Combining Anastrozole and Clomiphene: Safety and Efficacy

Breast Cancer Context

• The National Comprehensive Cancer Network recommends avoiding the combination of anastrozole with selective estrogen receptor modulators like tamoxifen in breast cancer treatment due to potential antagonistic effects 1, 2, 3
• Patients receiving combined tamoxifen and anastrozole gained no benefit over those in the tamoxifen group alone, suggesting antagonistic effects 3
• The American College of Clinical Oncology notes that this antagonism likely occurs due to the weak estrogenic effect of tamoxifen/SERMs interfering with the estrogen-depleting action of anastrozole 2
• Anastrozole works by inhibiting aromatase enzyme, reducing estrogen production, as recommended by the National Comprehensive Cancer Network 3
• Clomiphene, like tamoxifen, is a SERM that binds to estrogen receptors, according to the National Comprehensive Cancer Network 4
• When used together in breast cancer treatment, the SERM's partial estrogen agonist activity may counteract the complete estrogen suppression desired from aromatase inhibitors, as stated by the National Comprehensive Cancer Network 1

Clinical Recommendations

• The National Comprehensive Cancer Network recommends avoiding combining anastrozole with clomiphene or other SERMs based on evidence from the ATAC trial showing potential antagonistic effects 1, 2, 3
• If endocrine therapy is needed, the American College of Clinical Oncology suggests using either an aromatase inhibitor alone or a SERM alone, but not in combination 2

Important Considerations and Monitoring

• The American Society of Clinical Oncology recommends monitoring bone mineral density due to increased risk of osteoporosis and fractures in breast cancer patients on anastrozole 5, 6