Inflammation is crucial for health, as it initiates the healing process [1]. Inflammation aggravates pain sensation, and the mismanagement thereof can negatively affect an individual’s quality of life [2]. Symptomatic treatment of these conditions is achievable by inhibiting the inflammatory process [3, 4]. Non-steroidal anti-inflammatory drugs (NSAIDs) are the leading approach for the treatment of pain [5]. The anti-inflammatory effects of NSAIDs are due to the inhibition of prostaglandin by blocking the cyclooxygenase (COX)-2 enzymes [6]. Non-selective NSAIDs (nsNSAIDs) also bind to the COX-1 enzyme that is responsible for gastrointestinal protection [7]. Diclofenac is a nsNSAID with a fast onset and long duration of action. However, extended oral administration often leads to adverse gastrointestinal effects due to the inhibition of COX-1 enzymes [8], which stresses the need for an alternative route of drug delivery, such as the transdermal route, for the administration of NSAIDs [5].
The skin is the largest, most accessible organ of the human body [9], but drug delivery proves challenging due to the rate-limiting outermost layer (stratum corneum) that functions as a barrier [10–13]. To overcome the stratum corneum hindrance during transdermal drug delivery, active pharmaceutical ingredients (APIs) require certain ideal physicochemical properties [14], i.e., molecular weight (< 500 Da) and melting point (< 200°C) [14, 15]. It became evident that diclofenac did not possess these ideal physicochemical properties; therefore, it was incorporated into a drug delivery vehicle to assist with transdermal delivery by altering the skin’s protective barrier [16].
Nano-emulsions (NEs), nano-emulgels (NGs), and a colloidal suspension (CS) containing drug-loaded nanoparticles (NPs) were formulated with diclofenac. Furthermore, the incorporation of a natural oil (evening primrose oil (EPO) acting as a penetration enhancer) into the NEs and NGs at different concentrations, allowed for increased API penetration through the skin [17].
NEs are dispersions of two immiscible phases (oil and water) that form droplets with sizes ranging between 20–500 nm [18]. The nanometric-sized droplets offer increased skin permeation and stability [19], while the hydrophilic and lipophilic characteristics, attained from the two phases, allow for numerous applications in drug delivery through the stratum corneum [20]. Oil-in-water (o/w) NEs are mainly used to deliver lipophilic APIs, such as diclofenac [21], and can subsequently be formulated by means of a high energy emulsification method that allows for the formation of ultra-small/nanometric droplets [22].
NGs possess all the ideal properties associated with NEs, as they are formulated by adding a gelling agent to the aqueous phase of the optimized NEs [23], which increases the low viscosity associated with NEs, allows for easy topical application and extended contact time between the drug delivery vehicle and the surface of the skin [24–26]. Carbopol® Ultrez 20 is a fast absorbing and easy wetting gelling agent, well known for its increased reproducibility and enhanced stability over a large pH range [26, 27].
CS containing drug-loaded NPs have gained significant interest in recent years due to its various uses in the field of nanotechnology [28]. NPs are synthesized by means of an emulsion-solvent evaporation technique that results in nanometric-sized particles (1-100 nm) with a large volume-to-surface area ratio that enhances API release [29]. NPs have enhanced stability, improved skin permeation, decreased adverse effects and depending on the type of NP, it can have high solubility for either lipophilic or hydrophilic (double emulsion method) APIs [29, 30]. To produce a CS, the formulated NPs underwent suspension in a continuous aqueous phase, which assists in the transdermal delivery of the particles [28].
The aim of this study was to formulate and characterize NEs and NGs as drug delivery vehicles with different EPO concentrations. Furthermore, diclofenac-loaded NPs underwent synthetization, suspension into a CS and characterization. The drug delivery vehicles were investigated and compared during membrane release, in vitro skin diffusion and tape stripping studies. The in vitro cytotoxicity of the NE presenting with the highest skin diffusion results, a placebo and the NPs was determined by methyl thiazolyl tetrazolium (MTT) and neutral red (NR) assays using human keratinocytes (HaCaT).