Hyaluronic acid-coated cubosomal <i>in situ</i> gel for Brimonidine Tartrate: a sustained delivery system with enhanced ocular bioavailability and intraocular pressure reduction

Glaucoma causes irreversible blindness, with poor adherence driven by frequent dosing and low eye drop bioavailability. Brimonidine tartrate (BRT) requires multiple daily doses due to rapid elimination and limited corneal permeability. To address these limitations, this study aimed to develop a long-acting, mucoadhesive ocular delivery system using hyaluronic acid-coated cubosomes (HA-BRT-CUB), further incorporated into a thermoresponsive <i>in situ</i> gel (HA-BRT-CUB-ISG). The HA-BRT-CUB formulation was optimized using a Design of Experiments (DoE) approach, resulting in a particle size of 189.24 ± 2.05 nm, zeta potential of −25.45 ± 0.88 mV, and entrapment efficiency of 78.86 ± 2.42%. Incorporation into a poloxamer-based ISG system enhanced precorneal retention and sustained delivery. <i>In vitro</i> release from HA-BRT-CUB-ISG was extended over 24 h (h), best fitting the Korsmeyer–Peppas model (<i>R</i>² = 0.981). <i>Ex vivo</i> permeation studies revealed a flux (J) of 2.89 µg/cm²/h and a 3.92-fold enhancement ratio (ER) compared to the BRT solution. In normotensive New Zealand rabbits, HA-BRT-CUB-ISG achieved a 30.31% IOP reduction at 24 h (17.05 ± 0.35 mmHg), compared to 29.91% with HA-BRT-CUB and only 20.75% with Alphagan^®. Pharmacokinetic analysis revealed that HA-BRT-CUB-ISG attained a Cmax of 7.35 ± 0.82 μg/mL and an AUC₀–∞ of 70.22 ± 3.56 µg/mL* h, representing a 4.5- and 12-fold improvement, respectively, over Alphagan^®. The half-life extended to 8.75 ± 1.34 h with minimal clearance (Cl/<i>F</i> = 1.42 ± 0.59). Stability studies confirmed HA-BRT-CUB formulation integrity under refrigerated conditions over 90 days. Overall, HA-BRT-CUB-ISG presents a promising once-daily nanocarrier system for sustained glaucoma management.