Anastrozole marketed under the trade name Arimidex by AstraZeneca, is a drug used to treat breast cancer after surgery and for metastases in both pre and post-menopausal women. Anastrozole is an aromatase inhibitor, which means that it interrupts a critical step in the body's synthesis of estrogen. Some breast cancer cells require estrogen to grow, and eliminating estrogen suppresses their growth. Annual sales approx $2.2bn. The first patent for Arimidex expired in June 2010. Although this patent was originally set to expire in December 2009, the manufacturer was given a six-month extension for performing pediatric studies.