α-Hydroxylation of Tamoxifen and Toremifene by Human and Rat Cytochrome P450 3A Subfamily Enzymes

Abstract

An increased risk of developing endometrial cancer is observed in breast cancer patients treated with tamoxifen (TAM) and in healthy women undergoing TAM chemoprevention therapy. TAM-DNA adducts were detected in the endometrium of women taking TAM (Shibutani, S., et al. (2000) Carcinogenesis 21, 1461-1467) and are formed primarily through O-sulfonation of alpha-hydroxytamoxifen (alpha-OHTAM). To explore the genotoxicic mechanisms of TAM, TAM was incubated with one of multiple human cytochrome P450 enzymes, i.e., P450 1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4, 3A5, 3A7, 4A11, 4F2, 4F3A, or 4F3B, in a NADPH regenerating system, and the metabolites were identified using HPLC/UV analysis with authentic standards. Among the 18 human P450 enzymes, P450 3A4 generated a significant amount of alpha-OHTAM. When some rat P450 enzymes were examined, P450 3A2 also catalyzed alpha-hydroxylation of TAM. Similarly, human P450 3A4 and rat P450 3A1 and 3A2 converted toremifene (TOR, a chlorinated TAM analogue) to alpha-hydroxytoremifene (alpha-OHTOR). The formation of alpha-OHTAM and alpha-OHTOR by these P450 enzymes was confirmed by tandem mass spectroscopy. Only the P450 3A subfamily enzymes are able to alpha-hydroxylate TAM and TOR. Although the formation of alpha-OHTOR by these enzymes was much higher than that of alpha-OHTAM, TOR is known to be much less genotoxic than TAM. The results support our proposed mechanism that the lower genotoxicity of TOR is due to limited O-sulfonation of alpha-OHTOR by hydroxysteroid sulfotransferases, resulting in the poor formation of DNA adducts (Shibutani, S., et al. (2001) Cancer Res. 61, 3925-3931).

Mass spectrometric characterization of urinary toremifene metabolites for doping control analyses

“…Metabolic pathways studied were based on those metabolites reported in previous studies for toremifene [3,13,14,20,21,24,26,[28][29][30] and other related compounds, such as tamoxifen and clomifene [3,4,17,[31][32][33][34][35][36][37][38][39][40][41]. All previously reported metabolic pathways and their combination were included in SRM method, with the main exception of those proposed after metabolic removal of the amine function because of no ionization in positive ion mode was expected for these compounds [13][14][15][18][19][20][21][22][23][24][25]27,42] ( Table 1).…”

“…In the case of toremifene, pharmacokinetic and pharmacodynamic studies with detection of toremifene and its metabolites in plasma and faeces, have been reported [13][14][15][16][17][18][19][20][21][22][23][24][25]. Few analytical methods for the urinary detection of toremifene administration have been developed [3,14,[26][27][28][29].…”

“…Few analytical methods for the urinary detection of toremifene administration have been developed [3,14,[26][27][28][29]. Different metabolic pathways have been described as the most characteristic for toremifene and related compounds such as tamoxifen and clomifene including, hydroxylation, N-desmethylation, N,Ndidesmethylation, deaminohydroxylation, deaminocarboxylation, N-oxide formation, among others [3,4,17,[30][31][32][33][34][35][36][37][38][39][40][41][42]. However, no systematic study on toremifene metabolites appearing in urine after drug administration has been performed.…”

Sulfation of 4-Hydroxy Toremifene: Individual Variability, Isoform Specificity, and Contribution to Toremifene Pharmacogenomics

“…Subsequently, UDP-glucuronosyltransferase and sulfotransferases (SULTs) participate in 4-OH TOR metabolism (Crewe et al, 2002;Kim et al, 2003). SULTs, along with UDP-glucuronosyltransferases, are Phase II detoxification enzymes whose physiological function is to increase the water solubility of various substrates, thus facilitating their excretion (Jakoby and Ziegler, 1990).…”

Applications of accelerator mass spectrometry for pharmacological and toxicological research

“…With the closely related structural analog toremifene, CYP3A4 catalyzed the formation of a-hydroxytoremifene at a much higher rate than the equivalent reaction for tamoxifen (Kim et al, 2003). It was proposed that the lower genotoxicity and formation of DNA adducts following treatment of rats with toremifene (Hard et al, 1993) was because of the limited Osulfonation of a-hydroxytoremifene by hydroxysteroid sulfotransferases.…”