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Prescribing Advice for GPs

An NHS Prescribing Advisers' Blog

Exemestane for Breast Cancer Prevention

The New England Journal of Medicine has published the results of a study that aimed to assess whether exemestane (Aromasin®) is an effective treatment for the prevention of breast cancer in postmenopausal women.

Tamoxifen and raloxifene have limited acceptance in preventing breast cancer in high risk individuals (although this is not a licensed indication for either drug in the UK) but use in this way is very limited. This trial aimed to assess whether an aromatase inhibitor might be as effective but with fewer side effects.

This study recruited 4,560 women aged 35 years an older with at least on of the following risk factors:

  • 60 years of age or older
  • 5-year cancer risk score greater than 1.66% (using the Breast Cancer Risk Assessment Tool)
  • prior atypical ductal or lobular hyperplasia or lobular carcinoma in situ
  • ductal carcinoma in situ with mastectomy

The study had a median follow up period of 35 months. During that time there was a 65% relative risk reduction in the annual incidence of invasive breast cancer (0.19% versus 0.55%, hazard ratio 0.35, 95% CI 0.18-0.70, P=0.002) and a 53% relative risk reduction in the annual incidence of invasive plus non-invasive breast cancers (0.35% versus 0.77%, hazard ratio 0.47, 95% CI 0.27-0.79, P=0.004). Adverse events occurred in a high proportion of both arms of the study (88% of the exemestane group and 85% of the placebo group) and were not significantly different.

The authors conclude that, "exemestane significantly reduced invasive breast cancers in postmenopausal women who were at moderately increased risk for breast cancer".

The study does have some limitations. Patients in the study were not exclusively individuals who had never had cancer so the population cohort is a mixture of primary and secondary prevention. The study also compared treatment to placebo rather than an active comparator.

This study reports some impressive relative risk reductions but the actual risk reductions are less impressive. The 65% decrease in annual incidence of invasive breast cancer is an actual risk reduction of 0.35% (0.55% - 0.19% = 0.35%). This means 278 postmenopausal women like those recruited to this study would need to be treated with exemestane for a year instead of placebo to prevent one case of invasive breast cancer. 30 days supply of exemestane currently costs £88.80, so preventing this one case would require an investment of almost £300,000. This does not appear to be a cost effective use of NHS resources.

Action: The emotive nature of cancer and the impressive relative risk reductions quoted may generate patient queries. Clinicians should be aware of the actual risk reductions. Patients should be encouraged to participate in screening to aid early diagnosis.

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