Mucoadhesive effect of thiolated PEG stearate and its modified NLC for ocular drug delivery

Abstract

This study was to develop a thiolated non-ionic surfactant, cysteine-polyethylene glycol stearate (Cys-PEG-SA), for the assembling of nanoparticulate ocular drug delivery system with mucoadhesive property. Cys-PEG-SA was synthesized in two steps reaction involving a new derivative intermediate formation of p-nitrophenylcarbonyl-PEG-SA (pNP-PEG-SA). Up to 369.43 ± 25.54 μmol free thiol groups per gram of the conjugates was reached. The nanostructured lipid carrier (NLC) loaded cyclosporine A (CyA) was prepared by melt-emulsification method. The mucoadhesive NLC (Cys-NLC) was obtained by incubating NLC emulsion with Cys-PEG-SA. The mucoadhesive properties of these nanocarriers were examined by using mucin particles method. The particle size or zeta potential of the porcine mucin particles were changed with the added concentration of Cys-PEG-SA, and the disulphide bond breaker cysteine significantly reduced the adhesion of Cys-NLC to mucin particles (P < 0.05), whereas PEG-SA and NLC did not alternate the properties of the mucin particles. When Cys-NLC was administered topically to the rabbit eye, the encapsulated cyclosporine was found to remain on the ocular surface in the cul-de-sac for up to 6 h, both precorneal retention time and concentration were dramatically increased (P < 0.05), compared with the NLC without thiomer modification.

Introduction

Topical drug delivery into the eyes is, by far, the most common and well-accepted route of administration for the treatment of various eye diseases. A major problem in conventional ophthalmic drug delivery is low drug bioavailability due to ocular anatomical and physiological constraints, which include the poor permeability of cornea, nasolacrimal drainage effect, and short retention time in the precorneal area [1]. Attempts to improve ocular bioavailability have been focused on overcoming precorneal constraints through improving corneal penetration and prolonging the precorneal retention [2].

Over the past few years, mucoadhesive polymers received considerable attention as new excipients for ophthalmic drug delivery system due to their ability to prolong the residence time of dosage forms and enhance drug bioavailability as well [3], [4], [5]. The mucus in the eye is mainly produced by the conjunctival goblet cells, and it forms the bottom layer of the tear film wetting the corneal epithelium [6]. Mucoadhesive polymers can adhere to the mucin coat covering the conjunctival and the corneal surfaces of the eye by hydrogen bonding, electrostatic, covalent and hydrophobic interactions, which forms the basis of ocular mucoadhesion [7].

Recently, a new generation of mucoadhesive polymers-thiolated polymers or so-called thiomers, has been developed to improve the adhesive properties of polymers. Thiomers is a kind of mucoadhesive basis polymers with thiol bearing side chains, which are capable of forming disulfide bonds between such polymer and cysteine-rich subdomains of the mucus gel layer of the mucosa [8], [9]. The underlying mechanism is based on thiol/disulfide exchange reactions and an oxidation process between the reactive thiol groups of the thiomers and the mucin glycoproteins [10]. The extensive cross-linking process of the thiomers with mucin results in a tremendous increase in viscosity [11] and mucoadhesion [12], thus, it can significantly prolong the residence time and also provide an intimate contact between drugs and the absorbing tissues. The efficacy of this new generation of mucoadhesive polymers has been demonstrated by various in vivo studies in different species on the gastrointestinal [13], nasal [14], and ocular mucosa [15].

Being mostly studied in the literature to prolong the retention of drugs in the eye, colloidal drug delivery systems was found to be taken up by the corneal epithelial cells, which serve as a reservoir to release the drug slowly to the surrounding tissue [16], [17]. Further works in this field have shown that the surface composition of the colloidal system may affect its affinity towards the ocular mucosa. The positive surface charge was found to have a positive effect in prolong the residence time of emulsion droplet on the epithelial layer of the cornea [18]. Nanospheres coated with bioadhesive polymer, such as hyaluronic acid (HA), chitosan (CS), indicated that the nature of the coating layers affected the interaction of the colloidal systems with the corneal epithelial cells [19], [20].

The aim of this study was to develop a novel and simplified thiolated agent for the surface modification of ocular nanostructured lipid carrier (NLC). Polyethylene glycol stearate (PEG-SA) is an amphiphilic polymeric derivative of hydrophilic polyethylene glycol modified by attaching a hydrophobic moiety, which can be easily incorporated into the lipid core of colloidal carriers with the hydrophilic PEG chain on their surface. The cysteine–polyethylene glycol stearate conjugates (Cys-PEG-SA) was synthesized and characterized. The formation of mucoadhesive coating layer based on the thiolated non-ionic surfactant would be advantageous to improve the mucoadhesive properties of colloidal drug delivery systems. The mucoadhesive properties of Cys-PEG-SA and its modified nanostructured lipid carrier (Cys-NLC) were evaluated in vitro and in vivo, comparing with that of NLC and nanocarrier modified by PEG-SA (PEG-NLC).