Carcinogenicity of Bisphenol A Revisited

James Huff

National Institute of Environmental Health Sciences P.O. Box 12233 Research Triangle Park, NC 27709 E-mail: huff1{at}niehs.nih.gov

To the Editor:

Munro et al.(2002)Go challenged my interpretation of the carcinogenesis bioassay results of bisphenol A, specifically opining that I had altered the National Toxicology Program (NTP) evaluation. In my opinion these six authors have not examined in sufficient detail the NTP Technical Report on the bioassay of bisphenol A (NTP, 1982Go), nor have they studied my remarks (Huff, 2001Go). In fact, I had gone to great lengths not to misrepresent the results or conclusions of the carcinogenicity of bisphenol A. In reality, in my letter (Huff, 2001Go) and in a more lengthy article (Huff 2002Go), I specifically used quotations liberally to adhere to the NTP results and conclusions, and not to mislead. Where I gave my experienced scientific opinion I was clear to indicate such. To clarify, and to respond to Munro et al.(2002)Go, the following are taken from my previous letter (Huff, 2001Go):

In 1980 at a public peer review meeting, the NTP concluded, "Under the conditions of this bioassay, there was no convincing evidence that bisphenol A was carcinogenic for F344 rats or B6C3F1 mice of either sex" (NTP, 1982Go). However, the NTP also emphasized in the abstract, discussion, and conclusion that "the marginally significant increase in leukemia in male rats, along with an increase (not statistically significant) in leukemias in female rats and a marginally significant increase in the combined incidence of lymphomas and leukemias in male mice, suggests that exposure to bisphenol A may be associated with increased cancers of the hematopoietic system.

To me this clearly indicates that the NTP was emphasizing that these hematopoietic tumors may be associated with bisphenol A exposures. I went on to state that "using the current NTP levels of evidence categories (Huff, 1992Go) these findings would clearly be considered as ‘equivocal evidence’ or possibly ‘some evidence’ of carcinogenicity." This I believe to be a true evaluation of the hematopoietic tumor data.

Thus, for Munro et al.(2002)Go to state that I deviated from the NTP bioassay report regarding these neoplastic lesions is incorrect. At that time the NTP was careful to use the phrase "no convincing evidence" in order to stress that these hematopoietic lesions may be associated with the administration of bisphenol A. One must note that the NTP did not state "not carcinogenic," which was the vocabulary used in those days for a negative study, and that would have been used if bisphenol A did not exhibit any carcinogenic activity. Thus, Munro et al.(2002)Go stress the NTP conclusion of "no convincing evidence," without, however, taking into account or acknowledging this and the other findings of note emphasized in the NTP Technical Report.

Furthermore, regarding these hematopoietic cancers, the independent Peer Review Panel instructed the NTP "to reflect the facts that leukemia in male rats showed a significant positive trend, that leukemia incidence in high-dose male rats was considered not significant only on the basis of the Bonferroni criteria [note: use of which was subsequently abandoned], that leukemia incidence was also elevated in female rats and male mice" (NTP, 1982Go, p. xi).

I also stated clearly in my letter:

The NTP further opined in their abstract and conclusion: "A statistically significant increase in interstitial-cell tumors of the testes in male rats was also suggestive of carcinogenesis" and continued with this caveat "but was not considered to be convincing evidence of a compound-related effect because this lesion normally occurs at a high incidence in aging F344 rats." However, like the hematopoietic effects, this substantial and significant increase in testicular tumors in both exposure groups of male rats should likewise be considered as evidence of carcinogenicity: 35/49 (71%) in controls versus 48/50 (98%) in low dose and 46/49 (94%) in top dose (Haseman and Elwell, 1996Go; Huff, 2002Go).

Obviously, the last sentence is my opinion, based again on the phrase that the NTP stated "but was not considered to be convincing evidence of a compound-related effect." One must note the use of "convincing evidence" to indicate again this was "not negative." The NTP states that the "significant increase in interstitial-cell tumors of the testes in male rats was also suggestive of carcinogenesis." What part of this does Munro et al.(2002)Go not understand? I believe that these testicular effects, if evaluated today, would be considered "equivocal evidence" or possibly "some evidence of carcinogenic activity," certainly more than a negative response. Statistically, the dose-related trend was highly significant (p = 0.001), as were the pairwise comparisons between the control and the two exposed groups (p = 0.001 and p = 0.003, respectively).

For further comparison, only three other chemicals have been deemed related to the carcinogenicity of the testes in Fischer rats in NTP studies (controls listed first followed by exposed groups): ethylbenzene TR 466 [adenoma: 36/50, (72%); 33/50, (66%); 40/50, (80%); 44/50, (88%)], isoprene TR 486 (interstitial cell adenoma, bilateral 20/50, 29/50, 37/50, 48/50; interstitial cell adenoma, including bilateral 33/50, 37/50, 44/50, 48/50), and tetrafluoroethylene TR 450 (interstitial cell adenoma: 39/50, 40/50, 48/50, 47/50). For the first two, the testicular tumor responses were included in the experimental evidence that was used by the NTP to identify these chemicals (isoprene and ethylbenzene) as carcinogens, and the incidence rates were quite similar to those of bisphenol A. Importantly, the control rates for these tumors in the Fischer rat studies were 72%, 66%, and 78% compared similarly to the bisphenol A control rate of 71%. Therefore, for Munro et al.(2002)Go to base their dismissal on a "nonsignificant" survival difference is not persuasive. Their statement that "the NTP was correct in concluding that this response was not treatment related" does not reflect what is stated in the NTP Technical Report as reiterated above. That is, the NTP concluded that the increase in testicular tumors "was also suggestive of carcinogenesis," also meaning the leukemia/lymphoma increases. Following the more recent evaluations of ethylbenzene and isoprene and testicular tumors, the NTP might do likewise with bisphenol A if it were being evaluated now.

Also in my letter I mentioned these findings:

Additional estrogenic disruptor effects from exposing rodents to bisphenol A in these bioassays include an increasing trend for tumors of the mammary glands in male rats (an unusual tumor for males), and decreased trends for tumors of the adrenal gland in male (medulla) and female (cortical) rats and for endometrial stromal polyps of the uterus in female rats.

The data for the mammary gland tumors in male rats were 0/50 controls, 0/50 in low dose group, and 4/50 (8%) in the top dose group. The dose-related trend statistic is significant at p = 0.015. Comparing the 0/50 controls versus the 4/50 in the top dose, the p-value is 0.059, but if one combines the two 0/50 groups then the p-value becomes significant at 0.011. Also, if one compares the historical control rates at the time the studies were done (see Haseman et al., 1990Go), there were 51/1936 control male rats with tumors of the mammary glands; comparing this 2.6% (1.3 animals per group of 50) versus the 8% seen with bisphenol A gives a p-value of 0.047, a marginally significant increase. Furthermore, the 8% incidence with bisphenol A is 3.1 times the average historical control rate, a rate that would be of concern if found in humans.

Importantly, male rats are considered particularly non-responsive to chemicals that cause mammary tumors (Dunnick et al., 1995Go; Wolff et al., 1996Go; Huff, 2000Go) Only seven chemicals out of 500 (1.4%) tested by the NTP have caused tumors of the mammary gland in male rats whereas 44 of these 500 (8.8%) have caused tumors of the mammary gland overall, mainly in female rats (37 chemicals) and less so in female mice (12 chemicals), with none in male mice (http://ntp-server.niehs.nih.gov/htdocs/Sites/MAMM.html; Huff et al., 1991Go; ). Thus, in my opinion these endocrine tumors should be considered as related to the administration of bisphenol A.

Another interesting finding in male rats is for total malignant tumors; 19 were found in control animals, 25 in the low dose group, and 33 in the high dose group. This is a reasonable and fine dose-response trend. These data are typically not used by the NTP as evidence of carcinogenicity but are used routinely, and in my opinion appropriately, by the Ramazzini Foundation (Soffritti et al.., 2002Go). Statistically, there is a positive dose response trend for animals with malignant tumors (p = 0.028: 18/50 versus 22/50 and 28/50) as well as for total malignant tumors per animal (p = 0.014: 19/50 versus 25/50 and 33/50).

I ended my letter (Huff, 2001Go) and longer article on bisphenol A (Huff, 2002Go) with:

Overall, it appears that BPA exposure via the diet for two years should be considered associated with tumors of the hematopoietic system in rats and mice, and of the testes and of the mammary glands in male rats. Marginal decreases in tumors of the adrenal gland and of the uterus in rats should also be considered chemically related.

In my opinion, the data (three tumor sites in male rats plus total tumors, with one each for female rats and male mice) support this carcinogenesis and do not stray substantially from that detailed and intended in the NTP Technical Report. I stand by the scientific and primary prevention interpretations offered previously and reiterated herein. The particular lessons here are that the use of the terms "no convincing evidence" does not mean and should not be construed as a "negative response," and one must evaluate the totality of the experimental findings (Huff, 1999Go; Tomatis et al., 2001Go), because there are several in this bioassay of bisphenol A that are biologically meaningful.

REFERENCES

Dunnick, J. K., Elwell, M. R., Huff, J., and Barrett, J. C. (1995). Chemically induced mammary gland cancer in the National Toxicology Program’s carcinogenesis bioassay. Carcinogenesis 16, 173–179.[Abstract]

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Haseman, J. K., Eustis, S. L., and Arnold, J. (1990). Tumor incidences in Fischer rats: NTP historical data. In Pathology of the Fischer RatReference and Atlas, (G. A. Boorman, S. L. Eustic, M. R. Elwell, C. A. Montgomery, and MacKenzie, Eds.), pp. 555–564. Academic Press, San Diego.

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Munro, I. C., Haighton, L. A., Hlywka, J. J., Lynch, B. S., Doull, J., and Kroes, R. (2002). Carcinogenicity bioassay of bisphenol A. Toxicol. Sci. 66, 356.[Free Full Text]

NTP. (1982). Carcinogenesis Bioassay of Bisphenol A (CAS No. 80–05–7) in F344 Rats and B6C3F1 Mice (Feed Study). Technical Report No. TR-215. National Toxicology Program, US Department of Health and Human Services, Public Health Service. National Institute of Health, Research Triangle Park, NC.

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Wolff, M. S., Collman, G. W., Barrett, J. C., and Huff, J. (1996). Breast cancer and environmental risk factors: Epidemiological and experimental findings. Annu. Rev. Pharmacol. Toxicol. 36, 573–596.[ISI][Medline]