Tamoxifen has been used for several years to treat breast cancer which is hormone receptor-positive. This is because of its ability to competitively block estrogen binding to estrogen receptors, which stimulates breast cancer cell growth. As a selective estrogen receptor modulator (SERM), however, it not only antagonizes the actions of estrogen at its receptor by blocking its binding, but mimics other actions through its own binding to the estrogen receptor.
This is common to most SERMs, including raloxifene and toremifene, all of which act as estrogen antagonists in some organs or tissues, such as the breast, but as partial estrogen agonists in other target tissues, like the uterus and bone. Tamoxifen thus promotes uterine endometrial hyperplasia as well as bone growth, but restricts the growth of breast tissue.
Side effects due to the estrogen antagonistic activity of tamoxifen
The binding of tamoxifen to estrogen receptors has a profound effect on normal feedback processes. It induces the hypothalamus to sense a falsely high estrogen level, resulting in cessation of ovarian stimulation. In short, it induces an artificial menopause.
Tamoxifen-induced estrogen deprivation causes side effects such as hot flashes in 35% of patients. Other related hypoestrogenic symptoms include night sweats, dryness or excessive discharge from the vagina, memory lapses, depressive symptoms, and reduced libido. 17% of women on tamoxifen report insomnia, while 14% have weight gain. Bone loss and cataracts are other commonly reported side effects.
Such side-effects are more likely when women also take other medications that inhibit cytochrome P450 2D6, which metabolizes natural estrogen. However, such women also have lower recurrence rates of breast cancer, compared with women who have no side effects.
Side effects due to the estrogen agonist activity of tamoxifen
Tamoxifen also causes side effects due to its modest estrogen-like activity. These include endometrial proliferation, endometrial hyperplasia, endometrial polyp formation, invasive endometrial cancer, uterine sarcoma, venous thromboembolism and its complications, such as stroke.
Endometrial cell hyperplasia
Endometrial cell hyperplasia can result in troublesome vaginal bleeding and discharge. The incidence of endometrial hyperplasia when on tamoxifen was almost 12% in women with prior endometrial lesions, as compared to 0.7% in women without endometrial lesions at the time of initiation of tamoxifen.
It may progress to invasive cancer of the endometrium. Tamoxifen raises the risk of endometrial cancer in post-menopausal women who have not had a hysterectomy, but not in premenopausal women.
Endometrial polyps show an 18-fold increase in risk, which is sharply demarcated by the presence of initial lesions in the endometrium. Thus 17.6% of women with endometrial lesions developed polyps, compared to almost 13% of those with normal endometrium at the start of therapy.
Research shows that women on tamoxifen develop endometrial cancer at a rate of 1.26 per 1000 patient years, versus 0.58 in women on placebo. However, the rate of 5-year survival in women on breast cancer who are treated with tamoxifen is 38% greater than in breast cancer patients not on tamoxifen.
There is a 2-3% risk of endometrial cancer over the 15 years after the start of tamoxifen therapy in older women, as calculated for 5 years of tamoxifen adjuvant therapy. The risk is increased by approximately 2% at year 15 if tamoxifen is continued for 10 years. The risk ratio is thus about 2.5 for endometrial cancer. However, the death rate from endometrial cancer in this group is roughly 10% of the calculated incidence.
Uterine sarcomas are a rare form of cancer, accounting for only 8% of uterine cancers. However, they occur at a rate of 17 per 100,000 patients in women who are on tamoxifen, whereas there were no cases in the placebo group, and 1-2 per 100,000 patients in the general population.
Despite the low mortality rate from endometrial cancer in women on tamoxifen, and the improvement in the 5-year survival of women with breast cancer who are on tamoxifen adjuvant therapy, there remains a statistically significant rise in the number of endometrial cancers and deaths from this cause after approximately 5 years on tamoxifen, as compared to placebo.
VTE and post-surgical thromboembolism is also significantly more common in women on tamoxifen.
Tamoxifen in men has been known to cause headaches, nausea, vomiting, skin rashes, impotence and loss of libido.
Cost-benefit ratio with prolonged therapy
The risks of endometrial cancer and of venous thromboembolism are increased with the duration of tamoxifen therapy. This fact, along with the development of both new and acquired drug resistance, account for the recommendation to stop tamoxifen after 5 years. Lately, however, the period has been increased to 10 years, in the face of evidence of continuing benefit from tamoxifen in the shape of lowered breast cancer recurrence and mortality rates in these patients.
Favorable side effects of tamoxifen
The partial estrogen agonist action of tamoxifen also leads to beneficial effects such as:
- a lowering of serum cholesterol
- reduced bone loss and
- protection against atherosclerosis and cardiovascular disease in older women.