(550f) 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.