Interactions of Endoxifen and other major metabolites of Tamoxifen with human sulfotransferases SULT2A1, SULT1E1, and SULT1A1*1: implications for the therapeutic action and toxicity of Tamoxifen

Although tamoxifen has been successfully utilized in the treatment and prevention of estrogen-dependent breast cancer for decades, its use is limited by its low incidence of endometrial cancer. The carcinogenic effects of tamoxifen are complex and may involve a combination of estrogen receptor-mediated hormonal effects as well as the metabolic activation of tamoxifen to reactive electrophiles that are genotoxic. Moreover, a significant population of patients develop clinical resistance to tamoxifen, which leads to breast cancer recurrence and a decrease in patient survival. Therefore, the goal of the current study was to examine the interactions of major metabolites of tamoxifen with the human cytosolic sulfotransferases hSULT2A1, hSULT1E1, and hSULT1A1*1.

Changes in the catalytic activity of hSULT2A1 by tamoxifen metabolites may inhibit the formation of the genotoxic Α-sulfooxy tamoxifen intermediate catalyzed by this enzyme. Moreover, tamoxifen metabolites might interfere in the inactivation of hydroxysteroids catalyzed by hSULT2A1 as a part of the variable responses to tamoxifen therapy. Endoxifen was the most potent inhibitor of the hSULT2A1, which suggests that this metabolite may inhibit the role of hSULT2A1 in the metabolic pathway for genotoxicity that is seen with tamoxifen.N-desmethyltamoxifen (N-desTAM) was a substrate for the hSULT2A1, and the product of this reaction, N-desmethyltamoxifen sulfamate (N-desTAM-S), displayed greater inhibition of the enzyme than its unconjugated precursor. Thus, endoxifen, N-desTAM, and N-desTAM-S might serve protective roles in some tissues as they may inhibit the role of hSULT2A1 in the genotoxicity of tamoxifen.

Metabolites of tamoxifen were then examined as inhibitors of hSULT1E1 and hSULT1A1*1 due to the roles of these enzymes in the inactivation of estrogens. Each of the metabolites studied were weak inhibitors of hSULT1E1; thus, endoxifen is not likely to promote increased estrogen signaling in breast tissue when administered as an independent breast cancer therapeutic agent in ongoing clinical trials. However, 4-hydroxytamoxifen (4-OHTAM) was a very potent inhibitor of hSULT1A1*1 when examined with estradiol as substrate. This suggests the potential for 4-OHTAM to interfere in estrogen metabolism in tissues where hSULT1A1*1 is expressed and hSULT1E1 is not. This information will be useful when interpreting the clinical trials of endoxifen and will aid in the design of related molecules