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

Enhancing Transcutaneous Drug Delivery: Advanced Perspectives on Skin Models

Skin acts as a dynamic interface with the environment. Pathological alterations in the skin barrier are associated with skin diseases. These conditions are characterized by specific impairments in epidermal barrier functions. Despite its protective nature, the skin can be a relevant route of drug administration, both for topical and transdermal therapy, allowing for improved drug delivery and reducing the incidence of adverse reactions. This manuscript reviews transcutaneous drug delivery as a strategy for treating localized and systemic conditions, highlighting the importance of skin models in the evaluation of drug efficacy and barrier function. It explores advances in in vitro, ex vivo, in vivo, and in silico models for studying cellular uptake, wound healing, oxidative stress, anti-inflammatory, and immune modulation activities. Disease-specific skin models are also discussed.

Investigation of the suitability of confocal Raman spectroscopy for the demonstration of bioequivalence of topical products

Bioequivalence studies of topical formulations have attracted increased interest as the European Medicines Agencies "Guideline on quality and equivalence on locally applied, locally acting cutaneous products" describes them in the context of the approval of generics. Since the guideline only proposes tape stripping as a destructive method for bioequivalence testing in in vitro skin penetration, the aim of this study was to investigate the suitability of confocal Raman spectroscopy (CRS) as a non-destructive alternative. To validate the CRS results, tape stripping and CRS experiments using ketoprofen as a model API were performed consecutively on the same samples of ex vivo porcine skin after frozen storage and compared. All experiments were also carried out on two different animal test subjects to investigate a possible influence of inter-individual variations. Furthermore, the influence of the chosen incubation times inside and outside of the steady state was determined. We were able to show that CRS and tape stripping results were very similar both in overall detected API amounts as well as in the results of bioequivalence testing, proving CRS is not only suitable for quantitative skin penetration experiments but also for bioequivalence testing. Inter-individual variations were found to be relevant when comparing formulations measured on different subjects. Bioequivalence testing however reached the same results on both subjects. Finally, the chosen incubation time was limited by skin disintegration, the reaching of steady state however did not influence the results of bioequivalence testing.

A Novel Natural Penetration Enhancer for Transdermal Drug Delivery: In Vitro/In Vivo Evaluation and Penetration Enhancement Mechanism

Objectives: This study aimed to identify and develop a novel, safe, and effective transdermal penetration enhancer derived from the leaves of Perilla frutescens (L.) Britt, and to explore the underlying mechanisms of its penetration enhancement effects. Methods: To evaluate the safety profile of the penetration enhancer, both skin irritation tests and histopathological analyses were conducted. The transdermal enhancement capabilities of the penetration enhancer were assessed in vitro using five model drugs. Furthermore, to gain insights into the penetration enhancement mechanism of this novel penetration enhancer, a range of analytical methods were used, including a spectroscopic technique, differential scanning calorimetry, micro-optical techniques, and molecular docking simulations. Results: Perilla essential oil contained 93.70% perilla ketone (PEK), which exhibited a safety profile superior to that of azone. PEK significantly increased the cumulative skin permeation of all the model drugs (p < 0.05). PEK exhibited the most obvious impact on puerarin penetration, with quantitative enhancement ratios of 2.96 ± 0.07 and 3.39 ± 0.21 at concentrations of 3% and 5% (w/v), respectively. A strong correlation between the enhancement effect of PEK and the physicochemical properties of the drugs was observed. Mechanistic studies revealed that PEK facilitates drug distribution from the solution phase to the stratum corneum (SC). Conclusions: PEK, seldom discussed in former studies, was observed to show extensive penetration enhancement effects by inducing conformational changes in SC lipids and disrupting the tightly ordered bilayer arrangement of lipids. These findings highlight the potential of PEK as a promising and safe natural transdermal penetration enhancer.

Percutaneous absorption of two bisphenol a analogues, BPAF and TGSA: Novel In vitro data from human skin

Bisphenol AF (BPAF) and TGSA are analogues of Bisphenol A (BPA). BPAF is used in polymer synthesis, while TGSA is applied in thermal papers. The EU classifies BPAF as toxic to reproduction and TGSA as a skin sensitizer. However, TGSA's other health effects remain unclear. BPAF contamination has been noted among electronic waste workers, and TGSA exposure is documented in various professions. Despite the significance of skin contact, data on skin permeation rates for BPAF and TGSA are limited. This study aimed to generate percutaneous absorption data for BPAF and TGSA following OECD guidelines. [14C]-labeled BPAF or TGSA was applied to human skin samples in vitro using Franz diffusion cells for 20 and 40 h, respectively. Key parameters such as steady-state flux, lag time, and skin permeability coefficient (Kp) were calculated. Furthermore, the distribution of the dose across different compartments, particularly within the skin, was evaluated at the conclusion of the experiment. Sequential strippings and epidermis-dermis separation were conducted for BPAF to predict the potential absorption of the remaining dose present within the skin. The permeability coefficients for BPAF and TGSA were found to be 1.9 E-03 and 1.6 E-03 cm/h, with 22% and 23% of the applied doses absorbed, respectively. Both chemicals are classified as "fast" penetrants based on their Kp values. These findings suggest that BPAF and TGSA are absorbed through the skin, highlighting potential occupational risks through dermal exposure. The new percutaneous absorption data will enhance the assessment of the occupational risks.

A Consideration on Infinite and Finite Dosing in Skin Permeation Using Reconstructed Models

INTRODUCTION

When vitamin derivatives penetrate the epidermis, they release active compound such as ascorbic acids (AsA) and tocopherols via enzymatic digestion of chemical modifiers. To determine the transdermal penetration of the derivatives, the total permeation of both the derivatives and their active compounds that released from the derivatives should be considered. In this study, we established a skin penetration test method using a cultured, reconstructed skin model with active epidermal enzymes. And we analyzed two vitamin derivatives with different chemical properties: magnesium ascorbyl phosphate (APM) and sodium tocopheryl phosphate (TPNa), both of which has been confirmed their skin permeation in the reconstructed models and the digestion to AsA and α-tocopherol by the epidermal enzymes, respectively.

METHODS

We prepared the 1% of water solution containing either APM or TPNa. Then, we tested the cumulative permeation of the derivatives at 2 application volumes, 25 μL/cm2 (finite dosing) and 85 μL/cm2 (infinite dosing), on cultured reconstructed skin and observed the permeation of the permeants every 2 h up to 24 h.

RESULTS

When the applied formula was used to assess the evaporation rate to determine an end point of the test system, all the water evaporated in 6 h in finite model and in 8 h in infinite model. Both models showed that the cumulative permeation of the active compounds increased and a constant flux until 8 h after application; however, the flux decreased thereafter, indicating that the decreased flux depended on an end point of the test system. This indicated that our test system can analyze the permeation of the vitamin derivatives within 8 h before reaching the end point.

CONCLUSION

Using an infinite model of this system, we assessed the cumulative permeation of vitamin derivatives within 8 h using a reconstructed skin model.

INTRODUCTION

When vitamin derivatives penetrate the epidermis, they release active compound such as ascorbic acids (AsA) and tocopherols via enzymatic digestion of chemical modifiers. To determine the transdermal penetration of the derivatives, the total permeation of both the derivatives and their active compounds that released from the derivatives should be considered. In this study, we established a skin penetration test method using a cultured, reconstructed skin model with active epidermal enzymes. And we analyzed two vitamin derivatives with different chemical properties: magnesium ascorbyl phosphate (APM) and sodium tocopheryl phosphate (TPNa), both of which has been confirmed their skin permeation in the reconstructed models and the digestion to AsA and α-tocopherol by the epidermal enzymes, respectively.

METHODS

We prepared the 1% of water solution containing either APM or TPNa. Then, we tested the cumulative permeation of the derivatives at 2 application volumes, 25 μL/cm2 (finite dosing) and 85 μL/cm2 (infinite dosing), on cultured reconstructed skin and observed the permeation of the permeants every 2 h up to 24 h.

RESULTS

When the applied formula was used to assess the evaporation rate to determine an end point of the test system, all the water evaporated in 6 h in finite model and in 8 h in infinite model. Both models showed that the cumulative permeation of the active compounds increased and a constant flux until 8 h after application; however, the flux decreased thereafter, indicating that the decreased flux depended on an end point of the test system. This indicated that our test system can analyze the permeation of the vitamin derivatives within 8 h before reaching the end point.

CONCLUSION

Using an infinite model of this system, we assessed the cumulative permeation of vitamin derivatives within 8 h using a reconstructed skin model.

Hydroxytyrosol permeability comparisons and strategies to improve hydroxytyrosol stability in formulations

There has been a growing interest in hydroxytyrosol (HT) due to its powerful antioxidant and free-radical scavenging properties when added to formulations such as pharmaceuticals and cosmetics. To study the stability and transdermal properties of hydrogels and creams (HT-based formulations), a high-performance liquid chromatography method was developed for determining HT. In the Franz diffusion cell system, both hydrogel and cream show a rapid and similar penetration profile through the Bama miniature pig skin. However, the Strat-M® membrane exhibits slightly lower permeability and is selective to different formulations; that is, the cream has a permeability value of 10.69%, while the hydrogel has a value of 5.27%. The dynamics parameters from the permeation assays indicate that the model using the Strat-M® membrane can be used as a screening tool to evaluate the skin uptake and permeation efficacy of different formulations. Adding 3-O-ethyl-L-ascorbic acid to HT-based formulations can effectively prevent discoloration under prolonged high-temperature storage, while combining multiple antioxidants delays degradation most effectively. This study provides novel ideas for functional formulation optimization to enhance the realism and reproducibility of cosmetic products containing HT and provides scientific evidence for the production, packaging, shelf life, storage, and transportation of products.

An update of skin permeability data based on a systematic review of recent research

The cutaneous absorption parameters of xenobiotics are crucial for the development of drugs and cosmetics, as well as for assessing environmental and occupational chemical risks. Despite the great variability in the design of experimental conditions due to uncertain international guidelines, datasets like HuskinDB have been created to report skin absorption endpoints. This review updates available skin permeability data by rigorously compiling research published between 2012 and 2021. Inclusion and exclusion criteria have been selected to build the most harmonized and reusable dataset possible. The Generative Topographic Mapping method was applied to the present dataset and compared to HuskinDB to monitor the progress in skin permeability research and locate chemotypes of particular concern. The open-source dataset (SkinPiX) includes steady-state flux, maximum flux, lag time and permeability coefficient results for the substances tested, as well as relevant information on experimental parameters that can impact the data. It can be used to extract subsets of data for comparisons and to build predictive models.

Probing the effectiveness of barrier creams against human skin penetration of nickel powder

OBJECTIVE

Barrier creams (BCs) are marketed as locally applied medical devices or cosmetic products to protect the skin from exposure to chemicals and irritants. Generally, the mechanism of action of such products is mainly due to the formation of a superficial thin film between the skin and the irritant or sensitizer, thus reducing or totally blocking the cutaneous penetration of such agents. Specifically, studies focusing on the effectiveness of commercial protective creams to prevent nickel cutaneous penetration are extremely scarce. The aim of the current work, therefore, is to evaluate the protective role of a commercially available barrier cream for nickel and compare the results with a simple moisturizing, following exposure to Ni powder.

METHODS

Marketed BCs were evaluated and tested. Human skin absorption of Ni was studied in vitro using static Franz diffusion cells.

RESULTS

Our results demonstrate that the application of both formulations caused a reduction of Ni inside the skin (8.00 ± 3.35 μg cm-2 for the barrier cream and 22.6 ± 12.6 μg cm-2 for the general moisturizing product), with the specialized barrier cream being statistically (p = 0.015) more efficient on forming a protective barrier, thus evidencing the importance of some ingredients in such formulations on the nickel dermal accumulation.

CONCLUSIONS

The composition of the formulations based on film-forming or chelating agents may play an imperative role in reducing the cutaneous penetration of Ni.

Methods for the characterisation of dermal uptake: Progress and perspectives for organophosphate esters

Organophosphate esters (OPEs) are a group of pollutants that are widely detected in the environment at high concentrations. They can adversely affect human health through multiple routes of exposure, including dermal uptake. Although attention has been paid to achieving an accurate and complete quantification of the dermal uptake of OPEs, existing evaluation methods and parameters have obvious weaknesses. This study reviewed two main categories of methodologies, namely the relative absorption (RA) model and the permeability coefficient (PC) model, which are widely used to assess the dermal uptake of OPEs. Although the PC model is more accurate and is increasingly used, the most important parameter in this model, the permeability coefficient (Kp), has been poorly characterised for OPEs, resulting in considerable errors in the estimation of the dermal uptake of OPEs. Thus, the detailed in vitro methods for the determination of Kp are summarised and sorted. Furthermore, the commonly used skin membranes are identified and the factors affecting Kp and corresponding mechanisms are discussed. In addition, the experimental conditions, conclusions, and available data on Kp values of the OPEs are thoroughly summarised. Finally, the corresponding knowledge gaps are proposed, and a more accurate and sophisticated experimental system and unknown Kp values for OPEs are suggested.

Characterization of piroctone olamine for topical delivery to the skin

OBJECTIVE

Dandruff and its more severe related condition, seborrheic dermatitis affects a high proportion of the population at some point in their life. Piroctone olamine, also known as Octopirox® (OPX) is the monoethanolamine salt of piroctone and is an antifungal agent widely used for the management of dandruff. The aim of the present work was to characterize the physicochemical properties of piroctone olamine and to conduct pre-formulation studies for the development of novel topical formulations of this active.

METHODS

An HPLC method was developed and validated for the analysis of OPX. The melting point was determined using the DSC Q2000 (TA Instruments, USA). The distribution coefficient (logD(O/PBS) ) and partition coefficient (log Po/w ) was determined in phosphate-buffered saline (PBS) AND deionized (DI) water using the shake flask method. All experiments were performed at room temperature. The solubility was determined experimentally by adding amount of active to a solvent. The samples were kept at 32° ± 1°C for 48 h in a water bath. The stability of the compound was determined in a range of solvents by preparing solutions of 1 mg mL-1 in the relevant solvents. These solutions were kept and stirred throughout the experiment at 32 ± 1°C, and aliquots were taken at 24, 48 and 96 h.

RESULTS

The HPLC method was developed successfully; however, samples at the lower end of the calibration curve showed lower degrees of precision and accuracy. Based on experiments with DSC, the melting point was observed at an onset temperature of 132.4°C. The LogD was determined to be 1.84. The compound had the highest solubility in methanol (278.4 mg mL-1 ) and propylene glycol (PG), with a value of 248.8 mg mL-1 . The lowest solubility for OPX was in dimethyl isosorbide (9.9 mg mL-1 ), Labrafac™ (3.6 mg mL-1 ) and isostearyl isostearate (0.5 mg mL-1 ). Over the 4 days, OPX showed stability in ethanol and PG, while a notable decrease in OPX was observed in PBS and DI water at 32 ± 1°C.

CONCLUSION

The physicochemical properties of OPX were characterized to find suitable excipients able to target the epidermis for topical delivery. Building on these findings, future work will focus on the development of novel topical formulation of OPX.

Recent Options and Techniques to Assess Improved Bioavailability: In Vitro and Ex Vivo Methods

Bioavailability assessment in the development phase of a drug product is vital to reveal the disadvantageous properties of the substance and the possible technological interventions. However, in vivo pharmacokinetic studies provide strong evidence for drug approval applications. Human and animal studies must be designed on the basis of preliminary biorelevant experiments in vitro and ex vivo. In this article, the authors have reviewed the recent methods and techniques from the last decade that are in use for assessing the bioavailability of drug molecules and the effects of technological modifications and drug delivery systems. Four main administration routes were selected: oral, transdermal, ocular, and nasal or inhalation. Three levels of methodologies were screened for each category: in vitro techniques with artificial membranes; cell culture, including monocultures and co-cultures; and finally, experiments where tissue or organ samples were used. Reproducibility, predictability, and level of acceptance by the regulatory organizations are summarized for the readers.

4'-OH as the Action Site of Lipids and MRP1 for Enhanced Transdermal Delivery of Flavonoids

To date, the transdermal delivery study mainly focused on the drug delivery systems' design and efficacy evaluation. Few studies reported the structure-affinity relationship of the drug with the skin, further revealing the action sites of the drugs for enhanced permeation. Flavonoids attained a considerable interest in transdermal administration. The aim is to develop a systematic approach to evaluate the substructures that were favorable for flavonoid delivery into the skin and understand how these action sites interacted with lipids and bound to multidrug resistance protein 1 (MRP1) for enhanced transdermal delivery. First, we investigated the permeation properties of various flavonoids on the porcine skin or rat skin. We found that 4'-OH (hydroxyl group on the carbon 4' position) rather than 7-OH on the flavonoids was the key group for flavonoid permeation and retention, while 4'-OCH₃ and -CH₂═CH₂-CH-(CH₃)₂ were unfavorable for drug delivery. 4'-OH could decrease flavonoids' lipophilicity to an appropriate log P and polarizability for better transdermal drug delivery. In the stratum corneum, flavonoids used 4'-OH as a hand to specifically grab the C═O group of the ceramide NS (Cer), which increased the miscibility of flavonoids and Cer and then disturbed the lipid arrangement of Cer, thereby facilitating their penetration. Subsequently, we constructed overexpressed MRP1 HaCaT/MRP1 cells by permanent transfection of human MRP1 cDNA in wild HaCaT cells. In the dermis, we observed that 4'-OH, 7-OH, and 6-OCH₃ substructures were involved in H-bond formation within MRP1, which increased the flavonoid affinity with MRP1 and flavonoid efflux transport. Moreover, the expression of MRP1 was significantly enhanced after the treatment of flavonoids on the rat skin. Collectively, 4'-OH served as the action site for increased lipid disruption and enhanced affinity for MRP1, which facilitate the transdermal delivery of flavonoids, providing valuable guidelines for molecular modification and drug design of flavonoids.

Development and optimization of amphiphilic self-assembly into nanostructured liquid crystals for transdermal delivery of an antidiabetic SGLT2 inhibitor

The anti-hyperglycemic sodium glucose co-transporter 2 inhibitor Canagliflozin (CFZ) represents a recent antihyperglycemic modality, yet it suffers from low oral bioavailability. The current work aims to formulate CFZ-loaded transdermal nanostructured liquid crystal gel matrix (NLCG) to improve its therapeutic efficiency. Pre-formulation study included the construction of pseudoternary phase diagrams to explore the effect of two conventional amphiphiles against amphiphilic tri-block copolymer in the formulation of NLCG. The influence of different co-solvents was also investigated with the use of monooleine as the oil. Physical characterization, morphological examination and skin permeation were performed for the optimized formulations. The formula of choice was further investigated for skin irritation and chemical stability. Pharmacodynamic evaluation of the successful formula was conducted on hyperglycemic as well as normoglycemic mice. In addition, oral glucose tolerance test was conducted. Results revealed the supremacy of Poloxamer for stabilizing and maximizing liquid crystal gel (LCG) area percentage that reached up to 12.6%. CFZ-NLCG2 isotropic formula showed the highest permeation parameters; maximum flux value of 7460 μg/cm² h and Q₂₄ of 5327 μg/cm². Pharmacodynamic evaluation revealed the superiority of the antihyperglycemic activity of CFZ-NLCG2 in fasting mice and its equivalence in the oral glucose tolerance test (OGTT) compared to the oral one. The obtained results confirmed the success of CFZ-NLCG2 in the transdermal delivery of CFZ in therapeutically effective concentration compared to the oral route, bypassing first pass effect; in addition, eliminates the possible gastrointestinal side effects related to the inhibition of intestinal sodium glucose co-transporter (SGLT) and maximizes its selectivity to the desired inhibition of renal SGLT.

Predicting skin permeability using HuskinDB

A freely accessible database has recently been released that provides measurements available in the literature on human skin permeation data, known as the ‘Human Skin Database – HuskinDB’. Although this database is extremely useful for sourcing permeation data to help with toxicity and efficacy determination, it cannot be beneficial when wishing to consider unlisted, or novel compounds. This study undertakes analysis of the data from within HuskinDB to create a model that predicts permeation for any compound (within the range of properties used to create the model). Using permeability coefficient (K_p) data from within this resource, several models were established for K_p values for compounds of interest by varying the experimental parameters chosen and using standard physicochemical data. Multiple regression analysis facilitated creation of one particularly successful model to predict K_p through human skin based only on three chemical properties. The model transforms the dataset from simply a resource of information to a more beneficial model that can be used to replace permeation testing for a wide range of compounds.

Biodegradable core-multishell nanocarrier: Topical tacrolimus delivery for treatment of dermatitis

Two challenges in topical drug delivery to the skin include solubilizing hydrophobic drugs in water-based formulations and increasing drug penetration into the skin. Polymeric core-multishell nanocarrier (CMS), particularly the novel biodegradable CMS (bCMS = hPG-PCL_1.1K-mPEG_2k-CMS) have shown both advantages on excised skin ex vivo. Here, we investigated topical delivery of tacrolimus (TAC; > 500 g/mol) by bCMS in a hydrogel on an oxazolone-induced model of dermatitis in vivo. As expected, bCMS successfully delivered TAC into the skin. However, in vivo they did not increase, but decrease TAC penetration through the stratum corneum compared to ointment. Differences in the resulting mean concentrations were mostly non-significant in the skin (epidermis: 35.7 ± 20.9 ng/cm² for bCMS vs. 92.6 ± 62.7 ng/cm² for ointment; dermis: 76.8 ± 26.8 ng/cm²vs 118.2 ± 50.4 ng/cm²), but highly significant in blood (plasma: 1.1 ± 0.4 ng/ml vs 11.3 ± 9.3 ng/ml; erythrocytes: 0.5 ± 0.2 ng/ml vs 3.4 ± 2.4 ng/ml) and liver (0.01 ± 0.01 ng/mg vs 0.03 ± 0.01 ng/mg). bCMS were detected in the stratum corneum but not in viable skin or beyond. The therapeutic efficacy of TAC delivered by bCMS was equivalent to that of standard TAC ointment. Our results suggest that bCMS may be a promising carrier for the topical delivery of TAC. The quantitative difference to previous results should be interpreted in light of structural differences between murine and human skin, but highlights the need as well as potential methods to develop more a complex ex vivo analysis on human skin to ensure quantitative predictive value.

Preparation, Characterization and Permeation Study of Topical Gel Loaded with Transfersomes Containing Asiatic Acid

The objective of this study is to investigate the in vitro permeation of asiatic acid (AA) in the form of a topical gel after entrapment in transfersomes by Franz diffusion cells. Transfersomes composed of soybean lecithin and three different edge activators including Tween 80 (TW80), Span 80 (SP80) and sodium deoxycholate (SDC) at the ratio of 50:50, 90:10 and 90:10, respectively, together with 0.3% w/w of AA, were prepared by a high-pressure homogenization technique and further incorporated in gels (TW80AATG, SP80AATG and SDCAATG). All transfersomal gels were characterized for their AA contents, dynamic viscosity, pH and homogeneity. Results revealed that the AA content, dynamic viscosity and pH of the prepared transfersomal gels ranged from 0.272 ± 0.006 to 0.280 ± 0.005% w/w, 812.21 ± 20.22 to 1222.76 ± 131.99 Pa.s and 5.94 ± 0.03 to 7.53 ± 0.03, respectively. TW80AATG gave the highest percentage of AA penetration and flux into the Strat-M^® membrane at 8 h (8.53 ± 1.42% and 0.024 ± 0.008 mg/cm²/h, respectively) compared to SP80AATG (8.00 ± 1.70% and 0.019 ± 0.010 mg/cm²/h, respectively), SDCAATG (4.80 ± 0.50% and 0.014 ± 0.004 mg/cm²/h, respectively), non-transfersomal gels (0.73 ± 0.44 to 3.13 ± 0.46% and 0.002 ± 0.001 to 0.010 ± 0.002 mg/cm²/h, respectively) and hydroethanolic AA solution in gel (1.18 ± 0.76% and 0.004 ± 0.003 mg/cm²/h, respectively). These findings indicate that the TW80AATG might serve as a lead formulation for further development toward scar prevention and many types of skin disorders.

In vitro assessment of the dermal penetration potential of sodium fluoroacetate using a formulated product

This paper presents experimental data on the skin absorption of sodium fluoroacetate from a formulated product using an in vitro approach and human skin. Sodium fluoroacetate is a pesticide, typically applied in formulation (1080) for the control of unwanted vertebrate invasive species. It has been assigned a Skin Notation by the ACGIH, and other international workplace health regulatory bodies, due to its predicted ability to permeate intact and abraded human skin. However, there is a distinct lack of experimental data on the skin absorption of sodium fluoroacetate to support this assignment. This study found that sodium fluoroacetate, as a formulated product, permeated the human epidermis when in direct contact for greater than 10 hr. A steady-state flux (J_ss) of 1.31 ± 0.043 µg/cm²/hr and a lag time of 6.1 hr was calculated from cumulative skin permeation data. This study provides important empirical evidence in support of the assignment of a Skin Notation.

Impact of Alkanediols on Stratum Corneum Lipids and Triamcinolone Acetonide Skin Penetration

Alkanediols are widely used as multifunctional ingredients in dermal formulations. In addition to their preservative effect, considering their possible impact on drug penetration is also essential for their use. In the present study, the influence of 2-methyl-2,4-pentanediol, 1,2-pentanediol, 1,2-hexanediol and 1,2-octanediol on the skin penetration of triamcinolone acetonide from four different semisolid formulations was investigated. Furthermore, confocal Raman spectroscopy measurements were performed to examine the influence of the alkanediols on stratum corneum lipid content and order. Alkanediols were found to increase the penetration of triamcinolone acetonide. However, the extent depends strongly on the formulation used. In certain formulations, 1,2-pentanediol showed the highest effect, while in others the penetration-enhancing effect increased with the alkyl chain length of the alkanediol used. None of the tested alkanediols extracted lipids from the stratum corneum nor reduced its thickness. Notwithstanding the above, the longer-chained alkanediols cause the lipids to be converted to a more disordered state, which favors drug penetration. This behavior could not be detected for the shorter-chained alkanediols. Therefore, their penetration-enhancing effect is supposed to be related to an interaction with the hydrophilic regions of the stratum corneum.

An investigation on the effect of drug physicochemical properties on the enhancement strength of enhancer: The role of drug-skin-enhancer interactions

The aim of present work was to investigate the influence of drug physicochemical properties on the enhancement effect of enhancers, which guided the application of enhancers in different drug transdermal prescriptions. Firstly, Polyglyceryl-3 dioleate (POCC) was selected as a model enhancer and its enhancement effect on ten drugs was assessed by in vitro skin permeation experiment. Secondly, the correlation analysis of physicochemical properties of drugs was carried out from the aspects of partition and permeation. The interactions of drug-skin-POCC were elucidated by FT-IR, molecular docking, solubility parameters calculation, ATR-FTIR, Raman study, molecular dynamics simulation and confocal laser scanning microscopy (CLSM). The results showed that the enhancement ratio (ER) of drugs was ranging from 2.23 to 7.45. On one hand, the miscibility between drugs with low polar surface area (P.S.A) and donor solution was decreased more pronounced by the addition of POCC because of the drug was difficult to form hydrogen-bond with POCC, facilitating the vehicle/SC partition of drugs. On the other hand, the permeation of drugs with low P.S.A and polarizability was enhanced more significantly by POCC because the drug was less likely to interact with skin lipids compared to others, causing that POCC had more chance to interact with skin lipids to improve permeation drugs across the SC more easily. In conclusion, the different strength of drug-skin-POCC interactions was the main reason for the discrepancy in the enhancement effect of the POCC on ten drugs, which laid a basis for the research on the drug-specific molecular mechanisms of enhancers.

Lanolin-Based Synthetic Membranes for Transdermal Permeation and Penetration Drug Delivery Assays

Due to the high similarity in composition and structure between lanolin and human SC lipids, we will work with two models from wool wax. Two types of lanolin were evaluated: one extracted with water and surfactants (WEL) and the other extracted with organic solvents (SEL). Skin permeation and skin penetration studies were performed with two active compounds to study the feasibility of the use of lanolin-based synthetic membranes as models of mammalian skin. Diclofenac sodium and lidocaine were selected as the active compounds considering that they have different chemical natures and different lipophilicities. In the permeation assay with SEL, a better correlation was obtained with the less permeable compound diclofenac sodium. This assay suggests the feasibility of using artificial membranes with SEL as a model for percutaneous absorption studies, even though the lipophilic barrier should be improved. Penetration profiles of the APIs through the SEL and WEL membranes indicated that the two membranes diminish penetration and can be considered good membrane surrogates for skin permeability studies. However, the WEL membranes, with a pH value similar to that of the skin surface, promoted a higher degree of diminution of the permeability of the two drugs, similar to those found for the skin.

Effect of Application Amounts on In Vitro Dermal Absorption Test Using Caffeine and Testosterone

Dermal absorption of chemicals is a key factor in risk assessment. This study investigated the effects of different amounts of application on dermal absorption and suggested an appropriate application dose for proper dermal absorption. Caffeine and testosterone were chosen as test compounds. An in vitro dermal absorption test was performed using a Franz diffusion cell. Different amounts (5, 10, 25, and 50 mg (or µL)/cm²) of semisolid (cream) and liquid (solution) formulations containing 1% caffeine and 0.1% testosterone were applied to rat and minipig (Micropig^®) skins. After 24 h, the concentrations of both compounds remaining on the skin surface and in the stratum corneum, dermis and epidermis, and receptor fluid were determined using LC-MS / MS or HPLC. Dermal absorption of both compounds decreased with increasing amounts of application in both skin types (rat and minipig) and formulations (cream and solution). Especially, dermal absorptions (%) of both compounds at 50 mg (or µL)/cm² was significantly lower compared to 5 or 10 mg (or µL)/cm² in both rat and minipig skins. Therefore, a low dose (5 or 10 mg (or µL)/cm²) of the formulation should be applied to obtain conservative dermal absorption.

Skin Absorption of Bisphenol A and Its Alternatives in Thermal Paper

OBJECTIVES

Bisphenol A (BPA) is the most used colour developer in thermal paper for cashiers receipts, labels, and tickets. BPA can migrate onto the skin and be absorbed when handling these papers. BPA is a known endocrine disruptor and is therefore being replaced in thermal paper by some alternatives such as Bisphenol S (BPS), D-8, and Pergafast 201® (PF201). To our knowledge, no studies have characterized skin permeation of these BPA alternatives.

METHODS

We measured/characterized skin absorption for BPA, BPS, D-8, and PF201 through ex vivo human skin using flow-through diffusion cells according to OECD guideline 428. Skin samples were 7-12 per test substance from three different skin donors. Skin metabolism was studied for BPA. Dermal absorption was expressed as the amount of the BPA alternatives in the receptor fluid over applied dose in percent (%).

RESULTS

The absorbed dose after 24 h of exposure was 25% for BPA, 17% for D-8, 0.4% for BPS, and <LLOQ for PF201. The amount of BPA-glucuronide in the receptor fluid after 24 h was under the limit of quantification (LLOQ = 0.2 µg l-1). Despite the 10-fold lower concentration of the aq solution applied on the skin, D-8's permeation rate JMAX was 5-fold higher than the one for BPS (0.032 versus 0.006 µg cm-2 h-1). Neither D-8 nor BPS permeated readily through the skin (tlag = 3.9 h for D-8, 6.4 h for BPS). None of PF201's skin permeation kinetic parameters could be determined because this BPA analogue was not quantifiable in the receptor fluid in our test conditions.

CONCLUSIONS

Skin absorption was in decreasing order: BPA > D-8 >> BPS > PF201. These results are in agreement with their log Kow and molecular weights. We provided here the necessary data to estimate the extent of skin absorption of BPA analogues, which is a necessary step in risk assessment, and ultimately evaluate public health risks posed by D-8, BPS, and PF201.

Assessment of Finite and Infinite Dose In Vitro Experiments in Transdermal Drug Delivery

Penetration, usually with finite dosing, provides data about the total active amount in the skin and permeation, being the most used methodology, usually with infinite dosing, leads to data about pharmacokinetic parameters. The main objective of this work is to assess if results from permeation, most of them at finite dose, may be equivalent to those from penetration usually at infinite dose. The transdermal behavior of four drugs with different physicochemical properties (diclofenac sodium, ibuprofen, lidocaine, and caffeine) was studied using penetration/finite and kinetic permeation/infinite dose systems using vertical Franz diffusion cells to determine the relationships between permeation and penetration profiles. Good correlation of these two in vitro assays is difficult to find; the influence of their dosage and the proportion of different ionized/unionized compounds due to the pH of the skin layers was demonstrated. Finite and infinite dose regimens have different applications in transdermal delivery. Each approach presents its own advantages and challenges. Pharmaceutical industries are not always clear about the method and the dose to use to determine transdermal drug delivery. Being aware that this study presents results for four actives with different physicochemical properties, it can be concluded that the permeation/infinite results could not be always extrapolated to those of penetration/finite. Differences in hydrophilicity and ionization of drugs can significantly influence the lack of equivalence between the two methodologies. Further investigations in this field are still needed to study the correlation of the two methodologies and the main properties of the drugs that should be taken into account.

Biomimetic Artificial Membrane Permeability Assay over Franz Cell Apparatus Using BCS Model Drugs

A major parameter controlling the extent and rate of oral drug absorption is permeability through the lipid bilayer of intestinal epithelial cells. Here, a biomimetic artificial membrane permeability assay (Franz-PAMPA Pampa) was validated using a Franz cells apparatus. Both high and low permeability drugs (metoprolol and mannitol, respectively) were used as external standards. Biomimetic properties of Franz-PAMPA were also characterized by electron paramagnetic resonance spectroscopy (EPR). Moreover, the permeation profile for eight Biopharmaceutic Classification System (BCS) model drugs cited in the FDA guidance and another six drugs (acyclovir, cimetidine, diclofenac, ibuprofen, piroxicam, and trimethoprim) were measured across Franz-PAMPA. Apparent permeability (Papp) Franz-PAMPA values were correlated with fraction of dose absorbed in humans (Fa%) from the literature. Papp in Caco-2 cells and Corti artificial membrane were likewise compared to Fa% to assess Franz-PAMPA performance. Mannitol and metoprolol Papp values across Franz-PAMPA were lower (3.20 × 10^-7 and 1.61 × 10^-5 cm/s, respectively) than those obtained across non-impregnated membrane (2.27 × 10^-5 and 2.55 × 10^-5 cm/s, respectively), confirming lipidic barrier resistivity. Performance of the Franz cell permeation apparatus using an artificial membrane showed acceptable log-linear correlation (R² = 0.664) with Fa%, as seen for Papp in Caco-2 cells (R² = 0.805). Data support the validation of the Franz-PAMPA method for use during the drug discovery process.

Nanoencapsulation potentiates the cutaneous anti-inflammatory effect of p,p'-methoxyl-diphenyl diselenide: Design, permeation, and in vivo studies of a nanotechnological-based carrageenan gum hydrogel

This study aimed to investigate the feasibility of preparing a hydrogel based on (OMePhSe)₂-loaded poly(Ɛ-caprolactone) nanocapsules using carrageenan gum as a gel-forming agent. Furthermore, the anti-inflammatory action of hydrogel was assessed in an animal model of skin lesion induced by ultraviolet B (UVB) radiation in mice. Nanocapsules were prepared using the interfacial deposition of preformed polymer technique. The hydrogels were obtained by the direct addition of nanocapsules suspension in carrageenan gum (3%). Formulations with free compound, vehicle, and blank nanocapsules were also produced. The hydrogels were characterized by pH, compound content, diameter, spreadability, rheological behavior, and permeation profile. The pharmacological performance was assessed in an animal model of skin injury induced by UVB-radiation in male Swiss mice. All hydrogels had pH around 7.0, compound content close to the theoretical value (2.5 mg/g), an average diameter in nanometric range (around 350 nm), non-Newtonian flow with pseudoplastic behavior, and suitable spreadability factor. The nano-based hydrogel increased the compound content in the epidermis and dermis layers in comparison to the formulation prepared with non-encapsulated (OMePhSe)₂. Stability studies revealed that the hydrogels of nanoencapsulated compound had superior physicochemical stability in comparison to the formulation of free (OMePhSe)_2. Moreover, topical treatment with the hydrogel containing (OMePhSe)₂ loaded-nanocapsules was more effective in reducing ear thickness and the inflammatory process induced by UVB radiation in mice. Herein, a polysaccharide was applied as a gel-forming agent using a simple and low-cost method. Besides, a superior permeation profile and improved pharmacological action were achieved by the compound encapsulation.

In Vitro Permeation Test (IVPT) for Pharmacokinetic Assessment of Topical Dermatological Formulations

In vitro assessment of topical (dermal) pharmacokinetics is a critical aspect of the drug development process for semi-solid products (e.g., solutions, foams, sprays, creams, gels, lotions, ointments), allowing for informed selection of new chemical entities, optimization of prototype formulations during the nonclinical stage, and determination of bioequivalence of generics. It can also serve as a tool to further understand the impact of different excipients on drug delivery, product quality, and formulation microstructure when used in parallel with other techniques, such as analyses of rheology, viscosity, microscopic characteristics, release rate, particle size, and oil droplet size distribution. The in vitro permeation test (IVPT), also known as in vitro skin penetration/permeation test, typically uses ex vivo human skin in conjunction with diffusion cells, such as Franz (or vertical) or Bronaugh (or flow-through) diffusion cells, and is the technique of choice for dermal pharmacokinetics assessment. Successful execution of the IVPT also involves the development and use of fit-for-purpose bioanalytical methods and procedures. The protocols described herein provide detailed steps for execution of the IVPT utilizing flow-through diffusion cells and for key aspects of the development of a liquid chromatography-tandem mass spectrometry method intended for analysis of the generated samples (epidermis, dermis, and receptor solution). © 2020 Wiley Periodicals LLC. Basic Protocol 1: In vitro permeation test Support Protocol: Dermatoming of ex vivo human skin Basic Protocol 2: Bioanalytical methodology in the context of the in vitro permeation test.

Reflections on the OECD guidelines for in vitro skin absorption studies

At the 8th conference of Occupational and Environmental Exposure of the Skin to Chemicals (OEESC) (16-18 September 2019) in Dublin, Ireland, several researchers performing skin permeation assays convened to discuss in vitro skin permeability experiments. We, along with other colleagues, all of us hands-on skin permeation researchers, present here the results from our discussions on the available OECD guidelines. The discussions were especially focused on three OECD skin absorption documents, including a recent revision of one: i) OECD Guidance Document 28 (GD28) for the conduct of skin absorption studies (OECD, 2004), ii) Test Guideline 428 (TGD428) for measuring skin absorption of chemical in vitro (OECD, 2004), and iii) OECD Guidance Notes 156 (GN156) on dermal absorption issued in 2011 (OECD, 2011). GN156 (OECD, 2019) is currently under review but not finalized. A mutual concern was that these guidance documents do not comprehensively address methodological issues or the performance of the test, which might be partially due to the years needed to finalize and update OECD documents with new skin research evidence. Here, we summarize the numerous factors that can influence skin permeation and its measurement, and where guidance on several of these are omitted and often not discussed in published articles. We propose several improvements of these guidelines, which would contribute in harmonizing future in vitro skin permeation experiments.

Human in vitro percutaneous absorption of bisphenol S and bisphenol A: A comparative study

Bisphenol A (BPA) is widely used in industrial products. Due to the toxicity of this compound, and to comply with restrictions and regulations, manufacturers have progressively replaced it by substitutes. One of the main substitutes used is bisphenol S (BPS). Despite increasing use in many products, the effects of BPS on human health have been little investigated, and studies on percutaneous BPS absorption and particularly toxicokinetic data are lacking. However, the endocrine-disrupting activity of BPA and BPS appears comparable. Dermal contact is a significant source of occupational exposure and is the main route during handling of bisphenol-containing receipts by cashiers. Here, percutaneous BPS absorption was investigated and compared to that of BPA. Experiments were performed according to OECD guidelines. Test compounds dissolved in a vehicle - acetone, artificial sebum or water - were applied in vitro to fresh human skin samples in static Franz diffusion cells. Flux, cumulative absorbed dose and distribution of dose recovered were measured. BPA absorption was vehicle-dependent ranging from 3% with sebum to 41% with water. BPS absorption was much lower than BPA absorption whatever the vehicle tested (less than 1% of applied dose). However, depending on the vehicle 20% to 47% of the applied BPS dose remained in the skin, and was consequently potentially absorbable. Both BPA and BPS were mainly absorbed without biotransformation. Taken together, these results indicate that workers may be exposed to BPS through skin when handling products containing it. This exposure is of concern as its toxicity is currently incompletely understood.

Influence of Concentration on Release and Permeation Process of Model Peptide Substance-Corticotropin-From Semisolid Formulations

The transdermal route of administration of drug substances allows clinicians to obtain a therapeutic effect bypassing the gastrointestinal tract, where the active substance could be inactivated. The hormonal substance used in the study-corticotropin (ACTH)-shows systemic effects. Therefore, the study of the effect of the type of ointment base and drug concentration on the release rate and also permeation rate in in vivo simulated conditions may be a valuable source of information for clinical trials to effectively optimize corticotropin treatment. This goal was achieved by preparation ointment formulation selecting the appropriate ointment base and determining the effect of ACTH concentration on the release and permeation studies of the ACTH. Semi-solid preparations containing ACTH were prepared using Unguator CITO e/s. The release study of ACTH was tested using a modified USP apparatus 2 with Enhancer cells. The permeation study was conducted with vertical Franz cells. Rheograms of hydrogels were made with the use of a universal rotational rheometer. The dependence of the amount of released and permeated hormone on the ointment concentration was found. Based on the test of ACTH release from semi-solid formulations and evaluation of rheological parameters, it was found that glycerol ointment is the most favourable base for ACTH. The ACTH release and permeation process depends on both viscosity and ACTH concentration. The higher the hormone concentration, the higher the amount of released ACTH but it reduces the amount of ACTH penetrating through porcine skin.

Prediction of skin permeation and concentration of rhododendrol applied as finite dose from complex cosmetic vehicles

Finite dose experiments represent clinical use wherein depletion of dose, evaporation of excipients, and gradual change in vehicle composition may occur. In the present study, we attempted a mathematical approach for predicting skin permeation and concentration of a cosmetic active, rhododendrol (RD), from complex vehicle-based formulations applied in finite dose. In vitro skin permeation and concentration studies of RD were conducted from formulations containing water and polyols with concentrations ranging from 10 to 100% under infinite and finite dose conditions using vertical Franz diffusion cells. Observed data for skin permeation and the viable epidermis and dermis (VED) concentration of RD were estimated by the differential equations under Fick's second law of diffusion together with water evaporation kinetics and changes in the partition coefficient from vehicles to the stratum corneum. As a result, a goodness-of-fit was observed allowing accurate estimation of skin permeation and VED concentration of RD. This mathematical approach could become a useful tool to estimate the skin permeation and concentration of actives from topical formulation applied in finite dose conditions likened in actual use.

Usefulness of Artificial Membrane, Strat-M®, in the Assessment of Drug Permeation from Complex Vehicles in Finite Dose Conditions

The ban on the use of animals in testing cosmetic products has led to the development of animal-free in vitro methods. Strat-M^® is an artificial membrane engineered to mimic human skin and is recommended as a replacement for skin. However, its usefulness in the assessment of the permeation of cosmetics in in-use conditions remains unverified. No data have been published on its comparative performance with the membrane of choice, porcine skin. The comparative permeability characteristics of Strat-M^® and porcine skin were investigated using Franz diffusion cells. Caffeine (CF) and rhododendrol (RD) in complex vehicles with varying concentrations of polyols were applied as finite and infinite doses. Good rank orders of permeation from finite dose experiments were observed for RD. High correlations were observed in RD permeation between Strat-M^® and porcine skin under finite and infinite dose conditions, whereas only finite dose conditions for CF were associated with good correlations. Permeation from formulations with high polyol content and residual formulations was enhanced due to the disruption of the integrity of the Strat-M^® barrier. The usefulness of Strat-M^® in the assessment of dermal permeation may be limited to finite dose conditions and not applicable to infinite dose conditions or formulations applied in layers.

Potential application of novel liquid crystal nanoparticles of isostearyl glyceryl ether for transdermal delivery of 4-biphenyl acetic acid

Novel liquid crystal nanoparticles (LCNs) composed of isostearyl glyceryl ether (GE-IS) and ethoxylated hydrogenated castor oil (HCO-60) were developed for the enhanced transdermal delivery of 4-biphenyl acetic acid (BAA). The physical properties and pharmaceutical properties of the LCNs were measured. The interaction between the intercellular lipid model of the stratum corneum and the LCNs was observed to elucidate the skin permeation mechanism. In the formulation, the LCNs form niosomes with mean particles sizes of 180-300 nm. The skin absorption mechanisms of LCNs are different, depending upon the application and buffer concentration. The LCNs composed of GE-IS and HCO-60 are attractive tools for use as transdermal drug delivery systems carriers for medicines and cosmetics, due to their high efficiency and safety.

Fundamentals of fractional laser-assisted drug delivery: An in-depth guide to experimental methodology and data interpretation

In the decade since their advent, ablative fractional lasers have emerged as powerful tools to enhance drug delivery to and through the skin. Effective and highly customizable, laser-assisted drug delivery (LADD) has led to improved therapeutic outcomes for several medical indications. However, for LADD to reach maturity as a standard treatment technique, a greater appreciation of its underlying science is needed. This work aims to provide an in-depth guide to the technology's fundamental principles, experimental methodology and unique aspects of LADD data interpretation. We show that drug's physicochemical properties including solubility, molecular weight and tissue binding behavior, are crucial determinants of how laser channel morphology influences topical delivery. Furthermore, we identify strengths and limitations of experimental models and drug detection techniques, interrogating the usefulness of in vitro data in predicting LADD in vivo. By compiling insights from over 75 studies, we ultimately devise an approach for intelligent application of LADD, supporting its implementation in the clinical setting.

aQbD as a platform for IVRT method development - A regulatory oriented approach

The EMA draft guideline on quality and equivalence of topical products and the FDA non-binding product specific guidances release has encouraged the establishment of a regulatory background for in vitro release testing (IVRT). Herein, a novel framework applicable to the development of a discriminatory IVRT method is described, according to analytical quality by design (aQbD) principles. A commercially available diclofenac emulgel formulation was used as model product. Through the definition of IVRT analytical target profile, a risk assessment analysis was carried out, in which the critical analytical attributes (in vitro release rate, cumulative amount released at an initial/final point and dose depletion) and critical method variables (medium, membrane and dosage regimen) were identified. Based on this information, a 3 × 2 × 3 full factorial design was performed. Statistical modeling and system desirability assessment enabled the selection of the most suitable IVRT parameters, which were fully validated according with new EMA requirements. These consisted of PBS:Ethanol (80:20, pH = 7.4), Tuffryn membranes and 300 mg of applied product. aQbD provided a comprehensive framework for developing a reliable and effective IVRT method. A thorough analysis of the new EMA draft guideline requirements revealed that some of the established criteria may be challenging to attain.

Methods to Evaluate Skin Penetration In Vitro

Dermal and transdermal drug therapy is increasing in importance nowadays in drug development. To completely utilize the potential of this administration route, it is necessary to optimize the drug release and skin penetration measurements. This review covers the most well-known and up-to-date methods for evaluating the cutaneous penetration of drugs in vitro as a supporting tool for pharmaceutical research scientists in the early stage of drug development. The aim of this article is to present various experimental models used in dermal/transdermal research and summarize the novel knowledge about the main in vitro methods available to study skin penetration. These techniques are: Diffusion cell, skin-PAMPA, tape stripping, two-photon microscopy, confocal laser scanning microscopy, and confocal Raman microscopic method.

Small molecule delivery across a perforated artificial membrane by thermoreversible hydrogel poloxamer 407

Microperforations in the round window membrane have been suggested for enhancing the rate and reliability of drug delivery into the cochlea. Intratympanic injection, the most common delivery method, involves injecting therapy into the middle ear to establish a reservoir from which drug diffuses across the round window membrane into the cochlea. This process is highly variable because (i) the reservoir, if liquid, can lose contact with the membrane and (ii) diffusion across the membrane is intrinsically variable even with a stable reservoir. To address these respective sources of variability, we compared the thermoreversible hydrogel poloxamer 407 (P407) to saline as a drug carrier and studied the effect of membrane microperforations on drug diffusion rate. We used Rhodamine B as a drug proxy to measure permeance across an artificial membrane in a horizontal diffusion cell. We found that permeance of Rhodamine B from a saline reservoir was an order of magnitude higher than that from a P407 reservoir across unperforated membranes. Moreover, permeance increased with total perforation cross-sectional area regardless of number of perforations (p < 0.05 for all saline-based experiments), but the same association was not found with P407. Rather, for a P407 reservoir, only a large perforation increased permeance (p < 0.001), while multiple small perforations did not (p = 0.749). These results confirm that for drug dissolved in saline, multiple small perforations can effectively enhance diffusion. However, for drug dissolved in P407, larger perforations are necessary.

Soft X-ray microscopy for probing of topical tacrolimus delivery via micelles

The penetration of topically applied tacrolimus formulated in micelles into murine skin is reported, measured by X-ray microscopy. Tacrolimus and micelles are probed for the first time by this high spatial resolution technique by element-selective excitation in the C 1s- and O 1s-regimes. This method allows selective detection of the distribution and penetration depth of drugs and carrier molecules into biologic tissues. It is observed that small, but distinct quantities of the drug and micelles, acting as a drug carrier, penetrate the stratum corneum. A comparison is made with the paraffin-based commercial tacrolimus ointment Protopic^®, where local drug concentrations show to be low. A slight increase in local drug concentration in the stratum corneum is observed, if tacrolimus is formulated in micelles, as compared to Protopic^®. This underscores the importance of the drug formulations for effective drug delivery. Time-resolved penetration shows presence of drug in the stratum corneum 100 min after formulation application, with penetration to deeper skin layers at 1000 min. High resolution micrographs give indications for a penetration pathway along the lipid membranes between corneocytes, but also suggest that the compound may penetrate corneocytes.

Predicting Skin Permeability by Means of Computational Approaches: Reliability and Caveats in Pharmaceutical Studies

The skin is the main barrier between the internal body environment and the external one. The characteristics of this barrier and its properties are able to modify and affect drug delivery and chemical toxicity parameters. Therefore, it is not surprising that permeability of many different compounds has been measured through several in vitro and in vivo techniques. Moreover, many different in silico approaches have been used to identify the correlation between the structure of the permeants and their permeability, to reproduce the skin behavior, and to predict the ability of specific chemicals to permeate this barrier. A significant number of issues, like interlaboratory variability, experimental conditions, data set building rationales, and skin site of origin and hydration, still prevent us from obtaining a definitive predictive skin permeability model. This review wants to show the main advances and the principal approaches in computational methods used to predict this property, to enlighten the main issues that have arisen, and to address the challenges to develop in future research.

Dissipative Particle Dynamics Investigation of the Transport of Salicylic Acid through a Simulated In Vitro Skin Permeation Model

Permeation models are often used to determine diffusion properties of a drug through a membrane as it is released from a delivery system. In order to circumvent problematic in vivo studies, diffusion studies can be performed in vitro, using (semi-)synthetic membranes. In this study salicylic acid permeation was studied, employing a nitrocellulose membrane. Both saturated and unsaturated salicylic acid solutions were studied. Additionally, the transport of salicylic acid through the nitrocellulose membrane was simulated by computational modelling. Experimental observations could be explained by the transport mechanism that was revealed by dissipative particle dynamics (DPD) simulations. The DPD model was developed with the aid of atomistic scale molecular dynamics (AA-MD). The choice of a suitable model membrane can therefore, be predicted by AA-MD and DPD simulations. Additionally, the difference in the magnitude of release from saturated and unsaturated salicylic acid and solutions could also be observed with DPD. Moreover, computational studies can reveal hidden variables such as membrane-permeant interaction that cannot be measured experimentally. A recommendation is made for the development of future model permeation membranes is to incorporate computational modelling to aid the choice of model.

Topical delivery of climbazole to mammalian skin

Dandruff is a common condition, affecting up to half the global population of immunocompetent adults at some time during their lives and it has been highly correlated with the over-expression of the fungus Malassezia spp. Climbazole (CBZ) is used as an antifungal and preservative agent in many marketed formulations for the treatment of dandruff. While the efficacy of CBZ in vitro and in vivo has previously been reported, limited information has been published about the uptake and deposition of CBZ in the skin. Hence, our aim was to investigate the skin permeation of CBZ as well as the influence of various solvents on CBZ skin delivery. Four solvents were selected for the permeability studies of CBZ, namely propylene glycol (PG), octyl salicylate (OSal), Transcutol® P (TC) and polyethylene glycol 200 (PEG). The criteria for selection were based on their wide use as excipients in commercial formulations, their potential to act as skin penetration enhancers and their favourable safety profiles. 1% (w/v) solutions of CBZ were applied under infinite and finite dose conditions using Franz type diffusion cells to human and porcine skin. In line with the topical use of CBZ as an antidandruff agent, comparatively low amounts of CBZ penetrated across the skin barrier (<1% of the applied dose of CBZ). Finite dose studies resulted in a higher extraction of CBZ from human skin compared with infinite dose studies (p < 0.05). CBZ was also taken up to a higher extent in porcine skin (>7-fold) compared with human skin (p < 0.05). Nevertheless, no statistical differences were observed in the amounts that permeated across the different membranes. These preliminary results confirm the potential of simple formulations of CBZ to target the outer layers of the epidermis. The PG and OSal formulations appear to be promising vehicles for CBZ in terms of overall skin extraction and penetration. Future work will expand the range of vehicles studied and explore the reasons underlying the retention of CBZ in the outer layers of the skin.

Bioequivalence of topical generic products. Part 1: Where are we now?

Regulatory accepted methods for bioequivalence assessment of topical generic products generally involve long and expensive clinical endpoint studies. The only alternative relies on pharmacodynamic trials, solely applicable to corticosteroids. Considerable efforts have been channeled towards the development and validation of other analytical surrogates. The majority of these alternative methods rely on in vitro methodologies that allow a more sensitive and reproducible bioequivalence assessment, avoiding at the same time the financial burden that deeply characterizes clinical trials. The development and validation of these methods represent interesting areas of opportunities for generic drugs, since by enabling faster submission and approval processes, an enlargement of topical drug products with generic version is more easily attainable. This review aims to present a critical discussion of the most promising alternative methods, with particular emphasis on in vitro permeation studies and near infrared spectroscopy studies. Since the last technique is not broadly forecast as a bioequivalence assessment tool, its suitability is assessed by a careful analysis of patents that claim the use of NIR radiation in the skin. In fact, the extensive coverage of the devices that use this technology highlights its applicability towards a better understanding of the mechanism underlying topical drug delivery.