Clinical trials are designed to test whether a newly developed drug or other therapy is better than treatments currently on offer. Trials are complex and it can take years before a drug is ready for market. Before licensing, a treatment must pass through various different stages.
Researchers need to know as soon as possible whether a new treatment shows enough promise to be worth testing in humans so they run extensive tests on tissues, cell cultures and animal models.
Having established that a treatment works, they are especially interested in its efficacy, toxicity and the rate at which a drug is absorbed, distributed, metabolised and eliminated.
Phase I or “Is it safe in humans?”:
Once animal studies look positive, so-called “first in human” studies are carried out on a small number of healthy subjects. The volunteers are monitored continuously – the emphasis is on what effect the drug has on the body at different doses and how safe it is.
The volunteers are usually healthy but sometimes they are in the late or terminal stages of a disease, and are included because they have failed to respond to conventional therapies.
Phase II or “Is it safe in patients?”:
This phase is designed to show how effective the drug is at treating the target condition. If the drug has already shown, or is expected to show, a really big effect, or the disease is very rare, severe or currently untreatable the number of patients may be small. But in many cases, where the drug has only a small effect, a trial needs a large number of patients in order to show anything.
Again, the toxicity of the drug is important. New treatments are most likely to fail in this phase when drugs don’t perform as expected.
Phase III or “Is it really safe in patients?”:
Next, the efficacy and side effects of the new drug or treatment are compared with the current treatment or a placebo in a much bigger group of patients with the target condition.
These are usually multi-centre, randomised trials which may go on for two years or more. Success at phase III means that a licence may be issued for the drug.
Phase IV or “Just how safe is it, long term?”:
After the drug is on the market, large-scale epidemiological studies are needed. They may last decades and are intended to highlight any rare or late-developing side effects, monitor long-term efficacy, and, in some cases, pinpoint more conditions that the drug might treat.