Antihormones block the binding site of hormone receptors in hormone-sensitive tumors, thereby inhibiting binding of the endogenous hormone and thus the growth of the tumor cell.
For breast cancer the following antihormones are known:
- Tamoxifen, the traditional hormonal agent for breast cancer treatment, is a so-called selective estrogen receptor modulator. This drug blocks the estrogen receptors on the surface of cancer cells, thereby preventing estrogen from entering into the cells. However, tamoxifen is associated with side effects especially with prolonged use. Tamoxifen is associated with some side effects similar to symptoms of menopause, which include hot flashes, irregular menstrual periods and vaginal discharge or bleeding. Long-term use of tamoxifen appears to increase the number of blood clots (cataracts) and the risk of developing uterine cancer.
- Fulvestrant is an estrogen receptor antagonist, which prevents estrogen from stimulating the growth of ER-positive cells. It binds to and degrades estrogen receptors, possibly by downregulation. Common side effects of this drug are hot, flushes and GI complaints.
For prostate cancer among others the following antihormones are known:
- Cyproterone acetate is the antihormone used in the treatment of prostate cancer. This drug blocks the testosterone receptor competitively. Known adverse effects are inhibition of spermatogenesis and hepatotoxicity.
- Bicalutamide is the antihormone used in the treatment of prostate cancer. This drug blocks the testosterone receptor. Side effects of bicalutamide are hot flushes and gynaecomastia. The picture below shows the effect of anti-androgen treatment on the endocrine feedback loops. Anti-androgens increase the secretion of LHRH, LH and testosterone and the block the testosterone receptor in the prostate.