The use of eye drops is a well-established practice in the treatment of ophthalmic diseases, although the bioavailability of traditional eye drops, which are either solutions or suspensions, is insufficient, as the corneal barrier and dilution by lacrimation prevent the transcorneal penetration of drugs. Additionally, frequent instillation may cause undesirable systemic side effects and local corneal toxicity. To overcome these problems, micro- and nanoparticles, hydrogels, and viscous solutions have been tested, and solid nanoparticles are also expected to be applied. This review examines the usefulness of ophthalmic formulations based on solid nanoparticles, by using the specific example of indomethacin (IMC). Ophthalmic formulations based on solid IMC nanoparticles (IMC-NP dispersions) have been prepared using various additives (benzalkonium chloride, mannitol, methylcellulose, and cyclodextrin) and a rotation/revolution pulverizer (NP-100), to produce particles of 50-220 nm in size. The solubility of IMC in IMC-NP dispersions was 4.18-fold higher than that in the suspensions containing IMC microparticles (IMC-MP suspensions), and IMC-NP dispersions were better tolerated than commercially available NSAIDs eye drops, such as IMC, pranoprofen, diclofenac, bromfenac, and nepafenac eyedrops, in human corneal epithelial cells. Moreover, the corneal penetration in IMC-NP dispersions was higher than that in commercially available IMC and IMC-MP suspensions, and three energy-dependent endocytosis pathways (clathrin-dependent endocytosis, caveolae-dependent endocytosis, and macropinocytosis) were related to the high ophthalmic bioavailability of IMC-NP dispersions. This information can be used to support future studies aimed at designing novel ophthalmic formulations.
Keywords: drug delivery; endocytosis; indomethacin; nanoparticle; ophthalmic formulation.