Amino-Functionalized Mesoporous Silica Particles for Ocular Delivery of Brimonidine

To treat glaucoma, conventional eye drops are often prescribed. However, the eye drops have limited effectiveness as a result of low drug bioavailability due to their rapid clearance from the preocular space. To resolve this, we proposed amino-functionalized mesoporous silica (AMS) particles as delivery carriers of the glaucoma drug, brimonidine. Because of the presence of mesopores, brimonidine (BMD) could be encapsulated in the AMS with a loading amount of 41.73 μg/mg (i.e., drug loading capacity of about 4.17%) to give the BMD–AMS, which could release the drug in a sustained manner over 8 h. BMD–AMS was also shown to be mucoadhesive due to the presence of both hydroxyl and amino groups in the surface, allowing for formation of hydrogen bonds and an ionic complex with the mucin, respectively. Therefore, when topically administered to rabbit eyes in vivo, BMD–AMS could reside in the preocular space for up to 12 h because of its adherence to the mucous layer. To assess in vivo efficacy, we examined the variance in intraocular pressure (IOP) and brimonidine concentration in the aqueous humor (AH) after applying BMD–AMS to the eye, which was compared with that induced by Alphagan P, the marketed brimonidine eye drops. For BMD–AMS, the duration in the decrease in IOP and the area under the drug concentration in the AH–time curve (AUC) were 12 h and 2.68 μg·h/mL, respectively, which were about twice as large as those obtained with Alphagan P; this finding indicated enhanced ocular bioavailability of brimonidine with BMD–AMS.