**1. Introduction**

Ophthalmic drug delivery continues to pose challenges to formulation scientists due to the complex biochemical, anatomical, and physiological ocular barriers. The biological

**Citation:** Nair, A.B.; Shah, J.; Al-Dhubiab, B.E.; Jacob, S.; Patel, S.S.; Venugopala, K.N.; Morsy, M.A.; Gupta, S.; Attimarad, M.; Sreeharsha, N.; et al. Clarithromycin Solid Lipid Nanoparticles for Topical Ocular Therapy: Optimization, Evaluation, and In Vivo Studies. *Pharmaceutics* **2021**, *13* , 523 . https://doi.org/ 10.3390/pharmaceutics13040523

Academic Editors: Ana Catarina Silva and José Manuel Sousa Lobo

Received: 22 February 2021 Accepted: 6 April 2021 Published: 9 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

membranes that protects the anterior and posterior segments of the eye, in addition to the unique structure of the cornea, result in poor ocular bioavailability [1]. Topical drug administration to the anterior segment of the eye causes large pre-corneal clearance because of their high tear turn over (0.5–2.2 µL/min) and rapid blinking rate (6–15 times/min) [2]. Consequently, conventional ophthalmic dosage forms must be applied with frequent instillations to attain and/or control targeted drug levels within the anterior segment of the eye. Delivering the drug molecules to the posterior segment of the eye is challenging primarily due to the long diffusion pathway and the cellular characteristics of the vitreous humor [3]. Therefore, alternative techniques are utilized to transport drugs to the posterior vitreous, the uveal tract, retina, or choroid [4]. Endophthalmitis is a severe intraocular inflammatory infection that affects the vitreous and aqueous fluids within the anterior and posterior region of eye [5]. It is the infection generally caused due to organisms such as bacteria, fungi, or parasites that enter the eye through the blood stream, surgery in the eye or other parts near the eye, or sepsis [6]. A literature review indicated that few drug delivery systems have been developed and their efficacy evaluated in animals. In one attempt, the therapeutic effect demonstrated by an intravitreal drug delivery system of voriconazole was superior to intravitreal injection in rabbits [7]. Chitosan nanoparticles containing daptomycin were developed in ocular treatment of bacterial endophthalmitis. It was reported that these nanoparticles had appropriate characteristics to recommend as a non-invasive method for the ocular delivery of daptomycin to the eye [8].

Clarithromycin is a broad spectrum macrolide antibiotic and shows potential action against various organisms, including *Staphylococcus aureus*, *Streptococcus pneumoniae*, *Legionella pneumophila*, *Moraxella catarrhalis*, *Chlamydia trachomatis*, and *Mycobacterium avium* [9]. Clarithromycin is chemically 6-*O*-methylerythromycin (C38H69NO13), with a molecular weight of 747.95 Dalton, pKa value of 8.99, and melting point ranges between 217 and 220 ◦C [10]. Different solubility studies demonstrated that clarithromycin is soluble in acetone, slightly soluble in methanol, ethanol, and acetonitrile, and practically insoluble in water (0.33 mg/L). Lipophilicity is a major determining factor in a compound's absorption, distribution in the body, penetration across vital membranes and biological barriers, metabolism, and excretion. The pharmacokinetics of clarithromycin in adults showed average oral bioavailability of 53%, plasma concentration between 2.41 and 2.85 µg/L after 500 mg dose, and a half-life of ~4 h [10,11]. Pharmacokinetic and pharmacodynamic investigations suggest that clarithromycin exhibits a similar interaction profile as that of erythromycin [12]. Lipid-based drug delivery system have the potential capacity to entrap both lipophobic and lipophilic drugs, enhance the bioavailability of low aqueous soluble drugs, and protect them against degradation. In the last few decades, liquid nanoemulsions have been increasingly used as drug carriers for lipophilic drugs [13]. However, the feasibility of controlled drug release from nanoemulsions is restricted due to the submicron globule size and the fluid state of the carrier. Alternatively, solid matrices of nanostructured lipid carriers (NLCs) and solid lipid nanoparticles (SLNs) help to improve the stability and safety and provide controlled drug release, and they can incorporate both hydrophilic and hydrophobic molecules and can be used in various routes [14]. The basic difference between NLCs and SLNs is the type of lipids used, where liquid lipids are used in NLCs and solid lipids in SLNs [14].

SLNs are nano-sized (10–1000 nm) particles formed when the solid lipids are dispersed in aqueous media containing surfactants as stabilizers [15]. They widely accepted as a prospective delivery system similar to other colloidal carriers, including liposomes and other polymeric nanoparticles [16,17]. Due to the inclusion of physiologically compatible lipids of either natural or synthetic origin, nonirritant and nontoxic properties in SLNs reduce the potential threats of acute or chronic toxicity [15]. Furthermore, advantages such as sustained and controlled drug release, good physical stability, resistance to degradation of lipids, in vivo acceptability, and ability to increase pre-corneal retention time make SLNs a very adaptive carrier for various drug delivery systems [18]. Indeed, formulations of SLNs can be performed in the absence of organic solvents, can be sterilized, and can also use

excipients that are "generally recognized as safe" (GRAS), and hence can be easily scaledup in industry [19]. The incorporation of lipids in the solid state compared to the liquid form is very effective to obtain controlled drug release, since drug mobility is significantly retarded in solid lipid when compared to liquid oil. Extensive research has been carried in the last two decades to tap the potential of SLNs in delivering drugs through various routes, including the ocular route [19–22]. The literature suggests that SLNs have been widely studied in the treatment of ocular inflammations, infections, glaucoma, cataracts, age related macular degeneration, and as gene therapy carriers [23]. In light of this, the objective of the present study was to design, formulate, and evaluate the potential of drug-loaded SLNs to improve the therapeutic efficacy of clarithromycin in ocular therapy. Fractional factorial design was applied for preliminary screening of various factors utilized in the SLN formulation. A 3<sup>2</sup> full factorial design was employed to evaluate the influence of independent variables on the dependent variables. Selected SLN were assessed for drug permeation using goat cornea and evaluated in vivo in rabbits.
