Skin pharmacokinetics of diclofenac and co-delivered functional excipients

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.

Developing a method for the study of perfusion effects in topical product pharmacokinetics

Topical drug products are delivered to skin structures to treat numerous skin diseases. Due to the complexities of the skin environment and barrier, topical drug pharmacokinetics are difficult to determine, especially in vivo, as most pharmacokinetic assessment methods can only be performed ex vivo. Notably, in vivo conditions include perfusion via dermal capillaries, which influences topical drug uptake by acting as a clearance route and a "sink" driving permeation through the skin. In this study, we develop a method to examine the effects of perfusion on topical drug uptake in vivo using stimulated Raman scattering (SRS) microscopy, a chemically-specific imaging modality ideal for visualizing topical drug permeation over time. In this pilot study, we imaged the in vivo and ex vivo uptake of tazarotene in 70/30 v/v Transcutol:EtOH in paired mouse ear skin, comparing the effects of perfusion on tazarotene concentration (linearly proportional to SRS signal intensity) over time and pharmacokinetic parameters (Tmax, Cmax, AUC) in lipid-rich and lipid-poor regions in the stratum corneum and sebaceous glands. Obvious variations in SRS signal-time trends and statistically significant differences in stratum corneum pharmacokinetic parameters comparing uptake in lipid-rich and lipid-poor regions in vivo and ex vivo indicated slowed tazarotene flux through ex vivo skin in the absence of perfusion. The observed permeation differences in lipid-rich and lipid-poor regions in perfused and non-perfused skin reflects increased tazarotene permeation rate and removal in the presence of perfusion (in vivo) and decreased permeation rate and lack of elimination route in the absence of perfusion (ex vivo). Our method is demonstrated to be effective in assessing in vivo perfusion effects on topical drug uptake, promoting a better understanding of the influence of perfusion on topical drug delivery.

Cutaneous Pharmacokinetics of Topically Applied Novel Dermatological Formulations

Novel formulations are developed for dermatological applications to address a wide range of patient needs and therapeutic challenges. By pushing the limits of pharmaceutical technology, these formulations strive to provide safer, more effective, and patient-friendly solutions for dermatological concerns, ultimately improving the overall quality of dermatological care. The article explores the different types of novel dermatological formulations, including nanocarriers, transdermal patches, microsponges, and microneedles, and the techniques involved in the cutaneous pharmacokinetics of these innovative formulations. Furthermore, the significance of knowing cutaneous pharmacokinetics and the difficulties faced during pharmacokinetic assessment have been emphasized. The article examines all the methods employed for the pharmacokinetic evaluation of novel dermatological formulations. In addition to a concise overview of earlier techniques, discussions on novel methodologies, including tape stripping, in vitro permeation testing, cutaneous microdialysis, confocal Raman microscopy, and matrix-assisted laser desorption/ionization mass spectrometry have been conducted. Emerging technologies like the use of microfluidic devices for skin absorption studies and computational models for predicting drug pharmacokinetics have also been discussed. This article serves as a valuable resource for researchers, scientists, and pharmaceutical professionals determined to enhance the development and understanding of novel dermatological drug products and the complex dynamics of cutaneous pharmacokinetics.

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 and validation of a minimally invasive and image-guided tape stripping method to sample atopic skin in children

BACKGROUND

Molecular skin profiling techniques, typically performed on skin samples taken by punch biopsy, have enhanced the understanding of the pathophysiology of atopic dermatitis (AD), thereby enabling the development of novel targeted therapeutics. However, punch biopsies are not always feasible or desirable, and novel minimally invasive methods such as skin tape stripping have been developed.

AIM

To develop, optimize and validate a novel tape stripping method guided by noninvasive in vivo skin imaging to sample atopic skin in children.

METHODS

Skin tape stripping-based procedures were compared and optimized using data from 30 healthy controls (HCs: 5 adults, 25 children) and 39 atopic children. Evaluations were guided by high-resolution photography, reflectance confocal microscopy, optical coherence tomography and transepidermal water loss measurements. We assessed and compared adverse events (AEs), the time needed to perform the sampling and the cDNA levels obtained from the tapes.

RESULTS

Tape stripping methods based on previously described protocols resulted in erosions in all participants and required a median time of 65 min to perform (range 60-70 min), but provided good cDNA yield. Shorter durations appeared less invasive but provided lower cDNA yield. The final optimized tape stripping protocol, using 11 tapes of 22 mm in diameter, each applied twice for 5 s with 90° rotation, did not produce significant AEs, was completed within a median time of 7 min (range 5-15 min) and provided good cDNA yield both in HCs and atopic children.

CONCLUSION

Our minimally invasive method is safe and reliable, and provides reproducible acquisition of cDNA in atopic children. In addition, it enables rapid sample collection, a crucial factor in clinical practice.

Formulation and Evaluation of Diclofenac Potassium Gel in Sports Injuries with and without Phonophoresis

Background: Pain remains a global public heath priority. Phonophoresis, also known as sonophoresis or ultrasonophoresis, is when an ultrasound is used to maximize the effects of a topical drug. Purpose: The objective of this study was to test, in patients injured in sports or accidents (N = 200), the efficacy of diclofenac potassium (DK) 6%, 4%, and 2% formulated gels with and without phonophoresis in comparison with market available standard diclofenac sodium (DS or DN) gel. Methods: The patients were enrolled after informed consent. By using the lottery method, 100 patients were randomly segregated into five groups without phonophoresis and repeated similarly with phonophoresis at a frequency of 0.8 MHz, an intensity of about 1.5 W/cm2, and at continuous mode (2:1). Group-1 was treated with 6% DK gel, group-2 was treated with 4% DK gel, group-3 was treated with 2% DK gel, group-4 was treated with 4% DS gel and group-5 was given control gel three to four times a week for 4 weeks. The patients were screened by using NPRS and WOMAC scales. They were assessed on the baseline, 4th session, 8th session, 12th session, and 16th session. Results: Significant dose-dependently relief was observed in NPRS (Numeric Pain Rating Scale) and the WOMAC (Western Ontario McMaster Osteo-Arthritis) index for pain in disability and stiffness for each group treated with DK gel compared to DS gel. Phonophoresis increased these benefits significantly when used after topical application of DK gel or DS gel, and the dose-dependent effects of DK gel plus phonophoresis were stronger than the dose-dependent effects of DS gel plus phonophoresis. The faster and profounder relief was due to phonophoresis, which allows more penetration of the DK gel into the skin as compared to the direct application of DK gel in acute, uncomplicated soft tissue injury, such as plantar fasciitis, bursitis stress injuries, and tendinitis. In addition, DK gel with phonophoresis was well tolerated. Thus, in this personalized clinical setting, according to the degree of inflammation or injured-induced pain, disability, and stiffness, DK gel 6% with phonophoresis appeared more effective and thus more recommendable than DS gel 6% alone or DS gel 6% combined to phonophoresis.

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.

Investigator Impact on Reproducibility of Drug Bioavailability in Stratum Corneum Sampling by Tape Stripping

PURPOSE

Skin sampling by tape stripping measures the local bioavailability of topical drug products in the stratum corneum (SC). The goal of the current study was to evaluate the impact of different investigators in studies that utilize a tape stripping protocol designed to minimize investigator variability.

METHODS

Two open-label clinical studies compared two lidocaine patches and a diclofenac patch and solution in twelve healthy volunteers. The mass of drug was determined in SC samples collected on tape strips at three time points following product removal in duplicate by two investigators. Investigator results were compared with each other and with results for the diclofenac solution measured by another laboratory using a similar protocol.

RESULTS

For drug mass, the geometric mean ratio comparing two investigators is within the acceptable bioequivalence interval for most measurement times and drug products. Drug uptake into the SC from the diclofenac solution was not statistically different from that determined in another laboratory. The average flux from the SC over the clearance intervals for the four drug products correspond well with flux measurements from in vitro permeation tests.

CONCLUSIONS

Results from different investigators are reproducible within the limitations of measurement variability, which can be managed by increasing volunteer numbers.

Optimization of topical formulations using a combination of in vitro methods to quantify the transdermal passive diffusion of drugs

This paper describes a new approach to the early-stage optimization of topical products and selection of lead formulation candidates. It demonstrates the application of open flow microperfusion in vitro in conjunction with the Franz diffusion cell to compare time-resolved, 24-hour profiles of diclofenac passive diffusion through all skin layers (including the skin barrier, dermis, and subcutis) resulting from nine topical formulations of different composition. The technique was successfully validated for in vitro sampling of diclofenac in interstitial fluid. A multi-compartmental model integrating the two datasets was analyzed and revealed that the passive diffusion of diclofenac through the dermis and subcutis does not correlate with its diffusion through the skin barrier and cannot be predicted using Franz diffusion cell data alone. The combined application of the two techniques provides a new, convenient tool for product development and selection enabling the comparison of topical formulation candidates and their impact on drug delivery through all skin layers. This approach can also generate the experimental data required to improve the robustness of mechanistic PBPK models, and when combined with clinical sampling via open flow microperfusion - for the development of better in vivo-in vitro correlative models.