In the current 2-year benzophenone studies, the target organs of toxicity were liver, kidney, nose, and testes. Neoplastic responses occurred in the kidney, liver, and hematopoietic system.
Rats exposed to benzophenone exhibited a positive trend in the incidences of renal tubule adenoma. The NTP has shown that examination of the entire kidney, by step sectioning of residual tissues, enables a more precise evaluation of the potential chemical-related induction of renal proliferative lesions than observations made from single sections, particularly when the proliferative lesions are small and identified only by microscopic examination (Eustis et al., 1994
). For benzophenone, this extended evaluation of the male rat kidney showed significant increases in the incidences of renal tubule adenomas and hyperplasia in exposed groups. Incidences of renal tubule hyperplasia in all exposed female groups were significantly greater than that of the control group when the single and step section evaluations were combined. Renal tubule hyperplasia, adenoma, and carcinoma are considered to comprise a biological and morphological continuum in the progression of proliferative changes of the renal tubule epithelium. Both renal tubule hyperplasia and adenomas are characterized by increased numbers of relatively normal appearing tubular epithelial cells. Adenomas are large discrete lesions that are greater than five tubule diameters to 1 mm or more in size. Adenomas tend to have complex cellular patterns. Carcinomas are usually larger than adenomas with irregular borders, a prominent blood supply, and cellular anaplasia.
The pathogenesis of chemically induced renal tubule neoplasms has not been determined; however it appears to be complex with genotoxic and nongenotoxic modes (Barrett and Huff, 1991
, Hard, 1998
, Short, 1993
). Data from retrospective reviews of NTP 2-year carcinogenesis studies suggest that an increased severity of nephropathy may contribute to overall tumor response (Seely et al., 2002
). However, any contribution appears to be marginal, and additional factors are likely involved.
In female rats, the incidence of mononuclear cell leukemia was significantly increased in the 625 ppm group. Male rats exposed to 312 or 625 ppm benzophenone also exhibited significantly increased incidences of mononuclear cell leukemia. There was not a statistically significant increase in 1,250 ppm males; however, this is a late developing neoplasm and only two of the 50 animals in this group survived to the end of the study. It is possible that the early deaths of these animals precluded the ability to determine if benzophenone exposure increased mononuclear cell leukemia in male rats in a dose-dependent manner. Mononuclear cell leukemia is a common neoplasm in F344/N rats and has occurred at incidences of 30% to 68% in male controls from 2-year NTP feed studies and 12% to 38% in control females. Because of this variability, staging of the neoplasms is often done as a way of further evaluating a chemical-related increase in incidence. In this case, staging of the lesions provided no support to indicate that benzophenone was enhancing the progression of these lesions; therefore, the increased incidences were considered equivocal evidence of carcinogenicity.
Benzophenone exposure resulted in a positive trend in the incidence of histiocytic sarcoma in female mice, and one 625 ppm and two 1,250 ppm female rats had histiocytic sarcomas. This neoplasm is extremely rare; none have been observed in historical feed study control rats and only two have been observed in feed study control mice given the NTP-2000 diet. Histiocytic sarcomas are classified as hematopoietic tumors of the mononuclear phagocyte system based upon the morphology of the neoplastic cells and the presence of lysozyme, Mac-2, and mononuclear phagocyte antigens. They are slightly more common in female than male mice and in mice than rats (Frith et al., 1993
). The histiocytic sarcomas observed in mice exposed to benzophenone involved the liver and lung. In the 1,250 ppm female mice, the histiocytic sarcomas were highly invasive. Multiple organs throughout the body had neoplastic histiocytic lesions. All affected rats exposed to benzophenone had lung lesions. Only one rat in the 625 ppm group had organs affected throughout the body.
Chemical-associated increases in the incidences of histiocytic sarcomas are rare in NTP studies; however increased incidences have been observed in the studies with 1,3-butadiene (NTP, 1993
), tetrafluoroethylene (NTP, 1996a
), and phenolphthalein (NTP, 1996b
). The rarity of histiocytic sarcomas, combined with the fact that mice displayed a positive trend in the incidence of this lesion suggest that the occurrences in female rats may be related to benzophenone exposure, but the low number of neoplasms observed makes this an uncertain finding.
Male and female mice in all exposed groups had increased incidences of spleen hematopoietic cell proliferation. The incidence in the control female group in this study is consistent with previous NTP studies (Ward et al., 1999
). Increased hematopoietic cell proliferation has been associated with anemia and chronic inflammatory lesions. Evidence of an anemia with minimal severity was observed in rats and mice during the 14-week studies at higher doses than were used in the 2-year study (NTP, 2000
Increases in the incidences of hepatocellular adenoma were observed in male and female mice. Hepatoblastomas were also observed in exposed males; however the increased incidence was not statistically significant. Hepatocellular adenomas, hepatocellular carcinomas, and hepatoblastomas represent a biological and morphological continuum in progression of proliferative lesions. Because the malignant potential of hepatoblastomas and hepatocellular carcinomas appears similar and hepatoblastomas are often observed within hepatocellular neoplasms (mostly carcinomas), it is appropriate to combine the incidences of hepatoblastoma with those of adenoma and carcinoma when interpreting the carcinogenic potential of a chemical. The combined incidence of hepatocellular adenoma, hepatocellular carcinoma, and hepatoblastoma was significantly increased in 1,250 ppm males, and the incidences showed a positive trend.
In the current study, the incidences of metaplasia of the olfactory epithelium were significantly increased in the high dose male and female mice. This was a species-specific effect, as rats did not display similar lesions, possibly because of differences in the anatomy of the rat nasal cavity and potential lower relative exposures to benzophenone in rats. The mechanism by which benzophenone caused this lesion is unknown; however, enzymatic metabolism of benzophenone in the olfactory epithelium, which has a high activity of cytochrome P450, may be involved. Some compounds, such as phosphodiesterase inhibitors, that require metabolic activation by the cytochrome P450 enzyme system have been shown to cause olfactory epithelial injury, chronic hyperplastic/regenerative lesions, and olfactory neoplasms following oral or inhalation exposure in rodents (Pino et al., 1999
Decreases in the incidences and multiplicities of mammary gland fibroadenoma were observed in female rats exposed to benzophenone. Fibroadenomas are the most common spontaneous neoplasm of the mammary gland in female rats, occurring in 213/460 (46%, range 28% to 55%) NTP feed study control animals. The incidence of mammary gland tumors in NTP studies has been found to be positively associated with body weight. However, the decreased incidence of mammary gland tumors in this study could not be attributed to decreased body weights of exposed females, as the 1,250 ppm females had significantly lower incidences of this neoplasm after correcting for decreased body weight (Haseman et al., 1997
). Interestingly, benzophenone-based derivatives have shown impressive inhibitory activity of steroid sulfatase, an enzyme that regulates the formation of estrone and subsequent conversion to estradiol, and may be developed for therapeutic use in the treatment of hormone-dependent breast cancer (Hejaz et al., 2004
In conclusion, there was some evidence of carcinogenic activity of benzophenone in male F344/N rats based on increased incidences of renal tubule adenoma. There was equivocal evidence of carcinogenic activity of benzophenone in female F344/N rats based on the marginal increased incidences of mononuclear cell leukemia and histiocytic sarcoma. There was some evidence of carcinogenic activity of benzophenone in male B6C3F1 mice based on increased incidences of hepatocellular neoplasms, primarily adenoma. There was some evidence of carcinogenic activity of benzophenone in female B6C3F1 mice based on increased incidences of histiocytic sarcoma; the incidences of hepatocellular adenoma in female B6C3F1 mice may have been related to benzophenone exposure.