Dexamethasone‐melatonin‐vitamin E PLGA microspheres targeting neurodegeneration in glaucoma

Although the pathogenesis of glaucoma is not completely understood yet, all patients diagnosed with this chronic disease are observed with neurodegeneration. Neuroprotective treatment will benefit from joint administration of active substances directed towards several therapeutic targets. Our study focuses on the hypothesis that a combination of anti‐inflammatory and antioxidant drugs, namely dexamethasone (DX), melatonin (MEL) and vitamin E (VE), loaded in biodegradable drug delivery devices (microspheres) can be advantageous as a long‐term therapeutic strategy that can achieve protection of visual function and avoid retinal cells death. We further evaluated whether drugs' stability and release properties changed or not after sterilization, a key process for ophthalmic formulations. Biodegradable poly (lactic‐co‐glycolic) acid (PLGA) microspheres (MS) were elaborated using the oil‐in‐water emulsion solvent evaporation technique. DX and MEL were added in a ratio 1/2:10 (DX/MEL:PLGA)(w/w) and a volume of 40 μl of VE was also included as an oily additive. After lyophilisation, MS were subjected to 60Cobalt radiation at 25 kGy (S‐MS) in dry ice. Before and after gamma‐irradiation, MS were characterized by means of internal and external morphology, particle size, encapsulation efficiency, thermal properties and in vitro release profiles. The chosen 38–20 μm MS exhibited pores inside and at a surface level respectively when observed at transmission and scanning electron microscopes, due to a slow evacuation of organic solvent from the internal to the aqueous phase for the oily nature of VE. MS showed high drug loadings of DX (56.61 ± 2.42 μg/mg MS) and MEL (33.97 ± 2.64 μg/mg MS), which remained unaltered for S‐MS, and followed a unimodal distribution of particle size (30.14 ± 1.03 μm for MS; 32.75 ± 1.46 μm for S‐MS). Differential scanning calorimeter thermograms revealed no endothermic transitions at DX and MEL melting points, thus indicating effective encapsulation. Regarding in vitro release profiles, both drugs in MS were released in a controlled fashion for more than 50 days (0.47 ± 0.03 μg/day for DX; 0.60 ± 0.04 μg/day for MEL). DX was released during a total of 120‐day follow‐up period (94.12 ± 0.68% of total DX encapsulated). Both compounds in S‐MS showed a higher burst release values (p