Up to 70% of patients with estrogen receptor-positive (ER-positive) breast cancer initially benefit from hormone therapy. A sizable percentage of patients, however, will become resistant to these treatments. Excessive angiogenesis may contribute to the development of resistance to hormone therapy in breast cancer patients with ER-positive tumors.¹²

Two recent clinical studies evaluated the safety and effectiveness of combining hormone therapy with antiangiogenic drugs. The first of these combined bevacizumab with letrozole (Femara^®), a hormone therapy approved for use in postmenopausal women with early stage, hormone receptor-positive breast cancer following surgery. The study, which looked only at the safety of the drug combination, enrolled postmenopausal women with locally advanced or metastatic ER-positive breast cancer.¹³ The therapy was generally well tolerated, although some patients developed excess protein in the urine. A phase 3 clinical trial combining hormone therapy (letrozole or tamoxifen) with bevacizumab is underway.

Another small study combined sorafenib with the hormone therapy anastrozole (Arimidex^®) in postmenopausal women with ER- and progesterone (PR) positive tumors.¹⁴ The participants in this study were resistant to prior hormone therapy. At follow-up, 20% of patients had some clinical benefit from the sorafenib-anastrozole combination. Five patients had their tumors stop growing for more than 24 weeks, and two patients experience tumor shrinkage for more than 6 months.