Stratum Corneum Sampling to Assess Bioequivalence between Topical Acyclovir Products

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.

Comparative Analyses of Bioequivalence Assessment Methods for In Vitro Permeation Test Data

For topical, dermatological drug products, an in vitro option to determine bioequivalence (BE) between test and reference products is recommended. In particular, in vitro permeation test (IVPT) data analysis uses a reference-scaled approach for two primary endpoints, cumulative penetration amount (AMT) and maximum flux (J max), which takes the within donor variability into consideration. In 2022, the Food and Drug Administration (FDA) published a draft IVPT guidance that includes statistical analysis methods for both balanced and unbalanced cases of IVPT study data. This work presents a comprehensive evaluation of various methodologies used to estimate critical parameters essential in assessing BE. Specifically, we investigate the performance of the FDA draft IVPT guidance approach alongside alternative empirical and model-based methods utilizing mixed-effects models. Our analyses include both simulated scenarios and real-world studies. In simulated scenarios, empirical formulas consistently demonstrate robustness in approximating the true model, particularly in effectively addressing treatment-donor interactions. Conversely, the effectiveness of model-based approaches heavily relies on precise model selection, which significantly influences their results. The research emphasizes the importance of accurate model selection in model-based BE assessment methodologies. It sheds light on the advantages of empirical formulas, highlighting their reliability compared to model-based approaches and offers valuable implications for BE assessments. Our findings underscore the significance of robust methodologies and provide essential insights to advance their understanding and application in the assessment of BE, employed in IVPT data analysis.

Cutaneous pharmacokinetics-based bioequivalence: A clinical dermal open flow microperfusion verification study using lidocaine and prilocaine combination topical products

BACKGROUND

Using accurate, sensitive, reproducible and efficient in vivo cutaneous pharmacokinetics (PK)-based bioequivalence (BE) approaches can promote the development of topical generic drug products. A clinical dermal open flow microperfusion (dOFM) study has previously demonstrated the BE of topical drug products containing a hydrophilic drug. However, the utility of dOFM to evaluate the topical BE of drug products containing moderately lipophilic drugs, more representative of most topical drugs, has not yet been established.

OBJECTIVE

To evaluate the ability of a clinical dOFM study to assess BE of topical products containing two moderately lipophilic drugs that have only minor differences in chemical and physical properties.

METHODS

The study included 20 healthy subjects. Four application sites on each thigh were treated with fixed dose lidocaine/prilocaine combination products, and dermal drug concentrations were monitored with two dOFM probes per application site for 12 h. A reference cream was compared to itself and to an approved generic cream (both serving as positive controls for BE), and to a gel (negative control). BE was established based on AUC0to12h and Cmax using the scaled-average-BE approach. Systemic exposure of both drugs was assessed throughout the study.

RESULTS

BE was successfully demonstrated for the positive controls, and not for the negative control, for both drugs. The systemic exposure of both drugs was negligible.

CONCLUSIONS

dOFM accurately demonstrated BE between bioequivalent topical creams, sensitively discriminated between different formulations and differentiated the cutaneous PK of both study drugs, even though they differ only slightly in chemical and physical properties. These results support the utility of dOFM as a cutaneous PK-based BE approach for topical lipophilic drugs, including lidocaine and prilocaine.

Therapeutic-driven framework for bioequivalence assessment of complex topical generic drug products

Despite the continuous research on understanding how topical drugs and the skin interact, the development of a topical generic product remains a challenge. Due to their local action effect rather than systemic, establishing suitable frameworks for documenting bioequivalence between reference and test formulations is anything but straightforward. In previous years, clinical endpoint trials were considered the gold standard method to demonstrate bioequivalence between topical products. Nevertheless, significant financial and time resources were required to be allocated owing to the inherent complexity of these studies. To address this problem, regulatory authorities have begun to accept alternative approaches that could lead to a biowaiver, avoiding the need for clinical endpoint trials. These alternatives encompass various in vitro and/or in vivo techniques that have been analysed and the benefits and drawbacks of each method have been considered. Furthermore, other factors like the integration of a quality by design framework to ensure a comprehensive understanding of the product and process quality attributes have also been taken into account. This review delves into international regulatory recommendations for semisolid topical products, with a focus on those established by the European Medicines Agency, as well as the Food and Drug Administration. Both approaches were carefully examined, discussing aspects such as acceptance criteria, sample size, and microstructure evaluation. Additionally, novel and innovative therapeutic-driven approaches based on in vitro disease models for the rapid and effective development of topical generic products are presented.

Liposomal Encapsulation of Ascorbyl Palmitate: Influence on Skin Performance

L-ascorbic acid represents one of the most potent antioxidant, photoprotective, anti-aging, and anti-pigmentation cosmeceutical agents, with a good safety profile. However, the main challenge is the formulation of stable topical formulation products, which would optimize the penetrability of L-ascorbic acid through the skin. The aim of our research was to evaluate the performance of ascorbyl palmitate on the skin, incorporated in creams and emulgels (2%) as carriers, as well as to determine the impact of its incorporation into liposomes on the penetration profile of this ingredient. Tape stripping was used to study the penetration of ascorbyl palmitate into the stratum corneum. In addition, the sensory and textural properties of the formulations were determined. The liposomal formulations exhibited a better penetration profile (p < 0.05) of the active substance compared to the non-liposomal counterpart, leading to a 1.3-fold and 1.2 fold-increase in the total amount of penetrated ascorbyl palmitate in the stratum corneum for the emulgel and cream, respectively. Encapsulation of ascorbyl palmitate into liposomes led to an increase in the adhesiveness and density of the prepared cream and emulgel samples. The best spreadability and absorption during application were detected in liposomal samples. The obtained results confirmed that liposomal encapsulation of ascorbyl palmitate improved dermal penetration for both the cream and emulgel formulations.

In vitro studies into establishing therapeutic bioequivalence of complex topical products: Weight of evidence

Over the past decade, topically applied drug products have experienced extraordinary price increases, due to the shortage of multisource generic drug products. This occurrence is mainly related to the underlying challenges evolved in topical bioequivalence documentation. Although there has been continuing regulatory efforts to present surrogate in vitro methods to clinical endpoint studies, there is still a continued need for cost- and time-efficient alternatives that account for product specificities. Hence, this work intended to expose bioequivalence assessment issues for complex topical formulations, and more specifically those related with product efficacy guidance. As a model drug and product, a bifonazole 10 mg/g cream formulation was selected and two different batches of the commercially available Reference Product (RP) were used: RP1 that displayed lower viscosity and RP4 which presented high, but not the highest, viscosity. In vitro human skin permeation testing (IVPT) was carried out and the results were evaluated by means of the traditional bioequivalence assessment approach proposed by the EMA, as well as by the Scaled Average Bioequivalence assessment approach proposed by the FDA. Based on previous experience, there was an expectation of a high level of variability in the results, thus alternative methods to evaluate local drug skin availability were developed. More specifically, an infected skin disease model, where ex vivo human skin was infected and ATP levels were used as a biological marker for monitoring antifungal activity after product application. The results showed that permeation equivalence could not be supported between the different RP batches. In contrast, this statistical difference between the formulation batches was not indicated in the disease model. Nevertheless, in pivotal IVPT studies, the lowest permeant formulation (RP4) evidenced a higher antifungal in vitro activity as reported by the lower levels of ATP. A critical appraisal of the results is likewise presented, focusing on an outlook of the real applicability of the regulatory guidances on this subject.

Determining topical product bioequivalence with stimulated Raman scattering microscopy

Generic drugs are essential for affordable medicine and improving accessibility to treatments. Bioequivalence (BE) is typically demonstrated by assessing a generic product's pharmacokinetics (PK) relative to a reference-listed drug (RLD). Accurately estimating cutaneous PK (cPK) at or near the site of action can be challenging for locally acting topical products. Certain cPK approaches are available for assessing local bioavailability (BA) in the skin. Stimulated Raman scattering (SRS) microscopy has unique capabilities enabling continuous, high spatial and temporal resolution and quantitative imaging of drugs within the skin. In this paper, we developed an approach based on SRS and a polymer-based standard reference for the evaluation of topical product BA and BE in human skin ex vivo. BE assessment of tazarotene-containing formulations was achieved using cPK parameters obtained within different skin microstructures. The establishment of BE between the RLD and an approved generic product was successfully demonstrated. Interestingly, within the constraints of the current study design the results suggest similar BA between the tested gel formulation and the reference cream formulation, despite the differences in the formulation/dosage form. Another formulation containing polyethylene glycol as the vehicle was demonstrated to be not bioequivalent to the RLD. Compared to using the SRS approach without a standard reference, the developed approach enabled more consistent and reproducible results, which is crucial in BE assessment. The abundant information from the developed approach can help to systematically identify key areas of study design that will enable a better comparison of topical products and support an assessment of BE.

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.

Topical delivery of seriniquinone for treatment of skin cancer and fungal infections is enabled by a liquid crystalline lamellar phase

Seriniquinone (SQ) was initially described by our group as an antimelanoma drug candidate and now also as an antifungal drug candidate. Despite its promising in vitro effects, SQ translation has been hindered by poor water-solubility. In this paper, we described the challenging nanoformulation process of SQ, which culminated in the selection of a phosphatidylcholine-based lamellar phase (PLP1). Liposomes and nanostructured lipid carriers were also evaluated but failed to encapsulate the compound. SQ-loaded PLP1 (PLP1-SQ) was characterized for the presence of sedimented or non-dissolved SQ, rheological and thermal behavior, and irritation potential with hen's egg test on the chorioallantoic membrane (HET-CAM). PLP1 influence on transepidermal water loss (TEWL) and skin penetration of SQ was assessed in a porcine ear skin model, while biological activity was evaluated against melanoma cell lines (SK-MEL-28 and SK-MEL-147) and C. albicans SC5314. Despite the presence of few particles of non-dissolved SQ (observed under the microscope 2 days after formulation obtainment), PLP1 tripled SQ retention in viable skin layers compared to SQ solution at 12 h. This effect did not seem to relate to formulation-induced changes on the barrier function, as no increases in TEWL were observed. No sign of vascular toxicity in the HET-CAM model was observed after cutaneous treatment with PLP1. SQ activity was maintained on melanoma cells after 48 h-treatment (IC50 values of 0.59-0.98 µM) whereas the minimum inhibitory concentration (MIC) against C. albicans after 24 h-treatment was 32-fold higher. These results suggest that a safe formulation for SQ topical administration was developed, enabling further in vivo studies.

Bioequivalence Evaluation of Topical Metronidazole Products Using Dermal Microdialysis in New Zealand Rabbits

Comparative assessment of cutaneous pharmacokinetics (cPK) by dermal microdialysis (dMD) appears to be suitable to evaluate the bioequivalence (BE) of topical dermatological drug products applied to the skin (TDDPs). Although dMD studies in the literature have reported inconclusive BE assessments, we have addressed several methodological deficiencies to improve dMD's capability to assess BE between reference (R) and approved generic (referred to as test (T)) gel and cream products of metronidazole (MTZ). The 90% confidence interval (CI) of the geometric mean ratios for the Ln(AUC0-24) and Ln(Cmax) endpoints was centered within the BE limits of 80-125%. The CIs extended outside this range as the proof-of-principle study was not statistically powered to demonstrate BE (N = 7 rabbits). A power analysis suggests that, with the variability observed in this study, 21 rabbits for the cream and 11 rabbits for the gel would be sufficient to support an evaluation of BE with the 2 probe replicates we used, and only 10 and 5 rabbits would be sufficient to power the study for the cream and gel, respectively, if 4 probe replicates are used for each treatment per rabbit. These results indicate that dMD when properly controlling variables can be used to support BE assessments for TDDPs.

Application of a Customised Franz-Type Cell Coupled with HPTLC to Monitor the Timed Release of Bioactive Components in Complex Honey Matrices

The aim of this study was to assess the release profile of components in five different honeys (a New Zealand Manuka and two Western Australian honeys, a Jarrah honey and a Coastal Peppermint honey) and their corresponding honey-loaded gel formulations using a custom-designed Franz-type diffusion cell in combination with High-Performance Thin-Layer Chromatography (HPTLC). To validate the suitability of the customised setup, release data using this new approach were compared with data obtained using a commercial Franz cell apparatus, which is an established analytical tool to monitor the release of active ingredients from topical semisolid products. The release profiles of active compounds from pure honey and honey-loaded formulations were found to be comparable in both types of Franz cells. For example, when released either from pure honey or its corresponding pre-gel formulation, the percentage release of two Jarrah honey constituents, represented by distinct bands at RF 0.21 and 0.53 and as analysed by HPTLC, was not significantly different (p = 0.9986) at 12 h with over 99% of these honey constituents being released in both apparatus. Compared to the commercial Franz diffusion cell, the customised Franz cell offers several advantages, including easy and convenient sample application, the requirement of only small sample quantities, a large diffusion surface area, an ability to analyse 20 samples in a single experiment, and lower cost compared to purchasing a commercial Franz cell. Thus, the newly developed approach coupled with HPTLC is conducive to monitor the release profile of minor honey constituents from pure honeys and honey-loaded semisolid formulations and might also be applicable to other complex natural-product-based products.

Rational Design of Topical Semi-Solid Dosage Forms-How Far Are We?

Specific aspects of semi-solid dosage forms for topical application include the nature of the barrier to be overcome, aspects of susceptibility to physical and chemical instability, and a greater influence of sensory perception. Advances in understanding the driving forces of skin penetration as well as the design principles and inner structure of formulations, provide a good basis for the more rational design of such dosage forms, which still often follow more traditional design approaches. This review analyses the opportunities and constraints of rational formulation design approaches in the industrial development of new topical drugs. As the selection of drug candidates with favorable physicochemical properties increases the speed and probability of success, models for drug selection based on theoretical and experimental approaches are discussed. This paper reviews how progress in the scientific understanding of mechanisms and vehicle-influence of skin penetration can be used for rational formulation design. The characterization of semi-solid formulations is discussed with a special focus on modern rheological approaches and analytical methods for investigating and optimizing the chemical stability of active ingredients in consideration of applicable guidelines. In conclusion, the combination of a good understanding of scientific principles combined with early consideration of regulatory requirements for product quality are enablers for the successful development of innovative and robust semi-solid formulations for topical application.

Quantification of Chemical Uptake into the Skin by Vibrational Spectroscopies and Stratum Corneum Sampling

Evaluation of the bioavailability of drugs intended to act within the skin following the application of complex topical products requires the application of multiple experimental tools, which must be quantitative, validated, and, ideally and ultimately, sufficiently minimally invasive to permit use in vivo. The objective here is to show that both infrared (IR) and Raman spectroscopies can assess the uptake of a chemical into the stratum corneum (SC) that correlates directly with its quantification by the adhesive tape-stripping method. Experiments were performed ex vivo using excised porcine skin and measured chemical disposition in the SC as functions of application time and formulation composition. The quantity of chemicals in the SC removed on each tape-strip was determined from the individually measured IR and Raman signal intensities of a specific molecular vibration at a frequency where the skin is spectroscopically silent and by a subsequent conventional extraction and chromatographic analysis. Correlations between the spectroscopic results and the chemical quantification on the tape-strips were good, and the effects of longer application times and the use of different vehicles were clearly delineated by the different measurement techniques. Based on this initial investigation, it is now possible to explore the extent to which the spectroscopic approach (and Raman in particular) may be used to interrogate chemical disposition deeper in the skin and beyond the SC.

Regulatory Requirements for the Development of Second-Entry Semisolid Topical Products in the European Union

The development of second-entry topical products is hampered by several factors. The excipient composition should be similar to the reference product because excipients may also contribute to efficacy. Conventional pharmacokinetic bioequivalence studies were not considered acceptable because drug concentrations are measured downstream after the site of action. There was no agreed methodology to characterize the microstructure of semisolids, and waivers of therapeutic equivalence studies with clinical endpoints were not possible. Only the vasoconstrictor assay for corticosteroids was accepted as a surrogate. This paper describes the implementation of the European Union's stepwise approach for locally acting products to cutaneous products, discusses the equivalence requirements of the EMA Draft Guideline on the Quality and Equivalence of Topical Products, and compares them with the US Food and Drug Administration recommendations. Step 1 includes the possibility of waivers for simple formulations based on in vitro data only (Q1 + Q2 + Q3 + IVRT). Step 2 includes step 1 requirements plus a kinetic study (TS/IVPT/PKBE) to compare the local availability of complex formulations. Step 3 refers to clinical studies with pharmacodynamic/clinical endpoints. As excipients may affect the local tolerability and efficacy of the products, the similarity of excipient composition is required in all steps, except where clinical endpoints are compared.

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.

Raman Spectroscopic Tools to Probe the Skin-(Trans)dermal Formulation Interface

Medicines designed to deliver the active pharmaceutical ingredient either into or through the skin─often referred to as topicals and transdermals, respectively─are generally considered to be complex drug products. A particular challenge faced by these formulations is identifying a suitable method (ideally, in terms of specificity, accuracy, precision, and robustness) or combination of methods with which to assess the amount and rate of drug delivery to the target site. Significant research currently aims to identify and validate relevant and minimally invasive techniques that can be used to quantify both the levels of the drug attained within different parts of the skin and the kinetics with which the drug is taken up into the skin and cleared therefrom into the systemic circulation. Here, the application of confocal Raman microspectroscopy and imaging to interrogate events integral to the performance of topical and transdermal drug products at the formulation-skin interface is illustrated. Visualization, depth slicing, and profiling are used (a) to elucidate key chemical properties of both the delivery system and the skin that have impact on their interaction and the manner in which drug transfer from one to the other may occur, (b) for the transformation of a drug product from that manufactured into a residual phase post-application and inunction into the skin (including the potential for important changes in solubility of the active compound), and (c) for drug absorption into the skin and its subsequent '"clearance" into deeper layers and beyond. Overall, the Raman tools described offer both qualitative and potentially semi-quantitative insights into topical and transdermal drug product performance and provide information useful for formulation improvement and optimization.

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.

Assessment of dermal bioavailability: predicting the input function for topical glucocorticoids using stratum corneum sampling

Predicting the dermal bioavailability of topically delivered drugs is challenging. In this work, minimally invasive stratum corneum (SC) sampling was used to quantify the delivery of betamethasone valerate (BMV) into the viable skin. Betnovate® cream (0.1% w/w BMV) was applied at three doses (2, 5, and 10 mg cm-2) to the ventral forearms of 12 healthy volunteers. The mass of drug in the SC was measured using a validated tape-stripping method (a) after a 4-h "uptake" period, and (b) following a 6-h "clearance" period subsequent to cream removal. Concomitantly, the skin blanching responses to the same doses were assessed with a chromameter over 22 h post-application. BMV uptake into the SC was significantly higher for the 5 mg cm-2 dose compared to those of 2 and 10 mg cm-2. In all cases, ~30% of the drug in the SC at the end of the uptake period was cleared in the subsequent 6 h. From the SC sampling data, the average drug flux into the viable epidermis and its first-order elimination rate constant from the SC were estimated as 4 ng cm-2 h-1 and 0.07 h-1, respectively. In contrast, skin blanching results were highly variable and insensitive to the dose of cream applied. The SC sampling method was able to detect a 50% difference between two applied doses with 80% power; detection of a 20% difference would require a larger sample size. SC sampling enabled quantitative metrics describing corticosteroid delivery to the viable epidermis to be determined.

Skin pharmacokinetics of diclofenac and co-delivered functional excipients

An important question in the development of a dermatological drug product is whether a target concentration has been achieved in, for example, the viable epidermis following topical administration. When attempting to address this challenge, it is essential to consider the role of excipients in the formulation that may influence drug partitioning and diffusion in the different layers of the skin. The objective, therefore, was to correlate, in human subjects, the skin pharmacokinetics of diclofenac (specifically, its uptake into and clearance from the stratum corneum (SC)) from an approved drug product (Voltaren® medicated plaster) with the in vivo co-uptake of two key excipients, namely propylene glycol and butylene glycol. SC sampling was used to assess diclofenac input into the skin during patch application, and its subsequent clearance post-removal of the delivery system. In parallel the uptake of the two glycol excipients was also measured. Drug and excipient amounts in the SC increased with time of application up to 6 h and, for diclofenac, no further increase was observed when the administration was prolonged to 12 h. When the plaster was removed after 6 h of wear, diclofenac cleared relatively slowly from the SC suggesting that drug binding with a slow off-rate had occurred. The results indicate that the optimisation of drug delivery from a topical formulation must take into account the disposition of key excipients and their impact on dermato-pharmacokinetics in general.

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.

Skin Pharmacokinetics of Transdermal Scopolamine: Measurements and Modeling

Prediction of skin absorption and local bioavailability from topical formulations remains a difficult task. An important challenge in forecasting topical bioavailability is the limited information available about local and systemic drug concentrations post application of topical drug products. Commercially available transdermal patches, such as Scopoderm (Novartis Consumer Health UK), offer an opportunity to test these experimental approaches as systemic pharmacokinetic data are available with which to validate a predictive model. The long-term research aim, therefore, is to develop a physiologically based pharmacokinetic model (PBPK) to predict the dermal absorption and disposition of actives included in complex dermatological products. This work explored whether in vitro release and skin permeation tests (IVRT and IVPT, respectively), and in vitro and in vivo stratum corneum (SC) and viable tissue (VT) sampling data, can provide a satisfactory description of drug "input rate" into the skin and subsequently into the systemic circulation. In vitro release and skin permeation results for scopolamine were consistent with the previously reported performance of the commercial patch investigated. New skin sampling data on the dermatopharmacokinetics (DPK) of scopolamine also accurately reflected the rapid delivery of a "priming" dose from the patch adhesive, superimposed on a slower, rate-controlled input from the drug reservoir. The scopolamine concentration versus time profiles in SC and VT skin compartments, in vitro and in vivo, taken together with IVRT release and IVPT penetration kinetics, reflect the input rate and drug delivery specifications of the Scopoderm transdermal patch and reveal the importance of skin binding with respect to local drug disposition. Further data analysis and skin PK modeling are indicated to further refine and develop the approach outlined.

The Implications of Regulatory Framework for Topical Semisolid Drug Products: From Critical Quality and Performance Attributes towards Establishing Bioequivalence

Due to complex interdependent relationships affecting their microstructure, topical semisolid drug formulations face unique obstacles to the development of generics compared to other drug products. Traditionally, establishing bioequivalence is based on comparative clinical trials, which are expensive and often associated with high degrees of variability and low sensitivity in detecting formulation differences. To address this issue, leading regulatory agencies have aimed to advance guidelines relevant to topical generics, ultimately accepting different non-clinical, in vitro/in vivo surrogate methods for topical bioequivalence assessment. Unfortunately, according to both industry and academia stakeholders, these efforts are far from flawless, and often upsurge the potential for result variability and a number of other failure modes. This paper offers a comprehensive review of the literature focused on amending regulatory positions concerning the demonstration of (i) extended pharmaceutical equivalence and (ii) equivalence with respect to the efficacy of topical semisolids. The proposed corrective measures are disclosed and critically discussed, as they span from mere demands to widen the acceptance range (e.g., from ±10% to ±20%/±25% for rheology and in vitro release parameters highly prone to batch-to-batch variability) or reassess the optimal number of samples required to reach the desired statistical power, but also rely on specific data modeling or novel statistical approaches.

Assessment of Drug Delivery Kinetics to Epidermal Targets In Vivo

It has proven challenging to quantify ‘drug input’ from a formulation to the viable skin because the epidermal and dermal targets of topically applied drugs are difficult, if not impossible, to access in vivo. Defining the drug input function to the viable skin with a straightforward and practical experimental approach would enable a key component of dermal pharmacokinetics to be characterised. It has been hypothesised that measuring drug uptake into and clearance from the stratum corneum (SC) by tape-stripping allows estimation of a topical drug’s input function into the viable tissue. This study aimed to test this idea by determining the input of nicotine and lidocaine into the viable skin, following the application of commercialised transdermal patches to healthy human volunteers. The known input rates of these delivery systems were used to validate and assess the results from the tape-stripping protocol. The drug input rates from in vivo tape-stripping agreed well with the claimed delivery rates of the patches. The experimental approach was then used to determine the input of lidocaine from a marketed cream, a typical topical product for which the amount of drug absorbed has not been well-characterised. A significantly higher delivery of lidocaine from the cream than from the patch was found. The different input rates between drugs and formulations in vivo were confirmed qualitatively and quantitatively in vitro in conventional diffusion cells using dermatomed abdominal pig skin.

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.

Thiolated and PEGylated silica nanoparticle delivery to hair follicles

Targeting drug delivery to hair follicles is valuable to treat conditions such as alopecia's and acne, and this shunt route may also allow drug delivery to deeper skin layers and the systemic circulation by avoiding the intact stratum corneum. Here, we investigated the effects of nanoparticle surface chemistry on their delivery into hair follicles by synthesizing fluorescent thiolated silica nanoparticles and functionalizing with 750 Da and 5000 Da methoxypolyethylene glycol maleimide (PEG). The stability of the nanoparticles in skin homogenate was verified before tape stripping of porcine-dosed tissue showed the distribution of the free fluorescent dye and different nanoparticles in the skin. Analysis of microscopic images of the skin sections revealed penetration of nanoparticles functionalized with PEG into the appendages whereas thiolated nanoparticles stayed on the surface of the skin and were removed by tape stripping. Nanoparticles functionalized with PEG 5000 Da penetrated deeper into the hair follicles compared to counterparts functionalized with PEG 750 Da. PEGylation can thus enhance targeted delivery of nanoparticulates into hair follicles.

Cutaneous Pharmacokinetics of Acyclovir Cream 5% Products: Evaluating Bioequivalence with an In Vitro Permeation Test and an Adaptation of Scaled Average Bioequivalence

PURPOSE

The in vitro permeation test (IVPT) with a new statistical approach was investigated to evaluate the utility of an IVPT methodology as a sensitive tool to support a demonstration of bioequivalence (BE) for topical dermatological drug products.

METHODS

IVPT experiments were performed utilizing ex vivo human skin. The initial screening tests involved four differently formulated acyclovir 5% creams: the U.S. Zovirax® as the reference product and the U.K. Zovirax®, Aciclovir 1A Pharma® and Aciclostad® as test products. Subsequently, a pivotal BE study was conducted comparing the two Zovirax® creams. The resulting data was used to evaluate BE of test (T) versus reference (R), T versus T, and R versus R, with an adaption of scaled average BE approach to address high variability in IVPT data.

RESULTS

More acyclovir permeated into and through the skin from the two Zovirax® creams compared to the two non-Zovirax® creams. The U.S. Zovirax® cream showed a significantly higher J_max and total amount permeated over 48 h, compared to the U.K. Zovirax® cream. The statistical analysis indicated that the test and reference products were not bioequivalent, whereas each product tested against itself was shown to be bioequivalent.

CONCLUSIONS

The current study demonstrated that the IVPT method, with an appropriate statistical analysis of the results, is a sensitive and discriminating test that can detect differences in the rate and extent of acyclovir bioavailability in the skin from differently formulated cream products.

Franz Cell Diffusion Testing and Quantitative Confocal Raman Spectroscopy: In Vitro-In Vivo Correlation

Previously, we reported the use of Confocal Raman Spectroscopy (CRS) to investigate the topical delivery of actives and excipients. We have also correlated the results from CRS with findings from in vitro diffusion studies in human skin. However, until now CRS has only been used as a semi-quantitative method of determining the skin uptake of molecules, with results expressed as arbitrary units of signal intensity. Clearly, this posed challenges for using CRS to determine skin delivery and to assess the drug bioavailability and bioequivalence of topical formulations. In the present work, the permeation of niacinamide (NIA) from various formulations in human skin was studied in vitro using conventional Franz cells and in vivo using a quantitative CRS method under finite dose conditions. The selection of NIA was based on its wide use in pharmaceutical and personal care formulations for many years. This is the first fully quantitative study to compare these methods. The vehicles investigated were neat Transcutol^® P (TC); binary combinations of propylene glycol (PG) with propylene glycol monolaurate (PGML); and ternary mixtures of PG, PGML, and isopropyl myristate (IPM). These solvents were selected to encompass a range of physicochemical properties. NIA permeation was evident from all formulations in vitro and in vivo. The vehicles PG:PGML and PG:PGML:IPM delivered comparable amounts across the skin in vitro at 24 h (100.3–106.7 µg/cm², p > 0.05) that were significantly higher compared with those of TC (1.3 µg/cm², p < 0.05). An excellent in vitro in vivo correlation (R² = 0.98) was found following the linear regression of the cumulative amounts of NIA permeated in vitro and the amounts of NIA at 2 μm in the skin measured with CRS. A very good correlation between the cumulative permeation of NIA in vitro and the total amount of NIA that penetrated the stratum corneum (SC) per unit of surface area (μg/cm²) in vivo was also observed, with a Pearson correlation coefficient (R²) of 0.94. The findings support the use of CRS for the quantitative measurement of actives delivered to the skin in vivo. Future studies will focus on exploring the reproducibility and reliability of the method by investigating the delivery of different actives from a wider range of vehicles. Additionally, quantitative CRS will be evaluated further as a method for assessing the bioequivalence of topical formulations.