W/O microemulsions for ocular delivery: Evaluation of ocular irritation and precorneal retention

Abstract

Water-in-oil microemulsions (w/o ME) capable of undergoing a phase-transition to lamellar liquid crystals (LC) or bicontinuous ME upon aqueous dilution were formulated using Crodamol EO, Crill 1 and Crillet 4, an alkanol or alkanediol as cosurfactant and water. The hypothesis that phase-transition of ME to LC may be induced by tears and serve to prolong precorneal retention was tested. The ocular irritation potential of components and formulations was assessed using a modified hen's egg chorioallantoic membrane test (HET-CAM) and the preocular retention of selected formulations was investigated in rabbit eye using gamma scintigraphy. Results showed that Crill 1, Crillet 4 and Crodamol EO were non-irritant. However, all other cosurfactants investigated were irritant and their irritation was dependent on their carbon chain length. A w/o ME formulated without cosurfactant showed a protective effect when a strong irritant (0.1 M NaOH) was used as the aqueous phase. Precorneal clearance studies revealed that the retention of colloidal and coarse dispersed systems was significantly greater than an aqueous solution with no significant difference between ME systems (containing 5% and 10% water) as well as o/w emulsion containing 85% water. Conversely, a LC system formulated without cosurfactant displayed a significantly greater retention compared to other formulations.

Introduction

Ocular conditions are usually treated by topical application of drug solutions administered as eye drops. These conventional dosage forms account for around 90% of the available ophthalmic formulations, mainly due to their simplicity and convenience [1]. However, extensive precorneal loss caused by rapid drainage and high tear turnover are amongst the main drawbacks associated with topical ocular drug delivery. Only 1–5% of the topically applied drug penetrates the cornea/sclera and reaches the intraocular tissue with the remainder of the instilled dose undergoing non-productive absorption via the conjunctiva or drainage via the nasolacrimal apparatus. This results in drug loss into the systemic circulation and provides undesirable systemic side effects [2]. Many strategies have been implemented to overcome such delivery challenges. These included the use of thermosetting in situ gelling polymer-based systems [3], nanoparticles, liposomes and niosomes [4], [5], [6], [7]. On the other hand, microemulsions offer a promising alternative. They are thermodynamically stable and optically isotropic colloidal systems with excellent wetting and spreading properties. Moreover, they are comprised of aqueous and oily components and therefore can accommodate both hydrophilic as well as lipophilic drugs.

We have previously reported on the formulation and physico-chemical characterisation of two microemulsion-forming pseudoternary systems [8]. The first is characterised by a phase diagram in which, the single-phase, oil-rich microemulsion (ME) region is followed upon increasing water concentration by a region where liquid crystalline (LC) and coarse dispersed systems form. In the second, the oil-rich and water-rich ME systems merge together forming a continuous one-phase region upon increasing water concentration and the phase diagram is devoid of any LC systems. Systems comprising the two non-ionic surfactants sorbitan mono-laurate (Crill 1) and polyoxyethylene 20 sorbitan mono-oleate (Crillet 4 super) along with oily component ethyl oleate (Crodamol EO) and water as well as systems formulated with 1-hexanol, 1-octanol, 1,2-propandiol, 1,2-pentandiol and 1,2-octandiol as cosurfactants can be classified under the first category whereas systems formulated with 1-propanol, 1-butanol and 1,2-hexandiol as cosurfactants can be classified under the second category [8].

A prototype of the first category is the cosurfactant-free system and its phase behaviour is illustrated in Fig. 1. In this system, the LC region lies next to the ME region and towards the water apex. Thus, fluid ME systems that can undergo a phase-transition to viscous systems containing lamellar liquid crystals upon dilution with water can be formulated using these excipients (arrow, Fig. 1). It is postulated that the phase transition of a cosurfactant-free droplet w/o ME to a more viscous LC system could be induced in situ once such systems are instilled into the eye. Such transition could serve to prolong precorneal retention and hence have application in ophthalmic drug delivery. In contrast to the conventional in situ gel forming systems, the phase transition ME reported herein are oil-based, hence particularly suited for the solubilisation and delivery of lipophilic actives. Further, since these ME systems also contain aqueous domains, they may also have an application in the formulation of hydrophilic or amphiphilic bioactives for ophthalmic drug delivery.

Before testing this hypothesis it was necessary to establish the ocular irritation potential of the developed systems as well as the components used in their formulation. A lack of any ocular adverse effects such as irritation, burning, stinging and tearing is an essential requirement of any system intended for ocular use. This was also considered an essential prerequisite to any in vivo testing as the colloidal systems under investigation contained potential irritant chemicals such as non-ionic surfactants, fatty acid esters, n-alcohols, and 1,2-alkanediols.

Irritation testing of the formulations and the ingredients used to formulate these systems was carried out using a modified HET-CAM test (hen's egg chorioallantoic membrane test) [9]. The chorioallantoic membrane (CAM) is a highly vascular embryonic membrane formed by the fusion of the allantoic wall with the chorion. As a highly vascular and stratified tissue, it responds to injury with an inflammatory reaction similar to that produced by the conjunctiva. Thus the CAM has been suggested as a model for the conjunctiva of the eye [10]. The usefulness of the HET-CAM test to assess the irritation potential of surfactant based and hydro-alcoholic formulations is well established and it is reported that protocols with short exposure times seem to provide more accurate results compared to others with longer exposure time [11]. In addition, the HET-CAM test has been reported as a useful in vitro assay for assessing the eye irritation properties of cosmetic formulation and their ingredients with high agreement of the results with in vivo data [12].

Following irritation testing, a precorneal clearance study using gamma scintigraphy was conducted to evaluate the precorneal retention of the non-irritant formulations and test the hypothesis that dilution of the ME system and subsequent phase transition upon ocular administration by resident tear fluid would enhance retention. The investigated ME systems have a fluid consistency, thus enable ease of administration and the resulting phase-transition may facilitate prolonged retention by virtue of the high viscosity of the formed LC systems. It is well established that nasolacrimal drainage is the major factor behind precorneal drug loss that leads to poor ocular bioavailability and systemic exposure with possible side effects and potential toxicity [13]. Thus it was anticipated that the in situ phase transition would serve to increase ocular retention and subsequently, improve ocular bioavailability. The aims of this work were therefore to:

• establish the physiological compatibility of the components used to formulate the previously described pseudoternary systems so as to select formulations that could be evaluated in vivo and;

• investigate whether a phase-transition w/o ME based formulation indeed demonstrates prolonged preocular retention.