Percutaneous uptake rate of 2-butoxyethanol in the guinea pig.

P. Percutaneousuptake rate of 2-butoxyethanolin theguineapig. Scand J Work Environ Health 12(1986) 499-503. The percutaneousabsorption rate and elimination kinetics of 2-butoxyethanol (ethylene glycol monobutyl ether) wereestimated in the guinea pig. An intravenous bolus dose of 42or 92J.lmol/kg of body weightwas administeredinto the jugular vein of 10 pentobarbital anesthetizedanimals. Epicutaneous administration of 2-butoxyethanol followed 2.5 h later in one or two sealed glassringson theclippedbackof theanimal. Arterialbloodsamples wereobtainedand then analyzed for 2-butoxyethanolbygas chromatography.Following the intravenousdose, the apparent total clearance and mean residence time of 2-butoxyethanolwere calculatedto be 128 ml . min- t • kg- 1 (SD 30 0J0) and 4.7 min (SD 30 OJo), respectively. During the latter part of the 2-h skin exposure, the concentration of 2-butoxyethanol in the blood appeared to level off at an averageconcentration of 21J.lmolll (SD45 0,70). The absorption rate through the skin was estimated to be 0.25 (range 0.05-0.46) J.lmol . min- t • cm- 2 (SD 49 0J0). The skin uptake rate in the guinea pig was extrapolated to man for a comparison of the percutaneous absorption of liquid solvent with respiratory uptake of solvent vapor. The extrapolation indicated a risk of acute adverse effects when large areas of the skin arc exposed to 2-butoxyethanol.

Glycol ethers are commonly used in consumer products , as well as in industrial processes, because of their excellent solvent and evaporation characteristics (19). Among the glycol ethers, the 2-alkoxyethanols (ethylene glycol monoalkyl ethers), and more specifically 2-ethoxyethanol (along with its acetate) and 2-butoxyethanol (ethylene glycol rnonobutyl ether) , are the most common (13). Recent interest has focused on the reproductive hazards of some of the glycol ethers (9,10). In general these solvents ha ve rather low vapor pre ssures, compared to those of man y other organic solvents , but they may readily penetrate the skin. Such penetration has been shown indirectly from the recording of various toxic effects , eg, the median lethal dose, after skin application of several glycol ethers (10). Two cases of transcutaneous 2-methoxyethanol (ethylene glycol monomethyl ether) poisoning have been reported (16). However, quantitative data on skin uptake rates in vivo are scarce. The aim of the present investigation was to estimate quantitatively the percutaneous uptake rate of 2-butoxyethanol in the guinea pig.

Materials and methods
All chemicals were of analytical grade and purchased from Merck (Darmstadt, Federal Republic of Germany) unless otherwise stated. They were used without further The animals were anesthetized with pentobarbital (60 mg/ml mebumalnatrium, ACO , Solna, Sweden) intraperitoneally with a dose of 0.6 mg/kg of body weight and then kept under anesthesia throughout the experiment. A polyethylene catheter (PE50) was implanted in the left jugular vein. This catheter was used for the bolus administration of 2-butoxyethanol. A similar catheter was inserted into the right carotid artery for blood sampling. Flushing the catheters with 130 IV (0.5 ml) of heparin (Vitrum, SoIna, Sweden) in the beginning of the experiment prevented clotting. After an intravenous bolus dose (approximately 42 or 92 JLmoIlkg of body weight) of 2-butoxyethanol (10.7 mg/mi 2-butoxyethanol in 0.3 M sodium chloride), arterial blood was sampled after 5, 10, 20, 30, 40, 50,60,75,90, 105, and 120 min. Thereafter the hair on the back of the animal was clipped, and one or two glass rings with an exposure area of 3.14 emeach were glued with cyanoacrylate (cyanolit 201, Bennetter, Stockholm, Sweden) to the skin, as previously described (11). After the glue had dried, a new "blank" blood sa mple was taken. At 150 min after the intravenous administration, each ring was filled with 1 ml of undiluted 2-butoxyethanol and sealed with a cover glass. Arterial blood was sampled at the same time intervals as after the intravenous dose. On each sampling occasion, two arteri3.Iblood samples of 100 JLI were collected in capillary tubes. Heptanol (99 %, Sigma, St Louis, Montana) was added to the samples as an internal standard. Toluene extracts of the samples were derivatized with pentafluorobenzoyl chloride (Pierce, Beijerland, Holland) and analyzed by gas chromatography with electron capture detection as described elsewhere (12).
At steady state, the zero order of the intravenous (iv) infusion rate of a substance ma y be obtained as the product of its blood con centration (Css,b) and its total blood clearance (CL b ) . If skin metabolism is negligible, the rate (R) of substance flow through the skin into the systemic circulation is equal to the intra venous infusion rate . Accordingly, the percutaneous uptake rate of 2-butoxyethanol was calculated as in the present study. The average blood concentration after 90-120 min of skin exposure was assigned to Css,b' The clearance was calculated as: The area under the blood concentration time curve following the intravenous dose (AUCiv(o-OO» was obtained by the trapezoidal method (17). The quotient of the blood concentration at 60 min and the elimination rate constant was added as a residual term. The elimination rate constant was calculated as the average slope of a log-linear plot of concentration versus time 60-120 min after the intravenous administration in all experiments (see figure 2 in the Results and Discussion section.) Another approach to obtain the uptake rate is based on mass-balance. The amount (a) of substance which has penetrated into the body at any given time (t) may be expressed as the sum of the amount of solvent present in the body and that eliminated: The amount of substance in the body at time t may be expressed as: where C b is the measured concentration in blood and V ss is the apparent steady-state volume of distribution. The V ss of 2-butoxyethanol was assumed to be 56 % of the body weight. This was the average value obtained in previously performed experiments in which men were exposed to 20 ppm of 2-butoxyethanol (12).
The amount of substance eliminated at time t may be expressed as: The clearance value was obtained as has already been described, and the area under the blood concentration time curve (AUCskin(O-t) was calculated by the trapezoidal method. Combining the equations yields:

500
The " invasion curve, " ie, the plot of a body versus time, approached a straight line during the latter half of the skin exposure. (See figure 3 in th e Results and Discussion section). The percutaneous uptake rate (R skin) was obtained as the slope of the line fitted by linear regression to the value s on a OOd after 75-120 min of skin expo sure. The time lag of the skin penetration was determined as the intercept between the regression line and the x-axis.

Results and discussion
The concentration of 2-butoxyethanol in blood declined rapidly following an intravenous bolus dose (figures 1 and 2). Estimates of total clearance and mean residence time for the 10 individual experiments are listed in table I. The average clearance value of 128 mI· min-l . kg-I of body weight corresponds to 2.7 ml . mirr" . g-l of liver based on a liver weight of 47.2 g/kg of body weight in the guinea pig (1). The corresponding value for man has been shown to be 0.8 ml . mirr" . g-I of liver (12), while the intrinsic clearance in perfused rat liver was 2.0 ml . mirr"! . g-l (14). The relatively high clearance observed in the present study may be partly caused by pentobarbital (15). However, as long as the clearance of 2-butoxyethanol remains unchanged during an experiment, any interaction from pentobarbital will not affect the calculation of the skin uptake rate.
The time lag of 21-60 min observed in the present study is in accordance with that reported for the penetration of 2-butoxyethanol through human skin in vitro (7). The tissue binding of 2-butoxyethanol appears to be low in that the olive oil: wa ter and the  a Total blood cle arance , calculated as GL b = do se/AUG, where AUG is the area under the blood concentration time curve of 2·butoxyet hanol foll owin g a known ir], t ravenous dose . b Mean reside nc e tim e, calc ul ated as t = Vss/G~, on the assumption tha t the st eady-state volume of dis t ribution (V ss) is 56 % of the bod y weight, as in man (12). c Steady-s tate concentrat ion in blood, calc ulated as the average concent ration at 90 , 105, and 120 min . d Skin up take rat e, calc ulated as R sk1 n = G SS • b X GL b • e Skin upt ake rate , obtained as the s lope of th e st raight line fit ted to t he " invasion curve " (figure 3). f Time lag of the sk in penet ration, obtained as the intercept with t he x-ax is of the straight line fitted to the " invasio n curve" (figure 3). 9 Experi ment 1 was exc luded, as a d ifferent dose and expos ure area were used . bovine erythrocytes: water partition ratios are close to or less than unity (Johanson, unpublished observations) and in that the volume of distribution for 2-butoxyethanol is of the same order of magnitude as that of total body water (12). These pieces of information support the assumption that a constant absorption rate was virtually reached within the 120 min of skin exposure in the present study.
One may suspect that pentobarbital, used as the anesthetic in the present experiments, caused altered blood flow through the skin. It is however generally accepted that the stratum corneum is the main diffusive barrier (2,18) and therefore that, with the exception of very small and diffusible substances, such as inert gases, the absorption rate is not affected by the blood flow.
The calculations presented in this study are valid only when the kinetics of 2-butoxyethanol are linear, or first order. In experiments performed with perfused rat liver, the elimination kinetics of 2-butoxyethanol were Michaelis-Menten like, the estimated values on the apparent Michaelis constant ranging from 0.32 to 0.70 mmol/l (14). The blood concentrations of 2butoxyethanol were approximately 10 to 20 times lower in the present study. Linear kinetics are thus indicated under the present conditions, when extrapolating from rat to guinea pig.
It is of interest to relate the obtained results to the pulmonary uptake rate in man and to the occupational exposure limit. The absolute and relative respiratory uptake in men exposed to 2~butoxyethanol at 20 ppm (the current Swedish exposure limit) during light physical exercise was approximately 10 p.mol/min and 60 0J0, respectively (12). The corresponding area of skin exposure to the liquid solvent would be approximately 40 em' if it is assumed that the absorption rate through human skin equals the average value obtained in the guinea pig in the present study. This assumption is conservative in that it probably overestimates the absorption rate in man (2). Conversely, dipping both hands in 2-butoxyethanol [area 740 cm' (5)] would result in an uptake rate of 185 p.mol/min, which corresponds to a theoretical inhalation exposure at 370 ppm during light physical exercise. At rest, and on the assumption of a respiratory ventilation of 6 l/min and a relative uptake of 60 0J0, the corresponding exposure concentration would be above the saturation level of 1 000 ppm. For comparison, the 4-h median lethal concentration is 450-486 ppm for rats (6) and the corresponding 7-h value for mice is 700 ppm (20). Humans experimentally exposed to 98 ppm, 113 ppm, and 195 ppm of 2-butoxyethanol vapor have been shown to experience discomfort and, in some cases, increased erythrocyte osmotic fragility (3).
In conclusion, skin contact with chemical products containing 2-butoxyethanol should be considered in industrial hygiene. Furthermore, exposure of large areas of the skin to 2-butoxyethanol may cause acute adverse effects in man.