A drug to prevent breast cancer? One in nine New Zealand women are destined to be diagnosed with the disease in their lifetime, so a lot of people might be keen to get their hands on that. Even better, the drug is inexpensive and readily available.
Except there is a caveat. Anastrozole is an aromatase inhibitor, so stops production of the hormone oestrogen in the body. This class of drug is used currently by women who are in recovery from hormone-positive breast cancer as protection against the cancer returning. A full course of treatment lasts 5-10 years. There are unpleasant side effects, including menopausal symptoms like hot flushes as well as weakened bones leading to a higher fracture risk. This means that as a preventive, anastrozole has its limits. It is suited to post-menopausal women who are identified as being at high risk of developing the disease.
In the UK, the drug hit headlines recently when it was licensed as an option for preventing breast cancer in post-menopausal women who have a significant family history of breast cancer. In New Zealand, there has been nothing to stop the drug being taken by women as a preventive measure, says Adele Gautier, research manager at the Breast Cancer Foundation. “It’s not used very often, but some doctors do occasionally prescribe it to people with high risk. However, unless you have visited a breast specialist or you have a particularly clued-up GP, you’re unlikely to be offered it.”
In menopause, the ovaries stop producing oestrogen, but a weaker version of the hormone is still produced from the adrenal glands and in fatty tissue. This process is enabled by an enzyme called aromatase. Drugs like anastrozole block the enzyme and stop oestrogen production.
Two large clinical trials have shown anastrozole can be effective as a preventive therapy. A reduced rate of breast cancer was found in high-risk women who took the medication. One of them, IBIS-II, was a randomised, placebo-controlled trial involving almost 4000 women, including a number from New Zealand. Invasive oestrogen-receptor positive cancer was reduced by 58% and ductal carcinoma in situ was reduced by 70%. Adverse side effects were fractures, joint pain and menopausal symptoms.
Those side effects are why some women taking the drug as part of a breast-cancer treatment plan aren’t able to tolerate it.
“We know that half of patients stop taking hormone treatments before the five years are up,” says Gautier.
Still, she believes that for some women it is worth considering taking anastrozole purely for breast cancer prevention.
“In New Zealand, we have very inconsistent high-risk screening. In some areas you might be put in a programme where you get extra screening, but if that’s not available then maybe this medication will be a good option for you,” she says.
A bone-strengthening drug, Zoledronate, can be prescribed alongside the drug to lower the risk of the brittle-bone disease osteoporosis. Even so, healthy women might baulk at taking a pill every day to prevent a cancer they may never get. So now, work is underway to identify who will benefit most.
A recent analysis of the IBIS-II trial has shown that those with higher concentrations of oestrogen in their bloodstream have the best results from preventive therapy.
“Women with the lowest oestrogen measurements benefited little from taking anastrozole, but still suffered from the side effects of the drug,” says medical oncologist Nicholas Zdenkowski, study co-chair for the IBIS II trial. “This data suggests that inexpensive blood tests to measure the ratio of these hormones could be used to identify the women who will benefit most.”
Most breast cancers are found in women 50 or over. Gautier says the Breast Cancer Foundation is keen to spread the word about anastrozole so more post-menopausal women can consider whether preventive therapy might be worthwhile.
“It’s going to be important to inform GPs about it,” she says. “We run a meeting every two years for clinicians to talk about the latest research and I’m definitely putting it on the agenda as a topic for debate.”
