A trial of megestrol acetate and letrozole for women with breast cancer (PIONEER)

Cancer type:

Breast cancer




Phase 2

This trial is comparing letrozole and megestrol acetate with letrozole on its own before surgery to remove breast cancer. It is for women who:

  • can no longer become pregnant (post menopausal Open a glossary item)
  • have breast cancer that is sensitive to the female hormone oestrogen (oestrogen receptor positive Open a glossary item or ER positive) 

More about this trial

Women with early breast cancer Open a glossary item often have treatment with hormone therapy before surgery. This can help to slow the growth of the cancer so that you can have a smaller operation. 

A common hormone therapy that women who have had their menopause have is letrozole (Femara). But doctors are looking at news ways to help women in this situation. In this trial, they are looking at megestrol acetate.

Megestrol acetate (Megace) is also a hormone therapy. It is a man made version of the hormone progesterone Open a glossary item. Doctors think that taking megestrol acetate with letrozole may be better at slowing the growth of the cancer, than letrozole alone. 

Everyone taking part in this trial has 1 of the following:

  • letrozole (treatment A)
  • letrozole and a low dose of megestrol acetate (treatment B)
  • letrozole and a high dose of megestrol acetate (treatment C)

After finishing hormone treatment, everyone has surgery to remove the breast cancer. 

The main aim of this trial is to find out if megestrol acetate and letrozole is better than letrozole alone before surgery for breast cancer.

Who can enter

The following bullet points list the entry conditions for this trial. Talk to your doctor or the trial team if you are unsure about any of these. They will be able to advise you. 

Who can take part
You may be able to join this trial if you are a woman and all of the following apply:

  • you have early stage breast cancer and are post menopausal
  • your breast cancer is sensitive to the female hormone oestrogen (oestrogen receptor positive) and does not have receptors for a protein called HER2 Open a glossary item (HER2 negative) 
  • doctors think that surgery to remove breast cancer is the best treatment for you 
  • you are going to have breast cancer surgery in the next 2 to 6 weeks  
  • you are well enough to be up and about for at least half the day (performance status of 0, 1 or 2
  • you have satisfactory blood tests results 
  • you can swallow and absorb tablets 

Who can’t take part
You cannot join this trial if any of these apply:

  • you have breast cancer that has come back (recurrent) 
  • your breast cancer has spread to other parts of the body (metastatic) 
  • you have had hormone replacement therapy Open a glossary item, an aromatase inhibitor Open a glossary item or the drug tamoxifen in the last 6 months
  • you have had an experimental treatment in the past 4 weeks
  • you are known to be sensitive to lactose, megestrol acetate or any other similar drug 
  • you have an intra uterine device (IUD Open a glossary item or coil) that contains progesterone 
  • you have any other serious medical condition that the trial team thinks could affect you taking part

Trial design

This is a phase 2 trial. The researchers hope that around 189 women will take part. 

This trial is randomised. Women are put into 1 of the following treatment groups by computer: 

  • letrozole (treatment A)
  • letrozole and a low dose of megestrol acetate (treatment B)
  • letrozole and a high dose of megestrol acetate (treatment C)

Neither you nor your doctor are able to decide which group you are in. For every 2 women in treatment A, there are 3 women in treatment B and 3 in treatment C. 

study diagram

Letrozole and megestrol acetate are tablets that you take every day, once a day. You swallow them whole, with or without food. 

You take letrozole, or letrozole and megestrol acetate, for up to 19 days until the morning of your operation. Then everyone has surgery to remove the breast cancer. This is the same as the standard treatment. Your doctor can tell you more about this. 

The trial team will give you a diary to complete at home. They will ask you to keep a record of when you take the tablets. 

You return the diary to the team at the end of the trial. 

Tissue sample 
You need to give a tissue sample (biopsy) before the start of treatment and during surgery. 

The research team wants to:

  • look for any changes that may have been caused by letrozole and megestrol acetate
  • look at the way certain proteins (receptors) interact with the cancer DNA Open a glossary item 

Blood tests
You also have extra blood tests as part of this trial. You have them before the start of letrozole, or before the start of letrozole and megestrol acetate, and at the end of treatment.

Hospital visits

You see a doctor and have some tests before taking part. These tests include: 

  • physical examination
  • blood tests

You go to hospital to collect the study drug and the diary. The trial team will tell you when to start taking the drugs. 

You speak with the trial team after 5 and 10 days of the start of treatment. This is so they can find out how you are and what side effects you have had. 

You take letrozole, or letrozole and megestrol acetate, for up to 19 days and stop it on the morning of your operation. 

You then see the trial doctor 2 weeks after surgery.

Side effects

The trial team monitor you while you are having treatment. They will give you a phone number to call them if you are worried about anything. 

The team thinks you are unlikely to have any side effects from taking megestrol acetate. This is because you only take them for up to 19 days. The team will tell you about all the possible side effects before you start the trial.

The most common side effects of taking megestrol acetate for a long period of time are:

We have more information about the side effects of megestrol acetate. And information about:



Recruitment start:

Recruitment end:

How to join a clinical trial

Please note: In order to join a trial you will need to discuss it with your doctor, unless otherwise specified.

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Chief Investigator

Dr Richard Baird

Supported by

Anticancer Fund
Cambridge University Hospitals NHS Foundation Trust
Cancer Research UK Cambridge Centre
University of Cambridge
National Institute for Health Research

Freephone 0808 800 4040

Last review date

CRUK internal database number:


Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Caroline took part in a clinical trial for breast cancer

“I had treatment last year and I want to give something back.”

Last reviewed:

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