Self-assembled liquid crystalline nanoparticles as a novel ophthalmic delivery system for dexamethasone: Improving preocular retention and ocular bioavailability

Abstract

The object of this study was to design novel self-assembled liquid crystalline nanoparticles (cubosomes) as an ophthalmic delivery system for dexamethasone (DEX) to improve its preocular retention and ocular bioavailability. DEX cubosome particles were produced by fragmenting a cubic crystalline phase of monoolein and water in the presence of stabilizer Poloxamer 407. Small angle X-ray diffraction (SAXR) profiles revealed its internal structure as Pn3m space group, indicating the diamond cubic phase. In vitro, the apparent permeability coefficient of DEX administered in cubosomes exhibited a 4.5-fold (F1) and 3.5-fold (F2) increase compared to that of Dex-Na phosphate eye drops. Preocular retention studies revealed that the retention of cubosomes was significantly longer than that of solution and carbopol gel, with AUC_0→180 min of Rh B cubosomes being 2–3-fold higher than that of the other two formulations. In vivo pharmacokinetics in aqueous humor was evaluated by microdialysis, which indicated a 1.8-fold (F1) increase in AUC_0→240 min of DEX administered in cubosomes relative to that of Dex-Na phosphate eye drops, with about an 8-fold increase compared to that of DEX suspension. Corneal cross-sections after incubation with DEX cubosomes demonstrated an unaffected corneal structure and tissue integrity, which indicated the good biocompatibility of DEX cubosomes. In conclusion, self-assembled liquid crystalline nanoparticles might represent a promising vehicle for effective ocular drug delivery.

Introduction

Ocular diseases are usually treated with topical application of drug solutions (eye drops). However, the rapid and extensive precorneal losses caused by drainage and high tear fluid turnover limit drug ocular bioavailability. Meanwhile, for drugs entering the ocular tissue, the cornea is the major route of anterior drug absorption. The lipophilicity and tight conjunction of the corneal epithelium make it the major limiting barrier in corneal drug absorption; consequently, lipophilic (log D 2–3) drugs have a higher permeability than hydrophilic ionized drugs (Mannermaa et al., 2006).

To improve ocular bioavailability, several ophthalmic drug delivery systems have been proposed, such as emulsions (Yamaguchi et al., 2005), nanoparticles (Zimmer and Kreuter, 1995) and liposomes (Meisner and Mezei, 1995). These systems might be able to enhance drug bioavailability by facilitating transcorneal/transconjunctival penetration (Tamilvanan and Benita, 2004). Nevertheless, their potential in ocular drug delivery is limited by rapid clearance from the precorneal region, as the same rapid drainage has been observed as for aqueous eye drops. In order to enhance adherence to the corneal/conjunctival surface, dispersion of these vesicular systems into mucoadhesive gels has been proposed (Aggarwal and Kaur, 2005, Gan et al., 2009). However, the high gel viscosity might adversely accelerate the blinking frequency, leading to a feeling of discomfort.

Monoolein (MO), as a nontoxic, biodegradable and biocompatible material classified as GRAS (generally recognized as safe), show the mesomorphic phase, important in making more comprehensible the potential pharmaceutical application of the lipid. It may exist in several different phases depending on temperature and hydration. The phase sequence at room temperature when adding water is as follows: lamellar crystalline phase (Lc) in coexistence with a L2 phase, lamellar liquid crystalline phase (Lα phase) and the inverted bicontinuous cubic phase (C). What is perhaps the most intriguing is the ability of cubic phases to exist in equilibrium with excess water and can be dispersed to form cubosomes.

Liquid crystalline phases of MO, such as cubic phases, present interesting properties for a topical delivery system (Carr et al., 1997, Lee and Kellaway, 2000a, Lee and Kellaway, 2000b), as they (i) are bioadhesive, (ii) present a permeation enhancer as the structure forming lipid (MO), and (iii) afford the ability to incorporate compounds independently of their solubility, protecting them from physical and enzymatic degradation, and to sustain their delivery (Shah et al., 2001). The emulsification of cubic lipid phases in water results in the production of cubosomes, which can be defined as nanoparticulate dispersion systems. It has been demonstrated that the dispersed particles retain the internal structure of the bulk phase and its properties (Nakano et al., 2002, Siekmann et al., 2002, Boyd, 2003). In comparison with the bulk gel, the dispersions present some advantages, such as larger surface area and high fluidity (low viscosity), and can be incorporated into other product formulations (Lopes et al., 2006).

Despite the amazing properties of cubosomes as innovative drug carriers, little research has thus far been performed to demonstrate their potential as ophthalmic drug delivery systems (Lee et al., 2004, Leesajakul et al., 2004, Esposito et al., 2005). The aim of this work was to study the performance of cubosomes as innovative ocular delivery systems for dexamethasone, chosen as a model drug.

DEX is a lipophilic glucocorticoid steroid, which is similar to the natural steroid hormone made by the adrenal glands in the body. It is known to be an effective anti-inflammatory drug for the treatment of acute and chronic posterior segment eye diseases such as uvetis (Phillips and Katz, 2005). In the generally used clinical product, Dex-Na phosphate eye drops, DEX exists in a hydrophilic ionized form which cannot effectively penetrate the lipophilic corneal epithelium. Therefore, it needs to be instilled 3–4 times per day. It is reported that continuous application of eye drops of 0.1% dexamethasone for extended periods of time could cause glaucoma accompanied by optic nerve damage, defects in visual acuity and fields of vision, and posterior subcapsular cataract formation and thinning of the cornea or sclera (Kim and Chauhan, 2008).

In this study, DEX-containing monoolein cubosomes were prepared, and their internal structure was further characterized by SAXR and cryo-TEM. An in vitro penetration study was performed using freshly excised rabbit cornea. Ocular tolerance was evaluated by corneal cross-sections after incubation. A noninvasive fluorescence imaging system was utilized to assess the preocular retention of the cubosomes. Finally, in vivo aqueous humor pharmacokinetics was investigated using the microdialysis method.