Factors affecting carcinogenic potential of mixtures.

Historically, exposure to complex mixtures such as soot, coal tar, mineral oils, and cigarette smoke has been associated with increased cancer mortality. Benzo[a]pyrene (BaP) has been used to predict the carcinogenic potency of mixtures. Two complete carcinogenicity C3H/HEJ mouse skin bioassays were undertaken to determine the effect of low doses of BaP on the carcinogenic potential of mixtures. A toluene solution containing 0.1% each of five noncarcinogenic polycyclic aromatic hydrocarbons (PAHs), anthracene, chrysene, pyrene, fluoroanthene, and phenanthrene, produced tumors in 23% of the mice with a latent period of 73 weeks. With the addition of a 0.001% BaP to the above solution, 47% of the mice produced tumors with a latent period of 66 weeks. In the second study, coal tar in toluene, which was determined to contain 0.0006% BaP, produced tumors in 51% of mice with a latent period of 73 weeks. In both studies the BaP solutions by themselves did not produce tumors. In a third study, the 9-, 2-, and 3-methylbenz[a]-anthracene compounds were noncarcinogenic using toluene as the solvent. With the substitution of n-dodecane for toluene all three compounds produced significant numbers of tumors. The results indicate that (1) low dose levels of BaP can have an impact on the carcinogenic potential of mixtures, (2) the presence or absence of BaP is not always sufficient to account for the observed potency and the synergistic effects of other substances which might be present, and (3) that certain noncarcinogenic methylbenz[a]anthracenes can have their carcinogenic potential altered by a change in the solvent used.