Chapter 27
Use of Biological Monitoring in the Regulatory Process
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Leonard Ritter and Claire A. Franklin Environmental and Occupational Toxicology Division, Environmental Health Directorate, Tunney's Pasture, Ottawa, Ontario K1A 0L2, Canada
Occupational risks associated with pesticide use can only be estimated if exposure can be quantified. Dermal exposure has historically been estimated from patches placed on the worker or his clothing while more recent advances have focused on the use of biological monitoring. Biological monitoring of pesticides can be carried out in a variety of body fluids including blood, urine, sweat, saliva, adipose tissue and exhaled breath; indeed, urinary concentration of the chemical or its metabolites has become one of the most frequently reported biological measures of exposure. For pesticides such as 2,4-D, which is excreted largely unmetabolized, urinary concentrations may be quantitatively related to exposure. Similarly, for organophosphorus compounds, data are presently available which establish the relationship between dermal dose and urinary metabolites. It has been postulated that this standard curve approach could then be utilized to estimate exposure in workers based on the metabolite excretion. There are many products, however, for which the metabolite excretion pattern is not known, thus severely limiting the usefulness of the biological monitoring approach as a measure of exposure to such products. O c c u p a t i o n a l exposure t o p e s t i c i d e s may p r e s e n t h e a l t h r i s k s t o b o t h a p p l i c a t o r s and a g r i c u l t u r a l w o r k e r s . I n order t o p r o p e r l y assess t h i s exposure and e s t i m a t e r i s k , a c c u r a t e d a t a on exposure and a b s o r p t i o n a r e n e c e s s a r y . I n a d d i t i o n t o q u a n t i f y i n g exposure, p r e c i s e i n f o r m a t i o n s h o u l d be a v a i l a b l e on the dose l e v e l s f o r b i o l o g i c a l e n d p o i n t s e s t a b l i s h e d through e x p e r i m e n t a l t o x i c i t y studies. This chapter not subject to U.S. copyright Published 1989 American Chemical Society
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
27.
RITTER AND FRANKLIN
Monitoring in Regulatory Process
355
H i s t o r i c a l l y , r i s k from p e s t i c i d e r e s i d u e s i n food has been e s t i m a t e d by comparing the t h e o r e t i c a l d a i l y i n t a k e of the p e s t i c i d e r e s i d u e t o the a c c e p t a b l e d a i l y i n t a k e . The a c c e p t a b l e e f f e c t l e v e l seen i n e x p e r i m e n t a l a n i m a l s t u d i e s and utilization of an a p p r o p r i a t e s a f e t y f a c t o r f o r the t o x i c o l o g i c end-point b e i n g c o n s i d e r e d . I f the t h e o r e t i c a l d a i l y i n t a k e i s determined to be l e s s than the a c c e p t a b l e d a i l y i n t a k e , the p e s t i c i d e i s g e n e r a l l y c o n s i d e r e d s a f e f o r use on f o o d . S i m i l a r l y , r i s k t o agricultural workers can be e s t i m a t e d by comparing the t h e o r e t i c a l d a i l y exposure to a p e s t i c i d e and the a c c e p t a b l e d a i l y exposure. As i s the case w i t h food r e s i d u e s , i f the t h e o r e t i c a l d a i l y exposure i s e s t i m a t e d to be l e s s than the a c c e p t a b l e d a i l y exposure, the p e s t i c i d e may be c o n s i d e r e d s a f e f o r use by a g r i c u l t u r a l w o r k e r s . These comparisons a r e summarized i n F i g u r e 1. From the f i g u r e i t can be seen t h a t f o r the d e t e r m i n a t i o n of a g r i c u l t u r a l worker s a f e t y t h e r e i s a need to o b t a i n r e l i a b l e and a c c u r a t e e s t i m a t e s of worker exposure. The m a j o r i t y of exposure i n p e s t i c i d e workers has been shown t o take p l a c e v i a the dermal r o u t e ( 1 - 3) and t h i s exposure was o f t e n e s t i m a t e d by the use of adsorbent patches l o c a t e d at v a r i o u s s i t e s on the w o r k e r s b o d i e s or c l o t h i n g . C l a s s i c a l p a t c h t e c h n i q u e s may o v e r e s t i m a t e exposure (4) l e a d i n g t o exaggerated r i s k e s t i m a t e s and improper r e s t r i c t i o n on r e g i s t r a t i o n and use of the p e s t i c i d e . More r e c e n t l y , c o n s i d e r a b l e i n t e r e s t has developed i n the t e c h n i q u e of human b i o l o g i c a l m o n i t o r i n g t o supplement or indeed r e p l a c e many of these o l d e r p a t c h t e c h n i q u e s f o r e s t i m a t i n g dermal exposure ( 5 ) . B i o l o g i c a l m o n i t o r i n g , a s p e c i f i c form of m o n i t o r i n g of human exposure, can be used to determine i f an a g r i c u l t u r a l worker has been exposed to a c h e m i c a l and, i n s e l e c t e d c a s e s , the e x t e n t t o which exposure may have taken p l a c e . Measurements can be made i n a v a r i e t y of b i o l o g i c a l f l u i d s i n c l u d i n g u r i n e and b l o o d . The advantages of b i o l o g i c a l m o n i t o r i n g , when compared to c l a s s i c a l p a t c h t e c h n i q u e , l i e i n the a b i l i t y of t h i s approach to more a c c u r a t e l y e s t i m a t e body burden of the c h e m i c a l from a l l r o u t e s of exposure. L i m i t a t i o n s of t h i s t e c h n i q u e i n c l u d e i n d i v i d u a l p h a r m a c o k i n e t i c v a r i a b i l i t y (6) and a requirement f o r e x t e n s i v e knowledge on metabolism and d i s p o s i t i o n of the c h e m i c a l . The r e l i a b i l i t y of t h i s t e c h n i q u e i s t h e r e f o r e l i m i t e d by the g e n e r a l u n a v a i l a b i l i t y of such e x t e n s i v e d a t a f o r the m a j o r i t y of p e s t i c i d e s i n use today. Low c o n c e n t r a t i o n s of a m e t a b o l i t e i n u r i n e c o u l d be the r e s u l t of any one or c o m b i n a t i o n of the f o l l o w i n g : ( i ) the c h e m i c a l i s not w e l l absorbed; ( i i ) the c h e m i c a l i s absorbed but i s s e q u e s t e r e d i n the body; ( i i i ) the c h e m i c a l i s m e t a b o l i z e d i n t o a number of d i f f e r e n t m e t a b o l i t e s a l l of w h i c h are e x c r e t e d at low l e v e l s . I f the k i n e t i c s and metabolism of the c h e m i c a l are not w e l l understood i t would be p o s s i b l e to i n c o r r e c t l y conclude t h a t low l e v e l s of the measured m e t a b o l i t e i n u r i n e i n d i c a t e t h a t o n l y a s m a l l amount of the c h e m i c a l was absorbed. T h i s i n t u r n would r e s u l t i n an u n d e r e s t i m a t e of r i s k . In t h i s paper we w i l l focus on the use of b i o l o g i c a l m o n i t o r i n g f o r q u a n t i f y i n g exposure and w i l l d i s c u s s these cases which show how p r i o r knowledge of the metabolism of the c h e m i c a l can i n f l u e n c e the s u i t a b i l i t y of b i o l o g i c a l m o n i t o r i n g to p r e d i c t exposure. 1
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
356
BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE
1. CHEMICALS WHICH ARE RAPIDLY ABSORBED, NOT SEQUESTERED, AND EXCRETED IN URINE LARGELY UNMETABOLIZED ARE GOOD CANDIDATES FOR ESTIMATING EXPOSURE (2,4-D), An example of a c h e m i c a l t h a t i s a good c a n d i d a t e f o r e s t i m a t i n g exposure from b i o l o g i c a l m o n i t o r i n g i s 2,4-D. I t i s a broad spectrum a g r i c u l t u r a l h e r b i c i d e w i d e l y used throughout the world. I t s a b s o r p t i o n , d i s t r i b u t i o n and metabolism i n animals have been s t u d i e d by s e v e r a l a u t h o r s and r e p o r t e d i n t h e l i t e r a t u r e (7-14). The a b s o r p t i o n and e x c r e t i o n o f 2,4-D i n humans v i a the o r a l r o u t e have been i n v e s t i g a t e d i n a c o n t r o l l e d l a b o r a t o r y s i t u a t i o n . The f i r s t o f these s t u d i e s ( 9 ) i n v o l v e d a s i n g l e o r a l dose of 5 mg 2,4-D a c i d / k g body weight t o each of 5 v o l u n t e e r s . U r i n e was c o l l e c t e d over a p e r i o d o f 144 hours f o l l o w i n g i n g e s t i o n and t h e p e r c e n t a g e o f the o r a l dose of 2,4-D e x c r e t e d was then c a l c u l a t e d . I t was shown t h a t 2,4-D was e x c r e t e d u n m e t a b o l i z e d as f r e e o r g l u c u r o n i d e c o n j u g a t e d 2,4-D ( 9 ) . As can be seen from T a b l e I , complete r e c o v e r y o f the i n i t i a l dose was p o s s i b l e i n the f i r s t 96 h o u r s . I n a d d i t i o n , 70 t o 83% o f the i n g e s t e d 2,4-D was r e c o v e r e d i n t h e f r e e form. A l t h o u g h two s u b j e c t s e x c r e t e d up t o 26% as t h e c o n j u g a t e d form, two o t h e r s u b j e c t s e x c r e t e d no more than 10% o f t h e c o n j u g a t e and t h e r e m a i n i n g s u b j e c t e x c r e t e d o n l y f r e e 2,4-D. T h i s study i n man shows t h a t 2,4-D i s absorbed f o l l o w i n g o r a l a d m i n i s t r a t i o n and e l i m i n a t e d l a r g e l y as unchanged parent compound. I t has a l s o been shown i n man t h a t 6% o f a d e r m a l l y a p p l i e d dose o f 2,4-D a c i d i s absorbed ( 1 5 ) . R e s u l t s from our own l a b o r a t o r y ( u n p u b l i s h e d ) have a l s o shown t h a t 15% o f a d e r m a l l y a p p l i e d dose o f 2,4-D a c i d i s absorbed from rhesus monkey forearm and t h a t 29% o f the a p p l i e d dose i s absorbed from the forehand ( F i g u r e 2 ) . I t was a l s o shown t h a t 100% of an i n t r a v e n o u s dose was e x c r e t e d i n d i c a t i n g t h a t the compound was not sequestered ( 1 5 ) . The r e s u l t s r e p o r t e d above show t h a t 2,4-D i s a compound which i s absorbed by t h e o r a l and dermal r o u t e s , i s e x c r e t e d l a r g e l y as unchanged parent compound and i s not s e q u e s t e r e d . Although a d e f i n e d r e l a t i o n s h i p between t h e amount o f 2,4-D a p p l i e d d e r m a l l y and the c o n c e n t r a t i o n o f u r i n a r y m e t a b o l i t e has not been e s t a b l i s h e d , t h e type of i n f o r m a t i o n d e s c r i b e d above g i v e s one some c o n f i d e n c e i n c o n c l u d i n g t h a t low u r i n a r y m e t a b o l i t e l e v e l s i n workers r e f l e c t low absorbed doses. The r e l a t i o n s h i p between dermal exposure and u r i n a r y m e t a b o l i t e e x c r e t i o n under normal use c o n d i t i o n s has been i n v e s t i g a t e d by Grover and h i s co-workers ( 1 6 ) . I n t h i s s t u d y , a t o t a l o f 9 farmers who r e p e a t e d l y sprayed 2,4-D were m o n i t o r e d d u r i n g normal s p r a y a p p l i c a t i o n s . I n a l l cases the t o t a l amount of 2,4-D a v a i l a b l e f o r a b s o r p t i o n was e s t i m a t e d u t i l i z i n g a standard p a t c h t e c h n i q u e , w h i l e a b s o r p t i o n was e s t i m a t e d by measuring 2,4-D i n u r i n e c o l l e c t e d f o r 96 hours f o l l o w i n g the l a s t exposure t o t h e chemical. The r e s u l t s a r e summarized i n T a b l e I I . As can be seen from the t a b l e t h e r e was a v e r y wide range o f c a l c u l a t e d dermal d e p o s i t i o n i n workers a p p l y i n g 2,4-D (75 t o 13,286 ug 2,4-D/kg s p r a y e d ) w h i l e t h e r e was a much narrower range o f u r i n a r y e x c r e t i o n of 2,4-D i n these same w o r k e r s . I t i s noteworthy t h a t d e p o s i t i o n
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
hrs
82
Free
1
2
103
21
Conjugate
* adapted from (9)
Total
0-96
SUBJECT
88
Free
88
-
Conjugate
70
Free
3
96
26
Conjugate
87
Free
4
92
5
Conjugate
83
93
Free
5
10
Conjugate
Table I . Percentages of 2,4-D (Free and Conjugate) Excreted i n Urine Following A Single Oral Dose of 5 mg/kg 2,4-D*
358
BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE
Food
NOEL Safety Factor
• Acceptable D a i l y Intake
- animal t o x i c i t y data generated using o r a l route of exposure - o r a l route of exposure to humans through food and water - i f T h e o r e t i c a l D a i l y Intake (TDI) i s l e s s than Acceptable D a i l y Intake (ADI), p e s t i c i d e may be safe f o r use on food
Workers
NOEL Safety Factor
• Acceptable D a i l y Exposure
- animal t o x i c i t y data generated using o r a l route of exposure - dermal route of exposure g e n e r a l l y the greatest f o r workers and bystanders - i f T h e o r e t i c a l D a i l y Exposure (TDE) i s l e s s than Acceptable D a i l y Exposure (ADE), p e s t i c i d e may be safe f o r use by a g r i c u l t u r a l workers.
F i g u r e 1.
Risk Estimation
A B
S 0 R P T 1 0 N 0
1
2
3
4
5
6
7
8
9 10 11 12 13 14
TIME (DAYS) 0
FOREHEAD FOREARM F i g u r e 2.
2,4-D
A c i d - % Recovery i n Monkeys
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988. 75 391 1,529 13,286 1,339 444 1,572 951
CALCULATED DERMAL DEPOSITION OF ug 2,4-Da.e/Kg sprayed
11 6 14 3 17 17 33 17
a
CUMULATIVE URINARY EXCRETION OF ug 2,4-Da.e. /kg sprayed
T o t a l amount excreted during exposure period and f o r 4 days a f t e r l a s t exposure.
24 35 49 127 89 102 186 71
AMOUNT SPRAYED (kg a.e.)
• adapted front (16)
* Acid equivalent
a
B B B F D A G E
SUBJECT
Table I I . Cumulative Estimates of Dermal 2,4-Da.e.* Deposition and Urinary 2,4-Da.e. Excretion f o r Farmers Involved i n Spraying of 2,4-D BY Ground Rig.*
360
BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE
l e v e l s were n o t n e c e s s a r i l y c o r r e l a t e d w i t h t h e amount o f 2,4-D sprayed. These d a t a suggest t h a t compounds t h a t a r e r a p i d l y absorbed, not s e q u e s t e r e d and e x c r e t e d i n an unchanged form are w e l l s u i t e d t o the p r i n c i p l e o f b i o l o g i c a l m o n i t o r i n g and t h a t u r i n a r y m e t a b o l i t e l e v e l s may more a c c u r a t e l y r e f l e c t both exposure and body burden than would be e s t i m a t e d by more c l a s s i c a l patch t e c h n i q u e s . 2.
CHEMICALS FOR WHICH URINARY METABOLITES ARE QUANTITATIVELY RELATED TO DERMAL DOSE. (AZINPHOS-METHYL) Azinphos-methyl i s an i n s e c t i c i d e w i d e l y used f o r t h e c o n t r o l o f a v a r i e t y o f o r c h a r d i n s e c t s . E a r l y work w i t h organophosphorus i n s e c t i c i d e s has demonstrated t h a t i n humans (17-20) and i n r a t s (21-22) a r e l a t i o n s h i p e x i s t s between exposure t o organophosphorus p e s t i c i d e s and e x c r e t i o n o f a l k y l phosphate m e t a b o l i t e s . S p e c i f i c a l l y , i t has been shown (23) t h a t a z i n p h o s - m e t h y l i s degraded p r i m a r i l y i n m i c r o s o m a l and s o l u b l e f r a c t i o n s t o d i m e t h y l t h i o p h o s p h a t e (DMTP), d i m e t h y l phosphate (DMP) and t h e oxygen a n a l o g . The s t r u c t u r e o f a z i n p h o s - m e t h y l and t h e s i t e o f h y d r o l y s i s a r e shown i n F i g u r e 3. S t u d i e s u n d e r t a k e n i n our l a b o r a t o r y have examined t h e r e l a t i o n s h i p between i n c r e a s i n g doses o f t o p i c a l l y a p p l i e d a z i n p h o s - m e t h y l and t h e u r i n a r y e x c r e t i o n o f t h i s dose as DMTP ( m e t a b o l i t e ) i n r a t s (24) and i n man ( 2 5 ) . As can be seen from T a b l e I I I , d e r m a l l y a p p l i e d doses o f between 100 and 800 microgram per r a t r e s u l t e d i n a p p r o x i m a t e l y 10% o f t h e a p p l i e d dose b e i n g e x c r e t e d as DMTP m e t a b o l i t e . F o l l o w i n g o u r i n i t i a l o b s e r v a t i o n s t h a t a p p r o x i m a t e l y 10% o f a t o p i c a l l y a p p l i e d a z i n p h o s - m e t h y l dose i n r a t s c o u l d be r e c o v e r e d as u r i n a r y DMTP, a s i m i l a r i n v e s t i g a t i o n was u n d e r t a k e n i n man ( T a b l e I V ) . I t was shown t h a t a s i m i l a r 10:1 p r o p o r t i o n o f t h e d e r m a l l y a p p l i e d dose i n humans c o u l d be r e c o v e r e d as u r i n a r y DMTP m e t a b o l i t e . F u r t h e r , as can be seen from Table V, the r e l a t i o n s h i p between d e r m a l l y a p p l i e d dose and t h e amount o f DMTP m e t a b o l i t e e x c r e t e d was r e l a t i v e l y l i n e a r over a dermal dosage range o f 500 t o 6000 micrograms p e r p e r s o n . The r e s u l t s o b t a i n e d from b o t h t h e r a t and human i n v e s t i g a t i o n s have suggested t h a t t h e r e i s a q u a n t i t a t i v e r e l a t i o n s h i p between dermal dose and t h e p r o p o r t i o n o f t h i s dose t h a t c a n be r e c o v e r e d i n u r i n e as a DMTP m e t a b o l i t e . S p e c i f i c a l l y , these r e s u l t s suggest t h a t t h e r e i s a p p r o x i m a t e l y a 10 t o 1 r e l a t i o n s h i p between t h e dose t h a t i s a p p l i e d and t h e c o n c e n t r a t i o n o f u r i n a r y m e t a b o l i t e s ( T a b l e V ) . T h i s r e l a t i o n s h i p c a n be v e r y u s e f u l i n f i e l d m o n i t o r i n g o f a g r i c u l t u r a l workers i n t h a t i t a l l o w s e s t i m a t i o n o f dermal exposure s i m p l y by the d e t e r m i n a t i o n o f l e v e l s o f u r i n a r y m e t a b o l i t e s . I t s h o u l d , however, be noted t h a t w h i l e t h i s method may be u s e f u l f o r e s t i m a t i o n o f dermal exposure l e v e l s , u r i n a r y m e t a b o l i t e s may not be u s e f u l f o r purposes of e s t i m a t i n g r i s k . DMTP r e p r e s e n t s o n l y one o f s e v e r a l m e t a b o l i t e s formed on exposure t o a z i n p h o s - m e t h y l and may not r e f l e c t r e l a t i v e c o n c e n t r a t i o n s o f o t h e r b i o l o g i c a l l y i m p o r t a n t metabolites i n various species. 3. CHEMICALS WHICH UNDERGO COMPLEX METABOLISM AND WHERE DOSE AND ROUTE OF ADMINISTRATION MAY QUALITATIVELY QUANTITATIVELY AFFECT METABOLISM AND BIOLOGICAL OUTCOME (METHYLENE CHLORIDE). Mice exposed t o 3500 and 7000 mg/kg/6 h r day methylene c h l o r i d e (MC) by
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
8
5
5
3
100
200
400
800
Dermal
Dermal
Dermal
Dermal
Route
* Adapted from (24)
N
Dose (ug/rat)
8.3 15.5 34.8 73.6
12.2 23.0 18.9
48 hours
6.9
24 hours
86.1
81.7
11
9
37.3
9
36.4
91.2
120 hours
X Dose Excreted As DMTP
8
96 hours
X Dose
16.0
8.8
72 hours
Cumulative T o t a l (ug DMTP)
Table I I I . Excretion of DMTP Following Dermal Application of Azinphos-methyl i n Male Rats*
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
52 140
36 76
188 120
122 184
1,000
2,000
4,000
6,000
* adapted from (24)
82 25
246 322
264 270
104 76
80 180
110 41
48 hours
278 368
454 354
148 90
92 212
123 41
72 hours
Cumulative Total (ug DMTP)
24 hours
500
Dose (ug/person)
323
404
119
152
85
Average Total
5
10
6
15
17
% dose excreted as DMTP
Table IV. Excretion of DMTP Following Dermal Application of Azinphos-methyl to the Forearm of Human Volunteers*
o
w w
3
W H O
o ^ o
2 3
o
H
S o
P
5
o
ON
27. RITTER AND FRANKLIN
Azinphos-methyl:
Monitoring in Regulatory Process
s-(3,4-dihydro-4-oxobenzo [d]~ [ 123]-triazin-3-yl methyl)-0,O-dimethyl phosphorodithioate
-|—S-CHj
1
hydrolysis
Metabolite:
DMTP
Analysis:
Azinphos-methyl by HPLC
:
Metabolite by a modified shafik method to derlvatize alkyl phosphates followed by G.C. analysis
Figure 3
Table V.
Analysis of Azinphos-methyl by HPLC
Relationship Between Amount of DMTP Excreted and the Amount of Azinphos-methyl (A.M.) Applied*
Dermal Dose Range
Metabolite Excretion/ Amount A.M. Applied
Rat
100 - 800 ug
1 ug DMTP/10 ug A.M.
Human
500 - 6000 ug
1 ug DMTP/10 ug A.M.
* adapted from (25)
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE
i n h a l a t i o n f o r 5 days/week demonstrated an excess o f h e p a t o c e l l u l a r adenomas/carcinomas as compared t o unexposed c o n t r o l s ( 2 6 ) . These r e s u l t s c o n t r a s t e d w i t h an e a r l i e r study conducted by t h e N a t i o n a l C o f f e e A s s o c i a t i o n (27) i n w h i c h mice exposed t o MC i n d r i n k i n g water a t c o n c e n t r a t i o n s l e a d i n g t o exposure o f 250 mg/kg/day showed no i n c r e a s e i n t h e i n c i d e n c e o f l i v e r tumours. There a r e two pathways i n v o l v e d i n t h e metabolism o f MC. R e l a t i v e tumour response a s s o c i a t e d w i t h these two pathways i s shown i n F i g u r e 4. The two pathways i n v o l v e d i n c l u d e t h e s a t u r a b l e mixed f u n c t i o n o x i d a s e (MFO) pathway and t h e g l u t a t h i o n e - s - t r a n s f e r a s e (GST) pathway. E s t i m a t e s of t h e c o n c e n t r a t i o n o f MC and t h e r a t e s o f each o f t h e two m e t a b o l i c pathways have been examined ( 2 8 ) . The r e s u l t s o f these s t u d i e s a r e g i v e n i n T a b l e V I and have shown t h a t w h i l e t h e r a t e o f the MFO pathway i s comparable i n the two s t u d i e s , the d e l i v e r e d dose by t h e GST pathway and p a r e n t compound c o n c e n t r a t i o n s d i f f e r s u b s t a n t i a l l y . T h i s would tend t o i m p l y t h a t t h e MFO pathway i s not r e s p o n s i b l e f o r t h e tumour i n d u c t i o n seen. F u r t h e r m o r e , i n v i e w o f the f a c t t h a t t h e r a t e o f t h e MFO pathway i s comparable i n b o t h t h e d r i n k i n g water and i n h a l a t i o n s t u d i e s , u t i l i z a t i o n o f t h i s m e t a b o l i c pathway a l o n e f o r t h e purpose o f assessment o f p o t e n t i a l r i s k would l e a d t o erroneous c o n c l u s i o n s ; i t c o u l d not be u t i l i z e d t o e x p l a i n the b i o l o g i c a l response seen i n the i n h a l a t i o n s t u d y but absent i n the d r i n k i n g water s t u d y . C l e a r l y , i n t h i s c a s e , t h e e x a m i n a t i o n o f the GST pathway i s o f paramount importance i n a s s e s s i n g t h e o v e r a l l r i s k a s s o c i a t e d w i t h exposure t o MC. The case o f methylene c h l o r i d e , demonstrates r a t h e r e l e g a n t l y t h e need f o r a thorough u n d e r s t a n d i n g o f m e t a b o l i s m p r i o r t o u t i l i z i n g m e t a b o l i t e l e v e l s as an i n d i c a t i o n o f exposure f o r t h e purpose o f e s t i m a t i n g r i s k . 100 •••
80
Multistage Model Prediction
o
Drinking Water Study
+
Inhalation Study
60 -
40
Saturation of MFO Path
20
0
1 I T 0.005 0.010 0.015
/
/
0.85
GST Activity (g/L/day)
Figure 4. Metabolism of methylene chloride. (Adapted from ref. 28.)
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
1.80
Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
•adapted from (28)
0 3500 7000
0 60 125 185 250
Drinking water (mg/kg/day)
Inhalation (mg/kg/6 hr day)
Administered Dose
Route of Administration (dose u n i t s )
0.0 3.6 3.7
0.0 1.3 2.8 4.0 5.4
MFO Path (g/L/day)
0.00 0.85 1.80
0 0.003 0.007 0.011 0.016
0 360 770
0 1.3 2.9 4.6 6.4
Delivered Dose GST Path Parent Compound (g/L/day) (mg hr/L)
Table VI. Metabolism of Methylene Chloride i n Female Mice*
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BIOLOGICAL MONITORING FOR PESTICIDE EXPOSURE
SUMMARY We b e l i e v e t h a t b i o l o g i c a l m o n i t o r i n g , s p e c i f i c a l l y u r i n a r y m e t a b o l i t e l e v e l s , c a n be used t o e s t i m a t e exposure i f t h e a b s o r p t i o n , d i s t r i b u t i o n , m e t a b o l i s m and e x c r e t i o n a r e known f o r the chemical. The r e l a t i o n s h i p between exposure and u r i n a r y m e t a b o l i t e l e v e l s a r e more e a s i l y u n d e r s t o o d f o r c h e m i c a l s t h a t a r e a b s o r b e d , not s e q u e s t e r e d and e x c r e t e d v i r t u a l l y u n m e t a b o l i z e d . Selection of a few such c h e m i c a l s f o r i n depth e v a l u a t i o n c o u l d l e a d t o a b e t t e r e s t i m a t i o n o f dermal exposure than t h a t p r e s e n t l y o b t a i n a b l e from patches. T h i s i n t u r n would a s s i s t i n t h e e v e n t u a l v a l i d a t i o n o f the concept o f a g e n e r i c approach t o e s t i m a t e s o f e x p o s u r e . I t must be emphasized t h a t measurement o f u r i n a r y m e t a b o l i t e l e v e l s f o l l o w i n g e x p o s u r e , i n t h e absence o f adequate i n f o r m a t i o n on a b s o r p t i o n , d i s t r i b u t i o n , m e t a b o l i s m and e x c r e t i o n , i s u s e l e s s . Under such c i r c u m s t a n c e s , i t c o u l d not be d e t e r m i n e d whether a low c o n c e n t r a t i o n o f m e t a b o l i t e i n u r i n e was due t o poor a b s o r p t i o n (and t h e r e f o r e low r i s k ) , whether i t was w e l l absorbed and s e q u e s t e r e d w i t h i n t h e body o r whether t h e c h e m i c a l was w e l l absorbed and e x c r e t e d as many m e t a b o l i t e s , not a l l o f which were known o r measured. Another p o s s i b i l i t y would be t h a t e x c r e t i o n had taken place v i a other routes. Acknowledgments: The a u t h o r s g r a t e f u l l y m a n u s c r i p t by Wendy M i l k s .
acknowledge t h e t y p i n g o f t h i s
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Wang et al.; Biological Monitoring for Pesticide Exposure ACS Symposium Series; American Chemical Society: Washington, DC, 1988.