A trial looking at pembrolizumab for prostate cancer (PERSEUS1)

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

Cancer type:

Prostate cancer




Phase 2

This trial is looking at pembrolizumab for prostate cancer that has spread and hormone treatment is no longer working.

It is for men with prostate cancer that has spread and getting worse despite having hormones or surgery to remove the testicles. This is castration resistant metastatic prostate cancer. 

More about this trial

Pembrolizumab is an immunotherapy Open a glossary item. It works by stimulating the immune system  Open a glossary itemto attack cancer cells. It does this by blocking a protein called PD-1 on the T-cells  Open a glossary itemof the immune system. By blocking PD-1, pembrolizumab triggers the T-cells to find and kill cancer cells. 

Doctors use pembrolizumab to treat some cancer types. Researchers think they could use it to treat prostate cancer. 

In this trial everyone has pembrolizumab. 

The main aim of the trial is to find out how well pembrolizumab works for castration resistant metastatic prostate 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 all of the following apply. You:

  • have prostate cancer that has spread despite having hormone treatment or having your testicles removed 
  • are willing to have a fresh sample of cancer tissue (biopsy Open a glossary item) taken during the trial
  • have an area of cancer spread that can be measured on a scan or you have a blood test that shows there is a certain number of cancer cells in your blood stream and your PSA is 2ng/ml or more
  • have cancer that is getting worse and you have had treatment for it with at least one of the following abiraterone, enzalutamide, docetaxel, cabazitaxel or radium 223
  • can look after yourself but might not be able to work (performance status 0,1 or 2)
  • have satisfactory blood test results
  • are willing to use contraception during the trial and for 4 months after if you are sexually active and your partner might get pregnant 
  • are at least 18 years old

Who can’t take part

You cannot join this trial if any of these apply. 

Cancer related

  • have active cancer spread to the brain or spinal cord. You might be able to take part if you have had treatment and it is not getting worse. Your doctor can tell more about this. 
  • have cancer pressing on your spinal cord (spinal cord compression Open a glossary item) unless it is treated and not getting worse 
  • have had certain immunotherapies Open a glossary item already  
  • have had blood cell growth factors  Open a glossary itemsuch as G-CSF or GM-CSF within 2 weeks of starting the trial treatment
  • have ongoing moderate to severe side effects from previous treatment apart from hair loss (alopecia) and nerve damage (peripheral neuropathy Open a glossary item)
  • have had another cancer in the past 2 years apart from non melanoma skin cancer Open a glossary item and early bladder cancer  
  • started having bisphosphonates Open a glossary item within a month of starting trial treatment. You can join the trial if you are already having bisphosphonates and the dose has not changed within the month before you start the trial treatment. 

Medical conditions
You cannot join this trial if any of these apply. You:

  • are taking medication such as steroids that affects your immune system Open a glossary item
  • have a disease that affects your immune system
  • have a heart condition Open a glossary item or problem that isn’t controlled by medication
  • have high blood pressure (hypertension) that isn’t well controlled
  • have an autoimmune disease Open a glossary item such as inflammatory bowels disease or Crohn’s disease
  • have HIV, active hepatitis B or hepatitis C
  • have a problem with how well your platelets Open a glossary item work 
  • have any other medical condition or mental health problem that the trial team or your doctor thinks can affect you taking part

You cannot join this trial if you have a live vaccine Open a glossary item within 4 weeks of starting trial treatment. You can have vaccines that aren't live such as COVID-19.

Trial design

This is a phase 2 trial. The trial team need about 100 people to join. Everyone gets pembrolizumab. 

You have pembrolizumab as a drip into a vein. You have it every 3 weeks. You continue having pembrolizumab as long as it is working and the side effects aren’t too bad. 

Research samples
You give tissue samples and blood samples when you join the trial and during. 

You give a cheek swab when you join. To do this the research nurse uses a brush to wipe the inside of your cheek. 

Hospital visits

You see the doctor to have tests before taking part. These tests include:

  • a physical examination
  • blood tests
  • heart trace (ECG Open a glossary item)
  • CT scan or MRI scan
  • bone scan
  • tissue sample (biopsy) from the prostate or from where it has spread to

During treatment you see the doctor every 3 weeks for blood tests. Every 12 weeks you have a CT scan or an MRI scan and a bone scan. 

At the end of treatment you see the doctor or nurse for:

  • a physical examination
  • blood tests
  • heart trace

If you stopped treatment because your cancer was getting worse the team will ask for a tissue sample. You don’t have to agree to this. 

A month after the end of treatment you see the doctor to see how you are and for blood tests. 

You then see the doctor every 12 weeks for 2 years. 

Side effects

The trial team monitor you during treatment and afterwards. Contact your advice line or tell your doctor or nurse if any side effects are bad or not getting better. 
Pembrolizumab can affect the immune system. It may cause inflammation in different parts of the body which can cause serious side effects. They could happen during treatment, or some months after treatment has finished. Rarely, these side effects could be life threatening.

If you have any of these side effects, you should tell the doctor or nurse as soon as possible that you are on or have been on an immunotherapy. 

The common side effects of pembrolizumab are:

  • a drop in red blood cells (anaemia) causing tiredness
  • diarrhoea
  • shortness of breath
  • pain in the arms, legs, joints, hands and feet
  • a change to how much hormones your thyroid gland makes either too little which might cause fatigue, weight gain, constipation and being sensitive to the cold or too much which might cause diarrhoea, anxiety, sleeplessness, fatigue excessive sweating, trembling 
  • inflammation of the lungs causing difficulty breathing and a cough
  • a reaction to having pembrolizumab that might cause dizziness, fainting due to low blood pressure, rash, fever, shortness of breath, feeling sick and pain where the drip goes into the vein. This can happen when you are having pembrolizumab or just after. 

We have more information about pembrolizumab

Your doctor or a member of the trial team will talk to you about the side effects of pembrolizumab before you agree to take part. 

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 Johann de Bono

Supported by

Institute of Cancer Research (ICR)
Merck, Sharp & Dohme

If you have questions about the trial please contact our cancer information nurses

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.

Last reviewed:

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