In medicine, a clinical trial (synonyms: clinical studies, research
protocols, medical research) is a research study.
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
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.
Pharmaceutical clinical trials are commonly classified into four phases, and the
drug-development process will normally proceed through all four stages over many
years. If the drug successfully passes through the first three phases, it will
usually be successfully approved for use in the general population.
Before pharmaceutical companies start clinical trials on drugs, extensive pre-clinial
studies are conducted.
Phase I trials are the first-stage of testing in human subjects. Normally a
small (20-80) group of healthy volunteers will be selected. This phase includes
trials designed to assess the safety, tolerability, pharmacokinetics, and
pharmacodynamics of a therapy. These trials are almost always conducted in an
inpatient clinic, where the subject can be observed by full-time medical staff.
The subject is usually observed until several half-lives of the drug have
passed. Phase I trials also normally include dose-ranging studies such that
doses for clinical use can be refined. The tested range of doses will usually be
a small fraction of the dose that causes harm in animal testing. Phase I trials
most often include healthy volunteers, however there are some circumstances when
patients are used, such as with oncology (cancer) and HIV drug trials. In Phase
I trials of new cancer drugs, for example, patients with advanced (metastatic)
cancer are used. These trials are usually offered to patients who have had other
types of therapy and who have few, if any, other treatment choices.
There are two specific kinds of Phase I trials - SAD studies, and MAD
SAD - Single Ascending Dose studies are those in which groups of three or
six patients are given a small dose of the drug and observed for a specific
period of time. If they do not exhibit any adverse side effects, a new group of
patients is then given a higher dose. This is continued until intolerable side
effects start showing up, at which point the drug is said to have reached the
Maximum tolerated dose (MTD).
MAD - Multiple Ascending Dose studies are conducted to better understand
the pharmacokinetics/pharmacodynamics of the drug. In these studies, a group of
patients receives a low dose of the drug and the dose is subsequently escalated
upto a predetermined level. Samples (of blood, and other fluids) are collected
at various time points and analyzed to understand how the drug is processed
within the body.
Once the initial safety of the therapy has been confirmed in Phase I trials,
Phase II trials are performed on larger groups (100-300) and are designed to
assess clinical efficacy of the therapy; as well as to continue Phase I
assessments in a larger group of volunteers and patients. The development
process for a new drug commonly fails during Phase II trials due to the
discovery of poor efficacy or toxic effects.
Phase III studies are large double-blind randomized controlled trials on large
patient groups (1000-3000 or more) and are aimed at being the definitive
assessment of the efficacy of the new therapy, especially in comparison with
currently available alternatives. Phase III trials are the most expensive,
time-consuming and difficult trials to design and run; especially in therapies
for chronic conditions. Once a drug has proven satisfactory over Phase III
trials, the trial results are usually combined into a large document containing
a comprehensive description of the methods and results of human and animal
studies, manufacturing procedures, formulation details, and shelf life. This
collection of information makes up the "regulatory submission" that is provided
for review to various regulatory authorities in different countries, such as the
Therapeutic Goods Administration (TGA) in Australia, the European Medicines
Agency (EMEA) or the Food and Drug Administration (FDA) in the United States for
Phase IV trials involve the post-launch safety surveillance and ongoing
technical support of a drug. Phase IV studies may be mandated by regulatory
authorities or may be undertaken by the sponsoring company for competitive or
other reasons. Post-launch safety surveillance is designed to detect any rare or
long-term adverse effects over a much larger patient population and timescale
than was possible during the initial clinical trials. Such adverse effects
detected by Phase IV trials may result in the withdrawal or restriction of a
drug - recent examples include cerivastatin (brand names Baycol and Lipobay),
troglitazone (Rezulin) and rofecoxib (Vioxx).
* Rang HP, Dale MM, Ritter JM, Moore PK (2003). Pharmacology 5 ed. Edinburgh:
Churchill Livingstone. ISBN 0-443-07145-4
* Finn R, (1999). "Cancer Clinical Trials: Experimental Treatments and How They
Can Help You." Sebastopol: O'Reilly & Associates. ISBN 1-56592-566-1
Note: This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Clinical trial".