Nanoparticle Uptake in Human Corneal Epithelium

Topical ophthalmic formulations are convenient, but their performance is constrained by rapid precorneal clearance and the highly protective corneal epithelium. The reference study by Azadi and David addresses a central formulation question: how do nanoparticle size and surface chemistry determine interaction with, and internalization by, human corneal epithelial cells (HCECs)? Published in ACS Biomaterials Science & Engineering, the study provides a mechanistic framework for designing polymeric nanoparticles that can remain compatible with the ocular surface while reaching epithelial cells. The primary findings are reported in the reference study.

Study Background and Research Question

The ocular surface presents several sequential barriers to drug delivery. The tear film contains lipid and mucoaqueous components, while mucins can entrap foreign materials and promote their clearance. Beneath this interface, the cornea contains multiple tissue layers with distinct physicochemical properties. Although the epithelium represents only about 10% of total corneal thickness, it contributes approximately 90% of the corneal barrier function, according to the reference study.

These barriers help explain why topical drugs often show limited ocular bioavailability. Viscosity modifiers, penetration enhancers, ointments, prodrugs, and in situ gels can improve residence time or permeation, but they may also introduce irritation, inflammation, blurred vision, or systemic exposure. Polymeric nanoparticles offer a different strategy: they can protect a payload, provide sustained release, increase surface retention, and potentially alter how a drug encounters epithelial cells.

PLGA is particularly useful because its formulation can be adjusted across a broad range of sizes and surface properties. However, prior work had not fully resolved whether these physicochemical variables merely change the amount of nanoparticle associated with corneal cells or also redirect the internalization route. The study therefore asked whether particle size and polymeric surface modification produce distinct uptake behaviors in a human corneal epithelial model.

Key Innovation from the Reference Study

The main innovation is the integration of nanoparticle engineering with pathway-level analysis in a model that includes both HCECs and a simulated mucosal environment. Rather than evaluating uptake in a simplified cell suspension alone, the investigators examined particles at a barrier interface designed to reflect important aspects of the ocular surface. This is significant because a particle that performs well in buffer may behave differently in the presence of mucins, cell junctions, and extracellular clearance conditions.

The authors also separated two design concepts that are often discussed together but can have opposing consequences. Alginate and chitosan were used as mucoadhesive surface polymers, whereas polyethylene glycol (PEG) was used to create a more mucopenetrative interface. Comparing these modifications with unmodified PLGA particles allowed the investigators to examine how surface charge and interfacial chemistry influence cellular contact and internalization. The work consequently moves beyond the general statement that nanoparticles improve ocular delivery and instead asks which physical features favor epithelial uptake and through which routes.

Methods and Experimental Design Insights

PLGA nanoparticles were prepared using an emulsion-solvent evaporation method. The resulting particles were surface modified with alginate, chitosan, or PEG to generate different interfacial properties. The formulations were characterized for size, polydispersity, morphology, and zeta potential. The particles were spherical and monodisperse, with a polydispersity index below 0.2; measured sizes ranged from 100 to 250 nm and zeta potentials from −25 to +15 mV, as documented in the reference study.

Biocompatibility was assessed with the MTT assay after nanoparticle exposure. Uptake was then evaluated in a monolayer of HCECs integrated with simulated mucosal solution. This design is useful because it places particle uptake in the context of a surface barrier rather than treating the cell as an isolated target. The authors further applied inhibitors associated with major endocytic routes to distinguish energy-dependent internalization from passive association and to assess the relative contribution of macropinocytosis, caveolae-mediated endocytosis, clathrin-mediated endocytosis, and phagocytosis.

Protocol Parameters

• Particle platform: Use PLGA nanoparticles prepared by emulsion-solvent evaporation, with alginate, chitosan, or PEG as surface modifiers when comparing mucoadhesive and mucopenetrative designs.
• Characterization: Confirm spherical morphology, size distribution, polydispersity, and zeta potential before interpreting cell uptake; the reference formulations were below a 0.2 polydispersity index.
• Cell model: Evaluate uptake in an HCEC monolayer combined with simulated mucosal solution rather than relying only on a suspension assay.
• Compatibility check: Include an MTT or comparable viability assay before pathway interpretation. In the study, 24-hour exposure at concentrations up to 100 μg/mL produced only mild toxicity, with reported viability between 70% and 100%, according to the reference study.
• Mechanism testing: Use pathway inhibitors as comparative probes and interpret them alongside viability and particle-association controls, because pharmacological inhibitors can affect more than one cellular process.

Core Findings and Why They Matter

The first important result was that the tested nanoparticle formulations were broadly compatible with HCECs under the study conditions. This does not establish long-term ocular safety, but it supports the use of the PLGA platform for comparative uptake experiments without severe acute toxicity dominating the signal.

Uptake was primarily energy dependent, indicating that nanoparticles entered HCECs through active endocytic processes rather than simple diffusion across the plasma membrane. Among the tested formulations, 100 nm PLGA nanoparticles and PEG-PLGA-150 nanoparticles showed the highest uptake levels. The result is not a universal rule that smaller particles or PEGylated particles will always perform best. Instead, it shows that size and surface chemistry interact: an apparently favorable surface can produce a different outcome depending on the underlying particle dimensions and the mucosal environment.

Inhibitor experiments pointed to macropinocytosis and caveolae-mediated endocytosis as the dominant pathways. Clathrin-mediated endocytosis contributed partially, whereas phagocytosis did not appear to play a meaningful role within the tested size and surface-chemistry ranges. This pathway profile is valuable for formulation development because uptake quantity alone cannot reveal whether particles are entering through routes compatible with retention, intracellular trafficking, or eventual payload release.

For ocular drug-delivery researchers, the practical implication is a design workflow rather than a single optimal formulation. Particle size should be screened together with surface charge and polymer identity, and uptake should be measured in a model that incorporates mucosal conditions. The study also suggests that improving nanoparticle association with corneal cells requires attention to the mechanism of contact and internalization, not simply increasing mucoadhesion.

Comparison with Existing Internal Articles

The related resource Nanoparticle Uptake in Corneal Epithelial Cells: Mechanistic Insights emphasizes the same relationship between PLGA nanoparticle size, surface chemistry, and HCEC uptake. Its value is interpretive and practical, whereas the ACS article remains the primary source for the experimental design, particle ranges, and pathway results.

A second overview, Nanoparticle Uptake in Human Corneal Cells: Size and Surface Effects, foregrounds energy-dependent endocytosis and the roles of macropinocytosis and caveolae-mediated uptake. Together, these summaries can help readers orient themselves to the findings, but they should not replace the original article when selecting particle specifications, interpreting inhibitor data, or comparing biological models.

Limitations and Transferability

The model improves on a simple cell culture assay but does not reproduce the complete human ocular surface. It lacks the full multilayered corneal architecture, continuous tear turnover, living stromal and immune components, and the mechanical forces associated with blinking. Consequently, high uptake in HCECs should not be equated automatically with improved penetration across the intact cornea or with enhanced delivery to deeper ocular tissues.

The toxicity analysis was short term and assay dependent. MTT measures a metabolic endpoint, so it does not fully capture barrier integrity, inflammatory signaling, differentiation, or delayed effects. Likewise, an uptake signal may represent internalized particles, surface-associated material, or both unless orthogonal imaging and washing procedures are used. The absence of a detectable phagocytic contribution applies specifically to the tested size and surface-chemistry window; it should not be generalized to every nanoparticle formulation or ocular cell type.

Pathway inhibitors also require cautious interpretation. Inhibitors can alter membrane organization, cell metabolism, or viability in addition to blocking a nominal pathway. Follow-up studies should therefore combine inhibitor experiments with imaging, genetic perturbation where feasible, temperature or energy controls, and measurements of intracellular trafficking. Payload release and trans-epithelial transport should be evaluated separately from initial uptake.

Why this cross-domain matters, maturity, and limitations

The study creates a reasonable bridge to cytoskeletal and membrane-trafficking experiments because macropinocytosis and several forms of endocytosis depend on coordinated membrane remodeling. However, the reference data identify uptake pathways through inhibitor profiles; they do not by themselves prove that a particular actin-regulatory protein is the decisive control point. Actin-directed perturbation is therefore a follow-up mechanistic strategy, not a conclusion of the ocular nanoparticle study.

This bridge is most mature for hypothesis testing in the same HCEC or mucosal model. It is less mature for predicting in vivo biodistribution, therapeutic efficacy, or safety. Any cytoskeletal perturbation can change cell shape, junctions, viability, and barrier permeability, creating secondary effects that resemble altered nanoparticle uptake. These confounders make matched vehicle controls, viability measurements, barrier assays, and rescue or orthogonal experiments essential.

Research Support Resources

For follow-up uptake experiments, researchers can use Cytochalasin D (SKU B6645), a selective actin polymerization inhibitor, as a controlled perturbation of actin-dependent trafficking. The product information reports an IC50 of 25 nM and lists 0.2–0.5 μg/mL as a typical cell-culture range; these values should be treated as starting points requiring cell- and assay-specific optimization. Its reported activities also include cell cycle arrest at G1-S transition, tumor cell proliferation inhibition, apoptosis induction in cancer cells, and viral transcription inhibition. Those effects mean that altered nanoparticle uptake should not be attributed solely to endocytosis without parallel viability and barrier controls. The compound is described as requiring desiccated storage at −20 °C, with solutions used promptly rather than stored long term.