Dermatopharmacokinetic prediction of topical drug bioavailability in vivo

Effect of the barrier function of stratum corneum and viable epidermis and dermis on the skin concentration of topically applied chemicals

Three-dimensional cultured skin (3D skin) models have been utilized for in vitro skin permeation tests to evaluate the skin permeation rate and local effects (efficacy and toxicity) of applied chemicals, particularly from the perspective of the 3Rs (reduction, replacement, refinement) approach. The steady-state concentration of applied chemicals at different depths in the viable epidermis and dermis (VED) is affected by their skin permeation parameters, such as permeability coefficient (Kp) and partition coefficient (K) from the donor solution to the skin of the chemicals. In the present study, the steady-state concentration of chemicals in the VED of EpiDerm 606X (EpiDerm) as representative of a 3D skin model were compared with hairless rat skin. The VED concentrations of chemicals in EpiDerm were higher than those in hairless rat skin when a model hydrophilic compound, antipyrine, and a model lipophilic compound, flurbiprofen, were applied, suggesting that the barrier functions of the VED against the whole skin were higher in EpiDerm than in hairless rat skin. When an ester compound, ethyl nicotinate, was applied, the VED concentration of nicotinic acid, a metabolite of ethyl nicotinate, was lower in EpiDerm than in hairless rat skin. These differences in the VED concentrations of applied chemicals might be related to false-positives and -negatives of topical effects evaluated with 3D skin models. It is important to pay particular attention to differences in VED concentration in 3D skin models and real skin when evaluating local efficacy and toxicity using 3D skin models.

Transdermal Drug Delivery Systems: Different Generations and Dermatokinetic Assessment of Drug Concentration in Skin

Transdermal drug delivery systems (TDDS) are a highly appealing and innovative method of administering drugs through the skin, as it enables the drugs to achieve systemic effects. A TDDS offers patient convenience, avoids first-pass hepatic metabolism, enables local targeting, and reduces the toxic effect of drug. This review details several generations of TDDS and the advancements made in their development to address the constraints associated with skin delivery systems. Transdermal delivery methods of the first generation have been consistently growing in their clinical application for administering small, lipophilic, low-dose drugs. Second-generation TDDS, utilizing chemical enhancers and iontophoresis, have led to the development of clinical products. Third-generation delivery systems employ microneedles, thermal ablation, and electroporation to specifically target the stratum corneum, which is the skin's barrier layer. Dermatokinetics is the study of the movement of drugs and formulations applied to the skin over a period of time. It provides important information regarding the rate and extent to which drugs penetrate skin layers. Several dermatokinetic techniques, including tape stripping, microdialysis, and laser scanning microscopy, have been used to study the intricate barrier properties and clearance mechanisms of the skin. This understanding is essential for developing and improving effective TDDS.

Reverse Iontophoresis: Noninvasive Assessment of Topical Drug Bioavailability

Assessing drug disposition in the skin after the application of a topical formulation is difficult. It is hypothesized that reverse iontophoresis (RI), which can extract charged/polar molecules for monitoring purposes, may provide a noninvasive approach for the assessment of local drug bioavailability. The passive and RI extraction of salicylic acid (SA) and nicotine (NIC) from porcine skin in vitro was assessed after a simple solution of the former and a transdermal patch of the latter had been applied for 24 and 8 h, respectively. Immediately after this "passive skin loading", the amount of drug in the stratum corneum (SC) and "viable" tissue (VT) was measured either (a) after tape-stripping and subsequent solvent extraction of both skin layers or (b) following RI extraction over 4 h. Parallel experiments were then performed in vivo in healthy volunteers; in this case, the VT was not sampled and the skin loading period for NIC was only 4 h. RI extraction of both drugs was significantly higher (in vitro and in vivo) than that achieved passively, and the cumulative RI extraction profiles as a function of time were mathematically analyzed using a straightforward compartmental model. Best-fit estimates of drug amounts in the SC and VT (ASC,0 and AVT,0, respectively) at the end of "loading" and two first-order rate constants describing transfer between the model compartments were then determined. The in vitro predictions of ASC,0 and AVT,0 were in excellent agreement with the experimental results, as was the value of the former in vivo. The rate constants derived from the in vitro and in vivo results were also similar. In summary, the results provide proof-of-concept that the RI method has the potential to noninvasively assess relevant metrics of drug bioavailability in the skin.

Development of Essential Oil-Loaded Polymeric Nanocapsules as Skin Delivery Systems: Biophysical Parameters and Dermatokinetics Ex Vivo Evaluation

Essential oils (EOs) are natural antioxidant alternatives that reduce skin damage. However, EOs are highly volatile; therefore, their nanoencapsulation represents a feasible alternative to increase their stability and favor their residence time on the skin to guarantee their effect. In this study, EOs of Rosmarinus officinalis and Lavandula dentata were nanoencapsulated and evaluated as skin delivery systems with potential antioxidant activity. The EOs were characterized and incorporated into polymeric nanocapsules (NC-EOs) using nanoprecipitation. The antioxidant activity was evaluated using the ferric thiocyanate method. The ex vivo effects on pig skin were evaluated based on biophysical parameters using bioengineering techniques. An ex vivo dermatokinetic evaluation on pig skin was performed using modified Franz cells and the tape-stripping technique. The results showed that the EOs had good antioxidant activity (>65%), which was maintained after nanoencapsulation and purification. The nanoencapsulation of the EOs favored its deposition in the stratum corneum compared to free EOs; the highest deposition rate was obtained for 1,8-cineole, a major component of L. dentata, at 1 h contact time, compared to R. officinalis with a major deposition of the camphor component. In conclusion, NC-EOs can be used as an alternative antioxidant for skin care.

Multi-window sparse spectral sampling stimulated Raman scattering microscopy

Stimulated Raman scattering (SRS) is a nondestructive and rapid technique for imaging of biological and clinical specimens with label-free chemical specificity. SRS spectral imaging is typically carried out either via broadband methods, or by tuning narrowband ultrafast light sources over narrow spectral ranges thus specifically targeting vibrational frequencies. We demonstrate a multi-window sparse spectral sampling SRS (S4RS) approach where a rapidly-tunable dual-output all-fiber optical parametric oscillator is tuned into specific vibrational modes across more than 1400 cm-1 during imaging. This approach is capable of collecting SRS hyperspectral images either by scanning a full spectrum or by rapidly tuning into select target frequencies, hands-free and automatically, across the fingerprint, silent, and high wavenumber windows of the Raman spectrum. We further apply computational techniques for spectral decomposition and feature selection to identify a sparse subset of Raman frequencies capable of sample discrimination. Here we have applied this novel method to monitor spatiotemporal dynamic changes of active pharmaceutical ingredients in skin, which has particular relevance to topical drug product delivery.

Physiologically-Based Pharmacokinetic Modeling to Support Determination of Bioequivalence for Dermatological Drug Products: Scientific and Regulatory Considerations

Physiologically-based pharmacokinetic (PBPK) modeling and simulation provides mechanism-based predictions of the pharmacokinetics of an active ingredient following its administration in humans. Dermal PBPK models describe the skin permeation and disposition of the active ingredient following the application of a dermatological product on the skin of virtual healthy and diseased human subjects. These models take into account information on product quality attributes, physicochemical properties of the active ingredient and skin (patho)physiology, and their interplay with each other. Regulatory and product development decision makers can leverage these quantitative tools to identify factors impacting local and systemic exposure. In the realm of generic drug products, the number of US Food and Drug Administratioin (FDA) interactions that use dermal PBPK modeling to support alternative bioequivalence (BE) approaches is increasing. In this report, we share scientific considerations on the development, verification and validation (V&V), and application of PBPK models within the context of a virtual BE assessment for dermatological drug products. We discuss the challenges associated with model V&V for these drug products stemming from the fact that target-site active ingredient concentrations are typically not measurable. Additionally, there are no established relationships between local and systemic PK profiles, when the latter are quantifiable. To that end, we detail a multilevel model V&V approach involving validation for the model of the drug product of interest coupled with the overall assessment of the modeling platform in use while leveraging in vitro and in vivo data related to local and systemic bioavailability.

Relating transdermal delivery plasma pharmacokinetics with in vitro permeation test (IVPT) findings using diffusion and compartment-in-series models

Increasing emphasis is being placed on using in vitro permeation test (IVPT) results for topical products as a surrogate for their in vivo behaviour. This study sought to relate in vivo plasma concentration - time pharmacokinetic (PK) profiles after topical application of drug products to IVPT findings with mechanistic diffusion and compartment models that are now widely used to describe permeation of solutes across the main skin transport barrier, the stratum corneum. Novel in vivo forms of the diffusion and compartment-in-series models were developed by combining their IVPT model forms with appropriate in vivo disposition functions. Available in vivo and IVPT data were then used with the models in data analyses, including the estimation of prediction intervals for in vivo plasma concentrations derived from IVPT data. The resulting predicted in vivo plasma concentration - time profiles for the full models corresponded closely with the observed results for both nitroglycerin and rivastigmine at all times. In contrast, reduced forms of these in vivo models led to discrepancies between model predictions and observed results at early times. A two-stage deconvolution procedure was also used to estimate the in vivo cumulative amount absorbed and shown to be linearly related to that from IVPT, with an acceptable prediction error. External predictability was also shown using a separate set of in vitro and in vivo data for different nitroglycerin patches. This work suggests that mechanistic and physiologically based pharmacokinetic models can be used to predict in vivo behaviour from IVPT data for topical products.

Single dose pharmacokinetics of topical iodiconazole creams in healthy Chinese volunteers

1.In this study, the pharmacokinetics of new triazole antifungal iodiconzole creams at target sites after single-dose topical application was investigated.2.30 healthy Chinese volunteers were randomly divided into three groups after being stratified by sex, each group was given a single topical dose of 1%, 2%, 4% iodiconazole cream (0.4 g). Stratum corneum (SC) samples of treated sites were collected by tape-stripping method after the chosen contact times, and were extracted and analysed by a validated LC-MS method.3.After single-dose topical application of 1%, 2%, 4% iodiconazole creams, the C_max of iodiconazole in SC was 1.2 ± 0.7, 2.2 ± 1.0, 2.4 ± 1.0 mg/g; T_max was 3.3 ± 1.1, 2.9 ± 1.1, 3.8 ± 0.4 h; t_1/2 was 6.6 ± 3.4 h, 7.2 ± 4.1 h, 5.9 ± 2.9 h; AUC_0-t was 10.9 ± 3.0, 20.8 ± 10.4, 20.9 ± 7.9 mg·h/g; AUC_0-∞ was 11.6 ± 2.9, 23.5 ± 14.4, 22.2 ± 8.9 mg·h/g, respectively. The results showed that C_max, AUC_0-t and AUC_0-∞ did not increase proportionately with dose, which could also be due to the drug being saturated in the formulation at ∼2%.4.The results of this study could provide reference for the clinical medication and further study of the formulations.

Pharmacokinetics and pharmacodynamics in the treatment of cutaneous leishmaniasis - challenges and opportunities

Pharmacological efficacy is obtained when adequate concentrations of a potent drug reach the target site. In cutaneous leishmaniasis, a heterogeneous disease characterised by a variety of skin manifestations from simple nodules, skin discoloration, plaques to extensive disseminated forms, the parasites are found in the dermal layers of the skin. Treatment thus involves the release of the active compound from the formulation (administered either topically or systemically), it's permeation into the skin, accumulation by the local macrophages and further transport into the phagolysosome of the macrophage. The pharmacodynamic activity of a drug against the parasite is relatively straight forward to evaluate both in vivo and in vitro. The pharmacokinetic processes taking place inside the skin are more complex to elucidate due to the multi-lamellar structure of the skin, heterogeneous distribution of drugs within the tissue, the difficulty of accessing the site of infection complicating sampling and the lack of surrogate markers reflecting the activity of a drug in the skin. This review will discuss the difficulties encountered when investigating drug distribution, PK PD relationships and efficacy in the skin with a focus on cutaneous leishmaniasis treatment.

Ex Vivo and In Vivo Assessment of the Penetration of Topically Applied Anthocyanins Utilizing ATR-FTIR/PLS Regression Models and HPLC-PDA-MS

Anthocyanins are natural colorants with antioxidant properties, shown to inhibit photoaging reactions and reduce symptoms of some skin diseases. However, little is known about their penetration through the stratum corneum, a prerequisite for bioactivity. The aim was to investigate anthocyanin penetration from lipophilic cosmetic formulations through the skin using a porcine ear model and human volunteers. ATR-FTIR/PLS regression and HPLC-PDA-MS were used to analyze anthocyanin permeation through the stratum corneum. Penetration of all anthocyanins was evident and correlated with molecular weight and hydrophilicity. Lower-molecular-weight (MW) anthocyanins from elderberry (449–581 Da) were more permeable within the skin in both ex vivo and in vivo models (K_p = 2.3–2.4 × 10⁻⁴ cm h⁻¹) than the larger anthocyanins (933-1019 Da) from red radish (K_p = 2.0–2.1 × 10⁻⁴ cm h⁻¹). Elderberry and red radish anthocyanins were found at all levels of the stratum corneum and at depths for activity as bioactive ingredients for skin health.

Pesticide dermal absorption: Case study x in vitro study

In order to evaluate dermal absorption during typical working conditions in family farming, the amount of dimethoate on clothing and in the stratum corneum (SC) was measured in three rural workers. This was achieved by using cotton patches on the worker's clothes and SC quantification by the tape stripping approach. To mimic the above study, an in vitro approach was performed using Franz cells by applying dimethoate (0.4 and 1.8 μg) direct to pig skin or, on a section of cotton before contact with pig skin. The in vivo case results demonstrated the high levels and variability of agrochemicals to which these farmers are subjected, with the total potential dermal absorption between 0.87-2.85 mg/person/h and the estimated SC penetration factor (PF) between 0-54.0 and 0-28.9 % for the back of the neck and the arms respectively. This probably demonstrates the impact of correct protective clothing. For the in vitro study, the amount of pesticide retained in the SC was 52.63 ± 10.73and 135.15 ± 31.8 ng/cm² after applying 0.4 and 1.8 μg of pesticide directly on SC, and demonstrated close agreement with the in vivo approach. Further studies performed with this and other pesticides with different characteristics will contribute to the understanding of their transport through the skin.

Lipid Vesicles and Nanoparticles for Non-invasive Topical and Transdermal Drug Delivery

The delivery of drugs, via different layers of skin, is challenging because it acts as a natural barrier and exerts hindrance against molecules to permeate into or through it. To overcome such obstacles, different noninvasive methods, like vehicle-drug interaction, modifications of the horny layer and nanoparticles have been suggested. The aim of the present review is to highlight some of the non-invasive methods for topical, diadermal and transdermal delivery of drugs. Special emphasis has been made on the information available in numerous research articles that put efforts in overcoming obstacles associated with barrier functions imposed by various layers of skin. Advances have been made in improving patient compliance that tends to avoid hitches involved in oral administration. Of particular interest is the use of lipid-based vesicles and nanoparticles for dermatological applications. These particulate systems can effectively interact and penetrate into the stratum corneum via lipid exchange and get distributed in epidermis and dermis. They also have the tendency to exert a systemic effect by facilitating the absorption of an active moiety into general circulation.

Novel Approach for the Bioequivalence Assessment of Topical Cream Formulations: Model-Based Analysis of Tape Stripping Data Correctly Concludes BE and BIE

PURPOSE

The purpose of this study was (a) to suggest a novel dermatopharmacokinetic (DPK) approach from which pharmacokinetic parameters relevant to the bioequivalence (BE) assessment of a topical formulation can be deduced while circumventing the need for numerous measurements and assumptions, and (b) to investigate whether this approach enables the correct conclusion of BE and bioinequivalence (BIE).

METHODS

Bioequivalent and bioinequivalent formulations of acyclovir were compared versus a reference product (Zovirax®). Tape Stripping was conducted at only one dose duration during the uptake phase to generate drug content in stratum corneum versus time profiles, each time point corresponding to one stripped layer. Nonlinear mixed effect modeling (ADAPT5®) (MLEM algorithm) was used to fit the DPK data and to estimate the rate (K_in) and extent (F_S) of drug absorption/input into the skin. Results were evaluated using the average BE approach.

RESULTS

Estimated exposure metrics were within the usual BE limits for the bioequivalent formulation (F_S: 102.4 [90%CI: 97.5-107.7]; K_in: 94.2 [90%CI: 83.7-106.0]), but outside those limits for the bioinequivalent formulation (F_S: 43.4 [90%CI: 27.9-67.6]; K_in: 54.5 [90%CI: 36.6-81.1]).

CONCLUSIONS

The proposed novel DPK approach was shown to be successful, robust and applicable to assess BE and BIE correctly between topical formulations.

Microstructure and biopharmaceutical performances of curcumin-loaded low-energy nanoemulsions containing eucalyptol and pinene: Terpenes' role overcome penetration enhancement effect?

The objective of this work was to develop low-energy nanoemulsions for enhanced dermal delivery of curcumin, using monoterpene compounds eucalyptol (EUC) and pinene (PIN) as chemical penetration enhancers. Spontaneous emulsification was the preparation method. All formulations contained 10% of the oil phase (medium-chain triglycerides (MCT), or their mixture with EUC or PIN). Formulations were stabilized by the combination of polysorbate 80 and soybean lecithin (surfactant-to-oil-ratio=1). Concentration of curcumin was set to 3 mg/ml. Average droplet diameter of all tested formulations ranged from 102 nm to 132 nm, but the ones containing monoterpenes had significantly smaller size compared to the MCT formulation. Such finding was profoundly studied through electron paramagnetic resonance spectroscopy, which proved that the presence of monoterpenes modified the nanoemulsions' interfacial environment, resulting in droplet size reduction. The release study of curcumin (using Franz cells) demonstrated that the cumulative amount released after 6 h of the experiment was 10.1 ± 0.2% for the MCT nanoemulsions, 13.9 ± 0.1% and 14.0 ± 0.2% for PIN and EUC formulations, respectively. In vivo tape stripping revealed their performances in delivering curcumin into the skin, indicating the following order: EUC>MCT>PIN. The formulation with EUC was clearly the most successful, giving the highest cumulative amount of curcumin that penetrated per surface unit: 34.24±5.68 µg/cm². The MCT formulation followed (30.62±2.61 µg/cm²) and, finally, the one with PIN (21.61±0.11 µg/cm²). These results corelated with curcumin's solubility in the chosen oils: 4.18±0.02 mg/ml for EUC, 1.67±0.04 mg/ml for MCT and 0.21±0.01 mg/ml for PIN. Probably, higher solubility in the oil phase of the nanoemulsion promoted curcumin's solubility in the superficial skin layers, providing enhanced penetration.

Confocal Raman Spectroscopy as a tool to measure the prevention of skin penetration by a specifically designed topical medical device

BACKGROUND/AIM

The scope of this study was to utilize confocal Raman spectroscopy in the evaluation of the degree of non-penetration into the viable skin layers of a paraffin and petrolatum-based product for use in the intimate areas of the skin. The formulation was purposely designed with properties to prevent undesirable skin penetration.

METHODS

Product-The test product was a proprietary topical medical device comprising paraffinum liquidum, petrolatum, paraffin, and tocopheryl acetate. Volunteers-A total of 20 healthy volunteers were recruited onto the study-17 females and three males. Product Testing-Raman spectra were obtained at Baseline and 90 minutes after product application. Product Penetration-Skin penetration was calculated from Raman spectra taken at skin depths of -5, 0, 5, 10, 15, and 20 μm.

RESULTS

Raman spectra of the investigated product could be clearly differentiated from the skin spectrum. The minimum measurable concentration of the test product was determined at a detection level of 0.5%. In this study, the test product did not penetrate down to skin depths of 10 to 20 μm.

CONCLUSIONS

Within the precision range of the test method, the investigated product did not penetrate into the compact part of the stratum corneum. The study revealed Raman spectroscopy to be suitable to detect not only penetration but also non-penetration of substances into human skin.

Application of an Optimized Tape Stripping Method for the Bioequivalence Assessment of Topical Acyclovir Creams

This study indicates the application of tape stripping (TS) for bioequivalence (BE) assessment of a topical cream product containing 5% acyclovir. A TS method, previously used successfully to assess BE of topical clobetasol propionate and clotrimazole formulations, was used to assess BE of an acyclovir cream (5%) formulation as well as a diluted acyclovir formulation (1.5%) applied to the skin of healthy humans. An appropriate application time was established by conducting a dose duration study using the innovator product, Zovirax® cream. Transepidermal water loss was measured and used to normalize thicknesses between subjects. The area under the curve (AUC) from a plot of amount of acyclovir/strip vs cumulative fraction of stratum corneum (SC) removed was calculated for each application site. BE was assessed using Fieller's theorem in accordance with FDA's guidance for assessment of BE of topical corticosteroids. Adco-acyclovir cream (5%) was found to be BE to Zovirax® cream, where the mean test/reference (T/R) ratio of the AUC's was 0.96 and the bioequivalence interval using a 90% confidence interval was 0.91-1.01 with a statistical power > 95%, whereas the diluted test product fell outside the BE acceptance criteria with T/R ratio of AUC of 0.23 and a 90% CI of 0.20-0.26. This study indicates that the data resulting from the application of this TS procedure has reinforced the potential for its use to assess BE of topical drug products intended for local action, thereby obviating the necessity to undertake clinical trials in patients.

Noninvasive measurement of transdermal drug delivery by impedance spectroscopy

The effectiveness in transdermal delivery of skin permeation strategies (e.g., chemical enhancers, vesicular carrier systems, sonophoresis, iontophoresis, and electroporation) is poorly investigated outside of laboratory. In therapeutic application, the lack of recognized techniques for measuring the actually-released drug affects the scientific concept itself of dosage for topically- and transdermally-delivered drugs. Here we prove the suitability of impedance measurement for assessing the amount of drug penetrated into the skin after transdermal delivery. In particular, the measured amount of drug depends linearly on the impedance magnitude variation normalized to the pre-treated value. Three experimental campaigns, based on the electrical analysis of the biological tissue behavior due to the drug delivery, are reported: (i) laboratory emulation on eggplants, (ii) ex-vivo tests on pig ears, and finally (iii) in-vivo tests on human volunteers. Results point out that the amount of delivered drug can be assessed by reasonable metrological performance through a unique measurement of the impedance magnitude at one single frequency. In particular, in-vivo results point out sensitivity of 23 ml⁻¹, repeatability of 0.3%, non-linearity of 3.3%, and accuracy of 5.7%. Finally, the measurement resolution of 0.20 ml is compatible with clinical administration standards.

Influence of the skin barrier on the penetration of topically-applied dexamethasone probed by soft X-ray spectromicroscopy

The penetration of dexamethasone into human skin ex vivo is reported. X-ray microscopy is used for label-free probing of the drug and quantification of the local drug concentration with a spatial resolution reaching 70±5nm. This is accomplished by selective probing the dexamethasone by X-ray absorption. Varying the penetration time between 10min and 1000min provides detailed information on the penetration process. In addition, the stratum corneum has been damaged by tape-stripping in order to determine the importance of this barrier regarding temporally resolved drug penetration profiles. Dexamethasone concentrations distinctly vary, especially close to the border of the stratum corneum and the viable epidermis, where a local minimum in drug concentration is observed. Furthermore, near the basal membrane the drug concentration strongly drops. High spatial resolution studies along with a de-convolution procedure reveal the spatial distribution of dexamethasone in the interspaces between the corneocytes consisting of stratum corneum lipids. These results on local drug concentrations are interpreted in terms of barriers affecting the drug penetration in human skin.

Investigation of the Dermal Absorption and Irritation Potential of Sertaconazole Nitrate Anhydrous Gel

Effective topical therapy of cutaneous fungal diseases requires the delivery of the active agent to the target site in adequate concentrations to produce a pharmacological effect and inhibit the growth of the pathogen. In addition, it is important to determine the concentration of the drug in the skin in order to evaluate the subsequent efficacy and potential toxicity for topical formulations. For this purpose, an anhydrous gel containing sertaconazole nitrate as a model drug was formulated and the amount of the drug in the skin was determined by in vitro tape stripping. The apparent diffusivity and partition coefficients were then calculated by a mathematical model describing the dermal absorption as passive diffusion through a pseudo-homogenous membrane. The skin irritation potential of the formulation was also assessed by using the in vitro Epiderm™ model. An estimation of the dermal absorption parameters allowed us to evaluate drug transport across the stratum corneum following topical application. The estimated concentration for the formulation was found to be higher than the MIC100 at the target site which suggested its potential efficacy for treating fungal infections. The skin irritation test showed the formulation to be non-irritating in nature. Thus, in vitro techniques can be used for laying the groundwork in developing efficient and non-toxic topical products.

Estimating Maximal In Vitro Skin Permeation Flux from Studies Using Non-sink Receptor Phase Conditions

PURPOSE

This study explored the impact of non-sink receptor conditions on the in vitro skin permeation test (IVPT) and sought to estimate equivalent sink condition IVPT data.

METHODS

Simulated diffusion model and experimental IVPT data were generated for ethyl salicylate across human epidermal membranes in Franz diffusion cells using six different receptor phases, with a 10 fold variation in ethyl salicylate solubility.

RESULTS

Both simulated and experimental IVPT - time profiles were markedly affected by receptor phase solubility and receptor sampling rates. Similar sink condition equivalent estimated maximum fluxes were obtained by nonlinear regression and adjustment of linear regression estimates of steady state flux for relative saturation of the receptor phase over time for the four receptor phases in which the ethyl salicylate was relatively soluble. The markedly lower steady - state fluxes found for the other two phases in which ethyl salicylate was less soluble was attributed to an aqueous solution boundary layer effect.

CONCLUSIONS

Non-sink receptor phase IVPT data can be used to derive equivalent sink receptor phase IVPT data provided the receptor phase solubility and hydrodynamics are sufficient to minimise the impact of aqueous diffusion layers on IVPT data.

Colloidal nanocarriers for the enhanced cutaneous delivery of naftifine: characterization studies and in vitro and in vivo evaluations

In topical administration of antifungals, the drugs should pass the stratum corneum to reach lower layers of the skin in effective concentrations. Thus, the formulation of antifungal agents into a suitable delivery system is important for the topical treatment of fungal infections. Nanosized colloidal carriers have gained great interest during the recent years to serve as efficient promoters of drug penetration into the skin. Microemulsions are soft colloidal nanosized drug carriers, which are thermodynamically stable and isotropic systems. They have been extensively explored for the enhancement of skin delivery of drugs. This study was carried out to exploit the feasibility of colloidal carriers as to improve skin transport of naftifine, which is an allylamine antifungal drug. The microemulsions were formulated by construction of pseudoternary phase diagrams and composed of oleic acid (oil phase), Kolliphor^® EL or Kolliphor^® RH40 (surfactant), Transcutol^® (cosurfactant), and water (aqueous phase). The plain and drug-loaded microemulsions were characterized in terms of isotropy, particle size and size distribution, pH value, refractive index, viscosity, and conductivity. The in vitro skin uptake of naftifine from microemulsions was studied using tape stripping technique in pig skin. The drug penetrated significantly into stratum corneum from microemulsions compared to its marketed cream (P<0.05). Moreover, the microemulsion formulations led to highly significant amount of naftifine deposition in deeper layers of skin than that of commercial formulation (P<0.001). Microemulsion–skin interaction was confirmed by attenuated total reflectance – Fourier transformed infrared spectroscopy data, in vitro. The results of the in vivo tape stripping experiment showed similar trends as the in vitro skin penetration study. Topical application of the microemulsion on human forearms in vivo enhanced significantly the distribution and the amount of naftifine penetrated into the stratum corneum as compared to the marketed formulation (P<0.05). The relative safety of the microemulsion formulations was demonstrated with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide viability test. This study indicated that the nanosized colloidal carriers developed could be considered as an effective and safe topical delivery system for naftifine.

Quantification of uptake and clearance of acyclovir in skin layers

BACKGROUND

Quantification of drug uptake and clearance in the skin layers could provide better insight into the skin kinetics of dermatological formulations aimed for deeper skin tissues. This study assessed the skin kinetics of acyclovir in different skin layers following topical application on the abdominal region of Wistar rats.

METHODS

In vivo skin pharmacokinetics parameters were determined by two different protocols such as post drug load assessment and subsequent drug load assessment following topical application of 500 mg of cream formulation containing 5% (w/w) of acyclovir.

RESULTS

Topical application of acyclovir exhibited concentration gradient between the skin layers (stratum corneum > viable epidermis > dermis) which were inconsistent over the time-course of the study. The rate and extent of drug reaching target site (basal epidermis) was relatively low. The drug uptake and clearance profiles were found to be distinct in all the three skin layers suggesting no drug concentration correlation (P<0.05) between skin layers. Drug concentration in the viable epidermis continued to increase even after termination of therapy (Tmax=4 h) and then declined rapidly. The availability of acyclovir in the target was comparatively low (approximately 0.4% of the applied dose) although an order of magnitude higher percentage was determined in the stratum corneum.

CONCLUSIONS

The data observed in this study demonstrates low skin uptake and rapid clearance of acyclovir in the target site. Further, the methodology employed can be useful for studying other topical antiviral agents as well as for optimizing formulations for drugs (such as acyclovir) that may enhance their efficacy.

A strategy for in-silico prediction of skin absorption in man

For some time, in-silico models to address substance transport into and through the skin are gaining more and more importance in different fields of science and industry. In particular, the mathematical prediction of in-vivo skin absorption is of great interest to overcome ethical and economical issues. The presented work outlines a strategy to address this problem and in particular, investigates in-vitro and in-vivo skin penetration experiments of the model compound flufenamic acid solved in an ointment by means of a mathematical model. Experimental stratum corneum concentration-depth profiles (SC-CDP) for various time intervals using two different in-vitro systems (Franz diffusion cell, Saarbruecken penetration model) were examined and simulated with the help of a highly optimized three compartment numerical diffusion model and compared to the findings of SC-CDPs of the in-vivo scenario. Fitted model input parameters (diffusion coefficient and partition coefficient with respect to the stratum corneum) for the in-vitro infinite dose case could be used to predict the in-use conditions in-vitro. Despite apparent differences in calculated partition coefficients between in-vivo and in-vitro studies, prediction of in-vivo scenarios from input parameters calculated from the in-vitro case yielded reasonable results.

Enhanced dermal delivery of acyclovir using solid lipid nanoparticles

The present investigation was enthused by the possibility to develop solid lipid nanoparticles (SLNs) of hydrophilic drug acyclovir (ACV) and evaluate their potential as the carrier for dermal delivery. ACV-loaded SLNs (ACV-SLNs) were prepared by the optimized double emulsion process using Compritol 888 ATO as solid lipid. The prepared SLNs were smooth and spherical in shape with average diameter, polydispersity index, and entrapment efficiency of 262 ± 13 nm, 0.280 ± 0.01, and 40.08 ± 4.39% at 10% (w/w) theoretical drug loading with respect to Compritol 888 ATO content. Differential scanning calorimetry and powder X-ray diffraction pattern revealed that ACV was present in the amorphous state inside the SLNs. In vitro skin permeation studies on human cadaver and Sprague-Dawley rat skin revealed 17.65 and 15.17 times higher accumulation of ACV-SLNs in the dermal tissues in comparison to commercially available ACV cream after 24 h. Mechanism of topical permeation and dermal distribution was studied qualitatively using confocal laser scanning microscopy. While free dye (calcein) failed to penetrate skin barrier, the same encapsulated in SLNs penetrated deeply into the dermal tissue suggesting that pilosebaceous route was followed by SLNs for skin penetration. Histological examination and transdermal epidermal water loss measurement suggested that no major morphological changes occurred on rat skin surface due to the application of SLNs. Overall, it was concluded that ACV-loaded SLNs might be beneficial in improving dermal delivery of antiviral agent(s) for the treatment of topical herpes simplex infection.

Dermic diffusion and stratum corneum: a state of the art review of mathematical models

Transdermal biotechnologies are an ever increasing field of interest, due to the medical and pharmaceutical applications that they underlie. There are several mathematical models at use that permit a more inclusive vision of pure experimental data and even allow practical extrapolation for new dermal diffusion methodologies. However, they grasp a complex variety of theories and assumptions that allocate their use for specific situations. Models based on Fick's First Law found better use in contexts where scaled particle theory Models would be extensive in time-span but the reciprocal is also true, as context of transdermal diffusion of particular active compounds changes. This article reviews extensively the various theoretical methodologies for studying dermic diffusion in the rate limiting dermic barrier, the stratum corneum, and systematizes its characteristics, their proper context of application, advantages and limitations, as well as future perspectives.

Finite and infinite dosing: difficulties in measurements, evaluations and predictions

Due to the increased demand for reliable data regarding penetration into and permeation across human skin, assessment of the absorption of xenobiotics has been gaining in importance steadily. In vitro experiments allow for determining these data faster and more easily than in vivo experiments. However, the experiments described in literature and the subsequent evaluation procedures differ considerably. Here we will give an overview on typical finite and infinite dose experiments performed in fundamental research and on the evaluation of the data. We will point out possible difficulties that may arise and give a short overview on attempts at predicting skin absorption in vitro and in vivo.

Novel imaging method to quantify stratum corneum in dermatopharmacokinetic studies: proof-of-concept with acyclovir formulations

PURPOSE

Tape-stripping the stratum corneum (SC) is used in the assessment of dermatopharmacokinetics (DPK). The amount of SC per tape can be determined gravimetrically, but a novel imaging method offers advantages in terms of sensitivity, reproducibility, precision, stability and speed. High-resolution images, acquired under controlled conditions, are analysed in terms of pixel greyscale values and distributions, and their usefulness in DPK studies is demonstrated in this study using acyclovir.

METHODS

At all tape-stripped sites, the SC amount per tape was measured gravimetrically and by imaging. In a first series of experiments, untreated sites were stripped to determine total SC thickness. Subsequently, post-application of two acyclovir creams, drug-permeation profiles were constructed.

RESULTS

The greyscale values from the imaging data can be used directly to estimate total SC thickness and DPK parameters. The results compared favourably with the traditional weighing method. The concentration of drug on each tape, as a function of the relative position within the SC, permitted diffusivity and partitioning parameters characterising the penetration of acyclovir to be derived.

CONCLUSION

The new imaging approach offers a sensitive, reproducible, precise, and rapid technique to quantify the relative SC amount removed on tape-strips, and facilitates the acquisition of DPK data.

Novel imaging method to quantify stratum corneum in dermatopharmacokinetic studies

PURPOSE

Tape stripping the stratum corneum (SC) is used in topical bioequivalence studies. Formulations are compared using drug concentration profiles as a function of relative SC position; both of these parameters require quantification of SC amount removed per tape. Here, a novel imaging method to quantify SC on tape strips is described. Comparisons are made with established SC quantification methods, specifically weighing and UV pseudo-absorption.

METHODS

Six stratum corneum tape strips were measured 15 times by the three methods, which were compared for precision, signal:noise ratio, sample size and speed. Furthermore, 600 tape strips were assayed by each method, and correlations examined.

RESULTS

Weighing exhibited low precision, extremely low signal:noise ratio, and was slow. UV pseudo-absorption had high precision, acceptable signal:noise ratio and was quick. However, only a fraction of the total SC removed is analysed, and inhomogeneity can affect the results. The new imaging method was precise, with high signal:noise ratio, and measured the whole SC sample, unaffected by inhomogeneity. In addition, the approach was rapid and has the potential for fast automated scanning of multiple tapes and for further image analysis.

CONCLUSION

The novel imaging method has many advantages over established methods for quantifying SC amount per tape.

Targeting tacrolimus to deeper layers of skin with improved safety for treatment of atopic dermatitis-Part II: in vivo assessment of dermatopharmacokinetics, biodistribution and efficacy

The objective of present investigation was to study in vivo behavior of tacrolimus-loaded lipid-nanoparticles (T-LN) to understand its targeting potential for treatment of atopic-dermatitis-(AD). T-LN have shown significantly improved drug penetration to deeper epidermal and dermal skin-layers than commercial ointment-Protopic(®) and effectively reached target dendritic-immune-cells, responsible for immunopathogenesis of AD. Due to enhanced penetrability of T-LN, it became necessary to evaluate the toxicity of the nanocarrier and the drug at non-target tissues. This paper evaluates dermatopharmacokinetics (DPK), biodistribution, efficacy and safety of T-LN in comparison to Protopic(®) as reference. In vivo DPK in guinea pigs showed 3.02-fold higher bioavailability while γ-scintigraphy in albino-rats demonstrated 1.5-fold rapid penetration of radioactivity in skin for T-LN. Biodistribution in albino-rats revealed restricted localization at the target-skin-area with no general spreading to other body organs suggesting targeting potential of T-LN. In vivo efficacy studies in BALB/c mice showed highly efficient suppression of inflammatory AD-like skin-lesions with T-LN than reference and placebo. Dermal toxicity-studies revealed keratosis and collagenous mass-infiltration with repeated application of reference however interestingly, T-LN treated group showed no evident toxicity demonstrating significantly improved safety. Thus T-LN offered improved penetration to the target site without any toxic-effects and would represent an efficient and commercially viable alternative for AD treatment.

Imaging drug delivery to skin with stimulated Raman scattering microscopy

Efficient drug delivery to the skin is essential for the treatment of major dermatologic diseases, such as eczema, psoriasis and acne. However, many compounds penetrate the skin barrier poorly and require optimized formulations to ensure their bioavailability. Here, stimulated Raman scattering (SRS) microscopy, a recently developed, label-free chemical imaging tool, is used to acquire high resolution images of multiple chemical components of a topical formulation as it penetrates into mammalian skin. This technique uniquely provides label-free, nondestructive, three-dimensional images with high spatiotemporal resolution. It reveals novel features of (trans)dermal drug delivery in the tissue environment: different rates of drug penetration via hair follicles as compared to the intercellular pathway across the stratum corneum are directly observed, and the precipitation of drug crystals on the skin surface is visualized after the percutaneous penetration of the cosolvent excipient in the formulation. The high speed three-dimensional imaging capability of SRS thus reveals features that cannot be seen with other techniques, providing both kinetic information and mechanistic insight into the (trans)dermal drug delivery process.

Assessment of dermal exposure to pesticide residues during re-entry

Currently, the determination of health risks to pesticide applicators from dermal exposure to these chemicals is assessed using either a concentrate of the compound or a relevant aqueous dilution. Neither of these conditions reflects a normal exposure of an individual when re-entering an area after pesticide application, that is, contact with dried residue of the diluted product on foliage. Methodology has therefore been developed to determine a relevant estimate of this potential dermal re-entry exposure from pesticide residues. Potential delivery platforms have been characterized for the transfer of pesticide residue to skin. Spin coating has been used to deposit uniform pesticide layers on to each platform. Five pesticides have been chosen to encompass a wide range of physicochemical properties: atrazine, 2,4-dichlorophenoxyacetic acid (2,4-D), chlorpyrifos, monocrotophos, and acetochlor. In vitro (Franz diffusion cell) experiments have been performed to monitor the transfer of these pesticides from the delivery platforms onto and through excised porcine skin. Parallel experiments were also conducted with aqueous pesticide dilutions for comparison, and a final in vivo measurement using ibuprofen (as a model compound) complemented the in vitro data. The results demonstrate that transfer of chemical residue onto and subsequently through the skin is dependent on the physical attributes of the residue formed. Thus, assessing dermal exposure to pesticides based on skin contact with either the chemical concentrate or a relevant aqueous dilution may incorrectly estimate the risk for re-entry scenarios.

Visualization, dermatopharmacokinetic analysis and monitoring the conformational effects of a microemulsion formulation in the skin stratum corneum

The use of nano-systems such as the microemulsions is considered as an increasingly implemented strategy in order to enhance the percutaneous transport into and across the skin barrier. The determination of the major pathway of penetration and the mechanisms by which these formulations work remains crucial. In this study, laser confocal scanning microscopy was used to visualize the penetration and the distribution of a fluorescently-labelled microemulsion (using 0.1% w/v Nile red) consisting of (%, w/w) 15.4% oleic acid, 30.8% Tween 20, 30.8% Transcutol® and 23% water. The surface images revealed that the microemulsion accumulated preferentially in the intercellular domains of the stratum corneum. Additionally, by analysis of the images taken across the whole stratum corneum (SC), the penetration was found to occur along its whole depth. The latter result was confirmed using tape stripping and the subsequent sensitive analysis using liquid chromatography mass spectroscopy. Dermatopharmacokinetic parameters were obtained for the microemulsion different components. These values proved the breakage of the microemulsion during its penetration across the stratum corneum. Moreover, the mechanisms of penetration enhancement and the micro molecular effects on the skin stratum corneum were investigated using attenuated Fourier transform infra-red spectroscopy. The results revealed the penetration of all the microemulsion components in the stratum corneum and demonstrated the microemulsion interaction with the skin barrier perturbing its architecture structure.

Rapid sampling of molecules via skin for diagnostic and forensic applications

PURPOSE

Skin provides an excellent portal for diagnostic monitoring of a variety of entities; however, there is a dearth of reliable methods for patient-friendly sampling of skin constituents. This study describes the use of low-frequency ultrasound as a one-step methodology for rapid sampling of molecules from the skin.

METHODS

Sampling was performed using a brief exposure of 20 kHz ultrasound to skin in the presence of a sampling fluid. In vitro sampling from porcine skin was performed to assess the effectiveness of the method and its ability to sample drugs and endogenous epidermal biomolecules from the skin. Dermal presence of an antifungal drug-fluconazole and an abused substance, cocaine-was assessed in rats.

RESULTS

Ultrasonic sampling captured the native profile of various naturally occurring moisturizing factors in skin. A high sampling efficiency (79 +/- 13%) of topically delivered drug was achieved. Ultrasound consistently sampled greater amounts of drug from the skin compared to tape stripping. Ultrasonic sampling also detected sustained presence of cocaine in rat skin for up to 7 days as compared to its rapid disappearance from the urine.

CONCLUSIONS

Ultrasonic sampling provides significant advantages including enhanced sampling from deeper layers of skin and high temporal sampling sensitivity.

Development, stability and in vitro permeation studies of gels containing mometasone furoate for the treatment of dermatitis of the scalp

Dermatological inflammatory diseases such as atopic dermatitis, psoriasis and seborrhoeic dermatitis often affect the scalp and the eyebrows. Although there are many dosage forms available, these are particularly critical anatomic regions for application of topical formulations because of the presence of hair. Lotions are therefore the recommended type of drug delivery system for these areas. The presence of hair may limit the application and thus the acceptability of the formulation and its compliance. Because of its low apparent viscosity, lotion application is unpleasant. Gels, given their consistency and adhesiveness, are a suitable alternative to lotions in this situation. The aim of this study was to formulate a stable gel containing mometasone furoate, which is an anti-inflammatory and anti-pruritic corticosteroid, in order to improve topical treatment of scalp dermatitis. In this study, pharmaceutical development, physical-chemical characterization, stability and in vitro permeation studies were performed. In terms of the pH, viscosity, assay and macroscopic and microbiological properties, the gel was stable over the period of study. The in vitro permeation studies allowed the characterization of the mometasone furoate permeation profile for the gel through different membranes. Mometasone furoate presented a slow permeation through the skin. This gel appears safe for topical application.