Correlation Between Carcinogenicity and Chemical Structure in Cyclopenta[A]Phenanthrenes

[CANCER RESEARCH 33, 832-837, April 1973]

Correlation between Carcinogenicity and Chemical Structure in Cyclopenta[a]phenanthrenes

M. M. Coombs. T. S. Bhatt, and C. J. Croft Chemistry Department, Imperial Cancer Research Fund, Lincoln 's Inn Fields, London, WC2A 3PX, England Â¡M.M. C..T.S.B.], and the National Institute for Medical Research, Mill Hill, London, NW7IAA, England Â¡C.J. C./

SUMMARY steroids, but they also retain oxygen functions at positions commonly oxygenated in this class of natural products. This The relationship between structure and carcinogenicity in fact has led to speculation that abnormal steroid metabolism fifteen new cyclopenta[a]phenanthrene derivatives, all closely by mammalian cells might produce endogenous carcinogens. related to the potent carcinogen [15,16-dihydro-l 1-methyl- Recently, Bischoff (2) reviewed work on carcinogenic hydro cyclopenta[a]phenanthren-17-one (compound VI)], was carbons in connection with the carcinogenic effects of tested by mouse skin painting. Each mouse received 30 /zg of steroids. the compound on the dorsal skin twice weekly for 1 year, and In a recent paper, Hill et al. (17) proposed a connection the experiment was concluded at the end of the 2nd year. between the higher incidence of carcinoma of the colon in Nine compounds were active carcinogens. The chrysene Western Europe and North America compared with that in analog (1,2,3,4-tetrahydro-l 1-methylchrysen-l-one) displayed East Africa, Asia, and South America, and the higher levels of the same high potency as did the ketone (VI), while the dietary steroids consumed in the former areas. Furthermore, parent, unsubstituted chrysene was inactive. The 11-methyl- they demonstrated a higher anaerobe-to-aerobe ratio in the 17-ol derived from compound VI and the 1l-methoxy-7- bacterial flora of feces from people living in these areas and methyl-17-ketone also possessed marked activity. The 7- concluded that these differences in intestinal flora were methyl-17-ketone was much less active than VI while the 2-, brought about by differences in diet. They suggested that the 3-, 4-, and 6-methyl isomers and the 11,12-dihydro derivative etiology of cancer of the colon could be understood if certain Were inactive. The 11-ethyl homolog was also much less active gut bacteria were able to generate carcinogens from steroid than VI and the 11-Â«-butyl homolog was almost without precursors. activity. The positional isomer 15,16-dihydro-l 1-methyl- There has been little work on the modification of steroids cyclopenta [a] phenanthren-15-one was only weakly active, but by anaerobic bacteria. However, Hill et al. recently showed moderate carcinogenicity was associated with the 6-methoxy that Clostridium spp. are able to effect nuclear dehydrogena- and 16-hydroxy dÃ©rivÃ¢tesofVI. tion and C-10 demethylation of androgens to yield 1,3,5(10)- The results are discussed in connection with the possible oestratrienes (15) and -tetrenes (private communication). mode of action of these compounds and in relation to Similar reactions are brought about by a number of other hypothetical routes of steroid degradation, especially by microorganisms not known to inhabit the gut, notably Bacillus anaerobic gut bacteria. cyclo-oxidans (19), Pseudomonas testosteronii (23), and Norcardia restrictus (22), while aromatization of ring B of 7-dehydrocholesterol was observed with the protozoon Acanthamoeba castallanii (18). A remarkably varied and INTRODUCTION increasingly large number of chemical transformation of steroids are known to be carried out by microorganisms (4, Strong carcinogenic activity has been observed in certain 24), including hydroxylation of the 18-methyl group by a derivatives of cyclopenta[a]phenanthrene' which bear oxygen Aspergillus niger strain (1). This is of importance, since it at C-ll and C-17 (8). These compounds not only have the could lead to loss of the angular methyl group under mild same carbon ring system as have the naturally occurring conditions, thus possibly allowing the aromatization of ring C with the formation of compounds of the cyclopenta[a]phen- 'The term cyclopenta[a]phenanthrene (RRI 4781) (21) is properly anthrene series (8). reserved for the fully unsaturated hydrocarbon of which XXI is the 15//-isomer; XX is therefore the 16,17-dihydro derivative. Except in These new developments indicate the need to delineate the formal chemical names, the term is used in this paper loosely to imply structural limits associated with carcinogenicity in compounds both types of structure. The ring positions are numbered according to which could arise as steroid transformation products. In this the steroid convention, as indicated. study, we investigated the connection between carcinogenicity n and chemical structure in a number of new cyclopenta[a] phenanthrene derivatives related to the potent carcinogen 15,16-dihydro-l l-methyl-cyclopenta[a] phenanthren - 17 - one (VI). In particular, the consequences of positional isomerism XXI of the methyl group and ketone function, of partial reduction Received November 9, 1972; accepted January 18, 1973. of the conjugated system, of the introduction of additional

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1973 American Association for Cancer Research. Carcinogenic Cyclopenta[ a] phenanthrencs oxygen functions, and of varying the substituent at C-ll have received attention. The carcinogenicity of the related chry- senes, XV and XVI, has also been studied. The structures of these compounds are illustrated in Chart 1.

MATERIALS AND METHODS

Chemistry. With the exception of VII and VIII, the synthesis of Compounds I to XVI is described elsewhere (5-7, 9, 10). UV spectra (log e in parentheses) were recorded for ethanol solutions on a Perkin-Elmer Model 402 spectrophotometer and for infrared spectra as Nujol mulls on a Perkin-Elmer Model 257 spectrophotometer. TLC2 was carried out on glass plates (20 x 10 cm) coated with Merck Silica Gel G to a thickness of 0.25 mm, and these were dried for a minimum of 18 hr at room temperature before use. Preparation of 15,16-dihydro-l l-ethylcyclopenta[a jphenan- thren-17-one (VII). A solution of 17,17-ethylenedioxy-ll, 12,13,14,15,16-hexahydro-ll-oxocyclopenta[a]phenanthrene (4.25 g) (5) in dry benzene (30 ml) was added drop- wise to a solution of ethyl magnesium bromide [pre pared from magnesium turnings (0.72 g), ethyl bromide VII vni IX (3.27 g), and anhydrous ether (50 ml)], and the mixture was heated under reflux for 6 hr. The cooled reaction mixture was shaken with ice-cold M H2S04, the organic layer was washed with aqueous NaHCOs and with water, and the mixture was dried over anhydrous Na2 SCU. Evaporation gave an amber gum that possessed only weak infrared carbonyl absorption at 6 urn. This gum was dissolved in a mixture of nitrobenzene (15 ml) and glacial acetic acid (60 ml) and was heated under reflux for 45 min with concentrated HC1 (15 ml). After dilution with water, the nitrobenzene was removed in steam. Extraction of the tarry residue with chloroform gave a brown gum that was purified by chromatography on a column (40 x 2 cm in diameter) of WoÃ«lmGrade III alumina, eluting with benzene. Fractions homogeneous by TLC (CH2C12) were combined to yield a pale fawn solid (2.9 g) which recrystallized from ethanol as large, pale brown platelets ( 1.7 g) of the 11-ethyl-17-ketone (VII), m.p., 129-130Â°.

C19H160 Required: C 87.65, H 6.2 Found: C 87.4, H 5.95

\â€žax, 264.5 (4.91), 289 (4.47), 302 (4.33), 360 (3.51), and 378 (3.54) nm; i^max, 5.88 (aryl ketone), 12.38, 13.36, 13.90, XV XVI and 14.54 /Â¿m. Chart 1. Compounds tested by skin-painting experiments. Preparation of 15,16-Dihydro-ll-n-butylcyclopenta[a]- phenanthren-17-one (Vili). A sample of the above oxoketal (1.48 g) was added to a 0.54 M solution (10.0 ml) of Â«-butyl already described. The dark brown gum obtained after removal lithium in ether, contained in a flask fitted with a water-cooled of the nitrobenzene in steam was purified by chromatography, as outlined above, to give fawn crystals [from ethanol (150 condenser under a dry nitrogen atmosphere. The solvent mg)] of the 11-H-buty 1-17-ketone (VIII), m.p., 113-114Â°. boiled and the solid dissolved, yielding a yellow solution. After 18 hr at ambient temperature, this solution was treated first C2 iHjoO with M H2SÃ›4, then with a boiling mixture of nitrobenzene (6 ml), acetic acid (24 ml), and concentrated HC1 (6 ml), as Required: C 87.45, H 7.0 Found: C 87.35, H 6.6

1The abbreviation used is: TLC, thin-layer chromatogiaphy. Xinax, 264.5 (4.90), 289.5 (4.45), 302 (4.32), 360 (3.41), and

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1973 American Association for Cancer Research. M. M. Coombs, T. S. Bhatt, and C. J. Croft 20 dehydrogenation of the 17-ol (XII) to the ketone (VI) was demonstrated as follows. Compound XII (50 jug), dissolved in CH pure dimethyl sulfoxide (20 jul), was added to a suspension of rat liver microsomes [prepared from 400 mg of fresh liver by -15 the method of Gelboin (14)] in phosphate-buffered saline (3.0 00 ml) that contained NADPH (20 mg). After incubation at 37Â° for 30 min with shaking in air, the mixture was extracted with ethyl acetate and the extract was examined by TLC in the -10 solvent systems, tolueneiethyl acetate:methanol (15:5:1, by volume) and benzene:methanol:glacial acetic acid (11:2:1, by volume). In addition to the starting material and several slower-running metabolites, the 2 systems showed spots at RF's 0.80 and 0.73, respectively, fluorescing bright blue in UV -5 (and orange, after the plates had been sprayed with ethanolic H2SO4) characteristic of the ketone (VI). Elution of these spots with ethanol gave solutions with UV Xmax, 264, 288, 301, 358, and 376 nm and fluorescent excitation and emission 1ÃŽT 20 ^0 60 70 80 90 100 maxima at 270 and 411 nm, respectively, which are values Chart 2. Carcinogenicity of isomers and homologs of ketone (VI). identical with those of an ethanolic solution of the ketone Experimental details are given in "Materials and Methods." The 2-, 3-, (VI). 4-, 6-, and 12-methyl isomers are all inactive, as are the 11,12-dihydro derivative (IX) and unmethylated parents of ketones Compounds VI, X, and XVI. The 11-n-butyl ketone (VIII) produced a skin tumor at the RESULTS site of application in 1 animal at 42 weeks. The results of our skin-painting experiments are summarized 377 (3.46) nm; vmax, 5.90 (aryl ketone), 11.44,12.72, 13.40, in Charts 2 and 3 and in Table 1. and 14.08/im. In our previous study (8), the l l-methyl-17-ketone (VI) and Skin-painting Experiments. A closed colony of randomly the corresponding l l-methoxy-17-ketone (XIX) both proved bred albino (T.O.) mice maintained in these laboratories was to be strong carcinogens, whereas the 12-methyl-, 3-methoxy-, used. The animals were divided by a formal randomization and unsubstituted-17-ketones were without activity. The procedure into groups of 10 mice of either sex for each results of testing the remaining monomethyl isomers of compound tested. They were fed a diet of GR 3 EK [E. Dixon Compound VI (with the exception of the 1-methyl isomer & Sons, Ware, Herts., England] and water ad libitum, and were which has not yet been synthesized) are shown in Chart 2. All 3 months old at the start of the experiment. The dorsal hair were inactive except the 7-methyl-17-ketone (V), which was removed from each animal by electric clippers at regular displayed moderate carcinogenicity [tumor incidence (number intervals during the course of the experiment. Toluene was of animals with skin tumors/number of animals alive at the used as solvent for the chemicals, since no skin tumors have been observed among a large number of control mice painted with this alone, and the survival of these animals is the same as 20 that of untreated controls. One drop (6 n\) of a 0.5% w/v solution of the chemical was applied twice a week for 50 weeks to the clipped dorsal skin of each mouse in the -IS appropriate group, and the animals were observed for 1 more year. The date of appearance and the size of all skin tumors were recorded both when the animals were painted and once a week during the 2nd year. When the tumors had grown to about 1 cm in diameter, the animals were killed and autopsies were performed. Tumors were examined histologically, except in 16 cases in which suitable material was not available. i 15 Tumors that involved the panniculus carnosus muscle were Ã³ classified as carcinomas, and those in which this muscle was -5 intact were classified as papillomas. In addition to the groups already mentioned, a group of 20 mice painted in the same way with methylcholanthrene acted as a positive control, while 2 similar groups ( 1 painted with toluene alone and the 10 20 30 Weeks 60 70 80 90 100 other clipped but otherwise untreated) acted as negative control groups. These procedures and conditions are substan Chart 3. Carcinogenicity of oxygenated derivatives and analogs of tially the same as those used previously (8). ketone (VI). Experimental details are given in "Materials and Methods." In Vitro Oxidation of the Alcohol (XII). Microsomal MC, methylcholanthrene control.

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Table 1 Number of mice surviving without tumors and histology of induced tumors at site of application in the skin-painting experiment

No. of tumorless survivors at No. of mice with

Compound tested1IIIIIIVVVIVIIVIIIIXXXIXIIXIIIXIVXVXVIMethylcholanthreneToluenemo.1419182020919191920192(1191920124202012mo.1616IS181421418171913617II1910181818mo.101614131001014141184321500161524mo.5864207493531090(i78Squamouspapillema0000003003122000000Squamouscarcinoma00004Â°14Â°30Â°014Â°103a8"016Â°1800

controlClipping control6 " Tumors were unavailable for histology for these compounds as follows: V, 2; VI, 4; VIII, 1; XI 1; XIII, 6; and XVI, 2. " An additional 2 animals had tumors (spindle cell sarcomas) at the site of application. appearance of the 1st tumor, 6 of 20); average latent period, unexpected, for this compound lacks conjugation of the 42 weeks]. In this study, the 11-methyl-17-ketone (VI) had an phenanthrene system at C-17. However, oxidation of this average latent period of 31 weeks compared with 21 weeks for alcohol to the ketone (VI) by dehydrogenases in the skin is the methylcholanthrene control, and both had a 100% tumor probable and is readily demonstrated by incubation of the incidence (18 of 18, in both cases). Thus VI is by far the most 17-ol with rat liver microsomes in the presence of NADPH in active of the isomerie methyl ketones. Elongation of the side vitro. The fairly high carcinogenicity of the 7-methyl-ll- chain at C-ll to ethyl in the 11-ethyl-17-ketone (VII) methoxy-17-ketone (XIV) (10 of 20, 41 weeks) was to be considerably reduced carcinogenicity (6 of 19, 55 weeks), expected in view of the activity of both the 7-methyI- and while the 11-Â«-butyl homolog (VIII) produced only 1 skin 11-methoxy-17-ketones. tumor (1 of 19) at 42 weeks. Transposition of the conjugated carbonyl group from C-17 in VI to C-15 in the ll-methyl-15-ketone (X) greatly reduces activity; therefore this ketone (4 of 17, 62 weeks) is similar in DISCUSSION carcinogenic potential to the corresponding 11-methyl hydro carbon, which lacks the extended conjugation of the phen- In agreement with our earlier observations (8), these results anthrene ring system (8). Retention of the carbonyl group at show clearly that the presence of small electron-donating C-17 but enlargement of Ring D from 5 to 6 carbon atoms methyl or methoxy groups at C-ll promotes carcinogenicity in cyclopenta[a]phenanthrenes. Although Diel's hydrocarbon gives a chrysene (XVI) with the same high carcinogenicity (18 of 19, 30 weeks) as the corresponding cyclopenta[a]phen- (XVII) was known to be inactive as a carcinogen (16), weak anthrene (VI). As in the latter series, the unsubstituted activity was demonstrated (3) for the 11-methyl isomer, and chrysene ketone (XV) is devoid of activity. Interruption of the this activity was confirmed by us (8) for the 11,17-dimethyl hydrocarbon. Weak activity was also associated (12) with the conjugation in the phenanthrene system of the carcinogen A16 derivative of Diel's hydrocarbon (XVIIIa), while the (VI), giving the 11,12-dihydro derivative (IX), abolishes activity. combination of these 2 tumorigenic features in 11,17- Introduction into the carcinogen (VI) of a 6-methoxy group dimethyl-15//-cyclopenta [a] phenanthrene (XVlIIb) produced (XIII) (11 of 20, 56 weeks) or a 16-hydroxy group (XI) (6 of a hydrocarbon with much higher activity (8). Unexpectedly, 19, 42 weeks) reduces activity, but both of these derivatives high carcinogenic potency was also established (8) for the are still moderately tumorigenic (Chart 3). The activity of l l-methyl-17-ketone (VI), which retains oxygen at C-17, a Compound XI is of interest, because a metabolite with the ring position that is oxygenated in virtually all C18 and CI9 same Chromatographie properties has been detected in the steroids, and for the 11-methoxy-17-ketone (XIX), which urine of rats given Compound VI (11). The higher activity of bears oxygen at 2 "steroid" positions. The importance of a the ll-methyl-17-ol (XII) (12 of 20, 38 weeks) was C-l 1 methyl or methoxy group and the C-17 carbonyl group

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1973 American Association for Cancer Research. M. M. Coombs, T. S. Bhatt, and C. J. Croft in inducing carcinogenicity in cyclopenta [a] phenanthrenes is an enzyme engaged in metabolism of the compounds to the further exemplified by the results of this study. proximate carcinogens and thereby facilitates this metabolism. The lack of activity in the 11,12-dihydro derivative (IX) CH CH, could be a result of the altered distribution of electron density engendered by saturation of the C-l 1,12 double bond in the carcinogen (VI). Apart from this 1 instance, it is not known whether a completely conjugated phenanthrene nucleus is required for tumorigenicity. Additional work on the relation ship between carcinogenicity and structure in partially saturated cyclopenta [a] phenanthrenes is in progress, for it is XVII XVIII XIX considered that abnormal steroid metabolism might lead a,R=H preferentially to this type of compound. Also, 3 of the 9 b.R=CH, active compounds each bear 1 oxygen atom, in addition to the oxygen atom at C-17. In the hydrocarbon series, methyl substitution at C-7 also While no member of the cyclopenta [a] phenanthrene series (3) promoted weak carcinogenicity, whereas methyl substitu has yet been found to occur naturally, it is an empirical fact tion at other ring positions was without effect. The same that a number of compounds of this series, having the same pattern has now emerged in the 17-ketone series, with the carbon ring system as steroids, are potent carcinogens. difference that C-ll substitution is very effective in this Moreover, plausible biochemical pathways can now be regard, while C-7 substitution is less so. The complete lack of envisaged which should be capable of leading to their activity (8) in the closely similar C-6 and C-12 methyl isomers formation in vivo. It would now be logical to search for is of particular interest. Replacement of the 11-methyl group abnormal steroid metabolites in association with forms of in VI by larger alkyl residues considerably reduces activity; in cancer wherein a link with steroids is suspected on epidemio this behavior, these ketones resemble the classical polycyclic lÃ³gica!data. As mentioned above, cancer of the colon appears hydrocarbons. to be a case in point, yet very little is known of the chemical If metabolism of these compounds is required to generate transformations undergone by steroids in the gut. Concur proximate carcinogens, the importance of the 7- and 11- rently, further work on the relationship between carcino methyl groups in inducing activity might arise in a number of genicity and chemical structure in compounds of this class ways. Compared with the biologically inactive analogs, the would lead to a more precise understanding of the types to be substituent might alter the direction taken by metabolism to covered by the search. yield a metabolite with the correct stereochemistry and reactivity to allow it to bind to a cellular receptor specific for the induction of the carcinogenic process. It might alter the half-life of such an intermediate, thus making it capable of ACKNOWLEDGMENTS diffusing to, and reacting with, the cellular target (13). We wish to thank Mr. J. Daniels, Miss R. C. Cole, and Miss P. Adams Without influencing the type of metabolite formed, the for assistance in the care of the animals used in these experiments and substituent might alter the properties of a reactive inter Mr. F. P. Wharton and Miss W. A. Dick for making histological mediate, common to this class of compounds, in one or more preparations. of these ways. Alterations of these types might result from the influence of the substituent on the electron density at a site elsewhere in the molecule, or as a consequence of stereo- chemical factors, either in the original ketone or in an REFERENCES intermediary metabolite. Steric interaction of the C-ll 1. Auret, B. J., and Holland, H. L. Microbiological 18-Hydroxylation methyl group with the C-l proton and of the 7-methyl group of Steroids. Chem. Commun., 1157,1971. with the C-l 5 protons is clearly demonstra table by nuclear 2. Bischoff, F. Carcinogenic Effects of Steroids. Advan. Lipid Res., 7: magnetic resonance spectroscopy in these 17-ketones (9) but 165-244 (see especially 216-218), 1969. not in the biologically inactive isomers with the methyl group 3. Butenandt, A., and Dannenberg, H. Untersuchungen Ã¼berdie elsewhere in the molecule. Krebserzeugende Wirksamkeit der Methylhomologen des 1,2- The C-17 carbonyl group or the 17-methyl-A16 system also Cyc/opentenophenanthrenes. Arch. Geschwulstforsch., 6: 1â€”¿7, promote carcinogenicity in this series (8), but this cannot be 1953. solely the result of extension of the phenanthrene conjugation. 4. Capek, A., Hanc, O., and Tadra, M. In: Z. Prochazka and J. Dyr In comparison with the 1l-methyl-17-ketone (VI), which is (eds.), Microbial Transformation of Steroids. Prague: Academia, conjugated at C-17, the 11-methyl-15-ketone (X), conjugated Czechoslovak Academy of Sciences, 1966. at C-l 5, has much lower activity (similar to that of the 5. Coombs, M. M. Potentially Carcinogenic Cyc/openta(a)phenan threnes. Part I. A New Synthesis of 15,16-Dihydro-17-oxo- 11-methyl hydrocarbon, which lacks carbonyl conjugation). In cyc/openta [a] phenanthrene and the Phenanthrene Analogue of the 1,2,3,4-tetrahydrochrysene series also, a carbonyl group at 18-Noroestrone Methyl Ether. J. Chem. Soc., 955-962, 1966. C-l (equivalent to C-17 in a cyclopenta [a] phenanthrene) 6. Coombs, M. M. 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7. Coombs, M. M. Potentially Carcinogenic Cyc/openta[a]phen- 16. Hartwell, J. L. Survey of Compounds which have been Tested for anthrenes. Part III. Oxidation Studies. J. Chem. Soc., 2484-2488, Carcinogenic Activity. U.S. Public Health Serv. Pubi., 149: 184, 1969. 1951. 8. Coombs, M. M., and Croft, J. C. Carcinogenic Cyc7openta[a] phen- 17. Hill, M. J., Crowther, J. S., Drasar, B. S., Hawksworth, G., Aries, anthrenes. Progr. Exptl. Tumor Res., //: 69-85, 1969. V., and Williams, R. E. O. Bacteria and Aetiology of Cancer of the 9. Coombs, M. M., and Jaitly, S. B. Potentially Carcinogenic Large Bowel. Lancet, 95-100,1971. Cyc/openta[a]phenanthrenes. Part V. Synthesis of 15,16-Dihydro- 18. Korn, E. D., Ulsamer, A. G., Weihing, R. R., Wetzel, M. G., and 7-methylcyc/openta[a]phenanthrene-17-one. J. Chem. Soc., Wright, P. O. The Enzymatic Aromatisation of the ÃŸRing of 230-234, 1971. A5 .'-sterols. Biochim. Biophys. Acta, 187: 555-563, 1969. 10. Coombs, M. M., Jaitly, S. B., and Crawley, F. E. H. Potentially 19. Kupchan, S. M., Sih, C. J., Katsui, N., and El Tayeb, O. Carcinogenic Cyc/openta[a]phenanthrenes. Part IV. Synthesis of Microbiological Transformations. II. The Aromatisation of Ring A 17-Ketones by the Stobbe Condensation. J. Chem. Soc., 1266- ofStrophanthidin. J. Am. Chem. Soc., 84: 1752-1753, 1962. 1271,1970. 20. Pataki, J., and Huggins, C. In: E. D. Bergmann and B. Pullman 11. Crawley, F. E. H. Ph.D. Thesis, University of London, London, (eds.), The Jerusalem Symposia on Quantum Chemistry and 1972. Biochemistry. Physicochemical Mechanisms of Carcinogenesis, Vol. 12. Dannenberg, H. Ãœber Beziehungen Zwichen Steroiden und 1, pp. 64-71. Jerusalem: Israel Academy of Sciences and Krebserzeugenden Kohlenwasser-stoffen. Z. Krebsforsch., 63: Humanities, 1969. 523-531, 1960. 21. Patterson, A. M., Capell, L. T., and Walker, D. F. In: The Ring 13. Dipple, A., Lawley, P. D., and Brooks, P. Theory of Tumour Index, Ed. 2, p. 643. Washington: American Chemical Society, Initiation by Chemical Carcinogens. Dependence of Activity on 1960. Structure of Ultimate Carcinogen. European J. Cancer, 4: 22. Sih, C. J., Wang, K. C., and Tai, H. H. Mechanisms of Steroid 493-506,1968. Oxidation by Microorganisms. Biochemistry, 7: 796-818, 1968. 14. Gelboin, H. V. A Microsome-dependent Binding of Benzo[ajpy- 23. Talalay, P. Enzymatic Mechanisms in Steroid Biochemistry. Ann. rene to DNA. Cancer Res., 29: 1272-1276, 1969. Rev. Biochem., 34: 347-380, 1965. 15. Goddard, P., and Hill, M. J. Aromatisation of Androst-4-ene-3,17- 24. Tamm, C. Conversion of Natural Substances by Microbial dione by Human Intestinal Bacteria. Biochem. J., Â¡24:73P, 1971. Enzymes. Angew. Chem. Intern. Ed. Engl., 1: 178-195, 1962.

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1973 American Association for Cancer Research. Correlation between Carcinogenicity and Chemical Structure in Cyclopenta[ a]phenanthrenes

M. M. Coombs, T. S. Bhatt and C. J. Croft

Cancer Res 1973;33:832-837.

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