Extended breast cancer treatment with an aromatase inhibitor (Letrozole) after tamoxifen: why, who and how long?

Breast cancer remains a leading cause of cancer death in women worldwide, and the risk for disease recurrence continues despite improvements in screening and treatment and the use of prophylactic estrogen-inhibiting therapies such as tamoxifen. A number of long-term studies now indicate a significant risk for breast cancer recurrence among patients who have undergone the currently recommended five years of tamoxifen adjuvant therapy following successful treatment of their initial disease. This ongoing recurrence risk extends even to patients commonly considered at low risk for relapse, that is, those with low-grade, small tumors, and/or node-negative disease. Treatment with tamoxifen for more than five years appears detrimental rather than beneficial and, therefore, tamoxifen is not indicated for use beyond the initial five years. Endometrial cancer and thromboembolism are among the serious adverse events that have been observed with long-term tamoxifen treatment. The aromatase inhibitors are able to reduce overall estrogen levels and appear to be better tolerated over a long term. Letrozole is the most potent aromatase inhibitor and has been available in Europe since 1996 and in the United States since 1997. Letrozole has been approved for first-line treatment of postmenopausal women with hormone-receptor-positive or hormone-receptor-unknown, advanced or metastatic breast cancer in the United States and Europe, as well as for neoadjuvant treatment (primary systemic therapy) of early breast cancer prior to surgery in many countries. The results of the pivotal MA-17 trial demonstrate that letrozole is unique in its ability to improve disease-free survival in breast cancer patients who have undergone tamoxifen therapy for five years. The MA-17 results indicate that extended adjuvant therapy with letrozole reduces risk of recurrence in this setting by 42%, reduces risk of distant recurrence (metastasis), and may improve patient survival in the node-positive patient population. The results also show letrozole to be well tolerated and safe over the length of follow-up. The trial outcomes have led to the approval of letrozole for the extended adjuvant indication in more than 40 countries worldwide. Re-randomization of letrozole-treated patients from this pivotal trial is underway to investigate if ten years of extended adjuvant endocrine therapy leads to further improvement, and the results of this extension study should aid in resolving several open questions regarding extended adjuvant therapy, including who should be treated and for how long.