Tamoxifen is indicated for the treatment of breast cancer in a variety of settings. It should be noted that evidence suggests that patients with estrogen receptor positive tumors are more likely to benefit from tamoxifen. Tamoxifen also has many off-labeled uses, and they may require additional data.
Tamoxifen exhibits both estrogenic agonist and antagonist effects in different parts of the body. Because it has two actions, it is patient-specific as a selective estrogen receptor modulator (SERM). In the breast tissue, it competes with estrogen for binding sites and causes antiestrogenic and antitumor effects. Through downstream intracellular processes, it slows cell cycling which classifies it as cytostatic. In the bone it stimulates estrogen receptors instead of blocking them, exerting an estrogenic agonist effect, and may prevent osteoporosis in postmenopausal women.
Tamoxifen is available in a tablet (10 mg or 20 mg) or an oral solution (10 mg/5 mL). If it as administered as an oral solution, it is important to use the supplied dosing cup for adequate administration. The American Society of Clinical Oncology (ASCO) guidelines for Adjuvant Endocrine Therapy of Hormone-Receptor Positive Breast Cancer recommends a dose of 20 mg daily for breast cancer prevention after completion of chemotherapy. The duration of endocrine therapy depends on the patient's menopausal status and can last 5 to 10 years. For the treatment of metastatic breast cancer, 20 to 40 mg daily is recommended, although clinical benefit has not been shown for doses above 20 mg daily. In some off-label clinical trials, 10 mg was used as the dose. It may be taken with or without food.
Tamoxifen has a black box warning for uterine malignancies, pulmonary embolism, and stroke in patients who are at high risk for cancer or who have ductal carcinoma in situ. In patients who are female, tamoxifen is associated with an increased incidence of uterine or endometrial cancers, with some being fatal. In patients who were already diagnosed with breast cancer, however, the benefits outweigh the risks.
Like many cancer drugs, tamoxifen has many adverse effects associated with it, though serious and fatal ones are rare. The most common adverse effects seen in treatment are hot flashes, irregular periods, and vaginal discharge. Other common adverse effects include peripheral edema, hypertension, mood changes, pain, depression, skin changes and skin rashes, nausea and vomiting, weakness, arthritis, arthralgia, lymphedema, and pharyngitis.
Less common adverse effects may include insomnia, dizziness, headache, weight gain, abdominal pain, diarrhea, indigestion, urinary tract infections, thrombocytopenia, back pain, alopecia, ostealgia, and cataracts among many more. Due to the extensive adverse effects of tamoxifen, it is important for patients to discuss all adverse effects they are experiencing with their doctor.
Tamoxifen can also cause a local disease flare which may lead to increased bone and tumor pain. This can be associated with good tumor response and usually resolves quickly. In patients with bone metastasis, hypercalcemia may occur. If hypercalcemia becomes severe and not manageable, discontinue tamoxifen.
Tamoxifen should not be used in patients with a known allergy to it or any component in its formulation or concomitantly with warfarin. For patients taking tamoxifen for breast cancer risk reduction who are at high risk for breast cancer or with ductal carcinoma in situ, it should be avoided if the patient has a history of deep vein thrombosis (DVT) or pulmonary embolism (PE). In patients that have been diagnosed with breast cancer, the benefits outweigh the risks, but it should still be used in caution in patients with a history of thromboembolic events.
Tamoxifen also has many drug-drug interactions, so a comprehensive medication list in all patients receiving it is vital.
All patients on tamoxifen should have routine lab work including a complete blood count with platelets, serum calcium, and liver function tests. Female patients should monitor for abnormal vaginal bleeding and receive a breast and gynecologic exam at baseline and routinely after. Patients should also watch for signs and symptoms of a DVT or PE.
Other monitoring parameters tend to vary, depending on patient-specific factors. In patients with pre-existing hyperlipidemia, triglycerides and cholesterol should be monitored. Patients on vitamin K antagonists should have their INR and PT checked. Reproductive female patients need a pregnancy test before treatment and should use reliable birth control methods during treatment. Premenopausal women should receive a bone mineral density test. All patients should get an ophthalmic exam if vision problems or cataracts occur.
There is currently no antidote available for tamoxifen.
Interprofessional teamwork and communication provide success for all patients, especially those undergoing cancer treatments. Oncologists should thoroughly evaluate the patient and select an appropriate treatment regimen based on guidelines and patient-specific factors. For example, in estrogen receptor-negative breast cancer, tamoxifen may not be appropriate. Oncology nurses should be aware of the more severe adverse effects tamoxifen may cause and look out for them in their patients, reporting concerns to the clinical team. Pharmacists should assist the clinical team by examining the patient’s current and complete drug regimen and make sure there are no interactions as tamoxifen has many. These are just a few examples, and roles tend to overlap in many different professions, but every healthcare professional provides a vital role that, in turn, benefits the patient greatly. [Level 5]
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