Until recently tamoxifen occupied the central stage in adjuvant (post operative) hormonal treatment of breast cancer. Tamoxifen has been the unchallenged hormonal therapy of choice for adjuvant treatment of early stage breast cancer for over two decades.

Things have changed now. A new group of hormonal drugs that has been introduced few years ago has almost replaced tamoxifen as the treatment of choice for early stage breast cancer. This new group of drugs known as aromatase inhibitors has been shown to be more effective than tamoxifen over and over again.

This new group of drugs however is not effective in premneopausal women who have been diagnosed with breast cancer. Tamoxifen still remains the hormonal treatment of choice for women with breast cancer who have hormone receptor positive early stage breast cancer.

This new group of drugs known as aromatase inhibitors acts by inhibiting an enzyme called aromatase, which is responsible for production of small amounts of estrogen in post-menopausal women. Estrogen production from the ovaries in premneopausal women is not depended on aromatase hormone, and that is the reason why this drug is not effective in premneopausal women.

Three drugs belonging to the group of aromatase inhibitors are currently FDA approved and available for treatment of breast cancer. These drugs are anastrozole (Arimidex), letrozole (Femara) and exemestane (Aromasin). All these are active drugs are generally considered to be equally effective.

Aromatase inhibitors may be useful even if a woman has already been started on tamoxifen. Aromatase inhibitors can be taken for five years after completion of five years of tamoxifen. A clinical trial (MA-17) has shown that if Femara is taken for another extra five years after completion of five years of treatment with tamoxifen, this would result in improved outcome. Other trials (ABCSG trial, ARNO trial 95, and ITA trial) have shown that it is beneficial to switch from tamoxifen to aromatase inhibitors after two to three years of treatment with tamoxifen.

Aromatase inhibitors have already shown to be superior to tamoxifen in the treatment of hormone receptor positive metastatic (cancer that has spread to distant organs) breast cancer. In this situation Femara is currently the drug of choice and tamoxifen have moved to second or third position as an option for hormonal therapy.

One thing is clear. The very foundation of tamoxifen as the hormonal treatment of choice is shaking and the aromatase inhibitors are here to stay.