Selective Oestrogen Receptor Modulators
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Direct answer
Tamoxifen antagonises the oestrogen receptor in breast tissue while acting as a partial agonist in endometrium and bone — the defining trick of selective oestrogen receptor modulators, whose action depends on which coactivator and corepressor proteins each tissue recruits to the drug-bound receptor. The class spans tamoxifen 20 mg daily (adjuvant therapy of ER-positive breast cancer), raloxifene 60 mg daily (postmenopausal osteoporosis, with breast-cancer risk reduction and no endometrial stimulation), clomiphene 50 mg for five days (ovulation induction), and ormeloxifene 30 mg weekly — India's own SERM, marketed as Saheli. Fulvestrant 500 mg intramuscularly (days 0, 14, 28, then monthly) is a pure antagonist, a selective oestrogen receptor degrader.
What you must remember
- Tissue logic: tamoxifen = antagonist in breast, agonist in endometrium and bone, partial agonist in liver (raises triglycerides, clotting factors); raloxifene = antagonist in breast and uterus, agonist in bone.
- Tamoxifen 20 mg/day: five to ten years of adjuvant therapy in ER-positive disease; reduces ipsilateral recurrence and contralateral cancer; hot flushes and vaginal discharge are the everyday adverse effects.
- Tamoxifen's serious risks: venous thromboembolism roughly doubled, endometrial carcinoma (about 2-fold, from its agonist action), cataract — weighed against survival benefit.
- CYP2D6 trap: tamoxifen is a prodrug for endoxifen; strong CYP2D6 inhibitors (fluoxetine, paroxetine) reduce its efficacy — a textbook interaction question.
- Raloxifene 60 mg/day: spine-fracture protection without endometrial stimulation, but it carries VTE risk and raises hot flushes; it lowers invasive breast cancer risk in postmenopausal women.
- Clomiphene mechanism: blocks hypothalamic/pituitary ER so the body "reads" low oestrogen and releases FSH-LH — ovulation induction for five days from day 2 of the cycle, 50 mg escalating; risk of multiple pregnancy and ovarian hyperstimulation.
- Ormeloxifene (centchroman) 30 mg twice weekly then weekly: developed by CDRI Lucknow, non-hormonal, non-steroidal contraception with a delayed-action return of fertility; also used for mastalgia and fibroadenoma off-label.
- Fulvestrant: a SERD given by deep intramuscular injection because oral bioavailability fails; used for ER-positive metastatic disease after anti-oestrogen progression.
Sorting a patient through the class
A 58-year-old postmenopausal woman with a 2 cm ER-positive, node-negative tumour and a spine DEXA T-score of -2.8 illustrates every branch. If she were premenopausal, tamoxifen would be the only endocrine option of the two (aromatase inhibitors need a postmenopausal oestrogen source to block). Being postmenopausal, anastrozole is often preferred for superior efficacy, but her osteoporosis tips the balance: aromatase inhibitors accelerate bone loss, whereas tamoxifen is bone-neutral-to-positive — a genuine trade-off that exams love. Her endometrial history matters next: any abnormal bleeding on tamoxifen demands evaluation, because the same agonist action that protects bone stimulates endometrium. Had she asked for osteoporosis prevention alone with breast-cancer risk reduction, raloxifene 60 mg would fit — with the caution that it does not treat vasomotor symptoms and worsens them. Meanwhile her 29-year-old daughter with anovulatory infertility receives clomiphene 50 mg from day 2 to 6, with ultrasound tracking — the same receptor, blocked at a different site, producing a completely opposite therapeutic goal.
Where students slip
Calling tamoxifen "the oestrogen antagonist" without tissue qualification is the most heavily penalised simplification in this zone; the endometrial cancer risk exists precisely because it is not a pure antagonist. Second, candidates mix up clomiphene's site of action — it blocks pituitary-hypothalamic receptors to raise gonadotropins, it does not stimulate the ovary directly. Third, the Indian exam-setter's favourite: ormeloxifene/centchroman, a CDRI-developed weekly contraceptive that global textbooks omit — knowing its name, dose (30 mg) and non-steroidal character marks out prepared candidates.
Frequently asked questions
Why does tamoxifen cause endometrial cancer while raloxifene does not?
Tamoxifen acts as a partial agonist at endometrial oestrogen receptors, stimulating proliferation, whereas raloxifene is an antagonist in the uterus.
Which antidepressants should be avoided with tamoxifen, and why?
Fluoxetine and paroxetine — strong CYP2D6 inhibitors that block conversion of tamoxifen to its active metabolite endoxifen; sertraline or venlafaxine are the usual swaps.
How does clomiphene induce ovulation if it blocks oestrogen receptors?
By blocking ER in the hypothalamus and pituitary it removes negative feedback, so FSH (and LH) rise and drive follicular recruitment — used for five days early in the cycle.
What is fulvestrant and why is it given intramuscularly?
A selective oestrogen receptor degrader — pure antagonist promoting receptor degradation — with negligible oral bioavailability, given 500 mg IM on days 0, 14 and 28, then monthly.
Which SERM was developed in India and for what indication?
Ormeloxifene (centchroman, Saheli), developed at CDRI Lucknow — a non-hormonal weekly oral contraceptive also used for dysfunctional uterine bleeding and mastalgia.