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Clinical trials are designed to evaluate whether a new development is safe, effective, and better than the current standard of care. These interventions can include new drugs, new combinations of existing therapies, new approaches to radiation therapy or surgery, new methods of treatment, complementary or alternative therapies, and new prevention methods. Cancer clinical trials are designed to compare an investigational therapy with the standard treatment regimen being used at the time.

All clinical trials have guidelines about who can participate, called inclusion and exclusion criteria. These guidelines are based on a person's age, type of disease, medical history, and current medical condition.

Inclusion criteria help ensure that all the people in the clinical trial are medically similar.

Likewise, exclusion criteria help keep people safe. They are not used to reject people personally. For example, it is often not safe for women who are pregnant, or who may become pregnant, to receive certain cancer treatments, so they may be excluded from clinical trials.

Phase I trials

The goal of a phase I trial is to prove that a new drug or treatment, which has proven to be safe for use in animals, also may be given safely to humans. Doctors collect data on the dose, timing, and safety of the investigational therapy. People who participate in phase I trials are often the first to receive a new therapy or a new combination of therapies.

In phase I trials, the dose of an investigational drug is gradually increased to determine the optimal safe dose. This process is called dose escalation. The first participants are given a small dose of the drug. If there are no or few side effects, the next group is given higher amounts of the drug until the doctors determine the optimal dose with the fewest side effects. The doctors also learn the best way to administer the new treatment, such as by mouth or through a vein. Finally, the doctors collect data on how the drug is absorbed, processed, and spread through the body.

Phase I trials generally last several months to a year and involve a very small number of people, usually no more than 10 to 20. People whose cancers have not responded to prior chemotherapy are often offered participation in phase I trials. However, phase I trials do not test how well a drug works. Sometimes, a person's cancer will respond to investigational drugs in phase I trials, but this is rare.

Phase II trials

Phase II trials are designed to provide more detailed information about the safety of the treatment, as well as to evaluate the efficacy of the drug. These trials focus on determining whether the new treatment has an anticancer effect in a specific cancer such as shrinking a tumor or improving blood test results. Phase II trials take about two years to complete and usually involve about 20 to 40 people. The response rate in this phase needs to be equal to or higher than the standard treatment in order to proceed to phase III trials.

Phase III trials

The goal of phase III trials is to take a new treatment that has shown promising results when used to treat a small number of patients with a particular disease and compare it with the current standard of care for that specific disease. In this phase, data is gathered from large numbers of patients to determine whether the new treatment is more effective and possibly less toxic than the current standard treatment. Phase III trials are usually randomized and although they focus on patients with a specific disease, they typically include patients of various ages, multiple ethnicities, and both genders so that the results, once obtained, may be applicable to a large number of people. The number of people enrolled in a phase III trial can range in the hundreds to thousands. These trials take many years to complete.

Once a drug has been proven successful in a phase III trial, the researchers can submit an application for FDA approval. If data from the clinical trials meet the FDA's standards, the treatment will be approved for a specified use. However, physicians sometimes prescribe a drug for a use not specified by the FDA, but rather on the basis of studies published in peer-reviewed journals demonstrating effectiveness in treating other diseases, conditions, or symptoms; this is referred to as "off-label" use.