Chapter 9
Pesticides Downloaded from pubs.acs.org by HONG KONG UNIV SCIENCE TECHLGY on 09/16/18. For personal use only.
Mammalian Metabolism H. Wyman Dorough Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762
While pesticide risk assessment should never become totally a predictive process, it is essential that our prior knowledge of xenobiotic metabolism be used to help predict the hazards of chemicals before they are introduced into the environment. It is important to realize, however, that even a complete knowledge of the metabolism of a pesticide in mammals does not necessarily alert one to the potential safety or risks that might be associated with the use of that particular compound. This can be accomplished only when the metabolism data can be related to toxicological significance. Consequently, greater emphasis in the future must be placed on defining the influence of given metabolic reactions on toxicity, while at the same time continuing the awesome task of individual metabolite identification and toxicity assessment.
The recent space s h u t t l e d i s a s t e r w h i c h c l a i m e d t h e l i v e s o f seven American a s t r o n a u t s r e s u l t e d i n one o f t h e most e x t e n s i v e and e x p e n s i v e i n v e s t i g a t i o n s e v e r c o n d u c t e d . E v e r y phase o f t h e space program was i n c l u d e d i n t h e i n v e s t i g a t i o n and f l a w s c a u s i n g , o r a t l e a s t c o n t r i b u t i n g t o , t h e a c c i d e n t were e v e n t u a l l y r e v e a l e d t h a t s e e m i n g l y would never o c c u r i n such a s o p h i s t i c a t e d u n d e r t a k i n g . T h i s d r e a d f u l e v e n t i s mentioned h e r e j u s t as one example where p r o p e r c a u t i o n , even amongst t h e b e s t , sometimes i s n o t p r a c t i c e d . L i k e most a f t e r - t h e - f a c t i n v e s t i g a t i o n s , t h e space s h u t t l e i n c i d e n t r a i s e s a p o i n t t h a t i s germane t o a l l whose a c t i o n s and r e a c t i o n s may impact upon t h e s a f e t y o f o t h e r s . That i s , i t i s n o t o n l y i m p o r t a n t t h a t t h e r i g h t q u e s t i o n s a r e b e i n g asked, i t i s e s s e n t i a l t h a t t h e r i g h t q u e s t i o n s a r e b e i n g asked a t t h e r i g h t t i m e . One o f t h e most i m p o r t a n t a s p e c t s o f t h e symposium on p e s t i c i d e r i s k i s t h e opportunity i t provides f o r those i n t h e p e s t i c i d e c h e m i s t r y and t o x i c o l o g y f i e l d t o t h i n k about whether t h e r i g h t q u e s t i o n s a r e b e i n g asked and i f t h e y a r e b e i n g asked a t t h e r i g h t time. Such i s n o t always t h e c a s e as e v i d e n t by t h e numerous i n c i d e n c e s where c a u t i o n and, i n d e e d , most o f o u r r e g u l a t i o n s 0097-6156/87/0336-0106$06.00/0 © 1987 American Chemical Society
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c o n c e r n i n g p e s t i c i d e s a f e t y , have r e s u l t e d l a r g e l y from a f t e r - t h e f a c t c o n s i d e r a t i o n s . F o r t u n a t e l y , t h e frequency o f s c i e n t i s t s r e a c t i n g t o p e s t i c i d e - r e l a t e d c r i s i s , i n l i e u o f sound p l a n n i n g towards t h e i r p r e v e n t i o n , seems t o be d e c r e a s i n g . As amply e v i d e n c e d by t h e r e c e n t s i t u a t i o n r e g a r d i n g p e s t i c i d e - c o n t a m i n a t e d ground water, however, a l l i s n o t w e l l and i t i s c l e a r t h a t t h e r e a r e s t i l l many c a s e s where t h e r i g h t q u e s t i o n s have n o t been asked, o r a t l e a s t asked o f t h e r i g h t p e o p l e . I d e a l l y , t h e r i g h t p e o p l e would be t h o s e s c i e n t i s t s who a r e e x p e r t s i n t h e a r e a o f p e s t i c i d e s and who, i f s p e c i f i c a l l y asked, c o u l d a c c u r a t e l y p r e d i c t many h a z a r d s b e f o r e t h e y o c c u r . That t h i s i s n o t always done, even when t h e e x p e r t i s e e x i s t s , a t t e s t s t o a v o i d a b l e weaknesses i n t h e p r e s e n t h a z a r d assessment system. The p r e s e n t paper i s concerned w i t h p e s t i c i d e m e t a b o l i s m and i n k e e p i n g w i t h t h e i s s u e o f q u e s t i o n - a s k i n g as h e r e t o f o r e d i s c u s s e d , t h e q u e s t i o n w h i c h must be a d d r e s s e d i s as f o l l o w s : Can and w i l l a b e t t e r u n d e r s t a n d i n g o f mammalian metabolism o f p e s t i c i d e s minimize p e s t i c i d e r i s k ? On t h e s u r f a c e a s i m p l e q u e s t i o n s e e m i n g l y d e s e r v i n g a s i m p l e answer, t h i s q u e s t i o n i s v e r y d i f f i c u l t and p o t e n t i a l l y e x c e e d i n g l y complex because o f some a p p a r e n t and some n o t so a p p a r e n t a s s o c i a t e d r a m i f i c a t i o n s . S i n c e an answer would depend upon a p r e c i s e and perhaps v e r y d i f f e r e n t p e r s o n a l i n t e r p r e t a t i o n s o f t h e q u e s t i o n , some a n a l y s i s o f t h e q u e s t i o n i s i n o r d e r . M e t a b o l i s m and R i s k U n d e r s t a n d i n g v s Knowledge. The term " u n d e r s t a n d i n g " t a k e s on paramount importance i n any attempt t o r e l a t e p e s t i c i d e m e t a b o l i s m to t h e i s s u e o f h a z a r d s and p e s t i c i d e r i s k . A t h o r o u g h knowledge o f p e s t i c i d e m e t a b o l i s m i n mammals i s e s s e n t i a l t o any f u n d a m e n t a l understanding o f the processes involved, but i t i s safe t o say that we have a f a r g r e a t e r knowledge t h a n u n d e r s t a n d i n g o f t h e s u b j e c t . A l t h o u g h a monumental t a s k and one t h a t i s f a r from complete, i t i s presently quite possible t o i d e n t i f y e s s e n t i a l l y a l l metabolites of most p e s t i c i d e s i n any g i v e n system. T h i s i n i t s e l f , however, does n o t mean t h a t t h e r i s k a s s o c i a t e d w i t h t h e u s e o f t h e c h e m i c a l i n v o l v e d , o r i t s a n a l o g s , has been m i n i m i z e d . When t h e p r o d u c t s formed by m e t a b o l i c p r o c e s s e s a r e t o x i c o l o g i c a l l y i n s i g n i f i c a n t and, when t h i s i s a known f a c t , t h e f i n d i n g s may be v a l u a b l e i n a s s e s s i n g p e s t i c i d e r i s k . C o n t r a r i l y , a number of p e s t i c i d e s y i e l d m e t a b o l i t e s known t o be h i g h l y t o x i c and t h e s e m a t e r i a l s may be t a k e n i n t o account i n t h e r i s k assessment p r o c e s s . Too many t i m e s , however, t h e i s o l a t i o n and i d e n t i f i c a t i o n o f p e s t i c i d e m e t a b o l i t e s t e l l us v e r y l i t t l e about r i s k s t h a t may be a s s o c i a t e d w i t h t h e u s e o f a p a r t i c u l a r c h e m i c a l because t h e i n f o r m a t i o n c a n n o t be r e l a t e d t o i n v i v o t o x i c o l o g i c a l s i g n i f i c a n c e . Time, economic, and t e c h n i c a l l i m i t a t i o n s make t h e p r o p e r t e s t i n g o f a l l metabolites f o r a l l types o f t o x i c a c t i o n v i r t u a l l y i m p o s s i b l e . C o n s e q u e n t l y , few m e t a b o l i t e s , p e r s e , a r e t h o r o u g h l y t e s t e d and, even t h e n , t h e t y p e o f t o x i c i t y a s s e s s e d i s u s u a l l y l i m i t e d t o acute i n v i v o s i t u a t i o n s and t o s e l e c t b a t t e r i e s o f
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s h o r t - t e r m e d b i o a s s a y s . Moreover, one must be e s p e c i a l l y m i n d f u l t h a t t h e p o s s i b i l i t y always e x i s t s t h a t t h e t e s t s u t i l i z e d do n o t allow expression of the s p e c i f i c t o x i c c h a r a c t e r i s t i c s of the compound b e i n g e v a l u a t e d . Furthermore, t h e r e i s t h e e v e r - p r e s e n t p o s s i b i l i t y t h a t t h e t e s t system i s n o t a p p l i c a b l e t o humans. The c o n c l u s i o n must be drawn, t h e r e f o r e , t h a t i t i s f a r e a s i e r t o c o r r e c t l y p r e d i c t what m e t a b o l i t e w i l l be formed t h a n t o p r e d i c t what a f f e c t i t s f o r m a t i o n w i l l have on t h e h a z a r d s o f t h e p e s t i c i d e being evaluated. U s i n g Knowledge t o P r e d i c t R i s k . B e f o r e d e c l a r i n g t h e whole s i t u a t i o n h o p e l e s s l y d i s a s t r o u s , a b r i g h t e r s i d e o f t h e m e t a b o l i s m and r i s k s h o u l d be c o n s i d e r e d . T h a t i s , when s u f f i c i e n t knowledge o f p e s t i c i d e m e t a b o l i s m i n mammals i s accumulated t o b e g i n t o unders t a n d t h e b a s i c p r o c e s s e s i n v o l v e d and p r e d i c t how t h e s e p r o c e s s e s a r e l i k e l y t o i n f l u e n c e t o x i c i t y , t h e n , we may b e g i n t o use t h a t knowledge i n an i n t e l l i g e n t manner as s u g g e s t e d by W i l l i a m s ( 1 ) . I n 1968 t h i s m e t a b o l i s m s c i e n t i s t wrote t h a t "The i n t e l l i g e n t use o f our knowledge o f t h e b i o c h e m i s t r y o f f o r e i g n compounds s h o u l d p e r m i t us t o p r e d i c t what compounds a r e s a f e t o use and t o a v o i d t h o s e , l i k e t h a l i d o m i d e , w h i c h produce a d v e r s e e f f e c t s " . W h i l e t h e i n t e l l i g e n t use o f our knowledge o f p e s t i c i d e b i o c h e m i s t r y i s n o t l i k e l y t o e v e r a l l o w us t o always p r e d i c t w h i c h compounds a r e s a f e and t o a v o i d t h o s e w h i c h p r o d u c e a d v e r s e e f f e c t s as proposed by W i l l i a m s , g r e a t s t r i d e s towards t h i s u l t i m a t e o b j e c t i v e have been made i n r e c e n t y e a r s . F o r example, i t i s p r e s e n t l y w e l l r e c o g n i z e d t h a t many n i t r o s o compounds a r e h i g h l y c a r c i n o g e n i c c h e m i c a l s (2) and t o see today a n i t r o s o a m i n e o r n i t r o s o a m i d e i n t r o d u c e d as a p o t e n t i a l l y new p e s t i c i d e would be v e r y u n l i k e l y . A d d i t i o n a l l y , w h i l e many p e s t i c i d e s i n use t o d a y undergo n i t r o s a t i o n , any new compound w h i c h might do so would be viewed w i t h g r e a t s k e p t i c i s m by most t o x i c o l o g i s t s s h o u l d such a c h e m i c a l be proposed as a new p e s t i c i d e . A t a minimum, such a compound would be marked by our knowledge and u n d e r s t a n d i n g o f t h e t o x i c o l o g i c a l p r o p e r t i e s o f n i t r o s o compounds as a p r o d u c t t o r e c e i v e s p e c i a l a t t e n t i o n from a r i s k assessment v i e w p o i n t . Knowing when t o e x e r c i s e p a r t i c u l a r c a u t i o n i s t h e most v i t a l component o f predictive toxicology. A l o n g t h e s e same l i n e s , we would n e v e r today c o n s i d e r a compound l i k e 2-AAF ( 2 - a c e t y l a m i n o f l u o r e n e ) as a commercial p e s t i c i d e . T h i s compound i s l i s t e d as a c a r c i n o g e n by t h e E n v i r o n m e n t a l P r o t e c t i o n Agency (3) and i s used by many as a p o s i t i v e c o n t r o l i n t h e Ames m u t a g e n i c i t y a s s a y . N o n e t h e l e s s , t h i s compound was s c r e e n e d f o r i t s i n s e c t i c i d a l a c t i o n a f t e r i t s s y n t h e s i s i n 1933, l o n g b e f o r e i t s c a r c i n o g e n i c p r o p e r t i e s were known. F o r t u n a t e l y , i t s p e s t i c i d a l a c t i v i t y d i d n o t m e r i t commercial development. The p r o b a b i l i t y i s e x t r e m e l y low t h a t a compound as h i g h l y c a r c i n o g e n i c as 2-AAF would escape d e t e c t i o n as a c a r c i n o g e n today. Not o n l y would t h e c h e m i c a l s t r u c t u r e a l e r t t h e t o x i c o l o g i s t o f such a p o s s i b i l i t y , c a n c e r i s t h e most dreaded o f a l l d i s e a s e s and s o c i e t y has demanded t h a t s c i e n c e d e v e l o p t e s t s which, w i t h g r e a t a c c u r a c y , i d e n t i f y c a r c i n o g e n i c s u b s t a n c e s b e f o r e commercial introduction. U n l i k e what o f t e n appears t o be t h e c a s e , however, c a n c e r i s n o t t h e o n l y p o s s i b l e a d v e r s e e f f e c t o f human p e s t i c i d e
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exposure. I n t i m e , o t h e r a s p e c t s o f x e n o b i o t i c t o x i c i t y t o humans must be g i v e n t h e same t h o r o u g h t r e a t m e n t by m e d i c a l s c i e n c e as has cancer. S p e c i a l Problems; New Compounds. A major c h a l l e n g e p r e s e n t l y i s t o make c e r t a i n t h a t our c u r r e n t knowledge and u n d e r s t a n d i n g o f p e s t i c i d e r i s k s be e f f e c t i v e l y put t o use i n t h e development o f new materials. The s y n t h e t i c p y r e t h r o i d i n s e c t i c i d e s w h i c h a r e r a p i d l y r e p l a c i n g many o f t h e o l d e r p r o d u c t s may be used t o i l l u s t r a t e t h e point. W h i l e t h e s e compounds a r e i n s e c t i c i d a l a t a l m o s t u n b e l i e v a b l y low r a t e s , t h e i r mode o f a c t i o n i s t h e same o r s i m i l a r t o t h a t of DDT ( 4 ) . When c h l o r i n a t e d , as i s t h e c a s e w i t h some o f t h e more e f f e c t i v e ones, t h e y i n d e e d become c h l o r i n a t e d h y d r o c a r b o n s and, as such, t h e y must be s c r u t i n i z e d j u s t as c a r e f u l l y as i f DDT itself was t h e compound under c o n s i d e r a t i o n . One t r u s t s t h a t t h i s i s b e i n g done, but i t i s i m p o r t a n t t h a t t h e l o w - r a t e phenomenon n o t be a l l o w e d t o have an undue i n f l u e n c e as t h e o v e r a l l t o x i c o l o g y o f t h e s e new p e s t i c i d e s a r e b e i n g a s s e s s e d . C h e m i c a l s c a p a b l e o f k i l l i n g i n s e c t s a t r a t e s f a r below t h o s e around which our p r e s e n t c r i t e r i a f o r s a f e t y e v a l u a t i o n were based do pose a s p e c i a l problem. I f t h e y a r e i n s e c t i c i d a l a t such low doses, a r e o t h e r a s p e c t s o f t h e i r t o x i c o l o g i c a l p r o p e r t i e s perhaps as e q u a l l y s p e c t a c u l a r ? The problem i s n o t so much r e l a t e d t o t h o s e t o x i c i t i e s d e t e c t e d u s i n g c o n v e n t i o n a l t e s t i n g p r o t o c o l s , but t o t h o s e w h i c h have n o t e v o l v e d as problems w i t h c h e m i c a l s t h a t a r e f a r l e s s i n s e c t i c i d a l . C h e m i c a l s which might a d v e r s e l y a f f e c t t h e m e n t a l s t a t e o r immune c a p a c i t y o f humans f a l l w i t h i n t h i s l a t t e r c a t e g o r y . As o f t e n t h e c a s e , i t i s t h e unknown t h a t i s most f r i g h t e n i n g and t h e knowledge accumulated from y e a r s o f e x p e r i e n c e w i t h t h e o l d e r m a t e r i a l s i s o f l i t t l e c o m f o r t when a t t e m p t i n g t o e s t i m a t e a l l h a r m f u l e f f e c t s o f new c h e m i c a l s proposed as p e s t i c i d e s . M e t a b o l i t e s and
Toxicity
As p r e v i o u s l y p o i n t e d o u t , t h e problems a s s o c i a t e d w i t h u s i n g m e t a b o l i s m d a t a t o a s s e s s r i s k s a r e enormous. Knowing t h a t one cannot a c c u r a t e l y p r e d i c t t h e a c u t e t o x i c i t y o f a c h e m i c a l t o l a b o r a t o r y mice, o r t o d i f f e r e n t s t r a i n s even a f t e r t h e t o x i c i t y i n one s t r a i n i s known, v i v i d l y d e m o n s t r a t e s how v u l n e r a b l e our p r e d i c t i o n s may be r e l a t i n g t o more e l u s i v e i s s u e s such as r e p r o d u c t i v e d i s o r d e r s , growth and development, m e n t a l h e a l t h , a g i n g p r o c e s s e s , immune d e f i c i e n c i e s , and g e n e t i c i n t e g r i t y o f i n d i v i d u a l s and p o p u l a t i o n s . Because o f t h e numerous p o t e n t i a l r i s k s a s s o c i a t e d w i t h e a c h p e s t i c i d e m e t a b o l i t e , i t i s d i f f i c u l t t o comprehend a system t h a t would r e a s o n a b l y a s s u r e a d e q u a t e l y t e s t i n g . T h a t m e t a b o l i t e s do i n d e e d i n f l u e n c e t o x i c i t y , however, can be i l l u s t r a t ed u s i n g s p e c i f i c m e t a b o l i t e s and a g i v e n t y p e o f t o x i c r e s p o n s e . A few common examples w i l l s e r v e t o make t h e p o i n t . H e p t a c h l o r and H e p t a c h l o r E p o x i d e . H e p t a c h l o r i s a c y c l o d i e n e i n s e c t i c i d e i n t r o d u c e d around t h e same t i m e as DDT and, u n t i l r e c e n t l y , was used e x t e n s i v e l y . I t i s i m p o r t a n t t o t h e p r e s e n t d i s c u s s i o n because i t was t h e f i r s t c a s e where a m e t a b o l i t e o f a p e s t i c i d e was p r o v e n t o be i n v o l v e d i n t h e t o x i c r e s p o n s e assumed
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i n i t i a l l y t o r e s u l t s o l e l y from t h e a p p l i e d c h e m i c a l . The s i t u a t i o n came t o l i g h t when h e p t a c h l o r - s p e c i f i c means o f r e s i d u e a n a l y s i s r e v e a l e d t h a t t h e " t o x i c a n t " was no l o n g e r p r e s e n t , but t h a t b i o a s s a y s showed o t h e r w i s e . S o l v e n t e x t r a c t s o f s o i l s and t r e a t e d p l a n t s c o n t i n u e d t o k i l l h o u s e f l i e s and o t h e r t e s t s p e c i e s l o n g a f t e r t h e h e p t a c h l o r had d i s s i p a t e d below l e v e l s d e t e c t e d by c h e m i c a l and i n s t r u m e n t a l a n a l y s i s . Subsequent a n a l y s e s demonstrated t h a t t h e c u l p r i t was h e p t a c h l o r e p o x i d e , a m e t a b o l i t e j u s t as t o x i c , i f n o t more so, as h e p t a c h l o r and much more p e r s i s t e n t i n t h e environment. The r e s u l t o f t h i s d i s c o v e r y was t h a t a l l f u t u r e r i s k assessments o f h e p t a c h l o r had t o i n c l u d e t h e m e t a b o l i t e , and, u l t i m a t e l y , t h e e p o x i d e was l a r g e l y r e s p o n s i b l e f o r t h e demise o f h e p t a c h l o r as a major a g r i c u l t u r a l c h e m i c a l . Whether t h e d i s c o v e r y o f h e p t a c h l o r e p o x i d e has e v e r saved a human l i f e o r c o n t r i b u t e d t o t h e p r e v e n t i o n o f a " S i l e n t S p r i n g " w i l l n e v e r be known f o r c e r t a i n . That i s n o t i m p o r t a n t . The i m p o r t a n t t h i n g i s t h a t t h e d i s c o v e r y , and hundreds o f a s i m i l a r n a t u r e w h i c h f o l l o w e d , p r o v i d e d new i n f o r m a t i o n a p p l i c a b l e t o most c h e m i c a l s t h a t improved t h e v a l i d i t y o f t h e r i s k assessment p r o c e s s . P a r a t h i o n and Paraoxon. A g a i n , t h i s r e p r e s e n t s a r e a c t i o n ( t h e s u l f u r o x i d a t i o n of a thiophosphate p e s t i c i d e ) that i s f a m i l i a r to most i n t h e p e s t i c i d e a r e a . U n l i k e h e p t a c h l o r e p o x i d e , paraoxon i s n o t a s t a b l e compound and i t s a c t u a l p r e s e n c e i n a p o i s o n e d a n i m a l was v e r y d i f f i c u l t t o d e m o n s t r a t e . The oxons o f o t h e r organop h o s p h o r o t h i o a t e s a r e n o t so e l u s i v e . I n any e v e n t , t h e paraoxon m e t a b o l i t e i s an e x c e l l e n t example o f where an u n d e r s t a n d i n g o f m e t a b o l i c p r o c e s s e s and t h e i r p o t e n t i a l t o x i c o l o g i c a l s i g n i f i c a n c e a l e r t e d s c i e n t i s t s t o t h e l i k e l i h o o d t h a t such a m e t a b o l i t e e x i s t e d . Many y e a r s o f work w i t h s i m i l a r compounds had e s t a b l i s h e d t h a t t h e i n s e c t i c i d a l t h i o p h o s p h a t e s r e q u i r e d o x i d a t i o n t o t h e P=0 form i n order to i n h i b i t the n e u r o t r a s m i t t e r a c e t y l c h o l i n e s t e r a s e , the biochemical b a s i s of t h e i r t o x i c a c t i o n . Paraoxon was e v e n t u a l l y i s o l a t e d i n v i v o and now c o n s i d e r a t i o n o f t h e oxon i s a v i t a l p a r t of t h e o v e r a l l r i s k assessment o f t h i s group o f p e s t i c i d e s . C a r b a r y l and 1-Naphthol. The mode o f a c t i o n o f t h e carbamate i n s e c t i c i d e s i s , l i k e t h e organophosphorus compounds, i n h i b i t i o n o f a c e t y l c h o l i n e s t e r a s e . However, no m e t a b o l i c a c t i v a t i o n i s r e q u i r e d as w i t h t h e l a t t e r i n s e c t i c i d e s . C a r b a r y l i s t h e most w i d e l y u s e d carbamate and, i n f a c t , i s one o f t h e most w i d e l y used p e s t i c i d e s i n the world. I t s a c u t e o r a l LD50 t o r a t s i s u s u a l l y r e p o r t e d t o be i n t h e 400 t o 600 mg/kg r a n g e . Because c h e m i c a l h y d r o l y s i s o f t h e e s t e r l i n k a g e y i e l d s 1-naphthol, carbon d i o x i d e , and water, m e t a b o l i s m v i a t h i s r o u t e would be e x p e c t e d t o y i e l d p r o d u c t s o f l i t t l e t o x i c o l o g i c a l s i g n i f i c a n c e t o mammals. B o t h carbon d i o x i d e and water a r e o b v i o u s l y o f no h a z a r d and 1-naphthol has an a c u t e o r a l LD50 t o r a t s of over 2500 mg/kg. W h i l e t h e m e t a b o l i s m o f c a r b a r y l i s now known to be e x t r e m e l y complex and t o i n v o l v e f o r m a t i o n o f t o x i c o x i d a t i v e m e t a b o l i t e s as w e l l as t h e n o n t o x i c h y d r o l y t i c p r o d u c t s , t h e l a t t e r were once thought t o p r e d o m i n a t e and e a r l y r i s k assessments were based on t h i s assumption ( 5 ) .
9. DOROUGH
Mammalian Metabolism
M e t a b o l i c P r o c e s s e s and
111
Toxicity
I s o l a t i o n , i d e n t i f i c a t i o n , and d e t e r m i n a t i o n o f t h e t o x i c i t y o f a l l m e t a b o l i t e s o f a p e s t i c i d e may appear t o be n e c e s s a r y t o p r o p e r l y a s s e s s r i s k , but t h i s i s n o t p o s s i b l e and perhaps n o t always t r u e . The time and r e s o u r c e s t h a t would be r e q u i r e d t o do so would mean almost c e r t a i n d e a t h t o t h o s e c h e m i c a l s p r e s e n t l y under development, and no such c o n s i d e r a t i o n c o u l d be g i v e n t o t h e hundreds o f p e s t i c i d e s a l r e a d y on t h e market. T h i s means t h a t o t h e r methods f o r e s t i m a t i n g t h e t o x i c i t y o f i n d i v i d u a l m e t a b o l i t e s must be employed. The p r o c e s s would be made r a t h e r s i m p l e i f g i v e n t y p e s o f i n v i v o c h e m i c a l and b i o c h e m i c a l r e a c t i o n s were known t o always have t h e same e f f e c t on t o x i c i t y . That i s , t h a t a l l e s t e r h y d r o l y s i s r e s u l t e d i n d e t o x i f i c a t i o n , t h a t a l l s u l f u r o x i d a t i o n s were a c t i v a t i o n s , and t h a t a l l c o n j u g a t i o n s r e n d e r e d a compound n o n t o x i c and r e a d i l y e x c r e t a b l e from t h e body. U n f o r t u n a t e l y , t h i s i s n o t t h e c a s e . However, when a p p l i e d t o s p e c i f i c t y p e s o f c h e m i c a l s , our knowledge and u n d e r s t a n d i n g o f how c e r t a i n r e a c t i o n s l i k e l y a f f e c t t o x i c i t y a r e s u f f i c i e n t t o be e x t r e m e l y v a l u a b l e i n e s t i m a t i n g pesticide risk. The more t h a t i s known about a p a r t i c u l a r c h e m i c a l group i n t h i s r e g a r d , t h e more l i k e l y t h e p r e d i c t i o n w i l l be an a c c u r a t e one. O v e r a l l M e t a b o l i c Processes. In g e n e r a l , metabolic processes which f a c i l i t a t e e l i m i n a t i o n o f a p e s t i c i d e from t h e body a r e c o n s i d e r e d desirable. T h i s i s based a g r e a t d e a l on our l o n g h i s t o r y o f a s s o c i a t i n g t o x i c i t y w i t h c h e m i c a l s t h a t accumulate i n t h e body. A r s e n i c , l e a d , mercury and o t h e r m e t a l s s u b s t a n t i a t e t h e s e c o n c e r n s as do more modern s y n t h e t i c o r g a n i c c h e m i c a l s such as DDT and m i r e x . Because so many c h e m i c a l s r a p i d l y v o i d e d from t h e body a r e now known t o be e x t r e m e l y hazardous, r i s k s and e x c r e t i o n r a t e s a r e e v a l u a t e d very c a r e f u l l y . S t i l l , storage of m e t a b o l i t e s i s not a p o s i t i v e c h a r a c t e r i s t i c even f o r t h o s e compounds l i k e DDE whose danger, i f any, as a body burden has n o t been e s t a b l i s h e d . N o n e t h e l e s s , so l o n g as p e s t i c i d e s and t h e i r m e t a b o l i t e s remain i n t h e body, t h e p o t e n t i a l f o r damage remains. Once removed, t h e p o t e n t i a l c e a s e s and emphasis may be p l a c e d on d e t e r m i n i n g t h e t o x i c o l o g i c a l consequences o f t h e c h e m i c a l h a v i n g p a s s e d t h r o u g h t h e body. I t s h o u l d be c l e a r , t h e r e f o r e , t h a t t h e a c t u a l m e t a b o l i c p r o c e s s e s w h i c h a r e r e s p o n s i b l e f o r c l e a r a n c e o f t h e p e s t i c i d e need n o t always be known i n o r d e r t o be u s e f u l i n r i s k assessment. Such i n f o r m a t i o n , r o u t e s and r a t e s o f e x c r e t i o n , i s u s u a l l y a v a i l a b l e from r a d i o t r a c e r s t u d i e s l o n g b e f o r e t h e number and c h e m i c a l n a t u r e o f t h e m e t a b o l i t e s have been d e f i n e d . A t h o r o u g h e v a l u a t i o n o f t h e data at t h i s stage i s c r i t i c a l to the proper planning of f u r t h e r m e t a b o l i s m s t u d i e s and, combined w i t h o t h e r d a t a , might v e r y w e l l p r o v i d e a b a s i s f o r c o n d u c t i n g o n l y a f r a c t i o n o f t h e s t u d i e s which a r e "recommended" i n r e g i s t r a t i o n g u i d e l i n e s . Processes vs Products. While i t i s o f t e n s t a t e d t h a t c e r t a i n metabolic r e a c t i o n s g e n e r a l l y represent e i t h e r a c t i v a t i o n or d e t o x i f i c a t i o n , c a r e must be t a k e n t o d i f f e r e n t i a t e between t h a t w h i c h i s assumed and t h a t which i s known f o r c e r t a i n . F o r example, t h e m e t a b o l i c c o n v e r s i o n o f t h i o p h o s p h a t e s (P=S) t o phosphates (P=0)
112
PESTICIDES: MINIMIZING THE RISKS
i s r e c o g n i z e d as an a c t i v a t i o n s t e p and, as mentioned e a r l i e r f o r p a r a t h i o n , t h i s i s c e r t a i n l y t h e c a s e f o r some t h i o p h o s p h a t e s . S i n c e t h e t h i o p h o s p h a t e s a r e weak i n h i b i t o r s o f a c e t y l c h o l i n e s t e r a s e , i t i s p r o b a b l y s a f e t o say t h a t a l l compounds o f t h i s t y p e which i n d u c e d e a t h by s e v e r e i n h i b i t i o n o f a c e t y l c h o l i n e s t e r a s e undergo a c t i v a t i o n v i a s u l f u r o x i d a t i o n . Death w i t h o u t t h i s enzyme i n h i b i t i o n would s u g g e s t an a l t e r n a t e mode o f a c t i o n . Does t h i s mean, t h e n , t h a t m e t a b o l i s m o f a l l t h i o p h o s p h a t e s t o phosphates e f f e c t s g r e a t e r r i s k t o t h e o r g a n i s m i n v o l v e d ? C e r t a i n l y n o t . Many oxons produced i n t h e body a r e so u n s t a b l e t h a t t h e r e a c t a n t i s d e s t r o y e d b e f o r e r e a c h i n g t h e s i t e o f d e t o x i f i c a t i o n . The p o i n t h e r e i s t h a t a p r o d u c t may be formed w h i c h , based on s i m i l a r compounds and even i n v i t r o t e s t i n g , s h o u l d be h i g h l y t o x i c but, i n f a c t , i s not. A s i t u a t i o n o p p o s i t e t o t h a t i s seen w i t h t h e carbamate i n s e c t i c i d e s . As shown i n T a b l e I , d a t a from s t u d i e s i n our l a b o r a t o r y demonstrate t h a t carbamate i n s e c t i c i d e s most t o x i c t o r a t s are a l s o hydrolyzed at a f a s t e r r a t e . Ester h y d r o l y s i s i n these s t u d i e s was measured by q u a n t i t a t i o n o f r a d i o a c t i v e c a r b o n d i o x i d e i n t h e r e s p i r a t o r y gases o f a n i m a l s t r e a t e d o r a l l y w i t h t h e carbamate r a d i o l a b e l e d on t h e c a r b o n y l c a r b o n . Y e t , h y d r o l y s i s o f T a b l e I . Carbamate E s t e r H y d r o l y s i s (0-24 Hour) F o l l o w i n g O r a l Treatment (0.2 Mg/Kg) o f R a t s Compared w i t h T o x i c i t y
Insecticide
Aldicarb Carbofuran Croneton Carbaryl
Percent
Acute O r a l
o f Dose
LD50, Mg/Kg
71.5 54.3 39.9 27.8
1 5 400 600
carbamate i n s e c t i c i d e s i s always c o n s i d e r e d as a d e t o x i f i c a t i o n r e a c t i o n . The r e a s o n f o r t h i s o b v i o u s l y i s t h e f a c t t h a t t h e p r o d u c t s o f h y d r o l y s i s , 1-naphthol from c a r b a r y l , f o r example, a r e always much l e s s t o x i c t o mammals t h a n t h e p a r e n t compound. One p o s s i b i l i t y t h a t n a t u r a l l y comes t o mind when t h e i n v i v o d a t a a r e examined i s t h a t e i t h e r h y d r o l y s i s y i e l d s a more a c t i v e p r o d u c t o r t h a t o x i d a t i o n does so and i t i s t h e l a t t e r which i s h y d r o l y z e d t o carbon d i o x i d e . T h i s i s n o t s u p p o r t e d by our d a t a t o d a t e . None o f t h e h y d r o l y t i c p r o d u c t s t e s t e d has demonstrated any a n t i c h o l i n e s t e r ase a c t i v i t y . Moreover, h e p a t i c enzyme i n d u c t i o n u s i n g m i r e x and o t h e r i n d u c e r s , and c o n f i r m e d by a number o f i n v i t r o r e a c t i o n , d i d not a l t e r r a d i o a c t i v e c a r b o n d i o x i d e p r o d u c t i o n as would have been e x p e c t e d i f h y d r o l y s i s r e s u l t e d from t h e f o r m a t i o n o f an o x i d a t i v e metabolite.
9.
DOROUGH
Mammalian Metabolism
113
In s i m i l a r s t u d i e s , we have compared t h e t o x i c i t y o f t h e same carbamate i n s e c t i c i d e t o s e v e r a l l a b o r a t o r y a n i m a l s p e c i e s t h a t v a r i e d c o n s i d e r a b l y i n s o f a r as r a t e s o f i n v i t r o e s t e r h y d r o l y s i s was c o n c e r n e d . A g a i n , i t was t h e more s u s c e p t i b l e s p e c i e s w h i c h demonstrated t h e f a s t e r r a t e s o f h y d r o l y s i s ( 6 ) . N e i t h e r a b s o r p t i o n from t h e gut, o r t o t a l m e t a b o l i s m as e v i d e n c e d by e x c r e t i o n r a t e s and m e t a b o l i t e s e x c r e t e d , was s i g n i f i c a n t l y d i f f e r e n t when t h e same carbamate was a d m i n i s t e r e d t o e i t h e r r a t s , mice, g u i n e a p i g s , o r g e r b i l s . A t t h i s p o i n t , i t can o n l y be s u r m i s e d t h a t t h e same p r o p e r t i e s t h a t make t h e carbamates e x c e l l e n t i n v i v o i n h i b i t o r s o f a c e t y l c h o l i n e s t e r a s e a l s o make them e x c e l l e n t s u b s t r a t e s f o r o t h e r esterases. Minimizing
Risk
There i s l i t t l e question at t h i s p o i n t t h a t a b e t t e r understanding o f mammalian m e t a b o l i s m o f p e s t i c i d e s can m i n i m i z e p e s t i c i d e r i s k . T h e r e remains, however, c o n s i d e r a b l e c o n c e r n about t h e e f f e c t i v e n e s s o f t h e use o f m e t a b o l i s m d a t a i n t h i s r e g a r d and o f t h e l a c k o f c o o r d i n a t e d e f f o r t s t o improve t h e d e s i g n o f m e t a b o l i s m s t u d i e s t o more a p p r o p r i a t e l y a d d r e s s t h e i s s u e o f r i s k assessment. One o f t h e b i g g e s t o b s t a c l e t o a s s u r i n g t h a t a b e t t e r u n d e r s t a n d i n g o f p e s t i c i d e m e t a b o l i s m i n mammals w i l l be u s e f u l i n m i n i m i z i n g p e s t i c i d e r i s k i s t h e absence o f any d e s i g n a t e d group o f e x p e r t s whose m i s s i o n i t i s t o do j u s t t h a t . Compound-by-compound r e v i e w i n v o l v i n g , by n e c e s s i t y , a few r e g u l a t o r y s c i e n t i s t s and some number o f i n d u s t r y r e p r e s e n t a t i v e s i s a v e r y i n e f f i c i e n t and p o t e n t i a l l y dangerous means o f d e t e r m i n i n g p e s t i c i d e r i s k . D i f f e r e n t c h e m i c a l groups pose d i f f e r e n t problems and i t i s u n r e a s o n a b l e t o e x p e c t a r e v i e w e r , o r a s e r i e s of r e v i e w e r s , t o keep a b r e a s t o f t h e l a t e s t developments w i t h i n a p a r t i c u l a r f i e l d , much l e s s t o r e n d e r a d e c i s i o n as t o whether i t s p r o p o s e d use i s a l l o w e d or d i s a l l o w e d . The t e n d e n c y may be t o p l a y i t s a f e and t o d e l a y a d e c i s i o n as l o n g as a p p a r e n t r a t i o n a l r e a s o n s t o do so c o u l d be d e v i s e d . Groups o f e x p e r t s would have t h e same t e n d e n c y , but s u c h a group would b e n e f i t from t h e knowledge t h a t i s a v a i l a b l e o n l y when t h o s e who b e s t know a s u b j e c t a r e brought t o g e t h e r . W h i l e i m p e r f e c t , t h e p e e r r e v i e w system o f NSF and NIH, and t h e EPA,s p r o c e d u r e f o r d e v e l o p i n g Water C r i t e r i a Documents, e t c . , a r e t h e t y p e s o f d e c i s i o n - m a k i n g p r o c e s s e s w h i c h i n s t i l l c o n f i d e n c e o f t h e n a t u r e r e q u i r e d f o r making p e s t i c i d e r i s k assessments. W i t h p e s t i c i d e r e g i s t r a t i o n , i t would not be f e a s i b l e f o r a s c i e n t i f i c r e v i e w p a n e l t o r u l e on day-to-day m a t t e r s . The r o l e o f such a p a n e l on m e t a b o l i s m would be t o p e r i o d i c a l l y examine t h e r e q u i r e m e n t s , o r g u i d e l i n e s , f o r r e g i s t r a t i o n and, u s i n g s e l e c t chemicals then being reviewed, determine i f the process i s working i n t h e most e f f e c t i v e and e f f i c i e n t manner. I d e a l l y , t h e r e would be a general metabolism panel to consider issues p e r t i n e n t to the o v e r a l l t o p i c o f m e t a b o l i s m and p e s t i c i d e r i s k . S i m i l a r p a n e l s o f e x p e r t s would t h e n be formed t o d e a l w i t h each major c h e m i c a l group o f p e s t i c i d e s , and an ad hoc p a n e l t o a d d r e s s m e t a b o l i s m - r i s k i s s u e s of miscellaneous p e s t i c i d e s . The ad hoc p a n e l might c o n s i s t o f , among o t h e r s , one member from each c h e m i c a l - g r o u p p a n e l , and t h e g e n e r a l p a n e l c o u l d c o n s i s t
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of, among others, one member from each chemical- ov panel. In addition to risk matters pertaining to pesticide registration, the chemical-group panels would hopefully become the national advisory body for all major problems that arise with chemicals falling within their area of expertise. The problems are just too diverse for one panel of advisors to adequately address all pesticide issues. For some, a proposal to "panelize" the risk assessment process may be taken as a criticism of individuals presently responsible for such matters, and/or a mechanism to share the blame. The former is totally untrue and the latter only partially true. A regulatory reviewer is somewhat like a judge; that is, one who renders a decision based on that which has been deemed proper by appropriate authorities. Their role is not to make the rules. Panels of experts would serve to assist in making the rules and in lending advice to the reviewers. Thus, the views of the reviewer, backed by the panel, would become stronger. If not backed by the panel, then, the reviewer should take comfort in the fact that she/he, or they, do not stand alone but are making a decision representative of those who are most qualified to render a judgement. Whether a reviewer, a panel member, an industry representative, or just a concerned citizen, it is essential that each recognize that there are severe risks involved in decision making. Risk assessment of pesticides is difficult and not all decisions related thereto will be flawless. Only those who are extremely secure in their roles as scientists and as caring human beings should dare take on such awesome responsibilities. Literature Cited 1. 2. 3. 4. 5. 6.
Williams, R. T. In "The Biochemistry of Foreign Compounds"; Parke, D. V., Ed.; Pergamon Press: London, 1968; p. ix. Mirvish, S. S. J. Toxicol. Environ. Health 1977, Vol.2, 1267-77. "The Merck Index", Merck: Rahway, NJ, 1983; 10th ed., p.4058. Casida, J. E . ; Gammon, D. W.; Glickman, A. J.; Lawrence, L. J. Ann. Rev. Pharmaco. Toxicol. 1983, Vol. 23, 413-38. Kuhr, R. J.; Dorough, H. W. "Carbamate Insecticides: Chemistry, Biochemistry and Toxicology"; CRC Press: Cleveland, 1976; p. 146. Benson, W. H.; Dorough, H. W. Pestic. Biochem. Physiol. 1984, Vol. 21, 199-206.
RECEIVED
December 17, 1986
