report in September 2009 from Health and Human Services’ Agency for Healthcare Research and Quality suggests that tamoxifen, raloxifene, and tibolone used to treat breast cancer significantly reduce invasive breast cancer in midlife and older women, but also increase the risk of adverse side effects.
Some cases of lower-limb lymphedema have been associated with the use of tamoxifen, due to the blood clots and deep vein thrombosis (DVT) that can be caused by this medication. Resolution of the blood clots or DVT is needed before lymphedema treatment can be initiated.
A beneficial side effect of tamoxifen is that it prevents bone loss by acting as an ER agonist (i.e., mimicking the effects of estrogen) in this cell type. Therefore, by inhibiting osteoclasts, it prevents osteoporosis. When tamoxifen was launched as a drug, it was thought that tamoxifen would act as an ER antagonist in all tissue, including bone, and therefore it was feared that it would contribute to osteoporosis. It was therefore very surprising that the opposite effect was observed clinically. Hence tamoxifen’s tissue selective action directly led to the formulation of the concept of SERMs. In contrast tamoxifen appears to be associated with bone loss in premenopausal women who continue to menstruate after adjuvant chemotherapy.
Tamoxifen is a SERM. Even though it is an antagonist in breast tissue it acts as partial agonist on the endometrium and has been linked to endometrial cancer in some women. Therefore, endometrial changes, including cancer, are among tamoxifen’s side effects. With time, risk of endometrial cancer may be doubled to quadrupled, which is a reason tamoxifen is typically only used for five years.
The American Cancer Society lists tamoxifen as a known carcinogen, stating that it increases the risk of some types of uterine cancer while lowering the risk of breast cancer recurrence. The ACS states that its use should not be avoided in cases where the risk of breast cancer recurrence without the drug is higher than the risk of developing uterine cancer with the drug.
Cardiovascular and metabolic
Tamoxifen treatment of postmenopausal women is associated with beneficial effects on serum lipid profiles. However, long-term data from clinical trials have failed to demonstrate a cardioprotective effect. For some women, tamoxifen can cause a rapid increase in triglyceride concentration in the blood. In addition, there is an increased risk of thromboembolism especially during and immediately after major surgery or periods of immobility. Use of tamoxifen has been shown to slightly increase risk of deep vein thrombosis, pulmonary embolism, and stroke. Tamoxifen is also a cause of fatty liver, otherwise known as steatorrhoeic hepatosis or steatosis hepatis.
Central nervous system
Tamoxifen-treated breast cancer patients show evidence of reduced cognition, and semantic memory scores. However, memory impairment in patients treated with tamoxifen was less severe compared with those treated with anastrozole (an aromatase inhibitor).
Acute overdose of tamoxifen has not been reported in humans. In dose-ranging studies, tamoxifen was administered at very high doses in women (e.g., 300 mg/m2) and was found to produce acute neurotoxicity including tremor, hyperreflexia, unsteady gait, and dizziness. These symptoms occurred within three to five days of therapy and disappeared within two to five days of discontinuation of therapy. No indications of permanent neurotoxicity were observed. QT prolongation was also observed with very high doses of tamoxifen. There is no specific antidote for overdose of tamoxifen. Instead, treatment should be based on symptoms.
Patients with variant forms of the gene CYP2D6 (also called simply 2D6) may not receive full benefit from tamoxifen because of too slow metabolism of the tamoxifen prodrug into its active metabolites. On 18 October 2006, the Subcommittee for Clinical Pharmacology recommended relabeling tamoxifen to include information about this gene in the package insert.
Certain CYP2D6 variations in breast cancer patients lead to a worse clinical outcome for tamoxifen treatment. Genotyping therefore has the potential for identification of women who have these CYP2D6 phenotypes and for whom the use of tamoxifen is associated with poor outcomes.
Recent studies suggest that taking the selective serotonin reuptake inhibitors (SSRIs) antidepressants paroxetine (Paxil), fluoxetine (Prozac), and sertraline (Zoloft) can decrease the effectiveness of tamoxifen, as these drugs compete for the CYP2D6 enzyme which is needed to metabolize tamoxifen into its active forms. A U.S. study presented at the American Society of Clinical Oncology’s annual meeting in 2009 found that after two years, 7.5% of women who took only tamoxifen had a recurrence, compared with 16% who took either paroxetine, fluoxetine or sertraline, drugs considered to be the most potent CYP2D6 inhibitors. That difference translates to a 120% increase in the risk of breast cancer recurrence. Patients taking the SSRIs; Celexa (citalopram), Lexapro (escitalopram), and Luvox (fluvoxamine), did not have an increased risk of recurrence, due to their lack of competitive metabolism for the CYP2D6 enzyme. A newer study demonstrated a clearer and stronger effect from paroxetine in causing the worst outcomes. Patients treated with both paroxetine and tamoxifen have a 67% increased risk of death from breast cancer, from 24% to 91%, depending on the duration of coadministration.
Recent research has shown that 7–10% of women with breast cancer may not receive the full medical benefit from taking tamoxifen due to their genetic make-up. DNA Drug Safety Testing can examine DNA variations in the CYP2D6 and other important drug processing pathways. More than 20% of all clinically used medications are metabolized by CYP2D6 and knowing the CYP2D6 status of a person can help the doctor with the future selection of medications. Other molecular biomarkers may also be used to select appropriate patients likely to benefit from tamoxifen.
Tamoxifen interacts with certain other antiestrogens. The aromatase inhibitor aminoglutethimide induces the metabolism of tamoxifen. Conversely, the aromatase inhibitor letrozole does not affect the metabolism of tamoxifen. However, tamoxifen induces the metabolism of letrozole and significantly reduces its concentrations.