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Types of Clinical Trial

The most commonly performed Clinical Trials evaluate new drugs, medical devices, biologics, or other interventions on patients in strictly scientifically controlled settings, and are required for regulatory authority approval of new therapies. NIH organizes trials into five (5) different types:

  1. Treatment trials: test experimental treatments, new combinations of drugs, or new approaches to surgery or radiation therapy.
  2. Prevention trials: look for better ways to prevent disease in people who have never had the disease or to prevent a disease from returning. These approaches may include medicines, vitamins, vaccines, minerals, or lifestyle changes.
  3. Diagnostic trials: conducted to find better tests or procedures for diagnosing a particular disease or condition.
  4. Screening trials: test the best way to detect certain diseases or health conditions.
  5. Quality of Life: trials (or Supportive Care trials) explore ways to improve comfort and the quality of life for individuals with a chronic illness.

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. The trial objectives and design are usually documented in a Clinical Trial protocol. In the U.S. there is a 50% tax credit on certain Clinical Trials. 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 (in the U.S., this body is the Institutional Review Board) 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. Observational 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 observational studies is that, in Clinical Trials, the investigators manipulate the administration of a new intervention and measure the effect of that manipulation, whereas observational studies only observe associations (correlations) between the treatments experienced by participants and their health status or diseases. These are fundamental distinctions in evidence-based medicine.

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. This is to prevent biases in the administration of the drugs, since a physician may feel more useful to give the drug to a patient who could more easily benefit of it, and the placebo to a more advanced case. Moreover, it has been assessed how there can be a “placebo effect” that can cause tumor responses in the order of roughly 10%. A specialized form of double-blind study called a “double-dummy” design allows another measure of insurance against bias or placebo effect. Here, all patients are given both placebo and active doses in alternating periods of time during the study.

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 “sponsored” 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, and the trial sponsor is more likely to be a commercial company or a government, or other academic, research body. It is sometimes necessary to organize multicenter trials. Often the centers 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 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. The number of patients required to give a statistically significative result relates also to the question the trial wants to answer: to show the efficacy of a new drug in a non-curable disease as metastatic kidney cancer requires many fewer patients than in a highly curable disease as seminoma. 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, a trial 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.

Many Clinical Trials are randomised trials. Randomised trials aim to make a fair comparison between a new treatment and the current treatment on offer, or between two (or more) existing treatments, to see which one works best. They compare groups of people who are similar except for the treatment they receive.
If you take part in a randomised trial, you will have an equal chance of receiving any of the treatments being compared. Neither you nor your doctor will know which treatment you will receive. This is known as randomisation. Randomisation is usually done by a computer. Randomisation is the best way of ensuring that people in the different groups in the trial are broadly similar.

Randomisation is also the best way of ensuring that the results of trials are not biased. For example, if a doctor chose which treatment a patient should receive as part of a trial, she or he might give the new treatment to sicker patients, or to younger patients. This would make the results of a trial unreliable. Randomisation helps prevent this kind of bias.

Across the world, randomised trials are now seen as the most reliable way to test new treatments and to compare two (or more) existing treatments, to see which one works best.

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