28-Day Ocular Delivery of Brimonidine Tartrate From Rationally Designed Degradable Microparticles In a Rabbit Model

Glaucoma is the second leading cause of blindness worldwide, expected to affect up to 3 million Americans by 2020 (Quigley HA. Lancet. 2011;377[9774]:1367-77). One of the main risk factors in glaucoma is an unsafe increase in intraocular pressure (IOP).  IOP reduction in patients with glaucoma is typically accomplished through the administration of medicated eye drops several times daily, the difficult and frequent nature of which contributes to patient compliance rates estimated to be as low as 50% (Hermann MM. Int Ophthalmol. 2010;30[4]:385-90).  Medications such as brimonidine tartrate (BT), which requires dosing every 8-12 hours, have yet to be adapted into controlled-release formulations that could drastically improve compliance.  The purpose of this study was to develop and test a controlled release BT formulation that provides one month of therapeutic levels of drug, thereby eliminating compliance issues inherent to eye drop administration.

BT was loaded into poly(lactic-co-glycolic) acid (PLGA) microparticles, which were fabricated using a standard double emulsion procedure. Both blank and drug-loaded microparticles were characterized for surface morphology and volume average diameter.  In vitro release of BT from the drug-loaded microparticles was quantified by incubating a known mass of microparticles in buffer and measuring the absorption using a spectrophotometer.  For our in vivo studies, groups of 3 healthy Dutch belted rabbits were randomized to receive blank microparticles (no drug), BT-loaded microparticles, or 0.2% BT drops (Alphagan¨, Allergan, Irvine, CA).  Rabbits in both microparticle groups received a subconjunctival injection of microparticles suspended in sterile saline on Day 0 in one eye only.  Rabbits in the positive control group received a single drop of Alphagan¨ solution in one eye twice a day each day of the study.  Intraocular pressure was monitored over 28 days in both eyes and slit lamp examination was performed periodically to assess the effect, if any, of the microparticles on the surrounding tissue.  Following sacrifice on Day 28, both eyes were enucleated for histological analysis.  All slides were masked prior to performing histopathological analysis for evidence of irritation or foreign body response.

Microparticles were confirmed to have a diameter of 7.5±2.9 μm with a primarily poreless morphology.  They released an average of 0.62±0.34 μg BT/mg particles/day in our in vitro setup, within the calculated therapeutic range of Alphagan¨ drops.  Our in vivo study of the BT-loaded microparticles demonstrated that the decrease in IOP was significantly lower (p<0.05) in the treated eye for BT drops versus BT microparticles for all time points.  There was no significant difference in ΔIOP for the untreated eye between BT drops and BT microparticles.  In contrast, IOP steadily increased in rabbits injected with the blank microparticles.  The microparticle bleb was visible using light microscopy and no evidence of microparticle migration was seen.  Histological analysis showed no signs of foreign body response due to the microparticles or migration of the microparticles from the periocular area.

The BT-loaded microparticles presented in this study are capable of releasing a therapeutically relevant amount of a common glaucoma medication for over 30 days.  Additionally, treatments based on BTMPs demonstrate a significantly greater drop in IOP over the entire 28-day study compared to the baseline IOP in a healthy rabbit model.  Subconjunctival injections of the microparticles resulted in neither foreign body response nor infection after the full 28-day experimental period, and microparticles were confined to the injection area.  We are currently adapting this controlled-release BT delivery system into a simple, patient-administered formulation that provides the same IOP reduction potential and biocompatibility without the need for intraocular injections.