Phases involved in Drug Clinical Trials

Before a new treatment can be approved for use in patients to treat a specific condition, the manufacturers of the new treatment need to demonstrate that it is effective and safe in that condition – and ideally more effective or safer than the current best available treatment.

There are many steps involved between the discovery of a new drug and the point where it is approved for use in patients and the process takes approx. 12 years.

Preclinical phase: 

Before the drug can be tested in people at all, many years are spent evaluating the drug in a laboratory using animals or animal models. As well as trying to get a clear understanding of how the drug works, the most important aim of this phase is to test for possible toxicity – i.e. would this drug be safe to use in humans?

Phase I: 

If preclinical studies demonstrate that the drug’s toxicity profile is acceptable, it is then evaluated in small numbers of healthy volunteers (with the exception of cancer trials, where the people taking part are normally cancer patients). The main aims of a Phase I trial are to closely study the effect of the drug on the body to confirm that the drug behaves in people in the manner expected from the preclinical phase. Different doses of the drug are tested to get a clear picture of how the drug behaves in the body. Phase I studies are normally carried out in specialist research centres where patients can be closely monitored 24 hours a day.

Phase II: 

Phase II studies are  larger than phase I studies (typically several hundred patients) are are conducted in patients who have the specific condition the drug is intended to help. The new drug will be compared to either a placebo (dummy) drug or another drug which is currently used to treat this condition. As well confirming whether the drug is effective and safe in treating the condition it is intended for, another important objective is to confirm what the best dose is (i.e. the dose which is most effective with least side effects). 

Phase III: 

This is also conducted in patients with the specific condition the drug is being tested for use in but this time in a much larger number of patients (typically several thousand) and also more varied types of patients. If the drug is shown to be effective and safe is this larger group of patients compared to the comparator treatment, the drug may then be approved for use in patients with this condition and made available for doctors to use. Only a tiny percentage of drugs which enter preclinical testing make it through phase I, II and III and end up approved for use .

Phase IV: 

Additional trials conducted in a condition in which a drug is already approved for use are called Phase IV trials. They may be conducted for marketing reasons, for example to demonstrate that the drug is better than another drug on the market, or for the purpose of safety monitoring, for example to further investigate the frequency of a potential side effect.

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