Dermal Absorption and Pharmacokinetics of ... - ACS Publications

Chapter 14

Dermal Absorption and Pharmacokinetics of Pesticides in Rats 1

2

3

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P. V. Shah , H. L. Fisher , M . R. Sumler , and L. L. Hall

2

1Toxic Effects Branch, Health and Environmental Review Division (TS-796), Office of Toxic Substances, U.S. Environmental Protection Agency, Washington, DC 20460 Cellular Biology Branch, Health Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711 Northrop Environmental Sciences, Northrop Corporation, Research Triangle Park, NC 27709 2

3

Dermal a b s o r p t i o n and d i s p o s i t i o n o f carbaryl, c a r b o f u r a n , c h l o r d e c o n e , h e x a c h l o r o b i p h e n y l and d i n o s e b were s t u d i e d in female F i s c h e r 344 rats. Acetone s o l u t i o n o f t h e [14C] l a b e l e d pesticide was a p p l i e d to a p r e v i o u s l y c l i p p e d m i d - d o r s a l r e g i o n o f t h e back. At v a r i o u s t i m e i n t e r v a l s , a n i m a l s were killed and s e l e c t e d organs were a n a l y z e d f o r radioactive [14C] c o n t e n t . A physiological compartmental model was f i t t e d t o t h e s k i n p e n e t r a t i o n , tissue distribution, and e x c r e t i o n d a t a . The e q u i l i b r i u m t i s s u e t o b l o o d ratios and excretion r a t e c o n s t a n t s and s k i n a b s o r p t i o n rate were d e t e r m i n e d f r o m t h e model. In many c a s e s , differences in kinetics and retention were found between young and a d u l t rats. The dermal p e n e t r a t i o n function was s i n g l e o r biexponential, requiring a one o r two compartmental model f o r s i m u l a t i n g t h e s k i n penetration process. More t h a n one b i l l i o n pounds o f p e s t i c i d e s a r e b e i n g u s e d each year i n the U n i t e d S t a t e s alone t o i n c r e a s e the p r o d u c t i v i t y of a g r i c u l t u r a l l a n d ( 1 ) . G r e a t e r r e l i a n c e on use o f p e s t i c i d e s appears i n e v i t a b l e as t h e demand f o r f o o d p r o d u c t i o n i n c r e a s e s w i t h i n c r e a s i n g p o p u l a t i o n . O c c u p a t i o n a l exposure t o p e s t i c i d e s o c c u r s v i a o r a l , d e r m a l , and i n h a l a t i o n r o u t e s . A l t h o u g h t h e r a t e o f a b s o r p t i o n may be h i g h e r by pulmonary and o r a l r o u t e s , t h e t o t a l amount absorbed a f t e r s k i n c o n t a c t may be much g r e a t e r d u r i n g o c c u p a t i o n a l exposure t o p e s t i c i d e s (2,3). Knowledge of dermal a b s o r p t i o n ( r a t e s and e x t e n t ) o f p e s t i c i d e s i s v e r y i m p o r t a n t i n r i s k assessment and e s t a b l i s h m e n t 0097-6156/89/0382-0169$06.00/0 ° 1989 American Chemical Society

Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

170

BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE

of r e e n t r y standards f o r f i e l d workers. With r e g a r d t o o c c u p a t i o n a l e x p o s u r e t o t o x i c a n t s i n g e n e r a l , r i s k assessment has u s u a l l y been b a s e d on s t u d i e s i n a d u l t s . S k i n a b s o r p t i o n depends on a number o f f a c t o r s , i n c l u d i n g t h e c h e m i c a l and p h y s i c a l n a t u r e o f t h e p e n e t r a n t and t y p e o f s k i n . The l i m i t e d l i t e r a t u r e on t h e e f f e c t s o f age on s k i n p e n e t r a t i o n has been r e c e n t l y summarized (4)» I t a p p e a r s t h a t t h e e f f e c t o f age on p e r c u t a n e o u s a b s o r p t i o n v a r i e s w i t h s p e c i e s , compound, and method o f assessment. I n c r e a s e d s k i n p e r m e a b i l i t y o f n e o s y n e p h r i n e (as measured by t h e b l a n c h i n g r e s p o n s e ) i n p r e m a t u r e i n f a n t s compared t o f u l l t e r m i n f a n t s has been documented (50. No d i f f e r e n c e s i n p e r c u t a n e o u s a b s o r p t i o n o f t e s t o s t e r o n e between t h e newborn and a d u l t r h e s u s monkey was n o t e d {6J, w h i l e more r a p i d l o s s o f t o p i c a l l y a p p l i e d t r i a d i m e f o n from t h e s k i n o f young Sprague-Dawley r a t s has been r e p o r t e d (7). A recent c o m p a r a t i v e s t u d y on p e r c u t a n e o u s a b s o r p t i o n o f p e s t i c i d e s i n young and a d u l t F i s c h e r 344 r a t s r e p o r t e d age-dependent p e r c u t a n e o u s a b s o r p t i o n f o r 11 o u t o f 14 p e s t i c i d e s s t u d i e d (8) • We p r e s e n t h e r e t h e c o m p a r a t i v e p e r c u t a n e o u s a b s o r p t i o n and d i s p o s i t i o n of c a r b a r y l , carbofuran, chlordecone, hexachlorob i p h e n y l , and d i n o s e b i n young and a d u l t F i s c h e r 344 r a t s u t i l i z i n g i d e n t i c a l methodology. A l s o , comparison o f t h e r e s u l t s o f i n v i v o and two i n v i t r o methods f o r p e r c u t a n e o u s a b s o r p t i o n measurements a r e p r e s e n t e d . The u t i l i t y o f t h e p h y s i o l o g i c a l compartmental model i n s i m u l a t i n g dermal a b s o r p t i o n and disposition i s discussed. EXPERIMENTAL APPROACH; CHEMICALS The c h e m i c a l s s t u d i e d , t h e i r r e s p e c t i v e s p e c i f i c a c t i v i t i e s , r a d i o l a b e l i n g p o s i t i o n , and s o u r c e s a r e l i s t e d i n T a b l e I . The r a d i o c h e m i c a l p u r i t y was g r e a t e r t h a n 98% f o r a l l pesticides studied. U n l a b e l e d c h e m i c a l s were s u p p l i e d by t h e P e s t i c i d e and I n d u s t r i a l C h e m i c a l R e p o s i t o r y (U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency, R e s e a r c h T r i a n g l e Park, NC). Minimum E s s e n t i a l Medium (MEM) E a g l e Formula w i t h E a r l ' s s a l t s a n d L - g l u t a m i n e and w i t h o u t p h e n o l r e d and sodium c a r b o n a t e was p u r c h a s e d from G i b c o L a b o r a t o r i e s , C h a g r i n F a l l s , OH. HEPES ( N- 2h y d r o x y e t h y l p i p e r a z i n e - N - 2 - e t h a n e s u l f o n i c a c i d ) , sodium p y r u v a t e , L- s e r i n e , L - a s p a r t i c a c i d , and g e n t a m i c i n were p u r c h a s e d from G i b c o , Grand I s l a n d , NY. Anhydrous sodium c a r b o n a t e , sodium h y d r o x i d e , e t h y l e t h e r ( a n h y d r o u s ) , and a c e t o n e ( r e a g e n t grade) were p u r c h a s e d from F i s h e r S c i e n t i f i c Co., F a i r l a w n , NJ. IN VIVO APPLICATION Female F i s c h e r 344 r a t s o f 33 (young) and 82 ( a d u l t ) days o f age were u t i l i z e d f o r d e r m a l a b s o r p t i o n e x p e r i m e n t s . Three r a t s were u s e d f o r each t i m e p o i n t . Randomized o f f s p r i n g f r o m t i m e d p r e g n a n t r a t s were u s e d i n o r d e r t o have b e t t e r c o n t r o l o v e r t h e age and w e i g h t o f t h e a n i m a l s u s e d i n t h e s e e x p e r i m e n t s . The i n v i v o e x p e r i m e n t a l t e c h n i q u e s employed i n t h i s s t u d y have been d e s c r i b e d i n d e t a i l by Shah e t a l . ( 8 ) .


Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988. position

C

C

,

,

Klng(U)

Ring(U)

,

1

4

C C £ / ;

]

Benzofuranyl r i n g

2 , 4 , 5 , 2 , 4 , 5 - [

1 4

1 4

^^C(U)

1 4

Naphthyl[1- C]

Radiolabeling

2,4,5,2*,4',5'-Hexachlorobiphenyl.

9.2

4.29

Dinoseb

1

3.2

Chlordecone

PCB

39.4

5.8

Carbofuran

Carbaryl

Specific activity (mCi/mM)

New E n g l a n d N u c l e a r , B o s t o n , MA

P a t h f i n d e r Lab., I n c . , S t . L o u i s , MO

Midwest R e s e a r c h I n s t i t u t e , Kansas C i t y , MO

FMC Corp., P r i n c e t o n , NJ

Amersham, A r l i n g t o n H e i g h t s , I L

Source

P e s t i c i d e s T e s t e d i n P r e s e n t S t u d y , w i t h S p e c i f i c A c t i v i t y , R a d i o l a b e l i n g P o s i t i o n , and Source

Common name

TABLE I .

172


B r i e f l y , t h e a n i m a l ' s h a i r was c l i p p e d 24 h p r i o r t o dermal a p p l i c a t i o n w i t h O s t e r a n i m a l h a i r c l i p p e r s ( O s t e r , Milwaukee, WI). D o s i n g s o l u t i o n s (0.1 mL young and 0.2 mL a d u l t ) c o n t a i n i n g r a d i o a c t i v i t y and a p p r o p r i a t e dose (285 nmol/cm ) i n a c e t o n e were a p p l i e d t o t h e p r e v i o u s l y c l i p p e d mid-back r e g i o n o f t h e r a t . The t r e a t e d a r e a , w h i c h r e p r e s e n t s a p p r o x i m a t e l y 2.3% o f t h e t o t a l body s u r f a c e a r e a (2.8 cm young , 5.6 cm a d u l t ) , was p r o t e c t e d f r o m grooming by p l a c i n g a n o n o c c l u s i v e d e v i c e o v e r i t . At t h e end o f the d e s i g n a t e d t i m e i n t e r v a l , the a n i m a l was a n e s t h e t i z e d w i t h e t h e r and a b l o o d specimen was o b t a i n e d from t h e h e a r t . V a r i o u s t i s s u e s were removed t o determine t h e d i s t r i b u t i o n o f dermally absorbed r a d i o - l a b e l e d p e s t i c i d e s . A l l t h e t i s s u e s , e x c r e t o r y p r o d u c t s , and c a r c a s s e s were a n a l y z e d f o r r a d i o a c t i v i t y by l i q u i d s c i n t i l l a t i o n c o u n t i n g as d e s c r i b e d by Shah e t a l . ( 8 ) . 2

2

2

IN VITRO PROCEDURE I n v i t r o a b s o r p t i o n o f p e s t i c i d e s t h r o u g h young and a d u l t s k i n was d e t e r m i n e d by u s i n g b o t h F r a n z c e l l s and c o n t i n u o u s flow-through cells. The d e t a i l e d e x p e r i m e n t a l and s k i n p r e p a r a t i o n t e c h n i q u e s have been d e s c r i b e d i n d e t a i l by Shah e t a l . (_9). B r i e f l y , back h a i r was c l i p p e d 24 h p r i o r t o e x p e r i m e n t . A n i m a l s were k i l l e d w i t h c a r b o n d i o x i d e , and back s k i n was c a r e f u l l y e x c i s e d . Skin was f l o a t e d on t i s s u e c u l t u r e medium (minimum e s s e n t i a l media, Eagle formula w i t h E a r l ' s S a l t s ) . S p l i t t h i c k n e s s (350 um t h i c k n e s s ) s k i n s o b t a i n e d w i t h a dermatome ( P a d g e t t Dermatome, Kansas C i t y , MO) were mounted i n t h e i n v i t r o c e l l s w i t h e p i d e r m i s exposed t o t h e e n v i r o n m e n t . Only one s k i n d i s c was used from each animal. The amount o f p e s t i c i d e ( i n a c e t o n e ) per cm a r e a was k e p t t h e same f o r b o t h i n v i v o and i n v i t r o e x p e r i m e n t s . Receptor f l u i d was m a n u a l l y removed p e r i o d i c a l l y from t h e F r a n z c e l l s t o d e t e r m i n e t h e r a t e and e x t e n t o f p e n e t r a t i o n , w h i l e c o n t i n o u s f l o w t h r o u g h c e l l s w i t h a media f l o w r a t e of 3 mL/h were a u t o m a t i c a l l y sampled every 3 h. Sample p r e p a r a t i o n f o r r a d i o a c t i v i t y d e t e r m i n a t i o n and o x i d a t i o n o f t r e a t e d s k i n were done a c c o r d i n g t o p r o c e d u r e d e s c r i b e d by Shah e t a l . (8_). B r i e f l y , t h e t r e a t e d s k i n was combusted i n t h e sample o x i d i z e r t o measure the r e m a i n i n g r a d i o a c t i v i t y i n o r on t h e s k i n . A l l o t h e r samples were mixed w i t h 10 mL I n s t a - G e l ( P a c k a r d I n s t r u m e n t Co., Downers Grove, I L ) for s c i n t i l l a t i o n counting. 2

Data A n a l y s i s S t a t i s t i c a l c o m p a r i s o n s t o d e t e r m i n e t h e e f f e c t o f age on s k i n p e n e t r a t i o n of p e s t i c i d e s were made u s i n g t h e S t a t i s t i c a l A n a l y s i s System package (10) w i t h age and age X t i m e as c l a s s v a r i a b l e s i n the g e n e r a l l i n e a r model (GLM). R e c i p r o c a l v a r i a n c e was used f o r weighting. The s i g n i f i c a n c e o f t h e l a b o r a t o r y method u s e d ( f l o w , s t a t i c , i n v i v o ) was d e t e r m i n e d w i t h t h e SAS u s i n g age, method, and age X method, and age X method X t i m e as c l a s s v a r i a b l e s i n the GLM and r e c i p r o c a l v a r i a n c e f o r w e i g h t i n g .


14.

SHAH ET AL.

Absorption and Pharmacokinetics

173

A m a t h e m a t i c a l model was c o n s t r u c t e d a l o n g t h e l i n e s o f t h e p h y s i o l o g i c a l model d e v e l o p e d by Bungay e t a l . (11,12). L e a s t squares f i t t i n g o f t h e model t o t h e d a t a was done u s i n g SAAM 27, a computer package d e v e l o p e d a t t h e N a t i o n a l I n s t i t u t e s o f H e a l t h (NIH) by Berman and Weiss ( 1 3 ) . RESULTS S k i n a b s o r p t i o n o f c a r b o f u r a n as d e t e r m i n e d by t h e t h r e e methods i s shown i n T a b l e I I . The i n v i v o s k i n a b s o r p t i o n o f c a r b o f u r a n was 4.7% a t 6 h and 33.5% a t 72 h i n young a n i m a l s . I n a d u l t s , i t i n c r e a s e d from 2.0% a t 6 h t o 11.8% a t t h e 72 h t i m e p o i n t . The s t a t i c system gave 3.1% s k i n a b s o r p t i o n a t 6 h and 12.0% a t 72 h i n young a n i m a l s , w h i l e i n a d u l t s , a b s o r p t i o n ranged from 1.7% a t 6 h t o 8.8% a t t h e 72 h t i m e p o i n t . The s k i n a b s o r p t i o n o f c a r b o f u r a n as d e t e r m i n e d by t h e f l o w system i n young a n i m a l s ranged from 27.3% a t t h e 6 h t o 41.1% a t 72 h t i m e p o i n t . I n a d u l t s , i t ranged from 5.0% a t 6 h t o 11.2% a t t h e 72 h t i m e p o i n t . The y o u n g / a d u l t r a t i o o f s k i n p e n e t r a t i o n a t 72 h was 2.9 ( i n v i v o ) , 3.7 ( f l o w ) , and 1.4 ( s t a t i c ) . Thus, a l l t h r e e methods gave h i g h e r s k i n p e n e t r a t i o n v a l u e s i n young r a t s compared t o a d u l t s . However, t h e s t a t i c c e l l gave c o n s i s t e n t l y l o w e r s k i n p e n e t r a t i o n v a l u e s i n young r a t s compared t o i n v i v o o b s e r v a t i o n s . S k i n a b s o r p t i o n o f c a r b a r y l as d e t e r m i n e d by t h e t h r e e methods i s shown i n T a b l e I I I . The s k i n a b s o r p t i o n o f c a r b a r y l ( i n v i v o ) ranged from 2.4% a t 6 h t o 38.5% a t 72 h i n young a n i m a l s , and i n a d u l t s i t ranged from 3.6% a t 6 h t o 38.4% a t 72 h. The s k i n a b s o r p t i o n o f c a r b a r y l ( s t a t i c ) ranged from 5.3% a t 6 h t o 20.9% a t 72 h i n t h e young, and i n a d u l t s , i t ranged from 4.7% a t 6 h t o 20.3% a t 72 h. The s k i n a b s o r p t i o n o f c a r b a r y l ( f l o w ) ranged from 18.8% a t 6 h t o 29.2% a t 72 h i n young a n i m a l s , and i n a d u l t s , i t ranged from 8.1% a t 6 h t o 18.2% a t 72 h. Both i n v i v o and s t a t i c systems showed no s i g n i f i c a n t y o u n g / a d u l t d i f f e r e n c e s i n s k i n p e n e t r a t i o n o f c a r b a r y l a t t h e 72 h t i m e p o i n t , w h i l e t h e f l o w system gave s i g n i f i c a n t d i f f e r e n c e s i n s k i n p e n e t r a t i o n between young and a d u l t a n i m a l s , w i t h young a b s o r b i n g more c a r b a r y l compared t o a d u l t s . S k i n a b s o r p t i o n o f d i n o s e b as d e t e r m i n e d by t h e t h r e e methods i s shown i n T a b l e IV. Dinoseb showed r a p i d p e n e t r a t i o n i n b o t h young and a d u l t r a t s compared t o o t h e r p e s t i c i d e s . The r e s u l t s from t h e i n v i v o method showed a s t a t i s t i c a l l y s i g n i f i c a n t l o w e r s k i n a b s o r p t i o n o f d i n o s e b i n young a n i m a l s . In t h e young, s k i n a b s o r p t i o n ranged from 11.1% a t 1 h t o 55.7% a t 72 h, w h i l e i n a d u l t s , i t ranged from 22.3% a t 1 h t o 84.3% a t 72 h. The s t a t i c method i n d i c a t e s s i g n i f i c a n t d i f f e r e n c e s i n s k i n p e n e t r a t i o n o f d i n o s e b i n young and a d u l t , e x c e p t a t 1 h s a m p l i n g p e r i o d . In the young, t h e s k i n a b s o r p t i o n v a l u e ( s t a t i c ) , f o r d i n o s e b ranged from 3.0% a t 1 h t o 74.9% a t 72 h, and i n t h e a d u l t , i t ranged from 1.5% a t 1 h t o 70.8% a t 72 h. The f l o w system i n d i c a t e d s i g n i f i c a n t young/adult d i f f e r e n c e s i n s k i n absorption of dinoseb a t a l l t i m e p o i n t s , and young a b s o r b e d more t h a n t w i c e t h e amount o f d i n o s e b as a d u l t s . The s k i n a b s o r p t i o n v a l u e ( f l o w ) i n young ranged from 6.4% a t 1 h t o 47.3% a t 72 h, and 2.4% a t 1 h t o 20.6% a t 72 h i n t h e a d u l t .



11.2±0.9

3.7**

4. 1**

NS

f o r the hypothesis

0.05.

S i g n i f i c a n t a t p < 0.05

from J . T o x i c o l . E n v i r o n .

NS Not s i g n i f i c a n t , p >

**

* Reprinted

Age X t i m e

Health.

12.0±1.1

9.8±1.0

7.0±0.8

8.8±0.8

7.1±0.7

4.6±0.5

1.7±0.2

o f no y o u n g - a d u l t d i f f e r e n c e .

22:207-223, 1987 w i t h p e r m i s s i o n

of publisher.

o

w w

o

H O

o •* *•

P 3E

o 5

s

**

2.9**

3.8**

4.2**

2.4

Young adult

**

11.8±0.9

6.8±0.1

5.7±1.2

2.0±0.2

Adult

In v i v o

o H o 2

33.5±2.7

26.1±2.6

23.9±1.8

4.7±0.7

Young

Carbofuran*

3.4**

1.4**

1.4**

1.5**

1.8

Young/ adult

**

41.1±5.9

72

9.7±0.9

4.6**

3.1±0.5

Adult

Age

39.4±6.0

48

7.8±0.9

5.5**

Young

1.4**

36.0±5.7

24

5.0±0.6

Young/ adult

System

Penetration

Static

% Skin

Comparison o f S k i n P e n e t r a t i o n o f

4.3**

27.3±4.5

6

Adult

Methods

All

Young

Time (h)

Flow System

TABLE I I .


S i g n i f i c a n t a t p < 0.05 f o r t h e h y p o t h e s i s Not s i g n i f i c a n t , p > 0.05.

o f no y o u n g - a d u l t

difference.

*

*

Age X Time

* NS

NS

1.04

1.03

*

20.3±1.9

1.02

1.02

1.15

Age

20.9±1.5

16.1±1.6

11.1±1.1

4.7±0.6

Young Adult

1.80*

1.60*

System

Adult

Static

Penetration

All

18.2±2.6

29 2±2.9

72

16.5±1.2

1.70*

16.0±2.3

27 2±2.8

48

11.4±0.8

1.86*

13.3±1.9

24 6±2.6

24

5.3±0.5

2.33*

8.1±1.2

Young

Adult

18 .8±2.1

Young

6

Time (h)

Young Adult

Flow System

% Skin

38.5±2.4

23.6±1.0

14.9±2.5

2.4±0.3

Young

TABLE I I I . Methods Comparison o f S k i n P e n e t r a t i o n o f C a r b a r y l

Adult

Vivo

38.4±1.3

32.4±0.7

15.2±1.2

3.6±0.5

In

*

*

0.89*

1.00

0.73*

0.98

0.66*

Young Adult


29.0±3.5

41.0±3.7

45.0±3.8

47.3±3.9

6

24

48

72

20.6±3.5

18.6±3.0

15.7±2.5

11.19±2.2

2.4±0.6

Flow System Adult

0.70*

0.66*

difference.

*

84.3±2.7

0.65*

*

55.7±2.4

85.1±1.9

55.3±9.7

*

1.10

1.06

1.07

0.74*

0.90

43.1±9.4 85.5±1.8

0.50 22.3±8.8

In V i v o Ratio Adult

63.0±2.9

38.8±5.7

11.1±2.1

Young

*

70.8±3.8

66.2±3.8

1.13

1.22

17.0±1.1 52.3±3.1

2.00*

1.5±0.1

S t a t i c System Adult Ratio

*

74.9±8.0

70.9±8.4

59.1±7.4

20.7±2.0

3.0±0.2

Young

*

2.47*

2.29*

2.42*

2.62*

2.61*

2.64*

Ratio

Penetration

* S i g n i f i c a n t a t p < 0.05 f o r t h e h y p o t h e s i s o f no y o u n g - a d u l t

Age X Time

Age

All

6.4±1.0

1

\

Youna

Time (h)

% Skin

TABLE IV. Methods Comparison o f S k i n P e n e t r a t i o n o f Dinoseb

0

6

a

O

W

5

H

Gfl

W

o 2 2 o *i o

H

o

1

5

14.


SHAH ET AL.

177

Methods c o m p a r i s o n o f h e x a c h l o r o b i p h e n y l s k i n p e n e t r a t i o n i n young and a d u l t r a t s i s shown i n T a b l e V. I n v i t r o s k i n a b s o r p t i o n v a l u e s o f h e x a c h l o r o b i p h e n y l i n young and a d u l t r a t s were s i g n i f i c a n t l y lower t h a n i n v i v o r e s u l t s . Skin absorption v a l u e s i n young a t 6 h were 0.012%, 0.071%, and 2.5% as d e t e r m i n e d by f l o w , s t a t i c , and i n v i v o methods, r e s p e c t i v e l y . In a d u l t s , s k i n a b s o r p t i o n v a l u e s a t 6 h were 0.015, 0.019, and 0.63% as d e t e r m i n e d by f l o w , s t a t i c , and i n v i v o methods, r e s p e c t i v e l y . S k i n a b s o r p t i o n v a l u e s i n young a t 72 h were 1.25, 2.50, and 33.2% as d e t e r m i n e d by f l o w , s t a t i c and i n v i v o methods, r e s p e c t i v e l y . In a d u l t s , 72 h s k i n a b s o r p t i o n v a l u e s were 1.36, 1.15, and 19.6% as d e t e r m i n e d by f l o w , s t a t i c and i n v i v o methods, r e s p e c t i v e l y . Both i n v i t r o methods showed n o n s i g n i f i c a n t d i f f e r e n c e s i n s k i n p e n e t r a t i o n o f h e x a c h l o r o b i p h e n y l between young and a d u l t r a t s , w h i l e i n v i v o r e s u l t s showed s i g n i f i c a n t d i f f e r e n c e s i n s k i n p e n e t r a t i o n between young and a d u l t r a t s a t a l l t i m e p o i n t s . Methods c o m p a r i s o n o f s k i n p e n e t r a t i o n o f c h l o r d e c o n e (Kepone) i s shown i n T a b l e V I . I n v i t r o s k i n a b s o r p t i o n v a l u e s o f c h l o r d e c o n e i n young and a d u l t r a t s were s i g n i f i c a n t l y lower t h a n i n v i v o v a l u e s . S k i n a b s o r p t i o n v a l u e s i n young a t 6 h were 0.45, 0.022, and 0.51% as d e t e r m i n e d by f l o w , s t a t i c , and i n v i v o methods, r e s p e c t i v e l y . I n a d u l t s , s k i n a b s o r p t i o n v a l u e s a t 6 h were 0.052, 0.026, and 0.398% as d e t e r m i n e d by f l o w , s t a t i c , and i n v i v o methods, r e s p e c t i v e l y . S k i n a b s o r p t i o n v a l u e s i n young a t 72 h were 1.4, 2.2, and 9.1% as d e t e r m i n e d by f l o w , s t a t i c and i n v i v o methods, r e s p e c t i v e l y . I n a d u l t s , 72 h s k i n a b s o r p t i o n v a l u e s were 1.2, 2.2, and 7.9% as d e t e r m i n e d by f l o w , s t a t i c , and i n v i v o methods, r e s p e c t i v e l y . A l l t h r e e methods showed h i g h e r s k i n p e n e t r a t i o n o f c h l o r d e c o n e i n young compared t o a d u l t animals. PHYSIOLOGICAL MODELING A p h y s i o l o g i c a l compartmental model f o r c a r b o f u r a n dermal a b s o r p t i o n and d i s p o s i t i o n i s shown i n F i g u r e 1. The compartmental volumes ( V ) were t h e average w e i g h t o f k i d n e y , l i v e r , and body f o u n d i n t h e p r e s e n t s t u d y . The o t h e r compartmental volumes were d e r i v e d from t h e body p r o p o r t i o n s g i v e n by L u t z e t a l . ( 14)• C a r d i a c o u t p u t and b l o o d f l o w r a t e s were s c a l e d d i r e c t l y w i t h compartmental s i z e from t h e v a l u e s g i v e n by Bungay e t a l . ( 1 1 ) . A b l o o d f l o w r a t e f o r a 1.9 g k i d n e y was c a l c u l a t e d t o be 8.33 mL/min from a plasma f l o w r a t e o f 5 mL/min and h e m a t o c r i t o f 40% ( 1 5 ) . The system o f d i f f e r e n t i a l e q u a t i o n s d e s c r i b i n g t h e t r a n s f e r of m a t e r i a l between organ compartments i s g i v e n below. F o r b l o o d T

d i=K,L M,S

V. R.

For muscle and n o n t r e a t e d dt

y l

V

Y

B

Y

B " V.R. i i i

skin where

i = M,S.


Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988. *

Age X t i m e

1.15±0.21

0.592±0.138

0.188±0.054

0.019±0.005

Adult

2.20

2.18

2.11

2.43

3.63

* *

33.2±2.2

28.1±3.5

13.62±0.53

2.50±0.20

Young/ a d u l t Young

* S i g n i f i c a n t a t p < 0.05 f o r t h e h y p o t h e s i s o f no y o u n g - a d u l t d i f f e r e n c e . NS Not s i g n i f i c a n t , p > 0.05.

NS

1.36±0.16 2.50±0.32

1.25±0.24

1.05

0.58±0.08 0.92

0.4610.12

1.00

Age

1.25±0.23

72

0.071±0.022

0.83

0.015±0.002

0.96

0.60±0.13

48

Young/ a d u l t Young

0.17±0.03

Penetration

S t a t i c System

% Skin

Adult

All

0.1710.04

0.012±0.003

Young

24

6

Time (h)

1

19.611.5

18.4±0.6

8.3510.53

0.63±0.06

Adult

In v i v o

*

*

1.65*

1.69*

1.53*

1.63*

3.94*

Young adult

Methods Comparison o f S k i n P e n e t r a t i o n o f 2 , 2 , 4 , 4 * , 5 , 5 ' - H e x a c h l o r o b i p h e n y l

Flow System

T a b l e V.

o

B W W

H Q

s

o

2

1

r 3 0

o

s

3

s

00


0.921±0.052

1.201±0.057

1.398±0.057

24

48

72

1.234±0. 310

0.699±0. 243

0.205±0. 090

0.052±0. 020

1.84*

1.13*

1.80*

4.49*

8.69*

Young Adult

2.213±0.435

1.100±0.238

2.47±0.072

0.022±0.007

1.09

1.03

1.21

1.24

0.86

Young Adult

difference.

2.153±0.320

0.912±0.161

0.199±0.039

0.026±0.007

S t a t i c System Young Adult

% Skin Penetration

9.066±0.664

6.825±0.219

3.570*0.453

0.513*0.063

7.922±1.061

5.626±0.686

2.508±0.606

3.98±0.108

In V i v o Method Young Adult

Methods Comparison o f S k i n P e n e t r a t i o n o f C h l o r d e c o n e

* S i g n i f i c a n t a t p < 0.05 f o r t h e h y p o t h e s i s o f no y o u n g - a d u l t

All

0.450±0.043

Flow System Young Adult

6

Time (h)

Table V I .

1.21

1.14

1.21

1.42

1.29

Young Adult

180


Treated Skin

V

Blood

v

Liver

Feces

Kidney Urine

Skin

Muscle and Fat Figure 1. Physiological model for carbofuran dermal absorption and disposition in the rat. (Reproduced with permission from ref. 9. Copyright 1987 Hemisphere Publishing Corporation.)


14. For

181


S H A H E T A L .

kidney _d_ dt K Y

and f o r

VK

=

liver Y

Y

dt L

k

Y

V^ B " V R L B L L For t r e a t e d s k i n y

dt T

y

T

=

Y (0)

" V i '

T

Nomenclature

v

Dermal dose (n moles) R a d i o a c t i v i t y i n compartment i (n moles) B l o o d f l o w r a t e t o t i s s u e i : (mL/h) Volume f o r t i s s u e compartment i : (mL) Equilibrum tissue/blood r a t i o for compartment i Dermal a b s o r p t i o n r a t e c o n s t a n t ( h ~ ) Urinary e x c r e t i o n r a t e constant (h~ ) F e c a l e x c r e t i o n r a t e c o n s t a n t (h"^)

i R-

k k k

Subscripts B K L M S

1

A

T F U

1

y

F

Blood Kidneys Liver Muscle Nontreated skin Treated s k i n Feces Urine

C a r b o f u r a n l e v e l s from b l o o d , k i d n e y , l i v e r , s k i n , c a r c a s s , u r i n e , and f e c e s were f i t t e d t o the model u s i n g SAAM27 t o o p t i m i z e t h e parameters f o r e q u i l i b r u m t i s s u e / b l o o d r a t i o (R,p), t h e u r i n e ( K y ) , and f e c e s ( K ) , e x c r e t i o n - r a t e c o n s t a n t , and t h e d e r m a l a b s o r p t i o n - r a t e c o n s t a n t ( K ) . The parameter v a l u e s a r e shown i n Table V I I . The dermal a b s o r p t i o n - r a t e c o n s t a n t ( K ) f o r young, 0.0 0548 F

A

A

1

1

h " , i s about 3 t i m e s t h a t f o r a d u l t s , 0.00173 h " . The model r e s u l t s f o r dermal a b s o r p t i o n were i n agreement w i t h t h e o b s e r v e d v a l u e s a f t e r 24 h f o r b o t h young and a d u l t r a t s . B e f o r e 24 h, t h e model r e s u l t s were a p p r o x i m a t e l y h a l f o f t h e o b s e r v e d v a l u e s , perhaps i n d i c a t i n g an a d d i t i o n a l r a p i d component o f a b s o r p t i o n w h i c h was not i n c o r p o r a t e d i n t h e model. The u r i n a r y e x c r e t i o n r a t e c o n s t a n t s ( K ) were f o u n d t o be 36.1 h ~ f o r young and 26.1 h " f o r t h e a d u l t . The f e c a l e x c r e t i o n r a t e c o n s t a n t s ( K ) were found t o be 0.67 h ~ f o r young and 0.24 h ~ f o r the a d u l t . The i n c r e a s e d dermal a b s o r p t i o n - r a t e c o n s t a n t i n young compared t o a d u l t a n i m a l s appears t o be b a l a n c e d by the i n c r e a s e d e x c r e t i o n r a t e i n t h e young, such t h a t t h e body burdens i n young and a d u l t s a r e about t h e same a t 120 h. The e q u i l i b r i u m t i s s u e - t o - b l o o d r a t i o s ( T a b l e V I I ) f o r a d u l t a n i m a l s were h i g h e r t h a n t h o s e o f t h e r e s p e c t i v e organs i n t h e young. Most n o t a b l e i s t h e d i f f e r e n c e i n t h e k i d n e y - t o - b l o o d r a t i o of 10.3 f o r a d u l t s as compared t o 5.4 f o r young, i n d i c a t i n g the greater sequestering of carbofuran-derived r a d i o a c t i v i t y i n a d u l t s t h a n i n young. 1

y

1

p

1

1



_

t

F

b

b

c

b

b

1

1

6.38 ( 9 ) b 0.72 3.03 11.35 (16) 40.44 (57) 70.94° Kp = 0.674 h ~

K

b

14.18 ( 9 ) 1.31 5.93 25.21 (16) 89.82 (57) 154.58 = 0.239 h "

±

V (mL) Q ±

(mL/min)

K

K

A

A = 0.00548 h "

17.74 3.195.41 4.861.91 1.421.28 2.242.38

A = 0.00173 h "

39.4010.29 5.782.47 9.491.94 3.153.65 4.98

1

1

P a r a m e t e r s Employed i n t h e P h y s i o l o g i c Model f o r C a r b o f u r a n A b s o r p t i o n and D i s p o s i t i o n i n t h e Young and A d u l t R a t *

from J . T o x i c o l . Environ.

Health.

Observed.

c

* Reprinted

P e r c e n t o f body w e i g h t g i v e n i n p a r e n t h e s e s .

b

22:207-223, 1987 w i t h p e r m i s s i o n o f p u b l i s h e r .

V^, compartmental volume; Q^ b l o o d f l o w r a t e t o t h e o r g a n s ; R^, e q u i l i b r i u m t i s s u e t o b l o o d r a t i o ; Ky, r a t e c o n s t a n t f o r u r i n a r y e x c r e t i o n ; Kp, r a t e c o n s t a n t f o r f e c a l e x c r e t i o n ; K , r a t e c o n s t a n t f o r dermal a b s o r p t i o n .

a

1

Blood Kidneys Liver Skin M u s c l e and f a t Body Ky = 36.06h"

Young

1

Blood Kidneys Liver Skin M u s c l e and f a t Body ky = 26.09 h "

Organ

Adult

Rat

TABLE V I I .

r—

GO

o

w W

3

H

3

O

2

O

o

P

5

8

gg

14.

183


SHAH ET AL.

A s t e a d y s t a t e was r e a c h e d r a p i d l y . The model p r e d i c t e d o r g a n c o n t e n t t o be maximum a t 6 h, f o l l o w e d by a d e c r e a s e i n c o n t e n t as a r e s u l t o f t h e d e c l i n e i n t h e amount t h a t was a v a i l a b l e f o r p e n e t r a t i o n . C o n t i n u o u s e x p o s u r e was s i m u l a t e d w i t h t h e model by m a i n t a i n i n g a u n i t c o n s t a n t amount on t h e s k i n . E q u i l i b r i u m l e v e l s c a l c u l a t e d i n t h e o r g a n s as f r a c t i o n s o f t h e amount on t h e s k i n f o r young and a d u l t , r e s p e c t i v e l y , were 1.5 X 4

10" ,

and

liver, 10~

4

9.4

0.65

X 10~

X 10"

4

4

and

f o r kidney, 4.7

f o r t h e t o t a l body.

h) f o r young and 4

X

10~

4

X 10""

and

f o r s k i n , and

Corresponding

a d u l t s were 52 X 1

4

3.7

10" 4

4

1.4

X 10~

81 X 1 0 ~

4

excretion rates and

17 X 1 0 ~

4

h"

4

and

for 40

X

(fraction/ 1

for urine

1

and 2.5 X 1 0 ~ h " and 0.34 X 1 0 ~ h " f o r f e c e s . From t h i s t e m p o r a l e x p e r i m e n t w i t h a s i n g l e a p p l i e d dose o f c a r b o f u r a n , t h e l i n e a r compartmental f l o w model p r o v i d e s a good f i t t o t h e d a t a . A more complete v a l i d a t i o n o f t h e model would r e q u i r e d a t a a t a sequence o f doses as w e l l as d a t a from m u l t i p l e dose e x p e r i m e n t s . F o r c h l o r d e c o n e , a model d e v e l o p e d from i n t r a v e n o u s ( i v ) d o s i n g was used b u t m o d i f i e d t o accommodate t h e dermal a b s o r p t i o n pathway. A t i m e - v a r y i n g dermal a b s o r p t i o n r a t e c o n s t a n t was f o u n d t o enhance t h e model f i t . The u r i n a r y e x c r e t i o n r a t e c o n s t a n t was a l s o time-dependent. E x c r e t i o n began a t about 0.75 h"^ and f i n i s h e d a t about 0.01 a t 120 h. Equilibrium tissue/blood r a t i o s a g r e e d w i t h t h o s e f o u n d by i v d o s i n g e x c e p t f o r s k i n . Skin was f o u n d t o have an e q u i l i b r i u m t i s s u e / b l o o d r a t i o o f about 3.5 i n s t e a d o f 6 as d e t e r m i n e d by i v d o s i n g . T h i s was t h e l o w e s t r a t i o f o r any t i s s u e . The h i g h e s t r a t i o was 10 w h i c h o c c u r r e d f o r liver. Dermal a b s o r p t i o n f o r d i n o s e b was b i p h a s i c r e q u i r i n g two f i r s t o r d e r compartments t o a d e q u a t e l y s i m u l a t e t h e o b s e r v e d data. T i s s u e / b l o o d r a t i o s were l o w e s t i n muscle, about 0.15, and h i g h e s t i n k i d n e y , about 0.55. S i n c e a l l r a t i o s were l e s s t h a n 1.0, no s e q u e s t e r i n g o f d i n o s e b r a d i o a c t i v i t y i s i n d i c a t e d . The u r i n a r y pathway was t h e p r e f e r r e d r o u t e o f e x c r e t i o n by about a 4:1 r a t i o as compared t o t h e f e c a l r o u t e . H e x a c h l o r o b i p h e n y l was modeled w i t h s e p a r a t e compartments f o r p a r e n t and m e t a b o l i t e as d e t e r m i n e d from p u b l i s h e d i v e x p e r i m e n t s (14). Dermal a b s o r p t i o n was d e s c r i b e d by a s i n g l e f i r s t o r d e r compartment b u t d e l a y e d by a b o u t 1 h i n young r a t s and 4 h i n adult rats. The t i s s u e / b l o o d r a t i o f o r a d i p o s e f o r t h e p a r e n t compound was f o u n d t o be i n t h e 100's o r 1000's i n d i c a t i n g a v e r y s m a l l r e l e a s e o f p a r e n t from t h e a d i p o s e compartment. M u s c l e had t h e l o w e s t t i s s u e / b l o o d r a t i o f o r p a r e n t o f about 4. The m e t a b o l i s m c o n s t a n t f o r l i v e r was about 0.02 mL/min which i s q u i t e slow. The m e t a b o l i t e s a r e r e a d i l y e x c r e t e d and p r o d u c e d t i s s u e b l o o d r a t i o s o f o n l y 1 t o 4. DISCUSSION: A number o f s t u d i e s have s u g g e s t e d h i g h e r s k i n p e r m e a b i l i t y i n premature and f u l l - t e r m i n f a n t s compared t o a d u l t s . B e h l e t a l . (4) and Maibach and B o i s i t s (16) have r e v i e w e d t h e l i t e r a t u r e on


184


a g e - r e l a t e d p e r c u t a n e o u s a b s o r p t i o n . B e h l e t a l . (4,17) r e p o r t e d i n v i t r o s k i n p e n e t r a t i o n o f w a t e r , methanol, e t h a n o l , b u t a n o l , h e x a n o l , and o c t a n o l i n male h a i r l e s s mice o f d i f f e r e n t ages. They o b s e r v e d i n c r e a s e d s k i n p e r m e a b i l i t i e s r e a c h i n g maximum v a l u e s around 25 days o f age, t h e n d e c l i n i n g s h a r p l y up t o about 60 days o f age, and r e m a i n i n g more o r l e s s i n v a r i a n t t h e r e a f t e r . In t h e p r e s e n t s t u d y , a g e - r e l a t e d d i f f e r e n c e s i n p e r c u t a n e o u s a b s o r p t i o n o f some o f t h e p e s t i c i d e s t e s t e d were o b s e r v e d . I n many c a s e s s i g n i f i c a n t i n f l u e n c e o f methods employed t o d e t e r m i n e t h e s k i n a b s o r p t i o n o f t h e s e p e s t i c i d e s was a l s o o b s e r v e d . R e s u l t s a t 72 h f r o m t h e i n v i v o method showed a g e - r e l a t e d d i f f e r e n c e s i n s k i n p e n e t r a t i o n f o r c a r b o f u r a n , d i n o s e b , and hexachlorobiphenyl. The f l o w method showed s t a t i s t i c a l l y s i g n i f i c a n t s k i n p e n e t r a t i o n d i f f e r e n c e s f o r dinoseb, c a r b a r y l , c a r b o f u r a n and h e x a c h l o r o b i p h e n y l . The s t a t i c method showed s i g n i f i c a n t young/adult p e n e t r a t i o n d i f f e r e n c e s only f o r c a r b o f u r a n and h e x a c h l o r o b i p h e n y l . T h i s study c l e a r l y demonstrates t h a t s i g n i f i c a n t d i f f e r e n c e s i n a b s o l u t e p e n e t r a t i o n v a l u e s a r i s e from t h e d i f f e r e n t methods used. A number o f s t u d i e s have shown good c o r r e l a t i o n between i n v i v o and i n v i t r o s k i n p e r m e a b i l i t y v a l u e s f o r a number o f c h e m i c a l s and a n i m a l s p e c i e s because s k i n a b s o r p t i o n has been c o n s i d e r e d as t h e p a s s i v e d i f f u s i o n p r o c e s s t h r o u g h a r a t e l i m i t i n g , n o n l i v i n g b a r r i e r , s t r a t u m corneum. F r a n z (18) compared t h e p e r c u t a n e o u s a b s o r p t i o n ( i n v i t r o ) o f 12 o r g a n i c compounds w i t h i n v i v o r e s u l t s r e p o r t e d by Feldmann and Maibach ( 1 9 ) . He r e p o r t e d e x c e l l e n t q u a l i t a t i v e agreement between i n v i t r o and i n v i v o methods. He f u r t h e r r e p o r t e d t h e d i s c r e p a n c i e s i n t h e q u a n t i t a t i v e agreement between t h e two s e t s o f d a t a . L a t e r on F r a n z (20) r e p o r t e d t h a t d i s c r e p a n c i e s i n q u a n t i t a t i v e agreement was due s i m p l y t o e x p e r i m e n t a l v a r i a t i o n based on a d d i t i o n a l i n v i t r o e x p e r i m e n t s t o s i m u l a t e i n v i v o c o n d i t i o n s by w a s h i n g s k i n a f t e r 24 h. Bronaugh (21) r e p o r t e d i d e n t i c a l i n v i v o and i n v i t r o rat s k i n absorption values f o r benzoic a c i d , a c e t y l s a l i c y l i c a c i d and u r e a . The two i n v i t r o methods u t i l i z e d h e r e , namely, t h e s t a t i c and f l o w - t h r o u g h d i f f u s i o n c e l l s , have been r e c e n t l y compared by Bronaugh and S t e w a r t ( 2 2 ) . The a u t h o r s (22) r e p o r t e d t h e c o m p a r a t i v e s k i n a b s o r p t i o n v a l u e s f o r w a t e r , c o r t i s o n e , and b e n z o i c a c i d t h r o u g h r a t s k i n as d e t e r m i n e d by s t a t i c and f l o w through d i f f u s i o n c e l l s . They r e p o r t e d s i m i l a r a b s o r p t i o n p r o f i l e s and q u a n t i t a t i v e v a l u e s between t h e two d i f f u s i o n systems f o r w a t e r , c o r t i s o n e , and b e n z o i c a c i d under t h e e x p e r i m e n t a l conditions described. However, d i s c r e p a n c i e s i n a b s o r p t i o n v a l u e s were o b s e r v e d f o r a h y d r o p h o b i c compound, [3-phenyl-2-propenyl-2aminobenzoate ( c i n n a m y l a n t h r a n i l a t e ) ] between t h e two d i f f u s i o n c e l l systems; t h e f l o w - t h r o u g h d i f f u s i o n c e l l system gave enhanced a b s o r p t i o n v a l u e f o r c i n n a m y l a n t h r a n i l a t e compared t o t h e s t a t i c c e l l system. They have f u r t h e r r e p o r t e d good agreement between i n v i v o and i n v i t r o ( s t a t i c as w e l l as f l o w - t h r o u g h d i f f u s i o n c e l l s ) s k i n p e r m e a t i o n f o r c o r t i s o n e and b e n z o i c a c i d when each was applied i n petrolatum v e h i c l e . We have r e p o r t e d d i s c r e p a n c i e s i n t h e s k i n a b s o r p t i o n v a l u e s as d e t e r m i n e d by t h e two i n v i t r o methods. We a l s o o b s e r v e d p o o r


14.

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c o r r e l a t i o n between i n v i v o and i n v i t r o r e s u l t s f o r t h e s e h i g h l y complex p e s t i c i d a l c h e m i c a l s . There a r e a number o f f a c t o r s t h a t can i n f l u e n c e t h e s k i n a b s o r p t i o n measurements. The i n f l u e n c e o f r e c e p t o r f l u i d f o r i n v i t r o s k i n a b s o r p t i o n measurements has been w i d e l y documented. F o r example, t h e i n v i t r o a b s o r p t i o n o f c h l o r d e c o n e and h e x a c h l o r o b i p h e n y l i n t h e p r e s e n t s t u d y n e v e r exceeded more t h a n t h a n 2.5% i n b o t h d i f f u s i o n c e l l systems. These two c h e m i c a l s have p o o r w a t e r s o l u b i l i t y (0.9 ppb f o r h e x a c h l o r o b i p h e n y l ) and t h e r e c e p t o r f l u i d u t i l i z e d i n t h i s i n v e s t i g a t i o n may n o t be s u f f i c i e n t l y l i p o h i l i c t o remove t h e c h e m i c a l from t h e d e r m i s ( s p l i t - t h i c k n e s s s k i n ) . Bronaugh and S t e w a r t (23) s u g g e s t e d t h a t t h e l a c k o f s o l u b i l i t y o f t h e h y d r o p h o b i c p e n e t r a n t i n t o t h e r e c e p t o r f l u i d f o r i n v i t r o methods c o u l d be a c o n t r i b u t i n g f a c t o r a c c o u n t i n g f o r i n v i v o and i n v i t r o d i s c r e p a n c i e s i n s k i n a b s o r p t i o n v a l u e s . Bronaugh and S t e w a r t (23) r e p o r t e d maximum a b s o r p t i o n o f c i n n a m y l a n t h r a n i l a t e by u s i n g 6% PEG-20 o l e y l e t h e r r e c e p t o r f l u i d w i t h e i t h e r t y p e o f d i f f u s i o n c e l l system. R e c e n t l y , S c o t t and Ramsey (24) r e p o r t e d t h a t t h e h i g h e s t a b s o r p t i o n o f c y p e r m e t h r i n was a c h i e v e d when 50% aqueous e t h a n o l was used as t h e r e c e p t o r f l u i d w h i c h was i n agreement w i t h i n v i v o a b s o r p t i o n t h r o u g h t h e r a t s k i n . They f u r t h e r r e p o r t e d t h a t t h e a b s o r p t i o n o f c y p e r m e t h r i n measured w i t h two o t h e r r e c e p t o r f l u i d s (6% V o l p o 20 and 20% f e t a l c a l f serum) was v e r y s i m i l a r t o 50% aqueous e t h a n o l as t h e r e c e p t o r f l u i d . Thus t h e above mentioned study suggests t h a t , the s o l u b i l i t y of the penetrant i n t o r e c e p t o r f l u i d , r a t h e r than the d i f f u s i o n of penetrant through the e p i d e r m a l membrane, i s t h e r a t e l i m i t i n g s t e p . I t appears t h a t t h e r e s u l t s o b t a i n e d by d i f f e r e n t methods have t o be c a r e f u l l y e v a l u a t e d w i t h t h e e x p e r i m e n t a l c o n d i t i o n s prescribed. I t i s a l s o c l e a r from t h e l i t e r a t u r e t h a t t h e i n v i t r o r e s u l t s can be i n f l u e n c e d by a number o f f a c t o r s . Perhaps more c o m p a r a t i v e s t u d i e s may improve t h e u n d e r s t a n d i n g o f t h e i n v i t r o and i n v i v o c o r r e l a t i o n . V a l i d a t i o n o f r e c e n t l y recommended g u i d e l i n e s f o r i n v i t r o p e r c u t a n e o u s p e n e t r a t i o n s t u d i e s (25) w i t h a number o f c h e m i c a l s may h e l p i n u n d e r s t a n d i n g t h e i n v i t r o s k i n absorption process. U t i l i t y o f p h a r m a c o k i n e t i c models has been e x p l o r e d f o r e x t r a p o l a t i o n t o man, a n i m a l s p e c i e s s c a l i n g , e x t r a p o l a t i o n o f r e s u l t s beyond t h e o b s e r v e d t i m e p o i n t , s i m u l a t i o n o f c o n t i n u o u s or m u l t i p l e exposure s i t u a t i o n , t i s s u e dose e x t r a p o l a t i o n , i m p r o v i n g e x p e r i m e n t a l d e s i g n , and o v e r a l l r e d u c t i o n o f a n i m a l usage. P h y s i o l o g i c a l models have been shown t o be adequate f o r o t h e r c h e m i c a l s (26,27). The a p p l i c a t i o n s o f p h y s i o l o g i c a l l y based m o d e l i n g and i t s t e c h n i q u e has been r e v i e w e d by a number o f i n v e s t i g a t o r s (26,28,29). I n t h e p r e s e n t s t u d y , p h y s i o l o g i c a l l y based p h a r m a c o k i n e t i c models were used t o d e c r i b e dermal a b s o r p t i o n and d i s p o s i t i o n p r o c e s s e s f o r t h e c h e m i c a l s s t u d i e d . Rate c o n s t a n t s f o r dermal a b s o r p t i o n , and u r i n a r y and f e c a l e x c r e t i o n , as w e l l as t i s s u e t o b l o o d steady s t a t e r a t i o s were determined. A p h y s i o l o g i c a l model d e s c r i b i n g c a r b a r y l r a d i o a c t i v i t y was f i t t e d t o t h e t i s s u e d i s t r i b u t i o n d a t a . The dermal a b s o r p t i o n was f o u n d t o be d e s c r i b e d by a s i n g l e f i r s t o r d e r compartment f o r b o t h


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young and a d u l t r a t s o v e r t h e e x p e r i m e n t a l p e r i o d . The o r d e r o f t h e e q u i l i b r i u m t i s s u e / b l o o d r a t i o s was t h e same i n young and a d u l t r a t s . The t i s s u e w i t h t h e l a r g e s t t i s s u e / b l o o d r a t i o o f about 6 o r 7 was k i d n e y . The u n t r e a t e d s k i n has t h e l o w e s t e q u i l i b r i u m t i s s u e / b l o o d r a t i o o f about 0.8 t o 1.0. The r a t e c o n s t a n t f o r k i d n e y t o u r i n e e x c r e t i o n was l a r g e a t 15 t o 20 h"^. A more complex model t o i n c l u d e m e t a b o l i s m , b l o o d c l e a r a n c e , r e l e a s e o f chemical from s o l v e n t t o s k i n , i n c o r p o r a t i o n o f v i a b l e l a y e r , and m e t a b o l i s m d u r i n g a b s o r p t i o n , e t c . , may a i d i n i m p r o v i n g t h e model's u t i l i t y . CONCLUSIONS; P e r c u t a n e o u s a b s o r p t i o n was compared i n young and a d u l t F i s h e r 344 r a t s f o r f i v e p e s t i c i d e s u t i l i z i n g b o t h i n v i v o and i n v i t r o methods. I n v i v o p e n e t r a t i o n o f t h e p e s t i c i d e s d u r i n g a 72 h p e r i o d , ranged from a p p r o x i m a t e l y 8.0% ( c h l o r d e c o n e , a d u l t ) t o 84% ( d i n o s e b , a d u l t ) , d e p e n d i n g on compound and age o f t h e a n i m a l . Three p e s t i c i d e s , ( c a r b o f u r a n , h e x a c h l o r o b i p h e n y l , and c h l o r d e c o n e ) showed h i g h e r s k i n p e n e t r a t i o n v a l u e s i n young compared t o a d u l t . D i f f e r e n c e s i n s k i n p e n e t r a t i o n v a l u e s were o b s e r v e d among p e s t i c i d e s due t o methods o f measurement. Two i n v i t r o methods, namely, s t a t i c c e l l and f l o w - t h r o u g h d i f f u s i o n c e l l systems gave d i f f e r e n t p e n e t r a t i o n v a l u e s . A p h y s i o l o g i c a l compartmental model a p p e a r s t o be good f o r s i m u l a t i n g dermal a b s o r p t i o n and d i s p o s i t i o n o f t h e p e s t i c i d e s s t u d i e d . ACKNOWLEDGMENTS: T h i s work was c o n d u c t e d a t N o r t h r o p E n v i r o n m e n t a l Sciences, Research T r i a n g l e P a r k , NC. The r e s e a r c h d e s c r i b e d i n t h i s a r t i c l e has been r e v i e w e d by t h e O f f i c e o f T o x i c S u b s t a n c e s , U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency, and approved f o r p u b l i c a t i o n . A p p r o v a l does n o t s i g n i f y t h a t t h e c o n t e n t s n e c e s s a r i l y r e f l e c t t h e v i e w s and p o l i c i e s o f t h e Agency, n o r does mention o f t r a d e names o r c o m m e r c i a l p r o d u c t s c o n s t i t u t e endorsement o r recommendation f o r u s e .

Literature Cited 1. 2. 3. 4.

5. 6. 7.

Synthetic Organic Chemicals , United States International Trade Commission, 1985, pp 226-241. Durham, W.F.; Wolfe, H.R. Bull. WHO 1962, 26 , 75-91. Wolfe, H.R.; Durham, W.F.; Armstrong, J.F.Arch.Environ. Health 1976, 14 , 622-633. Behl, C.R.; Bellantone, N.H.; Flynn, G.L. In Percutaneous Absorption Mechanisms - Methodology - Drug Delivery . Bronaugh, R.L., Maibach, H.I.; Eds.; Dekker: New York, 1985; pp 183-212. Nachman, R.L.; Esterly, N.B.J. Pediatr . 1971, 79 , 628-632. Wester, R.C.; Noonan, P.K.; Cole, M.P.; Maibach, H.I. Pediatr. Res . 1977, 11 , 737-739. Knaak, J.B.; Yee, K., Ackerman, CR., Zweig, G., Wilson, B.W. Toxicol. Appl. Pharmacol . 1984, 72 , 406-416.


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