Abstract

Toremifene (TOR) is a selective estrogen receptor modulator used in adjuvant therapy for breast cancer and in clinical trials for prostate cancer prevention. The chemical structure of TOR differs from that of tamoxifen (TAM) by the presence of a chlorine atom in the ethyl side chain, resulting in a more favorable toxicity spectrum with TOR. In addition, some patients who fail on TAM therapy benefit from high-dose TOR therapy. Several studies have indicated that functional genetic variants in the TAM metabolic pathway influence response to therapy, but pharmacogenomic studies of patients treated with TOR are lacking. In this study, we examined individual variability in sulfation of 4-hydroxy TOR (4-OH TOR) (the active metabolite of TOR) in human liver cytosols from 104 subjects and found approximately 30-fold variation in activity. 4-OH TOR sulfation was significantly correlated (r = 0.98, P < 0.0001) with β-naphthol sulfation (diagnostic for SULT1A1) but not with 17β estradiol sulfation, a diagnostic substrate for SULT1E1(r = 0.09, P = 0.34). Examination of recombinant sulfotransferases (SULTs) revealed that SULT1A1 and SULT1E1 catalyzed 4-OH TOR sulfation, with apparent K_m values of 2.6 and 6.4 μM and V_max values of 8.5 and 5.5 nmol · min⁻¹ · mg protein⁻¹, respectively. 4-OH TOR sulfation was inhibited by 2,6-dichloro-4-nitrophenol (IC₅₀ = 2.34 ± 0.19 μM), a specific inhibitor of SULT1A1. There was also a significant association between SULT1A1 genotypes and copy number and 4-OH TOR sulfation in human liver cytosols. These results indicate that variability in sulfation could contribute to response to TOR in the treatment of breast and prostate cancer.