Clinical trial: Encyclopedia II - Clinical trial - Types of clinical trials

Clinical trial - Types of clinical trials

The most commonly performed clinical trials evaluate new drugs, medical devices, biologics, or other interventions to patients in strictly scientifically controlled settings, and are required for Food and Drug Administration approval of new therapies. Trials may be designed to assess the safety and efficacy of an experimental therapy, to assess whether the new intervention is better than standard therapy, or to compare the efficacy of two standard or marketed interventions.

To be ethical, they must involve the full and informed consent of participating human subjects. They are closely supervised by appropriate regulatory authorities. All interventional studies must be approved by an ethics committee before permission is granted to run the trial.

The study design that provides the most compelling evidence of a causal relationship between the treatment and the effect, is the randomized controlled trial. Studies in epidemiology such as the cohort study and the case-control study are clinical studies in that they involve human participants, but provide less compelling evidence than the randomized controlled trial. The major difference between clinical trials and epidemiological studies is that, in clinical trials, the investigators manipulate the administration of a new intervention and measure the effect of that manipulation, whereas epidemiological studies only observe associations (correlations) between the treatments experienced by participants and their health status or diseases.

Currently some Phase II and most Phase III drug trials are designed to be randomized, double-blind, and placebo-controlled. This means that each study subject is randomly assigned to receive one of the treatments, which might be the placebo. Neither the subjects nor scientists involved in the study know which study treatment is being administered to any given subject; and, in particular, none of those involved in the study know which subjects are being administered a placebo. Of note, during the last ten years or so it has become a common practice to conduct "active comparator" trials (also known as "active control" trials) - in other words, when a treatment exists that is clearly better than doing nothing (i.e. the placebo) for the subject, the alternate treatment would be a standard-of-care therapy.

While the term clinical trials is most commonly associated with large randomized studies, many clinical trials are small. They may be initiated by single physicians or a small group of physicians, and are designed to test simple questions. Other clinical trials require large numbers of participants followed over long periods of time. It is sometimes necessary to organize multicenter trials. Often the centres taking part in such trials are in different countries (in which case they may be termed international clinical trials).

The number of patients enrolled in the study also has a large bearing on the ability of the trial to reliably detect an effect of a treatment. This is described as the "power" of the trial. It is usually expressed as the probability that, if the treatments differ in their effect on the outcome of interest, the statistical analysis of the trial data will detect that difference. The larger the sample size or number of participants, the greater the statistical power. However, in designing a clinical trial, this consideration must be balanced with the greater costs associated with larger studies. The power of a trial is not a single, unique value; it estimates the ability of a trial to detect a difference of a particular size (or larger) between the treated and control groups. For example, of a lipid-lowering drug with 100 patients per group, might have a power of .90 to detect a difference between active and placebo of 10 mg/dL or more, but only have a power of .70 to detect a difference of 5 mg/dL.

Adapted from the Wikipedia article "Types of clinical trials", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki