Different phases of clinical trials

Before new cancer treatments or drugs are given to people in trials they are first tested in laboratories using cancer cells. They then go through 3 phases (or steps) of being tested in trials before they are approved for use.

Phase 1 trials

The treatment is given to a small number of people. The trial is to see how the treatment effects the body and how much of the treatment can be given without causing serious side effects.

Phase 2 trials

The treatment is tested on a larger number of people to see which cancers it would be best used for. The researchers also learn more about potential side effects and the best dose to use.

Phase 3 trials

The treatment is tested on a very large number of people. It is compared to the standard treatment already available to find out if it works better. Manufacturers can apply for a licence for the drug or treatment after phase 3 trials, if the drug is seen to be safe and effective.

Phase 4 trials

This phase happens after approval, to research the long-term benefits and risks.

Different phases of clinical trials

Researchers carry out treatment trials in a series of phases (or steps). They test possible new cancer drugs or treatments in a laboratory first.

To make sure drugs are safe, researchers test them on animals before they give them to people in trials. Using animals in research is highly regulated. Gov.uk and Cancer Research UK have more information about this.

If the drugs are effective in laboratory tests, researchers test them in phase 1 trials with people affected by cancer.

If these are successful, researchers test the drugs in phase 2 trials and then phase 3 trials. Phase 4 trials test drugs that are already licensed for use.


Phase 1 trials

Phase 1 trials are the first tests involving people for a new drug or type of treatment. Before this, it has only been tested in the laboratory.

These trials usually involve individual drugs rather than a combination of drugs. For example, it could be a trial of a new targeted drug, immunotherapy drug or chemotherapy drug.

Phase 1 trials usually happen in cancer clinical research units at specialist hospitals. Sometimes this means you have to travel further to a hospital to have the trial.

A phase 1 trial aims to find out:

  • how much of a drug can be given safely
  • what side effects it causes
  • whether it has any effect on the cancer.

These trials involve very few people. People taking part have usually already had treatments, but they are no longer working for them. Other standard cancer treatments are unlikely to be helpful for them.

To take part, you need to meet the inclusion criteria to make sure you are reasonably well. This is because these trials are used to find out about side effects.

Before you can enter a trial, doctors or nurses may ask questions to see how well you manage day-to-day activities. Doctors call this performance status. It makes sure people who are already feeling unwell do not have a treatment that may not work for them and may make them feel worse.

How phase 1 trials work

The research team give the first people in the trial a small dose of the drug that is expected to be safe. Depending on the side effects, the next group get a higher dose. The dose keeps increasing with each group.

Researchers look carefully at the different side effects and whether they are mild, moderate or severe. This helps them work out the best dose to give.

In a phase 1 trial, researchers do not know whether people will benefit from the new treatment. Finding out the best dose to give and its side effects is an important stage before testing how effective the drug is.

The drug or treatment goes on to a phase 2 trial if:

  • it is safe
  • there are signs that it may have an effect on the cancer.


Phase 2 trials

A phase 2 trial aims to find out:

  • what types of cancer the drug might be best used to treat
  • more about the side effects of the drug and how to manage them.

Phase 2 trials involve more people, who are monitored closely to see how well the drug or treatment is working against the cancer. Even though it has been through a phase 1 trial, researchers still look at the side effects. They test in bigger groups to find any side effects they have not seen before.

The drug or treatment goes on to a phase 3 trial if:

  • it has an effect on the cancer
  • it is safe.


Phase 3 trials

A phase 3 trial involves bigger groups of people. It compares trial treatments that are giving good results with the current best standard treatments. It may also give more information about any side effects the new treatment may cause.

A phase 3 trial looks at whether a new treatment:

  • is as good as the standard treatment
  • causes fewer side effects.

Phase 3 trials usually involve a randomisation process. A computer chooses the groups, to avoid bias. The trial compares the standard treatment with the new treatment. It may also involve a placebo. A placebo looks the same as the drug or treatment being tested, but it does not act on the cancer.

Once phase 3 trials have shown a drug to be safe and effective, manufacturers of the drug can apply for a drug license. Licensed drugs are available for use. Most licences are given by the European Medicines Agency (EMA). Or the Medicines and Healthcare products Regulatory Agency (MHRA) can give a licence for a drug to be used in the UK only.

Drugs that are licensed may go through phase 4 trials for further research.

I took part in a clinical trial of a new chemotherapy drug. I'd already had treatment so my progress was measured against the new one.

Ron, clinical trial participant


Phase 4 trials

These are done after a drug has proved to work well and is licensed. They aim to find out:

  • how well the drug works when it is used more widely
  • the long-term risks and benefits of the drug
  • more about possible rare side effects and the safety of the drug.

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