Distribution of particles, small molecules and polymeric formulation excipients in the suprachoroidal space after microneedle injection

Abstract

The purpose of this work was to determine the effect of injection volume, formulation composition, and time on circumferential spread of particles, small molecules and polymeric formulation excipients in the suprachoroidal space (SCS) after microneedle injection into New Zealand White rabbit eyes ex vivo and in vivo. Microneedle injections of 25–150 μL Hank’s Balanced Salt Solution (HBSS) containing 0.2 μm red-fluorescent particles and a model small molecule (fluorescein) were performed in rabbit eyes ex vivo, and visualized via flat mount. Particles with diameters of 0.02 – 2 μm were co-injected into SCS in vivo with fluorescein or a polymeric formulation excipient: fluorescein isothiocyanate (FITC)-labeled Discovisc or FITC-labeled carboxymethyl cellulose (CMC). Fluorescent fundus images were acquired over time to determine area of particle, fluorescein and polymeric formulation excipient spread, as well as their co-localization. We found that fluorescein covered a significantly larger area than co-injected particles when suspended in HBSS, and that this difference was present from 3 min post-injection onwards. We further showed that there was no difference in initial area covered by FITC-Discovisc and particles; the transport time (i.e., the time until the FITC-Discovisc and particle area began dissociating) was 2 d. There was also no difference in initial area covered by FITC-CMC and particles; the transport time in FITC-CMC was 4 d. We also found that particle size (20 nm – 2 μm) had no effect on spreading area when delivered in HBSS or Discovisc. We conclude that (i) the area of particle spread in SCS during injection generally increased with increasing injection volume, was unaffected by particle size and was significantly less than the area of fluorescein spread, (ii) particles suspended in low-viscosity HBSS formulation were entrapped in the SCS after injection, whereas fluorescein was not and (iii) particles co-injected with viscous polymeric formulation excipients co-localized near the site of injection in the SCS, continued to co-localize while spreading over larger areas for 2 – 4 days, and then no longer co-localized as the polymeric formulation excipients were cleared within 1 – 3 weeks and the particles remained largely in place. These data suggest that particles encounter greater barriers to flow in SCS compared to molecules and that co-localization of particles and polymeric formulation excipients allow spreading over larger areas of the SCS until the particles and excipients dissociate.

Polysaccharides in Ocular Drug Delivery

“…Rheological properties of the polymer are important in this regard: high MW and moderately non-Newtonian polymeric solutions (e.g., HA) enhance particle spreading, whereas strongly non-Newtonian polymer solutions (e.g., methylcellulose, CMC) are immobilized at the injection site. For example, an injected solution of Discovisc (1.6 MDa HA) is eliminated in one week, while 700 kDa CMC is retained at injection site for 3 weeks [186].…”

Suprachoroidal and Subretinal Injections of AAV Using Transscleral Microneedles for Retinal Gene Delivery in Nonhuman Primates

“…The team also employed a conventional 27-gauge needle to deliver AAV8 suprachoroidally in three nonhuman primates, 56 but this free-hand technique is less precise and cannot exclude the possibility of inadvertent subretinal delivery. In contrast, our study employed rhesus macaques with ocular dimensions and vasculature similar to humans, and transscleral microneedles, which reliably access the suprachoroidal space with well-characterized biodistribution, 57,58 and are already employed in human patients in phase I/II clinical trials. 42,59 We also employed in vivo SLO imaging, demonstrating the striking, unique expression pattern of suprachoroidal AAV8, consistent with the anatomic characteristics of the choroidal-scleral junction, which differs from subretinal or intravitreal AAV8 delivery.…”

The suprachoroidal space as a route of administration to the posterior segment of the eye

“…Multiple studies have shown that increasing injection volume resulted in increasing coverage area[65, 66, 68, 71, 75, 81]. However, it was difficult to cover 100%, likely due to anatomical barriers[80].…”

“…Polymeric formulations (such as hyaluronic acid) are hypothesized to spread well after injections since they exert an osmotic pressure that seeks to imbibe fluid, resulting in expansion of the formulation, and may also coat particles and/or tissue to prevent non-specific binding. By tracking the distribution of particles and fluid formulations simultaneously, the injected fluid traveled further into the SCS than the injected particles when using low-viscosity formulations, suggesting some degree of particle entrapment with the SCS; with high-viscosity formulations, the discrepancy between the travel distance of the particle and polymeric formulation was decreased [81]. These findings suggest that these viscous formulations increased transit time in the SCS, and reduced particle-tissue interactions[81].…”

“…In addition, particles ranging from 20 nm to 2,000 nm have been shown to distribute and behave similarly, independent of particle size[81]. Polymeric microparticles can be found in the eye at least 4 months post-injection, suggesting that they are resistant to clearance by the routes molecules clear by[66].…”