Polymer-free tacrolimus microsphere implants for long-acting prevention of corneal neovascularization in rats by subconjunctiva injection

Corneal neovascularization (CNV) severely compromises vision and remains difficult to manage with current pharmacotherapies, which are often limited by rapid drug clearance, poor tissue penetration, and adverse effects. Although polymer-based drug carriers can provide sustained drug release, they frequently suffer from low drug-loading capacity, suboptimal release kinetics, or induce inflammatory responses. Tacrolimus (FK506), a potent immunosuppressant with anti-inflammatory activity, is similarly limited by poor water solubility and rapid ocular clearance when administered as eye drops. In this study, we developed a polymer-free tacrolimus microsphere system using an emulsion-solvent evaporation method. The resulting microspheres exhibited uniform morphology (6.8 ± 1.0 μm) with a high drug-loading capacity (up to 98 ± 2 %) and a dense amorphous internal structure. In vitro, the microsphere showed a sustained release profile, reaching 85 % cumulative release by Day 30. In a rat CNV model, subconjunctival injection of tacrolimus microsphere maintained elevated drug concentrations in the cornea (81.0 ± 21.2 ng/g) and sclera (114.2 ± 34.9 ng/g) for at least one month and significantly suppressed CNV progression, reduced corneal opacity, and decreased the expression of inflammatory (TNF-α, IL-1β) and angiogenic (VEGF) markers. Histological analysis further confirmed improved corneal architecture, minimal inflammatory infiltration, and reduced neovascularization. In contrast, tacrolimus eye drops demonstrated limited therapeutic benefit and rapid clearance. Overall, these findings highlight polymer-free tacrolimus microsphere as a promising long-acting delivery platform for effective CNV management upon subconjunctiva injection, with the potential to improve therapeutic outcomes by enabling sustained drug release and reducing administration frequency.