A trial looking at palbociclib and letrozole before breast cancer surgery (PALLET)

Cancer type:

Breast cancer




Phase 2

This trial looked at palbociclib and letrozole for women before surgery to remove their breast cancer. 

It was for women:

  • whose breast cancer hadn’t spread elsewhere in the body
  • who had gone through the menopause Open a glossary item
  • whose breast cancer had receptors for the hormone oestrogen (oestrogen receptor positive Open a glossary item or ER positive)

Cancer Research UK supported this trial. 

This trial was open for people to join between 2015 and 2018. The team published the results in 2018.

More about this trial

Doctors sometimes treat breast cancer with hormone therapy before surgery. It can help shrink the cancer so that the operation is easier and the surgeon may not need to remove as much breast tissue.

You might have a standard hormone treatment called letrozole. Researchers thought that adding another drug called palbociclib to letrozole might improve treatment. Palbociclib is a type of targeted drug. It targets and blocks the proteins that help cancer cells divide and grow.

In this trial some women had letrozole on its own. And some had palbociclib and letrozole.
The aims of the trial were to:

  • find out which treatment worked better
  • learn more about the side effects

Summary of results

The trial team found that palbociclib and letrozole didn’t shrink the cancer more. The researchers think this might be because it takes longer than 14 weeks for the cancer to get smaller. But they did find more cancer cells that had died in the women who had the combined treatment.

And they found that palbociclib and letrozole helped to stop cancer cells dividing as fast than the letrozole on its own.

Trial design
This was a phase 2 trial. 307 women took part. They were put into 1 of 4 treatment groups at random. They had treatment as follows:

  • 103 had letrozole on its own
  • 68 had letrozole for 2 weeks followed by letrozole and palbociclib for the next 12 weeks
  • 69 had palbociclib for 2 weeks followed by letrozole and palbociclib for the next 12 weeks
  • 67 letrozole and palbociclib for 14 weeks

For every 3 women in group A, there were 2 women in group B, group C or group D.

The women gave blood samples and tissue samples (biopsies) Open a glossary item from the breast cancer:

  • just before the start of treatment
  • 2 weeks after starting treatment
  • when treatment finished

The trial team used these samples to find out:

  • how quickly cancer cells were dividing (this is called cell proliferation)
  • the number of cancer cells which die (this is called apoptosis) and are no longer able to divide
  • if the cancer had shrunk 
  • if the type of surgery the women had changed from removing the whole breast (mastectomy Open a glossary item) to removing the breast cancer only (breast conserving surgery)

253 patients completed 14 weeks of treatment prior to their planned surgery. 

The trial team looked at how treatment affected how quickly cells divided. This is called cell proliferation. They did a number of calculations. They found that having palbociclib and letrozole worked better than letrozole to stop the cells dividing as quickly. 

They also looked at how many women had cancer cells that died and could no longer divide. This happened in:

  • 9 out of 10 women (90%) who had palbociclib and letrozole
  • just under 6 out of 10 women (59%) who had letrozole on its own

At the end of the trial they checked how much the cancer had shrunk by half or had completely disappeared. This is a partial or complete response. They found this was:

  • about 54 out of every 100 women (54%) who had letrozole and palbociclib
  • just under 50 out of every 100 women (49.5%) who had letrozole on its own

Although these numbers look a bit different, the difference wasn’t significant. This means it could have happened by chance. 

The researchers didn’t find a difference between the groups in how many women had their planned surgery changed from removing the whole breast, to removing the cancer area only.

Side effects
The researchers say that most of the side effects were mild to moderate. 

Women who had palbociclib and letrozole had more problems with a drop in the number of white blood cells Open a glossary item.

This trial showed that 14 weeks of treatment with palbociclib and letrozole worked better at slowing down how quickly the cancer cells divided compared to letrozole on its own.

There was no difference in the size of the cancer between the groups at the end of 14 weeks of treatment. But the researchers think that this is because it may take much longer than 14 weeks for the cancer to get smaller in size with treatment.

All trial results help doctors and researchers understand more about different cancers and the best way to treat them. 

Where this information comes from
We have based this summary on information from the research team. The information they sent us has been reviewed by independent specialists (peer reviewed Open a glossary item) and published in a medical journal. The figures we quote above were provided by the trial team who did the research. We have not analysed the data ourselves.

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

Professor Stephen Johnson

Supported by

Cancer Research UK
Institute of Cancer Research (ICR)
The Royal Marsden NHS Foundation Trust
International Drug Development Institute (IDDI), USA

Other information

This is Cancer Research UK trial number CRUK/13/031. 

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.

Harriet wanted to try new treatments

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“I was keen to go on a clinical trial. I wanted to try new cancer treatments and hopefully help future generations.”

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