For another, tamoxifen mimics the effects of estrogen in some ways and Arimidex doesn't.
Aromatase inhibitors, like Arimidex, stop estrogen production completely by curtailing a hormone called aromatase.
Key to the panel's decision: The study didn't prove that the patients on Arimidex survived longer.
For one thing, tamoxifen can be used on women who have not reached menopause, while Arimidex can't.
Gershon Locker, a Northwestern University professor who was involved in the Arimidex trials, almost seemed to agree.
Women taking Arimidex were more likely to suffer fractures or have more vague muscle and skeletal pain.
The approval is vital because Arimidex's predecessor, Nolvadex, will lose its exclusivity in the United States in February.
No matter what future clinical trials show, Arimidex will never fully replace tamoxifen.
AstraZeneca is also seeking permission to promote more broadly Casodex, its prostate cancer drug, and Arimidex, its breast cancer drug.
This means that women with osteoporosis may see their bones get much weaker on Arimidex than they would on tamoxifen.
Thursday evening, the U.S. Food and Drug Administration approved AstraZeneca 's Arimidex for use against early-stage breast cancer in post-menopausal women.
Also, says Dr. Grana, Arimidex's high cost may make it too expensive for many women who are eligible to take it.
My oncotype dx said I have 9% chance of recurrence and with Arimidex for 5 years that is reduced to 4.5 %.
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Right now, it is not clear that Arimidex can take tamoxifen's place.
He expects approval by midyear for the use of Casodex and Arimidex to treat prostate and breast cancer, respectively, immediately upon diagnosis.
McKillop built on that knowledge of hormones to develop Casodex and Arimidex.
But an advisory board convened by the ASCO told doctors to stick with tamoxifen and use Arimidex only when the older drug doesn't work.
In a study of more than 9, 000 women with breast cancer over 31 months, Arimidex reduced the risk of new tumors appearing by about 20%.
Still, the American Society for Clinical Oncology said in May that it was still too soon to tell women that Arimidex is superior to tamoxifen.
So until long-term data on Arimidex are in, AstraZeneca will have to be satisfied with its new drug playing second fiddle to its old one.
It is not yet clear how these compounds compare with Arimidex.
Results from a 9, 366-patient trial showed that those on Arimidex had a 17% reduction in the risk of breast cancer recurrence compared with patients on Nolvadex.
On Friday the drug was approved for use in combination with Aromasin for postmenopausal women with recurring HER2-negative breast cancer, after treatment with the drugs Femara and Arimidex.
But the data AstraZeneca has presented for Arimidex are impressive.
Arimidex could well eclipse AstraZeneca's popular breast cancer drug Nolvadex.
The new approval makes it more likely that doctors will consider giving Arimidex to patients who would otherwise have received Nolvadex, which is better known by the chemical name tamoxifen.
Awaiting the FDA's nod this year are Crestor, a potent cholesterol buster, as well as permission to more broadly promote Casodex, its prostate cancer drug, and Arimidex, its breast cancer drug.
Arimidex failed another test as well: Data show that the two drugs given together worked no better than tamoxifen alone over a three-year period--even though Arimidex bested tamoxifen over the same time frame.
Aside from Arimidex, there are two others: Aromasin, made by Pharmacia (nyse: PHA - news - people ), and Femera, made by Novartis (nyse: NVS - news - people ).
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