Pharmaceuticals · 2 authors, 1 centre
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This review summarizes the potential of solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) for ocular drug delivery, highlighting their ability to enhance bioavailability, provide sustained release, and improve corneal penetration. Both systems are biocompatible, biodegradable, and can be surface-modified to prolong ocular residence time. Despite promising preclinical results, no SLN- or NLC-based ophthalmic formulations have yet reached clinical application.
**Background:** Ocular drug delivery is challenging due to the eye's complex anatomy and protective barriers, resulting in low bioavailability (less than 5% of topically applied dose reaches deeper tissues). Conventional eye drops suffer from short residence time and rapid clearance. Solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) are advanced lipid-based nanocarriers that offer biocompatibility, controlled release, and enhanced penetration. This review summarizes recent progress in using SLNs and NLCs for ocular therapeutics.
**Methods:** The authors conducted a narrative review of the literature, covering the anatomical and physiological barriers of the eye (tear film, cornea, conjunctiva, blood–aqueous and blood–retinal barriers), the composition and types of SLNs and NLCs, surface modification strategies, sterilization methods, and recent preclinical studies. The review includes data from in vitro, ex vivo, and in vivo studies, but does not follow a systematic review or meta-analysis methodology.
**Key Results:** SLNs are typically 50–1000 nm in size and composed of solid lipids (e.g., triglycerides, waxes, fatty acids) stabilized by surfactants. NLCs additionally contain liquid lipids (up to 30% of total lipid) to create a less-ordered matrix, improving drug loading and reducing drug expulsion. Both systems can encapsulate hydrophilic and hydrophobic drugs. Surface modification with cationic compounds (e.g., chitosan, stearylamine) or PEGylation enhances mucoadhesion and corneal residence time. For example, PEGylated chitosan-coated SLNs increased ofloxacin concentration in rabbit eyes two- to three-fold compared to plain drug. Hybrid systems (e.g., SLNs in thermosensitive gels) provided sustained release up to 30 h (mizolastine) and improved pharmacodynamic effects. NLCs loaded with polyphenols (diosmin, mangiferin) improved solubility and antioxidant activity. Sterilization methods include autoclaving, gamma irradiation, and sterile filtration, with variable effects on particle size and entrapment efficiency. No SLN- or NLC-based ophthalmic products are currently marketed; a search on ClinicalTrials.gov (accessed 1 March 2023) yielded no ocular trials for these carriers.
**Clinical Implications:** SLNs and NLCs hold significant promise for treating both anterior segment diseases (e.g., dry eye, infections, allergic conjunctivitis) and posterior segment diseases (e.g., glaucoma, age-related macular degeneration, diabetic retinopathy). Their ability to provide sustained release, enhance corneal penetration, and improve drug stability could reduce dosing frequency and improve patient compliance. However, challenges remain in batch reproducibility, long-term toxicity assessment, and regulatory approval. The authors recommend adopting quality-by-design approaches and establishing unified safety protocols to facilitate clinical translation.