Intravitreal safety profiles of sol-gel mesoporous silica microparticles and the degradation product (Si(OH)4)

Abstract

Mesoporous silica has attracted significant attention in the drug delivery area; however, impurities can be a source of toxicity. The current study used commercial microparticles produced at large scale in a well-controlled environment. Micrometer sized mesoporous silica particles were acquired through a commercial vendor and pore structures were characterized by SEM. The three silica particle formulations had a diameter of 15 micrometers and three different pore sizes of 10 nm, 30 nm, and 100 nm. The fourth formulation had particle size of 20-40 micrometers with 50 nm pores. Before in vivo tests, an in vitro cytotoxicity test was conducted with silicic acid, derived from the sol-gel particles, on EA.hy926 cells. Low concentration (2.5 µg/mL) of silicic acid showed no cytotoxicity; however, high concentration (25 µg/mL) was cytotoxic. In vivo intravitreal injection demonstrated that 15 um silica particles with 10 nm pore were safe in both rabbit and guinea pig eyes and the particles lasted in the vitreous for longer than two months. Formulations of with larger pores demonstrated variable localized vitreous cloudiness around the sol-gel particle depot and mild inflammatory cells in the aqueous humor. The incidence of reaction trended higher with larger pores (10 nm: 0%, 30 nm: 29%, 50 nm: 71%, 100 nm: 100%, p < .0001, Cochran Armitage Trend Test). Sol-gel mesoporous silica particles have uniform particle sizes and well-defined pores, which is an advantage for implantation via a fine needle. Selected formulations may be used as an intraocular drug delivery system with proper loading and encapsulation.

Keywords: Intravitreal drug delivery; rabbit and guinea pig eyes; silica pore size and ocular toxicity; silicic acid cytotoxicity; sol-gel mesoporous silica.

Figure 1.

SEM images of the sol-gel silica pores: (A) pore = 10 nm; (B) pore = 100 nm.

Figure 2.

The boxplots of the OD values from the WST-1 cell viability assay stratified by the source of silicic acid, concentration of silicic acid, and exposure time of silicic acid to the cultured EA.hy926 cells.

Figure 3.

In vitro silicic acid dissolution kinetics from the sol-gel particles.

Figure 4.

Rabbit fundus images. Panel (A) was taken 3 days after the intravitreal injection while panel (B) was taken 7 days after, showing the sol-gel particles settling down toward the inferior vitreous cavity over time.

Figure 5.

Intraocular pressure (IOP) of the eyes injected with the 15 µm/10 nm formulation (OD) versus the control eyes (OS) over the study course. There is no significant difference between OD and OS although IOP of both eyes increased over time.

Figure 6.

Particle depot size measured by optic nerve head area over time in the rabbit vitreous. The speed of degradation correlated with the release speed in vitro. 15 µm/10 nm low dose of pSi particles disappeared completely by week 8 while particles within the other groups were still visible. 15 µm/10 mm pSi particles showed the most rapid release, in both the high and low dose groups. P/p particle (particle/pore); 15/10/H = 15 µm/10 nm/high dose; 15/10/L = 15 µm/10 nm/low dose; 15/30 = 15 µm/30 nm; 35/50 = 35 µm/50 nm; 15/100 = 15 µm/100 nm.

Figure 7.

FA and OCT from a rabbit eye (A and C) injected with 15 µm/10 nm pSiO₂ and its contralateral eye (B and D) injected with saline, images taken 8 weeks after the intravitreal injection. Both eyes demonstrated normal FA and OCT.

Figure 8.

Guinea pig histology. (A) positive control for TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling), (B) negative control for TUNEL, and (C) test eye TUNEL staining of guinea pig retina 8 weeks after the injection of sol-gel 15 µm/10 nm formulation. No detectable difference of apoptotic staining between the test eye retina and the negative control retina. (D) is an H&E stained slide from 4 weeks after injection, showing subretinal hemorrhage in the eye which showed aqueous cells and vitreous haze in clinical examination.