Timolol maleate-loaded double Pickering emulsion for the treatment of glaucoma: development, characterization, and ocular biocompatibility

Glaucoma is a leading cause of irreversible blindness, associated with high intraocular pressure. Topical beta-adrenergic antagonists, such as timolol maleate (TM), are first-line therapies but often have limited effectiveness due to rapid precorneal clearance and low ocular absorption. To address these limitations, a TM-loaded water-in-oil-in-water (W/O/W) double Pickering emulsion (TM-loaded DPE) was developed to enhance corneal retention, sustain drug release, and enhance ocular bioavailability. The TM-loaded DPE consisted of a primary W/O emulsion prepared from TM solution and mineral oil, stabilized by soy lecithin, combined with an external aqueous phase containing chitosan-Hydroxypropyl methylcellulose (chitosan-HPMC) particles as Pickering stabilizers, to form a W/O/W emulsion. Chitosan-HPMC particles had a size of 1.02 ± 0.16 μm, a zeta potential of 36.20 ± 2.41 mV, and a contact angle of 78.47 ± 1.80°, making them suitable Pickering stabilizers. The TM encapsulation efficiency in the TM-loaded DPE was 96%, indicating the high loading capacity of the developed system. Confocal laser scanning microscopy revealed a mononuclear droplet morphology, and the system exhibited excellent macroscopic physical stability under refrigerated conditions (4 °C, 12 months), as corroborated by the creaming index test, despite slight signs of coalescence observed in microscopic analysis. Rheological testing confirmed a non-Newtonian and shear-thinning behavior. The formulation achieved approximately 84% TM release over 48 hours, indicating sustained release. The Hen egg test chorioallantoic membrane (HET-CAM) assay indicated no irritation potential, confirming its safety. Overall, these results highlight TM-loaded DPE as a promising drug delivery system for glaucoma treatment, offering improved drug retention and enhanced ocular bioavailability.