(542f) Targeted Drug Delivery to the Back of the Eye By Hydrogel Pushing in the Suprachoroidal Space

Drug delivery targeted to the choroid and retina of the eye and, in some cases the most posterior portion of these tissues at the macula is expected to increase drug efficacy and reduce side effects, because the site of action for ocular drugs treating diseases such as age-related macular degeneration, diabetic macular edema, and posterior uveitis is around macula. Previously, we have used microneedles to inject drug formulations into the suprachoroidal space (SCS) of the eye, which allows drug to flow circumferentially at the choroid-sclera interface. This targets drug to the choroid and adjacent retina, but does not specifically target the macula. This approach is currently the subject of multiple advanced clinical trials for treatment of ocular inflammation.

In this study, we sought to develop a method to better target drug delivery to the macula via the SCS. We hypothesize that injecting drug particles in a formulation that swells within the SCS can push the drug further toward the back of the eye and in that way flow to the macula. We accomplished this by injecting, from a single syringe, a first formulation containing drug particles followed by a second formulation containing a swellable hydrogel material without drug designed to push the drug particles in the first material posteriorly toward the macula within the SCS.

Albino New Zealand White rabbit eyes were used for ex vivo experiments. All SCS injections were accomplished using a 30-gauge hollow microneedle with 750 µm length. We filled two materials into each syringe. First, 30 µL of 4% (w/v) hyaluronic acid (HA, 2.6 MDa) hydrogel was filled into the syringe as a drug pushing material. Then 20 µL of a model drug formulation containing 0.5% (w/v) of red-fluorescent particles (2 µm diameter) in 1% (w/v) HA, was filled into the syringe without mixing with the first material. In this way, the model drug particles can come out of the syringe first during injection. The injection was performed 3 mm posterior to the limbus at supranasal location of the ocular globe. Intraocular pressure (IOP) of the eye ex vivo was measured using a tonometer and adjusted to 10-15 mmHg by adding HBSS buffer into the vitreous humor. To minimize backflow of the injected materials after injection, the syringe was maintained for 1 min at the injection site.

Particle distribution was analyzed right after the injection by freezing the eyes in chilled 100% isopropyl alcohol. Then, fully frozen eyes were dissected from posterior pole to limbus by making eight radial cuts like flower petals. To determine the particle distribution in the SCS, each petal was cut into 4 pieces depending on the distance from limbus – 0-3, 3-6, 6-9, over 9 mm – and the particles were extracted with 0.5X RIPA buffer. Particle content in each piece of tissue was measured by it signal by fluorometry.

To optimize the two formulations in the syringe, the HA concentration in the particle formulation and the pushing hydrogel formulation was varied. The objectives were (i) to minimize the hydrogel content in the particle formulation so that it would flow more easily to the back of the eye, (ii) to maximize the hydrogel content in the pushing formulation to increase hydrogel swelling and, therefore, pushing, (iii) to minimize viscosity of both formulations to reduce pressure during injection and (iv) to achieve an appropriate balance between the viscosities of the two formulations so that they would not mix during injection or pushing. Based on these considerations, we chose 1% (w/v) HA for a particle solution and 4% (w/v) HA as the pushing hydrogel.

Using this formulation, 14.1 ± 5.4% of particles was delivered toward the back of the eye (i.e., > 6 mm from the limbus) right after injection, 54.3 ± 10.2% of particles were delivered > 6 mm from the limbus after 6 h at 37 ºC. This demonstrated the ability of the pushing formulation to move more than half of the particles to the back part of the SCS. This pushing was facilitated not only by the increase in hydrogel volume upon contact with water in the eye, but also because the viscosity of the HA pushing hydrogel was decreased at 37 ºC (relative to room temperature at which it was injected).

In addition, we hypothesized that a high-salt (i.e., high osmotic pressure) HA hydrogel can increase osmotic flow of water into the hydrogel to increase pushing even more. To assess this hypothesis, we added 9 % sodium chloride to the HA hydrogel pushing material. We found that the high-salt HA hydrogel pushed 80.7 ± 6.1% of the particles > 6 mm from the limibus (and 38.0 ± 2.1% of particles were > 9 mm from the limbus) 6 h after injection at 37 ºC.

We conclude that hydrogel pushing in the suprachoroidal space can be target delivery of particles to the back of the eye, which may be of use for drug delivery of ocular diseases.