A two-layer diffusive model for describing the variability of transdermal drug permeation

Variability of Skin Pharmacokinetic Data: Insights from a Topical Bioequivalence Study Using Dermal Open Flow Microperfusion

PURPOSE

Dermal open flow microperfusion (dOFM) has previously demonstrated its utility to assess the bioequivalence (BE) of topical drug products in a clinical study. We aimed to characterize the sources of variability in the dermal pharmacokinetic data from that study.

METHODS

Exploratory statistical analyses were performed with multivariate data from a clinical dOFM-study in 20 healthy adults evaluating the BE, or lack thereof, of Austrian test (T) and U.S. reference (R) acyclovir cream, 5% products.

RESULTS

The overall variability of logAUC values (CV: 39% for R and 45% for T) was dominated by inter-subject variability (R: 82%, T: 91%) which correlated best with the subject's skin conductance. Intra-subject variability was 18% (R) and 9% (T) of the overall variability; skin treatment sites or methodological factors did not significantly contribute to that variability.

CONCLUSIONS

Inter-subject variability was the major component of overall variability for acyclovir, and treatment site location did not significantly influence intra-subject variability. These results support a dOFM BE study design with T and R products assessed simultaneously on the same subject, where T and R treatment sites do not necessarily need to be next to each other. Localized variation in skin microstructure may be primarily responsible for intra-subject variability.

Development of solid dispersions of artemisinin for transdermal delivery

Solid dispersions of the poorly soluble drug artemisinin were developed using polymer blends of polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) with the aim of enhancing solubility and in vitro permeation of artemisinin through skin. Formulations were characterised using a combination of molecular dynamics (MD) simulations, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Solubility of artemisinin was determined in two solvents: de-ionised water and phosphate buffered saline (PBS; pH 7.4), while in vitro drug permeation studies were carried out using rabbit skin as a model membrane. MD simulations revealed miscibility between the drug and polymers. DSC confirmed the molecular dispersion of the drug in the polymer blend. Decrease in crystallinity of artemisinin with respect to polymer content and the absence of specific drug-polymer interactions were confirmed using XRD and FT-IR, respectively. The solubility of artemisinin was dramatically enhanced for the solid dispersions, as was the permeation of artemisinin from saturated solid-dispersion vehicles relative to that from saturated solutions of the pure drug. The study suggests that high energy solid forms of artemisinin could possibly enable transdermal delivery of artemisinin.

Formulation study of topically applied lotion: in vitro and in vivo evaluation

INTRODUCTION

This article presents the development and evaluation of a new topical formulation of diclofenac diethylamine (DDA) as a locally applied analgesic lotion.

METHODS

To this end, the lotion formulations were formulated with equal volume of varying concentrations (1%, 2%, 3%, 4%; v/v) of permeation enhancers, namely propylene glycol (PG) and turpentine oil (TO). These lotions were subjected to physical studies (pH, viscosity, spreadability, homogeneity, and accelerated stability), in vitro permeation, in vivo animal studies and sensatory perception testing. In vitro permeation of DDA from lotion formulations was evaluated across polydimethylsiloxane membrane and rabbit skin using Franz cells.

RESULTS

It was found that PG and TO content influenced the permeation of DDA across model membranes with the lotion containing 4% v/v PG and TO content showed maximum permeation enhancement of DDA. The flux values for L4 were 1.20±0.02 μg.cm(-2).min(-1) and 0.67 ± 0.02 μg.cm(-2).min(-1) for polydimethylsiloxane and rabbit skin, respectively. Flux values were significantly different (p < 0.05) from that of the control. The flux enhancement ratio of DDA from L4 was 31.6-fold and 4.8-fold for polydimethylsiloxane and rabbit skin, respectively. In the in vivo animal testing, lotion with 4% v/v enhancer content showed maximum anti-inflammatory and analgesic effect without inducing any irritation. Sensatory perception tests involving healthy volunteers rated the formulations between 3 and 4 (values ranging between -4 to +4, indicating a range of very bad to excellent, respectively).

CONCLUSION

It was concluded that the DDA lotion containing 4% v/v PG and TO exhibit the best performance overall and that this specific formulation should be the basis for further clinical investigations.

Applying response surface methodology to optimize nimesulide permeation from topical formulation

Nimesulide is a non-steroidal anti-inflammatory drug that acts through selective inhibition of COX-2 enzyme. Poor bioavailability of this drug may leads to local toxicity at the site of aggregation and hinders reaching desired therapeutic effects. This study aimed at formulating and optimizing topically applied lotions of nimesulide using an experimental design approach, namely response surface methodology. The formulated lotions were evaluated for pH, viscosity, spreadability, homogeneity and in vitro permeation studies through rabbit skin using Franz diffusion cells. Data were fitted to linear, quadratic and cubic models and best fit model was selected to investigate the influence of permeation enhancers, namely propylene glycol and polyethylene glycol on percutaneous absorption of nimesulide from lotion formulations. The best fit quadratic model explained that the enhancer combination at equal levels significantly increased the flux and permeability coefficient. The model was validated by comparing the permeation profile of optimized formulations' predicted and experimental response values, thus, endorsing the prognostic ability of response surface methodology.

Effect of direction (epidermis-to-dermis and dermis-to-epidermis) on the permeation of several chemical compounds through full-thickness skin and stripped skin

PURPOSE

Compound permeation through stratum corneum-stripped skin is generally greater than that through full-thickness skin. In addition, epidermis-to-dermis permeation profile should be the same as dermis-to-epidermis permeation profile. However, stripped skin permeability of some compounds was lower than full-thickness skin permeability and different permeabilities were found for some compounds between the two directions of skin permeation. The reasons for these findings were investigated in this study.

METHODS

Full-thickness or stripped hairless rat skin was set in a Franz-type diffusion cell, and a solution of compound was applied on the epidermis or dermis side to determine the in vitro skin permeability.

RESULTS

Although the stripped skin permeability of pentyl paraben (PeP) with extremely high logK(o/w) was lower than full-thickness skin permeabilities, the addition of 3% ethanol resulted in the expected permeation order. Epidermis-to-dermis permeation of PeP through full-thickness skin was higher than dermis-to-epidermis permeation. Epidermis-to-dermis permeations of fluorescein isothiocyanate dextran (FD-4) and isosorbide 5-mononitrate with negative logK(o/w) were also higher than those in the opposite direction.

CONCLUSIONS

Morphological observation of skin after FD-4 permeation suggested that a conically shaped trans-follicular permeation pathway model could be advocated to explain the difference between the epidermis-to-dermis permeation and that in the opposite direction.