A new class of drugs, called aromatase inhibitors, is being used to treat women with advanced breast cancer and some with early breast cancer.

Breast cancer growth often depends on the presence of estrogens. In younger women, surgical removal of the ovaries, the major source of estrogens in young women, can lead to remission of the cancer. But older, postmenopausal women also produce a small amount of estrogens and these can support the growth of breast cancer.

In the past, surgical removal of the adrenal glands, which produced most of the estrogens in postmenopausal women, could lead to remissions in many older women with advanced breast cancer. In the last 20 years, this operation has been made unnecessary by the introduction of the drug tamoxifen, which blocks the action of estrogens and leads to shrinkage of the cancer.

But tamoxifen eventually stops working and the cancer grows back. This is where the aromatase inhibitors have proved so valuable. They work on the cancer cells differently from tamoxifen, not by blocking the effect of estrogen, but by preventing its production.

These drugs are designed to interfere with the body’s chemical process that leads to the production of estrogen. Since their introduction into practice in the US about five years ago, three different aromatase inhibitors are being used. They are called anastrozole, exemestane, and letrozole.

Are aromatase inhibitors useful for women whose ovaries are still functioning? No, this drug is only effective in postmenopausal women. The amount of estrogen produced by functioning ovaries is too great compared to the amount blocked by these drugs.

For women whose cancer had begin to grow on tamoxifen, what are the chances these drugs will help? About one-third of women will benefit, either by their tumors shrinking or at least by not growing.

Can aromatase inhibitors be used instead of tamoxifen? Many doctors are beginning to prescribe them instead of tamoxifen in women with advanced breast cancer.

Are they as effective as tamoxifen? Several studies have shown they are at least as effective and some have found them to be more effective.

Do these drugs work if the cancer doesn’t have estrogen receptors, which is the kind of cancer that tamoxifen can’t treat? No, the cancer must be sensitive to estrogens; it must have estrogen receptors.

What about side effects? The good news is that these drugs have few side effects. A few women will have nausea and hot flashes, but these are mild. In general side effects are less than with tamoxifen.

Are the three drugs equally effective? So far it appears that they are, although there have not been any trials where they have been compared with each other.

Can they be used for early breast cancer? There are several clinical trials in progress testing these as adjuvant (preventive) therapy in place of tamoxifen after breast cancer surgery in postmenopausal women. Because we won’t know whether these drugs are as good as tamoxifen in this setting for several years, women should not take these drugs as adjuvant therapy outside of a clinical trial.

Can they shrink breast cancers even before surgery? Yes, they can. In a study reported in the Sept. 15 issue of the Journal of Clinical Oncology, postmenopausal women with very large tumors were treated with letrozole and about half had enough shrinkage that they could be treated with lumpectomy instead of mastectomy.

What does the future hold? Because these drugs are so new, we really don’t know what their long-term side effects might be. Also, they haven’t been carefully studied combined with chemotherapy. Although this might make for a more effective treatment, it might create problems.

The aromatase inhibitors could reduce the effect of the chemotherapy or even create more side effects. It is important for doctors and patients to wait for the results of clinical trials before trying these drugs in untested settings. Patients who are interested in participating in clinical trials of these drugs should talk to their doctor and look for clinical trials on the web.