Please note that this summary only contains information from the full scientific abstract: View ESMO Scientific Abstract

A real-world study comparing palbociclib plus letrozole with letrozole alone in women with advanced breast cancer in the United States

Date of summary: September 2019

A retrospective analysis of electronic medical records between February 2015 through November 2018


The full title of this abstract is: Comparative effectiveness of palbociclib plus letrozole vs letrozole for metastatic breast cancer in US real-world clinical practices

This study drug is approved to treat the condition under study that is discussed in this summary.


Researchers must look at the results of many types of studies to understand whether a study drug works, how it works, and whether it is safe to prescribe to patients.

This summary reports the results of only one study. The results of this study might be different from the results of other studies that the researchers look at.


More information can be found in the scientific
abstract of this study, which you can access here:
View ESMO Scientific Abstract

What did this study look at?

  • Advanced breast cancer is cancer that has spread either within or beyond the breast.
  • Some people have a type of advanced breast cancer that is sensitive to chemical messengers in the body (hormones) such as estrogen.
    • This cancer is called hormone receptor positive (HR+)/human epidermal growth factor 2 negative (HER2−) advanced breast cancer.
  • Palbociclib is a type of medicine known as a CDK4/6 inhibitor. It works by blocking a group of proteins that cancer cells need for growth.
    • This may help to slow down the growth of the cancer.
  • Palbociclib is approved to treat HR+/HER2− advanced breast cancer in combination with medicines called aromatase inhibitors (AIs for short). An example of an AI is letrozole.
  • AI treatment lowers the amount of estrogen that the body produces.
    • Estrogen can help breast cancer cells to grow. Lowering the amount of estrogen production with AIs can slow or stop tumor growth.
  • This study looked at women with advanced breast cancer who took palbociclib + letrozole, or letrozole alone, as their first treatment for advanced breast cancer. This was a ‘real-world’ study, not a clinical trial.
    • In clinical trials, the treatments people receive are often tightly controlled so that the effect of a new medicine can be tested.
    • In real-world studies, people receive approved medicine in a routine clinical setting. This type of study helps researchers understand how medicines are used in clinical practice and whether the results seen in real life are similar to those seen in clinical trials. It also provides information on people who may not be eligible for clinical trials, presenting a more realistic real-world view.
  • This summary describes how long the women in this study lived without their cancer getting worse.

Who took part in this study?

  • Researchers identified women with HR+/HER2− advanced breast cancer from a US database. This database has information on 2 million people with cancer from 275 cancer clinics.
  • Of 1,416 women who met the criteria for this study, the researchers matched 906 women in pairs, based on their characteristics.

What were the results of the study?

More results from this study can be found here:
View ESMO Scientific Abstract

What were the main conclusions reported by the researchers?

  • In this comparative study, women taking palbociclib + letrozole as their first treatment for HR+/HER2− advanced breast cancer lived for longer without their cancer getting worse than women taking letrozole alone.
  • The results of this real-world study support the findings of previous clinical studies of palbociclib-based therapy.

Who sponsored this study?

Pfizer Inc.
235 East 42nd Street NY,
NY 10017
Phone (United States): +1 212-733-2323

Pfizer would like to thank all of the women who took part in this study.

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