Tumor promotion as a target for estrogen/antiestrogen effects in rat hepatocarcinogenesis

Tamoxifen is a well-tolerated palliative and adjuvant treatment for human breast cancer and requires continuous, long-term administration for optimal therapeutic effectiveness. A two-stage model of experimental hepatocarcinogenesis, based upon the natural history of cancer development, has been employed to assess the carcinogenic potential of tamoxifen. In this study, the effectiveness of tamoxifen both as an initiator and a promoter in hepatocarcinogenesis was assessed in female F-344 rats. Tamoxifen was tested as an initiator at a single intragastric dose of 40 mg/kg, followed by promotion with 0.05% phenobarbital. The number and size of the resulting altered hepatic lesions were quantified, and tamoxifen was found to lack initiating action at the dose tested. Other groups of animals were initiated with a nonnecrogenic, subcarcinogenic dose of diethylnitrosamine (10 mg/kg) and were fed tamoxifen at either 250 or 500 mg/kg in the AIN-76A purified diet for 6 months. The livers of these animals showed an increase in the size and number of altered hepatic lesions compared with those animals that were initiated but not exposed to tamoxifen; this indicates that tamoxifen acts as a tumor promoter in the rat liver. The promotion index of tamoxifen, a measure of relative potency, was less than one-tenth that of ethinyl estradiol and more than four times that of phenobarbital, an agent commonly employed as a representative promoting agent in experimental carcinogenesis. Since tamoxifen lacked initiating activity in the rat liver at the dose tested, the mechanism of tumor induction in long-term feeding studies by tamoxifen may be due to its promotion of spontaneously initiated hepatocytes. The chronic therapeutic use of tamoxifen should therefore be limited by the potential carcinogenic risk of this agent as an effective tumor promoter.