What are the Different Phases of Clinical Trials?
The programmes of clinical trials testing new drugs or treatments are split into different stages, called phases. During the early phase of trials, researchers will be looking to see if a drug causes any side effects and whether it is safe to use. As the programme progresses, the objectives of the later phase trials are to assess whether a new treatment is better than treatments already available.
There are 4 phases of clinical trials in humans, however, some trials may have an earlier stage called phase 0 and often phases 1 to 3 overlap, with phase 4 trials taking place after a drug has been licensed for use.
Phase 0 trials are very small, including around 10-20 participants and usually take place during the early stages of phase 1 studies. However, the exposure of the drug to the participants is limited to a small dose to assess the effect that the drug has on the body, or pharmacodynamics (PD) and the way in which the drug moves through the body, or pharmacokinetics (PK). Screening trials are also usually referred to as Phase 0.
Phase 1 trials usually involve a small cohort of between 20 and 50 healthy volunteers. The main aim of the phase is to find out how much of the drug is safe to be administered, collect more evidence about any side effects and to continue to collect PK data about how the drug is metabolised in the body and how it is excreted.
Phase 1 studies are often described as dose escalation studies. Meaning that the first group, or cohort, of patients are administered very small doses of the study drug. If all goes smoothly, the next cohort will be administered a slightly higher dose. The researchers will collect information about the side effects observed and with each cohort, the dose will be gradually increased until the optimum dose is found. If unacceptable toxicity levels are not recorded during phase 1, phase 2 studies can then begin.
Phase 2 trials are all about establishing the effectiveness of the trial drug. These trials are usually larger than phase 1, with around 100 or so patients with a particular condition or disease taking part. Sometimes phase 2 trials will compare the new treatment with another treatment currently being used, or in other trials a comparison will be made with a placebo. Data will continue to be collected about new and existing side effects, as well as refining the optimum dose to be carried over into phase 3.
Phase 3 trials compare the current gold standard treatment with the new treatment. Phase 3 studies are much larger with hundreds and sometimes thousands of participants who not only have a particular condition or disease but will be from a wider population group and may also have other conditions or be taking other medications. This means that new side effects may become apparent as drug-drug interactions are reported.
After phase 3, the complete information on the new drug is submitted to the health authorities for licence approval. Once approval for marketing has been received, then trials will move onto phase 4. Phase 4 studies will usually have a large cohort of participants with the aim to find out more about how well the drug works when it’s used more widely, the side effects and safety of the drug, as well as what the long-term risks and benefits are.
Most trials are just one phase. But some trials cover more than one phase. For example, the same trial can include both phase 1 and phase 2. So, you may see trials written as phase 1/2 or phase 2/3.
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