A biophysically based dermatopharmacokinetic compartment model for quantifying percutaneous penetration and absorption of topically applied agents. I. Theory

A Comparative Evaluation of Desoximetasone Cream and Ointment Formulations Using Experiments and In Silico Modeling

The administration of therapeutic drugs through dermal routes, such as creams and ointments, has emerged as an increasingly popular alternative to traditional delivery methods, such as tablets and injections. In the context of drug development, it is crucial to identify the optimal doses and delivery routes that ensure successful outcomes. Physiologically based pharmacokinetic (PBPK) models have been proposed to simulate drug delivery and optimize drug formulations, but the calibration of these models is challenging due to the multitude of variables involved and limited experimental data. One significant research gap that this article addresses is the need for more efficient and accurate methods for calibrating PBPK models for dermal drug delivery. This manuscript presents a novel approach and an integrated dermal drug delivery model to address this gap that leverages virtual in vitro release (IVRT) and permeation (IVPT) testing data to optimize mechanistic models. The proposed approach was demonstrated through a study involving Desoximetasone cream and ointment formulations, where the release kinetics and permeation profiles of Desoximetasone were determined experimentally, and a computational model was created to simulate the results. The experimental studies showed that, even though the cumulative permeation of Desoximetasone at the end of the permeation study was comparable, there was a significant difference seen in the lag time in the permeation of Desoximetasone between the cream and ointment. Additionally, there was a significant difference seen in the amount of Desoximetasone permeated through human cadaver skin at early time points when the cream and ointment were compared. The computational model was optimized and validated, suggesting that this approach has the potential to bridge the existing research gap by improving the accuracy and efficiency of drug development processes. The model results show a good fit between the experimental data and model predictions. During the model optimization process, it became evident that there was variability in both the permeability and the partition coefficient within the stratum corneum. This variability had a significant and noteworthy influence on the overall performance of the model, especially when it came to its capacity to differentiate between cream and ointment formulations. Leveraging virtual models significantly aids the comprehension of drug release and permeation, mitigating the demanding data requirements. The use of virtual IVRT and IVPT data can accelerate the calibration of PBPK models, streamline the selection of the appropriate doses, and optimize drug delivery. Moreover, this novel approach could potentially reduce the time and resources involved in drug development, thus making it more cost-effective and efficient.

An Integrated Biophysical Model for Predicting the Clinical Pharmacokinetics of Transdermally Delivered Compounds

The delivery of therapeutic drugs through the skin is a promising alternative to oral or parenteral delivery routes because dermal drug delivery systems (D³S) offer unique advantages such as controlled drug release over sustained periods and a significant reduction in first-pass effects, thus reducing the required dosing frequency and level of patient noncompliance. Furthermore, D³S find applications in multiple therapeutic areas, including drug repurposing. This article presents an integrated biophysical model of dermal absorption for simulating the permeation and absorption of compounds delivered transdermally. The biophysical model is physiologically/biologically inspired and combines a holistic model of healthy skin with whole-body physiology-based pharmacokinetics through dermis microcirculation. The model also includes the effects of chemical penetration enhancers and hair follicles on transdermal transport. The model-predicted permeation and pharmacokinetics of select compounds were validated using in vivo data reported in the literature. We conjecture that the integrated model can be used to gather insights into the permeation and systemic absorption of transdermal formulations (including cosmetic products) released from novel depots and optimize delivery systems. Furthermore, the model can be adapted to diseased skin with parametrization and structural adjustments specific to skin diseases.

Transdermal Delivery of Etoposide Phosphate II: In Vitro In Vivo Correlations (IVIVC)

A dependable in vitro in vivo correlation (IVIVC) is a vital tool to optimize drug formulation and expedite product development time. Although many IVIVC examples are available for oral delivery systems, IVIVC for transdermal delivery is far less common, especially for electrical-assisted delivery. The objective of this study was to develop an IVIVC for the iontophoretic delivery of the anticancer drug etoposide. Iontophoresis was performed at 4 current densities (100, 200, 300, and 400 μA/cm(2)) both in vitro using a standard Franz-cell apparatus with excised porcine skin as membrane, and in vivo in a rabbit model. There was strong correlation between the in vitro % permeated across porcine skin and in vivo absorption (AUC, Cmax) in the range 100-300 μA/cm(2). The correlation between in vitro flux and in vivo input rate (R0) permitted to predict the R0 from a different set of in vitro data (external validation). Convolution of such input rate accurately predicted in vivo plasma profiles (PE% <15) in the absorption phase, whereas the elimination phase was slightly under-predicted (PE% >20). In vivo absorption profiles obtained with deconvolution did not overlap directly with the in vitro profiles; however, correction for the lag time and the application of a scaling factor estimated from Levy' s plots resulted in excellent correlation.

PBTK modeling demonstrates contribution of dermal and inhalation exposure components to end-exhaled breath concentrations of naphthalene

BACKGROUND

Dermal and inhalation exposure to jet propulsion fuel 8 (JP-8) have been measured in a few occupational exposure studies. However, a quantitative understanding of the relationship between external exposures and end-exhaled air concentrations has not been described for occupational and environmental exposure scenarios.

OBJECTIVE

Our goal was to construct a physiologically based toxicokinetic (PBTK) model that quantitatively describes the relative contribution of dermal and inhalation exposures to the end-exhaled air concentrations of naphthalene among U.S. Air Force personnel.

METHODS

The PBTK model comprised five compartments representing the stratum corneum, viable epidermis, blood, fat, and other tissues. The parameters were optimized using exclusively human exposure and biological monitoring data.

RESULTS

The optimized values of parameters for naphthalene were a) permeability coefficient for the stratum corneum 6.8 x 10(-5) cm/hr, b) permeability coefficient for the viable epidermis 3.0 x 10(-3) cm/hr, c) fat:blood partition coefficient 25.6, and d) other tissue:blood partition coefficient 5.2. The skin permeability coefficient was comparable to the values estimated from in vitro studies. Based on simulations of workers' exposures to JP-8 during aircraft fuel-cell maintenance operations, the median relative contribution of dermal exposure to the end-exhaled breath concentration of naphthalene was 4% (10th percentile 1% and 90th percentile 11%).

CONCLUSIONS

PBTK modeling allowed contributions of the end-exhaled air concentration of naphthalene to be partitioned between dermal and inhalation routes of exposure. Further study of inter- and intraindividual variations in exposure assessment is required to better characterize the toxicokinetic behavior of JP-8 components after occupational and/or environmental exposures.

A Dermatotoxicokinetic Model of Human Exposures to Jet Fuel

Workers, both in the military and the commercial airline industry, are exposed to jet fuel by inhalation and dermal contact. We present a dermatotoxicokinetic (DTK) model that quantifies the absorption, distribution, and elimination of aromatic and aliphatic components of jet fuel following dermal exposures in humans. Kinetic data were obtained from 10 healthy volunteers following a single dose of JP-8 to the forearm over a surface area of 20 cm2. Blood samples were taken before exposure (t = 0 h), after exposure (t = 0.5 h), and every 0.5 h for up to 3.5 h postexposure. The DTK model that best fit the data included five compartments: (1) surface, (2) stratum corneum (SC), (3) viable epidermis, (4) blood, and (5) storage. The DTK model was used to predict blood concentrations of the components of JP-8 based on dermal-exposure measurements made in occupational-exposure settings in order to better understand the toxicokinetic behavior of these compounds. Monte Carlo simulations of dermal exposure and cumulative internal dose demonstrated no overlap among the low-, medium-, and high-exposure groups. The DTK model provides a quantitative understanding of the relationship between the mass of JP-8 components in the SC and the concentrations of each component in the systemic circulation. The model may be used for the development of a toxicokinetic modeling strategy for multiroute exposure to jet fuel.

Dermal, subdermal, and systemic concentrations of granisetron by iontophoretic delivery

PURPOSE

The purpose of this work was to demonstrate the iontophoretic delivery of granisetron hydrochloride by novel, self-contained iontophoretic patches and to determine the subcutaneous and dermal absorption kinetics using microdialysis.

METHODS

In vitro iontophoretic delivery of granisetron hydrochloride was evaluated at 5, 10, or 20 mg/ml concentrations of donor using Franz diffusion cells and hairless rat skin as a membrane. In vivo studies were performed in hairless rats. Animals received either subcutaneous or dermal microdialysis probes and iontophoretic patches filled with drug formulation were applied on the abdominal area such that the probe lies below the anode chamber. Blood and microdialysate samples were collected at different time intervals. Intravenous administration of granisetron was also done to determine the basic pharmacokinetic parameters.

RESULTS

Iontophoretic patches delivered current constantly throughout the patch application. The patches delivered granisetron hydrochloride at a rate of 14.91+/-4.53 microg/min/kg. Similar concentrations of granisetron hydrochloride in dermal and subcutaneous tissue were observed. Depot formation was identified in the subcutaneous and dermal profiles, indicating that subcutaneous structures are also responsible for the depot formation of the drug in the dermis.

CONCLUSION

The patches successfully delivered granisetron hydrochloride by iontophoresis and depot formation was observed in the dermal and subcutaneous structures in the skin.

Effects of vehicle on the uptake and elimination kinetics of capsaicinoids in human skin in vivo

While the physiologic and molecular effects of capsaicinoids have been extensively studied in various model systems by a variety of administration routes, little is known about the uptake and elimination kinetic profiles in human skin following topical exposure. The present study evaluated the uptake and elimination kinetics of capsaicinoids in human stratum corneum following a single topical exposure to 3% solutions containing 55% capsaicin, 35% dihydrocapsaicin, and 10% other analogues prepared in three vehicles: mineral oil (MO), propylene glycol (PG), and isopropyl alcohol (IPA). Capsaicinoid solutions were evaluated simultaneously in a random application pattern on the volar forearms of 12 subjects using a small, single 150-microg dose. Capsaicin and dihydrocapsaicin were recovered from human skin using commercial adhesive discs to harvest stratum corneum from treated sites. Capsaicinoids were extracted from the stratum corneum-adhesive discs and quantified by liquid chromatography/mass spectroscopy (LC/MS). Both capsaicinoids were detected in stratum corneum 1 min after application with all vehicles and achieved a pseudo-steady state shortly thereafter. IPA delivered three times greater capsaicin and dihydrocapsaicin into the human stratum corneum than PG or MO at all time points investigated. The Cmax of capsaicin in IPA, PG, and MO was 16.1, 6.2, and 6.5 microg, respectively. The dihydrocapsaicin content was 60% of capsaicin with all vehicles. The estimated T(half) of capsaicin and dihydrocapsaicin in the three vehicles was similar (24 h). Thus, maximal cutaneous capsaicinoid concentrations were achieved quickly in the human stratum corneum and were concentration and vehicle dependent. In contrast, capsaicinoid half-life was long and vehicle independent.

Acyclovir permeation through rat skin: mathematical modelling and in vitro experiments

The aim of this work is to characterise the skin permeation properties of a male rat by means of a purely diffusive mathematical model based on Fick's second law. Additionally, in the attempt of proposing a reliable tool allowing the skin permeability (or resistance) determination on the basis of experimental data, the model automatically accounts also for two typical experimental conditions. In particular, drug dissolution in the donor environment and receiver sampling technique (part of the receiver volume is withdrawn and immediately replaced by fresh solvent) are considered. The results of this characterisation are then compared with those coming from a common simplified approach. Acyclovir is chosen as model drug and a thermostatic (37 degrees C) Franz cell apparatus is used to perform permeation experiments. This study suggests that Acyclovir permeation through the rat skin can be well described by the proposed model and that some differences arise in the evaluation of the full-skin resistance performed by means of our model or the usual simpler approach.

Pharmacokinetic models of dermal absorption

Many studies have used pharmacokinetic (compartment) models for skin to predict or analyze dermal absorption of chemicals. Comparing these models is difficult because the relationships between rate constants and the physicochemical parameters were not always defined clearly, simplifying assumptions built into models sometimes were not stated, and which skin layers were included often were not specified. In this paper we review and compare published one- and two-compartment models for which rate constants were expressed in terms of the physicochemical and physical properties of the skin (i.e., diffusion coefficients, partition coefficients and thickness). Nine one-compartment and two two-compartment models are presented with a consistent nomenclature and clearly defined assumptions. In addition, methods used for estimating the physicochemical parameters required by the various are summarized. These eleven compartment models are compared with calculations from a two-membrane skin model that corresponds better with skin function. Many of the compartment models do not predict key characteristics of the two-membrane skin model, especially the effect of blood flow on skin concentration and penetration rates, even when the same input parameters were used. The compartment models developed by Kubota and by McCarley are better predictors of the two-membrane model results, because these models were developed to match characteristics of the membrane model.

Efficacy of topical phenol decontamination strategies on severity of acute phenol chemical burns and dermal absorption: in vitro and in vivo studies in pig skin

Pure phenol is colorless and used in the manufacture of phenolic resins, plastics, explosives, fertilizers, paints, rubber, textiles, adhesives, pharmaceuticals, paper, soap, and wood preservatives. The purpose of this study was to compare the efficacy of several phenol decontamination strategies following dermal exposure using the pig as a model for human exposure, and then assess the effect of the two best treatments on phenol absorption in the isolated perfused porcine skin flap (IPPSF). Six anesthetized Yorkshire pigs were exposed to 89% aqueous phenol for 1 min using Hilltop chambers (10 skin sites/pig; 400 microl/site). Exposure to phenol was followed by one of 10 different decontamination procedures: 1-, 5-, 15-, and 30-min water wash; Ivory soap solution; polyethylene glycol (PEG 400); PEG 400/industrial methylated spirits (IMS); PEG 400/ethanol (EtOH); polyvinyl pyrrolidone (PVP)/70% isopropanol (IPA); and 70% IPA. For each of the last five strategies, 1-min treatment washes were repeatedly alternated with 1-min water washes for a total of 15 min. Evaluation was based on scoring of erythema, edema, and histological parameters such as intracellular and intercellular epidermal edema, papillary dermal edema, perivascular infiltrates, pyknotic stratum basale cells, and epidermal-dermal separation. It was concluded that PEG 400 and 70% IPA were superior to the other treatments investigated and equally efficacious in the reduction of phenol-induced skin damage. In addition, phenol absorption was assessed utilizing the two most effective in vivo treatments in the IPPSF. The assessment of percutaneous absorption of phenol found the PEG 400, 70% IPA, and 15-min water treatments significantly (P < 0.05) reduced phenol absorption relative to no treatment.

Dermal absorption and distribution of topically dosed jet fuels jet-A, JP-8, and JP-8(100)

Dermal exposure to jet fuels has received increased attention with the recent release of newer fuels with novel performance additives. The purpose of these studies was to assess the percutaneous absorption and cutaneous disposition of topically applied (25 microl/5 cm(2)) neat Jet-A, JP-8, and JP-8(100) jet fuels by monitoring the absorptive flux of the marker components 14C naphthalene and (3)H dodecane simultaneously applied nonoccluded to isolated perfused porcine skin flaps (IPPSF) (n = 4). Absorption of 14C hexadecane was estimated from JP-8 fuel. Absorption and disposition of naphthalene and dodecane were also monitored using a nonvolatile JP-8 fraction reflecting exposure to residual fuel that might occur 24 h after a jet fuel spill. In all studies, perfusate, stratum corneum, and skin concentrations were measured over 5 h. Naphthalene absorption had a clear peak absorptive flux at less than 1 h, while dodecane and hexadecane had prolonged, albeit significantly lower, absorption flux profiles. Within JP-8, the rank order of absorption for all marker components was (mean +/- SEM % dose) naphthalene (1.17 +/- 0.07) > dodecane (0.63 +/- 0.04) > hexadecane (0.18 +/- 0.08). In contrast, deposition within dosed skin showed the reverse pattern. Naphthalene absorption into perfusate was similar across all fuel types, however total penetration into and through skin was highest with JP-8(100). Dodecane absorption and total penetration was greatest from JP-8. Absorption of both markers from aged JP-8 was lower than other fuels, yet the ratio of skin deposition to absorption was greatest for this treatment group. In most exposure scenarios, absorption into perfusate did not directly correlate to residual skin concentrations. These studies demonstrated different absorption profiles for the three marker compounds, differential effects of jet fuel types on naphthalene and dodecane absorption, and uncoupling of perfusate absorption from skin disposition.

A sequential algorithm for the non-linear dual-sorption model of percutaneous drug absorption

A sequential algorithm is developed for the non-linear dual-sorption model developed by Chandrasekaran et al. [1,2] which monitors pharmacokinetic profiles in percutaneous drug absorption. In the experimental study of percutaneous absorption, it is often observed that the lag-time decreases with the increase in the donor concentration when two or more donor concentrations of the same compound are used. The dual-sorption model has sometimes been employed to explain such experimental results. In this paper, it is shown that another feature observed after vehicle removal may also characterize the dual-sorption model. Soon after vehicle removal, the plots of the drug flux versus time become straight lines on a semilogarithmic scale as in the linear model, but the half-life is prolonged thereafter when the dual-sorption model prevails. The initial half-life after vehicle removal with a low donor concentration is longer than that with a higher donor concentration. These features, if observed in experiments, may be used as evidence to confirm that the dual-sorption model gives an explanation to the non-linear kinetic behaviour of a permeant.