Relatively few postmenopausal women at high risk for breast cancer are taking a generic drug proven to reduce the disease's occurrence.

After a study of 4,600 healthy women concluded that exemestane, sold under the brand name Aromasin, decreased the risk of breast cancer by 65 per cent was published in the New England Journal of Medicine last June, Canadian researchers examined its bone density loss side-effects more closely.

Although the bone density side-effects were significant in 65 per cent of the women, the benefits in preventing cancer were considerable. For every 94 women who took the drug for three years, one cancer was prevented.

"If they are at increased risk and their mother and sister had it, yeah, I would consider taking this medication," said lead study author Dr. Angela Cheung, director of the osteoporosis program at Toronto's University Health Network.

Other options for these women, such as having their breasts removed, are more extreme.

Exemestane belongs to a class of drugs known as aromatase inhibitors that suppress production of estrogen, which can fuel breast cancer.

Exemestane is approved in Canada to prevent recurrence of tumours in people with breast cancer, but it isn't approved to prevent first cancers as Cheung's study sought to do.

Dr. Paul Goss of Massachusetts General Hospital Cancer Center was a co-author of the New England Journal of Medicine paper that showed the benefits of exemestane to prevent breast cancer.

Weighing drug prevention

"Pfizer

[has] lost interest in the compound because drug is generic," Goss said in a email to CBC News. "So, yes further studies will be limited."

Pfizer said exemestane is not listed as a preventive therapy in postmenopausal women at high risk for developing breast cancer in the U.S. and Europe.

But Canadian doctors can prescribe it off-label for that purpose.

"Pfizer cannot comment on regulatory plans for Aromasin at this time," said Jenifer Antonacci, director of media relations for the company.

Anti-estrogen drugs such as tamoxifen and raloxifene have more serious side-effects such as uterine cancer and stroke.

Cheung said the blood clotting risk of tamoxifen and raloxifene was similar to that of birth control pills.

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Women taking exemestane need to have regular bone monitoring and get adequate vitamin D and calcium, says Dr. Angela Cheung. (CBC)

Estrogen is actually beneficial to bones, Cheung said in stressing the need for women taking exemestane to have regular bone monitoring and get adequate vitamin D and calcium.

Dr. Eitan Amir, a medical oncologist at Toronto's Princess Margaret Hospital, wrote a commentary last May in the Journal of Clinical Oncology that called uptake of chemoprevention in breast cancer "stubbornly low."

An estimated 20 per cent of women are at increased risk of breast cancer but only a quarter of those are taking preventive drugs, Amir said.

"I suspect most women are probably not fully aware," he said in an interview.

In the study, the 351 women had an average age of 61, did not have osteoporosis and had not been prescribed bone-building medications.

The participants took daily supplements of 1,200 to 1,500 milligrams of calcium and 800 international units of vitamin D to preserve bone health.

When the researchers measured the integrity of key bones using sophisticated techniques, they found significant changes in about 65 per cent of the women.

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Aromasin or exemestane can be used to treat and prevent breast cancer (CBC)

Previous studies may have underestimated bone loss caused by aromatase inhibitors, Jane Cauley, an epidemiology professor at the University of Pittsburgh, said in a commentary published Tuesday with Cheung's study in the journal Lancet Oncology.

For Dorothy Funk of Portage la Prairie, Man., those odds are worth it. After Funk's mother and two aunts died of breast cancer, she and her sisters enrolled in the clinical trial. The 57-year-old decided to start taking exemestane, saying that watching her mother die made her want to act.

"I am not sure why people would hesitate," Funk said. "If you can be proactive, that is the way to go."

In the latest study, anyone over 60 and Caucasian would be considered at increased risk for breast cancer.

Lorna Wilson is a healthy 67-year-old in Toronto with no family history of breast cancer. Since taking part in the original study,  she takes a pill every morning to reduce her risk.

"I am concerned about my overall health," Wilson said. "If this helped me or helped any other women not get breast cancer you know that's fantastic."

Wilson is monitored for bone loss and uses proper nutrition and exercise to further reduce her risk of breast cancer.

The researchers said they still need a better way to protect individual risk for women like Funk and Wilson. In the meantime, they continue to stress that lifestyle changes such as physical activity, weight loss and changes in diet are proven prevention strategies.

Other side-effects of exemestane reported so far resemble the symptoms of menopause. More serious problems such as heart disease could show up with long-term use. 

The study was funded by the Canadian Breast Cancer Research Alliance, the Canadian Cancer Society, Canadian Institutes of Health and an unrestricted grant from Pfizer.

With files from CBC’s Kelly Crowe and The Canadian Press