An automated in vitro dermal absorption procedure: II. Comparative in vivo and in vitro dermal absorption of the herbicide fenoxaprop-ethyl (HOE 33171) in rats

Exposure of frogs and tadpoles to chiral herbicide fenoxaprop-ethyl

Pesticides have long been considered to a risk factor of amphibian population declines. The bioaccumulation and elimination of fenoxaprop-ethyl (FE) in frogs and tadpoles were studied and the main metabolites fenoxaprop (FA) and 6-chloro-2,3-dihydrobenzoxazol-2-one (CDHB) were monitored. The acute toxicity and genotoxicity of the enantiomers to tadpoles was also studied. After both oral administration and aqueous solution exposure, FE was not found in frogs, while FA was formed and accumulated in liver, kidney, brain, eggs, skin, thigh muscle and blood with preferential accumulation of R-FA. The presence of FA in frog eggs suggested maternal transfer in females and potential impacts to offsprings. The elimination of FA in frog tissues was also enantioselective with a preferential metabolism of R-FA (kidney) or S-FA (liver, eggs, skin, muscle and whole blood). FE and FA were hardly detectable in tadpoles after aqueous solution exposure, while CDHB was accumulated and eliminated as first-order kinetics with half-life of 37.1 h. Mortality of tadpoles and micronucleus rate in peripheral blood erythrocytes of tadpoles were used to evaluate the enantioselective acute toxicity and genotoxicity. Only CDHB induced significant acute toxicity to tadpole with 96-h LC₅₀ value of 30.4 μg/mL, and rac-FA, S-FA and CDHB showed genotoxicity.

The application and limitations of mathematical modelling in the prediction of permeability across mammalian skin and polydimethylsiloxane membranes

OBJECTIVES

Predicting the rate of percutaneous absorption of a drug is an important issue with the increasing use of the skin as a means of moderating and controlling drug delivery. One key feature of this problem domain is that human skin permeability (as K(p)) has been shown to be inherently non-linear when mathematically related to the physicochemical parameters of penetrants. As such, the aims of this study were to apply and evaluate Gaussian process (GP) regression methods to datasets for membranes other than human skin, and to explore how the nature of the dataset may influence its analysis.

METHODS

Permeability data for absorption across rodent and pig skin, and artificial membranes (polydimethylsiloxane, PDMS, i.e. Silastic) membranes was collected from the literature. Two quantitative structure-permeability relationship (QSPR) models were used to compare with the GP models. Further performance metrics were computed in terms of all predictions, and a range of covariance functions were examined: the squared exponential (SE), neural network (NNone) and rational quadratic (QR) covariance functions, along with two simple cases of Matern covariance function (Matern3 and Matern5) where the polynomial order is set to 1 and 2, respectively. As measures of performance, the correlation coefficient (CORR), negative log estimated predictive density (NLL, or negative log loss) and mean squared error (MSE) were employed.

KEY FINDINGS

The results demonstrated that GP models with different covariance functions outperform QSPR models for human, pig and rodent datasets. For the artificial membranes, GPs perform better in one instance, and give similar results in other experiments (where different covariance parameters produce similar results). In some cases, the GP predictions for some of the artificial membrane dataset are poorly correlated, suggesting that the physicochemical parameters employed in this study might not be appropriate for developing models that represent this membrane.

CONCLUSIONS

While the results of this study indicate that permeation across rodent (mouse and rat) and pig skin is, in a statistical sense, similar, and that the artificial membranes are poor replacements of human or animal skin, the overriding issue raised in this study is the nature of the dataset and how it can influence the results, and subsequent interpretation, of any model produced for particular membranes. The size of the datasets, in both absolute and comparative senses, appears to influence model quality. Ideally, to generate viable cross-comparisons the datasets for different mammalian membranes should, wherever possible, exhibit as much commonality as possible.

Skin decontamination: Importance of the wash-in effect

That skin washing/decontamination may increase percutaneous absorption is commonly referred to as the 'wash-in' (W-I) effect. This article traces the development of the W-I effect both in vivo and more recently in vitro. The mechanism(s) responsible for this effect are examined particularly in relation to the laboratory method used in vitro. The importance of the W-I effect is presented and it is recommended that caution be practiced when skin is washed as the W-I effect may increase both local cutaneous and general systemic toxicity. Experimental data on a wide variety of chemicals are urgently needed.

In vitro dermal penetration study of carbofuran, carbosulfan, and furathiocarb

In this study, the dermal penetration rate of carbofuran, carbosulfan, and furathiocarb has been measured with rat abdominal skin using the static diffusion cell. The technical grades of three compounds were applied at different doses on skin surface mounted in static diffusion cell and incubated at 32 degrees C for 48 h with shaking. The same procedures were carried out with furathiocarb EC (emulsifiable concentrate) and WP (wettable powder). At regular intervals, the receptor fluid in cell was sampled and analyzed by HPLC. Only carbofuran was found in carbosulfan- or furathiocarb-treated samples, suggesting they converted into carbofuran while passing through the skin layer. The quantity of insecticide penetrating skin increased with time and applied dose. The skin penetration rate increased with the water solubility of insecticides. The dermal penetration rates of carbofuran, furathiocarb, and carbosulfan were determined as 1.05 micro g/cm(2) per h ( r(2)=0.991), 0.46 micro g/cm(2) per h ( r(2)=0.984) and 0.14 micro g/cm(2) per h ( r(2)=0.967), respectively. There was no significant difference in rate of skin penetration between furathiocarb EC (1.42 micro g/cm(2) per h, r(2)=0.988) and WP (1.35 micro g/cm(2) per h, r(2)=0.982), while furathiocarb technical grade showed a lower skin penetration rate. In vitro models may be used to predict percutaneous absorption and are useful in selecting safer formulations for field application of pesticide.

Automated In Vitro Dermal Absorption (AIVDA): predicting skin permeation of atrazine with finite and infinite (swimming/bathing) exposure models

The Automated In Vitro Dermal Absorption (AIVDA) HPLC method developed in our laboratory for finite dose tests was developed further for infinite dose swimming/bathing exposure tests and for monitoring skin viability during the permeation assay. Skin absorption of the herbicide atrazine was tested under both finite and infinite dose conditions and the data compared to that obtained using the Bronaugh flow-through cell. Confirmation that skin viability was maintained throughout the AIVDA 24-hr infinite dose tests without phenol was obtained using a novel Radio-HPLC method that was developed to monitor glycolytic 14C-lactic acid formation in 14C-glucose spiked receiver solution. Advantages of the AIVDA method (e.g. rapid, sensitive, versatile, cost-effective analysis) together with its disadvantages (e.g. need for manual dexterity, lack of receiver albumin, compound retention time, limit of sampling interval) are discussed.

Effect of environmental conditions on the penetration of benzene through human skin

The in vitro penetration of [14C]benzene through freshly prepared human skin was examined under a variety of skin conditions associated with swimming and bathing. The experimental system utilized a recirculating donor solution and a flow-through receiver solution, and was modified to accommodate the analysis of volatiles. The permeability coefficient of 0.14 cm/h under standard conditions at 26 degrees C was found to increase to 0.26 cm/h at 50 degrees C and decrease to 0.10 cm/h at 15 degrees C. Storage of the skin at- 20 degrees C did not affect the penetration of benzene. Application of baby oil, moisturizer, or insect repellant to the skin before exposure under standard conditions did not affect the flux of benzene, but a significant increase was observed when the skin was pretreated with sunscreen (permeability coefficient 0.24 cm/h). These results suggest that risk assessment or exposure modeling for benzene and other environmental contaminants should account for appropriate changes in the environmental conditions when considering the dermal route of exposure.

Automated in Vitro Dermal Absorption (AIVDA): A New in Vitro Method for Investigating Transdermal Flux

A new in vitro method for investigating transdermal flux is described. The method, called Automated In Vitro Dermal Absorption (AIVDA), employed small autosampler vial inserts (Moody cells) to hold human skin (0.07cm2) in a standard autosampler carousel. HPLC was used to monitor the permeation rate of test compounds. Analysis was fully automated and remotely monitored and controlled via a modem. A modified Hank's balanced salt solution was used to minimise HPLC background interference. Permeation assays conducted with Deep Woods®, a commercial N,N-diethyl-m-toluamide (DEET)-based mosquito repellent, showed no significant difference in DEET absorption between fresh and frozen human abdominal skin (Student's t test [p ≤ 0.05] 56 ± 9.1% [n = 4] and 48 ± 6.3% [n = 4], respectively). No significant difference was obtained for DEET absorption in human skin pretreated with barrier cream. Preliminary tests conducted to validate AIVDA versus the standard Bronaugh procedure demonstrated excellent agreement for DEET permeation in rat skin (62 ± 12.8% [n = 4] Moody cells; 59 ± 8.1% [n = 3] Bronaugh cells). AIVDA has many advantages over other in vitro permeation tests, including automation, higher analytical sensitivity, rapid sample processing and the ability to use small (5mm outside diameter) skin specimens.

Fate of fluazifop butyl in rat and human skin in vitro

Enzyme mediated hydrolysis of fluazifop butyl has been measured with rat and human skin post-mitochondrial fractions. Rat skin had a ten times greater capacity to metabolise fluazifop butyl than human skin, but the enzyme affinities were similar. The post-mitochondrial fraction metabolism was compared to that seen during absorption in a flow through diffusion cell with viable skin. Limited hydrolysis of absorbed fluazifop butyl was seen in rat skin, but increased two fold if the stratum corneum was removed. The stratum corneum was found to retain fluazifop butyl. When the skin was pre-incubated with the esterase inhibitor bis (p-nitrophenol) phosphate (BNPP), reduced metabolism was seen. No metabolism of fluazifop butyl was seen in human skin during absorption. Retention of the compound by the stratum corneum is postulated to restrict the accessibility of the compound to the enzyme site, thus influencing the observed metabolism during the absorption process.