Polycyclic Hydrocarbons and Carcinogenesis - American Chemical

4 The Bay Region Theory of Polycyclic Aromatic Hydrocarbon Carcinogenesis 1

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ROLAND E. LEHR , SUBODH KUMAR , WAYNE LEVIN , ALEXANDER W. WOOD , RICHARD L. CHANG , ALLAN H. CONNEY , HARUHIKO YAGI , JANE M. SAYER , and DONALD M. JERINA 2

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Downloaded by UNIV OF MISSOURI COLUMBIA on May 16, 2013 | http://pubs.acs.org Publication Date: July 19, 1985 | doi: 10.1021/bk-1985-0283.ch004

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Department of Chemistry, University of Oklahoma, Norman, OK 73019 Department of Experimental Carcinogenesis and Metabolism, Hoffmann-La Roche, Inc., Nutley, NJ 07110 Laboratory of Bioorganic Chemistry, National Institute of Arthritis, Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, MD 20205

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The hydrolysis rates, mutagenicities and tumorigeni c i t i e s of a variety of diol epoxides and tetrahydroepoxides derived from polycyclic aromatic hydrocarbons (PAH) are examined. Bay-region diol epoxides and tetrahydroepoxides, as predicted by the "bay-region theory," are the most reactive and biologically active of those derivatives of a given PAH. A comparison of reactivity and mutagenicity data with a quantum chem i c a l parameter that estimates chemical reactivity, ΔE /β, reveals a generally good correlation for the hydrolysis data, provided that diol epoxides with in groups of similar conformational preference are compared. A similar result i s found for the mutagen i c i t y data when the analysis i s confined to fourand five-ring PAH, with the significant exception of diol epoxides with the epoxide in a sterically hin dered bay region, which are considerably more muta genic than other diol epoxides with similar ΔE /β values. Three- and six-ring tetrahydro- and diol epoxides, however, exhibit consistently lower muta genicities than their four- and five-ring counter parts with similar conformational preferences and ΔE /β values. Additional restrictions apply to tumorigenicity. To date, only bay-region diol epoxides that have preferred conformations with pseudodiequatorial hydroxyl groups have been found to be significantly tumorigenic, and those only when they have a relatively high calculated ΔE /β value or a highly hindered bay-region epoxide. Mutagenicity data for benz[a]- and benz[c]acridine diol epoxides and tetrahydroepoxides reveals a de activating effect of nitrogen dependent upon i t s position relative to the epoxide. Benz[a]acridine bay region diol and tetrahydroepoxides have signi ficantly reduced mutagenicities relative to their deloc

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0097-6156/85/0283-0063$06.25/0 © 1985 American Chemical Society

In Polycyclic Hydrocarbons and Carcinogenesis; Harvey, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYCYCLIC HYDROCARBONS AND CARCINOGENESIS

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benz[a]anthracene counterparts, whereas the benz[c]acridine non-bay tetrahydroepoxide on the angular benzo ring has significantly reduced mutagenicity. For more than f i f t y years i t has been recognized that members of the polycyclic aromatic hydrocarbon (PAH) class of molecule exhibit widely varied carcinogenicities. However, i t i s only relatively recently that the chemistry and biochemistry of PAH has become sufficiently understood to permit an assessment of the structureactivity relationships involved in their diverse carcinogenicities. A conceptual basis for understanding the chemical carcinogenesis of PAH was laid i n the 1950's by the Millers (1). Their work with aromatic amides led them to hypothesize that many organic molecules become carcinogenic only after "metabolic activation" to reactive, electrophilic species capable of binding with cellular macromolecules. The identification of these reactive, "ultimate" carcino genic forms of PAH proved d i f f i c u l t , as a number of exciting possi b i l i t i e s emerged, but were eventually discarded. Nonetheless, by 1976, evidence was accumulating that, for benzo[a]pyrene (BaP), an important "ultimate" carcinogenic form was BaP 7,8-diol-9,10 -epoxide, produced by three steps of metabolism:

BaP-

This important discovery was made possible by significant contri butions from a number of research laboratories. Discussions of these contributions are included i n a number of articles (2-6). The bay region theory Although BaP 7,8-diol-9,10-epoxides provided a possible clue to explaining the carcinogenesis of PAH, i t was i n i t i a l l y unknown whether metabolic activation to diol epoxides would prove to be generally important and whether any structural feature i n the BaP 7,8-diol-9,10-epoxide could be identified that would provide a guide to predicting which of the various diol epoxides derivable from a PAH would be most biologically active. Jerina and Daly, by interpreting substituent effects upon the carcinogenicity of benz[a]anthracene (BA) derivatives and considering observed re activities at benzylic positions on tetrahydrobenzo rings of PAH, provided the i n i t i a l postulation that i t was the "bay region" nature of the epoxide i n BaP 7,8-diol-9,10-epoxide that was the c r i t i c a l structural feature (3). The prototype of a "bay region" in a PAH i s the sterically hindered region between C-4 and C-5 of phenanthrene, and bay region epoxides on tetrahydrobenzo rings of PAH are characterized by this structural feature, though additional aromatic rings may be fused to the aromatic nucleus. Because of the route by which PAH are metabolically activated, there w i l l also be a trans, vicinal dihydroxyl substitution at the two remaining


4.

LEHR ET AL.

s a t u r a t e d carbon

atoms on the t e t r a h y d r o b e n z o

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The Bay Region Theory

ring.

W h i l e , as

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w i l l be seen l a t e r , these h y d r o x y l groups have an I m p o r t a n t I n f l u e n c e on r e a c t i v i t y and b i o l o g i c a l a c t i v i t y , f o r the p r e s e n t I t I s a p p r o p r i a t e t o f o c u s a t t e n t i o n upon the epoxide m o i e t y , and I t s b e n z y l i c p o s i t i o n , as the s i t e o f r e a c t i v i t y . Quantum c h e m i c a l c a l c u l a t i o n s p r o v i d e d a t h e o r e t i c a l b a s i s f o r the bay r e g i o n t h e o r y . These c a l c u l a t i o n s were used t o e s t i m a t e the r e l a t i v e ease o f r i n g - o p e n i n g o f the b e n z y l i c C-0 epoxide bond t o f o r m a r e s o n a n c e - s t a b i l i z e d c a r b o c a t l o n (7^8). The p r e c i s e mechanism by w h i c h d i o l e p o x i d e s r e a c t w i t h DNA and o t h e r macrom o l e c u l e s was unknown then and r e m a i n s a t o p i c of a c t i v e r e s e a r c h . Nonetheless, carbocatIons are a t t r a c t i v e Intermediates t o c o n s i d e r , s i n c e they are I n v o l v e d I n the r a p i d h y d r o l y s i s o f the BaP bay r e g i o n d i o l e p o x i d e s and a r e h i g h l y e l e c t r o p h l l l c s p e c i e s c o n s i s t e n t w i t h the M i l l e r s ' h y p o t h e s i s . F u r t h e r , H u l b e r t (9) and o t h e r s have argued t h a t r e a c t i o n v i a a Sjjl mechanism o r t r a n s i t i o n s t a t e w i t h a p p r e c i a b l e c a r b o c a t l o n - l l k e c h a r a c t e r p r o v i d e s one way o f accommodating a s i g n i f i c a n t ( I f low) l e v e l o f r e a c t i v i t y o f an e l e c t r o p h l l e w i t h w e a k l y n u c l e o p h l l l c m a c r o m o l e c u l e s l i k e DNA. Thus, an e s t i m a t i o n of the r e l a t i v e ease w i t h w h i c h v a r i o u s ben z y l i c epoxides on t e t r a h y d r o b e n z o r i n g s a r e c o n v e r t e d t o e l e c t r o p h l l l c c a r b o c a t l o n s seemed a p p r o p r i a t e . A s i m i l a r approach had been p r e v i o u s l y a p p l i e d t o a r y l m e t h y l d e r i v a t i v e s of PAH, when those d e r i v a t i v e s appeared t o be o f Importance I n the m e t a b o l i c a c t i v a t i o n o f m e t h y l a t e d PAH (10). The p r i n c i p a l o p p o r t u n i t y f o r d i f f e r e n c e s I n a c t i v a t i o n energy f o r the c o n v e r s i o n r e s i d e s I n the TT - e l e c t r o n energy changes t h a t o c c u r , and t h e s e can be e a s i l y e s t i m a t e d through p e r t u r b a t l o n a l m o l e c u l a r o r b i t a l (PMO) c a l c u l a t i o n s . The PMO c a l c u l a t i o n y i e l d s a p a r a m e t e r , A E ^ ^ /&, w h i c h I s a d i f f e r e n c e I n i T - e l e c t r o n energy between the r e a c t a n t i r - s y s t e m (the u n s u b s t i t u t e d a r o m a t i c n u c l e u s ) and the ir-system o f the p r o d u c t (an a r y l m e t h y l c a t i o n , s u b s t i t u t e d a t the p o s i t i o n on the a r o m a t i c n u c l e u s c o r r e s p o n d i n g t o t h a t o f the b e n z y l i c carbon atom o f the epoxide). P r o v i d e d t h a t these d i f f e r e n c e s I n ir-energy a r e r e f l e c t e d I n the t r a n s i t i o n s t a t e f o r the c o n v e r s i o n , H i / 3 v a l u e s w i l l p r o v i d e an e s t i m a t e of r e l a t i v e r e a c t i v i t y , w i t h those e p o x i d e s t h a t can f o r m more h i g h l y s t a b i l i z e d c a r b o c a t l o n s b e i n g more r e a c t i v e . Thus, f o r c o n v e r s i o n o f BA H ^ - l , 2 - e p o x l d e t o the b e n z y l i c c a r b o c a t l o n ( E q u a t i o n 1), the PMO c a l c u l a t i o n c o r r e s p o n d s t o the TT-energy change shown I n E q u a t i o n 2. That I s , t h e TT-energy d i f f e r e n c e corresponds t o t h a t between anthracene and 1 - a n t h r a c e n y l m e t h y l c a t i o n . F o r f u r t h e r I l l u s t r a t i o n , the a n a l o gous c o n v e r s i o n o f BA H^-l0,11-epoxide ( E q u a t i o n 3) I s c a l c u l a t e d as the ir-energy d i f f e r e n c e between phenanthrene and 3-phenan A E

e

O C



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- t h r e n y l m e t h y l c a t i o n ( E q u a t i o n 4). The ir-systems o f t h e a r y l m e t h y l c a r b o c a t i o n s a r e i n v a r i a b l y s t a b i l i z e d r e l a t i v e t o t h e TTsystems o f t h e i r p r e c u r s o r , 0(H)


Eq. 1

Eq. 2

0(H) Eq. 3

AE

,

/3=

Eq. 4

0.571

w i t h l a r g e r values of A E j / 3 corresponding to greater s t a b i l i z a t i o n . N o t a b l y , these c a l c u l a t e d v a l u e s v a r y c o n s i d e r a b l y , w i t h s i g n i f i c a n t l y g r e a t e r s t a b i l i z a t i o n b e i n g found f o r c a r b o c a t l o n s d e r i v e d f r o m b a y - r e g i o n t e t r a h y d r o e p o x i d e s o f a g i v e n PAH than f r o m t e t r a h y d r o e p o x i d e s i n w h i c h t h e epoxide i s n o t a t a bay r e g i o n p o s i t i o n (7,8). A t h e o r e t i c a l b a s i s f o r t h i s o b s e r v a t i o n has been o f f e r e d (11). F u r t h e r , t h e c a l c u l a t i o n s o f A E J ^ /& f o r b e n z y l i c p o s i t i o n s o f d i f f e r e n t PAH r e v e a l e d t h a t more h i g h l y c a r c i n o g e n i c PAH t y p i c a l l y had h i g h e r c a l c u l a t e d AE^ ^ / 3 v a l u e s a t t h e b a y r e g i o n p o s i t i o n than d i d weak o r n o n - c a r c i n o g e n i c PAH (]_ 8). I n f a c t t h e c o r r e l a t i o n o f p r e d i c t e d c a r c i n o g e n i c i t y f o r a PAH based upon t h e c a l c u l a t e d A E j ^ / 3 v a l u e a t t h e b a y - r e g i o n p o s i t i o n o f the d e r i v e d t e t r a h y d r o b e n z o r i n g i s about e q u a l l y " s u c c e s s f u l " as t h a t o f t h e Pullmann's " K - r e g i o n " c a l c u l a t i o n s , s i n c e a good c o r r e l a t i o n between t h e p a r a m e t e r s used i n t h e two methods e x i s t s (8). The s t r u c t u r e s o f a v a r i e t y o f PAH o f v a r y i n g c a r c i n o g e n i c i t y t h a t have been i n v e s t i g a t e d i n o u r l a b o r a t o r i e s a r e shown i n F i g u r e 1, E L O C

(

Q C

y

E

Q C


LEHRET AL.

The Bay Region Theory

'4

.571

.545 '10


8

9

7

Phenanthrene Ph noncarc.

>8

7

6

Benzo[c]phenanthrene BcPh weak care.

6

BenzOaJanthracene BA weak care. Triphenylene Tp noncarc. 12

1

11

.794

.4BB Benzo Dajoyrene BaP strong care. 7

6

5

Benzo &1 pyrene BeP noncarc. .500

' 8

7

5

.S45

6

Dibenzo&,hJ pyrene DBahP strong care.

9

8

7

DibenzoCa,0pyrene DBaiP strong care.

F i g u r e 1. S t r u c t u r e s , w i t h numbering and c a l c u l a t e d A £ v a l u e s a t b e n z y l i c p o s i t i o n s o f the t e t r a h y d r o b e n z o - r i n g t i v e s o f PAH d i s c u s s e d i n t h i s c h a p t e r .

/3 deriva

d e l o c


68


along w i t h c a l c u l a t e d values of A E ^ ^ / 3 , which r e f e r to the v a l u e s c a l c u l a t e d a t t h e b e n z y l i c p o s i t i o n on t h e t e t r a h y d r o b e n z o r i n g d e r i v a t i v e o f t h e PAH. A l s o i n d i c a t e d i n t h i s F i g u r e a r e t h e a b b r e v i a t i o n s t o be used f o r t h e PAH i n t h i s a r t i c l e , and t h e numbering o f t h e PAH. The h i g h e r c a l c u l a t e d v a l u e s o f A E ^ ^ / 3 a t the b a y - r e g i o n p o s i t i o n s o f each PAH a r e e v i d e n t , and i t i s a p p r o p r i a t e t o note t h a t e x p e r i m e n t a l r e s u l t s w i t h a l l the carcinogens i n F i g u r e 1 a r e c o n s i s t e n t w i t h m e t a b o l i c a c t i v a t i o n t o bay r e g i o n d i o l e p o x i d e s as a major r o u t e o f a c t i v a t i o n o f t h e PAH. Indeed, f o r t h e more than t e n e v e n - a l t e r n a n t PAH so f a r s t u d i e d i n some depth, the e x p e r i m e n t a l r e s u l t s s t r o n g l y support "bay-region" a c t i v a t i o n . Recent r e v i e w s (12-14) have summarized t h e s e r e s u l t s , w h i c h came f r o m d e t a i l e d s t u d i e s o f t h e m e t a b o l i s m o f PAH and t h e i r d i h y d r o d i o l d e r i v a t i v e s , s t u d i e s o f t h e m u t a g e n i c i t y o f PAH and t h e i r d e r i v a t i v e s b o t h w i t h and w i t h o u t m e t a b o l i c a c t i v a t i o n , and s t u d i e s o f t h e t u m o r i g e n i c i t y o f PAH and t h e i r d e r i v a t i v e s . This m a n u s c r i p t w i l l f o c u s p r i m a r i l y upon t h e " u l t i m a t e " mutagens and c a r c i n o g e n s , t h e d i o l e p o x i d e s , as w e l l a s t h e r e l a t e d t e t r a h y d r o epoxides, w i t h a view to e x p l o r i n g the s t r u c t u r e - a c t i v i t y r e l a t i o n s h i p s t h a t g o v e r n t h e i r c h e m i c a l and b i o l o g i c a l b e h a v i o r , and t h e e x t e n t t o w h i c h quantum c h e m i c a l c a l c u l a t i o n s are u s e f u l i n e x p l a i n i n g those r e l a t i o n s h i p s .


E

Q C

T e t r a h y d r o e p o x i d e s as models. S i n c e the quantum c h e m i c a l c a l c u l a t i o n s a p p l y most r i g o r o u s l y t o t h e s i m p l e b e n z o - r i n g t e t r a h y d r o e p o x i d e s and s i n c e t h e c a l c u l a t i o n s n e g l e c t i n f l u e n c e s o f t h e h y d r o x y l groups i n t h e d i o l e p o x i d e s , i t i s i n s t r u c t i v e f i r s t t o examine the b e n z o - r i n g t e t r a h y d r o e p o x i d e s as s i m p l i f i e d models f o r the r e a c t i v e s i t e i n t h e d i o l e p o x i d e s . Most o f t h e i n f o r m a t i o n about t e t r a h y d r o e p o x i d e r e a c t i v i t y d e r i v e s f r o m s t u d i e s o f t h e k i n e t i c s o f t h e i r h y d r o l y s i s r e a c t i o n s , i n w h i c h c i s - and t r a n s d i o l s , as w e l l as t e t r a h y d r o k e t o n e s can be formed ( E q u a t i o n 5).

OH

OH

The r e a c t i o n f o l l o w s t h e r a t e law: k i ^R^"*" o» ^ — ^ v a l u e s o f k f o r e i g h t PAH t e t r a h y d r o e p o x i d e s a r e p l o t t e d i n F i g u r e 2 against the AE^eloc^ values at the b e n z y l i c p o s i t i o n bearing the epoxide. S i n c e cne e x p e r i m e n t a l c o n d i t i o n s used f o r t h e naph t h a l e n e (Np) and phenanthrene (Ph) s t u d i e s d i f f e r e d f r o m those f o r the s t u d i e s o f BA, t r i p h e n y l e n e (Tp), benzo[e]pyrene (BeP) and BaP, a d i r e c t c o m p a r i s o n o f a l l compounds i s d i f f i c u l t . Interestingly, f o r a s e r i e s o f t e t r a h y d r o e p o x i d e s whose h y d r a t i o n p r o d u c t s have been s t u d i e d , t h e percentage o f c i s - h y d r a t i o n d u r i n g a c i d i c hy d r o l y s i s i n c r e a s e s as t h e c a l c u l a t e d ease o f f o r m a t i o n o f t h e c a r b o c a t l o n i n c r e a s e s , a r e s u l t c o n s i s t e n t w i t h l o n g e r - l i v e d (more s t a b l e ) c a r b o c a t i o n s as one proceeds a l o n g t h e s e r i e s (16). =

+

k

a

0 D S

Polycyclic Hydrocarbons and Carcinogenesis - American Chemical

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