Early-stage breast cancer: Switching to aromatase inhibitors after tamoxifen

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In the "switch" treatment approach, the patient uses tamoxifen for two to three years, and then switches to an aromatase inhibitor. This anti-estrogen treatment lasts for five years in total. The decision regarding whether or not to switch to an aromatase inhibitor is made after the treatment with tamoxifen. The aromatase inhibitors anastrozole and exemestane have been approved for this treatment approach in Germany.

In 2016, the Institute for Quality and Efficiency in Health Care (IQWiG, Germany) looked into the advantages and disadvantages of using aromatase inhibitors in the "switch" treatment approach for early-stage breast cancer in women.

The Institute analyzed how switching to aromatase inhibitors after treatment with tamoxifen compared to continued treatment with tamoxifen. The Institute found seven relevant studies. Five studies involving a total of 4,845 women compared anastrozole with tamoxifen. The other two studies involving a total of 4,784 women compared exemestane with tamoxifen. All of the women had taken tamoxifen for two to three years before participating in the studies. The results are described below.

What are the advantages of switching to aromatase inhibitors?

  • Life expectancy: Based on the research results, it can be concluded that aromatase inhibitors can prolong life more than tamoxifen can: About 17 out of 100 women who continued to take tamoxifen had passed away after seven years, compared to about 14 out of 100 women who switched to an aromatase inhibitor.
  • Life without recurrences: Based on the results of two studies, it can be concluded that aromatase inhibitors can prevent breast cancer from returning (recurrences): For instance, one study suggests that about 27 out of 100 women who continue to take tamoxifen will get breast cancer again or pass away within seven years. This can be expected to happen in about 23 out of 100 women who switch to exemestane after two to three years.
  • Severe side effects: Based on the research results it can be concluded that serious side effects are less common during treatment with aromatase inhibitors than during treatment with tamoxifen: About 22 out of 100 women who continued to take tamoxifen experienced severe side effects, compared to about 18 out of 100 women who switched to aromatase inhibitors.

What are the disadvantages of switching to aromatase inhibitors?

  • Bone fractures: Anti-estrogen treatment for breast cancer can have a number of side effects, such as bone fractures. One study implies that bone fractures are more common during treatment with aromatase inhibitors than during treatment with tamoxifen: About 2 out of 100 women who continued to take tamoxifen had a bone fracture, compared to 3 out of 100 women who switched to aromatase inhibitors. This means that an additional 1 out of 100 women had a bone fracture while taking aromatase inhibitors.

No difference

  • Quality of life: Two studies looked at the effect of aromatase inhibitors on the women's quality of life. They found no difference here between treatment with aromatase inhibitors and treatment with tamoxifen.
  • Hormone-related problems: Breast cancer treatment can cause hormone-related side effects such as thyroid problems. Based on two studies, it can be concluded that there was no difference between aromatase inhibitors and tamoxifen over a time period of two years in terms of severe hormone-related diseases.

What remains unanswered?

  • Breast-cancer-related problems: The studies contained no suitable data about the effect of aromatase inhibitors on the women's breast-cancer-related problems.
  • Treatment stopped due to side effects: It remains unclear whether switching to aromatase inhibitors makes people more or less likely to stop their treatment due to side effects. Different studies produced contradictory results.
  • The studies also didn't include any suitable data concerning how aromatase inhibitors affect the following serious side effects of the treatment:

The Institute also looked for differences between different aromatase inhibitors. There were no suitable studies for this comparison.