Endocrine Therapy for Metastatic Breast Cancer

Introduction to Treatment

The American Society of Clinical Oncology recommends endocrine therapy as first-line treatment for metastatic breast cancer, unless there is visceral crisis or rapidly progressive disease 1

The European Society for Medical Oncology notes that endocrine therapies, such as tamoxifen and aromatase inhibitors, are contraindicated during pregnancy due to potential risks 2
The European Society for Medical Oncology also states that HER2-targeted agents are contraindicated during pregnancy, highlighting the need for alternative treatment approaches for HER2+ patients 2
The National Comprehensive Cancer Network recommends discussing treatment incompatibility with patients, explaining that effective treatments for metastatic breast cancer cannot be safely administered during pregnancy 2

The National Comprehensive Cancer Network suggests that while fertility preservation might have been an option earlier in treatment, the current metastatic status makes pregnancy inadvisable, and patients should be counseled accordingly 3
Premenopausal women with hormone receptor-positive breast cancer should receive ovarian suppression or ablation in combination with hormone therapy as the standard of care, according to the American Society of Clinical Oncology 4
Ovarian function can be suppressed or eliminated through GnRH agonists, surgical oophorectomy, or ovarian radiation, with similar clinical outcomes in metastatic breast cancer, though GnRH agonists offer the advantage of reversibility 4

For patients without prior hormone therapy exposure, ovarian suppression + tamoxifen or ovarian suppression + aromatase inhibitor (AI) are recommended 4
For patients who relapsed during/within 12 months of adjuvant tamoxifen, ovarian suppression + AI (nonsteroidal preferred) is recommended 4
For patients who relapsed during/after AI + ovarian suppression, ovarian suppression + fulvestrant ± palbociclib is recommended 4
Tamoxifen alone can be considered in hormone therapy-naïve patients, but evidence suggests improved outcomes when combined with ovarian suppression, as per the National Comprehensive Cancer Network 4

The following methods can be used for ovarian suppression/ablation:

Method	Description
GnRH agonists	Reversible, preferred initially
Surgical oophorectomy	Permanent option for patients who tolerate ovarian suppression
Ovarian radiation	Less commonly used currently

Monthly administration of GnRH agonists is preferred over every 3 months dosing, which is not recommended, as stated by the American Society of Clinical Oncology 4

Discuss fertility preservation options such as cryopreservation of embryos or oocytes before initiating treatment, even in patients with metastatic disease, as recommended by the American Society for Reproductive Medicine 4
Acknowledge that systemic treatment will preclude pregnancy for most patients, according to the National Cancer Institute 4
Consider calcium and vitamin D supplementation to mitigate the risk of osteopenia/osteoporosis, as suggested by the European Society for Medical Oncology 5 and the American College of Rheumatology 6
Common side effects of hormone therapy include menopausal symptoms, such as hot flashes, sleep disturbances, vaginal dryness, and decreased libido, as well as accelerated osteopenia and osteoporosis 7

Incomplete ovarian suppression, particularly when combining GnRH agonists with AIs, can lead to treatment failure, as warned by the American Society of Clinical Oncology 4
Using AIs without ovarian suppression in premenopausal women can lead to compensatory increases in ovarian estrogen production and treatment failure, as cautioned by the European Society for Medical Oncology 4, the American Society of Clinical Oncology 5, and the National Comprehensive Cancer Network 6
Leuprolide is indicated in premenopausal patients with metastatic hormone-receptor positive disease, and can be administered with tamoxifen or aromatase inhibitors, with a standard dose of 3.75-7.5 mg IM every 4 weeks or 11.25-22.5 mg IM every 12 weeks 8, 4
The optimal duration of leuprolide treatment is 5 years, with a minimum recommended duration of 2 years, according to the SOFT and TEXT trials 8
Leuprolide improves disease-free survival (DFS) in comparison with tamoxifen alone (DFS at 8 years: 85.4% with ovarian suppression + tamoxifen vs 80.2% with tamoxifen alone) 8