Percutaneous Absorption - American Chemical Society

Chapter

13

Percutaneous Absorption In Vitro Technique as an Alternative to In Vivo Assessments

Downloaded by UNIV OF MELBOURNE on March 27, 2016 | http://pubs.acs.org Publication Date: December 23, 1988 | doi: 10.1021/bk-1988-0382.ch013

R. C. Scott Central Toxicology Laboratory, Imperial Chemical Industries PLC, Alderley Park, Nr. Macclesfield, Cheshire SK10 4TJ, United Kingdom Chemicals, during manufacture and use may come into contact with human skin and have the potential to be absorbed. Hence an integral part of their safety evaluation should involve quantitation of percutaneous absorption. Currently, Regulatory Agencies require in vivo dermal absorption studies for this quantitation. Such in vivo data, obtained using animal skin, are used to predict the potential hazard to man. As the permeability of animal skin and human skin has been shown to be different (for many chemicals) extrapolation is difficult. In the studies reported in this paper in vivo and in vitro absorption data using rat skin have been obtained with two molecules, cypermethrin and carbaryl, which have different physicochemical properties. For both these chemicals the in vitro absorption data which were obtained predicted the in vivo data. In vitro carbaryl absorption data were also obtained with human skin and compared with published in vivo data. Again, the in vitro data predicted the available in vivo results. As in vitro data are more readily obtained and as human skin can be used (thus eliminating any extrapolation step from animal to man) the in vitro system offers a geniune alternative and improvement to in vivo animal experiments. Safety evaluation of pesticides and their formulations requires an integrated series of toxicity studies. By necessity, animals are used as models for man and the data are then extrapolated to predict the potential hazard to man. Toxicity after the percutaneous absorption of toxic active ingredients, such as pesticides, is assessed currently using in vivo absorption data determined using an animal skin (usually rat or rabbit). This method is used since chemicals with only a limited toxicology data base cannot, for ethical reasons, be tested in man directly. 0097-6156/89/0382-0158$06.00/0 ° 1989 American Chemical Society

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


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However, once s u f f i c i e n t i n f o r m a t i o n has been g a t h e r e d t o enable a f u l l e r u n d e r s t a n d i n g of the t o x i c o l o g i c a l p r o f i l e of an a c t i v e i n g r e d i e n t , human v o l u n t e e r s t u d i e s might then be p o s s i b l e . I n e v i t a b l y t h e a b i l i t y t o conduct such s t u d i e s i s determined by c o n s i d e r a t i o n s of e t h i c a l committees who approve p r o t o c o l s . Often p e s t i c i d e s a r e prepared i n many f o r m u l a t i o n s and t h e problem o f , f o r example, a s s e s s i n g t h e s a f e t y of s e v e r a l n o v e l f o r m u l a t i o n s ( i e the f o r m u l a t i o n which r e s u l t e d i n the l o w e s t a b s o r p t i o n r a t e ) w i t h the same a c t i v e i n g r e d i e n t by t h e human v o l u n t e e r e x p e r i m e n t a l r o u t e would be a d a u n t i n g t a s k . I n v i v o , a n i m a l s t u d i e s a r e o n l y m a r g i n a l l y l e s s complex, a l t h o u g h the a c q u i s i t i o n of d a t a i s f a c i l i t a t e d by more ready a c c e s s t o t e s t s u b j e c t s . I n o r d e r t o f a c i l i t a t e the development of d a t a on t h e a b s o r p t i o n of p e s t i c i d e s from v a r i o u s f o r m u l a t i o n s , i n v i t r o t e c h n i q u e s have been developed t o p r e d i c t i n v i v o percutaneous absorption. The major advantage w i t h the i n v i t r o t e c h n i q u e i s t h e s a f e use of human t i s s u e , t h e b e s t model f o r man, i n a d d i t i o n t o a r e d u c t i o n i n t h e use of a n i m a l s . I t i s t h e i n t e n t i o n of t h i s paper t o show t h a t t h e percutaneous a b s o r p t i o n of p e s t i c i d e s can be measured i n v i t r o w i t h t h e same degree o f c e r t a i n t y and a c c u r a c y demanded from i n v i v o s t u d i e s . Some of t h e p o t e n t i a l e x p e r i m e n t a l p i t f a l l s f o r t h e unwary e x p e r i m e n t a l i s t a r e a l s o d i s c u s s e d and comparisons w i t h i n v i v o d a t a p r e s e n t e d . The i n v i v o and i n v i t r o a b s o r p t i o n , through r a t s k i n , of a p o o r l y absorbed l i p o p h i l i c m o l e c u l e , t h e i n s e c t i c i d e c y p e r m e t h r i n , has been s t u d i e d ( 1 ) . I n t h i s study 3\il of a p o t e n t i a l c o n c e n t r a t e f i e l d f o r m u l a t i o n (nominal c y p e r m e t h r i n c o n c e n t r a t i o n , 3 5 9 g / l ) , o r 2 0 u l of an o i l - b a s e d spray d i l u t i o n (nominal c y p e r m e t h r i n c o n c e n t r a t i o n , 2 9 g / l ) , was a p p l i e d t o t h e shorn backs of 4-5 week o l d r a t s (Alpk/AP s t r a i n , W i s t a r d e r i v e d ) . The a p p l i c a t i o n s i t e was l e f t open t o the atmosphere but p r o t e c t e d by a p l a s t i c guard. To f a c i l i t a t e the d e t e r m i n a t i o n of a b s o r p t i o n , [-^C] c y p e r m e t h r i n was added t o t h e f o r m u l a t i o n . A b s o r p t i o n was a s s e s s e d a f t e r 8 hours. T h i s time p e r i o d was s e l e c t e d t o r e p r e s e n t a t y p i c a l working day p e r i o d . With t h e spray d i l u t i o n t h e t i m e c o u r s e of a b s o r p t i o n over a 24 hour c o n t a c t p e r i o d was determined. I n b o t h s t u d i e s , d e t e r m i n a t i o n of t o t a l a b s o r p t i o n up t o a p a r t i c u l a r time p o i n t r e q u i r e d a n i m a l s t o be k i l l e d humanely p r i o r to t h e a n a l y s i s of the s k i n s i t e , u r i n e , f a e c e s and v a r i o u s t i s s u e s f o r [ C ] c o n t e n t (which was regarded as e q u i v a l e n t t o cypermethrin). D u r i n g t h e 8 hour c o n t a c t p e r i o d w i t h the c o n c e n t r a t e f o r m u l a t i o n a mean of 1.0% (SEM ± 0.4; n = 3) o f t h e a p p l i e d r a d i o l a b e l was d e t e c t e d t o have been absorbed. I n our experiments we d e f i n e d t h e absorbed c h e m i c a l as t h a t which had passed through the s k i n , i e e n t e r e d the c i r c u l a t o r y system and, t h e r e f o r e , c a p a b l e of e x e r t i n g s y s t e m i c t o x i c i t y . A c h e m i c a l which i s l o c a t e d i n t h e s t r a t u m corneum ( t h e outermost l a y e r of t h e e p i d e r m i s ) , a l t h o u g h p o t e n t i a l l y a b s o r b a b l e , cannot e x e r t s y s t e m i c t o x i c i t y , o t h e r than l o c a l , and so i s not regarded as absorbed. 1 4



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A s e r i e s of i n v i t r o experiments were done t o o b t a i n d a t a f o r comparison w i t h t h a t o b t a i n e d i n v i v o . The a p p l i c a t i o n c o n d i t i o n s of the f o r m u l a t i o n on the s k i n i n v i t r o d u p l i c a t e d those i n v i v o . The a p p a r a t u s used f o r the i n v i t r o experiments was a g l a s s d i f f u s i o n c e l l ( F i g u r e 1) i n which the s k i n membrane was sandwiched between a donor and r e c e p t o r chamber, w i t h the e p i d e r m a l s u r f a c e a d j o i n i n g the donor chamber. The donor chamber was open at the top i n o r d e r to expose the e p i d e r m a l s u r f a c e t o ambient atmospheric c o n d i t i o n s and the r e c e p t o r chamber was p l a c e d i n a w a t e r - b a t h m a i n t a i n e d a t a c o n s t a n t temperature (30° ± 1°C). Two v a r i a b l e s were i n t r o d u c e d i n t o the experiment. F i r s t l y , whole s k i n ( e p i d e r m i s and d e r m i s ) o r an e p i d e r m a l membrane (dermis removed) was used (2^). Secondly, s e v e r a l r e c e p t o r f l u i d s ( p h y s i o l o g i c a l s a l i n e , 6% V0LP0 i n s a l i n e , 6% PEG i n s a l i n e , 20% f o e t a l c a l f serum and 50% aqueous e t h a n o l ) were used w i t h b o t h whole and epidermal s k i n membranes. The i n v i t r o d a t a are p r e s e n t e d i n T a b l e 1. With whole s k i n i n the d i f f u s i o n c e l l s no a b s o r p t i o n was d e t e c t e d d u r i n g the 8 hour c o n t a c t p e r i o d i n t o any of the r e c e p t o r f l u i d s . These r e s u l t s do not agree w i t h the i n v i v o d a t a . However, when e p i d e r m a l membranes were used, a b s o r p t i o n ( s i m i l a r to t h a t measured i n v i v o ) was d e t e c t e d w i t h 3 of the 5 r e c e p t o r f l u i d s . C y p e r m e t h r i n i s e s s e n t i a l l y i n s o l u b l e i n the 2 r e c e p t o r f l u i d s ( p h y s i o l o g i c a l s a l i n e and 6% PEG i n s a l i n e ) i n t o which no a b s o r p t i o n was d e t e c t e d . These d a t a i n d i c a t e t h a t the i n v i v o percutaneous a b s o r p t i o n of a l i p o p h i l i c m o l e c u l e can be p r e d i c t e d i n v i t r o by u s i n g e p i d e r m a l membranes and a s u i t a b l e r e c e p t o r f l u i d i n which the p e n e t r a n t i s s o l u b l e . The i n a b i l i t y of l i p o p h i l i c m o l e c u l e s t o p e n e t r a t e i n v i t r o whole s k i n (though e s s e n t i a l l y the d e r m i s ) has been demonstrated p r e v i o u s l y ( 2 , 3 ) . I n v i v o c h e m i c a l s must d i f f u s e through the s t r a t u m corneum and v i a b l e e p i d e r m i s , t o reach the s y s t e m i c c i r c u l a t i o n , but not the f u l l w i d t h of the dermis. Thus the use of e p i d e r m a l membranes i n v i t r o mimics the i n v i v o pathway to the c i r c u l a t o r y system. The need f o r a r e c e p t o r f l u i d i n which the p e n e t r a n t i s s o l u b l e , (but which does not a l t e r the o v e r a l l i n t e g r i t y of the d i f f u s i o n b a r r i e r ) i s necessary and has been r e p o r t e d p r e v i o u s l y (4) but t h i s i s s t i l l not a p p r e c i a t e d f u l l y . The t e c h n i c a l m i s t a k e of u s i n g whole s k i n and an u n s u i t a b l e r e c e p t o r f l u i d might, i n the p a s t , have p r e j u d i c e d workers i n the i n v e s t i g a t o r y f i e l d of percutaneous a b s o r p t i o n from u s i n g the i n v i t r o technique. C e r t a i n l y , the i n a b i l i t y to reproduce i n v i v o a b s o r p t i o n r e s u l t s i n v i t r o would have been a f r u s t r a t i n g experience. The p r o f i l e s of i n v i v o and i n v i t r o c y p e r m e t h r i n a b s o r p t i o n a g a i n s t time, through r a t s k i n , from the o i l - b a s e d spray d i l u t i o n are shown i n F i g u r e 2. D u r i n g the i n i t i a l 12 hour c o n t a c t p e r i o d t h e r e was no d i f f e r e n c e between the i n v i v o and i n v i t r o d a t a , but a f t e r t h i s time the data o b t a i n e d w i t h 6% V0LP0 20 as the r e c e p t o r f l u i d i n v i t r o underestimated the i n v i v o a b s o r p t i o n . The 50% aqueous e t h a n o l s o l u t i o n a g a i n demonstrated t h a t over prolonged c o n t a c t p e r i o d s , the i n v i t r o t e c h n i q u e i s capable of p r e d i c t i n g the i n v i v o a b s o r p t i o n .



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F i g u r e _1

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Diagram of t h e g l a s s d i f f u s i o n c e l l used i n i n v i t r o percutaneous a b s o r p t i o n experiments

T a b l e I . I n v i t r o A b s o r p t i o n of C y p e r m e t h r i n through Rat S k i n D u r i n g 8hr Contact P e r i o d from a C o n c e n t r a t e F i e l d Formulation % a p p l i e d dose absorbed

1 ND (5)

Whole s k i n Receptor 2 3 4 ND (5)

ND ND (5) (5)

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Epidermis Receptor 2 3

1.7 2.7 ±0.2 ±0.4 (4) (4)

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R e c e p t o r f l u i d s used: (1) 50% aqueous e t h a n o l (2) 6% V0LP0 20 i n s a l i n e (3) p h y s i o l o g i c a l s a l i n e (4) 20% f o e t a l c a l f serum (5) 6% PEG i n s a l i n e


5 ND


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Such an i n v i v o time course e x p e r i m e n t , u s i n g t h i s type of p r o t o c o l , r e q u i r e s the use and s a c r i f i c e of many a n i m a l s (eg. 2-5 per time p o i n t ) , w h i l s t the i n v i t r o t e c h n i q u e r e q u i r e s much fewer (3-5 r e p l i c a t e s f o r each r e c e p t o r f l u i d w i t h 3-5 s k i n samples from each r a t ) . The a b i l i t y t o study the e f f e c t of a l t e r i n g the c o n c e n t r a t i o n , time of s k i n c o n t a c t or changes i n f o r m u l a t i o n s c o u l d be measured i n v i t r o much more e f f i c i e n t l y and the e f f e c t on i n v i v o a b s o r p t i o n p r e d i c t e d . In such a manner development of l e s s t o x i c f o r m u l a t i o n s , i e those which cause the l e a s t percutaneous a b s o r p t i o n , would be f a c i l i t a t e d . A s i m i l a r degree of a c c u r a t e p r e d i c t i o n of i n v i v o a b s o r p t i o n , u s i n g the i n v i t r o method, has a l s o been a c h i e v e d w i t h a more water s o l u b l e and w e l l absorbed p e s t i c i d e , c a r b a r y l . I n i t i a l l y , animals were g i v e n a s i n g l e i n t r a v e n o u s b o l u s i n j e c t i o n of c a r b a r y l and the u r i n e and f a e c e s c o l l e c t e d . The p r o p o r t i o n of the dose e l i m i n a t e d i n the u r i n e was then c a l c u l a t e d . The k i n e t i c s of e l i m i n a t i o n f o l l o w i n g percutaneous a b s o r p t i o n were assumed to be the same as a f t e r i n t r a v e n o u s d o s i n g . The [^- C] c a r b a r y l was a p p l i e d to r a t s k i n i n a v o l a t i l e v e h i c l e ( a c e t o n e ) and a b s o r p t i o n was a s s e s s e d by c o l l e c t i n g u r i n e and a s s a y i n g f o r [^ C] which was regarded as e q u i v a l e n t to c a r b a r y l . The amount of [^- C] i n the u r i n e a f t e r dermal c o n t a c t was then a d j u s t e d by the c a l c u l a t e d ' c o r r e c t i o n f a c t o r * from the i n t r a v e n o u s study ( i . e . p r o p o r t i o n i n the u r i n e ) t o determine percutaneous a b s o r p t i o n . T h i s t e c h n i q u e has been employed s u c c e s s f u l l y i n monkey and human s t u d i e s ( 5 ) . I n v i t r o a b s o r p t i o n was measured i n t o two r e c e p t o r f l u i d s (see F i g u r e 3 ) . The use of 50% aqueous e t h a n o l p r o v i d e d the b e t t e r agreement than the 6% V0LP0 i n s a l i n e r e c e p t o r f l u i d and p r e d i c t e d a c c u r a t e l y the i n v i v o d a t a . The percutaneous a b s o r p t i o n of c a r b a r y l has a l s o been s t u d i e d p r e v i o u s l y through human forearm s k i n (6^). We have d u p l i c a t e d the a b s o r p t i o n c o n d i t i o n s and repeated the study i n v i t r o w i t h human abdominal s k i n . A comparison of our i n v i t r o d a t a w i t h the p u b l i s h e d i n v i v o d a t a i s p r e s e n t e d i n F i g u r e 4. A g a i n , t h e r e i s agreement between the i n v i v o and i n v i t r o d a t a , e s p e c i a l l y when the s m a l l amount of c a r b a r y l a p p l i e d and absorbed i s c o n s i d e r e d t o g e t h e r w i t h the d i f f e r e n t a p p l i c a t i o n s i t e . I t i s p o s s i b l e that a repeat i n v i v o study u s i n g the abdomen as the a p p l i c a t i o n s i t e would g i v e an even b e t t e r c o r r e l a t i o n . These e x p e r i m e n t a l d a t a support the view t h a t the i n v i t r o t e c h n i q u e c o u l d be a genuine a l t e r n a t i v e to the i n v i v o d e t e r m i n a t i o n of percutaneous a b s o r p t i o n . In v i t r o d a t a which p r e d i c t e d i n v i v o data have been o b t a i n e d w i t h d i f f e r e n t types of c h e m i c a l s ( p h y s i c o c h e m i c a l p r o p e r t i e s ) and w i t h d i f f e r e n t s p e c i e s ( r a t and man). E x t r a p o l a t i o n of dermal t o x i c i t y d a t a from animal s t u d i e s to man i s d i f f i c u l t . The a b i l i t y of the i n v i t r o t e c h n i q u e to q u a n t i f y d i f f e r e n c e s i n a b s o r p t i o n between a n i m a l models and man i s i l l u s t r a t e d i n F i g u r e 5. T h i s shows the a b s o r p t i o n p r o f i l e s of [ l ^ C ] c a r b a r y l from a 40ug cm"^ dose, a p p l i e d i n a c e t o n e , t o r a t and human s k i n . The p r o f i l e and r a t e s of a b s o r p t i o n are very different. I f the r a t d a t a were used t o p r e d i c t human a b s o r p t i o n 4

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Cumulative i n v i v o and i n v i t r o a b s o r p t i o n i n s e c t i c i d e c y p e r m e t h r i n from an o i l - b a s e d d i l u t i o n through r a t s k i n

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A b s o r p t i o n o f c a r b a r y l f o l l o w i n g d e p o s i t i o n on s k i n i n a v o l a t i l e ( a c e t o n e ) v e h i c l e through r a t s k i n




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t h e r e would be a l a r g e o v e r e s t i m a t i o n . These d a t a show c l e a r l y t h a t when a b s o r p t i o n s t u d i e s a r e done to p r e d i c t human a b s o r p t i o n then the best s k i n t o use i s human; t h e r e i s , as y e t , no a n i m a l s k i n which has proved t o be a good model f o r human s k i n ( 7 , 8 ) . The d i f f i c u l t i e s of human i n v i v o s t u d i e s have been d i s c u s s e d . The most r a t i o n a l way f o r w a r d , t h e r e f o r e , t o study and understand percutaneous a b s o r p t i o n as i t r e l a t e s to man would appear t o be the use of an i n v i t r o system u s i n g human s k i n . T h i s approach a l l o w s the e f f e c t of f o r m u l a t i o n changes, dose, time of c o n t a c t and o t h e r v a r i a b l e s on percutaneous a b s o r p t i o n , as they r e l a t e t o man, t o be a s s e s s e d r e a d i l y . Most d e t e r m i n a t i o n s of a b s o r p t i o n , p a r t i c u l a r l y f o r the assessment of s y s t e m i c h a z a r d by R e g u l a t o r y A u t h o r i t i e s , l e a d t o p r e s e n t a t i o n of the d a t a i n terms of '% a p p l i e d dose a b s o r b e d a t a s p e c i f i e d t i m e . The p r e c i s e times of i n t e r e s t a r e i l l - d e f i n e d but o f t e n r e l a t e t o the w o r k i n g day p e r i o d (8-10 h o u r s ) and up to a 24 hour p e r i o d . At p r e s e n t , t h e r e i s no w o r k i n g or a c c e p t e d ' c l a s s i f i c a t i o n scheme' f o r the e x t e n t and r a t e of a b s o r p t i o n adopted amongst i n t e r e s t e d s c i e n t i s t s i n t h i s a r e a of i n v e s t i g a t i o n . The d a t a p r e s e n t e d i n F i g u r e s 2 and 3 a r e f u r t h e r a n a l y s e d i n T a b l e I I . With b o t h compounds, the e x t e n t of i n v i v o a b s o r p t i o n , t o g e t h e r w i t h the a p p r o p r i a t e '% a p p l i e d dose absorbed' would have been p r e d i c t e d from the i n v i t r o d a t a ( 6 % V0LP0 d a t a was l e s s a c c u r a t e than 50% aqueous e t h a n o l ) a t any of the r e l a t e d time p o i n t s . I f d a t a a r e r e q u i r e d i n t h i s form then the i n v i t r o system c l a s s i f i e d a c c u r a t e l y the i n v i v o a b s o r p t i o n . P r e c i s e l y how t h i s type of d a t a can be used f o r assessment of hazard i s , however, not e n t i r e l y c l e a r . T o x i c i t y i s not r e l a t e d s i m p l y t o 'the percentage of a p p l i e d dose absorbed' but a b s o r p t i o n r a t e and t o t a l amount absorbed or a c o m b i n a t i o n of t h e s e . Data p r e s e n t e d i n T a b l e I I I a r e from i n v i t r o experiments t o determine the a b s o r p t i o n of l^C] c a r b a r y l through r a t s k i n a f t e r a p p l i c a t i o n ( i n an acetone v e h i c l e ) a t doses of 4, 40, 400 and 4000 u-g/cm^. The a b s o r p t i o n r a t e s and '% a p p l i e d dose absorbed' have been determined and c a l c u l a t e d over two time p e r i o d s . I f the d a t a a r e c o n s i d e r e d i n terms of '% dose absorbed' t h e n , the h i g h e r v a l u e s a r e o b t a i n e d w i t h the lower doses. However, the h i g h e s t a b s o r p t i o n r a t e s , which would determine any t o x i c i t y , a r e from the h i g h e r doses. What i s a l s o o b v i o u s w i t h t h i s p a r t i c u l a r compound from t h i s v e h i c l e , i s the n o n - l i n e a r i t y of the a b s o r p t i o n r a t e s w i t h dose. The i n v i t r o system f a c i l i t a t e s the a c q u i s i t i o n of such d a t a . The '% a p p l i e d dose absorbed' can o n l y be r e l e v a n t t o any p a r t i c u l a r dose a p p l i e d and s p e c i f i c time p e r i o d . A b s o r p t i o n r a t e s a r e more m e a n i n g f u l f o r t o x i c o l o g i c a l / h a z a r d assessments and e n a b l e a b s o r p t i o n from d i f f e r e n t f o r m u l a t i o n s and dose l e v e l s (as can happen i n the f i e l d from s p r a y equipment) t o be compared. These r a t e s can be o b t a i n e d u s i n g the i n v i t r o t e c h n i q u e . I n our l a b o r a t o r y we have d e v e l o p e d , an e x p e r i m e n t a l i n v i t r o percutaneous a b s o r p t i o n scheme which we use t o p r e d i c t the p o t e n t i a l h a z a r d from p e s t i c i d e f o r m u l a t i o n s as a r e s u l t of dermal c o n t a c t ( T a b l e I V ) . A b s o r p t i o n measured from b u l k ' i n f i n i t e ' doses to the s k i n p r o v i d e s d a t a r e l e v a n t t o the worst exposure s i t u a t i o n . 1


166

BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE 1

Table I I . Comparison o f t h e '% A p p l i e d Dose A b s o r b e d i n I n V i v o and I n V i t r o Experiments a t V a r i o u s Times A f t e r A p p l i c a t i o n t o the Skin


Compound

Expt

% A p p l i e d Dose Absorbed 20hr 5hr lOhr

Cypermethrin

In v i v o 50% aq. e t h 6% VOLPO

3 4 2.5

Carbaryl

In v i v o 50% aq. e t h 6% VOLPO

10 10 5

6 9 5

19 20 11

20 20 7.5

33 25 12

T a b l e I I I . I n V i t r o A b s o r p t i o n o f C a r b a r y l a t a Range o f Doses Expressed as t h e Rate and % i A p p l i e d Dose A b s o r b e d Over S p e c i f i c Time P e r i o d s 1

1

Dose (u-g/cm^)

2 - 6

Time P e r i o d ( h r ) 20 - 40 %

Js

%

-

-

4

1.77

100

40

1.16

15

0.28

35

400

1.75

25

0.84

10

4000

5.56

1.17

2

0.8

J , approximate a b s o r p t i o n r a t e (u-g/cm^/hr) %, t o t a l ' % a p p l i e d dose a b s o r b e d a t end o f time p o i n t s

1

Table IV. In Vitro Dermal Application Regime for Predicting Potential Percutaneous Absorption From Different Formulations Formulation Concentrate Spray d i l u t i o n Spray d i l u t i o n ( x l O ) Spray d i l u t i o n ( i l O ) Spray d i l u t i o n Spray d i l u t i o n (unoccluded) Cone: Spray d i l u t i o n (1:3)

z

A p p p l i c a t i o n Volume ( u l / c m ) 100 100 100 100 10 10 100



13. SCOTT


167

To study t h e e f f e c t s of o t h e r than t h e s p e c i f i e d s p r a y d i l u t i o n c o n t a c t i n g s k i n , as can happen i f l a b e l i n s t r u c t i o n s a r e not f o l l o w e d , a b s o r p t i o n i s measured from a range of s p r a y d i l u t i o n s which i n c l u d e s the recommended d i l u t i o n . A d d i t i o n a l l y , t o a s s e s s a b s o r p t i o n under exposure s i t u a t i o n s l i k e l y i n t h e f i e l d , t h e s p r a y d i l u t i o n i s a p p l i e d t o t h e s k i n a t a more r e a l i s t i c exposure r a t e (lOu-l/cm^) w i t h both t h e f o r m u l a t i o n s covered ( o c c l u d e d ) t o prevent v e h i c l e components e v a p o r a t i n g and a l s o open t o t h e atmosphere (unoccluded) t o mimic a c t u a l f i e l d exposure. A f u r t h e r a p p l i c a t i o n regime which we have r e c e n t l y i n t r o d u c e d i s t o measure a b s o r p t i o n from a m i x t u r e o f t h e c o n c e n t r a t e f o r m u l a t i o n and s p r a y d i l u t i o n (1:3) t o study p o t e n t i a l a b s o r p t i o n f o r t h e ' m i x e r - l o a d e r ' o p e r a t o r . O b v i o u s l y , t h i s approach i s geared t o t h e spray o p e r a t o r s i t u a t i o n which we b e l i e v e t o be the most r e l e v a n t f o r c o n s i d e r a t i o n of r i s k w i t h p e s t i c i d e f o r m u l a t i o n s . I n a l l our a p p l i c a t i o n s we a p p l y s p e c i f i e d volumes o f f o r m u l a t i o n s , r a t h e r than amounts of a c t i v e i n g e d i e n t ( a l t h o u g h t h i s i s known) and t h e d a t a can be r e - i n t e r p r e t e d i n terms of '% a p p l i e d dose' i f desired. I t i s f i r m l y our b e l i e f t h a t the h a z a r d o f dermal exposure and subsequent a b s o r p t i o n t o p e s t i c i d e s needs t o be a s s e s s e d . C u r r e n t l y , we b e l i e v e t h a t t h e i d e a l way t o do t h i s i s w i t h f u l l human p h a r m a c o k i n e t i c v o l u n t e e r t r i a l s , however, t h e a c c u m u l a t i o n of s u f f i c i e n t d a t a by t h i s method p r e s e n t s many problems which makes t h i s approach i m p r a c t i c a l . I n order t o f u r t h e r a i d our u n d e r s t a n d i n g of t h e f a c t o r s g o v e r n i n g t h e a b s o r p t i o n o f p e s t i c i d e s , from d i f f e r e n t f o r m u l a t i o n s and t h e subsequent h a z a r d to spray o p e r a t o r s , we b e l i e v e t h a t t h e i n v i t r o t e c h n i q u e o f f e r s a genuine a l t e r n a t i v e t o i n v i v o s t u d i e s .

Literature cited 1.

2.

3.

4. 5.

Scott, R. C. and Ramsey, J. D. (1987). Comparison of the in vivo and in vitro absorption of a lipophilic molecule (cypermethrin, a pyrethroid insecticide). J. Invest. Dermatol. 89: 142-146 Scott, R. C., Walker, M. and Dugard, P. H. (1986). In vitro percutaneous absorption experiments: A technique for production of intact epidermal membranes from rat skin. J. Soc. Cosmet. Chem. 37: 35-41. Scheuplein, R. J. (1965). Mechanism of percutaneous absorption, iv. Penetration of non-electrolytes (alcohols) from aqueous solutions and from pure liquids. J. Invest. Dermatol. 60: 286-296. Bronaugh, R. L. and Stewart, R. F. (1984). Methods for in vitro percutaneous absorption studies. 111. Hydrophobic compounds. J. Pharm. Sci. 73: 1255-1258 Bronaugh, R. L. and Maibach, H. I. (1985). Percutaneous absorption of nitroaromatic compounds: in vivo and in vitro studies in human and monkey. J. Invest. Dermatol. 84: 180183.


168 6. 7.

8.


Feldman, R. J. and Maibach, H. I. (1970). Absorption of some organic compounds through skin in man. J. Invest. Dermatol. 54: 399-404. Scott, R. C., Walker, M. and Dugard, P. K. (1986). A comparison of the in vitro permeability properties of human and from laboratory animal skins. Int. J. Cos. Sci. 8 189-194 (1986). Bartek, M. J., La Budde, J. A. and Maibach, H. I. (1972). Skin permeability in vivo: comparison in rat, rabbit, pig and man. J. Invest. Dermatol. 58 114-123.


RECEIVED March 8, 1988


Percutaneous Absorption - American Chemical Society

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