Aromatase inhibitor and bone

Aromatase is a key enzyme of intratumoral production of estrogen in breast cancers. Aromatase inhibitors are commonly used as hormone therapy in postmenopausal estrogen sensitive breast cancer patients. Type I aromatase inhibitors such as exemestane are steroidal inhibitors, which have androstenedione like structure and bind to androgen receptor with low affinity. Type II aromatase inhibitors such as anastrozole and letrozole are known as non-steroidal inhibitors, which are non-competitive inhibitors of aromatase. Sex steroid hormones such as estrogen and androgen play important roles in the maintenances of female and male bone tissues. It is well known that adult women have less bone mass than men. Especially after menopause, adult women loss their bone mass more rapidly than men of comparable age do. Therefore, many clinical reports of breast cancer patients treated with aromatase inhibitors have emphasized potential bone loss caused by aromatase inhibition. Several basic researches using animal model or in vitro model demonstrated the different effects of steroid and non-steroid aromatase inhibitors on bone tissues and cells. In this review, we summarize the effects of AIs on bone tissues reported in clinical studies and animal/in vitro studies.