Eye drops containing Tranilast (TL), N-(3,4-dimethoxycinnamoyl) anthramilic acid, are used as an anti-allergic conjunctivitis drug in the ophthalmic field. Traditional eye drops are very patient compliant, although the bioavailability (BA) of most eye drops is low since eye drops cannot be instilled beyond the capacity of the conjunctival sac due to its limited volume. Thus, traditional eye drops have low BA and a short duration of the drug on the ocular surface, so solutions to these problems are highly anticipated. In this study, we designed a sustained-release drug-delivery system (DDS) for TL nanoparticles. TL nanoparticles were prepared by bead mill treatment, and the gel formulations containing TL nanoparticles (TL-NPs-Gel, particle size 50 nm-100 nm) were provided by carboxypolymethylene. The crystal structure of TL with and without bead mill treatment is the same, but the TL solubility in formulations containing nanoparticles was 5.3-fold higher compared with gel formulations containing TL microparticles (TL-MPs-Gel). The photo and thermal stabilities of TL-NPs-Gel are also higher than those of dissolved TL. Moreover, when TL-NPs-Gel is applied to the upper eyelid skin (outside), the TL is released as nanoparticles, and delivered to the lacrimal fluid through the meibomian glands. In addition, the TL release profile for TL-NPs-Gel was sustained over 180 min after the treatment. These findings can be used to develop a sustained-release DDS in the ophthalmic field.
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