Correlation of the carcinogenic potential of di(2-ethylhexyl)phthalate (DEHP) with induced hyperplasia rather than with genotoxic activity.

It has been reported that in a long-term feeding study 12,000 ppm of di(2-ethylhexyl)phthalate (DEHP) in the diet produced hepatocellular carcinomas in male and female F-344 rats while 6000 ppm DEHP produced the same tumor type in male and female B6C3F1 mice. In terms of the actual numbers of animals with tumors DEHP produced a greater response in mice than rats. DEHP and its principal hydrolysis product, mono(2-ethylhexyl)phtalate (MEHP) produce multiple effects in the animal such as liver peroxisomal proliferation and hyperplasia. Accordingly, genotoxicity as DNA repair or unscheduled DNA synthesis (UDS) and cell replication as the percentage of cells undergoing scheduled DNA synthesis (SDS or S phase) were determined in mouse hepatocytes in vitro and in vivo in response to DEHP and MEHP. UDS and SDS were determined by autoradiographic quantitation of [3H]-thymidine incorporation in primary hepatocyte cultures treated directly or isolated from B6C3F1 male mice treated in vivo. No DNA repair was observed in mouse hepatocyte cultures treated with up to 1.0 mM DEHP or 0.5 mM MEHP. No DNA repair was observed in cultures from mice treated with up to 500 mg/kg DEHP 12, 24 or 48 h previously or from animals treated up to 28 days with 6000 ppm DEHP in the diet. At 24 h following treatment with 500 mg/kg DEHP, 3.1% of the hepatocytes were in S phase compared to control values of 0.2%. Administration of DEHP in the diet at 6000 ppm produced 9.2% of the cells in S phase at day 7 with the value returning to control levels by day 14. On day 28 of the feeding study the liver to body weight ratios had almost doubled in the group treated with DEHP compared to controls. No increase in the liver-specific enzyme alanine aminotransferase was seen in the serum following treatment with 500 mg/kg DEHP, indicating that the hyperplasia was due to mitogenic stimulation rather than regenerative hyperplasia in response to cytotoxicity. Increases in the endpoints relating to hyperplasia in response to DEHP were greater in the mouse than those that have been reported in the rat. Thus, the carcinogenic response of DEHP correlates better with induced hyperplasia rather than with genotoxicity.