In Vitro Bioactivation of Dihydrobenzoxathiin Selective Estrogen Receptor Modulators by Cytochrome P450 3A4 in Human Liver Microsomes: Formation of Reactive Iminium and Quinone Type Metabolites

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• 作者：Zhoupeng Zhang ; Qing Chen ; Ying Li ; George A. Doss ; Brian J. Dean ; Jason S. Ngui ; Maria Silva Elipe ; Seongkon Kim ; Jane Y. Wu ; Frank DiNinno ; Milton L. Hammond ; Ralph A. Stearns ; David C. Evans ; Thoma
• 刊名：Chemical Research in Toxicology
• 出版年：2005
• 出版时间：April 2005
• 年：2005
• 卷：18
• 期：4
• 页码：675 - 685
• 全文大小：252K
• 年卷期：v.18,no.4(April 2005)
• ISSN：1520-5010

文摘

Estrogens and selective estrogen receptor modulators (SERMs) are prescribed widely in theclinic to alleviate symptoms in postmenopausal women, and they are metabolized to reactiveintermediates, which may elicit adverse effects. As part of our efforts to develop safer SERMs,in vitro covalent protein binding of (2S,3R)-(+)-3-(4-hydroxyphenyl)-2-[4-(2-piperidin-1-ylethoxy)phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol (I) was evaluated. Radioactivity from [³H]I becamecovalently bound to proteins in a fashion that was both time- and NADPH-dependent in humanliver microsomes and reached a value of 1106 pmol equiv/mg protein following a 45 minincubation. At least three pathways are involved in the bioactivation of I, namely, oxidativecleavage of the dihydrobenzoxathiin moiety to give a hydroquinone/para-benzoquinone redoxcouple, hydroxylation at position 5 or 7 of the benzoxathiin moiety leading to an o-quinoneintermediate, and metabolism of the piperidine ring to give an iminium ion. The latter reactiveintermediate was identified as its bis-cyano adduct when human liver microsomal incubationswere performed in the presence of sodium cyanide. Structural modification of I, including areplacement of the piperidine with a pyrrolidine group, led to (2S,3R)-(+)-3-(3-hydroxyphenyl)-2-[4-(2-pyrrolidin-1-ylethoxy)phenyl]-2,3-dihydro-1,4-benzoxathiin-6-ol (II), which did not forma reactive iminium ion. Following the incubation of II with human liver microsomes, covalentbinding to proteins was reduced (461 pmol equiv/mg protein), the residual level of bindingapparently due to the formation of a rearranged biphenyl quinone type metabolite. Studieswith inhibitory antibodies and chemical inhibitors showed that P450 3A4 was the primaryenzyme responsible for oxidative bioactivation of I and II in human liver microsomes. Thesestudies thus demonstrated that gaining an understanding of bioactivation mechanisms maybe exploited in terms of guiding structural modifications of drug candidates to minimize covalentprotein binding and, hopefully, to lower the potential for drug-mediated adverse effects.