In Vitro Percutaneous Absorption of Cadmium from Water and Soil into Human Skin

The objective was to determine percutaneous absorption of cadmiumas the chloride salt from water and soil into and through humanskin. Soil (Yolo County 65-California-57-8) was passed through10-, 20-, and 48-mesh sieves. Soil retained by 80 mesh was mixedwith radioactive cadmium-109 at 13 ppb. Water solutions of cadmium-109at 116 ppb were prepared for comparative analysis. Human cadaverskin was dermatomed to 500-µm, and used in glass diffusioncells with human plasma as the receptor fluid (3 ml/hr flowrate) for a 16-hr skin application time. Cadmium in water (5µ1/cm²) penetrated skin to concentrations of 8.8 ±0.6 and 12.7 ± 11.7% of the applied dose from two humanskin sources. Percentage doses absorbed into plasma were 0.5± 0.2 and 0.6 ± 0.6%, respectively. Cadmium fromsoil (0.04 g soil/cm²) penetrated skin at concentrations of0.06 ± 0.02 and 0.13 ± 0.05% for the two humanskin sources. Amounts absorbed into plasma were 0.01 ±0.01 and 0.07 ± 0.03%. Most of the non-absorbed cadmiumwas recovered in the soap and water skin surface wash. Bindingof cadmium from water to soil was greater than binding fromwater to powdered human stratum corneum, supporting the lowerabsorption from soil than from water. Short-term exposure ofcadmium in water to human skin for 30 min (bath or swim) resultedin skin uptake, which upon further perfusion (48 hr), absorbedinto the plasma receptor fluid (systemic). Cadmium in soil wasincreased from 6.5 to 65 ppb. Skin levels correspondently increased,but plasma receptor fluid levels remained constant. Soil capacitywas decreased from 40 to 4 mg/cm². Skin levels correspondinglydecreased, suggesting decreased skin contact, but plasma receptorfluid levels remained constant. The above suggest that, within vitro diffusion, the surface concentration of cadmium willinfluence skin cadmium concentration, but that absorption intoplasma receptor fluid is relatively independent of skin surfaceconcentrations. Calculations suggest that a daily whole bodyexposure to cadmium at 116 ppb with 0.5% absorption will resultin daily systemic intake of about 10 µg Cadmium.     

In Vivo and in Vitro Percutaneous Absorption and Skin Evaporation of Isofenphos in Man

Studies were done to determine the percutaneous absorption ofisofenphos in human volunteers from whom informed consent hadbeen obtained. In vivo absorption in man was 3.6±3.6%of applied dose for 24-hr exposure and 3.6±0.5% for 72-hrexposure. Skin wash recovery data show that isofenphos evaporatesfrom in vivo skin during the absorption process; the surfacedose is minimal (<1%) by 24 hr. Skin stripping showed noresidual isofenphos in stratum corneum. This explains the similarabsorption for 24 and 72-hr dose prewash exposures. Skin surfacerecovery in vivo with soap and water was 61.4±10.4 forthe first dosing time (15 mm). Time-recovery response declinedwith time to 0.5±0.2% at 24 hr. In vitro absorption utilizingflow-through diffusion methodology with human cadaver skin andhuman plasma receptor fluid gave 2.5±2.0% dose absorbed,an amount similar to In vivo studies. An additional 6.5±24%was recovered in the skin samples (total of 9%). Skin surfacewash at 24 hr recovered 79.7±2.2% and skin content was6.5±2.4% (total dose accountability of 88.7±4.6%).Thus, isofenphos was available for absorption during the wholedosing period. Neither in vitro absorption nor in vitro evaporationstudies predicted the potential skin evaporation of isofenphos.Published dermal studies in the rat had predicted isofenphosabsorption at 47% of applied dose (12-fold greater than actualin man). Subsequent toxicokinetic modeling predicted possibleconcern with the use of isofenphos. This is an example wherethe choice of the rat produced a nonrelevant absorption prediction.In vivo studies in human volunteers seem more relevant for predictingpercutaneous absorption in man.     

In Vivo and In Vitro Percutaneous Absorption of Cancer Preventive Flavonoid Apigenin in Different Vehicles in Mouse Skin

Purpose. In vivo and in vitro percutaneous absorption of apigenin was investigated in three vehicles previously used in cancer prevention studies to determine the drug delivery properties for optimal chemo-preventive activity. Methods. In vivo percutaneous absorption of apigenin on SENCAR mice was studied with DMSO and acetone/DMSO (A/D, 4:1) vehicle. In vitro percutaneous absorption studies used whole mouse skin, without subcutaneous fat, mounted on Franz diffusion cells with 37°C Dulbecco's phosphate-buffered saline as the receptor fluid. The skin was treated with [G-³H]-apigenin in DMSO, A/D (4:1), or propylene glycol/DMSO (PG/D, 4:1). Results. Apigenin uptake by epidermal cells and distribution in epidermis following in vivo topical treatment in two vehicles was in the order of A/D > DMSO, while apigenin distribution in dermis and subcutaneous fat was not different between DMSO and A/D. Total apigenin absorption in mouse skin in vitro was in the order of A/D > DMSO > PG/D. However, apigenin sub-tissue distribution within epidermis determined by tape-stripping and by determination of apigenin in dermal and epidermal tissue indicated that DMSO delivered more apigenin into viable epidermis than A/D while A/D deposited more apigenin in the stratum corneum. Apigenin absorption in mouse skin with DMSO or A/D showed saturation kinetics while apigenin in PG/D showed very low absorption initially and non-saturated absorption in a period of 6 hr. HPLC-scintillation profiles of in vitro samples showed no evidence of apigenin metabolism in mouse skin. Conclusions. Delivering apigenin into viable epidermis appears to be a necessary property for an apigenin formulation to be effective in skin cancer prevention.     

Percutaneous Absorption of Sunscreens Through Micro-Yucatan Pig Skin In Vitro

Purpose. The objectives of this study were to develop an in vitro model for studying sunscreen permeation in skin, and evaluate the influence of formulation differences. Methods. The sunscreens studied were two of the most widely used agents, octyl methoxycinnamate (OMC) and benzophenone-3. Preparations containing radiolabeled actives were applied to micro-Yucatan pig skin dermatomed to a thickness of 250–300 m as a finite dose in a flow-through diffusion system. At the end of each experiment the amounts removed by washing, retained inside stratum corneum (SC) and penetrated into receptor and viable skin were determined. Results. The two sunscreens reached a peak level in SC within an hour. Benzophenone-3 penetrated skin to a greater extent than OMC. The opposite was true when comparisons of SC retention were made. The ratio of retained to penetrated amount of sunscreens from a hydroalcoholic formulation at the end of 10 hours was higher when the sunscreens were present together than alone. Conclusions. Despite the highly lipophilic nature of sunscreens, particularly OMC, SC is the rate limiting skin layer for penetration. Penetration and SC retention were formulation dependent. The ratio of SC content to the amount penetrated is a useful tool for evaluating sunscreen permeation.     

Effect of Vehicles and Penetration Enhancers on the In Vitro and In Vivo Percutaneous Absorption of Methotrexate and Edatrexate Through Hairless Mouse Skin

Purpose. Low-dose methotrexate (MTX) is approved for the treatment of recalcitrant rheumatoid arthritis (RA). The objective of this study was to determine the effect of vehicles and penetration enhancers on the percutaneous absorption of MTX and its analog edatrexate (EDAM), and develop transdermal (TD) delivery systems of the drugs for the treatment of RA. Methods. From previously published pharmacokinetic parameters with low-dose MTX therapy, and considering a 50 cm² diffusional area, the target steady state in vitroTD flux for MTX was calculated to be 35 g/cm²/hr. Modified Franz diffusion chambers and hairless mouse skin were used for in vitro skin permeation studies. Hairless mice were used for in vivo studies. Delivered amounts of MTX and EDAM were determined by assaying the receiver phase fluid (or blood) with validated reversed phase HPLC methods. Results. Intrinsic partition coefficient of MTX was low (log P = –1.2). Target MTX fluxes of 35 g/cm²/hr were achievable only with 1–15% (v/v) Azone^® in propylene glycol (PG). Flux of EDAM (85 g/cm²/hr) was higher than MTX from an isopropyl alcohol (IPA)—5% (v/v) Azone^® system. Clinically significant steady state in vivo blood concentration of MTX and EDAM was achieved using delivery systems containing 2.5% Azone^® in PG. Area under the drug concentration-time curves (AUC_0–24hr) for MTX were 2379 and 3534 ng*hr/ml from PG—2.5% Azone^® and PG—7.5% Azone^® systems respectively. AUC_0-24hr of EDAM was 6893 ng*hr/ml using a PG—2.5% Azone^® system. Conclusions. Results of this study show the feasibility of using a transdermal delivery system of MTX and EDAM for the treatment of rheumatoid arthritis.     

In Vivo Percutaneous Absorption of Boric Acid, Borax, and Disodium Octaborate Tetrahydrate in Humans Compared to in Vitro Absorption in Human Skin from Infinite and Finite Doses

Literature from the first half of this century report concernfor toxicity from topical use of boric acid, but assessmentof percutaneous absorption has been impaired by lack of analyticalsensitivity. Analytical methods in this study included inductivelycoupled plasma-mass spectrometry which now allows quantitationof percutaneous absorption of ¹⁰B in ¹⁰B-enriched boric acid,borax, and disodium octaborate tetrahydrate (DOT) in biologicalmatrices. This made it possible, in the presence of comparativelylarge natural dietary boron intakes for the in vivo segmentof this study, to quantify the boron passing through skin. Humanvolunteers were dosed with ¹⁰B-enriched boric acid, 5.0%, borax,5.0%, or disodium octaborate tetrahydrate, 10%, in aqueous solutions.Urinalysis, for boron and changes in boron isotope ratios, wasused to measure absorption. Boric acid in vivo percutaneousabsorption was 0.226 (SD = 0.125) mean percentage dose, withflux and permeability constant (K_p) calculated at 0.009 µg/cm²/hand 1.9 x 10^–7 cm/h, respectively. Borax absorption was0.210 (SD = 0.194) mean percentage of dose, with flux; and K_pcalculated at 0.009 µg/cm²/h and 1.8 x 10^–7 cm/h,respectively. DOT absorption was 0.122 (SD = 0.108) mean percentage,with flux and K_p calculated at 0.01 µg/cm²/h and 1.0 x10^–7 cm/h, respectively. Pretreatment with the potentialskin irritant 2% sodium lauryl sulfate had no effect on boronskin absorption. In vitro human skin percentage of doses ofboric acid absorbed were 1.2 for a 0.05% solution, 0.28 fora 0.5% solution, and 0.70 for a 5.0% solution. These absorptionamounts translated into flux values of, respectively, 0.25,0.58, and 14.58     

In Vivo Percutaneous Absorption,Skin Barrier Perturbation,and Irritation from JP-8 Jet Fuel Components

Abstract

JP-8 jet fuel has been reported to cause systemic and dermal toxicities in animal models and humans. There is a great potential for human exposure to JP-8. In this study, we determined percutaneous absorption and dermal toxicity of three components of JP-8 (i.e., xylene, heptane, and hexadecane) in vivo in weanling pigs. In vivo percutaneous absorption results suggest a greater absorption of hexadecane (0.43%) than xylene (0.17%) or heptane (0.14%) of the applied dose after 30 min exposure. Transepidermal water loss (TEWL) provides a robust method for assessing damage to the stratum corneum. Heptane showed greater increase in TEWL than the other two chemicals. No significant (p<0.05) increase in temperature was observed at the chemically treated site than the control site. Heptane showed greater TEWL values and erythema score than other two chemicals (xylene and hexadecane). We did not observe any skin reactions or edema from these chemicals. Erythema was completely resolved after 24 h of the patch removal in case of xylene and hexadecane.     

In Vitro Percutaneous Absorption of Pine Bark Extract (Pycnogenol) in Human Skin

One promising class of antioxidant compounds is polyphenols, contained abundantly in pine bark extract (Pycnogenol®—pine bark extract). This medicinal extract is utilized for its anti‐inflammatory properties. Its pharmacological action in skin depends on the kinetics of its absorption. In this study the dermal bioavailability of pine bark extract was investigated. Viable human skin, adapted on continuously perfused Franz cells, was applied with 5% (w/v) pine bark solution. Samples were taken at 0.5, 1, 2, 4, 6, 8, 10, and 12‐hour intervals and analyzed for detection of pine bark extract constituents by high performance liquid chromatography (HPLC) (reversed phase column, isocratic conditions) coupled with an electrochemical detector (EC). Several constituents of pine bark extract such as gallic acid, protocatechuic acid, catechin, p‐hydroxybenzoic acid, vanillin, and one unidentified constituent were detected. These findings indicate that pine bark extract is readily absorbed by human skin and can be used for topical application.     

In Vitro Absorption of Triethanolamine Through Human Skin

The human skin penetration of triethanolamine (TEA) was measured using in vitro diffusion cell techniques. [¹⁴C]TEA was applied to viable skin in an oil-in-water emulsion containing TEA stearate as an emulsifying agent to simulate cosmetic exposure. The percent of the applied dose of TEA absorbed into the receptor fluid was similar with both 1% and 5% TEA formulations. Absorption of TEA was reduced by lowering the pH of the formulation, presumably due to the increased ionization of TEA. Absorption of TEA into the receptor fluid (1% formulation, pH 7.0) was 0.43% of the applied dose in a 24 h study. Substantial amounts of TEA remained in the skin at the end of the study (9.4% of dose), but only minimal amounts diffused into the receptor fluid when the collection time was extended to 72 h in separate studies. The amount of TEA remaining in skin at the end of the 24 h studies should not be included in estimates of systemic absorption.     

Vehicle Effects on in Vitro Percutaneous Absorption Through Rat and Human Skin

We studied the effects of three vehicles (propylene glycol, octanol and ethyl decanoate) with differing polarity on the in vitro percutaneous absorption of three chemicals (fluazifop-butyl, dimethyl phthalate and fomesafen sodium salt) with a range of physico-chemical properties. Absorption rate measurements were made from high vehicle volume (200 µl/cm²) and low vehicle volume (<10 µLl/cm²) applications. For the lipophilic fluazifop-butyl absorption rate was highest from the more polar vehicle propylene glycol, but this effect was only significant under high-volume conditions. There was a variable vehicle effect on absorption of the intermediate chemical dimethyl phthalate. The largest vehicle effect was seen for the more hydrophilic fomesafen sodium salt where absorption was fastest from the least polar vehicle ethyl decanoate. These results support the hypothesis that the absorption process can in part be predicted from a knowledge of solute solubility. Vehicle effects were greater from high volume applications than from those more comparable to occupational exposure conditions.     

Percutaneous Absorption of Benzoic Acid Across Human Skin. I. In Vitro Experiments and Mathematical Modeling

The percutaneous absorption of benzole acid across human skin in vitro was experimentally and mathematically modeled. Skin partition coefficients were measured over a range of benzoic acid concentrations in both saline and distilled water. The permeation of benzoic acid was measured across isolated stratum corneum, stratum corneum and epidermis, and split-thickness skin. These experiments demonstrated that the stratum corneum was the rate-limiting barrier and that the flux is proportional to the concentration of the undissociated species. The permeation data were analyzed with a comprehensive non-steady-state mathematical model of diffusion across skin. Two adjustable parameters, the effective skin thickness and diffusivity, were fit to the permeation data by nonlinear regression.     

Changes in Skin A.C. Impedance Parameters In Vivo During the Percutaneous Absorption of Local Anesthetics

Pharmaceutical Research -     

Investigations on the Percutaneous Absorption of the Antidepressant Rolipram in Vitro and in Vivo

In vitro experiments using full-thickness human skin showed that it was feasible to deliver therapeutic amounts of the new antidepressant drug rolipram. Simple transdermal devices were constructed, and the presence of isopropyl myristate (IPM) in a silicone adhesive (Dow Corning X7-2920) enhanced the flux across excised human skin. The steady-state fluxes from adhesive mixtures containing 0, 5, and 10% IPM were 3, 5.2, and 6 µg/cm²/hr, respectively. The in vitro experiments were confirmed in a clinical study involving six healthy male volunteers. The formulations tested were an alcoholic solution and adhesive patches containing 5 and 10% IPM. The dose of drug administered was 0.5 mg/cm² and the device size 25 cm². Blood samples were withdrawn over a 24-hr period and analyzed using radioimmunoassay. The topical applications were well tolerated, with only mild or no side effects. A lag time of approximately 2 hr was found for the detection of rolipram in the plasma (detection limit, 50 pg/ml). Interindividual variations both for the peak drug levels and throughout the delivery were quite high but this magnitude of variation has been observed in many other transdermal studies. Plasma levels between 1 and 2 ng/ml were found for all formulations and the AUC_0–30hr was significantly higher for the patch containing 5% IPM.     

In Vivo Percutaneous Absorption of Hydrocortisone: Multiple-Application Dosing in Man

Percutaneous absorption of hydrocortisone was measured in six healthy adult men from whom informed consent had been obtained. The study compared a single topical dose to multiple-topical dose treatments (one vs three applications) on the same day. ¹⁴C-Labeled hydrocortisone in acetone was applied to 2.5 cm² of ventral forearm skin and protected with a nonocclusive polypropylene chamber. The amount of ¹⁴C measured in urine collected over 7 days was used to determine hydrocortisone absorption. The treatments, performed 2 to 3 weeks apart, each utilized adjacent sites on the same individuals. A single dose of 13.33 µg/cm² delivered 0.056 µg/cm² of hydro-cortisone through the skin. When the single dose was tripled to 40 µg/cm², the amount delivered through the skin increased by nearly three times, from 0.056 to 0.140 µg/cm²; the expected delivery was 3 x 0.056 µg/cm² = 0.168 µg/cm². Three serial doses of 13.33 µg/ cm² (total, 40 µg/cm²) were also expected to deliver 0.168 µg/cm² with or without soap and water washing between doses, but the observed amount of hydrocortisone delivered through the skin significantly exceeded our expectations. This indicates that multiple-dosing treatments resulted in a significant increase in bioavailability. It is postulated that increased vehicle application and washing dissolved and mobilized previously dosed hydrocortisone and increased bioavailability.     

Human Cadaver Skin Viability for In Vitro Percutaneous Absorption: Storage and Detrimental Effects of Heat-Separation and Freezing

Purpose. For decades, human cadaver skin has been banked and utilized by hospitals for burn wounds and to study percutaneous absorption and transdermal delivery. Skin storage maintenance and confirmation of skin viability is important for both uses, especially for the absorption process where the in vivo situation is simulated. Methods. Our system uses dermatomed human cadaver skin immediately placed in Eagles MEM-BSS, and refrigerated after donor death, then transfered to the laboratory and placed in Eagles MEM-BSS with 50 g/ml gentamicin at 4°C for storage. Results. Skin viability, determined by anaerobic metabolism where glucose is converted to lactose, was highest (p<0.000) during the 18 hours of the first day after donor death, decreased some 3-fold by day 2 (p<0.000), but then maintained steady-state viability through day 8. Viability then decreased by approximately one-half by day 13. Thus, using the above criteria, human skin will sustain viability for 8 days following donor death in this system. Heat-treated (60°C water for one minute) and heat-separated epidermis and dermis lose viability. Conclusions. Human skin viability can be maintained for absorption studies. It is recommended that this system be used, and that heat-separation and skin freezing not be used, in absorption studies where skin viability and metabolism might be contributing factors to the study.     

A Mechanistic Bayesian Inferential Workflow for Estimation of In Vivo Skin Permeation from In Vitro Measurements

Computational models can play an integral role in the chemical risk assessment of dermatological products. However, a limitation on the ability of mathematical models to extrapolate from in vitro measurements to in human predictions arises from context-dependence: modeling assumptions made in one setting may not carry over to another scenario. Mechanistic models of dermal absorption relate the skin penetration kinetics of permeants to their partitioning and diffusion across elementary sub-compartments of the skin. This endows them with a flexibility through which specific model components can be adjusted to better reflect dermal absorption in contexts that differ from the in vitro setting, while keeping fixed any context-invariant parameters that remain unchanged in the two scenarios. This paper presents a workflow for predicting in vivo dermal absorption by integrating a mechanistic model of skin penetration with in vitro permeation test (IVPT) measurements. A Bayesian approach is adopted to infer a joint posterior distribution of context-invariant model parameters. By populating the model with samples of context-invariant parameters from this distribution and adjusting context-dependent parameters to suit the in vivo setting, simulations of the model yield estimates of the likely range of in vivo dermal absorption given the IVPT data. This workflow is applied to five compounds previously tested in vivo. In each case, the range of in vivo predictions encompassed the range observed experimentally. These studies demonstrate that the proposed workflow enables the derivation of mechanistically derived upper bounds on dermal absorption for the purposes of chemical risk assessment.     

Percutaneous Absorption of Pentachlorophenol from Soil

Pentachlorophenol (PCP) is one of the most heavily used pesticides.About 80% of PCP is used for wood preservation, whereas theremainder is used as an herbicide, fungicide, and disinfectant.PCP is a probable human carcinogen, based on animal studies.Illness and death have been reported where PCP is in directcontact with skin. PCP is the most ubiquitous compound foundwhen the general population is screened for pesticide residue.PCP is found in soil as well as other environmental sources.Our objective was to determine the skin bioavailability of PCPfrom soil and from the control vehicle acetone. In vivo in theRhesus monkey, percutaneous absorption of PCP was 24.4±6.4%of applied dose from soil and 29.2±5.8% of applied dosefrom acetone vehicle for a 24-hr exposure period. This amountof absorption makes PCP one of the more extensively absorbedcompounds to date. Additionally, the ¹⁴C half-life was 4.5 daysfollowing both intravenous and skin administration of [¹⁴C]PCP.These data suggest high bioavailability and an extended biologicalinteraction period with the long half-life. In vitro percutaneousabsorption with human cadaver skin and human plasma receptorfluid underestimated the in vivo absorption. Receptor fluidaccumulation was 0.6±0.09% and 1.5±0.2% for twoskin sources for PCP in acetone vehicle and 0.01±0.00%and 0.00±0.08% for two skin sources with soil vehicle.Skin content after skin surface wash ranged from 2.6 to 3.7%for acetone vehicle and 0.07–0.11% for soil vehicle. Overallaccountability for in vitro dose ranged from 81 to 96%.     

干扰素α脂质体凝胶剂的透皮吸收作用研究

目的探讨基因重组人干扰素α-2b脂质体凝胶剂经家兔皮肤给药后的吸收作用。方法给家兔皮肤分别涂抹干扰素α-2b脂质体凝胶（试验组）和干扰素α-2b凝胶（对照组），用双抗体夹心酶联免疫吸附法（ELISA法）检测干扰素含量，测定给药后不同时间内血浆中干扰素含量及24h后皮肤层中的干扰素含量。结果与对照组比较，试验组干扰素在家兔皮肤层中有较多的滞留量（P〈0．01）；两组家兔不同时间血浆中干扰素的含量均极微少（P〉0．05）。结论脂质体作为干扰素皮肤局部给药的载体，能够提高干扰素在皮肤中的含量，但并不增加进入血液循环的干扰素量。     

In Vitro Cutaneous and Percutaneous Delivery and in Vivo Efficacy of Tetracaine from Liposomal and Conventional Vehicles

The aim of this study was to quantitate drug delivery and correlate drug concentration in the skin with anesthetic effect. The rate of delivery of radiolabelled tetracaine from two different liposome formulas (1F2 and 23C2) and two conventional dosage forms (PEG Ointment USP and Glaxal^® base) was investigated in Flow-thru diffusion cells using human breast skin from mammoplasty. The results indicated a 1.5 and 4 times higher concentration of tetracaine within the skin when the liposomal formula (1F2) was used, compared to tetracaine in Glaxal^® base and PEG Ointment USP, respectively. The amount of drug delivered into the skin in 24 h from the liposomal formula was 5.3% of total applied whereas from Glaxal^® base it was 3.3% and from PEG Ointment base it was 1.2%. The amount of liposomal (1F2) phospholipids in the skin after 24h was 68.3 µg/cm² (0.2% of total applied). The steady state flux of tetracaine from liposomes (1F2) was 16.06 µg/cm²/h with a lag time of 3.1h, from Glaxal^® base 10.24 µg/cm/h with a lag time of 11.2h and from PEG Ointment it was 5.70 µg/cm²/h with a lag time of 9.0h. The second liposome formula (23C2) showed similar flux and permeability coefficient than the Glaxal^® base, however the lag time was about half. The results indicated that optimized liposome formulation is necessary to achieve maximum drug delivery. The concentration of drug within the skin and the flux measured in vitro showed correlation with in vivo efficacy. The in vivo data showed that liposomal (1F2) tetracaine produced the deepest anesthesia with shortest onset in volunteers, followed by Glaxal^® base, liposome formula 23C2 while tetracaine in PEG Ointment had a lack of effect.     

Percutaneous Absorption of Benzoic Acid Across Human Skin. II. Prediction of an in Vivo,Skin-Flap System Using in Vitro Parameters

The possibility of predicting the behavior of in vivo systems based on physical and chemical parameters determined by in vitro experiments is examined using benzoic acid. The physical and chemical parameters governing percutaneous absorption of benzoic acid—permeability, partition coefficient, and skin thickness—were determined by in vitro experiments as described in Ref. 1. These parameters were used, in combination with benzoic acid elimination kinetics, to predict the results of in vivo experiments using a comprehensive mathematical model. The in vivo system consists of a congenitally athymic (nude) rat with a surgically constructed human skin sandwich (HSSF) flap on which a donor cell is placed. To apply the in vitro parameters to an in vivo system requires a suitable pharmacokinetic model describing distribution and elimination for benzoic acid in the nude rat. Blood concentrations of benzoic acid following a bolus intravenous injection are closely described by a two-compartment open pharmacokinetic model with elimination occurring from only one compartment. The mathematical model of the rat-donor cell system combines this two-compartment model of the rat with a percutaneous absorption model to provide useful estimates of the measured in vivo blood levels. Comparisons of predicted and measured results suggest that the parameters determined by in vitro experimentation can be used to predict the behavior of complex in vivo systems, if a suitable mathematical model is available.