In a study sponsored by the National Toxicology Program, di(2-ethylhexyl)phthalate (DEHP) fed in the diet at 1.2% significantly increased the incidence of female rats with hepatocellular carcinomas. Extensive evaluation of DEHP for carcinogenicity has yielded negative results. The present investigations were designed to elucidate the mechanism of DEHP hepatocarcinogenesis under the conditions of the original bioassay.
Short-term studies designed to evaluate the promoting capability of DEHP, when administered after initiation, were negative when livers of female Fischer-344 rats were evaluated using multiple histochemical stains to identify foci of cellular alteration. Two different protocols were used to evaluate the initiating potential of DEHP in the liver using histochemically defined foci as the endpoint. In both experiments the results were negative. Chronic exposure to DEHP at 1.2% in the diet for 2 years resulted in elevation of hepatic peroxisomal enzymes while DNA replication, an indication of cell proliferation, was not affected in hepatocytes. The number of foci was not elevated in the DEHP group compared to the controls, even though a low incidence of rats with liver tumors occurred in the treated group.
The results of this series, as well as other published results, suggest that DEHP and other peroxisomal proliferating chemicals have unique effects on the development of hepatic neoplasms. The absence of altered foci after chronic administration or in initiation-promotion protocols distinguishes DEHP and perhaps other peroxisomal proliferating chemicals from both classic liver carcinogens and promoters.
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