Received for publication July 16, 2008.
Revised October 10, 2008.
Accepted for publication October 14, 2008.

Structural Modulation of Oxidative Metabolism in Design of Improved Benzothiophene Selective Estrogen Receptor Modulators (SERMs)

Abstract

Raloxifene and arzoxifene are benzothiophene selective estrogen receptor modulators (SERMs) of clinical use in postmenopausal osteoporosis and treatment of breast cancer and potentially in hormone replacement therapy (HRT). The benefits of arzoxifene are attributed to improved bioavailability over raloxifene, whereas the arzoxifene metabolite, desmethylarzoxifene (DMA) is a more potent antiestrogen. As polyaromatic phenolics, benzothiophene SERMs undergo oxidative metabolism to electrophilic quinoids. The long-term clinical use of SERMs demands increased understanding of correlations between structure and toxicity; metabolism being a key component. An homologous series of 4'-X-4'-desmethoxyarzoxifene (X-DMA) derivatives was developed and metabolism studied in liver and intestinal microsomes. Formation of glutathione (GSH) conjugates was assayed in rat liver microsomes and novel adducts were characterized by LC-MS-MS. Formation of glucuronide conjugates was assayed in human intestine and liver microsomes, demonstrating formation of glucuronides ranging from 5% to 100% for the benzothiophene SERMs: this trend was inversely correlated with the loss of parent SERM in rat liver microsomal incubations. Molecular orbital calculations generated thermodynamic parameters for oxidation which correlated with Hammett substituent constants, however, metabolism in liver microsomes correlated with a combination of both Hammett and Hansch lipophilicity parameters. The results demonstrate a rich oxidative chemistry for the benzothiophene SERMs, the amplitude of which can be powerfully modulated, in a predictable manner, by structural tuning of the 4’-substituent. The predicted extensive metabolism of DMA was confirmed in vivo and compared to the relatively stable arzoxifene and F-DMA.

Key words: bioactivation, bioavailability, chemical toxicology, chemoprotection, computational models, computer modeling and simulation, cytochrome P450 catalyzed oxidations, glutathione conjugates, metabolite identification, structure-activity relationships