Besifloxacin-Loaded Nanostructured Lipid Carrier for Ocular Delivery: Formulation Optimization, In Vitro Characterization, and Safety Assessment

Conjunctivitis, keratitis, endophthalmitis, and blepharitis are among the most prevalent bacterial eye infections. Topical eye drops are convenient but often exhibit low ocular bioavailability due to anatomical and physiological barriers, which may contribute to subtherapeutic exposure and antimicrobial resistance. Besifloxacin (BFX), a fourth-generation fluoroquinolone approved exclusively for ophthalmic use, has poor aqueous solubility at tear pH and is currently marketed as a suspension requiring frequent instillation. This study aimed to develop and optimize besifloxacin-loaded nanostructured lipid carriers (BFX-NLCs) to improve delivery. BFX-NLCs were prepared by high-pressure homogenization and optimized using response surface methodology, with particle size (Z-average) as the primary response. The optimized formulation exhibited a mean particle size of ~100 nm, a polydispersity index <0.25, and an entrapment efficiency of ~75%. In vitro release in simulated tear fluid showed a sustained release profile, reaching ~65% BFX release at 24 h, best described by the Korsmeyer-Peppas model (diffusion exponent n = 0.86), indicating anomalous (non-Fickian) diffusion. The minimum inhibitory concentrations of BFX-NLC against Staphylococcus aureus ATCC 23235 and Pseudomonas aeruginosa ATCC 9027 were comparable to those of free BFX, demonstrating preservation of antimicrobial activity. BFX-NLCs were non-toxic in the Galleria mellonella larvae model and exhibited suitable viscosity and osmolality for ophthalmic use, as well as physical stability and entrapment efficiency over nine months of storage. These findings support BFX-NLCs as a promising lipid-based platform for topical ocular delivery of besifloxacin, with potential to enhance therapeutic efficacy and reduce dosing frequency in bacterial eye infections.