Formulation, Development & Characterization of Nanostructured Lipid Carrier Loaded Topical Gel for Atopic Dermatitis

Pratik Singh¹

¹Project Trainee at Sun Pharma R&D Vadodara Gujarat, India.

 *Corresponding Author: pratikkshatri1234@gmail.com  

Abstract:

Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterized by pruritus, eczematous lesions, and impaired skin barrier function. Conventional topical corticosteroid therapies are often limited by poor skin penetration, frequent application, and potential adverse effects. The present study aimed to develop and evaluate a nanostructured lipid carrier (NLC)–based topical gel to enhance dermal delivery of a poorly water-soluble anti-inflammatory corticosteroid. NLCs were prepared using a biocompatible solid–liquid lipid matrix and optimized surfactants (Poloxamer 188 and Tween 80) through homogenization and ultrasonication. The optimized formulation exhibited favorable particle size, high drug entrapment efficiency, and sustained drug release. Incorporation of NLCs into a Carbopol-based gel resulted in a formulation with suitable pH, viscosity, spreadability, and rheological properties for topical application. The NLC-loaded gel demonstrated enhanced skin permeation and retention, indicating improved therapeutic efficacy and reduced dosing frequency. This NLC-in-gel system offers a promising and patient-compliant approach for the topical treatment of atopic dermatitis.

Keywords: Atopic dermatitis; Nanostructured lipid carriers; Topical drug delivery; Corticosteroid; Skin permeation; Carbopol gel

1. INTRODUCTION

Atopic dermatitis (AD) is a chronic, relapsing inflammatory skin disorder characterized by intense pruritus, erythema, scaling, and xerosis. Its global prevalence has increased markedly in recent decades, affecting both pediatric and adult populations. The complex pathophysiology of AD involves impairment of the skin barrier function and immune dysregulation, resulting in recurrent flare-ups and significant deterioration in quality of life. Current therapeutic strategies primarily include topical corticosteroids, emollients, and immunosuppressive agents. Although topical corticosteroids remain the cornerstone of AD management, their long-term use is associated with adverse effects such as skin atrophy, systemic absorption, and delayed wound healing, underscoring the need for safer and more effective delivery approaches.

Topical corticosteroid therapy is often limited by inadequate skin penetration and low local bioavailability, which can compromise therapeutic efficacy and increase the risk of systemic exposure. In this context, nanostructured lipid carriers (NLCs) have gained considerable attention as advanced lipid-based drug delivery systems for topical applications. NLCs, composed of a combination of solid and liquid lipids, offer advantages including enhanced drug solubility, improved stability, controlled release, and increased penetration into the stratum corneum. These properties enable sustained drug release at the site of action while minimizing systemic side effects.

2. MATERIAL & METHODS

Drug “X” was obtained from Sun Pharmaceutical Industries Ltd. Solid lipids (glyceryl dibehenate, glyceryl monostearate, stearic acid) were sourced from Gattefosse SAS and Indo Pharma Chem. Liquid lipids (oleic acid, sesame oil, MCT) and surfactants (Tween 80, Poloxamer 188) were procured from Croda Inc. and BASF. Carbopol 974P, glycerine, triethanolamine, and analytical-grade methanol, sodium acetate, and acetic acid were used. Water for injection was from SPARC. Equipment included an IKA Ultra Turrax T25 homogenizer, CV33 probe sonicator, Zetasizer Nano ZS, Shimadzu UV–Vis spectrophotometer, Anton Paar rheometer, and Franz diffusion cells.

3. METHOD

3.2.1 Pre-Formulation Studies

3.2.1.1 Organoleptic Evaluation

The visual inspection was performed to assess the physical properties of Drug on various parameters such as colour, odour, texture(50).

3.2.1.2 Solubility Studies

The drug’s solubility was assessed by adding 100 mg to 10 mL of various solvents. Samples were mixed, equilibrated for 24 hours at room temperature, examined visually and microscopically, and classified according to USP solubility terms(51).

3.2.1.3 Estimation of Melting Point

The melting point of the drug was determined using the capillary method. A dry, powdered, and homogeneous sample was packed into capillary tubes to a height of 2.0–3.0 mm and placed in a melting point apparatus. The temperatures at which melting began and was completed were recorded(52).

3.2.1.4 Partition Coefficient

The partition coefficient of the drug was determined using the n-octanol/acetate buffer system. A 10 mg drug sample was equilibrated between both phases, separated, and the drug concentration in each phase was measured by UV spectrophotometry to assess lipophilicity(53).

3.2.1.5 Determination of (λ_max) of Drug

The UV spectrum of the drug was recorded in ethanol. An accurately weighed 10 mg sample was dissolved in ethanol to prepare a 50 μg/mL solution, and the spectrum was scanned from 200 to 400 nm using a UV–visible spectrophotometer with ethanol as the blank(54).

3.2.1.6 Construction of calibration curve of Drug in ethanol

A stock solution (100 µg/mL) was prepared by dissolving 10 mg of the drug in ethanol and diluting to 100 mL. Aliquots were further diluted to obtain 2–10 µg/mL standards, and absorbance was measured at 236 nm using ethanol as the blank to construct a calibration curve.

3.2.1.7 Construction of calibration curve of Drug in Acetate Buffer Solution pH 5.5 + 0.5% Tween 20

A 100 µg/mL stock solution was prepared in acetate buffer (pH 5.5) with 0.5% Tween 20, diluted to 2–10 µg/mL, and analyzed spectrophotometrically to generate a calibration curve.(55).

3.2.1.8 Screening of Solid Lipid

Solid lipid screening was performed by assessing the saturation solubility of the drug in various solid lipids. The lipid showing maximum drug solubilization was selected for formulation development (56).

3.2.1.9 Selection of Liquid Lipid

Liquid lipid screening was conducted by evaluating drug solubility in various oils, and the oil showing maximum drug solubilization was selected for formulation development (57).

3.2.1.10 solid and liquid lipid Ratio Selection

Solid and liquid lipids were blended at different ratios, heated, and evaluated for homogeneity. The single-phase mixture without oil separation was selected for NLC development (54,57).

3.2.1.11 Selection of Surfactant

Surfactant selection significantly influences lipid nanoparticle quality. Tween 80 and Poloxamer 188 were chosen based on their high HLB values, as they effectively reduce interfacial tension and promote stable NLC formation.

3.2.1.12 Compatibility Study(58)

FTIR spectroscopy was used to identify drug peaks and assess drug–lipid compatibility by comparing spectra of the pure drug and its physical mixtures with lipids.

3.3 Formulation of Drug Loaded Nanostructured Lipid Carrier NLCs

Nanostructured lipid carriers (NLCs) were prepared by hot homogenization followed by ultrasonication. A 4% w/w solid–liquid lipid mixture (glyceryl monostearate and miglyol 812) containing Drug X was melted at 70 °C, while the aqueous phase with Poloxamer 188 and Tween 80 was heated to the same temperature. The molten lipid was added to the aqueous phase under stirring, homogenized at 15,000 rpm for 15 min, and then ultrasonicated at 65% amplitude for 15 min. The resulting nanoemulsion was cooled under gentle stirring to form stable NLCs.(59–61).

          Table 1 Optimized Formulation Table For 50gm Batch Size

*Where, SL = Solid lipid, LL = Liquid lipid, SAA = Surface active agent, GMS= Glyceryl monostearate, MCT = Medium Chain Triglycerides, P188= Poloxamer 188

4.4 Characterization of NLCs

4.4.1 particle size distribution and polydispersity index

Particle size and polydispersity index (PI) of Drug-loaded NLCs were measured using photon correlation spectroscopy (Malvern Nano ZS) after dilution with water. Results were expressed as d10%, d50%, d90%, and PI was calculated from the span of particle size distribution (62).

4.4.2 Zeta potential

The zeta potential of the formulated NLCs was measured in distilled water at 25 °C using electrophoretic light scattering with a Malvern Nano ZS (63).

4.4.3 Entrapment Efficiency and Drug Loading

Entrapment efficiency (EE%) of NLCs was determined in three steps. First, a standard calibration curve of the drug in ethanol was prepared using UV–Vis spectrophotometry. Second, the free (unentrapped) drug was quantified by centrifuging the NLCs, collecting the supernatant, diluting, and measuring absorbance. Third, the total drug content was measured by disrupting the nanoparticles in ethanol and analyzing spectrophotometrically. EE% was calculated using (10)(61).

Entrapment Efficiency (%) =

4.4.4 In-Vitro Release Studies of NLCs

In-vitro release of Drug-NLC was studied using the dialysis bag method (MWCO 12–14 kDa). Two milliliters of NLCs were placed in pre-soaked dialysis bags and immersed in 200 mL of release medium (ABS pH 5.5 + 0.5% Tween 20) at 37 ± 0.5 °C with stirring at 300 rpm. Samples (1 mL) were withdrawn at set intervals up to 8 hours, replaced with fresh medium, and analyzed by UV–Vis spectrophotometry at 240 nm (61).

3.5 Formulation of NLCs Loaded Topical Gel

A 20 g NLC-loaded gel was prepared by hydrating 0.2 g Carbopol 974P, adding glycerine and NLCs containing 0.025% drug, and neutralizing with 2% TEA to pH 5.5, yielding a smooth, homogenous topical gel (64).

                           Table 2 Optimized NLC Topical Gel formulation 20gm

3.6 Characterization of NLCs Loaded Topical Gel

3.6.1 Visual Appearance and Homogeneity

The gel’s physical appearance and texture were evaluated visually for color, clarity, homogeneity, and phase separation, and manually for smoothness and absence of grittiness or lumps (65).

3.6.2 PH-Measurement

gel-formulations was assessed for pH values at room temperature utilizing a bench top digital pH meter(64,65).

3.6.3 Spreadability And Rheological Behaviour

The spreadability and rheology of the Carbopol 974P gel were evaluated using an Anton Paar MCR rheometer with a 25 mm parallel plate at 25 °C. Gel samples were pre-sheared, rested, and subjected to a shear rate ramp (0.1–100 s⁻¹) to record viscosity and determine yield stress. Measurements were performed in triplicate (66).

3.6.4 Viscosity study

Gel viscosities were measured using a Brookfield LDV Prime I viscometer with spindle No. 6 at 10 RPM, and the mean viscosity was reported in centipoise (cP) (67).

3.6.5 Drug Content Determination

One gram of gel was dissolved in 10 mL ethanol, shaken for 2 hours to solubilize the drug, filtered through a 0.45 µm membrane, diluted, and analyzed by UV spectrophotometry (68).

3.6.6 In Vitro Drug Release Studies

In-vitro diffusion of the NLC gel was performed using a Franz diffusion cell with acetate buffer (pH 5.5) at 37 °C and 300 rpm. Gel (0.5 g, 125 µg drug) was applied to a pre-soaked 0.22 µm cellulose acetate membrane. Samples (1 mL) were withdrawn at 1, 2, 3, 4, and 6 h, replaced with fresh buffer, diluted, and analyzed by UV spectrophotometry at 240 nm (61,64)

3.6.7 Stability Studies

A 30-day accelerated stability study was conducted per ICH Q1A(R2) at 40 °C ± 2 °C and 75% ± 5% RH, monitoring appearance, pH, assay, and degradation to assess early product stability.

4. RESULT & DISCUSSION

4.1 PREFORMULATION STUDIES 

4.1.1 Organoleptic Evaluation

The formulation exhibited a white to off-white color, was odorless, and had a solid crystalline texture, consistent with reference standards.

4.1.2 Solubility Studies

The drug showed very low solubility in water (practically insoluble), sparing solubility in ethanol, and high solubility in acetone, consistent with reference data.

4.1.3 Melting Point Determination

The drug “X” exhibited a melting point of 195 ± 2 °C, consistent with the reference range of 195–196 °C.

4.1.4 Partition Coefficient Determination

The partition coefficient (log P) of Drug “X” was determined by the shake flask method as 4.2, closely matching the theoretical value of 4.3.

4.1.5 Determination of (λmax) of Drug.

The Spectroscopic analysis for Drug “X” is done by UV Spectroscopy (model name Shimadzu UV-1900i), the maximum wavelength was checked in medium i.e. ethanol. The observed λmax was found 236 nm.

Screening of solid lipids showed that Glyceryl Monostearate (GMS) had the highest drug solubility (60 mg/g, 1:16.67), followed by Compritol 888 ATO (50 mg/g, 1:20) and Stearic Acid (40 mg/g, 1:25). GMS was selected for formulation due to its superior solubilization capacity.

4.1.9 Screening of Liquid Lipid

Solubility screening in liquid lipids showed that Medium Chain Triglycerides (MCT) had the highest drug solubility (55 mg/mL), followed by Oleic Acid (45 mg/mL) and Sesame Oil (42 mg/mL). MCT was selected for formulation due to its superior solubilizing capacity.

4.1.10 solid and liquid lipid Ratio Selection

Solid–liquid lipid mixtures at ratios of 90:10, 80:20, and 70:30 formed clear, single-phase melts with no phase separation or oil droplets upon cooling, indicating all ratios were compatible and stable for NLC formulation.

4.1.11 Compatibility Study

FTIR analysis of the drug–GMS–MCT formulation showed characteristic peaks for the drug and lipids, including ester (~1744 cm⁻¹), ketone (~1663 cm⁻¹), aromatic C=C (~1612 cm⁻¹), aliphatic C–H (~2949 and 2858 cm⁻¹), O–H (~3332 cm⁻¹), and halogen (~878 cm⁻¹) bands. No significant peak changes were observed, indicating chemical compatibility between the drug and lipids. FTIR analysis of the drug with GMS and MCT showed that key functional groups (O–H, aliphatic C–H, ester and ketone C=O, aromatic C=C, C–H bending, C–O–C/C–O stretching, and C–Cl) remained largely unchanged, with only minor expected shifts in ester (+19 cm⁻¹) and ketone (−15 cm⁻¹) bands. The presence of CH₂ rocking at 720 cm⁻¹ reflected lipid chain ordering. These results indicate no significant chemical interactions, confirming compatibility between the drug and lipid excipients. 

5.2 Formulation of Nanostructured Lipid Carrier

Drug-loaded NLC formulations were developed using a lipid phase of Glyceryl Monostearate (GMS) and Medium Chain Triglycerides (MCT), selected based on solubility screening. The drug concentration was 5% w/w of the 4% lipid phase. Poloxamer 188 and Tween 80 were used as surfactants. NLCs were prepared via homogenization followed by probe sonication to reduce particle size and prevent crystal growth. Formulations with small particle size and high negative zeta potential demonstrated stable colloidal dispersions. Multiple formulations were prepared with varying lipid ratios (80:20, 70:30, 90:10) and surfactant concentrations (0.5–1.5% w/w) as detailed in Table.

6. Characterization of NLCs

6.1 particle size distribution and polydispersity index 

 Particle size analysis showed 11 of 18 NLC formulations formed nanoparticles (148–574 nm), with F3 and F10 (0.75% Poloxamer 188, 70:30 lipid ratio) producing the smallest, well-dispersed particles (<300 nm, PDI <0.5), highlighting the importance of optimized surfactant and lipid composition.

Table 3  particle size distribution and polydispersity index

6.2 Zeta potential

Zeta potential analysis showed non-aggregated NLC formulations ranged from –11.9 to –31.8 mV. Formulations F3 (–31.8 mV) and F10 (–28.2 mV) exhibited the highest negative values, indicating strong electrostatic repulsion and enhanced colloidal stability, while lower values (e.g., F12: –11.9 mV) suggested a higher risk of aggregation.

6.3 Entrapment Efficiency and Drug Loading

Entrapment Efficiency (%) Calculation

Table 4 Entrapment Efficiency (%)

6.4 Drug Assay

To estimate the total drug content of F3, 1 mL of the NLC formulation was mixed thoroughly with 9 mL of ethanol. The resulting mixture was vortexed or sonicated to effectively disrupt the nanostructured lipid carriers (NLCs) and ensure complete extraction of the encapsulated drug. After extraction, the absorbance of the prepared solution was measured to determine the drug concentration.                                            

6.5 In-Vitro Release Studies of NLCs

The in-vitro release profile of the developed Drug-NLC formulation was evaluated using the dialysis bag technique in ABS (pH 5.5) containing 0.5% Tween 20 to ensure sink conditions. The release data (Figure 1 and Table 1) show a sustained and gradual release of the drug over an 8-hour period. An initial of 7.5% was observed within the first hour, followed by a controlled release reaching 52.55% at 5 hours and up to 99.85% at 8 hours. This indicates that the Drug-NLC formulation effectively prolonged the release of the drug, demonstrating its potential for sustained drug delivery applications.

                                    Table 5 In-Vitro Release Studies of NLCs

A 30-day accelerated stability study (40 °C, 75% RH) showed that the optimized NLC formulation maintained a particle size of 158.3 nm, PDI 0.26, zeta potential –30.2 mV, and 98.5% drug assay, indicating good early stability.

7. Formulation of Topical Gel

The optimized topical gel (20 g) was formulated with 1% Carbopol 974P (0.2 g) as gelling agent, 5% glycerine (1 g) as humectant, TEA added to adjust pH, 2.52 g of NLCs (equivalent to 0.025% drug, 5 mg), and water q.s. to 20 g. The NLC volume was calculated based on the drug content and concentration in NLCs.

7.1 Visual Appearance and Homogeneity

The gel was translucent, smooth, and uniform, with no phase separation, grittiness, or visible particles. It showed good homogeneity and an even texture on tactile assessment.    

7.2 PH-Measurement

The NLC-loaded topical gel had a mean pH of 5.68, within the safe dermal range of 5.5–6.5, indicating it is non-irritating for skin application.

7.3 Viscosity Determination

The NLC-loaded topical gel exhibited a mean viscosity of 12,553 cP, within the standard range of 10,000–30,000 cP, indicating smooth, easily spreadable texture with prolonged skin contact.

7.4 Spreadability Rheological Behaviour Testing by Anton Paar Rheometer

The Carbopol 974P NLC gel exhibited pseudoplastic (shear-thinning) behavior, with viscosity decreasing from 82,400 cP at 0.1 s⁻¹ to 4,700 cP at 100 s⁻¹ and a yield stress of 118 Pa. This indicates the gel is stable at rest yet spreads easily under shear, demonstrating appropriate consistency, smooth texture, and excellent topical spreadability.

7.5 Drug Content Determination

Drug content analysis of the NLC-loaded topical gel showed 95.7% assay. One milliliter of gel was diluted, vortexed, and sonicated to extract the drug, and UV absorbance at 236 nm (0.3021) was used with the calibration curve to calculate a concentration of 239 µg/g, corresponding to 4.7 mg in 20 g of gel. 

7.7 Stability Testing

A 30-day accelerated stability study (40 °C, 75% RH) showed the NLC gel maintained a particle size of 156.4 nm, PDI 0.23, zeta potential –32.1 mV, and 98.6% drug assay, indicating good early stability

CONCLUSION

A nanostructured lipid carrier (NLC)-based topical gel for Drug X was developed to improve solubility, skin penetration, and sustained release for atopic dermatitis. Optimized nanoparticles showed high encapsulation, stability, and controlled release. Integrated into a user-friendly Carbopol gel, the formulation enhanced dermal delivery, barrier repair, and patient adherence, offering a promising, safe alternative to conventional corticosteroid.