Synthesis and Reactivity of Potential Toxic Metabolites of Tamoxifen Analogues:  Droloxifene and Toremifene o-Quinones

Abstract

Tamoxifen remains the endocrine therapy of choice in the treatment of all stages of hormone-dependent breast cancer. However, tamoxifen has been shown to increase the risk of endometrial cancer which has stimulated research for new effective antiestrogens, such as droloxifene and toremifene. In this study, the potential for these compounds to cause cytotoxic effects was investigated. One potential cytotoxic mechanism could involve metabolism of droloxifene and toremifene to catechols, followed by oxidation to reactive o-quinones. Another cytotoxic pathway could involve the oxidation of 4-hydroxytoremifene to an electrophilic quinone methide. Comparison of the amounts of GSH conjugates formed from 4-hydroxytamoxifen, droloxifene, and 4-hydroxytoremifene suggested that 4-hydroxytoremifene is more effective at formation of a quinone methide. However, all three substrates formed similar amounts of o-quinones. Both the tamoxifen-o-quinone and toremifene-o-quinone reacted with deoxynucleosides to give corresponding adducts. However, the toremifene-o-quinone was shown to be considerably more reactive than the tamoxifen-o-quinone in terms of both kinetic data as well as the yield and type of deoxynucleoside adducts formed. Since thymidine formed the most abundant adducts with the toremifene-o-quinone, sufficient material was obtained for characterization by (1)H NMR, COSY-NMR, DEPT-NMR, and tandem mass spectrometry. Cytotoxicity studies with tamoxifen, droloxifene, 4-hydroxytamoxifen, 4-hydroxytoremifene, and their catechol metabolites were carried out in the human breast cancer cell lines S30 and MDA-MB-231. All of the metabolites tested showed cytotoxic effects that were similar to the parent antiestrogens which suggests that o-quinone formation from tamoxifen, droloxifene, and 4-hydroxytoremifene is unlikely to contribute to their cytotoxicity. However, the fact that the o-quinones formed adducts with deoxynucleosides in vitro implies that the o-quinone pathway might contribute to the genotoxicity of the antiestrogens in vivo.

Quinone Methides and their Prodrugs: A Subtle Equilibrium Between Cancer Promotion, Prevention, and Cure

“…The same metabolic profile has been observed with toremifene (39, see Fig. 4) and droloxifene (40), for which adducts between their respective QMs with GSH and deoxynucleosides have been detected in vitro [22].…”

“…However, it has limited bioavailability and poor pharmacokinetics. A lipophilic derivative of ECG, TMECG (22), has been designed to improve its pharmacological characteristics [29]. This transformation led to a stronger antitumor compound but only on melanoma cells.…”

A ferrocenyl derivative of hydroxytamoxifen elicits an estrogen receptor-independent mechanism of action in breast cancer cell lines

“…However this non-genomic effect (i.e. an effect not mediated by a direct interaction with DNA) is observed only at high concentrations [10]. To date, the molecular targets and mechanism of action of OH-TAM in breast cancer cells are not fully understood.…”

Bioactivation of the Cancer Chemopreventive Agent Tamoxifen to Quinone Methides by Cytochrome P4502B6 and Identification of the Modified Residue on the Apoprotein

“…This is not surprising, because Fan et al, 2000 have shown that 1,8-Michael or 1,6-Michael addition of GSH to the quinone methide is a reversible process and that the adduct has a short half-life. In addition, 4-hydroxytamoxifen has been shown to form adducts with deoxynucleosides (Yao et al, 2001), and Notley et al (2002) have shown that recombinant P450s 2B6 and 2C19 catalyzed drug-protein adduct formation through 4OHtam and not catechol formation, whereas P450s 2D6 and 3A5 generated catechol metabolite formation and not drug-protein adduct formation. We were able to successfully identify and characterize a P450 adduct using recombinant purified CYP2B6.…”