A trial looking at taselisib and tamoxifen for breast cancer that has spread (POSEIDON)

Please note - this trial is no longer recruiting patients. We hope to add results when they are available.

Cancer type:

Breast cancer




Phase 1/2

This trial is looking at a new drug called taselisib (also known as GDC-0032) and a hormone therapy drug called tamoxifen for advanced breast cancer. It is for women

  • Who have already had hormone therapy and whose cancer has grown into surrounding tissue (is locally advanced) or has spread to another part of their body
  • Whose cancer is also oestrogen receptor (ER) positive Open a glossary item and HER2 negative Open a glossary item

More about this trial

If you have breast cancer that is very sensitive to oestrogen (oestrogen receptor positive), you usually have hormone therapy to try to stop or slow cancer growth.

Tamoxifen is a hormone therapy drug that doctors use often to treat cancer. We know from research that it might be useful to combine it with taselisib.

Taselisib is a type of targeted drug. It is a cancer growth blocker. It stops signals that cancer cells use to divide and grow.

In this trial, researchers are looking at tamoxifen alongside taselisib. The aims of the trial are to

  • Find the highest safe dose of taselisib
  • See if the combination of tamoxifen and taselisib looks more promising than tamoxifen alone for this group of women
  • Learn more about the side effects

Who can enter

You may be able to join the trial if all of the following apply.

  • You have breast cancer that has spread to another part of your body, or your cancer has grown into surrounding tissue but can’t be removed with surgery
  • You have breast cancer that is very sensitive to oestrogen (oestrogen receptor positive Open a glossary item) or to progesterone Open a glossary item (or both)
  • You have breast cancer that is HER2 negative Open a glossary item
  • Your cancer came back or got worse while you were taking hormone therapy such as anastrazole or exemestane
  • The trial team are able to take a new sample (a biopsy Open a glossary item) from the cancer in your breast or can use a sample that was taken when you had surgery or a biopsy in the past
  • You are well enough to be up and about for at least half the day (performance status 0, 1 or 2)
  • You have satisfactory blood test results
  • You are willing to use reliable contraception during the trial and for 2 months afterwards, if there is any chance that you could become pregnant
  • You are at least 18 years old

As well as the above, you may join the 2nd part of the trial if the following also apply.

  • Your cancer can be seen on a scan
  • If you are having types of drugs called bisphosphonates Open a glossary item, you must have been taking them for at least a month

You cannot take part in the trial if any of the following apply. You

  • Have cancer that has spread to the brain or spinal cord unless it has been successfully treated and you don’t need to take steroids Open a glossary item
  • Have had hormone therapy in the last 2 weeks
  • Have had chemotherapy in the last 3 weeks (6 weeks if you had chemotherapy drugs called nitrosoureas Open a glossary item or mitomycin C)
  • Have had more than 5 types of chemotherapy for advanced breast cancer
  • Have had radiotherapy Open a glossary item in the last 2 weeks
  • Have had monoclonal antibody treatment in the last 4 weeks
  • Have had major surgery in the last month
  • Have had any other cancer in the last 5 years apart from non melanoma skin cancer or a very early stage of cancer called carcinoma in situ Open a glossary item that has been successfully treated
  • Have not recovered from side effects of other drugs apart from hair loss or tingling in your hands and feet (peripheral neuropathy)
  • Have a problem with your stomach or bowel that could affect how you absorb tablets
  • Have a long term infection
  • Have certain heart problems (the trial team can advise you about this)
  • Have diabetes and you take medication everyday
  • Take steroids on a long term basis
  • Have a problem with the way your blood clots unless you are having a drug such as warfarin, rivaroxabane or low dose heprarin to control this
  • Are known to be sensitive to any of the drugs in this trial
  • Are known to have HIV
  • Are pregnant or breastfeeding

As well as the above, you cannot join the 2nd part of the trial if any of the following apply

  • You have had drugs called PI3K inhibitors, AKT inhibitors, PDK inhibitors or mTOR inhibitors (your doctor can tell you more about this)
  • You have had tamoxifen in the past, unless you had it after your surgery for breast cancer to try to stop your cancer coming back and you stopped taking it more than a year before your cancer started to spread
  • Your cancer suddenly gets worse and you need to have chemotherapy

Trial design

This phase 1/2 trial is in 2 parts. The researchers need between 22 and 35 women to join the 1st part of the trial and 290 to join the 2nd part.

In part 1, the researchers are trying to find the highest safe dose of taselisib to have alongside tamoxifen. Part 1 is now closed.

The first few patients taking part had a low dose of taselisib alongside a standard dose of tamoxifen. If they didn't have any serious side effects, the next few patients had a higher dose of taselisib. And so on, until the researchers found the best dose to give with tamoxifen. This is called a dose escalation study and means the researchers find the highest dose they can safely give. This part of the trial is now closed and part 2 of the trial has begun.

In the 2nd part of the trial, the researchers want to compare taselisib and tamoxifen with tamoxifen alone. This part is randomised. The women taking part are put into 2 different treatment groups by a computer. Neither you nor your doctor will be able to decide which group you are in.

  • One group have tamoxifen and taselisib
  • The other group have tamoxifen and a dummy drug (placebo)



Both taselisib and tamoxifen are tablets that you take every day. Whichever part of the trial you join, you take taselisib or the dummy drug once a day for 4 weeks. You take tamoxifen once a day for 4 weeks.

In both parts of the trial, you have treatment for as long as it is helping you, up to a maximum of 2 years.

In the 2nd part of the trial, if you are having tamoxifen with the dummy drug and your cancer gets worse, you may then join the group having taselisib alongside tamoxifen.

During the trial, the researchers take a number of blood samples to learn more about what happens to taselisib in your body. This is called pharmacokinetics Open a glossary item. They will ask for a sample of your cancer that was removed when you had surgery or a biopsy in the past. They will also ask you to have another biopsy 2 months after you start treatment and when you stop it.

The researchers will use these samples to look for substances called biomarkers Open a glossary item that may help them understand more about how taselisib and tamoxifen work together and how the drugs affect your cancer.

Hospital visits

You see the trial team and have some tests before you start treatment. These tests include

  • A physical examination
  • Heart trace (ECG Open a glossary item)
  • Blood and urine tests
  • A test to see how well your lungs are working
  • CT scan or MRI scan

 You may also have a biopsy.

If you join the 1st part of the trial, you go to hospital regularly to begin with for blood tests and a quick check up. This may be as often as 4 times a week for the first 2 months. After that, or if you are in part 2 of the trial, you go to hospital once or twice a month.

You have a CT or MRI scan every 2 months while you are having treatment.

When you finish the treatment, you go back to see the trial team within a week. You have more blood tests and you may have another scan.

After 1 year, you have a scan every 3 months if your cancer hasn't got worse.

Side effects

As taselisib is a new drug, there might be side effects we don’t know about yet. The most common side effects of taselisib so far include

  • Diarrhoea
  • High levels of sugar in the blood
  • Feeling or being sick
  • Tiredness (fatigue)
  • Loss of appetite
  • Skin rash
  • Pain, redness or swelling inside your mouth and around your nose, anus and vagina
  • Weight loss
  • Tummy pain (dyspepsia)
  • High temperatures (fever)
  • Increased risk of infection such as urine or skin infections 
  • Hair loss 
  • Fever
  • Heartburn or indigestion 

There is a small chance that taselisib can make your skin more sensitive to the sun. The trial team recommend that you apply sunscreen and lip balm that is sun protection factor 20 (SPF20) or higher and avoid long periods out in the sun.

We have information about the side effects of tamoxifen.

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

Cambridge University
Cambridge University Hospitals NHS Foundation Trust
Experimental Cancer Medicine Centre (ECMC)
Netherlands Cancer Institute
NIHR Clinical Research Network: Cancer
Vall d’Hebron Institute of Oncology

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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