A physiologically relevant pharmacokinetic model of xenobiotic percutaneous absorption utilizing the isolated perfused porcine skin flap

Modeling gold nanoparticle biodistribution after arterial infusion into perfused tissue: effects of surface coating, size and protein corona

A detailed understanding of the factors governing nanomaterial biodistribution is needed to rationally design safe nanomedicines. This research details the pharmacokinetics of gold nanoparticle (AuNP) biodistribution after arterial infusion of 40 or 80 nm AuNP (1 μg/ml) into the isolated perfused porcine skin flap (IPPSF). AuNP had surface coatings consisting of neutral polyethylene glycol (PEG), anionic lipoic acid (LA), or cationic branched polyethylenimine (BPEI). Effect of a porcine plasma corona (PPC) on 40 nm BPEI and PEG-AuNP were assessed in the IPPSF. Au concentrations were determined by ICP/MS and arterial to venous concentration-time profiles were analyzed over 8 hr (4 hr infusion, 4 hr washout) using a two-compartment pharmacokinetic model. IPPSF viability and vascular function were assessed by change in glucose utilization, vascular resistance, or weight gain after perfusion. All AuNP demonstrated some degree of AuNP arterial extraction and skin flap retention, as well as enhanced kinetic parameters of tissue uptake; with BPEI-AuNP consistently having the greatest biodistribution even with a PPC. Toxicological effects were not detected. Transmission electron microscopy confirmed intracellular uptake of AuNP. These studies paralleled previous in vitro cell culture studies using the same AuNP in human endothelial and renal proximal tubule cells, hepatocytes, keratinocytes, showing BPEI-AuNP having the greatest uptake, although the presence of a PPC did not reduce IPPSF biodistribution as in the cell culture studies. These findings clearly indicate arterial to the venous extraction of AuNP after infusion with the magnitude of extraction being greatest with the BPEI surface coating and provide data and model structure necessary to construct the whole body physiologically based pharmacokinetic models capable of utilizing available in vitro data.

The isolated perfused human skin flap model: A missing link in skin penetration studies?

Development of effective (trans)dermal drug delivery systems requires reliable skin models to evaluate skin drug penetration. The isolated perfused human skin flap remains metabolically active tissue for up to 6h during in vitro perfusion. We introduce the isolated perfused human skin flap as a close-to-in vivo skin penetration model. To validate the model's ability to evaluate skin drug penetration the solutions of a hydrophilic (calcein) and a lipophilic (rhodamine) fluorescence marker were applied. The skin flaps were perfused with modified Krebs-Henseleit buffer (pH7.4). Infrared technology was used to monitor perfusion and to select a well-perfused skin area for administration of the markers. Flap perfusion and physiological parameters were maintained constant during the 6h experiments and the amount of markers in the perfusate was determined. Calcein was detected in the perfusate, whereas rhodamine was not detectable. Confocal images of skin cross-sections shoved that calcein was uniformly distributed through the skin, whereas rhodamine accumulated in the stratum corneum. For comparison, the penetration of both markers was evaluated on ex vivo human skin, pig skin and cellophane membrane. The proposed perfused flap model enabled us to distinguish between the penetrations of the two markers and could be a promising close-to-in vivo tool in skin penetration studies and optimization of formulations destined for skin administration.

Mathematical models for skin toxicology

INTRODUCTION

Our skin is exposed daily to substances; many of these are neutral and safe but others are potentially harmful. In order to estimate the degree of toxicity and damage to skin tissues when exposed to harmful substances, skin toxicology studies are required. If these studies are coupled with suitably designed mathematical models, they can provide a powerful tool that allows appropriate interpretation of data. This work reviews mathematical models that can be employed in skin toxicology studies.

AREAS COVERED

Two types of mathematical models and their suitability for assessing skin toxicology are covered in this review. The first is focused on predicting penetration rate through the skin from a solute's physicochemical properties, while the second type of models transport processes in skin layers using appropriate equations with the specific aim of predicting the concentration of a given solute in viable skin tissues.

EXPERT OPINION

Mathematical models are an important tool for accurate valuation of skin toxicity experiments, estimation of skin toxicity and for developing new formulations for skin disease therapy. Comprehensive mathematical models of drug transport in skin, especially those based on more physiologically detailed mechanistic considerations of transport processes, are required to further enhance their role in assessing skin toxicology.

Physicochemical and Biological Data for the Development of Predictive Organophosphorus Pesticide QSARs and PBPK/PD Models for Human Risk Assessment

A search of the scientific literature was carried out for physiochemical and biological data [i.e., IC50, LD50, Kp (cm/h) for percutaneous absorption, skin/water and tissue/blood partition coefficients, inhibition ki values, and metabolic parameters such as Vmax and Km] on 31 organophosphorus pesticides (OPs) to support the development of predictive quantitative structure-activity relationship (QSAR) and physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) models for human risk assessment. Except for work on parathion, chlorpyrifos, and isofenphos, very few modeling data were found on the 31 OPs of interest. The available percutaneous absorption, partition coefficients and metabolic parameters were insufficient in number to develop predictive QSAR models. Metabolic kinetic parameters (Vmax, Km) varied according to enzyme source and the manner in which the enzymes were characterized. The metabolic activity of microsomes should be based on the kinetic activity of purified or cDNA-expressed cytochrome P450s (CYPs) and the specific content of each active CYP in tissue microsomes. Similar requirements are needed to assess the activity of tissue A- and B-esterases metabolizing OPs. A limited amount of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and carboxylesterase (CaE) inhibition and recovery data were found in the literature on the 31 OPs. A program is needed to require the development of physicochemical and biological data to support risk assessment methodologies involving QSAR and PBPK/PD models.

Prediction of dermal absorption from complex chemical mixtures: incorporation of vehicle effects and interactions into a QSPR framework

Significant progress has been made on predicting dermal absorption/penetration of topically applied compounds by developing QSPR models based on linear free energy relations (LFER). However, all of these efforts have employed compounds applied to the skin in aqueous or single solvent systems, a dosing scenario that does not mimic occupational, environmental or pharmaceutical exposure. We have explored using hybrid QSPR equations describing individual compound penetration based on the molecular descriptors for the compound modified by a mixture factor (MF) which accounts for the physicochemical properties of the vehicle/mixture components. The MF is calculated based on percentage composition of the vehicle/mixture components and physical chemical properties selected using principal components analysis. This model has been applied to 12 different compounds in 24 mixtures for a total of 288 treatment combinations obtained from flow-through porcine skin diffusion cells and in an additional dataset of 10 of the same compounds in five mixtures for a total of 50 treatment combinations in the ex vivo isolated perfused porcine skin flap. The use of the MF in combination with a classic LFER based on penetrant properties significantly improved the ability to predict dermal absorption of compounds dosed in complex chemical mixtures.

Percutaneous malathion absorption in the harvested perfused anuran pelvic limb

The objective of this study was to establish an accurate in vitro model for cutaneous absorption in anurans. The harvested perfused anuran pelvic limb (HPAPL) model maintains the anatomic and physiologic integrity of the skin from the pelvic limb, including the intact capillary network. Radiolabeled malathion was applied to the skin of the dorsal thigh, and perfusate was collected over a 6h period. Residues from the skin surface, stratum externum, and dosed area beneath the stratum externum were analyzed. Kinetic parameters were calculated from these data. Absorption was significantly less for the HPAPL than previously reported for Teflon flow-through diffusion cells. However, partitioning effects were comparable. The HPAPL is an appropriate in vitro model for examining cutaneous absorption kinetics in the bullfrog.

Kinetics of finite dose absorption through skin 2: Volatile compounds

A diffusion model to account for the disposition of an arbitrary dose of a (potentially) volatile compound applied to skin from a volatile vehicle is presented. In its most general form, the model allows for variable diffusivity of the permeant in the stratum corneum (SC) and must be solved numerically. However, for permeants having a constant diffusivity, absorption, and evaporation is characterized in terms of four dimensionless parameters-a reduced time tau, a fractional deposition depth in the SC f, a ratio of membrane capacity for the permeant to the applied dose beta, and a ratio of evaporative mass transfer coefficient to diffusive permeability chi. An important combination of these parameters arises as the reduced dose M(r) = (fbeta)(-1). Two cases are distinguished. In Case 1, corresponding to M(r) < or = 1, the dose is less than that required to saturate the upper layers of the SC, and the shape of the absorption and evaporation profiles is independent of the dose. Analytical solutions to Case 1 may be derived for arbitrary initial distributions of the permeant; the solution for a square wave is presented. In Case 2, corresponding to M(r) > 1, absorption and evaporation approach steady-state values as the dose is increased. Numerical evaluations of this behavior are shown. Limiting behavior for the case of a highly volatile solvent applied to skin is discussed. A companion paper discusses the application of the model to the absorption and evaporation of benzyl alcohol from human skin in vitro.

A Physiologically Based Pharmacokinetic Model of Organophosphate Dermal Absorption

The rate and extent of dermal absorption are important in the analysis of risk from dermal exposure to toxic chemicals and for the development of topically applied drugs, barriers, insect repellents, and cosmetics. In vitro flow-through cells offer a convenient method for the study of dermal absorption that is relevant to the initial processes of dermal absorption. This study describes a physiologically based pharmacokinetic (PBPK) model developed to simulate the absorption of organophosphate pesticides, such as parathion, fenthion, and methyl parathion through porcine skin with flow-through cells. Parameters related to the structure of the stratum corneum and solvent evaporation rates were independently estimated. Three parameters were optimized based on experimental dermal absorption data, including solvent evaporation rate, diffusivity, and a mass transfer factor. Diffusion cell studies were conducted to validate the model under a variety of conditions, including different dose ranges (6.3-106.9 microg/cm2 for parathion; 0.8-23.6 microg/cm2 for fenthion; 1.6-39.3 microg/cm2 for methyl parathion), different solvents (ethanol, 2-propanol and acetone), different solvent volumes (5-120 microl for ethanol; 20-80 microl for 2-propanol and acetone), occlusion versus open to atmosphere dosing, and corneocyte removal by tape-stripping. The study demonstrated the utility of PBPK models for studying dermal absorption, which can be useful as explanatory and predictive tools that may be used for in silico hypotheses generation and limited hypotheses testing. The similarity between the overall shapes of the experimental and model-predicted flux/time curves and the successful simulation of altered system conditions for this series of small, lipophilic compounds indicated that the absorption processes that were described in the model successfully simulated important aspects of dermal absorption in flow-through cells. These data have direct relevance to topical organophosphate pesticide risk assessments.

A rabbit ear flap perfusion experiment to evaluate the percutaneous absorption of drugs

A rabbit ear flap single-pass perfusion system was examined as an experimental method for studying the relationship between the physiological conditions of tissues and drug disposition after topical applications. Tyrode solutions containing bovine serum albumin (BSA) and sucrose or flurbiprofen (FP), used as a model drug, were perfused through the vessel in the ear flap to evaluate the physiological conditions prior to the application of FP to the skin surface. The extracellular volume and distribution properties of FP in the perfused ear were similar to those in an in vivo experimental system. In addition, the perfused ear flap exhibited a pharmacological response to bradykinin (BK). The amount of FP in the outflow Tyrode solution containing BSA after application to the skin surface of the perfused ear decreased with the addition of BK, while that in the tissues under the application site increased. FP binds to BSA, which leaked from the intravascular space, and could be retained in the tissues under the application site. The protein binding also affected the redistribution of FP to other tissues in the ear flap after application to the skin. The rabbit ear perfusion system is a useful method for studying the percutaneous absorption of drugs especially variations in the disposition of drugs in oedematous tissues.

Gulf War related exposure factors influencing topical absorption of 14C-permethrin

Topical exposure to permethrin has often been implicated as a mitigating factor in the illnesses reported in Gulf War veterans. These studies were designed to assess the effect of co-exposure to low level sulfur mustard, JP-8 jet fuel, N,N-diethyl-m-toluamide (DEET) and fabric occlusion on the percutaneous absorption and skin disposition of topically applied 14C-permethrin (40 microg/cm(2)) in the isolated perfused porcine skin flap (IPPSF) model. Extent of dermal absorption in vehicle controls in the IPPSF was comparable to literature values for humans. These studies demonstrated a two-fold increased 14C-permethrin percutaneous absorption and almost three-fold increased penetration when JP-8 was present, compared to a one-third decreased permethrin flux in the presence of sulfur mustard. Complete occlusion slightly increased 14C-permethrin absorption, while occlusion with fabric showed no significant effect. A previously noted effect of DEET to inhibit permethrin absorption was still seen in the presence of sulfur mustard exposure. These studies suggest that co-exposure to JP-8 or sulfur mustard may modulate transdermal flux of 14C-permethrin. However, the JP-8 increase in absorption and penetration was less than the five-fold increase previously seen with arterial infusion of pyridostigmine bromide and diisopropylfluorophosphate in the IPPSF. The toxicologic significance of this moderate increase in permethrin absorption remains unclear.

Potential and problems of developing transdermal patches for veterinary applications

A new frontier in the administration of therapeutic drugs to veterinary species is transdermal drug delivery. The primary challenge in developing these systems is rooted in the wide differences in skin structure and function seen in species ranging from cats to cows. The efficacy of a transdermal system is primarily dependent upon the barrier properties of the targeted species skin, as well as the ratio of the area of the transdermal patch to the species total body mass needed to achieve effective systemic drug concentrations. A drug must have sufficient lipid solubility to traverse the epidermal barrier to be considered for delivery for this route. A number of insecticides have been developed in liquid "pour-on" formulations that illustrate the efficacy of this route of administration for veterinary species. The human transdermal fentanyl patch has been successfully used in cats and dogs for post-operative analgesia. The future development of transdermal drug delivery systems for veterinary species will be drug and species specific. With efficient experimental designs and available transdermal patch technology, there are no obvious hurdles to the development of effective systems in many veterinary species.

Use of methyl salicylate as a simulant to predict the percutaneous absorption of sulfur mustard

Exposure to chemical vesicants such as sulfur mustard (HD) continues to be a threat to military forces requiring protectant strategies to exposure to be evaluated. Methyl salicylate (MS) has historically been the simulant of choice to assess HD exposure. The purpose of this study was to compare the percutaneous absorption and skin deposition of MS to HD in the isolated perfused porcine skin flap (IPPSF). The HD data were obtained from a previously published study in this model wherein 400 microg cm(-2) of ](14)C[-MS or ](14)C[-HD in ethanol were topically applied to 16 IPPSFs and experiments were terminated at 2, 4 or 8 h. Perfusate was collected at increasing time intervals throughout perfusion. Radioactivity was determined in perfusate and skin samples. Perfusate flux profiles were fitted to a bi-exponential model Y(t) = A(e(-bt) - e(-dt)) and the area under the curve (AUC), peak flux and time to peak flux were determined. Sulfur mustard had more pronounced and rapid initial flux parameters (P < 0.05). The AUCs determined from observed and model-predicted parameters were not statistically different, although the mean HD AUC was 40--50% greater than MS. The HD skin and fat levels were up to twice those seen with MS, but had lower stratum corneum and residual skin surface concentrations (P < 0.05). Compared with other chemicals studied in this model, HD and MS cutaneous disposition were very similar, supporting the use of MS as a dermal simulant for HD exposure.

Evaluation of Göttingen minipig skin for transdermal in vitro permeation studies

The optimal skin type for in vitro permeability studies depends on the purpose of the specific transdermal study. In a number of cases, it may be advantageous to use animal skin as an alternative to human skin although they have different characteristics. Recently, Göttingen minipigs have been reported as good models in toxicological and pharmacokinetic studies of drug substances. In this paper, the potential use of skin from the Göttingen minipig is evaluated by studying three model drug substances (nicotine, salicylic acid and testosterone) through skin from humans, domestic pigs and three ages of the Göttingen minipig. An analysis of variance and a Student's t-test showed that both the skin from the Göttingen minipig and the domestic pig possessed transdermal permeabilities, which correlated with human skin and exhibited a lower intra- and intervariation. Furthermore, it was shown that permeability and variation of fluxes through skin from Göttingen minipigs were dependent on the age of the minipig and of the drug substance. It is concluded that the Göttingen minipig, like the domestic pig, is a good skin model for in vitro permeation through human skin.

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.

Percutaneous penetration enhancement and its quantification

True penetration enhancing effects resulting from structural alterations of the barrier stratum corneum manifest themselves in an increase of the drug diffusion coefficient DB and/or of the drug solubility in the barrier csB. The quantification of enhancing effects on drug penetration is possible either by the direct determination of the drug fluxes or by an indirect determination through the measurement of the pharmacodynamic response. In both cases the thermodynamic drug activity has to be considered. In the case of pharmacodynamic measurements, enhancing effects may be determined from the horizontal distance of activity-response lines obtained without and with enhancer, respectively, i.e. the quotient of the drug concentrations that induce the same effect. The activity-standardized bioavailability factors fa obtained from the horizontal distances correspond to the enhancer-induced relative changes in the permeabilities PB, or more exactly in the product DB X csB. On the other hand, the vertical distance between the activity-response lines, i.e. the differences in the drug response after application of preparations with equal (even maximum) thermodynamic drug activities may be used to quantify penetration enhancing effects.

Ultrastructural characterization of sulfur mustard-induced vesication in isolated perfused porcine skin

The isolated perfused porcine skin flap (IPPSF) is a novel alternative, humane in vitro model consisting of a viable epidermis and dermis with a functional microvasculature. For this study, 200 microliters of either 10.0, 5.0, 2.5, 1.25, 0.50, or 0.20 mg/ml of bis (2-chloroethyl) sulfide (HD) in ethanol or ethanol control was topically applied to a 5.0 cm2 dosing area of the IPPSF and perfused for 8 h with recirculating media. HD dermatotoxicity was assessed in the flap by cumulative glucose utilization (CGU), vascular resistance (VR), light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). HD produced a statistically significant dose relationship for gross blisters and microvesicles. The HD-treated IPPSFs were also characterized by a decrease in CGU and an increase in VR. Light microscopic changes included mild intracellular and slight intracellular epidermal edema, multifocal epidermal-dermal separation, and dark basal cells. Ultrastructural alterations consisted of cytoplasmic vacuoles, pyknotic basal cells, nucleolar segregation, and epidermal-dermal separation occurring between the lamina lucida and lamina densa of the basement membrane. The severity of these changes increased in a dose-dependent manner. Morphologically, the IPPSF appeared similar to human skin exposed to HD with the formation of macroscopic blisters and microscopic vesicles. In conclusion, the IPPSF appears to be an appropriate in vitro model with which to study the pathogenesis of vesicant-induced toxicity.

Indirect immunohistochemistry and immunoelectron microscopy distribution of eight epidermal-dermal junction epitopes in the pig and in isolated perfused skin treated with bis (2-chloroethyl) sulfide

Sulfur mustard (bis [2-chloroethyl] sulfide, HD) is a potent cutaneous vesicant that causes gross blisters by separation of the epidermal-dermal junction (EDJ). The EDJ of the skin is a highly specialized and complex structure composed of several components and plays a major role in the integrity of the skin. The isolated perfused porcine skin flap (IPPSF) was dosed with 0.2 mg/ml (n = 4), 5.0 mg/ml (n = 4), and 10.0 mg/ml (n = 5) HD or ethanol (n = 4) for 8 hr (dose-response study) and 10.0 mg/ml HD or ethanol for 1, 3, 5, and 8 hr (n = 4/treatment) (time-response study). Successful EDJ mapping was carried out in normal pig skin (NPS), ethanol-treated IPPSFs, and HD-treated IPPSFs using the following antibodies: laminin, type IV collagen, fibronectin, GB3 (Nicein), bullous pemphigoid (BP), and epidermolysis bullosa acquisita (EBA). Two mouse anti-human monoclonal antibodies, L3d and 19-DEJ-1 (Uncein), did not cross-react with the EDJ of the pig. Antibody staining in NPS, ranging from very intense for laminin and type IV collagen to weak for fibronectin, was generally more discrete than in the IPPSF. No differences in staining were noted between the ethanol and nonblistered areas of the HD-treated IPPSFs. In HD-blistered areas, BP stained only the epidermal hemidesmosomes, and laminin, fibronectin, and GB3 stained primarily the dermis with fragments attached to the basal pole of the stratum base cells, while type IV collagen and EBA stained only the dermis. Mapping of these epitopes determined that the precise plane of EDJ separation in the HD-treated skin occurred beneath the hemidesmosomes within the upper portion of the lamina lucida. The conservation of human epitopes in the EDJ of the pig further emphasizes the similarities between human skin and pig skin. Therefore, pig skin and the IPPSF may be used to study HD-induced vesication and blistering diseases.

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

We present a general comprehensive mathematical model to stimulate and predict percutaneous absorption and subsequent disposition of chemicals in vivo that is chiefly based on biophysical parameters estimated or measured with in vitro and ex vivo perfused skin preparations. Current physicochemical principles of drug diffusion and partitioning across the skin barrier, solute and solvent concentration dynamics, the influence of solute and solvent on the stratum corneum barrier, and dynamic vascular perfusion effects are integrated in this model. Such a comprehensive approach is necessary to achieve optimal biological relevance in a quantitative model of percutaneous absorption, particularly when a chemical is applied as a binary (solute and solvent) or more complex formulation or chemical mixture. The proposed model should have applications in (a) designing drugs and permeation enhancers for passive or active (e.g., electrically assisted) transdermal drug delivery, (b) assessing the systemic exposure of topical drugs used in dermatology, and (c) integration into other mathematical models being developed to assess the risk after topical exposure to mixtures of environmental pollutants. We also have included experimental data to provide a preliminary illustration of the performance of the model.

Significant effects of application site and occlusion on the pharmacokinetics of cutaneous penetration and biotransformation of parathion in vivo in swine

Increasing attention has been paid to the variables of application site and dosing method in quantitation of chemical percutaneous absorption. Following topical and intravenous application of [ring-U-14C]parathion (PA) in weanling pigs, we have determined, in a previous publication, the profiles of 14C and HPLC-separated paraoxon (PO), p-nitrophenol (PNP), and p-nitrophenyl beta-D-glucuronide (PNP-G) in plasma, urine, tissues, and dosing device. The purpose of the present paper was to analyze these data further, focusing on a quantitation of the effects of application site (back versus abdomen) and dosing method (occluded versus nonoccluded) on in vivo disposition of both the parent PA and its sequential metabolites PO, PNP, and PNP-G. Cutaneous and systemic disposition parameters were determined using a numerical simulation modeling approach and moments analysis. Mean systemic bioavailability values of 8.9-9.2% for abdomen and 14.7-19.7% for back were determined. Under different dosing conditions, 1-35% of the topical dose was metabolized dermally, and 9-19% systemically. Radioactivity in plasma and urine was predominantly contributed by PNP-G and PNP. Site differences in 14C percutaneous absorption were governed by the differences in transport of PA, PO, and PNP from epidermis into blood, by local tissue distribution, and by the cutaneous metabolism to PNP. Systemic bioavailability of PA was higher from the back than from the abdomen. Occlusion not only increased the amount of 14C absorption and shortened the mean residence time in most compartments but also altered the systemic versus cutaneous biotransformation pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Percutaneous absorption, dermatopharmacokinetics and related bio-transformation studies of carbaryl, lindane, malathion, and parathion in isolated perfused porcine skin

The percutaneous absorption of topically applied pesticides is a primary route for systemic exposure and potential toxicity. The isolated perfused porcine skin flap (IPPSF) is an in vitro model for studying percutaneous absorption of xenobiotics as well as cutaneous metabolism and toxicity in an anatomically intact viable skin preparation. In the present studies, percutaneous absorption of four different pesticides, carbaryl (C), lindane (L), malathion (M), and parathion (P), was assessed topically in an ethanol vehicle. A 4-compartment pharmacokinetic model was utilized to model their absorption profile. The order of absorption was C > P > L > M for the 8-h experimental period, but C > L > P > M for a model-extrapolated 6-day prediction. Metabolism of C and P was also assessed by high performance liquid chromatography (HPLC). The HPLC results indicate a significant first-pass effect for both pesticides after topical application, with parathion being metabolized to paraoxon and para-nitrophenol and carbaryl to naphthol. In addition, comparison of the metabolic data of P with previous results underscores the difference between non-recirculating and recirculating IPPSF systems in xenobiotic metabolism studies.

Model describing transdermal iontophoretic delivery of lidocaine incorporating consideration of cutaneous microvascular state

A three-compartment pharmacokinetic model describing percutaneous absorption of iontophoretically driven topically applied lidocaine in the isolated perfused porcine skin flap is presented. Delivery from the active (drug-dosed) electrode to skin is estimated as a ramp input profile. Model parameters were estimated separately for dosing (4 h current-on) and washout (4 h current-off) periods in experiments with coadministered vasoactive drugs [tolazoline (vasodilator) and norepinephrine (vasoconstrictor)] and controls (lidocaine alone). The model presented was able to predict 8-h lidocaine absorptions and compartmental mass profiles for each of the three treatments, was able to document vascular effects of co-iontophoresed vasoactive compounds, and gives insight into the factors that modulate cutaneous disposition of iontophoretically delivered lidocaine in a biologically relevant model approximating in vivo delivery.

Pig ear skin as an in-vitro model for human skin permeability

Pig skin has been shown to have similar histological and physiological properties to human skin and has been suggested as a good model for human skin permeability. In this series of experiments, the in-vitro permeability of pig ear skin was compared with human (abdominal) skin and rat (dorsal) skin using both hydrophilic (water, mannitol, paraquat) and lipophilic (aldrin, carbaryl, fluazifop-butyl) penetrants. Pig skin was found to have a closer permeability character than rat skin to human skin, particularly for lipophilic penetrants. Electrical conductivity measurements across pig skin membranes showed that skin conductivity could be a useful method for assessing the integrity of membranes, particularly when used in conjunction with water permeability assessments.

Characterization of lewisite toxicity in isolated perfused skin

Lewisite (L) is a potent organic arsenical that causes rapid onset of pain and severe vesication on contact with epithelial tissues. The isolated perfused porcine skin flap (IPPSF) is an in vitro model that has shown potential as a model for cutaneous vesicant research. The objective of this study was to characterize IPPSF responses after topical exposure to six concentrations of L ranging from 0.07 to 5.0 mg/ml (n = 4/treatment plus controls). Biochemical markers of viability (glucose utilization (CGU) and lactate dehydrogenase (LDH) release), vascular resistance (VR), venous arsenic flux, and morphological parameters (light and electron microscopy) were evaluated. In addition, lewisite lesions were characterized at 1, 3, 5, and 8 hr after exposure (n = 4/time plus controls) using these morphological parameters, as well as enzyme histochemistry. Macroscopic and microscopic lesions caused by L exposure were dose related. Mild decreases in CGU were noted with the higher concentrations of L, while generally increased responses in LDH release and VR were seen. Marked increases in LDH activity were noted in the blister fluid of IPPSFs treated with 5.0 mg/ml of L. Also, significant cutaneous arsenic flux was noted at the 5.0 mg/ml dose of L. The formation of gross blisters, the location and characterization of epidermal-dermal junction separation, and the time course of lesion production paralleled the description of L-induced lesions in humans. The sensitivity of the IPPSF to L exposure and the similarity of lesions to those described for humans suggests that this model provides a relevant in vitro model with which to study mechanisms of chemical vesication and arsenic toxicity, as well as protective and therapeutic intervention for vesicant exposure.

Cutaneous toxicity and absorption of paraquat in porcine skin

Paraquat, a commonly used herbicide, has been shown to be toxic in exposed field workers. The objectives of this study were to (a) assess the cutaneous toxicity of paraquat in vivo in pig skin and in vitro in the isolated perfused porcine skin flap (IPPSF) and (b) quantitate its absorption in the IPPSF. The amounts of 3, 24, and 200 mg of paraquat were topically applied (5 cm2 surface area) on the ventral abdomen of pigs and biopsied after 6-8 hr for light microscopy (LM) and transmission electron microscopy (TEM). IPPSFs were topically dosed with the same concentrations and perfused for 8 hr (n = 4/treatment). The dosed area of the skin was sampled for LM, TEM, and enzyme histochemistry. IPPSFs were also treated topically with [14C]paraquat dichloride at the aforementioned concentrations (n = 4/dose) and hourly perfusate samples were collected for radiolabel determination and assessment of biochemical and physiological parameters. The epidermal changes were similar both in vivo and in vitro. The changes included epidermal intercellular edema which increased with dose and epidermal-dermal separation at the 200-mg dose. Acid phosphatase and nonspecific esterase activities were increased in the upper layers of the epidermis, while alkaline phosphatase showed a greater activity in the stratum basale layer. Glucose utilization of all treated IPPSFs was lower than that of the controls and a variation in the vascular resistance profiles was seen in all the treated flaps. Radiotracer studies indicated that a majority of the compound remained on top of the application site and minimal absorption or penetration into skin was observed. Thus, at high concentrations and prolonged exposure, paraquat may have deleterious effects on epidermal morphology in the absence of significant percutaneous absorption.

Effects of vasoactive drugs on transdermal lidocaine iontophoresis

The effect of co-administration of vasoactive drugs on the transdermal iontophoretic delivery of lidocaine.HCI was studied in in vitro cells, in the isolated perfused porcine skin flap (IPPSF), and in vivo in pigs. Iontophoresis of lidocaine in vitro across human and porcine skin were similar, supporting the use of porcine skin as an appropriate animal model. Co-iontophoresis of the vasodilator tolazoline marginally decreased lidocaine flux in vitro, but significantly increased it in the IPPSF and in vivo. In contrast, norepinephrine decreased lidocaine flux in the IPPSF. Vasomodulation also changed the shape of the venous efflux profile in the IPPSF as evidenced by changes in fractional absorption index, as well as the AUC. These studies demonstrate that co-iontophoresis of vasoactive compounds may significantly alter the transdermal delivery of lidocaine and that use of vitro animal model systems which possess a functional microcirculation are essential to study this process if reliable extrapolation to the in vivo setting is desired.

The isolated perfused porcine skin flap as an in vitro model for percutaneous absorption and cutaneous toxicology

The isolated perfused porcine skin flap (IPPSF) is a new perfused skin model which allows in vitro cutaneous pharmacology and toxicology studies to be conducted in a viable skin preparation which has a normal anatomical structure and a functional microcirculation. The purpose of this review is to (1) outline the background of this field which indicated the need for this type of model; (2) outline the surgical procedures needed to create and harvest viable preparations; (3) overview the criteria (biochemical, physiological, and histological) used to assess viability during an experiment; (4) present results of percutaneous absorption, cutaneous metabolism, transdermal delivery (passive and active), and skin distribution experiments conducted to date; (5) present the strategy developed to quantitate percutaneous absorption and cutaneous drug distribution using compartmental and physiological-based pharmacokinetic models; (6) assess the correlation of IPPSF data to in vivo results; (7) define the biochemical, physiological and histological (LM, TEM, enzyme histochemistry) response of the IPPSF to topically applied cutaneous vesicants; (8) overview where this type of in vitro model fits into the overall framework of cutaneous toxicology and pharmacology research; and (9) outline possible paths for future development. This review should provide the reader with an appreciation of some unique problems in this field which require an in vitro model that is closely integrated in structure and function to the in vivo setting.

Effects of organic solvent vehicles on the viability and morphology of isolated perfused porcine skin

Although many organic solvents are known to be cutaneous irritants, they are commonly utilized as vehicles in percutaneous absorption and toxicity studies. The isolated perfused porcine skin flap (IPPSF) is an alternative animal model that has been used to study percutaneous absorption and cutaneous toxicity. The purpose of this study was to evaluate the effect of five organic solvents (ethanol, acetone, dimethyl sulfoxide (DMSO), toluene, and cyclohexane) on biochemical viability parameters, vascular response, and epidermal morphology of the IPPSF. Cumulative glucose utilization (CGU), the ratio of lactate production/glucose utilization (L/CGU ratio), and the leakage of lactate dehydrogenase (LDH) were used as biochemical indicators of alterations in glucose metabolism and flap viability. Only ethanol resulted in a statistically significant decrease in the average rate of CGU over the perfusion period. All of the solvent treatments resulted in slight increases in LDH release versus the controls. Vascular resistance (VR) was measured to examine the response of the cutaneous vasculature to these solvents, and most treatments resulted in a decreased VR in the terminal phases of perfusion. Ethanol was the only solvent to cause an apparent increase in terminal VR. Light microscopy demonstrated a moderate increase in intracellular edema in the DMSO, toluene, and acetone flaps. Ultrastructural evaluation showed focal blebbing of the nuclear envelope and vesiculation of the rough endoplasmic reticulum in cells of the stratum basale and stratum spinosum layers with DMSO treatment. The IPPSF allowed the evaluation of subtle biochemical, vascular, and morphological changes associated with non-occlusive topical exposure to these organic solvents. These findings support the necessity of documenting vehicle effects which might mask or otherwise alter subtle, but potentially important, compound-specific responses.

Cutaneous toxicity of 2-chloroethyl methyl sulfide in isolated perfused porcine skin

Previous research has shown the isolated perfused porcine skin flap (IPPSF) to be a novel in vitro experimental model for investigating xenobiotic percutaneous absorption. In this study, the IPPSF was used to biochemically and morphologically assess the dermatotoxicity of 2-chloroethyl methyl sulfide (CEMS), a monofunctional analog of the vesicant, sulfur mustard. IPPSFs were perfused in a recirculating perfusion system and were treated with 97% CEMS (n = 4) or served as controls (n = 4). Additional IPPSFs were perfused in a nonrecirculating perfusion system and were treated with CEMS (n = 4) or were controls (n = 4). After dosing, each IPPSF was perfused for 8 hr. Cumulative glucose utilization (GU) and lactate production/glucose utilization ratio (L/GU ratio) were used as viability parameters. The average rate of GU for CEMS was significantly lower than control (p less than 0.05) in the recirculating and nonrecirculating IPPSFs. The L/GU ratio for CEMS was not significantly different (p greater than 0.05) from control for either perfusion system. CEMS resulted in a marked increase in vascular resistance versus control in both perfusion systems. Gross vesicles and bullae formation occurred in six of the CEMS-treated IPPSFs. Light microscopy revealed subepidermal vesicle formation above the basement membrane and extensive basal cell pyknosis in all IPPSFs treated with CEMS. No macroscopic or microscopic lesions were noted in the control flaps. Transmission electron microscopy revealed separation between the lamina lucida and the lamina densa of the basal lamina, with intracellular vacuolization and mitochondrial swelling occurring in the stratum basale and stratum spinosum cells of IPPSFs treated with CEMS. These lesions are similar to those described after human exposure to sulfur mustard. Full characterization of the morphological and biochemical changes seen after topical exposure of the IPPSF to vesicants may shed light on the pathogenesis of cutaneous toxicity of these compounds in vivo and serve as a relevant model to assess protective strategies against vesicant exposure.