The
benzothiophene selective estrogen receptor modulators (
SERMs),
raloxifene and
arzoxifene, in the clinic or clinical trials for treatment of
breast cancer and postmenopausal symptoms, are highly susceptible to oxidative metabolism and formation of electrophilic metabolites.
4'F-DMA, fluoro-substituted
desmethyl arzoxifene (DMA), showed attenuated oxidation to quinoids in incubation with rat hepatocytes as well as in rat and human liver microsomes. Incubations of
4'F-DMA with hepatocytes yielded only one
glucuronide conjugate and no GSH conjugates, whereas DMA underwent greater metabolism giving two
glucuronide conjugates, one
sulfate conjugate, and two GSH conjugates. Phase I and phase II metabolism were further evaluated in human small intestine microsomes and in human intestinal Caco-2 cells. In comparison to DMA,
4'F-DMA formed significantly less
glucuronide and
sulfate conjugates. The formation of quinoids was further explored in hepatocytes in which DMA was observed to give concentration- and time-dependent depletion of GSH accompanied by damage to
DNA, which showed inverse dependence on GSH; in contrast, GSH depletion and DNA damage were almost completely abrogated in incubations with
4'F-DMA.
4'F-DMA shows
ligand binding affinity to
estrogen receptor (ER)alpha and
ERbeta with similarity to both
raloxifene and to DMA. ER-mediated
biological activity was measured with the ERE-
luciferase reporter system in transfected MCF-7 cells and Ishikawa cells, and in MCF-7 cells, proliferation was measured. In all systems,
4'F-DMA exhibited anitestrogenic activity of comparable potency to
raloxifene but did not manifest estrogenic properties, mirroring previous results on inhibition of
estradiol-mediated induction of
alkaline phosphatase activity in Ishikawa cells. These results suggest that
4'F-DMA might be an improved
benzothiophene SERM with similar antiestrogenic activity to
raloxifene but improved metabolic stability and attenuated toxicity, showing that simple chemical modification can abrogate oxidative bioactivation to potentially toxic metabolites without loss of activity.