Development of lipid nanoparticles for topical delivery of anti-acne drug

Lipid nanoparticles a term that encompasses both solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) are colloidal delivery systems composed of physiologically compatible lipids that form a solid or semi-solid matrix at body temperature. This research developed and compared SLN and NLC formulations for topical delivery of adapalene, a third-generation retinoid prescribed for moderate acne vulgaris. Five formulations were prepared by the hot homogenisation–ultrasonication technique: two SLN variants (Compritol 888 ATO and Precirol ATO 5 as solid lipids) and three NLC variants incorporating oleic acid or Miglyol 812 as liquid lipids. The optimised NLC-1 formulation (Compritol:oleic acid 7:3, 2% Tween 80) produced particles of 156.8 ± 11.4 nm with a polydispersity index of 0.18 ± 0.03, zeta potential of −28.4 ± 1.7 mV, and entrapment efficiency of 91.3 ± 1.6%. In vitro release through a synthetic membrane showed sustained delivery over 24 h following Korsmeyer-Peppas kinetics (n = 0.54, R² = 0.994). Ex vivo permeation through porcine ear skin revealed a 3.4-fold enhancement in dermal retention compared with a marketed adapalene gel. Accelerated stability testing at 40 °C/75% RH over 90 days confirmed particle size growth below 12% and drug content above 96%. The NLC platform appears well suited to improving adapalene skin deposition while minimising the irritation associated with conventional retinoid vehicles. These data warrant further clinical evaluation for acne management.

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Size (nm) and EE (%): SLN-1 (Compritol) vs SLN-2 (Precirol) vs NLC-1 (Comp+Oleic) vs NLC-2 (Prec+Miglyol) vs NLC-3 (Comp+Miglyol)Close ×

Replotted from Table 1 (page 2) of the original paper. Every bar is the paper's own table value.

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Skin Deposition (µg/cm²): NLC-1 gel vs Marketed gel (Differin®) vs Plain adapalene gelClose ×

Replotted from Table 2 (page 2) of the original paper. Every bar is the paper's own table value.

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Introduction

Acne vulgaris is defined as a chronic inflammatory condition of the pilosebaceous unit, affecting approximately 85% of adolescents and persisting into adulthood in a substantial fraction of cases [1] . Topical retinoids tretinoin, adapalene, and tazarotene normalise follicular keratinisation and reduce comedone formation, making them first-line agents in acne treatment guidelines worldwide [2] . Adapalene, a naphthoic acid derivative, binds selectively to retinoic acid receptor subtypes β and γ and shows greater photostability and lower irritation potential than tretinoin [3] . But even adapalene can cause dryness, erythema, and peeling, particularly during the first weeks of therapy, leading many patients to discontinue treatment prematurely. Lipid-based nanocarriers offer a promising route to improve the skin tolerability profile of retinoids. SLNs, introduced by Müller and colleagues in the early 1990s, consist of a solid lipid core stabilised by surfactants [4] . They provide occlusive properties, controlled release, and UV protection for photolabile drugs. NLCs, the second generation, blend solid and liquid lipids to create an imperfect crystal structure with more space for drug accommodation, resulting in higher loading capacity and reduced drug expulsion during storage [5] . Several groups have reported SLN and NLC formulations of tretinoin, but adapalene which has distinct lipophilicity (log P ≈ 8.6) and molecular geometry has received less attention in

Preparation Method

Lipid nanoparticles were prepared by the hot homogenisation-ultrasonication method. The lipid phase (solid lipid alone for SLN, or solid lipid plus liquid lipid for NLC) was melted at 85 °C in a glass vial, and adapalene (0.1% w/v of the final dispersion) was dissolved in the molten lipid. Separately, the aqueous phase containing Tween 80 (2% w/v) was heated to the same temperature. The hot aqueous phase was poured into the lipid melt under high-speed stirring (Ultra-Turrax T25, IKA, 12,000 rpm, 5 min) to form a coarse pre-emulsion. This pre-emulsion was immediately subjected to probe ultrasonication (Sonics VCX 750, 40% amplitude, 10 min, pulsed 5 s on / 3 s off) while maintaining temperature above the lipid melting point. The resulting nanoemulsion was cooled slowly to room temperature in an ice bath, allowing lipid recrystallisation and nanoparticle formation. Each formulation was prepared in triplicate batches. For the gel vehicle, the NLC dispersion was incorporated into a pre-formed Carbopol 940 gel (0.5% w/w, neutralised to pH 5.5 with triethanolamine) at a 1:1 weight ratio under gentle stirring [7] .

Quality Control and Calibration

HPLC quantification of adapalene was performed on an Agilent 1260 Infinity II system with a C18 column (250 × 4.6 mm, 5 µm) using a mobile phase of acetonitrile:0.1% phosphoric acid (85:15 v/v) at 1.0 mL/min, detection at 254 nm. The method was validated per ICH Q2(R1): linearity R² = 0.9997 over 0.5-50 µg/mL, intra-day RSD 0.74%, interday RSD 1.18%, recovery 98.7-101.4%, LOD 0.12 µg/mL, LOQ 0.38 µg/mL. Particle size analyser (Malvern Zetasizer) was calibrated with 100 nm and 500 nm NIST-traceable latex standards before each measurement session. Differential scanning calorimetry (DSC) temperature calibration used indium (mp 156.6 °C) and tin (mp 231.9 °C) standards [8] .

Material and Methods

Material and Methods

Material

Material

Methods

Entrapment efficiency was determined by ultrafiltrationcentrifugation. A 2 mL aliquot of NLC dispersion was placed in an Amicon Ultra-4 centrifugal filter (MWCO 100 kDa) and centrifuged at 4000 × g for 30 min. Free drug in the filtrate was quantified by HPLC, and EE was calculated as [(Total drug -Free drug) / Total drug] × 100. Particle size, PDI, and zeta potential were measured by photon correlation spectroscopy and electrophoretic light scattering (Malvern Zetasizer Nano ZS) at 25 °C after 100-fold dilution. Morphology was examined by transmission electron microscopy (TEM, Zeiss EM10C, 80 kV) using negative staining with 2% phosphotungstic acid. DSC thermograms were recorded on a Mettler Toledo DSC 3+ from 25 to 100 °C at 5 °C/min under nitrogen. In vitro release used Franz diffusion cells (effective area 1.77 cm², receptor volume 12 mL, pH 5.5 phosphate buffer with 1% Tween 80 as solubiliser, 32 ± 0.5 °C) with a polycarbonate membrane (0.4 µm pore). Ex vivo permeation used dermatomed porcine ear skin (~0.7 mm) under identical conditions. After 24 h, skin was tape-stripped (20 strips), and adapalene in the stripped skin and remaining dermis was extracted with methanol and quantified by HPLC to determine skin deposition. Stability was evaluated at 25 °C/60% RH and 40 °C/75% RH over 90 days. Statistical analysis used one-way ANOVA with Tukey's post-hoc test (α = 0.05) [9] . The NLC-1 gel achieved 3.41-fold higher skin deposition than the marketed adapalene gel (Table 2). Cumulative permeation was also higher, but the most striking difference lay in the skin retention, suggesting the NLC particles deposit drug within the stratum corneum and viable epidermis rather than simply passing through to the receptor compartment. The box plot in Figure 1 visualises the spread of particle sizes across formulations. NLC-1 shows the tightest distribution and lowest median, while NLC-2 (Precirol + Miglyol) displays the widest spread, reflecting greater batch-to-batch variability with that lipid combination.

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Box plot of particle size distributions for five lipid nanoparticle formulations (n = 30 measurements each)

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Formulation	Cumulative Permeation (µg/cm²)	Skin Deposition (µg/cm²)	Enhancement Ratio
NLC-1 gel	14.37 ± 1.82	28.64 ± 3.17	3.41
Marketed gel (Differin®)	11.28 ± 1.43	8.41 ± 1.26	1.00
Plain adapalene gel	9.87 ± 1.21	6.73 ± 0.94	0.80

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Ex Vivo Skin Permeation and Deposition Data (24 h)

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Results

Results

Comprehensive interpretation

The NLC-1 formulation outperformed both SLN variants and the other NLC combinations across all measured parameters. The oleic acid liquid lipid appears to serve a dual function: disrupting the Compritol crystal lattice to increase drug accommodation space and acting as a penetration enhancer at the skin surface. The combination of small particle size, high entrapment, sustained release, and markedly improved skin deposition positions NLC-1 as a strong candidate for further development.

Discussion

superior performance of NLC-1 over the SLN formulations is consistent with the well-established principle that incorporating a liquid lipid into the solid matrix creates crystal imperfections that prevent drug expulsion during polymorphic transitions [5] . Adapalene's high lipophilicity (log P ~ 8.6) favours its partitioning into the lipid core, and the disordered NLC structure retains it more effectively than the more ordered SLN lattice [10] . The 3.41-fold skin deposition enhancement observed for NLC-1 over the marketed gel is promising. Similar or slightly lower enhancement ratios (2.0-3.0) have been reported for NLC formulations of other retinoids such as tretinoin and isotretinoin [11] . The mechanism likely involves occlusive film formation on the skin surface, which hydrates the stratum corneum and widens intercellular channels, as well as direct nanoparticle-skin interaction at the follicular orifice [12] . Stability data at 40 °C/75% RH showed less than 12% growth in particle size over 90 days, which is within acceptable limits for lipid nanoparticle dispersions [13] . Drug content remained above 96%, confirming adequate chemical stability of adapalene within the lipid matrix. The slightly negative zeta potential (-28.4 mV) provides sufficient electrostatic repulsion to minimise aggregation, though the presence of the Tween 80 steric layer is likely the dominant stabilisation mechanism [14] . A limitation of this work is the use of porcine ear skin as a surrogate for human skin. While porcine skin is widely accepted as the closest animal model, differences in follicular density and stratum corneum thickness may affect quantitative extrapolation. Clinical trials with the NLC-1 gel formulation are the logical next step to confirm tolerability and efficacy in acne patients [15] .

Conclusion

Five lipid nanoparticle formulations two SLN and three NLC were prepared for topical delivery of the retinoid adapalene. The NLC-1 formulation, combining Compritol 888 ATO and oleic acid at a 7:3 ratio with 2% Tween 80, emerged as the best candidate, exhibiting a particle size of 156.8 nm, entrapment efficiency of 91.3%, sustained Korsmeyer-Peppas release kinetics over 24 h, and a 3.41fold enhancement in skin drug deposition compared with the marketed adapalene gel. Accelerated stability testing over 90 days confirmed acceptable physical and chemical stability. These results demonstrate that NLC-based gels can substantially improve the dermal delivery of adapalene, potentially reducing the dose-related irritation that limits patient compliance with conventional retinoid therapy. Clinical evaluation in human volunteers is warranted to translate these laboratory findings into improved acne management.

Acknowledgement

Acknowledgements Funding sourcesThis research was supported by the Persian Gulf Institute of Technology internal research fund and a collaborative grant from Isfahan University of Advanced Sciences. Institutional supportThe authors thank both institutions for access to HPLC, TEM, and DSC facilities used in this research.

Figures and tables from the paper

Formulation	Size (nm)	PDI	ZP (mV)	EE (%)
SLN-1 (Compritol)	187.3 ± 14.2	0.24 ± 0.04	−23.6 ± 1.4	78.4 ± 2.1
SLN-2 (Precirol)	213.6 ± 18.7	0.29 ± 0.05	−21.8 ± 1.8	72.7 ± 2.8
NLC-1 (Comp+Oleic)	156.8 ± 11.4	0.18 ± 0.03	−28.4 ± 1.7	91.3 ± 1.6
NLC-2 (Prec+Miglyol)	242.1 ± 22.3	0.31 ± 0.06	−19.7 ± 2.1	83.6 ± 2.3
NLC-3 (Comp+Miglyol)	198.4 ± 16.8	0.22 ± 0.04	−25.1 ± 1.5	87.2 ± 1.9

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Physicochemical Characteristics of SLN and NLC Formulations of Adapalene

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Formulation development cycle for lipid nanoparticle preparation, from lipid screening through stability testing

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