Raloxifene binds to estrogen receptors, resulting in differential expression of multiple estrogen-regulated genes in different tissues. Raloxifene produces estrogen-like effects on bone, reducing resorption of bone and increasing bone mineral density in postmenopausal women, thus slowing the rate of bone loss. The maintenance of bone mass by raloxifene and estrogens is, in part, through the regulation of the gene-encoding transforming growth factor-_3 (TGF-_3), which is a bone matrix protein with antiosteoclastic properties. Raloxifene activates TGF-_3 through pathways that are estrogen receptor-mediated but involve DNA sequences distinct from the estrogen response element. The drug also binds to the estrogen receptor and acts as an estrogen agonist in preosteoclastic cells, which results in the inhibtion of their proliferative capacity. This inhibition is thought to contribute to the drug's effect on bone resorption. Other mechanisms include the suppression of activity of the bone-resorbing cytokine interleukin-6 promoter activity. Raloxifene also antagonizes the effects of estrogen on mammary tissue and blocks uterotrophic responses to estrogen. By competing with estrogens for the estrogen receptors in reproductive tissue, raloxifene prevents the transcriptional activation of genes containing the estrogen response element. As well, raloxifene inhibits the estradiol-dependent proliferation of MCF-7 human mammary tumor cells in vitro. The mechansim of action of raloxifene has not been fully determined, but evidence suggests that the drug's tissue-specific estrogen agonist or antagonist activity is related to the structural differences between the raloxifene-estrogen receptor complex (specifically the surface topography of AF-2) and the estrogen-estrogen receptor complex. Also, the existence of at least 2 estrogen receptors (ER_, ER_) may contribute to the tissue specificity of raloxifene.
Raloxifene, a selective estrogen receptor modulator (SERM) of the benzothiophene class, is similar to tamoxifen in that it produces estrogen-like effects on bone and lipid metabolism, while antagonizing the effects of estrogen on breast and uterine tissue. Raloxifene differs chemically and pharmacologically from naturally occuring estrogens, synthetic steroidal and nonsteroidal compounds with estrogenic activity, and antiestrogens. Estrogens play an important role in the reproductive, skeletal, cardiovascular, and central nervous systems in women, and act principally by regulating gene expression. When estrogen binds to a ligand-binding domain of the estrogen receptor, biologic response is initiated as a result of a conformational change of the estrogen receptor, which leads to gene transcription through specific estrogen response elements of target gene promoters. The subsequent activation or repression of the target gene is mediated through 2 distinct transactivation domains of the receptor: AF-1 and AF-2. The estrogen receptor also mediates gene transcription using different response elements and other signalling pathways. The role of estrogen as a regulator of bone mass is well established. In postmenopausal women, the progressive loss of bone mass is related to decreased ovarian function and a reduction in the level of circulation estrogens. Estrogen also has favourable effects on blood cholesterol.
Hepatic, raloxifene undergoes extensive first-pass metabolism to the glucuronide conjugates: raloxifene-4'-glucuronide, raloxifene-6-glucuronide, and raloxifene-6, 4'-diglucuronide. No other metabolites have been detected, providing strong evidence that raloxifene is not metabolized by cytochrome P450 pathways
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