Molecular Mechanism of Action
The main therapeutic goal of developing SERMs is to create an 'ideal' drug that retains only the beneficial properties of estrogens (such as bone protection and improvement of hepatic lipid metabolism) while eliminating the dangerous ones (cell proliferation in the breast and uterus).
Drugs in this group compete with endogenous estradiol for binding to $\alpha$- and $\beta$-estrogen receptors. When the drug molecule (ligand) binds to the receptor, its spatial structure changes. The final conformation of the receptor complex depends entirely on which ligand has attached.
Next, this modified receptor interacts with intranuclear proteins—coactivators or corepressors. These, in turn, influence the promoters of target genes. The varying effects of the drugs in different organs are explained by the unique set of these cofactors in each cell type.
Classification and Pharmacological Profiles
Based on their chemical structure, modulators are divided into triphenylethylene derivatives (clomifene, tamoxifen, toremifene) and benzothiophene derivatives (raloxifene).
| Drug | Hypothalamus / Pituitary | Mammary Glands | Endometrium | Bone Tissue |
|---|---|---|---|---|
| Estradiol | Agonist | Agonist | Agonist | Agonist |
| Clomifene | Antagonist | Antagonist | Partial agonist | - |
| Tamoxifen | - | Antagonist | Partial agonist | Agonist |
| Raloxifene | - | Antagonist | Antagonist | Agonist |
- Clomifene: Acts predominantly at the level of the hypothalamic-pituitary system. By blocking receptors, it deprives the body of the inhibitory influence of its own estrogens. Negative feedback is removed, leading to a sharp increase in the secretion of gonadotropins (FSH and LH), which stimulates follicular maturation. The main indication is anovulatory infertility. Common side effects include hot flashes, blurred vision, and menstrual cycle irregularities.
- Tamoxifen and Toremifene: Used for postmenopausal breast cancer. They block receptors in tumor tissue, but stimulate them in bones and the endometrium. Due to their agonistic effect on the uterus, prolonged use carries a risk of endometrial hyperplasia and cancer. They also cause hypoestrogenic reactions (sweating, palpitations).
- Raloxifene: A safer alternative for the prevention and treatment of postmenopausal osteoporosis. It preserves the protective effect on bones, but acts as a strict antagonist in both the mammary glands and the uterus, without increasing oncological risks.
Alternative: Estrogen Synthesis Inhibitors
While SERMs block the receptors themselves, synthesis inhibitors deprive receptors of their natural ligand. The use of gonadotropin-releasing hormone (GnRH) analogs shuts down ovarian function, but estrogens continue to be synthesized from adrenal androgens in peripheral tissues. The enzyme aromatase is responsible for this process.
Blockade of aromatase halts the conversion of androstenedione to estrone and testosterone to estradiol. This creates a pronounced estrogen deficiency, suppressing the growth of hormone-dependent tumors in postmenopausal women.
- First generation (Aminoglutethimide): Acted non-selectively, simultaneously shutting down the synthesis of glucocorticoids and mineralocorticoids in the adrenal cortex.
- Modern drugs: Work selectively in the periphery. They are divided into steroidal agents (formestane, exemestane), which bind covalently and irreversibly to the active site of the enzyme, and non-steroidal agents (anastrozole, letrozole), which act as competitive inhibitors.