Development of Dermal Lidocaine Nanosuspension Formulation by the Wet Milling Method Using Experimental Design: In Vitro/In Vivo Evaluation

Abstract

Lidocaine (LID), frequently used in dermal applications, is a nonpolar local anesthetic agent that is practically insoluble in water. The main aim of this study is to develop the nanosuspension formulation of LID using the design of experiments (DoE). The improved solubility and dissolution rate provided by nanosizing are expected to result in enhanced dermal bioavailability. Nanosuspension formulations were developed by a wet media milling method using different stabilizer types [poloxamer (POL) and poly(vinyl alcohol) (PVA)]. Characterization studies of the nanosuspensions were carried out using DSC, FTIR, XRD, and SEM in vitro release from the dialysis membrane and ex vivo permeation studies using rat skin were performed. Analgesic/anesthetic effects were evaluated using the tail-flick test in in vivo studies. Particle size (PS), polydispersity index (PDI), and zeta potential (ZP) values were found as 171.7 ± 3.52 nm, 0.251 ± 0.036, and −32.2 ± 0.907 mV for POL/LID nanosuspensions and 262.1 ± 29.42 nm, 0.453 ± 0.071, and −20.2 ± 3.50 mV for PVA/LID nanosuspensions, respectively. Compared to the coarse suspension of LID, it was determined that it accumulated in the skin approximately 1.81 times more in the POL/LID nanosuspension formulation and 1.79 times more in the PVA/LID nanosuspension formulation. According to analgesic effect and related AUC data, nanosuspension formulation was found to be statistically more effective than coarse suspension. It is concluded that DoE is a useful tool in determining process parameters when developing nanosuspensions by the wet media milling method, and POL is a suitable nonionic polymer to stabilize nanosuspensions.

Introduction

Dermal application is a way in which active ingredients can be applied effectively and efficiently through the skin. (1) When the active substance is applied dermally, it has advantages such as providing good patient compliance, being noninvasive, creating minimal drug–drug interactions, ease of application, reducing systemic side effects in case the disease originates from the skin, and providing continuous/controlled release at the site of action. (2) Dermal application may result in reduced pharmacological efficacy due to poor skin penetration of the active ingredients. It has been reported that various nanotechnological approaches such as liposomes, solid lipid nanoparticles, niosomes, transfersomes, ethosomes, nanostructured lipid carriers, nanoemulsions, dendrimers, and micelles can overcome these disadvantages. (3)

An alternative approach used for this purpose is to develop a nanosuspension formulation. In nanosuspensions, active substance nanocrystals are pure active substance particles smaller than 1000 nm and are stabilized with appropriate surfactants and/or polymers. (4) The nanometer-sized stabilized particles of the active substance can be absorbed more quickly and easily through the skin and enter the underlying tissues. In nanosuspensions, active substances barely soluble in water have a large-surface area, and therefore both the dissolution rate and water solubility of the active substance increase. They provide accumulation of active substances in the skin in nanoparticulate form, increasing skin penetration and bioavailability of drug molecules by causing an increased concentration gradient. (5,6) In addition, it has been reported that these systems increase the dermal pharmacological effect of the active substance as it accumulates in skin appendages and skin layers, especially in epidermis. (7,8)

The pH of an intact skin surface is generally 5.5, which is considered the classic cutaneous pH. This acidic pH value usually varies between 4 and 6 due to many factors such as age and gender. (9) pH is an important parameter affecting the rate of absorption of acidic and basic drugs, and the nonionized form of the drug penetrates better through the skin. The movement of ionizable particles in aqueous solutions is largely dependent on pH. (10,11) When the pH of the nanosuspensions is close to the pH of the stratum corneum, the nanosuspensions are in a nonionic form and the permeability of the drugs increases. (10) For this reason, in our study, the poorly water-soluble (nonionized) base form of lidocaine was used and nanosuspension formulations were developed.

Lidocaine is a local anesthetic agent that is practically insoluble in water. (12) In percutaneous or dermal applications, LID penetrates the stratum corneum and desensitizes pain receptors in the skin. Disadvantages such as polymorphism and low bioavailability seen in crystalline pharmaceuticals limit the transdermal application of LID. (13) With drug carrier systems such as nanosuspensions, it is possible for an active substance to penetrate the skin more easily and to provide sustained effect with slow release of the drug substance. In recent years, targeting topically applied active substances to different skin layers as particulate carriers has become an important research topic. (14) For this purpose, many drug carrier systems such as nanoethosomes, (15) solid lipid nanoparticles, (14) microemulsions, (16) nanostructured lipid carriers, (17) silica nanoparticles, (18) and liposomes (19) have been prepared containing LID. In this study, a nanosuspension formulation of LID was prepared to benefit from the advantages of nanosuspensions.
Nanosuspensions generally consist of active ingredient nanocrystals, surfactant or polymeric type stabilizers, and liquid dispersion medium. (20) The type and amount of stabilizing agents have a significant impact on the physical stability and in vivo behavior of the nanosuspension. Examples of the most commonly used stabilizers are poloxamers, polysorbates, cellulose derivatives, povidone, and lecithin. (21)

In this study, LID nanosuspension formulations were prepared using the media milling method. This technique has advantages such as high flexibility in handling, simplicity, high reproducibility, low use of excipients, low batch-to-batch variation, and easy scale-up compared to other nanosuspension production methods. (22,23) When preparing nanosuspension by media milling, there are many process parameters that need to be optimized such as bead size, milling time, milling speed, and bead volume. (24) For this purpose, a factorial design with two 23 (2 levels, 3 factors) three repetitions was performed separately using Design Expert software to determine the most appropriate process parameters.

The approach of using design of experiments (DOE) in quality by design (QbD) provides pharmaceutical researchers with the opportunity to obtain products in a shorter time with fewer experiments. (25) DOE helps identify and classify (critical or noncritical) various formulation and process parameters that affect system quality. Interactions between various input variables can be detected and quantified with a well-implemented DOE. It also provides the opportunity to predict desired quality attributes over the design space. (26) The choice of experimental design depends on the objectives of the experiment and the number of factors to be investigated.

In this study, it was aimed to develop a nanosuspension formulation that would allow LID to accumulate in the skin and have a greater anesthetic effect. The use of the base form was preferred because it penetrates the skin more easily, accumulates in the stratum corneum, and maintains its local anesthetic effect for a long time. In order to increase its dermal efficacy, LID nanosuspensions were prepared by using experimental design. The effect of process parameters on the PS, PDI, and ZP values of nanosuspensions in the wet media milling method was determined. The effect of nanosuspensions on the dermal bioavailability of LID was explained by permeation and skin accumulation experiments. Additionally, the effectiveness of the formulations was evaluated in vivo by the tail flick test.

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Development of Dermal Lidocaine Nanosuspension Formulation by the Wet Milling Method Using Experimental Design: In Vitro/In Vivo Evaluation, Özlem Kral, Sibel Ilbasmis-Tamer, Sevtap Han, and Figen Tirnaksiz, ACS Omega Article ASAP, DOI: 10.1021/acsomega.4c05296

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