Cumene Cumene Can Be Generated by Several Metabolic Pathways
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Report on Carcinogens, Fourteenth Edition For Table of Contents, see home page: http://ntp.niehs.nih.gov/go/roc Cumene cumene can be generated by several metabolic pathways. Cumene is extensively metabolized by cytochrome P450 in liver and other tis- CAS No. 98-82-8 sues, including the lung, but the specific isoforms have not been iden- tified. Thirteen cumene metabolites were identified in rat urine and Reasonably anticipated to be a human carcinogen fifteen metabolites in mouse urine. The most abundant metabolite First listed in the Thirteenth Report on Carcinogens (2014) in rat and mouse urine and rat bile was 2-phenyl-2-propanol gluc- Also known as isopropylbenzene uronide, and a conjugate or conjugates of 2-phenyl-2-propanol were found in the urine of humans exposed to cumene vapor (Seńczuk H3C H CH3 and Litewka 1976). C Cumene metabolism proceeds primarily through side-chain oxida- tion, but ring oxidation also occurs in vivo. Metabolism of cumene re- sulting in side-chain oxidation of α-methylstyrene to α-methylstyrene oxide or ring oxidation to arene oxides could potentially cause DNA Carcinogenicity damage. Arene oxides can form phenols, and when the para isomers of methylphenol, ethylphenol, and isopropylphenol were incubated Cumene is reasonably anticipated to be a human carcinogen based on with rat-liver microsomes, reactive quinone methide intermediates sufficient evidence of carcinogenicity from studies in experimental were readily formed (Thompson et al. 1995). animals. Cumene caused tumors at several tissue sites, including lung Studies on Mechanisms of Carcinogenesis and liver in mice and kidney in male rats. Several proposed mecha- nisms of carcinogenesis support the relevance to humans of lung and The mechanisms by which cumene causes cancer in experimental liver tumors in experimental animals. Specifically, there is evidence animals are not known. Differences between species in the types of that humans and experimental animals metabolize cumene through tumors observed may be due, in part, to species differences in me- similar metabolic pathways. There is also evidence that cumene is tabolism and disposition. Several potential modes of action or molec- genotoxic in some tissues, based on findings of DNA damage in ro- ular alterations associated with carcinogenesis have been identified, dent lung and liver. Furthermore, mutations of the K-ras oncogene including genetic and epigenetic effects, metabolic activation to re- and p53 tumor-suppressor gene observed in cumene-induced lung active metabolites, cell proliferation, and α2u-globulin nephropathy. tumors in mice, along with altered expression of many other genes, Chemical agents that cause cancer at several tissue sites in more resemble molecular alterations found in human lung and other can- than one species frequently are genotoxic carcinogens. Although cers. The relevance of the kidney tumors to cancer in humans is uncer- cumene was not mutagenic or genotoxic in most of the standard tain; there is evidence that a species-specific mechanism not relevant in vitro and in vivo assays, single-cell gel electrophoresis (the comet to humans contributes to their induction, but it is possible that other assay) provided evidence that cumene caused DNA damage in the mechanisms relevant to humans, such as genotoxicity, may also con- liver of male rats and the lungs of female mice (NTP 2012). Although tribute to kidney-tumor formation in male rats. α-methylstyrene was not mutagenic in bacteria (NTP 2007), there is evidence that it causes chromosomal damage in rodents and cul- Cancer Studies in Experimental Animals tured cells, and its proposed metabolite, α-methylstyrene oxide, is Inhalation exposure to cumene caused tumors at several different tis- mutagenic in bacteria. Therefore, some evidence exists for a geno- sue sites in mice (NTP 2009). In mice of both sexes, cumene caused toxic mechanism of action for cumene (presumably via its conver- benign and malignant lung tumors (alveolar/bronchiolar adenoma, sion to α-methylstyrene or to other metabolites). The extent to which carcinoma, and adenoma and carcinoma combined). In female mice, genotoxicity plays a role in causing tumors at various tissue sites is cumene caused dose-related increases in the incidence of benign or unknown. malignant liver tumors (hepatocellular adenoma alone or combined Mouse Lung Tumors with carcinoma). These findings are supported by observations of dose-related in- The occurrence of alveolar/bronchiolar neoplasms in mice but not creases in the incidence of benign nasal tumors (adenoma of the in rats may be partly explained by differences in disposition and me- respiratory epithelium) in rats of both sexes (NTP 2009). However, tabolism. Following administration of 14C-labelled cumene, 14C con- this type of nasal tumor typically does not progress to malignancy centrations in lung tissue were highest in female mice after seven (Brown 1991). Inhalation exposure to cumene also increased the com- consecutive daily doses, but did not increase with repeated dosing in bined incidence of benign and malignant kidney tumors (renal-tu- rats. In vitro studies with mouse and rat lung and liver microsomes bule adenoma and carcinoma) in male rats. However, the relevance demonstrated that mouse lung microsomes were the most efficient of these kidney tumors to cancer in humans is uncertain (see Stud- at metabolizing cumene, which is consistent with the accumulation ies on Mechanisms of Carcinogenesis). Additional tumors that may of cumene metabolites in mouse lung (Chen et al. 2011). have been related to cumene exposure include malignant blood-vessel Based on a comparison with other compounds that also in- tumors (hemangiosarcoma, primarily of the spleen) and benign thy- duced lung tumors in mice but not in rats, including ethylbenzene roid-gland tumors (follicular-cell adenoma) in male mice and benign and styrene, some investigators (Cruzan et al. 2009, 2012) proposed tumors of the testes (interstitial-cell adenoma) in male rats. that species-specific metabolism by the cytochrome P450 isoform CYP2F2 in the Clara cells of mouse lung results in the production of Absorption and Metabolism cytotoxic metabolites that cause tumors. They hypothesized that this Cumene is readily absorbed following inhalation or oral exposure, mechanism was not relevant to human lung cancer, because humans and studies in rats and rabbits showed that cumene is also absorbed generate smaller amounts of these cytotoxic metabolites. However, through the skin (IPCS 1999, EC 2001, Chen et al. 2011). Metabo- very few data are available to indicate which P450 isoforms are re- lism of cumene is complex and not fully elucidated; however, there sponsible for metabolizing cumene. CYP2E1 and CYP2F2 are likely are clear similarities across species, and reactive intermediates of candidates, based on similarities between cumene and other alkyl- National Toxicology Program, Department of Health and Human Services Report on Carcinogens, Fourteenth Edition benzenes, but metabolism of cumene by CYP2F2 in mouse lung has tumor formation. Although it is likely that genotoxicity plays a role not been demonstrated to date. The orthologous isozyme CYP2F1 in cumene-induced carcinogenicity at some tissue sites, the stron- is found in human lung. Furthermore, in a two-year carcinogenic- gest evidence for genotoxicity was found for lung and liver tumors, ity study of cumene (NTP 2009), bronchiolar hyperplasia and alveo- and the extent to which genotoxicity contributes to the formation of lar epithelial bronchiolar metaplasia were significantly increased in kidney tumors is unknown. Thus, the relevance of the kidney tumors mice of both sexes, but there was no evidence of cytotoxicity (e.g., in male rats to human cancer is uncertain, and the renal-tumor find- necrosis or inflammation) in the lung in the two-year study or in a ings are considered to support, rather than contribute directly to, the three-month study. sufficiency of the evidence for the carcinogenicity of cumene from Cumene-induced mouse lung tumors have more K-ras and/or studies in experimental animals. p53 mutations than do spontaneous lung tumors, and the muta- Cancer Studies in Humans tional spectra of K-ras and p53 in lung tumors from mice exposed to cumene differ from those observed in spontaneous lung tumors. No epidemiological studies or case reports were identified that eval- These findings suggest the involvement of DNA damage (either di- uated the relationship between human cancer and exposure specif- rect damage from adduct formation or indirect damage through re- ically to cumene. active oxygen species) and genomic instability (Hong et al. 2008). The Properties K-ras and p53 mutations observed in cumene-induced lung tumors were accompanied by increased expression of genes involved in al- Cumene is an alkylated benzene that exists at room temperature teration of the mitogen-activated kinase signaling pathway, invasion as a volatile, colorless liquid with a sharp, penetrating aromatic or and metastasis, inhibition of apoptosis, increased angiogenesis, and gasoline-like odor (NTP 2009). It is a flammable liquid that is stable increased metastatic potential (Wakamatsu et al. 2008). The molec- under normal conditions but may become unstable at high temper- ular alterations in mouse lung tumors resemble molecular alterations atures and pressures. It forms cumene hydroperoxide when exposed found in human lung cancers (Hoenerhoff et al. 2009). Therefore, to air for long periods and is