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Carcinogenicity Indices in Polycyclic Hydrocarbons [CANCER RESEARCH 39, 4760-4761, November 1979] 0008-5472/79/0039-OOOOS02.00 Letter to the Editor Carcinogenicity Indices in Polycyclic Hydrocarbons Correspondence re: Iden A. Smith, Gregory D. Berger, Paul G. Seybold, and M. P. Servé.Relationships between Carcinogenicity and Theoretical Reactivity Indices in Polycyclic Aromatic Hydrocarbons. Cancer Res., 38: 2968-2977, 1978. The above paper describes an attempt to correlate the charge is more evenly distributed around the rest of the mole carcinogenicity of a hydrocarbon with each of several param cule. eters concerned with its reactivity; these parameters were The relative ease of carbonium ion formation within a series derived by molecular orbital calculations on the structure of the of related compounds depends mainly on the energy needed hydrocarbon and its derivatives. Of the 21 parameters consid in solvating the ion. This can be estimated from the sum of the ered, 7 showed some correlation with carcinogenic activity. squares of the charges at each carbon atom around the mole The authors found that the most useful of these was O6, which cule (2). Hence, the ease of carbonium ion formation is best was derived as follows. The hydrocarbon was assumed to be estimated not by the charge at one particular position (Qb) but converted by metabolism to a diol-epoxide which gave rise to rather by the evenness of distribution around the molecule as a carbonium ion such as those illustrated below; this species a whole, since an even distribution will minimize the sum of the was presumed to react with cellular macromolecules causing squared charges. cancer. The carcinogenic potency of the hydrocarbon should A parameter which expresses the evenness of distribution of therefore be related to the stability of the carbonium ion and charge is 2 a0 , the sum of the nonbonding molecular orbital hence to the extent to which its positive charge can be delo- coefficients. This quantity is as easy to calculate as is O0; the calized around the rest of the molecule. There should therefore coefficients are calculated for alternate positions on the mole be a negative correlation between the carcinogenicity of the cule as described by Dewar (3), and 2 |a0 is the sum of the hydrocarbon and Qb, the charge that remains at the "bay- values of these coefficients (without regard to sign) around the region" position, as calculated by molecular orbital theory. A molecule. Intuitively, it represents the volume of space occu similar argument was presented by Jerina ef al. (6) and earlier pied by the orbital. It varies from 1.0 for a localized positive by Dipple ef al. (4) for the aryl alkyl esters; the parameters charge (as would be obtained from an aliphatic alkylating used by these authors are closely related (Jerina's &Edetoc/ß agent) to a little over 3 for ions derived from the highly delo = 2(1 - v/Q¡);Dipple'saor =VO¡). calized tribenzo(a)pyrenes. Smith et al. reported a fair correlation between the carcino In Chart 2, the carcinogenic potencies of 39 hydrocarbons genicity of the hydrocarbon and Qb of the most likely diol- are plotted against 2 |a0| for the most reactive diol-epoxides epoxide carbonium ion for the 25 hydrocarbons examined in they would be expected to form. This chart includes hydrocar the paper, as is evident from their Table 9, p. 2973. However, bons up to C24Hi4 having a benzo ring capable of forming a diol-epoxide, for which a value for Iball's index could be found the use of Q6 as a criterion of carcinogenic activity would appear to give rise to a number of "false positives": (a) linearly in the literature or estimated. It excludes those with aliphatic annelated hydrocarbons such as hexacene and benzo(a)- side chains and the fluoranthenes with orbital parameters that naphthacene yield low values of Q6 but are not carcinogenic. could not be calculated reliably by a simple method. It would Smith ef al. follow earlier authors in attributing this to high "L- appear that there is a good correlation and that the anomalies region" reactivity; (b) benz(a)anthracene, although at most weakly carcinogenic, has a O0 almost as low as those of the powerfully carcinogenic dibenzopyrenes and lower than that of the carcinogen dibenz(a,/i)anthracene; (c) hydrocarbons with 5-membered rings such as the fluorenes and fluoranthenes yield very low values of Ob, although most of these compounds are noncarcinogenic or weakly so. These anomalies appear to derive from the same source, namely, that in the presumed diol-epoxide carbonium ions from these hydrocarbons, the positive charge resides largely at 2 or 3 positions instead of being delocalized all around the aromatic nucleus. Compare, for instance, the theoretical charge distri bution for possible carbonium ions derived from the noncarcin- ogen benzo(a)tetracene and the powerful carcinogen di- benzo(a,rt)pyrene, as calculated by Longuet-Higgin's method (3) and shown in Chart 1. The charge at the "bay" position, •04-15-10 Ob, is 0.333 in either case, but in dibenzopyrene the rest of the Chart 1. Theoretical charge distribution for the carbonium ions derived from hypothetical diol-epoxides: (top) the 3,4-dihydrodiol 1.2-oxide of benzo(a)- Received March 12. 1979; accepted August 9, 1979. tetracene; (bottom) the 1,2-dihydrodiol 3,4-oxide of dibenzo(a,h)pyrene. 4760 CANCER RESEARCH VOL. 39 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1979 American Association for Cancer Research. Letter to the Editor -80 a comparison is that it can lead to a rule for predicting whether or not a hydrocarbon is appreciably carcinogenic, according -60 to the value of the chosen parameter; in this case, it would appear that the criterion 2 a0\ > 2.6 distinguishes carcinogens from noncarcinogens with few exceptions. 20 REFERENCES 1. Arcos, J. C.. and Argus, M. F. Molecular geometry and carcinogenic activity 3 of aromatic compounds. New perspectives. Adv. Cancer Res.. It: 305- K C Idol 471, 1968. Chart 2. Relationship between carcinogenic potency, as measured by Iball's 2. Bethell. D., and Gold. V. Carbonium Ions. Chap. 5. pp. 117-159. New York: index and the parameter £|a0| calculated as described in the text. The values of Academic Press. Inc.. 1967. Iball's index are taken from Arcos and Argus (1) or estimated from data in Ref. 3. Dewar, M. J. S. The Molecular Orbital Theory of Organic Chemistry, pp. 5; values of £|a„|areas follows. A. naphthalene (1.890); B, fluorene (2.111); C. 214-217. New York: McGraw-Hill Book Co.. 1969. anthracene (2.183); D. phenanthrene (2.236); E, benzo(a)fluorene and 4. Dipple. A.. Lawley, P. D., and Brookes. P. Theory of tumour initiation by benzo(o)fluorene (2.353); F, benzo(c)phenanthrene (2.380); G, naphthacene chemical carcinogens: dependence of activity on structure of ultimate car (2.401) and triphenylene (2.405); H, benzo(c)fluorene (2.414); /, chrysene cinogen. Eur. J. Cancer, 4: 493-506. 1968. (2.449); J, benzo(e)pyrene (2.466); K, benz(a)anthracene (2.469); /., 5. Hartwell, J. J. Survey of compounds which have been tested for carcinogenic dibenzo(a,/)fluorene (2.492), carcinogenic potency uncertain; M, dibenzo- activity. PHS Publication No. 149. Washington, D. C.: Government Printing (c.h)phenanthrene (2.513) and naphtho(1,2-a)pyrene (2.534); N, benzo(c>- Office, 1951. chrysene (2.535); O, naphtho(2.3-D)fluorene (2.550). dibenzo(c,g)phenan- 6. Jerina, D. M., Lehr, R., Schaefer-Ridder. M., Yagi. H., Karle, J M., Thakker, threne (2.554). naphtho(2,3-e)pyrene (2.555), dibenzo(e./)pyrene (2.556), di- D. R., Wood, A. W., Lu, A. Y. H., Ryan, D.. Levin, W., and Conney, A. H. benzo(c,g)fluorene (2.557), and naphtho(2,1-D)fluorene (2.560); P. Bay-region epoxides of dihydrodiols: a concept explaining the mutagenic dibenz(a,c)anthracene and dibenz(a,/)anthracene (2.556); Q. dibenzo(a,g)flu- and carcinogenic activity of benzo(a)pyrene and benzo(a)anthracene. In: H. orene (2.557), carcinogenic potency uncertain; R. pentacene (2.582). picene H. Hiatt, J. D. Watson, and J. A. Winsten (eds.). Origins of Human Cancer, (2.592), pentaphene (2.600), benzo(b)chrysene (2.610), and benzo(a)tetracene pp. 639-658. Cold Spring Harbor, N. Y.: Cold Spring Harbor Laboratory. (2.656); S, dibenz(a,h)anthracene (2.615); T. benzo(g)chrysene (2.722); U. 1977. benzo(a)pyrene(2.714); V, naphtho(2.3-a)pyrene(2.837); W. dibenzo(a,/)pyrene (2.872), for carcinogenicity. see the report of Arcos and Argus (1), p. 448; X, Martin R. Osborne* dibenzo(a,e)pyrene (2.930); /, dibenzo(a,n)pyrene (3.015); Z, dibenzo(a,/)py- rene (3.024). Chemical Carcinogenesis Division Institute of Cancer Research Pollards Wood Research Station referred to above have been largely removed. In particular, the Nightingales Lane low carcinogenicity of benz(a)anthracene would now be in ChalfontSt. Giles, Buckinghamshire HP84SP, United Kingdom keeping with the value of 2 a0| calculated for its diol-epoxide. The considerable scatter of points to the right of Chart 2 is not ' Supported by NIH (USA) Contract N01-CP-33367 and by grants to the surprising; apart from the inaccuracy of the theory, the measure Institute of Cancer Research from the Medical Research Council and the Cancer of carcinogenic potency is far from precise. The value of such Research Campaign. NOVEMBER 1979 4761 Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1979 American Association for Cancer Research. Carcinogenicity Indices in Polycyclic Hydrocarbons: Correspondence re: Iden A.
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