Design and Characterisation of Niosomes for Ocular Delivery of Naltrexone Hydrochloride

Abstract

Background: Recent reports have demonstrated that topical ocular administration of naltrexone (an opioid growth factor antagonist) is able to reverse the diabetic complications on the cornea (diabetic keratopathy). The topical administration of naltrexone accelerates corneal wound healing, restores corneal sensitivity and enhances corneal epithelialisation in diabetic rats, rabbits and humans. Naltrexone can be considered as a new therapeutic agent for treatment of diabetic keratopathy. To our best knowledge, an ophthalmic formulation has not yet been developed for naltrexone hydrochloride (NTX) nor has it been properly formulated in a liquid dosage form. Niosomes are non-ionic surfactant vesicles and have the same closed bilayer structures and properties of the well-known phospholipid vesicles, liposomes. Niosomes are considered more chemically stable and more economically viable alternatives to liposomes. At the same time, they could offer the same advantages as vesicular ocular delivery systems such as prolonged precorneal residence time and enhanced corneal penetration. Objective: The aim of this work is to design and characterise niosomal formulations for ocular delivery of NTX having the convenience of being delivered as eye drops. Methods: An HPLC method was developed and validated for NTX; preformulation studies were performed to study the physicochemical properties of NTX, such as aqueous solubility, lipid solubility, melting behaviour, spectrometric identification and construction of the Arrhenius plots for predicting NTX stability. Two classes of non-ionic surfactants, sorbitan fatty acid esters (Span®) and polyoxyethylene alkyl ethers (Brij®), were selected and investigated for their ability to form niosomes and encapsulate NTX using different levels of cholesterol (a bilayer membrane stabiliser). Also, different membrane additives [dicetyl phosphate (DCP), Solulan C24 (C24) and sodium cholate (CH)] were studied to modify the physical properties of niosomes, such as their percentage entrapment efficiency (EE %), size, morphology, rheology and spreading ability characteristics. Four different niosomes; F-S60, FDCP, F-C24 and F-CH were prepared and evaluated for ocular irritation, using the hen’s egg chorioallantoic membranes (HET-CAM) test, bovine eye test and histological corneal examination. Finally, they were studied for in vitro NTX release; transcorneal permeation of NTX; and physical stability. Results and discussion: The preformulation studies showed that NTX is a hydrophilic drug with a log P value of 1.61 at 35oC. The main degradation pathway of NTX in aqueous solutions was found to be autoxidation. Span 60-based niosomes, consisting of Span 60: cholesterol 7:3 mol/mol (F-S60), demonstrated superior EE % for NTX. Cryogenic scanning electron microscope (Cryo-SEM) images of niosomes showed onion-like structures, indicating multilamellarity. Incorporation of the membrane additives into bilayer membranes produced both spherical niosomes and discomes (giant disc-like niosomes), as confirmed by confocal laser scanning microscope (CLSM). The vesicular incorporation of NTX imparted a protective effect against light-induced oxidation, and the prepared niosomes demonstrated minimal to no irritation potential; significantly controlled NTX release; enhanced transcorneal permeation of NTX through excised cow corneas compared with the aqueous NTX solution; and were physically stable at least for three months at 4oC. Conclusion: The developed niosomes were able to protect NTX chemically, impart a penetration-enhancing effect and control NTX corneal permeation through bovine corneas, and demonstrate good ocular tolerability, suggesting that they are potential ocular delivery systems for NTX.