoplatforms

Liposomes and Phytosomes

The phospholipid bilayer of liposomes facilitates transmembrane drug transport. Lutein nanoliposomes coated with chitosan/epigallocatechin gallate (EGCG) complexes have been systematically characterized, and supercritical carbon dioxide (scCO₂) technology enables efficient lutein encapsulation, with reported encapsulation efficiencies of 56.7–97.0%.^78,79 A lutein liposome system coated with natural polysaccharide-modified zeolitic imidazolate framework-8 (ZIF-8) integrates the protective advantages of metal-organic frameworks (MOFs) and the drug-loading capacity of liposomes. ZIF-8 remains stable under gastric acidic conditions and protects lutein from degradation. Upon entry into the neutral-to-weakly alkaline intestinal milieu, the framework disintegrates and releases liposomes for transmembrane absorption, thereby enabling pH-responsive gastrointestinal targeting.⁸⁰ However, the potential cytotoxicity of zinc ions and the translational challenges of MOFs-based materials remain to be addressed.

Liposomes represent the most clinically validated nanocarrier class in ophthalmology, suggesting a favorable regulatory trajectory for ocular applications. Nevertheless, lutein-specific liposomal formulations for DR remain at the preclinical stage, and their translational potential depends on validation in diabetic models with retinal efficacy endpoints.

Solid Lipid Nanoparticle (SLN) and Nanostructured Lipid Carriers (NLC)

The principal advantages of SLN/NLC in DR lie in their capacity for retinal cell uptake through lipid metabolic pathways and their ability to enhance oral bioavailability. Lutein-loaded nanoemulsions (NE), SLN, and NLC exhibit bioavailability ranging from 42.61% to 62.81%, with all formulations showing favorable flowability.⁸¹ Lutein-loaded SLN significantly enhance dissolution in simulated intestinal fluid in vitro, with dissolved amounts at 2 h being at least 50-fold higher than those of crystalline lutein, and enhance oral relative bioavailability by 3.7-fold.⁸² Although these data demonstrate improved systemic exposure, they do not provide direct evidence of enhanced retinal NVU protection in diabetic models, because existing bioavailability data are derived predominantly from plasma pharmacokinetics, while direct evidence of retinal tissue distribution remains limited.

This improvement is particularly relevant for patients with DR, as diabetes is frequently associated with gastrointestinal motility disorders and impaired absorptive function, which further compromising the bioavailability of conventional lutein supplements. Nevertheless, the application of SLN/NLC in DR faces several challenges. Dyslipidemia in diabetic patients can alter the in vivo distribution and clearance kinetics of nanolipid particles, and polymorphic transitions of solid lipids pose a risk of drug expulsion during long-term storage, necessitating stringent formulation stability. From a translational standpoint, given their food-grade composition and established safety profile, SLN/NLC hold the strongest near-term potential as oral nutraceutical formulations for early-stage DR; however, retinal targeting efficiency must be validated in diabetic models before clinical advancement.

Exosome-Based Nanocarriers

Naturally Derived Exosome-Like Nanovesicles for Lutein Delivery

Exosomes, a subpopulation of extracellular vesicles (EVs), are endosome-derived, naturally secreted membrane-bound vesicles 50–150 nm in size with biocompatibility, stability, low immunogenicity, and easy detectability in bodily fluids. The potential to engineer specific exosomal cargo renders them attractive as therapeutic delivery vehicles for ocular drug delivery.⁸³ Plant-derived exosome-like nanovesicles (PENs) have demonstrated feasibility as lutein/zeaxanthin (xanthophyll) carriers. Carrot PENs from both juice and callus cultures achieve up to ~80% encapsulation efficiency via electroporation, while xanthophyll-encapsulated callus-derived PENs maintain >95% cell viability in ARPE-19 cells under oxidative stress, outperforming both juice-derived PENs and free carotenoid extracts.⁸⁴ In a rat model of DR, MSC-derived small EVs (MSC-sEVs) serve as a drug delivery system that prolongs the therapeutic effect of bevacizumab in DR, supporting the potential of sEVs to sustain drug delivery.⁸⁵

Advantages and Limitations

As a cell-free regenerative therapy, sEVs offer marked advantages in the treatment of ocular diseases, including intrinsic BRB penetrability, low immunogenicity, and dual functionality as both a therapeutics and delivery vehicles.^86,87 However, their clinical translation still faces substantial challenges, including batch-to-batch heterogeneity, Good Manufacturing Practice (GMP)-compliant manufacturing complexity, limited loading capacity for hydrophobic drugs such as lutein, ambiguous regulatory classification, and the potential for glycation and oxidative modification under diabetic conditions to compromise membrane integrity.^86,88,89 To date, no exosome-based ophthalmic therapy has received regulatory approval. Therefore, standardized isolation protocols, robust quality control metrics, and large-animal efficacy studies incorporating lutein co-loading are necessary prerequisites for clinical translation.

Carrier-Free and Biomimetic Nanoplatforms

Lutein Nanocrystals

Nanocrystals enable drug nanosizing without carrier materials, thereby avoiding potential carrier toxicity, while their exceptionally high drug loading makes them suitable for intraocular administration.^90,91 These formulations markedly enhance oral lutein bioavailability, which can be further augmented by orally disintegrating films.^92,93 Oral administration offers convenience and compliance advantages for long-term supplementation in DR patients; nevertheless, nanocrystal applications are subject to significant limitations. The carrier-free design lacks surface functionalization sites, precluding BRB traversal and targeted uptake by retinal cells. Moreover, polymorphic transitions of lutein within the fluctuating gastrointestinal milieu of diabetic patients may compromise dissolution and absorption. The intraocular physical stability and tolerability of nanocrystal suspensions remain to be validated in diabetic models.

Cell Membrane-Coated Biomimetic Nanoparticles

Biomimetic strategies coat nanoparticle cores with natural cell membranes, thereby conferring self-recognition and immune evasion properties. Lutein-loaded macrophage membrane-functionalized biomimetic NPs impede macrophage phagocytosis and facilitate targeting of activated endothelial cells.⁹⁴ Erythrocyte–endothelial hybrid membrane coatings have also additionally demonstrated endothelial targeting and immune evasion capabilities, enabling selective accumulation within retinal vessels and suppression of VEGF overexpression.⁹⁵

In DR, biomimetic NPs exhibit distinctive mechanistic adaptability. Activated macrophages and microglia in the diabetic retina and choroid serve as homing signals, guiding nanoparticle enrichment at lesion sites; surface anti-inflammatory molecules on the macrophage membrane may suppress local inflammation and act synergistically with lutein to exert anti-inflammatory effects. Nevertheless, clinical translation confronts substantial challenges. Membrane extraction and preservation are prone to batch-to-batch variation; lipid peroxidation and glycation of erythrocyte membranes in diabetic patients may compromise their self-recognition properties; and achieving uniform and stable cell membrane coatings is considerably more difficult than for synthetic materials.

Platform Selection for Diabetic Retinopathy

The major lutein nano-delivery platforms discussed above are schematically classified in Figure 3 into polymer-based, lipid-based, EV-based, and carrier-free or biomimetic systems, each contributing to retinal NVU protection through distinct barrier-crossing strategies. Oral platforms (SLN/NLC, nanocrystals) hold near-term potential for early-stage DR, whereas actively or biomimetically targeted systems better serve mid- to late-stage disease requiring efficient BRB traversal. A comparative summary of platform characteristics, therapeutic potential, and translational challenges is provided in Table 1 to guide rational selection across DR stages.

Figure 3.

Schematic classification of lutein nano-delivery platforms for retinal neurovascular unit (NVU) protection. Platforms are grouped into four classes according to constituent materials: polymer-based (PLGA, chitosan, and protein NPs), lipid-based (liposomes and SLN/NLC), extracellular vesicle-based (exosomes and MSC-sEVs), and carrier-free or biomimetic systems (lutein nanocrystals and cell membrane-coated NPs). Each platform is positioned around the central goal of retinal NVU protection, reflecting distinct delivery routes and blood-retinal barrier-crossing strategies.

Table 1.

Technical Characteristics and DR Compatibility of Nano-Delivery Platforms

Nanoplatform	Nanocarrier System	Key Characteristics and Mechanism	Therapeutic Potential in DR	Limitations and Clinical Translation Challenges	Primary Evidence Base	Refs.
Polymer-based	PLGA NPs	FDA-approved polymer with good biocompatibility	Lowers ROS; reaches anterior segment after topical dosing	Poor iBRB crossing; rapid clearance	In vitro ROS assays; ocular distribution in non-diabetic animals	[68–70]
	Chitosan-based NPs	Mucoadhesive with pH-sensitive release	Sustained release; higher oral bioavailability	GI pH and gastric emptying alter release	In vitro release and ARPE-19 protection; oral PK in diabetic rats	[71–73]
	Protein-based NPs	Ligand-receptor targeting with tunable size	Systemic BRB crossing; RPE protection	Glycation-mediated receptor competition; immunogenicity	In vitro RPE protection; in vivo BRB crossing unvalidated	[74–77]
Lipid-based	Liposomes	Bilayer structure with pH-responsive release	High loading (56.7% to 97.0%); GI-targeted release	Zinc ion toxicity and unclear translation path for MOF systems	In vitro encapsulation; GI release in non-diabetic models	[78–80]
	SLN/NLC	Uptake via lipid metabolic pathways	Oral bioavailability increased 3.7-fold; dissolution increased more than 50-fold	Dyslipidemia alters distribution; drug expulsion during storage	In vitro dissolution and plasma PK; retinal distribution scarce	[81,82]
EV-based	Exosomes/sEVs	Natural vesicles with immune evasion and BRB crossing	Anti-inflammatory; prolongs anti-VEGF effect; allows lutein co-delivery	Batch heterogeneity; low hydrophobic loading; costly GMP; unclear regulation	PEN lutein loading in vitro; MSC-sEV efficacy in diabetic rats	[83–89]
Carrier-free and Biomimetic	Lutein nanocrystals	Carrier-free with high drug loading	No carrier toxicity; better oral uptake and compliance	No sites for BRB targeting; ocular tolerability unproven	In vitro dissolution and oral bioavailability	[90–93]
	Cell membrane-coated NPs	Self-recognition and immune evasion	Targets activated endothelium and macrophages	Complex manufacture with batch variation; glycation effects	In vitro uptake; endothelial targeting in non-diabetic models	[94,95]

Targeted Delivery Mechanisms of Nano-Lutein Across the Blood-Retinal Barrier

Anatomical Architecture and Delivery Window of the Blood-Retinal Barrier

The BRB constitutes both the principal obstacle and a potential conduit for nanoparticle trans-barrier delivery (Figure 4A).⁹⁶ The outer blood-retinal barrier (oBRB) is formed by RPE tight junctions separating the choriocapillaris from the subretinal space and mediating selective exchange through transporters such as glucose transporter 1 (GLUT1).^97–99 At the iBRB, retinal microvascular endothelial cells (RMECs) are connected by specialized tight junctions containing TJ proteins, including claudin-5, occludin, and zonula occludens-1 (ZO-1).⁹⁷ Nanomedicine distribution within the eye and penetration across the BRB are governed by multiple physicochemical factors, including particle size and surface charge. Surface charge further affects trans-BRB transport: negatively charged carbon dots penetrate the BRB more rapidly than positively charged counterparts.¹⁰⁰ Moreover, nanomaterial surface hydrophilicity/hydrophobicity affects ocular surface bioavailability and RPE permeation efficiency.¹⁰¹

Figure 4.

Schematic overview of nano-lutein delivery across the blood-retinal barrier (BRB) in diabetic retinopathy. (A) BRB architecture: the inner BRB (iBRB) comprises retinal microvascular endothelial cells (RMECs) joined by tight junction (TJ) proteins (claudin-5, occludin, ZO-1), with pericyte and glial cell coverage; the outer BRB (oBRB) is formed by retinal pigment epithelial cells (RPEs) expressing TJ proteins and GLUT1, adjacent to photoreceptors (rods and cones). (B) Stage-dependent BRB permeability: an intact barrier in early-stage disease, focal breakdown with heterogeneous enhanced permeability and retention (EPR) windows in the middle stage, and extensive neovascularization with inflammatory infiltration in the late stage. (C) Active targeting: ligand-functionalized NPs selectively bind receptors on RPEs, photoreceptors, and retinal vessels. (D) Stimuli-responsive release: nanocarriers release lutein in response to pathological cues of the diabetic retina, including reactive oxygen species (ROS), hyperglycemia (Glu), and acidification (pH). Black arrows indicate nanoparticle transport and drug release.

Passive Targeting: Pathological Microenvironment-Enhanced Permeation and Retention

Passive targeting exploits the increased vascular permeability caused by BRB breakdown in DR, enabling NPs to extravasate preferentially into retinal lesions and accumulate in a size-dependent manner (Figure 4B). During DR progression, BRB integrity is compromised by multiple pathological insults, manifesting as vascular leakage, inflammatory infiltration, and disruption of TJ protein expression, creating a pathological window for nanomedicine delivery across the BRB.^102,103

The applicability of the EPR effect in DR is stage-dependent and spatially heterogeneous. In early-stage DR, barrier breakdown is modest and may be insufficient to produce substantial nanoparticle accumulation. In the mid-stage, focal vascular leakage and inflammatory infiltration create discrete EPR windows, yet permeability varies across retinal quadrants and vascular hierarchies, creating a risk of heterogeneous drug distribution. In proliferative DR, extensive neovascularization paradoxically increases interstitial pressure, potentially impairing convective nanoparticle penetration. Moreover, diabetic dyslipidemia alters plasma protein corona formation, opsonization profiles, and mononuclear phagocyte clearance kinetics, further compromising the predictability of EPR-mediated targeting. Consequently, reliance on EPR alone is insufficient for precision NVU protection; it must be integrated synergistically with active targeting and stimuli-responsive release.

Mitochondrial damage, NADPH oxidase activation, and dysregulation of the polyol pathway, PKC signaling, and AGE pathways result in excessive ROS generation, directly damaging vascular endothelium and RPE cells while triggering pro-inflammatory cytokine release and microglial activation.^104–106 Hyperglycemia downregulates ZO-1 and occludin expression via IL-33-mediated activation of the NOX1-PKCδ-ZO-1 signaling axis, while TGF-β1 upregulation promotes basement membrane thickening and increased vascular permeability.^39,40 Moreover, mitochondrial dysfunction and impaired mitophagy induced by hyperglycemia have been demonstrated to accelerate iBRB breakdown.^41,107

Active Targeting: Neurovascular Unit Cell-Specific Ligand Functionalization

Active targeting achieves precise BRB traversal and retinal cell-selective delivery by functionalizing nanocarrier surfaces with NVU cell-specific ligands that exploit high-affinity ligand-receptor recognition (Figure 4C). The surface of NVU component cells expresses diverse specific receptors, providing a molecular basis for ligand-mediated trans-barrier transport. In brain delivery studies in vivo, receptor-directed engineering through ligands targeting transferrin receptor 1 (TfR1) or alkaline phosphatase has improved nanocarrier blood-brain barrier (BBB) traversal, a principle that may be extrapolated to BRB-targeted design.¹⁰⁸ Extrapolation of these BBB strategies to the BRB is currently a theoretical proposition, complicated by substantial anatomical and functional disparities. The expression variability of TfR1 under DR pathological conditions, its transport characteristics across the BRB, and plasma transferrin competition require direct in vivo validation in diabetic retinal models before clinical relevance can be assumed.

Peptide-based systems, leveraging biological specificity and nanoscale precision, have emerged as pivotal tools for breaching the retinal barrier.¹⁰⁹ When conjugated to microvesicles, the photoreceptor-targeting peptide MH42 significantly improves photoreceptor targeting and repair efficiency and ameliorates retinal dysfunction in DR models.¹¹⁰ However, the streptozotocin (STZ) model employed in this study recapitulates only acute hyperglycemia and lacks the typical microvascular lesions and neurodegeneration characteristic of DR. The targeting mechanism of MH42 relies on binding to photoreceptor outer segment disc membranes, and structural damage and phagocytic dysfunction of photoreceptor outer segments in DR may compromise its binding efficiency. Consequently, the targeting efficiency of MH42 in chronic DR requires independent validation, and its performance across non-proliferative and proliferative stages warrants further evaluation.

Biomimetic hybrid NPs ([RBC-EC]-NPs) achieve endothelial targeting and immune evasion via erythrocyte-endothelial cell membrane coatings, selectively accumulating within retinal vessels while suppressing VEGF overexpression.⁹⁵ Ligand-functionalized nanocarriers can enhance the targeting precision and therapeutic potential of nano-lutein, efficiently delivering it to targets including the RPE and photoreceptors. This strategy offers the dual advantages of endothelial targeting and inflammatory tropism. Nevertheless, glycation and oxidative damage compromise RBC membrane deformability and alter surface molecular conformation in diabetic patients; consequently, NPs coated with membranes from healthy donors may show reduced immune evasion efficiency in vivo. Moreover, inter-batch variations in membrane ratio and orientation of biomimetic hybrid NPs may compromise product uniformity.

Stimuli-Responsive Drug Release: Lesion Microenvironment-Triggered Precision Drug Delivery

Stimuli-responsive drug delivery strategies enable spatiotemporally controlled drug release at target sites through the design of intelligent nanocarriers sensitive to specific cues within the DR lesion microenvironment (Figure 4D). DR retinal tissue exhibits three characteristic pathological cues: elevated oxidative stress, microenvironmental acidification, and high glucose concentration, providing ideal triggering conditions for intelligent responsive drug delivery systems. Nevertheless, the temporal dynamics and spatial heterogeneity of these three cues during DR progression present significant challenges to the precision of stimuli-responsive design.

ROS-responsive drug delivery represents a pivotal design strategy for lutein nanoformulations. Methionine-based ROS-responsive polymeric micelle systems offer a valuable paradigm for the development of ROS-responsive lutein nanoformulations.¹¹¹ Nano-lutein formulations have shown ROS-responsive release, allowing controlled lutein release in the highly oxidative retinal microenvironment and thereby exerting potent antioxidant and anti-apoptotic effects. Nevertheless, oxidative stress profiles vary across DR stages: early-stage DR is characterized by mitochondrial superoxide predominance, mid-stage by NADPH oxidase-derived ROS, and late-stage by a potential decline due to antioxidant system exhaustion, with the therapeutic window potentially narrowing as the disease progresses. Systemic inflammation, surgical intervention, or infection can additionally elevate local ROS levels, potentially resulting in off-target release. Moreover, alignment of release kinetics with pharmacodynamics is equally critical: rapid release may result in supratherapeutic local concentrations, whereas slow release may fail to achieve therapeutic concentrations during periods of ROS fluctuation.

pH-responsive drug delivery can be synergistically combined with ROS-responsive systems. Nano-lutein formulations undergo structural transformation within the acidic microenvironment of retinal lesion areas, facilitating targeted lutein release.⁵⁶ Nevertheless, the pH reduction in retinal ischemic regions is modest, presenting challenges for the design of activation thresholds of pH-sensitive materials. Glucose-responsive drug delivery can enable intelligent release in hyperglycemic microenvironments through reversible phenylboronic acid (PBA)-glucose binding or glucose oxidase (GOx)-catalyzed acid production and can deliver NPs to the RPE to mitigate BRB damage; however, DR-specific evidence remains limited.^112,113 Critical issues remain to be validated in DR models, including alignment of the glucose affinity of PBA systems with the physiological glucose range, GOx stability and immunogenicity in retinal tissue, and synchronization of response kinetics with glycemic fluctuations.

Synergy and Optimization of Targeting Strategies

Efficacy Bottlenecks of Single Targeting Modalities

Single targeting strategies encounter efficacy bottlenecks in addressing the complex NVU pathological network of DR. Passive targeting depends on BRB breakdown, which varies substantially across DR stages and among individuals, as critically appraised in Passive Targeting: Pathological Microenvironment-Enhanced Permeation and Retention; active targeting can enhance cell-specific uptake, but receptor expression is dynamically modulated by disease state, and high-affinity ligands may induce receptor downregulation or lysosomal degradation.

Toward Multifunctional Nanosystems with Integrated Targeting-Penetration-Release Synergy

Consequently, synergistically integrating passive and active targeting with stimuli-responsive release to construct multifunctional nanosystems that optimize targeting, penetration, and release represents the pivotal technical pathway for precise lutein-mediated NVU protection. Nevertheless, the complexity of synergistic integration introduces additional risks. Surface ligand functionalization may alter nanoparticle size and surface charge, and the incorporation of stimuli-responsive materials may compromise targeting ligand stability. From a manufacturing perspective, multistep functionalization and multifunctional integration increase batch-to-batch variability and complicate categorical assignment, thereby exacerbating regulatory uncertainty.

Stage-Adapted and Personalized Implementation Strategies

Future optimization of nano-lutein systems should adhere to stage-adapted, personalized, and simplification-first principles. In the early disease stage, oral or systemic lutein administration should be prioritized as a preventive nutritional adjunct, emphasizing neuroprotection and BRB maintenance before anti-VEGF therapy becomes necessary; in the mid-stage, active targeting systems should be integrated with existing anti-VEGF regimens to address barrier repair and inflammation that anti-VEGF therapy alone cannot fully resolve; and in the late stage, sustained-release systems combined with anti-VEGF therapy and surgical intervention should be used to control complications and reduce re-treatment frequency. Targeting strategies should be individualized based on patient-specific BRB permeability. Moreover, when functionally equivalent, designs with fewer components and more streamlined manufacturing should be prioritized to enhance clinical translation feasibility.

No single carrier platform or targeting strategy is universally optimal for DR. Efficacy depends instead on the interplay among carrier properties, disease-stage-dependent BRB integrity, and the selected targeting modality. Table 2 synthesizes the major systems discussed in Lutein Nano-Delivery Platforms and Their Suitability for Diabetic Retinopathy and Targeted Delivery Mechanisms of Nano-Lutein Across the Blood-Retinal Barrier, aligning each platform with its targeting strategy, retinal delivery efficiency, and therapeutic outcomes. Passive strategies generally improve systemic or anterior segment exposure but show limited inner retinal accumulation. Active and biomimetic approaches achieve superior BRB penetration yet face greater manufacturing and regulatory complexity. Stimuli-responsive systems offer the highest theoretical precision, but direct lutein-DR evidence is lacking and stage-specific triggers remain to be validated. This overview aims to guide selection of carrier-strategy combinations suited to specific translational objectives.

Table 2.

Comparative Overview of Nanocarrier Systems, Targeting Strategies, Retinal Delivery Efficiency, and Therapeutic Outcomes in Diabetic Retinopathy

Nanoplatform	Nanocarrier System	Targeting Strategy	Retinal Delivery Efficiency	Therapeutic Outcomes	Refs.
Polymer-based	PLGA NPs	Passive (topical, EPR-limited)	Detectable in cornea and choroid; poor iBRB crossing with rapid clearance	ROS attenuation in anterior segment; good ocular surface tolerance	[68–70]
	Chitosan-based NPs	Passive (oral, pH-responsive)	Enhanced intestinal permeability and oral bioavailability in diabetic rats	ARPE-19 protection against H2O2-induced oxidative stress	[71–73]
	Protein-based NPs	Active (ligand-receptor, systemic)	Size-tunable for BRB traversal and RPE uptake; in vivo crossing unvalidated	RPE protection against oxidative damage	[74–77]
Lipid-based	Liposomes	Passive (oral, pH-responsive) or active	Encapsulation efficiency 56.7% to 97.0%; GI-targeted release; retinal data scarce	Lutein protected from gastric degradation	[78–80]
	SLN/NLC	Passive (oral, lipid pathway)	Oral bioavailability increased 3.7-fold; dissolution more than 50-fold at 2 h; retinal distribution limited	Better dissolution for patients with impaired GI function	[81,82]
EV-based	Exosomes/sEVs	Active (innate BRB crossing)	Cross BRB without synthetic ligands; prolong anti-VEGF effect in diabetic rats	Lower injection frequency; no lutein-EV data in DR	[83–89]
Carrier-free and Biomimetic	Lutein nanocrystals	Passive (oral dissolution)	Oral bioavailability improved and further augmented by orally disintegrating films; no BRB-targeting functionalization	High loading; no carrier toxicity; no retinal efficacy data in DR	[90–93]
	Cell membrane-coated NPs	Active (biomimetic)	Selective accumulation in retinal vessels; immune evasion; batch-to-batch variation	VEGF suppression; endothelial and macrophage targeting	[94,95]

Preclinical Evidence for Nano-Lutein in Neurovascular Injury Intervention

In vitro Models and Cellular Safety

Lutein nanoformulations demonstrate antioxidant and protective effects in in vitro models, including RPE cells.^74,114–116 Nevertheless, existing in vitro evidence lacks validation through neurovascular unit multicellular co-culture systems, and predominantly focuses on cell viability, with insufficient assessment of tight junction integrity, inflammatory cytokine secretion profiles, and metabolic function; exposure conditions lack physiological relevance and fail to recapitulate pharmacokinetic processes following systemic administration.

Regarding cytotoxicity assessment, optimally designed nanoformulations typically demonstrate favorable cytocompatibility within the recommended dose range.^{114,117–119} Nevertheless, different endpoints reflect distinct injury mechanisms, and exposure concentrations and durations are not standardized; most studies employ only a single epithelial cell line, precluding toxicity assessment of key NVU components. Moreover, the physicochemical properties of NPs may elicit distinct biological effects, including lysosomal membrane permeabilization, inflammasome activation, and uptake saturation, which may evade detection by conventional assays.

In vivo Efficacy and Pharmacokinetics

Nanotechnology significantly enhances lutein bioavailability through mechanisms encompassing improved dissolution rate, enhanced intestinal absorption, and receptor-mediated endocytosis.^{81,82,92,93,120–125} Co-assembled natural medicinal NPs enable effective drug distribution within deep retinal tissue.¹²⁶ Regarding efficacy, nanoformulation strategies significantly enhance photoprotective effects by augmenting outer retinal accumulation, and demonstrate advantages in neuroprotection and functional restoration of dopaminergic and cholinergic systems.^127–130 Nevertheless, most studies employ free lutein rather than clinical commercial supplements as comparators, and retinal tissue efficacy endpoint data remain limited. Moreover, the intervention duration in most animal studies spans merely 2–8 weeks, whereas clinical management of DR extends over years; the long-term efficacy stability and potential tolerability of nanoformulations remain to be established.

Local and Systemic Safety

Local Safety Considerations

Regarding local safety, optimally designed nano-lutein formulations typically demonstrate favorable corneal biocompatibility at recommended doses.^131–134 Nevertheless, corneal neuropathy, epithelial basement membrane abnormalities, and compromised tear film stability in DR patients may prolong nanoparticle corneal residence time, thereby increasing epithelial toxicity and inflammatory risk; diminished corneal endothelial cell functional reserve necessitates particular attention to endothelial contact risk following nanoparticle penetration; physicochemical interactions with artificial tears, antibiotics, or anti-glaucoma agents may alter safety profiles. Diabetes-specific corneal safety data and evidence for long-term repeated administration remain to be accumulated.

Systemic Safety: Immunogenicity and Clearance Kinetics

Systemically administered lutein-loaded NPs are rapidly recognized and sequestered by the mononuclear phagocyte system (MPS), with macrophage-mediated clearance eliminating the majority of the administered dose and compromising therapeutic efficacy in chronic diabetic retinopathy.¹³⁵ After entering the circulation, NPs are rapidly recognized and cleared by mononuclear phagocytes. They accumulate mainly in the liver and spleen, which drastically reduces retinal exposure.^136,137 Chronic diabetic retinopathy requires repeated dosing. This repeated exposure increases the risk of the accelerated blood clearance (ABC) phenomenon.¹³⁸ Initial exposure can induce anti-PEG or anti-protein antibodies. These antibodies opsonize subsequent doses and accelerate their clearance. This process may also trigger complement activation-related pseudoallergy.¹³⁹ These immunogenicity risks apply broadly to PEGylated, protein-based, and biomimetic platforms.^140,141

The diabetic microenvironment adds further complexity. Hyperglycemia drives glycation and oxidative modification of membrane proteins.¹⁴² Diabetes-associated metabolic dysregulation alters protein corona composition. The modified corona becomes enriched in inflammatory mediators and receptor signal-related proteins. This enhances immune recognition and promotes inflammatory responses.¹⁴³ Long-term administration also raises concerns about chronic organ accumulation. Certain nanomaterials can persist in the liver and spleen over extended periods. This retention is associated with low-grade inflammation and fibrotic changes.¹⁴⁴ This persistence carries risks of subclinical inflammation and fibrosis. Therefore, the clinical translation of lutein nanocarriers for diabetic retinopathy requires rigorous preclinical evaluation. Such evaluation should cover immunogenicity, diabetes-specific surface modifications, and long-term biodistribution monitoring. These measures are necessary to ensure that therapeutic benefits are not compromised by immune-mediated clearance and chronic toxicity.^145,146

Evidence Hierarchy and Translational Weight Assessment

Preclinical evidence for lutein-mediated NVU protection requires clear stratification. Mechanistic data derive primarily from in vitro RPE monocultures and isolated neuronal cultures. These represent the lowest tier of translational evidence. Nano-delivery platforms have been evaluated mainly through in vitro physicochemical and cytoprotection assays. Pharmacokinetic data mostly come from non-diabetic rodents. Retinal efficacy in diabetic animal models remains largely untested.

Efficacy data specific to diabetic retinopathy are confined to short-term streptozotocin-induced rodent studies. These models replicate acute hyperglycemia but fail to recapitulate chronic microvascular remodeling, progressive neurodegeneration, and metabolic dysregulation characteristic of human DR. No clinical trials have evaluated nanoformulated lutein for diabetic retinopathy. The evidence descends from in vitro cultures to non-diabetic in vivo pharmacokinetics, then to short-term diabetic rodent models. A substantial translational gap remains before clinical application.

Clinical Translation Bottlenecks

Whereas preceding sections have established the mechanistic rationale and engineering feasibility of nano-lutein platforms, this section addresses the decisive translational question: why have these promising formulations not yet reached the clinic? By examining manufacturing scalability, regulatory classification ambiguity, and the preclinical-to-clinical evidence gap, we identify the non-technical bottlenecks that must be resolved to realize nano-lutein as a precision adjuvant therapy.

Safety, Manufacturing, and Scalability Challenges

Stratified Safety Assessment of Nanocarrier Materials and Lutein

Safety assessment of lutein nanodelivery systems must be stratified according to risk level. Carrier materials including PLGA, chitosan, and lipids possess extensive clinical use histories, and their biocompatibility profiles are well characterized; the primary concerns center on nanoscale-specific effects and organ distribution patterns. Lutein, classified as a dietary component with Generally Recognized as Safe (GRAS) status, has demonstrated no significant adverse effects following long-term high-dose administration; however, the altered pharmacokinetic profiles induced by nanoencapsulation may result in tissue exposure levels exceeding conventional safety thresholds. Of greater concern, interactions between carrier materials and lutein may elicit unexpected toxicities that cannot be predicted through single-component safety evaluations and necessitate systematic combination toxicology studies.^147,148

Manufacturing Scale-Up and Batch-to-Batch Consistency

The transition from laboratory synthesis to commercial manufacturing presents critical scale-up bottlenecks, particularly in maintaining particle size control, encapsulation efficiency stability, and batch-to-batch consistency during process amplification.¹⁴⁹ These multi-step processes are substantially more costly than conventional lutein extraction and tableting. This cost disparity raises significant concerns regarding patient accessibility and healthcare economics, particularly when compared with established anti-VEGF therapies that, despite their own high costs, have well-defined reimbursement pathways and proven efficacy endpoints. Regarding quality control, Silva et al established and validated a ultraviolet-visible spectrophotometric method for quantifying lutein concentrations in both free and nanoparticle-encapsulated forms, thereby providing standardized analytical protocols for quality assurance.¹⁵⁰ Although spray-drying techniques for producing amorphous lutein NPs have offered valuable insights for process optimization, achieving continuous production at industrial scale remains problematic.¹⁵¹

Stability, Storage, and Quality Control Frameworks

Compounding these challenges, localized thermal exposure during processing may accelerate lutein degradation or isomerization; the stringent sterility requirements for ophthalmic formulations impose competing demands on thermolabile materials; and drug leakage coupled with carrier degradation during storage necessitate the development of stability-indicating methodologies rather than relying solely on particle size monitoring. Biomimetic formulations are especially vulnerable to batch-to-batch variability arising from membrane extraction conditions, protein orientation heterogeneity, and lipid oxidation during storage. Current quality control frameworks rely heavily on conventional physicochemical assays, yet these methods may not adequately capture critical quality attributes specific to nanomedicines, including ligand density, steric stability under biological conditions, and drug crystallinity transitions. The lack of standardized pharmacopeial methods for nano-nutraceuticals creates regulatory uncertainty and impedes cross-study comparability.

Regulatory Classification and Jurisdictional Barriers

The regulatory classification of lutein nanodelivery systems—whether as novel nanomedicines, dietary supplements, or functional food ingredients—directly determines the applicable regulatory framework and clinical development requirements, with substantial divergence in safety evaluation standards and efficacy claim substantiation across these categories.^127,152 This classification ambiguity directly determines the rigor of preclinical safety testing, the design of clinical trials, and the scope of permitted marketing claims. Eye drop formulations containing NPs may be regulated as drugs requiring full New Drug Application pathways, or alternatively as medical devices if the carrier is deemed to provide a primarily physical function. This creates jurisdictional uncertainty across regulatory agencies. This classification ambiguity represents a critical prerequisite that must be resolved to enable clinical translation. Ophthalmic nanoformulations are subject to particularly stringent requirements regarding sterility assurance, local tolerability, and intraocular biocompatibility assessment.¹⁵³ Within the functional food sector, the regulatory oversight of food-grade nanomaterials must simultaneously address both food safety and nutritional efficacy, yet unified safety evaluation standards for composite systems in food-contact applications remain underdeveloped.^154,155 Ultimately, the regulatory trajectory for lutein nanodelivery systems must be defined according to their intended therapeutic or nutritional application; the current lag in regulatory science relative to technological innovation constitutes a major impediment to clinical translation, as manufacturers face divergent submission requirements across jurisdictions without harmonized guidance on nanomaterial characterization or safety thresholds.

The Preclinical-to-Clinical Evidence Gap

A critical barrier that precedes clinical trial design is the substantial gap between preclinical evidence and human applicability. Most efficacy data derive from streptozotocin-induced acute diabetic models or non-diabetic retinal injury models that fail to recapitulate the chronic metabolic dysregulation, microvascular remodeling, and progressive neurodegeneration characteristic of human DR. Rodent retinas differ fundamentally from human retinas in vascular architecture, cellular composition, and BRB permeability profiles, complicating direct extrapolation of dosing regimens and biodistribution patterns. Furthermore, preclinical studies typically assess endpoints over short intervention periods of two to eight weeks, whereas human DR management requires sustained intervention over years. The absence of validated biomarkers that translate reliably from animal models to human patients further obscures the predictive value of current preclinical data. These collective limitations necessitate cautious interpretation of existing evidence and underscore the urgent need for large-animal models or human retinal organoid systems that better approximate diabetic retinal pathology.

Real-World Clinical Trial Design and Implementation Barriers

Biomarker Selection and Multidimensional Endpoint Design

Biomarker selection represents a pivotal determinant for assessing the clinical efficacy of nanoformulations. Although egg white protein-sodium alginate nanoparticle-stabilized algal oil Pickering emulsions encapsulating lutein have demonstrated anti-inflammatory activity in vitro, the absence of validated biomarkers specific to lutein nanotherapy complicates clinical efficacy determination.¹⁵⁶

Long-Term Follow-Up, Patient Compliance, and Cost-Effectiveness

From a pharmacodynamic perspective, the evaluation of lutein nanodelivery systems necessitates extended follow-up protocols, which introduce substantial challenges regarding patient compliance and cost containment.¹⁵⁷ Long-term follow-up protocols extending over several years impose considerable burdens on patient adherence and study budgets in real-world settings. Regarding endpoint design, multi-targeted natural nanoparticle assemblies for choroidal neovascularization therapy require the integration of multidimensional outcome measures encompassing visual acuity, optical coherence tomography (OCT) morphometric parameters, and fluorescein angiography findings.¹²⁶ Furthermore, emerging research on nanoformulations targeting retinal blue light damage highlights the lack of standardized clinical definitions and quantification criteria for blue light-induced retinal injury, thereby complicating endpoint standardization.¹²⁸ An additional methodological concern involves the potential for compromised blinding due to physical disparities between conventional placebos and nanocarrier-based placebo formulations. Moreover, the cost-effectiveness of nano-lutein relative to standard anti-VEGF regimens has not been established. Without compelling pharmacoeconomic data, payer acceptance and clinical adoption will remain limited regardless of mechanistic promise.

Adaptive Trial Designs for Nano-Nutraceutical Translation

In light of these multifaceted challenges, conventional randomized controlled trial designs may prove suboptimal for early-phase clinical development. Adaptive seamless Phase II/III designs, sample size re-estimation strategies, and biomarker-driven enrichment approaches appear particularly well-suited to products characterized by novel delivery mechanisms yet composed of established ingredients. Nevertheless, the implementation of adaptive designs imposes heightened complexity requirements for data monitoring, statistical methodology, and regulatory communication, mandating close interdisciplinary collaboration among clinicians, biostatisticians, and regulatory scientists.

Limitations and Future Perspectives

Several limitations qualify the findings and translational prospects summarized above. First, the evidence base is dominated by in vitro monoculture assays and short-term rodent studies; to our knowledge, no clinical trial of nanoformulated lutein in DR has been published, so the magnitude of benefit and the optimal dosing regimen remain undefined. Second, pharmacokinetic data derive largely from plasma rather than retinal tissue, with retinal distribution rarely quantified in diabetic animals, limiting inference on BRB crossing efficiency. Third, heterogeneity in formulation composition, particle characterization, dose metrics and outcome measures precluded quantitative synthesis; a structured narrative approach with explicit evidence-base labelling was therefore adopted instead. Fourth, long-term safety, immunogenicity upon repeated dosing and batch-to-batch reproducibility remain insufficiently characterized, and no standardized pharmacopeial methods exist for nano-nutraceuticals. Fifth, study selection was not governed by a pre-registered protocol, and non-English or unpublished industry data may have been missed.

These gaps define an actionable roadmap. Near term (1–3 years): establish NVU multicellular co-culture and human retinal organoid platforms under chronic hyperglycemia; harmonize minimal characterization and reporting standards (particle size and polydispersity, encapsulation efficiency, drug crystallinity, ligand density, storage stability); and validate retinal pharmacokinetics in diabetic models against prespecified barrier-function endpoints (tight junction protein expression, TEER, vascular leakage). Medium term (3–5 years): achieve GMP-compliant, scalable manufacturing with defined critical quality attributes; compare formulations head-to-head against commercial lutein supplements rather than free lutein; and complete long-term immunogenicity, biodistribution and large-animal studies in models that better approximate human retinal architecture. Longer term: run biomarker-driven, adaptive multicenter trials enriched for early- to mid-stage disease, with multimodal endpoints spanning visual function, OCT morphometry and molecular biomarkers, alongside prospective pharmacoeconomic evaluation against standard anti-VEGF regimens. Delivering this agenda requires early, iterative regulator engagement to resolve nano-nutraceutical classification. Its completion would establish the evidence, quality and regulatory foundations needed before nano-lutein can be tested as an adjunct in clinical trials.

Conclusion

DR has shifted from a microvascular complication to a holistic NVU pathology. Beyond descriptive material characterization, this review bridges nanoscale engineering and clinical implementation by integrating the progression of NVU damage, the multitarget mechanisms of lutein and trans-BRB delivery strategies with manufacturing, regulatory and pharmacoeconomic feasibility. Lutein synergistically protects RPE, photoreceptors, ganglion cells, Müller glia and vascular endothelium through Nrf2-driven antioxidant defense and suppression of neuroinflammation; however, its hydrophobicity, chemical instability and poor BRB permeability limit retinal bioavailability, which nano-targeted systems circumvent through passive targeting, receptor-mediated active targeting and stimuli-responsive release. Preclinical studies show improved oral bioavailability, enhanced retinal accumulation and mitigation of oxidative and inflammatory injury in diabetic models. Targeting pathology refractory to anti-VEGF therapy, nano-lutein holds promise as an adjuvant to standard regimens.

Translation nevertheless faces unresolved challenges in three areas, namely the quality of preclinical evidence, manufacturing standardization and batch consistency, and the regulatory classification of nano-nutraceuticals, as detailed above. Addressing them will determine whether lutein advances from a dietary supplement to a precision adjuvant therapy for DR, with stage-adaptive nanoplatforms matched to individual patients.

Funding Statement

This study was supported by the Shaoxing Municipal Science and Technology Projects (Grant No. 2025A14035) and Zhejiang Provincial “Small‑but‑Strong” Clinical Innovation Cultivation Team (Grant No.CXRCTD202602023).

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work. Yanan Wang: Data curation; Writing – original draft; Writing – review and editing. Wenfang Xu: Conceptualization; Writing – review and editing. Yonggen Xu: Supervision; Funding acquisition; Writing – review and editing.

Disclosure

The authors have no conflict of interest.