Ocular disposition of ciprofloxacin from topical, PEGylated nanostructured lipid carriers: Effect of molecular weight and density of poly (ethylene) glycol

Abstract

Ciprofloxacin (CIP) is an antibacterial agent prescribed for the treatment of ocular infections. The objective of the present project is to investigate the effect of surface PEG functionalization of the Nano structured lipid carriers (NLCs) on formulation stability, ocular penetration and distribution. CIP NLCs were tested with different molecular weight (poly ethylene glycol) PEGs ranging from (2 K to 20 K) grafted onto the phospholipid and with different chain lengths (14–18 carbons) of phospholipids derivatized with PEG–2K. Drug load in the formulations was maintained at 0.3%w/v. Formulations prepared were evaluated with respect to in vitro release, transcorneal permeation, autoclavability, morphological characteristics and in vivo ocular tissue distribution. Scanning Transmission electron microscopy (STEM) studies revealed that the PEG-CIP-NLCs were spherical in shape. Transcorneal penetration of CIP was optimum with PEG molecular weight in between 2 K–10 K. Carbon chain length of the phospholipid, however, did not affect transcorneal penetration of CIP. In vivo ocular tissue CIP concentrations attained from the various formulations was consistent with the in vitro data obtained. The results suggest that surface functionalization of PEGs, within a specified range of molecular weight and surface packing density, significantly enhance trans-ocular penetration and impart sterilization-stabilization characteristics into the formulations.

Introduction

Delivery of drugs, especially to the back-of-the eye tissues comprising sclera, choroid, retina, and vitreous body, is restricted by multiple physiological processes, anatomic, static, dynamic and efflux barrier functionalities (Gaudana et al., 2010, Adelli et al., 2015a). Efflux protein pumps expressed on ocular tissues restrict transmembrane permeability of drugs, thus lowering penetration of substrates from the systemic, topical or periocular routes (Cholkar et al., 2013, Chen et al., 2013). Topical application is the most favored route because of the ease of administration, lack of associated complications and minimal non-specific systemic exposure. Only 5–10% of the topically administered dose, however, reaches the inner ocular tissues (Patel et al., 2013, Yellepeddi and Palakurthi, 2016). Although advances have been made with respect to delivery into the anterior segment ocular tissues, significant challenges still exist for very lipophilic molecules in view of the formulation restrictions placed by the sensitivity of the ocular tissues. Several formulation approaches such as inclusion of viscosity enhancers in aqueous ophthalmic solution or suspension formulations, ion-exchange resin based formulations, implants, transporter targeted systems, emulsions, films and other nanoparticle mediated drug delivery strategies have been described in the literature, and some are commercially available (Geroski and Edelhauser, 2000, Adelli et al., 2017, Sharma et al., 2016, Singh et al., 2013). Despite technological advancements in the formulation strategies, delivery of therapeutic agents efficiently into the back-of-the eye ocular tissues through the topical route remains elusive (Rowe-Rendleman et al., 2014). Ointments have been successful to some extent but various drawbacks, including difficulty in application and problems in vision, have limited its usefulness. Success in back-of-the eye delivery mainly depends on formulation platform, candidate’s physicochemical properties and absorption pathway. Penetration of drugs across alternatively polarized (lipophilic and hydrophilic) ocular layers, and through the corneal tight junctions, is highly dependent upon their physicochemical properties. Thus, the molecules should exhibit optimum physicochemical aspects and are to be formulated in appropriate dosage forms for enhanced retinal delivery (Barar et al., 2016, Adelli et al., 2014).

Kinetics, bio-distribution and release profile of drugs could be dramatically modulated with nano particulate systems (Maeda et al., 2009, Barenholz, 2012). Nanoparticles have been observed to exhibit superior penetration characteristics into the inner ocular tissues compared to solution or suspension formulations (Kompella et al., 2013). Lipid based systems such as nanostructured lipid carriers (NLCs) are potential carriers for therapeutic agents, especially hydrophobic molecules, and possess favorable properties including but not limited to biocompatibility, mucoadhesion, penetration/retention capability, lower clearance rate, controlled release, greater stability and protection of the drug candidate from chemical degradation. NLC’s can be formulated from a wide variety of lipids (solid/liquid) and phospholipid combinations with varying composition, to achieve desired morphometrical, physicochemical, surface charge and release characteristics. Mixture of solid and liquid lipids used in NLC’s create imperfections in the crystal lattice accommodating higher drug loads while maintaining similar penetration capabilities as the solid lipid nanoparticulates (SLNs). In addition, NLCs allow higher drug loading compared to SLNs, exhibit better encapsulation efficiency, lesser drug expulsion and higher stability (Yoon et al., 2013, Müller et al., 2002, Tiwari and Pathak, 2011). Reports suggest that PEGylated amphiphilic lipids possess the ability to transform into lipid based lyotropic crystals with thermodynamically stable self-assembled structures in aqueous environment (Chen et al., 2014, Balguri et al., 2015). In recent years, PEGylation technology (functionalization of nano carriers with PEG’s and appropriate ligands) has been widely used to improve the pharmacokinetics, bioavailability and tissue distribution characteristics of a variety of nanoparticles, because the hydrophilic and inert PEG creates a steric barrier on the surface of nanoparticles and minimizes protein binding (Zheng et al., 2014). The bulky and highly hydrated corona of the PEG extending from the lipid bilayer into the aqueous phase is critical for enhancing steric stabilization of the nanoparticles (Schilt et al., 2016). Also, incorporation of PEG could allow better stabilization against aggregation, on storage and on sterilization – by amorphization and inducing imperfections in crystal lipid lattices (Kakkar et al., 2015, Balguri et al., 2016).

Ciprofloxacin (CIP) belongs to class of fluoroquinolone antibiotics and is active against a broad spectrum of gram-positive and gram-negative bacteria. It is usually prescribed as the first line of treatment for corneal keratitis, allergic conjunctivitis and other bacterial infections of the eye. CIP is a zwitterion with pKa values of 6.0 (acidic group) and 8.8 (basic group) and an isoelectric point of 7.2 where it is least soluble (neutral species). The compound is currently marketed as an ophthalmic solution and needs frequent dosing due to its poor ocular bioavailability (Hosny, 2010). Because of solubility issues, the formulation has to be maintained at an acidic pH. On topical application, however, because of the buffering action of the tear fluid, the pH of the instilled formulation is quickly neutralized as a result of which the solubility of CIP in that environment is significantly reduced and precipitation can take place. Consequently, penetration of CIP into the interior ocular tissues is hampered. In general, there exists a need to enhance drug penetration into the ocular tissues through the topical route. Moreover, improved delivery and penetration of ocular drugs with solubility issues, such as CIP, would be highly beneficial for intervening in complications associated with bacterial infections.

The objective of the current research is to assess the effect of type and density of surface PEGylation of CIP loaded NLCs in terms of process (including autoclave sterilization) stability characteristics and ocular disposition.