Effect of Dietary Factors on Drug Disposition in Normal Human

6 Effect of Dietary Factors on Drug Disposition in Normal Human Subjects

Downloaded by RUTGERS UNIV on December 28, 2017 | http://pubs.acs.org Publication Date: May 6, 1985 | doi: 10.1021/bk-1985-0277.ch006

ELLIOT S. VESELL Department of Pharmacology, The Pennsylvania State University, College of Medicine, Hershey, PA 17033

In normal human subjects, several dietary manipulations have been identified that can alter the disposition of such model drugs as antipyrine, phenacetin, and theophylline. Other environmental factors were kept as constant as possible, and each subject served as his own control. Nutritional factors thus far investigated in this way include: 1) varying the proportions of protein, carbohydrate and fat in a daily diet of 2500 calories; 2) charcoal broiling of beef; 3) high intake of cruciferous vegetables; 4) daily doses of theobromine acetate; 5) obese subjects on a diet of 15 gms carbohydrate/day for 10 days. These and related studies showed clearly that physicians should take dietary factors that influence drug disposition into consideration before arriving at the appropriate dose of some drugs. In different countries and regions of the U.S.A. particular dietary practices such as vegetarianism may make certain subjects and groups especially susceptible or resistant to the therapeutic as well as the toxic effects of drugs. Future studies should address this different question: to what extent do these isolated single factors interact with each other in the normal diet and thereby contribute to the well recognized large interindividual variations that occur in drug metabolism and disposition? Experimental designs are described that could help to define what role dietary factors play in accounting for these large interindividual variations. Beginning in 1976, the laboratories of Kappas at Rockefeller University and Conney at Roche published in collaboration several pioneering experiments that demonstrated an effect of different dietary factors on drug disposition. Antipyrine, theophylline, phenacetin, and acetaminophen served as model drugs and were given to carefully selected normal volunteers before, during, and after a single dietary manipulation lasting from 3 to 14 days (see 1-5 for 0097-6156/ 85/ 0277-0061 $06.00/ 0 © 1985 American Chemical Society

Finley and Schwass; Xenobiotic Metabolism: Nutritional Effects ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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r e v i e w s o f t h e s e s t u d i e s ) . A l l o t h e r e n v i r o n m e n t a l f a c t o r s d u r i n g the s t u d i e s were c o n t r o l l e d as r i g i d l y as p o s s i b l e . Because the r e s u l t s o f t h e s e now c l a s s i c a l d i e t a r y s t u d i e s a r e c l e a r and have been r e v i e w e d f r e q u e n t l y ( 1 - 5 ) , t h e y w i l l be summarized o n l y b r i e f l y h e r e . Instead other a s p e c t s o f the r e l a t i o n s h i p between d i e t a r y f a c t o r s and v a r i a t i o n s i n d r u g metabolism w i l l be d e s c r i b e d , and the d e s i g n o f e x p e r i m e n t s t o s o l v e t h e s e problems w i l l be d i s c u s s e d . To summarize b r i e f l y , i n s t u d i e s performed as d e s c r i b e d above on 5 o f the d i e t a r y f a c t o r s shown i n the o u t e r c i r c l e o f F i g . 1, i t was p o s s i b l e t o show t h a t a) on a d i e t o f 2500 c a l o r i e s per day a h i g h r a t i o o f p r o t e i n (44?) t o c a r b o h y d r a t e (35?) enhanced a n t i p y r i n e and t h e o p h y l l i n e m e t a b o l i s m , whereas a h i g h r a t i o o f c a r b o h y d r a t e (70?) t o protein (10?) retarded biotransformation of t h e s e drugs (6). S u b s t i t u t i o n o f f a t f o r c a r b o h y d r a t e d i d not s i g n i f i c a n t l y change t h e s e r e s u l t s ( 7 ) . b) I n g e s t i o n o f c h a r c o a l b r o i l e d beef i n c r e a s e d the r a t e o f a n t i p y r i n e and t h e o p h y l l i n e metabolism ( 8 ) , but not t h a t o f acetaminophen (£). The r a t e o f b i o t r a n s f o r m a t i o n o f p h e n a c e t i n was unchanged, a l t h o u g h c h a r c o a l b r o i l e d beef produced a l a r g e f i r s t pass e f f e c t , t h e r e b y a p p r e c i a b l y r e d u c i n g plasma p h e n a c e t i n c o n c e n t r a t i o n s (10). c) C r u c i f e r o u s v e g e t a b l e s i n l a r g e amounts induced the metabolism o f a n t i p y r i n e and t h e o p h y l l i n e (_1J_) and the g l u c u r o n i d a t i o n o f acetaminophen (J_2). d) Theobromine a c e t a t e g i v e n i n h i g h concent r a t i o n s as the s a l t r e t a r d e d the r a t e o f i t s own biotransformation ( 1 3 ) * Theobromine i s a major c o n s t i t u e n t o f c h o c o l a t e . However, t h i s dose o f theobromine g i v e n as c h o c o l a t e r a t h e r than the s a l t f a i l e d t o alter i t s own m e t a b o l i s m , presumably because o f i t s reduced b i o a v a i l a b i l i t y i n chocolate, e) S t a r v a t i o n o f obese s u b j e c t s f o r 10 days f a i l e d t o a l t e r a n t i p y r i n e or t o l b u t a m i d e d i s p o s i t i o n 0 4 ) > but i n I n d i a n (15) and Sudanese (16) c h i l d r e n w i t h p r o t e i n c a l o r i e malnutrition antipyrine metabolism was impaired. Nutritional r e h a b i l i t a t i o n of these c h i l d r e n r e s t o r e d a n t i p y r i n e k i n e t i c s to normal v a l u e s . A few o f the f a c t o r s shown i n the o u t e r c i r c l e o f F i g . 1 have not been s t u d i e d i n human s u b j e c t s . Those have been demonstrated i n l a b o r a t o r y a n i m a l s t o a f f e c t drug metabolism and d i s p o s i t i o n . To What E x t e n t Do D i e t a r y F a c t o r s V a r i a t i o n s i n Drug Metabolism?

Contribute

to I n t e r i n d i v i d u a l

The p r e c e d i n g s t u d i e s , each performed on a d i f f e r e n t d i e t a r y f a c t o r , were c a r e f u l l y c o n t r o l l e d . Thus, o n l y a s i n g l e v a r i a b l e was a l t e r e d i n d e p e n d e n t l y o f a l l o t h e r s . F u r t h e r m o r e , s u b j e c t s were p a i n s t a k i n g l y selected so that their hepatic cytochrome P-450 dependent monooxygenases would be under s t a b l e , n e a r b a s a l c o n d i t i o n s ( n e i t h e r markedly i n d u c e d nor i n h i b i t e d through the p o t e n t i a l i n f l u e n c e o f numerous e n v i r o n m e n t a l p e r t u r b a t i o n s ) ( 1 - 5 ) . For example, none o f the s u b j e c t s took any drug r e g u l a r l y . None smoked c i g a r e t t e s or consumed ethanol chronically. A s i n g l e d i e t a r y manipulation could be i n t r o d u c e d under t h e s e w e l l d e f i n e d , u n i f o r m e n v i r o n m e n t a l c o n d i t i o n s . Then any a l t e r a t i o n i n r a t e o f metabolism o f a model drug, such as a n t i p y r i n e o r t h e o p h y l l i n e , c o u l d s e r v e as a r e l i a b l e , s e n s i t i v e i n d e x o f the e f f e c t o f t h a t s p e c i f i c n u t r i t i o n a l change on the h e p a t i c d r u g m e t a b o l i z i n g c a p a c i t y o f the s u b j e c t . Use o f each c a r e f u l l y s e l e c t e d s u b j e c t as h i s own control e l i m i n a t e d not o n l y a l l g e n e t i c but most e x t r a n e o u s e n v i r o n m e n t a l


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Dietary Factors and Drug Disposition


Daily ..Caloric Intake(appetite)

Macromolecular Constituents of Diet

Food Additives

Proportion of Fresh to Processed Foods

Timing and Contents of Meals

Cruciferous Vegetables of the Brassica genus Charcoal Broiling of Meats

Methylxanthines

Figure 1. This c i r c u l a r design suggests the p o s s i b i l i t y o f dynamic i n t e r a c t i o n s among t h e s e v e r a l well-established or s u s p e c t e d d i e t a r y f a c t o r s t h a t may i n f l u e n c e d r u g r e s p o n s e i n humans. Arrows from each f a c t o r i n t h e o u t e r c i r c l e a r e wavy t o i n d i c a t e t h a t e f f e c t s o f each d i e t a r y f a c t o r on d r u g r e s p o n s e may o c c u r a t m u l t i p l e s i t e s and t h r o u g h d i f f e r e n t p r o c e s s e s . The i n n e r c i r c l e s u g g e s t s t h a t e f f e c t s o f d i e t a r y f a c t o r s may be m o d i f i e d by many o t h e r e n v i r o n m e n t a l , as w e l l a s by g e n e t i c , factors.



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s o u r c e s o f v a r i a t i o n i n drug m e t a b o l i s m . Therefore, the a l t e r a t i o n observed i n t h e d i s p o s i t i o n o f a model drug a f t e r i m p o s i t i o n o f a s i n g l e d i e t a r y manipulation c o u l d r e a s o n a b l y be a s c r i b e d t o t h a t n u t r i t i o n a l change a l o n e . Furthermore, t h i s experimental design p e r m i t t e d r e s u l t s t o be v e r i f i e d i n s e v e r a l ways. F i r s t l y t h e e f f e c t c o u l d be reproduced e i t h e r i n t h e same o r i n o t h e r s u b j e c t s . S e c o n d l y , dose-response r e l a t i o n s h i p s c o u l d be g e n e r a t e d between t h e p a r t i c u l a r d i e t a r y f a c t o r i n v e s t i g a t e d and t h e subsequent change i n metabolism o f the model d r u g . R e s u l t s o f t h e e x p e r i m e n t s d e s c r i b e d above on d i e t a r y f a c t o r s have been r e p r o d u c e d . Q u e s t i o n s a r i s e , n e v e r t h e l e s s , about how f a r t h e s e r e s u l t s on d i e t a r y f a c t o r s i n normal s u b j e c t s under near b a s a l c o n d i t i o n s extend t o o t h e r , more c o m p l i c a t e d , dynamic c i r c u m s t a n c e s such as t h o s e t h a t p r e v a i l among p a t i e n t s whose environments may be r a p i d l y c h a n g i n g i n complex ways. What p o r t i o n o f t h e i n t e r i n d i v i d u a l v a r i a t i o n t h a t e x i s t s among such p a t i e n t s and a l s o among normal s u b j e c t s with u n r e s t r i c t e d l i f e s t y l e s c a n be a t t r i b u t e d t o d i e t a r y f a c t o r s ? Most l i k e l y t h i s q u e s t i o n cannot be answered s i m p l y . P r o b a b l y among a l l s u b j e c t s complex i n t e r a c t i o n s o f numerous f a c t o r s e x p l a i n t h e t o t a l i t y o f i n t e r i n d i v i d u a l v a r i a t i o n t h a t o c c u r s i n r a t e s o f drug m e t a b o l i s m . I n some s u b j e c t s , d i e t a r y c o n s i d e r a t i o n s may p l a y a major r o l e ; i n o t h e r s , smoking; i n o t h e r s , age; i n o t h e r s , g e n e t i c f a c t o r s ; i n s t i l l o t h e r s , i n t e r a c t i o n s among t h e s e and o t h e r f a c t o r s . T h i s d i s c u s s i o n emphasizes t h e c o m p l e x i t y o f such i n t e r a c t i o n s t h a t may o c c u r when a l l s u b j e c t s a r e i n c l u d e d . I n t e r a c t i o n s c a n t a k e p l a c e n o t o n l y among d i f f e r e n t f a c t o r s , but a l s o among t h e d i e t a r y f a c t o r s themselves ( F i g . 1). The magnitude o f t h e a l t e r a t i o n i n drug m e t a b o l i s m produced by s i n g l e d i e t a r y f a c t o r s i n s u b j e c t s under c a r e f u l l y c o n t r o l l e d e n v i r o n m e n t a l c o n d i t i o n s needs t o be compared t o the magnitude o f t h e t o t a l i n t e r i n d i v i d u a l v a r i a t i o n i n r a t e s o f metabolism o f t h e s e same model drugs b e f o r e i m p o s i t i o n o f any d i e t a r y change. S i n c e s e v e r a l d i e t a r y m a n i p u l a t i o n s can change a s u b j e c t ' s r a t e o f drug m e t a b o l i s m , i t might be e x p e c t e d t h a t t h e s e d i e t a r y f a c t o r s s h o u l d account f o r a l a r g e p r o p o r t i o n o f t h e e x t e n s i v e v a r i a t i o n s o b s e r v e d among normal s u b j e c t s . However, t h e l a t t e r does not n e c e s s a r i l y f o l l o w from t h e former w e l l e s t a b l i s h e d o b s e r v a t i o n s , and i t remains t o be determined whether such e x t r a p o l a t i o n s a r e v a l i d . W i t h r e s p e c t t o t h e changes i n a n t i p y r i n e and t h e o p h y l l i n e metabolism a f t e r i m p o s i t i o n o f d i e t s h i g h i n c r u c i f e r o u s v e g e t a b l e s and c h a r c o a l b r o i l e d b e e f , r e l a t i v e l y s m a l l a l t e r a t i o n s , i n t h e range of 10? t o 20? o f b a s a l v a l u e s , a r e caused by t h e s e d i e t a r y manipulations. Such changes a r e an o r d e r o f magnitude l e s s t h a n t h o s e observed among n o r m a l , c a r e f u l l y s e l e c t e d s u b j e c t s o f t h e same age and s e x , who a r e unmedicated, nonsmokers and n o t c h r o n i c consumers o f e t h a n o l 0 7 , J 8 ) . F i g s . 3 and 4 i n A l v a n 0 9 ) i l l u s t r a t e t h e 6 - f o l d i n t e r i n d i v i d u a l v a r i a t i o n s t h a t o c c u r i n a n t i p y r i n e c l e a r a n c e when a r e p r e s e n t a t i v e number o f s u b j e c t s a r e i n v e s t i g a t e d . Thus, t h e s e p a r t i c u l a r d i e t a r y f a c t o r s appear t o p l a y o n l y a v e r y s m a l l r o l e i n accounting f o r the 6-fold i n t e r i n d i v i d u a l v a r i a t i o n s i n a n t i p y r i n e metabolism among normal s u b j e c t s under b a s a l e n v i r o n m e n t a l c o n d i t i o n s . A l t h o u g h each f a c t o r by i t s e l f may p l a y a s m a l l r o l e , i n c o m b i n a t i o n w i t h o t h e r f a c t o r s a v e r y much l a r g e r s y n e r g i s t i c e f f e c t c o u l d emerge. F i g u r e 1 was d e s i g n e d t o suggest t h i s p o s s i b i l i t y . Here each d i e t a r y f a c t o r i s d i s p l a y e d as i f i n dynamic i n t e r a c t i o n w i t h o t h e r f a c t o r s .



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W h i l e t h e magnitude o f change c o n t r i b u t e d by l a r g e s h i f t s i n t h e p r o p o r t i o n o f p r o t e i n t o c a r b o h y d r a t e on an i s o c a l o r i c d i e t i s l a r g e r than t h a t from d i e t s h i g h i n c r u c i f e r o u s v e g e t a b l e s , i t i s s t i l l cons i d e r a b l y l e s s t h a n t h e 6 - f o l d i n t e r i n d i v i d u a l v a r i a t i o n s commonly encountered when a n t i p y r i n e k i n e t i c s a r e compared i n a t l e a s t 12 normal s u b j e c t s (V7,J_8). Here a g a i n i t might appear i n i t i a l l y t h a t even a v e r y marked m a n i p u l a t i o n i n t h e r a t i o o f p r o t e i n t o c a r b o h y d r a t e might be i n s u f f i c i e n t t o c o n t r i b u t e s u b s t a n t i a l l y t o t h e much l a r g e r i n t e r i n d i v i d u a l v a r i a t i o n s t h a t o c c u r among normal s u b j e c t s i n r a t e s o f d r u g m e t a b o l i s m . S u p e r f i c i a l l y , such a c o n c l u s i o n might seem t o be s u p p o r t e d by t h e o b s e r v a t i o n o f no change i n a n t i p y r i n e c l e a r a n c e a f t e r s t a r v a t i o n o f obese s u b j e c t s . S t u d i e s on s t a r v a t i o n , however, a r e c o m p a t i b l e w i t h a major e f f e c t o f changing t h e p r o p o r t i o n o f t h e macromolecular c o n s t i t u e n t s of i s o c a l o r i c d i e t s . This r e c o n c i l i a t i o n o f seemingly c o n t r a d i c t o r y r e s u l t s o f f e r s an o p p o r t u n i t y t o s t r e s s t h e complex e f f e c t s o f s e v e r a l d i e t a r y f a c t o r s and t h e p o s s i b i l i t y f o r i n t e r a c t i o n s among them. Why does s t a r v a t i o n f a i l t o change a n t i p y r i n e m e t a b o l i s m , whereas switching the proportion o f carbohydrate t o p r o t e i n r e s u l t s i n profound a l t e r a t i o n s o f a n t i p y r i n e metabolism? P o s s i b l y t h e body can d e t e c t t h e former d i e t a r y m a n i p u l a t i o n b e t t e r than t h e l a t t e r . Through d e t e c t i o n o f t h e g r o s s d i e t change o f s t a r v a t i o n f o r a l i m i t e d t i m e , t h e bpdy can compensate by p r o v i d i n g from a n o t h e r s o u r c e t h e amino a c i d s r e q u i r e d f o r p r o t e i n s y n t h e s i s . I n c o n t r a s t , t h e body may be unable t o d e t e c t , and hence compensate f o r , a much more s u b t l e s w i t c h i n t h e p r o p o r t i o n o f c a l o r i e s s u p p l i e d as e i t h e r c a r b o h y d r a t e or p r o t e i n . I f t h i s change i s uncompensated, d e p l e t i o n o f p r o t e i n c o u l d reduce r a t e s o f s y n t h e s i s o f h e p a t i c d r u g - m e t a b o l i z i n g enzymes, w h i c h i n t u r n c o u l d cause r e t e n t i o n o f a n t i p y r i n e and t h e o p h y l l i n e . A l t h o u g h f a s t i n g i n r o d e n t s g r e a t l y reduces r a t e s o f h e p a t i c metabolism o f some drugs (20-22), no change i n drug m e t a b o l i s m o c c u r r e d i n obese, o t h e r w i s e h e a l t h y , human s u b j e c t s a f t e r 7 t o 10 days on a d i e t i n w h i c h t h e t o t a l d a i l y c a r b o h y d r a t e i n t a k e was under 15 gm 04). T h i s d i e t induced k e t o a c i d o s i s , as w e l l as w e i g h t l o s s t h a t ranged from 4 t o 15 k g . When u n c o r r e c t e d f o r body w e i g h t , t h e apparent volume o f d i s t r i b u t i o n (aVd) o f both a n t i p y r i n e and t o l b u t a m i d e was s i g n i f i c a n t l y l o w e r a f t e r f a s t i n g t h a n b e f o r e , presumably because d u r i n g f a s t i n g t h e e a r l y l o s s o f body w e i g h t i s m a i n l y from body w a t e r , r a t h e r than from f a t s t o r e s o r muscle mass 0 4 ) . The e x t e n t o f decrease i n t h e aVd was p r o p o r t i o n a l i n each s u b j e c t t o t h e l o s s o f body w e i g h t . T h e r e f o r e , when c o r r e c t i o n was made f o r body w e i g h t , f a s t i n g had no e f f e c t on t h e aVd o f e i t h e r a n t i p y r i n e o r t o l b u t a m i d e . These r e s u l t s , extended t o o t h e r h e p a t i c m i c r o s o m a l o x i d a t i o n s , i n c l u d i n g t h o s e f o r s u l f i s o x a z o l e , i s o n i a z i d , and p r o c a i n e ( 2 3 ) , d i s c l o s e d t h a t when a l l o w a n c e was made f o r body w e i g h t , n e i t h e r h a l f l i f e v a l u e s n o r r a t e s o f h e p a t i c metabolism o f t h e s e drugs changed i n o t h e r w i s e normal obese s u b j e c t s f a s t e d f o r 7 t o 10 d a y s . G e n e r a l c o n c l u s i o n s on f a i l u r e o f a b s o l u t e f a s t i n g t o a l t e r h e p a t i c drug metabolism i n normal s u b j e c t s o r i n s u b j e c t s w i t h o u t l i v e r d y s f u n c t i o n were extended by a s t u d y o f 7 female p a t i e n t s w i t h confirmed c l a s s i c anorexia nervosa (24). I n these p a t i e n t s prolonged refusal t o e a t produced differing degrees of dehydration, hyponatremia, h y p o c h l o r e m i a , h y p o k a l e m i a , and anemia. Compared w i t h



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age and sex matched normal n u r s e s who s e r v e d as c o n t r o l s , when t h e s e v a l u e s were c o r r e c t e d f o r body w e i g h t , t h e a n o r e x i c p a t i e n t s had normal a n t i p y r i n e k i n e t i c s . A s t u d y on A s i a n I n d i a n s u b j e c t s (25) r e v e a l e d t h a t i n 15 men s u f f e r i n g from n u t r i t i o n a l edema - a s e v e r e m a n i f e s t a t i o n o f p r o t e i n d e f i c i e n c y and r e s u l t a n t hypoalbuminemia - the mean plasma a n t i p y r i n e h a l f - l i f e o f 12.8 hours d i d not d i f f e r s i g n i f i c a n t l y from t h a t o f age and sex matched nonsmoking c o n t r o l s (11.2 h o u r s ) , but was h i g h e r t h a n t h a t o f age and sex matched smoking c o n t r o l s (8.9 h o u r s ) . The same s t u d y examined a n o t h e r group o f 13 u n d e r n o u r i s h e d , underweight men w i t h o u t edema. T h e i r s h o r t mean a n t i p y r i n e h a l f - l i f e o f 8.6 h o u r s , s i m i l a r t o t h a t o f smoking c o n t r o l s (8.9 h o u r s ) , c o u l d be due t o the f a c t t h a t some smoked c i g a r e t t e s , some drank e t h a n o l , and some were a g r i c u l t u r a l l a b o r e r s exposed t o p e s t i c i d e s known t o i n d u c e h e p a t i c d r u g - m e t a b o l i z i n g enzymes. Thus, i n t h i s s t u d y , s e v e r e m a l n u t r i t i o n d i d not a l t e r a n t i p y r i n e d i s p o s i t i o n , s u p p o r t i n g t h e o b s e r v a t i o n s a l r e a d y d e s c r i b e d f o r p a t i e n t s w i t h a n o r e x i a n e r v o s a o r f o r obese but o t h e r w i s e normal s u b j e c t s a f t e r a 7 t o 10 day f a s t . However, s e v e r a l problems, i n c l u d i n g c h r o n i c exposure o f some s u b j e c t s t o i n d u c i n g chemicals, render the r e s u l t s i n c o n c l u s i v e . I n a s i g n i f i c a n t c o n t r i b u t i o n t o t h i s t o p i c , phenylbutazone p h a r m a c o k i n e t i c s were measured i n 4 normal male A s i a n I n d i a n c o n t r o l s (mean age, 30) and i n 5 u n d e r n o u r i s h e d hypoalbuminemic male A s i a n I n d i a n s u b j e c t s (mean age, 3 6 ) , none o f whom smoked c i g a r e t t e s o r consumed e t h a n o l f o r p r o l o n g e d p e r i o d s ( 2 6 ) . Compared t o c o n t r o l s , the m a l n o u r i s h e d group e x h i b i t e d s h o r t e r mean plasma phenylbutazone h a l f - l i v e s , but a l a r g e r mean phenylbutazone aVd and m e t a b o l i c clearance. These changes in phenylbutazone disposition in u n d e r n u t r i t i o n presumably a r o s e as a r e s u l t o f reduced b i n d i n g o f phenylbutazone to albumin, w i t h a corresponding increase in a v a i l a b i l i t y o f d r u g f o r m e t a b o l i s m and e l i m i n a t i o n . The r e s u l t s need t o be c o n f i r m e d i n s t u d i e s on l a r g e r groups o f u n d e r n o u r i s h e d s u b j e c t s since the conclusions could be important therapeutically. N u t r i t i o n a l l y d e p r i v e d hypoalbuminemic p a t i e n t s who r e c e i v e drugs h i g h l y bound t o a l b u m i n may r e q u i r e dosage adjustment due t o enhanced rates of elimination. R e n a l e l i m i n a t i o n o f c e r t a i n drugs can be d e c r e a s e d by f a s t i n g o r s t a r v a t i o n (23). Also, since concentrations of free f a t t y acids i n plasma r i s e d r a m a t i c a l l y a f t e r 12 hours o f f a s t i n g and s i n c e t h e s e f r e e f a t t y a c i d s b i n d albumin w i t h an a v i d i t y c a p a b l e o f d i s p l a c i n g many h i g h l y bound d r u g s , f a s t i n g f o r 24 t o 72 hours would be e x p e c t e d t o a c c e l e r a t e t h e r a t e o f e l i m i n a t i o n o f such h i g h l y bound drugs as bishydroxycoumarin, diazepam, phenylbutazone, phenytoin, and warfarin. Drug removal from t h e body would be hastened because d i s p l a c e m e n t o f drug from a l b u m i n by f r e e f a t t y a c i d s makes t h e p r e v i o u s l y bound, and hence s e q u e s t e r e d , d r u g i m m e d i a t e l y a v a i l a b l e f o r b o t h metabolism and r e n a l e l i m i n a t i o n , as i n t h e c a s e o f u n d e r n u t r i t i o n accompanied by hypoalbuminemia. T h i s d i s c u s s i o n o f s t a r v a t i o n i l l u s t r a t e s the complex problems o f i n t e r p r e t i n g the r e s u l t s o f even w e l l c o n t r o l l e d e x p e r i m e n t s on s i n g l e dietary manipulations. I t also shows the difficulties of e x t r a p o l a t i n g r e s u l t s from one s e t o f e n v i r o n m e n t a l c o n d i t i o n s t o another. C l e a r l y , such e x t r a p o l a t i o n i s hazardous i n a s s e s s i n g the c o n t r i b u t i o n o f any s i n g l e d i e t a r y f a c t o r t o l a r g e i n t e r i n d i v i d u a l v a r i a t i o n s i n metabolism o f model d r u g s . Other approaches must be


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developed t o a v o i d the problems i n h e r e n t i n t h e s e e x t r a p o l a t i o n s . The f i n a l two s e c t i o n s o f t h i s r e v i e w d e s c r i b e o t h e r a t t e m p t s t o answer t h i s important q u e s t i o n .


A S t a t i s t i c a l Approach t o Determine C a u s a t i o n o f L a r g e I n t e r i n d i v i d u a l V a r i a t i o n s i n Rates o f Drug M e t a b o l i s m The approach o f s t u d y i n g each e n v i r o n m e n t a l f a c t o r i n d e p e n d e n t l y o f all others under r i g i d l y c o n t r o l l e d c o n d i t i o n s has advantages enumerated i n the p r e c e d i n g s e c t i o n . The p r i n c i p a l advantage c o n s i s t s o f the use o f each s u b j e c t as h i s own c o n t r o l b e f o r e and a f t e r i m p o s i t i o n o f s i n g l e d i e t a r y f a c t o r s , whose e f f e c t s can then be estimated s e n s i t i v e l y and v e r i f i e d by r e p e t i t i o n o f the e n t i r e experiment i n the same s u b j e c t a t a l a t e r t i m e as w e l l as by c o n s t r u c t i o n o f a dose-response c u r v e . However, as we have a l s o seen, t h i s method does not p e r m i t easy e x t r a p o l a t i o n t o a d i f f e r e n t c i r c u m s t a n c e and a d i f f e r e n t q u e s t i o n : how much o f the t o t a l e x t e n s i v e i n t e r i n d i v i d u a l v a r i a t i o n observed i n drug m e t a b o l i s m i n a l a r g e n o n b a s a l p o p u l a t i o n can be accounted f o r by each d i e t a r y f a c t o r alone? An e n t i r e l y s t a t i s t i c a l approach t o t h i s q u e s t i o n has been a t t e m p t e d . I n g a t h e r i n g 128 f a c t o r y workers i n London, the i n v e s t i g a t o r s d e l i b e r a t e l y chose s u b j e c t s w i t h d i f f e r i n g e n v i r o n m e n t s ; i n t h e i r words, "the aim i n t h i s s t u d y was t o use i n d i v i d u a l s drawn from the Community and t o i n c l u d e a range o f r a c e and environment (27). A n t i p y r i n e was then g i v e n once t o each o f 128 London f a c t o r y and o f f i c e w o r k e r s . A c r i t i c a l , but as y e t unproven, assumption o f t h i s approach i s t h a t i n s u b j e c t s who a r e e n v i r o n m e n t a l l y p e r t u r b e d , a n t i p y r i n e k i n e t i c s o b t a i n e d o n l y once a r e r e p r e s e n t a t i v e and r e p r o d u c i b l e . A c o m p u t e r i z e d program based on m u l t i p l e r e g r e s s i o n a n a l y s i s was employed t o r e l a t e the a n t i p y r i n e c l e a r a n c e o f each s u b j e c t t o numerous a n t h r o p o m e t r i c and b i o c h e m i c a l i n d i c e s o f n u t r i t i o n a l s t a t u s (27). F i v e v a r i a b l e s c o r r e l a t e d independently with antipyrine clearance: r a c e , c o n t r a c e p t i v e p i l l usage, smoking s t a t u s , p o n d e r a l i n d e x (body w e i g h t / h e i g h t ) and serum albumin c o n c e n t r a t i o n ( 2 7 ) . Because the c o e f f i c i e n t s f o r r a c e and d i e t were v i r t u a l l y i d e n t i c a l , the a u t h o r s concluded t h a t i t was i m p o s s i b l e t o a n a l y z e the r o l e o f one f a c t o r i n d e p e n d e n t l y o f the o t h e r . The m a j o r i t y o f t h e i r A s i a n I n d i a n s u b j e c t s were v e g e t a r i a n s , whereas t h e i r p o p u l a t i o n o f White Londoners almost a l l a t e meat and, f u r t h e r m o r e , were confounded by a d d i t i o n a l e n v i r o n m e n t a l p e r t u r b a t i o n s such as c i g a r e t t e smoking and i n g e s t i o n o f o r a l a n t i f e r t i l i t y agents. R e c o g n i z i n g t h a t f i r m c o n c l u s i o n s r e g a r d i n g the r o l e o f d i e t a r y f a c t o r s i n causing large i n t e r i n d i v i d u a l v a r i a t i o n s i n a n t i p y r i n e k i n e t i c s were i m p o s s i b l e i n t h i s mixed, and hence h o p e l e s s l y confounded, e x p e r i m e n t a l s i t u a t i o n , t h e i n v e s t i g a t o r s d e c i d e d t o use t h i s same s t a t i s t i c a l method but w i t h a n o t h e r group o f s u b j e c t s . T h i s second group was c o n s i d e r e d more amenable t o i n v e s t i g a t i o n by m u l t i p l e r e g r e s s i o n a n a l y s i s because i t was r a c i a l l y homogeneous, t h e r e b y p e r m i t t i n g the r o l e o f d i e t t o be a s s e s s e d w i t h o u t i n t e r f e r e n c e from the unresolvable d i f f e r e n c e s they encountered i n their first p o p u l a t i o n o f 128 r a c i a l l y mixed s u b j e c t s . The s u b j e c t s o f t h i s second s t u d y were 36 h e a l t h y a d u l t I n d o - P a k i s t a n i immigrants t o B r i t a i n . Among these 36 s u b j e c t s , a n t i p y r i n e c l e a r a n c e was much s l o w e r i n the 16 l a c t o v e g e t a r i a n s (0.54 + 0.06 mL min" k g " ) t h a n i n the 20 r e g u l a r meat e a t e r s (0.91 + 0.07 ml min kg"" ) ( 2 8 ) . S i n c e absence o f meat 11



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from the d i e t was a s s o c i a t e d w i t h a s i g n i f i c a n t l y s m a l l e r d a i l y i n t a k e o f d i e t a r y p r o t e i n , w h i c h i n the 16 l a c t o v e g e t a r i a n s was abnormally low by Western s t a n d a r d s , t h e i n v e s t i g a t o r s concluded that t h i s d i f f e r e n c e i n d a i l y p r o t e i n i n t a k e was t h e p r i n c i p a l cause o f the d i f f e r e n c e s observed i n a n t i p y r i n e c l e a r a n c e ( 2 8 ) . As the a u t h o r s r e c o g n i z e d , however, t h e i r methods a r e open t o numerous c r i t i c i s m s . For example, as t h e y s t a t e : "Retrospective d i e t a r y assessments made a t a s i n g l e i n t e r v i e w , even when t h i s i s conducted by an e x p e r i e n c e d d i e t i c i a n , a r e open t o c r i t i c i s m s i n c e they r e l y h e a v i l y upon the memory o f t h e s u b j e c t i n t e r v i e w e d , the e s t i m a t e s o f q u a n t i t y made by the d i e t i c i a n , and the subsequent e x p r e s s i o n o f those q u a n t i t i e s i n mass u n i t s . The o p p o r t u n i t y f o r e r r o r i s c l e a r l y i n c r e a s e d when the i n t e r v i e w i s conducted t h r o u g h an i n t e r p r e t e r " (28). Very high standard d e v i a t i o n s were c a l c u l a t e d f o r the 16 lactovegetarians. Since extensive i n t e r i n d i v i d u a l v a r i a t i o n s i n a n t i p y r i n e k i n e t i c s p e r s i s t w i t h i n a population uniform with respect to t h i s p a r t i c u l a r d i e t a r y f a c t o r , f a c t o r s other than p r o t e i n i n t a k e must cause them. The a u t h o r s attempt t o p r o v i d e some p o s s i b l e s o u r c e s f o r these i n t e r i n d i v i d u a l v a r i a t i o n s : "significant correlations o c c u r r e d between a n t i p y r i n e c l e a r a n c e and age, s e x , r e l i g i o n , smoking, haemoglobin, k i l o c a l o r i e s , c a r b o h y d r a t e , and f a t . No s i g n i f i c a n t c o r r e l a t i o n was observed between a n t i p y r i n e c l e a r a n c e and w e i g h t , c o f f e e / t e a i n d e x , a l c o h o l i n t a k e o r plasma a l b u m i n " ( 2 8 ) . But the a u t h o r s d i d not attempt t o r e l a t e the s o c i o e c o n o m i c o r e d u c a t i o n a l l e v e l of subjects to t h e i r d i e t a r y h a b i t s , p a r t i c u l a r l y t h e i r p r o t e i n i n t a k e . T h i s and a d d i t i o n a l c o n f o u n d i n g f a c t o r s might i n f l u e n c e t h e i r results. Even f o r t h i s l i m i t e d group o f 36 I n d o - P a k i s t a n i , the methodology employed l e f t the major c o n c l u s i o n open t o q u e s t i o n because, as the a u t h o r s s t a t e , "when n o n - v e g e t a r i a n s a l o n e were c o n s i d e r e d , t h e r e was no s i g n i f i c a n t c o r r e l a t i o n between a n t i p y r i n e c l e a r a n c e and d a i l y meat i n t a k e , r a i s i n g the p o s s i b i l i t y t h a t one o r more o t h e r d i e t a r y f a c t o r s might be o f g r e a t e r i m p o r t a n c e . Moreover, many o f the v a r i a b l e s c o r r e l a t e d s i g n i f i c a n t l y w i t h each o t h e r as w e l l as w i t h c l e a r a n c e . S t e p w i s e m u l t i p l e r e g r e s s i o n a n a l y s i s was t h e r e f o r e performed and i d e n t i f i e d smoking, age and f a t i n t a k e as independent c o r r e l a t e s w i t h clearance" (28). Other d i s c r e p a n c i e s m e r i t a t t e n t i o n . A g a i n , a c c o r d i n g t o the a u t h o r s , s i n c e " d a i l y p r o t e i n i n t a k e was s i g n i f i c a n t l y lower amongst v e g e t a r i a n s , w h i l e d a i l y f a t i n t a k e was s i m i l a r i n the two subgroups, i t was s u r p r i s i n g t h a t r e g r e s s i o n a n a l y s i s i d e n t i f i e d f a t i n t a k e as the o n l y independent d i e t a r y c o r r e l a t e w i t h a n t i p y r i n e c l e a r a n c e " (28). T h i s anomaly s h o u l d be c o n s i d e r e d not o n l y i n l i g h t o f t h e c a r e f u l l y c o n t r o l l e d d i e t a r y s t u d i e s (7) t h a t a s s i g n e d a s m a l l r o l e t o f a t i n a f f e c t i n g a n t i p y r i n e k i n e t i c s but a l s o i n c o n n e c t i o n w i t h t h e i n c o n s i s t e n c y o f the present r e s u l t s on a l b u m i n w i t h t h e s e o f the a u t h o r s p r e v i o u s s t u d y ( 2 7 ) . C o l l e c t i v e l y , these d i f f i c u l t i e s r e n d e r q u e s t i o n a b l e the s t a t i s t i c a l approach used here and elsewhere ( 2 9 ) . I n an e a r l i e r s t u d y u s i n g m u l t i p l e r e g r e s s i o n a n a l y s i s t o determine the b a s i s o f i n t e r i n d i v i d u a l v a r i a t i o n s i n a n t i p y r i n e c l e a r a n c e among 49 Garabians ( 2 9 ) , the p r i n c i p a l c a u s a t i v e f a c t o r i d e n t i f i e d was the number o f c o l a n u t s chewed per day ( r = 0.4). However, i n a p r o s p e c t i v e s t u d y on an e n t i r e l y d i f f e r e n t p o p u l a t i o n o f c a r e f u l l y c o n t r o l l e d , e n v i r o n m e n t a l l y s t a b l e normal male s u b j e c t s 1



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l i v i n g i n s o u t h c e n t r a l P e n n s y l v a n i a , chewing c o l a n u t s f a i l e d t o a l t e r a n t i p y r i n e c l e a r a n c e (jJO). These a p p a r e n t l y c o n f l i c t i n g r e s u l t s are n o t n e c e s s a r i l y c o n t r a d i c t o r y because o f t h e m u l t i p l e g e n e t i c and e n v i r o n m e n t a l d i f f e r e n c e s between t h e groups i n v e s t i g a t e d . However, t h e i r d i v e r g e n c e emphasizes t h e c r u c i a l r o l e p l a y e d by t h e c r i t e r i a used t o s e l e c t methods t o a n a l y z e d a t a , as w e l l as s u b j e c t s t o r e c e i v e drugs. Because t h e B r i t i s h group a p p l i e d e x t e n s i v e l y t h e i r s t a t i s t i c a l method t o determine c a u s a t i o n o f l a r g e i n t e r 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 t i o n s w i t h o u t d e s c r i b i n g i t s s t r e n g t h s and weaknesses, o t h e r s have attempted t o a s s e s s c r i t i c a l l y t h e a p p l i c a t i o n o f m u l t i p l e r e g r e s s i o n a n a l y s i s f o r t h i s p a r t i c u l a r purpose (.31,32). W h i l e t h i s s t a t i s t i c a l method has g r e a t p o t e n t i a l , i t r e q u i r e s c o n s i d e r a b l e modif i c a t i o n beyond i t s i n i t i a l a p p l i c a t i o n s i n t h i s a r e a (27-29), i f t h a t p o t e n t i a l i s t o be r e a l i z e d (H,32). Thus f a r , i t s a p p l i c a t i o n s i n p h a r m a c o k i n e t i c s (27-29) have been d i s a p p o i n t i n g because t h o s e who have employed i t n e i t h e r f o r m u l a t e d n o r a d d r e s s e d , much l e s s demonstrated f u l f i l l m e n t o f , s e v e r a l fundamental assumptions i n h e r e n t i n i t s use (jJl, 3 2 ) . S e v e r a l fundamental assumptions o f t h e p a r t i c u l a r model used a r e dubious when a p p l i e d t o i n v e s t i g a t i o n s o f s o u r c e s o f i n t e r i n d i v i d u a l d i f f e r e n c e s i n a n t i p y r i n e k i n e t i c s . Four major concerns a r i s e . F i r s t l y , t h e numbers s t u d i e d i n most i n v e s t i g a t i o n s have been s m a l l and do n o t r e p r e s e n t f o r m a l p r o b a b i l i t y samples from a known p o p u l a t i o n . Thus, t h e t r u e r e p r e s e n t a t i v e c h a r a c t e r o f t h e r e s u l t s i s questionable, and t h e r o b u s t n e s s o f t h i s model remains t o be e s t a b l i s h e d through a demonstration that the r e s u l t s obtained with i t can be r e p l i c a t e d i n another sample group drawn from t h e same l a r g e r population. The s e r i o u s n e s s o f t h i s r e s e r v a t i o n a r i s e s from t h e m u l t i p l e e n v i r o n m e n t a l f a c t o r s t h a t c o u l d e x e r t some e f f e c t and t h e d i f f i c u l t y i n o b t a i n i n g a s u f f i c i e n t l y l a r g e group o f s u b j e c t s t o r e p r e s e n t a d e q u a t e l y most p e r t i n e n t e n v i r o n m e n t a l f a c t o r s . I n f a c t , as i n d i c a t e d below, r e p l i c a b i l i t y o f r e s u l t s has n o t been o b t a i n e d w i t h t h i s model. S e c o n d l y , t h i s method f a i l s t o account f o r t h e m a j o r i t y o f t h e i n t e r s u b j e c t v a r i a b i l i t y i n a n t i p y r i n e d i s p o s i t i o n t h a t e x i s t s among even t h o s e few s u b j e c t s i n whom t h e method has been a p p l i e d . F o r example, when smoking and o r a l c o n t r a c e p t i v e use were c o n s i d e r e d a l o n e ( 3 3 ) , o n l y 9% o f t h e t o t a l v a r i a n c e i n a n t i p y r i n e k i n e t i c s among 207 normal v o l u n t e e r s was accounted f o r ; none o f t h e o t h e r f a c t o r s examined by m u l t i p l e r e g r e s s i o n a n a l y s i s c o u l d p r o v i d e a c l u e as t o what was r e s p o n s i b l e f o r t h e o t h e r 91? o f t h e v a r i a n c e . The t h i r d and p o s s i b l y most s e r i o u s f l a w i n t h e p a r t i c u l a r m u l t i p l e r e g r e s s i o n model l i e s i n t h e assumption t h a t i t a c c u r a t e l y accounts f o r the l a r g e i n t e r s u b j e c t v a r i a b i l i t y i n a n t i p y r i n e k i n e t i c s t h a t a r i s e s i n response n o t o n l y t o any s i n g l e e n v i r o n m e n t a l f a c t o r a c t i n g a l o n e but a l s o t o s e v e r a l such f a c t o r s a c t i n g concomitantly. The m u l t i p l e r e g r e s s i o n a n a l y s i s model used t o a s s e s s s o u r c e s o f phenotypic v a r i a t i o n i n a n t i p y r i n e k i n e t i c s i s s e n s i t i v e only t o variations that exhibit a linear relationship (27,29,33). D i s t r i b u t i o n c u r v e s o f a n t i p y r i n e c l e a r a n c e s t h e m s e l v e s , as w e l l as o f e n v i r o n m e n t a l e f f e c t s on a n t i p y r i n e c l e a r a n c e , are not e n t i r e l y linear. Nonlinear portions o f t h i s v a r i a b i l i t y are i n s e n s i t i v e t o r e s o l u t i o n by t h e model t h a t was s e l e c t e d . Other more s e n s i t i v e models based on m u l t i p l e r e g r e s s i o n a n a l y s i s might have been, and s t i l l can


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be, employed t o r e s o l v e n o n - l i n e a r systems o r n o n - l i n e a r p o r t i o n s o f linear systems, providing that these independent variables are properly represented, e.g. by f o r m i n g p r o d u c t s o r powers. However, w i t h r e s p e c t t o p h e n o t y p i c v a r i a t i o n s i n a n t i p y r i n e k i n e t i c s , our p r e s e n t knowledge o f the c u r r e n t d i s t r i b u t i o n p r o p e r t i e s produced by each e n v i r o n m e n t a l f a c t o r i s t o o i n c o m p l e t e t o p e r m i t i t s a p p r o p r i a t e r e p r e s e n t a t i o n as a l i n e a r o r n o n - l i n e a r f u n c t i o n . The following c o n s i d e r a t i o n s s u g g e s t t h a t t h i s fundamental assumption o f l i n e a r i t y o f the model based on m u l t i p l e r e g r e s s i o n a n a l y s i s i s i n c o r r e c t . I n d i v i d u a l r e s p o n s e s t o many e n v i r o n m e n t a l f a c t o r s a r e u n p r e d i c t a b l e and h i g h l y v a r i a b l e . F o r example, i n d u c t i o n o f h e p a t i c d r u g m e t a b o l i z i n g enzymes has been shown t o be h i g h l y v a r i a b l e even though a p p a r e n t l y normal s u b j e c t s a r e exposed t o s i m i l a r doses o f an i n d u c i n g agent g i v e n by the same r o u t e . Hence, l a r g e i n t e r i n d i v i d u a l v a r i a t i o n s e x i s t i n the e x t e n t t o which t h e a c t i v i t i e s o f the hepatic mixed-function oxidases are enhanced by the same dose o f c e r t a i n i n d u c i n g a g e n t s a d m i n i s t e r e d by t h e same r o u t e . S i m i l a r l y , large i n t e r i n d i v i d u a l v a r i a t i o n s occur i n the magnitude o f enzyme i n h i b i t i o n produced by a d r u g a d m i n i s t e r e d i n t h e same dose by t h e same r o u t e t o normal s u b j e c t s under n e a r b a s a l conditions. The computer model cannot decipher from a s i n g l e antipyrine kinetic measurement the sources for these large i n t e r i n d i v i d u a l d i f f e r e n c e s beyond the few s p e c i f i c environmental f a c t o r s t h a t t h e i n v e s t i g a t o r s have c o n s i d e r e d . Accordingly, it appears t h a t such a model, by a p p l y i n g an average v a l u e f o r any g i v e n f a c t o r to a l l s u b j e c t s , c o r r e c t s i n a c c u r a t e l y f o r v a r i a b l e responses t o i n d u c t i o n o r i n h i b i t i o n and i s t h e r e f o r e b i a s e d when a p p l i e d t o the i n v e s t i g a t i o n of a genetic hypothesis. T h i s model overcorrects c e r t a i n s u b j e c t s f o r the i n f l u e n c e e x e r t e d by an e n v i r o n m e n t a l f a c t o r on a n t i p y r i n e c l e a r a n c e , w h i l e u n d e r c o r r e c t i n g o t h e r s u b j e c t s , t h e r e b y yielding contradictory conclusions. When s e v e r a l c o n c o m i t a n t l y acting factors influence antipyrine metabolism i n environmentally p e r t u r b e d s u b j e c t s , the e x t e n t o f the v a r i a b i l i t y produced p r o b a b l y exceeds t h a t e x p e c t e d from the r e s u l t s o f c a r e f u l l y c o n t r o l l e d e x p e r i m e n t s i n s u b j e c t s under n e a r basal c o n d i t i o n s where o n l y one f a c t o r i s changed a t a t i m e . Although the a n t i p y r i n e t e s t can i d e n t i f y such d e p a r t u r e s from e x p e c t a t i o n due t o i n t e r a c t i o n s among f a c t o r s , the model based on m u l t i p l e r e g r e s s i o n a n a l y s i s , by c o n t r a s t , assumes t h a t t h e n e t e f f e c t o f a l l f a c t o r s can be a c c o u n t e d f o r s i m p l y by a d d i n g the e f f e c t s o f each i n d i v i d u a l f a c t o r taken alone. Thus, the p a r t i c u l a r model based on m u l t i p l e r e g r e s s i o n a n a l y s i s t h a t was s e l e c t e d f o r use cannot d e t e c t s y n e r g i s m from interacting factors. I t must be emphasized t h a t the p o t e n t i a l o f m u l t i p l e r e g r e s s i o n a n a l y s i s t o r e s o l v e s o u r c e s o f p h a r m a c o k i n e t i c v a r i a t i o n s i s much g r e a t e r t h a n has been r e a l i z e d by the p a r t i c u l a r 'canned model used previously. The t e c h n i q u e i t s e l f i s both s e n s i t i v e and powerful. However, f o r m u l t i p l e r e g r e s s i o n a n a l y s i s t o be used a p p r o p r i a t e l y , a model must be d e v e l o p e d t h a t encompasses n o n - l i n e a r as w e l l as l i n e a r r e l a t i o n s h i p s (34). E r r o r terms e s p e c i a l l y need t o be a p p r o p r i a t e l y modelled, r a t h e r than t r e a t e d i n a simply a d d i t i v e manner as i n p r e v i o u s a p p l i c a t i o n s o f t h i s method. F o u r t h l y , i n view o f the p r e c e d i n g 3 major weaknesses i n the model, i t i s not s u r p r i s i n g t h a t the r e s u l t s o b t a i n e d w i t h i t d i f f e r markedly from much p r e v i o u s work on age, sex and g e n e t i c c o n s t i t u t i o n . 1


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F o r example, on the 3 o c c a s i o n s where r e g r e s s i o n a n a l y s i s was a p p l i e d , 3 d i v e r g e n t c o n c l u s i o n s were r e a c h e d on the e f f e c t o f sex on a n t i p y r i n e metabolism. Despite d i f f e r e n t populations i n each s t u d y , ethnic d i f f e r e n c e s a l o n e p r o b a b l y do not e x p l a i n the d i s c r e p a n c i e s . P o s s i b l y too much r e l i a n c e has been p l a c e d on the s t a t i s t i c a l s i g n i f i c a n c e o f p values d e r i v e d from the model based on m u l t i p l e r e g r e s s i o n a n a l y s e s , p a r t i c u l a r l y when the c o r r e l a t i o n s themselves a r e low and the s u b j e c t s a r e under h i g h l y p e r t u r b e d e n v i r o n m e n t a l c o n d i t i o n s (35,36). Whereas the r e s u l t s o f such s t u d i e s on causes o f v a r i a b i l i t y i n a n t i p y r i n e k i n e t i c s may p r o v i d e v a l u a b l e c l u e s , the c o n c l u s i o n s and c l u e s drawn from the model t h a t uses m u l t i p l e r e g r e s s i o n a n a l y s e s s h o u l d be v a l i d a t e d by a c a r e f u l l y c o n t r o l l e d , p r o s p e c t i v e experiment before safe acceptance. f


1

I n an a p p l i c a t i o n o f t h i s model f o r a s s e s s i n g f a c t o r s t h a t cause l a r g e i n t e r i n d i v i d u a l v a r i a t i o n s i n the c l e a r a n c e of t h e o p h y l l i n e (37), the a u t h o r s c l e a r l y r e c o g n i z e d i t s l i m i t a t i o n s and t h i s r e q u i r e ment t h a t the c o n c l u s i o n s be t e s t e d by a p r o s p e c t i v e c o n t r o l l e d e x p e r iment: "The f a c t o r s i d e n t i f i e d as i m p o r t a n t i n t h e o p h y l l i n e body clearances are associations found by retrospective statistical analysis which need not imply a cause-and-effect relationship, e s p e c i a l l y where a p a t h o p h y s i o l o g i c a l o r d r u g i n t e r a c t i o n r a t i o n a l e does not e x i s t . O f t e n t h e s e f a c t o r s need f u r t h e r c o n f i r m a t i o n by p r o s p e c t i v e e x a m i n a t i o n o f c o h o r t s o f s u b j e c t s w i t h the d i s e a s e o r history i n question." The r e l a t i o n s h i p between i n t r a i n d i v i d u a l and interindividual v a r i a t i o n m e r i t s d i s c u s s i o n because r e c e n t a p p r o a c h e s e i t h e r i g n o r e d the former or assumed t h a t i t was n e g l i g i b l e compared w i t h the l a t t e r (27,29,23). The magnitude o f i n t r a i n d i v i d u a l v a r i a t i o n i s not always small. The more e n v i r o n m e n t a l l y p e r t u r b e d the s u b j e c t s i n v e s t i g a t e d , t h e l a r g e r the i n t r a i n d i v i d u a l v a r i a b i l i t y r e l a t i v e t o i n t e r i n d i v i d u a l v a r i a b i l i t y , a l t h o u g h the former can n e v e r exceed the l a t t e r . The magnitude o f i n t r a i n d i v i d u a l v a r i a t i o n a l s o depends on the d r u g under s t u d y , b e i n g low f o r drugs w i t h low h e p a t i c e x t r a c t i o n r a t i o s (such as p h e n y l b u t a z o n e and a n t i p y r i n e ) and h i g h f o r drugs w i t h h i g h h e p a t i c e x t r a c t i o n r a t i o s ( e . g . p h e n a c e t i n ) (38). A s i m i l a r c o n c l u s i o n was r e a c h e d e a r l i e r (39) i n a s t u d y t h a t s t r e s s e d l a r g e i n t r a i n d i v i d u a l v a r i a t i o n s i n subjects of pharmacokinetic s t u d i e s on clindamycin, e p h e d r i n e , e t h o s u x i m i d e , l i n c o m y c i n , and w a r f a r i n . I n t e r p r e t a t i o n of the v a l u e s f o r i n t r a i n d i v i d u a l v a r i a t i o n i n the s u b j e c t s o f t h a t s t u d y i s d i f f i c u l t because no i n f o r m a t i o n c o n c e r n i n g s p e c i f i c e n v i r o n m e n t a l c o n d i t i o n s o f the s u b j e c t s was provided. A Third Approach to Assessment of Dietary I n t e r i n d i v i d u a l V a r i a t i o n s i n Drug D i s p o s i t i o n

Contributions

to

The magnitude o f i n t e r i n d i v i d u a l v a r i a t i o n r e p o r t e d can depend on the drug s e l e c t e d f o r study, the number of subjects studied, the p a r t i c u l a r p o p u l a t i o n from which the s u b j e c t s a r e drawn, and the • c o n d i t i o n o f the s u b j e c t s , i n c l u d i n g t h e i r p r e s e n t and p a s t h e a l t h , g e n e t i c c o n s t i t u t i o n , age, sex, d i e t , and exposure t o e n v i r o n m e n t a l c h e m i c a l s and drugs t h a t i n d u c e o r i n h i b i t h e p a t i c mixed-function oxidases. As s u g g e s t e d i n the p r e c e d i n g s e c t i o n , r e s u l t s o f such p u b l i s h e d s t u d i e s a l s o r e f l e c t t h e s p e c i f i c method used and the degree t o which the assumptions u n d e r l y i n g each method a r e fulfilled. 1



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A c c o r d i n g l y , t h i s r e v i e w s t r e s s e s the c h a r a c t e r i s t i c s o f each method and t h e assumptions upon which i t i s b a s e d . It should be emphasized t h a t when s u b j e c t s a r e under b a s a l e n v i r o n m e n t a l c o n d i t i o n s w i t h r e s p e c t t o t h e many f a c t o r s t h a t can a f f e c t h e p a t i c d r u g o x i d a t i o n and when a c c o r d i n g l y t h e i r h e p a t i c d r u g m e t a b o l i z i n g enzymes a r e r e l a t i v e l y uninduced and u n i n h i b i t e d , l a r g e i n t e r i n d i v i d u a l v a r i a t i o n s s t i l l remain. Although w e l l e s t a b l i s h e d , t h i s important fact i s often ignored. Thus, under n e a r basal conditions interindividual pharmacokinetic v a r i a t i o n s cannot be attributed safely to any obvious environmental influence. F u r t h e r m o r e , numerous t w i n and f a m i l y s t u d i e s have i n d i c a t e d t h a t these l a r g e i n t e r i n d i v i d u a l pharmacokinetic variations i n subjects under near b a s a l e n v i r o n m e n t a l c o n d i t i o n s a r i s e from g e n e t i c f a c t o r s Q1>1§> 31 i22,2j5). Nevertheless, under other conditions, where environmental f a c t o r s are permitted t o e x e r t a d i f f e r e n t i a l (unequal) i n f l u e n c e on the s u b j e c t s o f a study, this pattern of genetic transmission can be concealed. The major portion of the i n t e r i n d i v i d u a l v a r i a t i o n o b s e r v e d can t h e n be m i s t a k e n l y a t t r i b u t e d s o l e l y t o e n v i r o n m e n t a l s o u r c e s , because u n d e r l y i n g g e n e t i c f a c t o r s t h a t a r e a l s o o p e r a t i v e become u n r e c o g n i z a b l e . Accordingly, the answer t o the d i f f i c u l t q u e s t i o n o f what r o l e n u t r i t i o n a l f a c t o r s p l a y i n maintaining l a r g e i n t e r i n d i v i d u a l pharmacokinetic variations in normal s u b j e c t s living unrestricted l i f e styles depends on the c h a r a c t e r i s t i c s of the s u b j e c t s s e l e c t e d , the extent o f t h e i r e n v i r o n mental p e r t u r b a t i o n , and the p a r t i c u l a r c o m b i n a t i o n o f e n v i r o n m e n t a l f a c t o r s e x e r t i n g t h e i r e f f e c t s on t h e s e s u b j e c t s a t any g i v e n t i m e . To e l u c i d a t e t h e s e p r e c i s e r e l a t i o n s h i p s a d i f f e r e n t a p p r o a c h from t h o s e u n d e r t a k e n t h u s f a r i s recommended. This approach i n c o r p o r a t e s p a r t s o f s e v e r a l m e t h o d o l o g i e s d e s c r i b e d i n the p r e c e d i n g sections. The p a r t adopted from s t u d i e s t h a t employed m u l t i p l e r e g r e s s i o n a n a l y s i s i s s e l e c t i o n o f normal s u b j e c t s under u n r e s t r i c t e d l i f e styles. The p a r t adopted from c a r e f u l l y c o n t r o l l e d s t u d i e s on subjects under uniform near basal environmental conditions is manipulation of s i n g l e n u t r i t i o n a l f a c t o r s independently of a l l others w i t h r e p e a t e d a d m i n i s t r a t i o n s o f the model drug t o o b t a i n k i n e t i c measurements b e f o r e , d u r i n g and a f t e r t h i s s i n g l e d i e t a r y change. Such an e x p e r i m e n t a l d e s i g n s h o u l d p e r m i t assessment o f the influence o f t h e s i n g l e d i e t a r y change, g e n e r a l l y w i t h d r a w a l o f a p a r t i c u l a r element, on the k i n e t i c s o f the model d r u g . F o r example, i n normal s u b j e c t s l i v i n g an u n r e s t r i c t e d l i f e s t y l e , s u b s t i t u t i o n f o r s e v e r a l weeks o f a v e g e t a r i a n d i e t f o r one i n which meat p l a y s a p r i n c i p a l p a r t c o u l d be a s s e s s e d w i t h r e s p e c t t o a n t i p y r i n e k i n e t i c s . A l l o t h e r d i e t a r y and e n v i r o n m e n t a l f a c t o r s o f the s u b j e c t s i n such a s t u d y would have t o remain as c o n s t a n t as p o s s i b l e . In separate e x p e r i m e n t s e f f e c t s o f c r u c i f e r o u s v e g e t a b l e s and c h a r c o a l b r o i l e d b e e f c o u l d be e s t i m a t e d by p r o h i b i t i n g e a c h from the d i e t f o r s e v e r a l weeks and comparing the k i n e t i c s o f a model d r u g b e f o r e , d u r i n g and a f t e r such a p e r i o d o f withdrawal. The i n f l u e n c e o f such v a r i a b l e s as age, sex, smoking, e t h a n o l , o r a l a n t i f e r t i l i t y a g e n t s , t o t a l c a l o r i e s , the r e l a t i v e p r o p o r t i o n o f the macromolecular constitutents of the diet, exercise, and e n v i r o n m e n t a l exposure t o c e r t a i n p r e v a l e n t c h e m i c a l s would e v e n t u a l l y need t o be r e c o g n i z e d and t o some e x t e n t t e s t e d b e f o r e the g e n e r a l i t y o f any c o n c l u s i o n s drawn i n one group o f s u b j e c t s c o u l d be extended t o other groups. A l s o , as p r e v i o u s l y emphasized, normal s u b j e c t s under



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perturbed environmental conditions are more variable on repetition of their kinetic values than subjects under near basal environmental conditions. Therefore, the suggested experimental design needs to establish in each subject the extent of intraindividual variation. That is, kinetic values should be measured in each subject several times prior to the dietary manipulation. Possibly in some subjects the magnitude of intraindividual variation may reach, or perhaps exceed, that obtained after imposition of a specific dietary change. Such studies are laborious, time-consuming and technically difficult to undertake, but they may represent the next step in the extension of the conclusions on dietary factors from normal subjects under near basal environmental conditions to subjects under environmentally perturbed conditions. In this connection a study revealed markedly altered therapeutic responses secondary to a change in drug kinetics produced by switching the proportion of macromolecular constituents in the diet. In 14 children with asthma, theophylline kinetics were accelerated by a diet high in protein, and retarded by a diet high in carbohydrate (40). Of particular interest with respect to this discussion was the observation that within each of the 3 dietary groups the extent of interindividual variation was similar (3-fold) (40). Therefore, under none of these 3 different dietary conditions did the proportion of the macromolecular constitutents of the diet alone account for the 3-fold interindividual variations observed in theophylline clearance. More studies of this kind need to be performed to determine what proportion of the total interindividual pharmacokinetic variation arises from dietary factors. A major recent advance is the demonstration that parenteral nutrition is associated with altered antipyrine metabolism and that the extent of this change exhibits large interindividual variations (41). Literature Cited 1. 2.

3. 4.

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Anderson, K.D.; Conney, A.H.; Kappas, A. Nutr. Rev. 1982, 40, 161-171. Conney, A.H.; Buening, M.K.; Pantuck, E.J.; Pantuck, C.B.; Fortner, J.G.; Anderson, K.E.; Kappas, A. In "Environmental Chemicals, Enzyme Function and Human Disease"; Evered, D.; Lawrenson, G., Eds.; PRECEDINGS OF THE CIBA FOUNDATION SYMPOSIUM No. 76, Excerpta Medica, Amsterdam, 1980; p. 147. Conney, A.H.; Pantuck, E.J.; Kuntzman, R.; Kappas, A.; Anderson, K.E.; Alvares, A.P. Clin. Pharmacol. Ther. 1977, 22, 707. Kappas, A.; Alvares, A.P.; Anderson, K.E.; Garland, W.A.; Pantuck, E.J.; Conney, A.H. In "Microsomes and Drug Oxidation"; Ulrich, V.; Roots, I.; Hildebrand, A.; Estabrook, R.W.; Conney, A.H., Eds.; Pergamon Press, New York, 1977, p. 703. Vesell, E.S. Rational Drug Ther. 1980, 14, No. 5, 1. Kappas, A.; Anderson, K.E.; Conney, A.H.; Alvares, A.P. Clin. Pharmacol. Ther. 1976, 20, 643. Anderson, K.E.; Conney, A.H.; Kappas, A. Clin. Pharmacol. Ther. 1979, 26, 493. Kappas, A.; Alvares, A.P.; Anderson, K.E.; Pantuck, E.J.; Pantuck, C.B.; Chang, R.; Conney, A.H. Clin. Pharmacol. Ther. 1978, 23, 445.


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Effect of Dietary Factors on Drug Disposition in Normal Human

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