Probing Bioactive Mechanisms - ACS Publications - American

Chapter 24

Correlations and Mechanisms of Chemical Toxicity in Animals 1

Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 30, 2015 | http://pubs.acs.org Publication Date: November 14, 1989 | doi: 10.1021/bk-1989-0413.ch024

Philip S. Magee and James W. King

2

1BIOSAR Research Project, Vallejo, CA 94591 and School of Medicine, University of California, San Francisco, CA 94143 U.S. Army Chemical Research, Development and Engineering Center, Aberdeen Proving Ground, MD 21010-5423 2

Toxic response of animals to important chemical classes can be analyzed in terms of mechanistic descriptors, even with data from many different laboratories. We estimate that 8-10 data points are needed to support each descriptor with inter-laboratory data, a doubling of the requirement for single labs. Given the large data sets available from toxicity compilations like Sax and RTECS, we were able to achieve many successful correlations. Differences among animals or between different routes of administration were easily perceived in mechanistic terms. Important conclusions can be drawn in a three-way comparison of chemical class, animals and routes by examining the form of correlation and the mean level of toxicity. Toxicity data derived from single laboratories on selected animal subjects are known to correlate well with mechanistic descriptors such as logP. Examples are reported for barbiturates on rats, mice and rabbits (1_) , bicyclophosphates on mice (2) , 2,6-dialkylanilines on rats Q ) , and phenols on mice (4). Optimal dependance on logP is frequently observed. Data reported from different laboratories may involve different strains, size, sex, age, vehicle and technique on the reported animal. A l l of these factors make unknown contributions to the variance of inter-laboratory data. Moreover, there is no consistent protocol for reporting toxicity data. Perusal of a recent compila tion of pesticides (5) reveals LD50 toxicities reported as: about 50, 400-800, 3362, approximately 335, 318-557, 1.125, 568.9, 2900±800, 1717 (1366-2156) and 1077±78. A l l levels of confidence from rough estimates to precise knowledge are expressed in these reports. However reported, these data are converted to single LD50 numbers in RTECS (6) and Sax (7), with a bias toward selecting the lowest of several values. These compilations provide the only convenient source of large data sets of diverse chemical structure in most areas of interest. A key objective of this study is to analyze 0097-6156/89/0413-O390$06.00/0 © 1989 American Chemical Society

In Probing Bioactive Mechanisms; Magee, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


24.

MAGEE & KING

Chemical Toxicity in Animals

391

such s e t s i n terms o f m e c h a n i s t i c d e s c r i p t o r s t o a s s e s s d i f f e r e n c e s between c h e m i c a l c l a s s e s , a n i m a l s and r o u t e s o f a d m i n i s t r a t i o n . We hope t o a c h i e v e a p a r t i a l a n a l y s i s o f t h e d a t a w i t h o u t a t t e m p t i n g to p i c k - u p a l l t h e i n t e r l a b v a r i a n c e r e l a t e d t o a n i m a l and e x p e r i mental f a c t o r s . I n t e r - l a b o r a t o r y t o x i c i t y d a t a has been used t o d e v e l o p a g e n e r a l , n o n - m e c h a n i s t i c model f o r p r e d i c t i n g r a t LD50 ( 8 ) . F u r t h e r , the c o r r e l a t a b i l i t y o f RTECS d a t a i n m e c h a n i s t i c terms was r e c e n t l y demonstrated by L i p n i c k i n d e v e l o p i n g a b i l i n e a r l o g P r e l a t i o n f o r a l c o h o l t o x i c i t y t o r a t s (n • 127) ( 9 ) . One o f o u r o b j e c t i v e s i s t o uncover more complex m e c h a n i s t i c f a c t o r s than t h e f r e q u e n t l y r e p o r t e d dependance on l o g P . Phenol

Toxicity

In o r d e r t o g e t a f i x on t h e d i f f e r e n c e i n v a r i a n c e , a s e t o f s i m p l e phenols (n » 26) w i t h i p t o x i c i t y t o a l b i n o mice was s e l e c t e d t o study the s t r u c t u r e - a c t i v i t y v a r i a n c e f o r s i n g l e l a b o r a t o r y data ( 4 ) . In a comparative study (n • 5 0 ) , a d i v e r s e s e t o f p h e n o l s w i t h i p t o x i c i t y t o "mice" p r o v i d e d t h e e x p e c t e d v a r i a n c e f o r i n t e r - l a b o r a t o r y d a t a ( 7 ) . M u l t i p l e r e g r e s s i o n was used t o a n a l y z e a l l d a t a sets. I n t h e s i n g l e l a b d a t a s e t , measured l o g P was found t o c o r r e l a t e b e t t e r than EE, b u t t h e l a t t e r was a c c e p t a b l e and n e c e s s a r y f o r comparison w i t h t h e i n t e r l a b a n a l y s i s . M o l e c u l a r weight c o r r e c t i o n was found t o be i m p o r t a n t f o r t h e s i n g l e l a b a n a l y s i s and i s used throughout t h i s s t u d y . C o r r e c t i o n s f o r p h e n o l i o n i z a t i o n and f o r i n c r e m e n t a l t o x i c i t y o f n i t r o p h e n o l s a r e important i n the i n t e r l a b analysis. I n b o t h s e t s , t h e p h e n o l t o x i c i t y depends on a low p o s i t i v e s l o p e o f SII w i t h o u t c u r v a t u r e o v e r a broad s t r u c t u r a l v a r i a tion. The lower s l o p e o b s e r v e d f o r t h e i n t e r l a b d a t a may be a s i m p l e consequence o f the weaker c o r r e l a t i o n ( r » 0.764 v s . r • 0.931) which g e n e r a l l y causes r e d u c t i o n i n t h e r e g r e s s i o n c o e f f i cients. The i n t e r l a b d a t a s e t (n * 50) (T) comes from 17 d i f f e r e n t s o u r c e s and y e t , the many a n i m a l and e x p e r i m e n t a l v a r i a b l e s do n o t obscure the c o r r e l a t i o n with mechanistic d e s c r i p t o r s . A l b i n o mouse, i p t o x i c i t y (4) Log MW/LD50 - 0.429 III - 0.546 T * 12.49 n - 26 r - 0.931 F - 156.0 Mouse, i p t o x i c i t y (7) Log MW/LD50 - 0.199 III - 0.196 Za + 0.438 IN02 - 0.274 T = 5.65 4.47 4.12 n * 50 r - 0.764 F - 21.44 Mouse i v t o x i c i t y a l s o c o r r e l a t e s w i t h a low p o s i t i v e s l o p e o f III. C o r r e c t i o n s a r e needed f o r hydroxyphenols (I0H) and n i t r o phenols (IN02). Both c o r r e c t i o n s a r e p o s i t i v e s u g g e s t i n g t h e o p e r a t i o n o f i n t o x i c a t i o n mechanisms, perhaps v i a quinones f o r h y d r o x y phenols . The n i t r o p h e n o l c o r r e c t i o n i s s i m i l a r t o t h a t o b s e r v e d f o r i p t o x i c i t y (0.438).


392

PROBING BIOACTIVE MECHANISMS

Mouse, i v t o x i c i t y (7)


Log MW/LD50 - 0.126 ZII + 0.503 IOH + 0.605 IN02 + 0.060 T - 4.57 5.55 4.86 n - 30 r - 0.815 F - 17.13 By c o n t r a s t , mouse, r a t and g u i n e a p i g o r a l t o x i c i t i e s c o r r e l a t e o n l y w i t h r e a c t i v i t y f a c t o r s , Ea and/or 126 ( o r i>2,6) • The 2 , 6 - f a c t o r appears t o be more than s t e r i c as t h e i n d i c a t o r v a r i a b l e , 126, o f t e n c o r r e l a t e s b e t t e r than U2 6* There i s no dependance on lipophilicity. Mouse, o r a l t o x i c i t y (7) Log MW/LD50 - 0.425 Id + 0.388 126 - 0.707 T - 7.14 3.05 n - 42 r - 0.854 F « 52.54 Rat,

o r a l t o x i c i t y (7)

Log MW/LD50 - 0.396 id + 0.520 126 - 0.876 T - 5.55 4.54 n - 71 r = 0.729 F « 38.58 Guinea p i g o r a l t o x i c i t y (n « 13) c o r r e l a t e s o n l y w i t h t h e e l e c t r o n i c e f f e c t (0.618 Id - 0.822, r * 0.914), w h i l e dermal ( r a t ) and sub-cutaneous ( r a t , mouse) t o x i c i t i e s a l l depend on p o s i t i v e s l o p e s o f Id o r U2,6* R a b b i t dermal depends on b o t h . R a b b i t , dermal

toxicity(7)

Log MW/LD50 - 0.482 Id + 0.338 u - 1.10 T « 5.58 3.39 n - 19 r - 0.889 F = 30.26 2 6

The t r a n s i t i o n between s i m p l e dependance on l i p o p h i l i c i t y t o e x c l u s i v e dependance on r e a c t i v i t y f a c t o r s l e d us t o c o n s i d e r two s e p a r a t e c l a s s e s o f p h e n o l t o x i c i t y as d i s c u s s e d i n t h e c o n c l u s i o n s . In a d d i t i o n , t h e c l e a r dependance on s t e r i c o r " o r t h o " e f f e c t s f o r 2 , 6 - s u b s t i t u e n t s a l s o b r i n g s t h e commonly a c c e p t e d u n c o u p l i n g o f o x i d a t i v e p h o s p h o r y l a t i o n i n t o q u e s t i o n as a mechanism o f d e a t h . Diarylamine

Rodenticides

Mouse o r a l d a t a on d i a r y l a m i n e r o d e n t i c i d e s was r e c e n t l y r e p o r t e d by D r e i k o r n and 0 D o h e r t y w i t h o u t any attempt a t a n a l y s i s ( 1 3 ) . S e t 1 (n • 14) has one 2 , 4 , 6 - t r i n i t r o p h e n y l r i n g w i t h s u b s t i t u e n t s v a r i e d on t h e o t h e r r i n g . S e t 2 (n * 18) d i f f e r s i n h a v i n g one 2-CF3,4,6dinitrophenyl ring. These a r e presumed t o be u n c o u p l e r s o f o x i d a t i v e p h o s p h o r y l a t i o n w i t h N H - a c i d i t y as an i m p o r t a n t f a c t o r ( 1 4 ) . One o u t l i e r was d e l e t e d from each s e t . f


24.

Mouse O r a l Log

Log

(Set

1)

(Set

2)

393

(14)

MW/LD100 - 1.30 l a - 0.284 T « 2.41 n - 13 r - 0.588

Mouse O r a l



MAGEE & KING

F - 5.81

(14)

MW/LD100 - 2.20 lo + 0.390 T » 7.82 n - 17 r « 0.896

F - 61.07

Both s e t s depend e x c l u s i v e l y on an e l e c t r o n i c e f f e c t c o n t r o l l i n g t h e p o p u l a t i o n o f t h e t o x i c a n i o n , an e f f e c t a n a l o g o u s t o t h a t of p h e n o l s i n u n c o u p l i n g o x i d a t i v e p h o s p h o r y l a t i o n (14). The weaker c o e f f i c i e n t o f S e t 1 r e l a t i v e t o S e t 2 i s p r o b a b l y a consequence o f the weaker c o r r e l a t i o n ( r * 0.588 v s . r » 0.896), r a t h e r than a r e a l d i f f e r e n c e i n response. The d a t a were e x t r a c t e d from bar graphs i n the p u b l i c a t i o n (13) and may have more e r r o r than t a b u l a t e d v a l u e s . A r y l N-Methylcarbamate T o x i c i t y C o r r e l a t i o n s o f i n s e c t i c i d a l carbamates a g a i n s t t h e h o u s e f l y (AChE pI50) (10) and brown p l a n t h o p p e r (AChE pKd, Logl/LC50) 0 1 , 1 2 ) have r e v e a l e d complex f a c t o r s such as e l e c t r o n i c and s t e r i c e f f e c t s com b i n e d w i t h s e l e c t i v e hydrogen-bonding o f p o l a r s u b s t i t u e n t s . These problems a r e c o m p l i c a t e d by t h e f a c t t h a t o r t h o - , meta- and p a r a s u b s t i t u e n t s c o r r e l a t e by s e p a r a t e mechanisms i n honey-bee t o x i c i t y (P. S. Magee, u n p u b l i s h e d s t u d y , 1986, PM330). I t was n o t s u r p r i s i n g then, t o o b s e r v e a s i m i l a r c o m p l e x i t y i n a l a r g e s e t o f a r y l N-methylcarbamates (n « 27) a g a i n s t r a t s . Both s t e r i c ( i ^ ) and nons t e r i c (12) e f f e c t s a t t h e o r t h o - p o s i t i o n were c o u p l e d w i t h enhanced t o x i c i t y f o r H-bonding s u b s t i t u e n t s (HB). Rat, Log

O r a l T o x i c i t y (7) MW/LD50 T « n -

-0.223 III + 1.35 u - 1.40 12 + 0.438 HB + 0.493 3.85 2.10 2.79 2.12 27 r - 0.765 F « 7.78 2

In b o t h mouse (n * 14) and c h i c k e n (n * 8) o r a l t o x i c i t i e s , t h e o n l y d i s c e r n i b l e f a c t o r i s a s t r o n g p o s i t i v e dependance on H-bonding groups (1.01 HB, 0.98 HB). None o f t h e m e c h a n i s t i c f a c t o r s were s i g n i f i c a n t f o r these s m a l l s e t s . A c t i v e s i t e b i n d i n g i s the p r i n c i p a l m e c h a n i s t i c s t e p i n carbamate t o x i c i t y . Organophosphate T o x i c i t y A n a l y s i s o f l a r g e s e t s o f o r a l t o x i c i t y d a t a (7) f o r r a t s (n « 214), mice (n * 121), c h i c k e n (n « 65), g u i n e a p i g (n * 46) and r a b b i t (n « 39) was g e n e r a l l y u n s u c c e s s f u l i n i d e n t i f y i n g m e c h a n i s t i c f a c t o r s . None o f t h e d e r i v e d e q u a t i o n s a r e c o n s i d e r e d s t r o n g enough t o p r o v i d e a u s e f u l p r e d i c t i v e model. The extreme d i v e r s i t y o f s t r u c t u r e w i t h i n 18 s u b c l a s s e s o f p h o s p h a t e s , phosphonates and phosphoramidates



394


i s presumed r e s p o n s i b l e f o r t h e f a i l u r e . Future success w i l l pro b a b l y depend on f a c t o r i n g t h e s e s e t s i n t o l e s s d i v e r s e groups. The major u s e f u l o u t p u t o f these s t u d i e s were the mean v a l u e s o f L o g MW/ LD50 used f o r i n t e r - c l a s s , i n t e r - a n i m a l and i n t e r - r o u t e comparisons. A s t u d y i n t e n d e d f o r comparison w i t h t h e a n i m a l s e t s s u g g e s t e d the i n v o l v e m e n t o f l i p o p h i l i c i t y i n phosphate t o x i c i t y . Data on honeybee t o x i c i t y o f phosphates, phosphonates and phosphoramidates were a c q u i r e d from UC R i v e r s i d e ( 1 5 ) . T h i s d a t a s e t from a s i n g l e l a b o r a t o r y p r o v i d e d a measure o f o u r method f o r a b r o a d range o f structural variation. P i v a l u e s o f t h e groups around P+0 were summed w i t h a c o r r e c t i o n f o r P+S t o p r o v i d e a III e s t i m a t e d t o p a r a l l e l logP. Seven o u t l i e r s , a l l o f u n u s u a l s t r u c t u r e ( 1 0 % ) , were e l i minated d u r i n g r e g r e s s i o n . I n d i c a t o r v a r i a b l e s f o r a p h e n o l a t e l e a v i n g group (IOAr) and f o r phosphoramidates (INP) were i m p o r t a n t descriptors. Honeybee T o p i c a l Log MW/LD50 « T n -

(Dust) (15)

-0.146 III + 0.811 IOAr - 0.521 INP + 2.51 5.84 6.68 3.02 66 r - 0.691 F - 18.85

Phosphates d i f f e r from carbamates w i t h p h o s p h o r y l a t i o n a s t h e r a t e d e t e r m i n i n g s t e p . The i n v o l v e m e n t o f l o g P (III) i s p r o b a b l y r e l a t e d t o t r a n s p o r t r a t h e r than b i n d i n g . Aniline

Toxicity

A n i l i n e s t u d i e s were d i s a p p o i n t i n g , d e s p i t e t h e a v a i l a b i l i t y o f large data s e t s . We had hoped t o compare a n i l i n e s w i t h p h e n o l s p o i n t by p o i n t , b u t t h e c o r r e l a t i o n q u a l i t y does n o t p e r m i t i t . The e q u a t i o n f o r r a t o r a l t o x i c i t y i s s i g n i f i c a n t , though q u i t e weak (100 r « 35.3). I t indicates a negative s t e r i c e f f e c t f o r 2 , 6 - s u b s t i t u e n t s , o p p o s i t e t o t h a t o b s e r v e d f o r p h e n o l s , and a n e g a t i v e dependance on H-bonding s u b s t i t u e n t s . T h i s suggests i n v o l v e m e n t o f t h e amino-group i n t o x i c i t y w i t h o t h e r H-bonding groups as d i s r u p t i n g f a c t o r s . 2

Rat, O r a l T o x i c i t y (7) Log MW/LD50 T « n -

-0.239 u , 6 - 0.337HB - 0.685 2.40 * 5.01 53 r - 0.594 F - 13.66 2

Another s t u d y on mouse i n t r a p e r i t o n e a l t o x i c i t y i s below s t a n d a r d s i g n i f i c a n c e and i s shown o n l y because i t s u p p o r t s t h e n e g a t i v e s t e r i c e f f e c t f o r 2 , 6 - s u b s t i t u e n t s and shows a low depen dance on III s i m i l a r t o t h e p h e n o l s . Mouse, i p T o x i c i t y

(7)

Log MW/LD50 - 0.191 IK - 0.259 l a - 0.501 u , 6 " 0.062 T - 2.26 1.84 1.84 - 27 r - 0.550 F - 3.33 2

n


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D e s p i t e t h e s e weak r e s u l t s , the mean v a l u e o f L o g MW/LD50 was e v a l u a t e d f o r two a n i m a l s (rat,mouse) and two r o u t e s ( o r a l , i p ) needed to l o c a t e a n i l i n e s on t h e i n t e r - c o m p a r i s o n s c a l e s .


Pyridine

Toxicity

U n l i k e a n i l i n e s , t h e p y r i d i n e s e t s c o r r e l a t e d e a s i l y and w i t h g r e a t consistency. The c o n s i s t e n t f a c t o r i s a s t r o n g , n e g a t i v e dependance on s u b s t i t u e n t e l e c t r o n i c b e h a v i o r , i n d i c a t i n g a n u c l e o p h i l i c t o x i c i t y mechanism. C o r r e l a t i o n s a r e r e m a r k a b l y h i g h c o n s i d e r i n g t h e s m a l l s i z e o f t h e s e t s (n « 10-14). The mouse i v s e t c o n t a i n s no sigma minus groups and depends s i m p l y on Zo. The r a t o r a l s e t has a major o u t l i e r . D e l e t i o n s t r o n g l y improves t h e c o r r e l a t i o n b u t does n o t a l t e r t h e i n t e r p r e t a t i o n . L i p o p h i l i c dependance i n t h e mouse i p r e l a t i o n s u g g e s t s t h a t o t h e r f a c t o r s may emerge when l a r g e r sets are studied. Mouse, i v T o x i c i t y (7_) Log

MW/LD50 - -0.775 lo + 0.156 T - 3.49 n - 14 r - 0.710

F - 12.19

Mouse, i p T o x i c i t y (7) Log

Rat, Log

Log

MW/LD50 - 0.226 EII - 1.14 l a + 0.288 T - 2.51 5.01 n - 14 r - 0.881 F - 19.05 O r a l T o x i c i t y C7) MW/LD50 T n -

-0.618 lo - 0.999 2.66 10 r - 0.686

MW/LD50 T -

-0.707 lo 8.45

n Special

9

F - 7.08

- 1.08

r - 0.955

Inter-Lab/Inter-Animal

F - 71.44 Factors

O r i g i n a l l y , we p l a n n e d t o s t u d y t h e s p e c i a l f a c t o r s a f f e c t i n g t o x i c i t y from m u l t i p l e l a b o r a t o r i e s i n some d e t a i l . Different experi m e n t a l t e c h n i q u e s and a n i m a l v a r i a b l e s c l e a r l y a f f e c t t h e t o x i c i t y e n d - p o i n t when c a r r i e d o u t by d i f f e r e n t i n v e s t i g a t o r s . Of a l l t h e s e t s s t u d i e d , p h e n o l t o x i c i t y t o mice ( i p ) seemed i d e a l f o r a n a l y s i s because o f t h e s e t s i z e (n » 50) and s i m p l i c i t y o f t h e c l a s s 1 mechanism. The p r o j e c t was abandoned on l e a r n i n g t h a t t h e 50 d a t a p o i n t s come from 17 d i f f e r e n t l a b o r a t o r i e s . There a r e too many v a r i a b l e s to support a systematic a n a l y s i s . However, i t was p o s s i b l e t o s t u d y one o f the more i n t e r e s t i n g a n i m a l f a c t o r s , t h e d i f f e r e n t i a l t o x i c i t y o f c h e m i c a l s t o male and female o f t h e same s t r a i n . T o x i c i t y measures f o r 51 p e s t i c i d e s were assembled f o r 78 male/female p a i r s (65 r a t , 10 mouse, 3 o t h e r ) . Of


396



the 78 p a i r s , a s i m p l e count showed M>F (n « 37) and F>M (n * 40), an a p p a r e n t l y e q u a l d i s t r i b u t i o n . The b i a s o f h i g h e r t o x i c i t y t o the female i s r e v e a l e d i n summing Log MW/LD50 [E (F) « 8.05, Z(M) 5.60], and a l s o i n t h e s l o p e o f r e g r e s s i o n o f M v s . F 0.940). . I t i s i n t e r e s t i n g t h a t t h e e x p r e s s i o n c o r r e l a t e s h i g h l y (100 r ^ * 93.0) and p a s s e s a c c u r a t e l y t h r o u g h t h e o r i g i n a f t e r d e l e t i o n o f two e x treme o u t l i e r s . Normal v a r i a t i o n i n a n i m a l t o x i c i t y t e s t i n g i s Log MW/LD50(M) « 0.940 L o g MW/LD50(F) - 0.014 T « 33.84 n - 76 r * 0.969 s « 0.233

F - 1145

r o u g h l y a f a c t o r o f 3.0 ( l o g - 0.477) ( 8 ) , which i s n e a r l y i d e n t i c a l w i t h 2s (0.466) as d e f i n e d above. I f the r e s i d u a l s are normally d i s t r i b u t e d , then 5% o r 4 cases/76 would be e x p e c t e d t o exceed 2 s . There a r e a c t u a l l y 7 c a s e s p l u s t h e two o u t l i e r s o r 9/78 ( 1 1 . 5 % ) , which i n c o m b i n a t i o n w i t h t h e female t o x i c i t y b i a s , i n d i c a t e s a r e a l genetic factor. In o t h e r s t u d i e s , H o l l i n g w o r t h has documented c o l i n e a r r e l a t i o n s (r « 0.785-0.929) f o r p a i r s o f a n i m a l s (n - 9) t e s t e d a g a i n s t t h e same s e r i e s o f organophosphates (n * 7-32) ( 1 6 ) . T o x i c i t y o r d e r s can be deduced from these r e l a t i o n s and from s i m i l a r s t u d i e s o f 88 i n s e c t i c i d e s by Magee (P. S. Magee, u n p u b l i s h e d s t u d y , 1976, PM 9 7 ) . These o r d e r s a r e n o t g e n e r a l , however, even w i t h i n t h e organophosphate c l a s s . U c h i d a and O ' B r i e n d e t e r m i n e d t h e LD50 o f t h e i n s e c t i c i d e , Dimethoate, on seven a n i m a l s and r e l a t e d t h e d a t a a c c u r a t e l y (r « 0.974) t o t h e r a t e o f l i v e r d e g r a d a t i o n i n each a n i m a l (17,18). T h i s s u g g e s t s t h a t d i f f e r e n t i a l d e g r a d a t i o n c a n p l a y a key r o l e i n d e f i n i n g a n i m a l t o x i c i t y o r d e r s . Major d i f f e r e n c e s have a l s o been r e p o r t e d i n t h e t o x i c response t o b u t y l a t e d h y d r o x y t o l u e n e (BHT) o f f i v e d i f f e r e n t s t r a i n s o f male mice ( 2 1 ) . I t i s r a r e t o have d a t a s u c h as H o l l i n g w o r t h r e p o r t s f o r most c l a s s e s o f compounds. I n the u s u a l d a t a s e t , each a n i m a l i s exposed to a d i f f e r e n t s e r i e s o f p h e n o l s , carbamates o r phosphates, and c o l i n e a r r e g r e s s i o n s a r e i m p o s s i b l e . Moreover, n o t a l l d a t a s e t s p r o v i d e s a t i s f a c t o r y c o r r e l a t i o n s t h e r e b y e l i m i n a t i n g t h e i n t e r c e p t as a r e l i a b l e measure o f r e l a t i v e a n i m a l t o x i c i t y . There i s , however, one number t h a t e x p r e s s e s the most p r o b a b l e t o x i c i t y o f each c l a s s , namely the mean v a l u e o f L o g MW/LD50. F o r a random s e t o f r e l a t e d compounds, t h i s number s h o u l d be f a i r l y s t a b l e f o r n>30. These v a l u e s a r e a l l t a k e n a t f a c e v a l u e (no T - t e s t ) and t h e r e i s o b v i o u s o v e r l a p i n some comparisons. However, t h e r e i s a remarkable degree of c o n s i s t e n c y i n t h e o r d e r s o b s e r v e d t o d a t e . The f o l l o w i n g i n t e r - a n i m a l t o x i c i t y o r d e r s a r e based on mean Log MW/LD50 v a l u e s . Organophosphates ( O r a l ) P i g e o n , Duck > Q u a i l , Cat > Rat > Dog, Mouse > G p i g > R a b b i t (1.47) (1.43) (1.17) (1.02) (0.630) (0.521) (0.487) (0.366) (0.226) Carbamates ( O r a l )

C h i c k e n > Rat > Mouse (0.921) (0.306) (0.227)


24.


MAGEE & KING

Phenols

(Oral)

Mouse, G p i g > Rat > Rabbit (-0.499)(-0.485)(-0.720)(-0.974)

Phenols

(Dermal, i v , s c , i p )

Anilines, Pyridines (Oral, ip)


Inter-Route

Mouse > Rat > R a b b i t Mouse > Rat

and I n t e r - C l a s s Comparisons

The e f f e c t o f a d m i n i s t r a t i o n r o u t e on drug a c t i o n i s d i s c u s s e d i n some d e t a i l by Benet (19) and by Rowland ( 2 0 ) . Oral administration f o r c e s a f i r s t - p a s s r o u t e through t h e l i v e r , s u b j e c t i n g t h e t o x i c a n t to enhanced m e t a b o l i s m . Other r o u t e s a r e weaker m e t a b o l i c a l l y , though i n some c a s e s , s k i n c a n d i s p l a y up t o 80% o r more o f l i v e r metabolite a c t i v i t y . I n t h e r a t , f o r example, s k i n i s more e f f i c i e n t than l i v e r i n d e g r a d i n g a r y l carbamates. Our r e s u l t s s u p p o r t t h i s t h e s i s i n terms o f mean Log MW/LD50 v a l u e s f o r p h e n o l t o x i c i t y but n o t f o r carbamate t o x i c i t y . Phenols

(Rat)

iv > ip > sc, dermal > oral (0.373) (-0.157) (-0.429) (-0.469) (-0.720)

Phenols

(Mouse)

iv > ip > sc > oral (0.431) (0.048) (-0.130) (-0.499)

Phenols

(Rabbit)

dermal > oral (-0.869) (-0.974)

Carbamates (Rat)

oral >> (0.306)

dermal (-0.463)

I n t e r - C l a s s Comparisons. C l a s s e s o f t o x i c compounds a r e e a s i l y o r d e r e d by the same measure. Mouse ( O r a l )

Rat

(Oral)

Birds

(Oral)

D i a r y l a m i n e s > Phosphates > Carbamates > P h e n o l s > A n i l i n e s (1.15) (0.487) (0.227) (-0.499) (-0.723) Phosphates >Carbamates > P h e n o l s > A n i l i n e s > P y r i d i n e s (0.630) (0.306) (-0.720) (-0.875) (-2.88) Phosphates >Carbamates (1.17-1.47) (0.921)

Data a r e l a c k i n g f o r d i a r y l a m i n e s and phosphates f o r o t h e r routes of a d m i n i s t r a t i o n . Conclusions I n t e r - l a b o r a t o r y t o x i c i t y data give s t r u c t u r e - a c t i v i t y c o r r e l a t i o n s o f s u f f i c i e n t p r e c i s i o n t o c l a s s i f y mechanism and i n d i c a t e t h e mode of death. S t r o n g e r c o r r e l a t i o n s would be e x p e c t e d by u s i n g i n d i c a t o r v a r i a b l e s f o r gender, age, s i z e , e t c . , b u t t h e s e a r e u n l i k e l y t o enhance the m e c h a n i s t i c d e s c r i p t i o n . Good a n a l y s e s were a c h i e v e d


397


398


i n a l l c a s e s o f s u f f i c i e n t s i z e (n) and d a t a span. W i t h t e n o r more d a t a p o i n t s p e r d e s c r i p t o r , r e g r e s s i o n c o e f f i c i e n t s were u n i f o r m l y s t r o n g and c o n s i s t e n t among r e l a t e d s e t s . I n t r a p e r i t o n e a l and i n t r a v e n o u s t o x i c i t y o f p h e n o l s t o t h e mouse depend on s m a l l p o s i t i v e s l o p e s o f Ell w i t h no o b s e r v a b l e optimum. T h i s s i m p l e b e h a v i o r ( C l a s s 1) cannot be c a u s a l l y d e f i n e d b u t s u g g e s t s a b s o r p t i o n - d e s o r p t i o n from a l i p i d p o o l as t h e r a t e - l i m i t i n g step. A l l o t h e r t o x i c i t i e s e x p l o r e d ( o r a l , dermal, s c ) t o mouse, r a t , g u i n e a p i g and c h i c k e n c o r r e l a t e w i t h p o s i t i v e s l o p e s o f Za and/ o r 2 , 6 - e f f e c t s ( C l a s s 2 ) . D i a r y l a m i n e s , a n i l i n e s and p y r i d i n e s a l s o appear t o behave as C l a s s 2 t o x i c a n t s a g a i n s t mice. These r e a c t i v i t y f a c t o r s i n d i c a t e t a r g e t s i t e e x p r e s s i o n , c o n s i s t e n t w i t h d e a t h by irreversible inhibition. O r a l t o x i c i t y o f a r y l N-methylcarbamates t o t h e r a t i n d i c a t e a c o m p l e x i t y s i m i l a r t o t h a t o b s e r v e d a g a i n s t t h e h o u s e f l y (10) and brown p l a n t h o p p e r (11,12). Complex o r t h o - e f f e c t s and s t r o n g H-bond i n g f o r some s u b s t i t u e n t s combine t o m o d i f y t o x i c i t y . The major m e c h a n i s t i c s t e p i n carbamate p o i s o n i n g i s s t r o n g b i n d i n g t o an AChE site. The p a r a l l e l f a c t o r s i n v o l v e d f o r i n s e c t s and a n i m a l s a r e c o n s i s t e n t w i t h d e a t h due t o AChE i n h i b i t i o n . Honeybee t o x i c i t y was amenable t o p a r t i a l a n a l y s i s and gave hope t h a t t h e a n i m a l d a t a c o u l d be r e s o l v e d . However, t h e bee s e t i s f a r s i m p l e r i n s t r u c t u r a l scope than those r e p o r t e d f o r a n i m a l t o x i c i t y . F a i l u r e t o r e s o l v e these s e t s s u g g e s t s t h a t f a c t o r i n g i n t o s u b s e t s w i l l be needed t o reduce t h e c o m p l e x i t y o f some 18 organophosphate classes. Mean v a l u e s o f L o g MW/LD50 were used t o e s t a b l i s h o r d e r s among a n i m a l s , r o u t e s o f a d m i n i s t r a t i o n and t o x i c a n t c l a s s e s . T h i s method seems i d e a l f o r comparing n o n - o v e r l a p p i n g s t r u c t u r a l s e t s . Moreover, the o r d e r s appear u s e f u l f o r c l a s s i f y i n g mechanism. F o r example, i n t e r - a n i m a l and i n t e r - r o u t e o r d e r s c o n f i r m t h a t phosphates and c a r bamates k i l l by a s i m i l a r mechanism t h a t d i f f e r s from d e a t h by phe nol toxicity. T h i s i s c l e a r l y i n d i c a t e d by r e v e r s a l s i n ( r a t , mouse) and ( o r a l , dermal) o r d e r s . By combining m e c h a n i s t i c i n s i g h t from r e g r e s s i o n a n a l y s i s w i t h t o x i c i t y o r d e r s from Log MW/LD50, we hope t o d e v e l o p an e x t e n s i v e knowledge base i n t h r e e d i m e n s i o n s : animals, routes, t o x i c a n t s . Work i n t h e immediate f u t u r e w i l l c o n t i n u e w i t h organophosphates and extend t o o r g a n o h a l i d e s , o r g a n o m e r c u r i a l s , c o n j u g a t e d v i n y l s and nitroaromatics. Adknowledgement. We w i s h t o thank t h e C h e m i c a l Systems L a b o r a t o r y (Aberdeen P r o v i n g Ground, MD) f o r generous s u p p o r t t h r o u g h B a t t e l l e Columbus L a b o r a t o r i e s (Columbus, OH) under D e l i v e r y Order No. 1398.

Literature Cited 1. 2. 3.

Hansch, C . , Steward, A. R., Anderson, S. M. and Bentley, D., J. Med. Chem., 1967, 11, 1. Casida, J . E., Eto, M., Moscioni, A. D., Engel, J . L., Milbrath D. S. and Verkade, J . G . , Toxicol. Appl. Pharmacol, 1976, 36, 261. Durden, J . A . , J. Med. Chem., 1973, 16, 1316.


24. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18. 19. 20. 21.

MAGEE & KING


399

Biagi, G. L., Gandolfi, O.; Guerra, M. C., Barbaro, A. M. and Cantelli-Forti, G. J. Med. Chem. 1975, 18, 868. Spencer, E. Y . , Guide to the Chemicals Used in Crop Protection, Publication 1093, Canadian Government Publishing Centre, Ottowa, Canada, 1982. U. S. Department of Health and Human Services. Registry of Toxic Effects of Chemical Substances (RTECS), 1981-1982, 1983. Sax, N. I. Dangerous Properties of Industrial Materials, 6th Edi tion, Van Nostrand Reinhold, New York 1984. Enslein, K. and Craig, P. N. J . Environ. Path.-Tox. 1978, 2, 115. Lipnick, R. L., Pritzker, C. S. and Bentley, D. L. In QSAR and Strategies in the Design of Bioactive Compounds; Seydel, J . K. Ed.; VCH Verlagsgesellschaft, West Germany, 1985. Goldblum, A . , Yoshimoto, M. and Hansch, C. J . Agric. Food Chem. 1981, 29, 277. Nishioka, T . , Fujita, T . , Kamoshita, K. and Nakajima, M. Pest. Biochem. Physiol. 1977, 7, 107. Kamoshita, K . , Ohno, I . , Kasamatsu, K., Fujita, T. and Nakajima, M. Pest. Biochem. Physiol. 1979, 11, 104. Dreikorn, B. A. and O'Doherty, G. O. P. CHEMTECH 1985, 15, 424. Hansch, C . , Kiehs, K. and Lawrence, G. L. J . Am. Chem. Soc. 1965, 87, 5770. Atkins, E. L. et a l . Toxicity of Pesticides and Other Agricultu ral Chemicals to Honey Bees 1973; Reducing Pesticide Hazards to Honey Bees 1981, Agricultural Extension, University of Califor nia, Riverside. Hollingworth, R. M. In Insecticide Biochemistry and Physiology; Wilkinson, C. F . , Ed.; Plenum Press, New York, 1978, Chapter 12. Uchida, T . , Dauterman, W. C. and O'Brien, R. D. J. Agric. Food Chem. 1964, 12, 48. Uchida, T. and O'Brien, R. D. Toxicol. Appl. Pharm. 1967, 10, 89. Benet, L. Z. J . Pharmacokin. Biopharm. 1978, 6, 559. Rowland, M. J. Pharm. Sci. 1972, 61, 70. Kawano, S., Nakao, T. and Hiraga, K. Toxicol. Appl. Pharmacol. 1981, 61, 475.

RECEIVED June 14, 1989


Probing Bioactive Mechanisms - ACS Publications - American

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