How are Clinical Trials Designed?

There are two basic study designs by which drugs are evaluated in clinical trials, especially in Phase III trials. In a Parallel Design, subjects randomized to receive separate treatment arms (A or B), where A may be the active drug and B the placebo. Subjects then remain on that arm for the duration of the study. The advantage of a parallel design is that it provides the best way to assess the effect of a drug on survival, if that is the critical endpoint in its evaluation. A potential disadvantage of a parallel design is that it requires a large number of subjects to test hypotheses regarding survival.

Alternatively, in a Crossover Design, subjects are randomized to one treatment arm (A) and later switched to the other arm (B); in fact such a crossover may occur more than once during a trial. Two advantages of a crossover design are that each subject essentially serves as his or her own control, because they receive both the active drug and the placebo during the study and fewer subjects are required than in a parallel trial. However, crossover designs also suffer from at least two potential drawbacks. First, a crossover design cannot determine the effect of a drug on mortality, although it can assess its effects on non-fatal endpoints (or outcome measures) such as blood pressure or blood sugar control, recovery time from a stroke or heart attack, length of hospitalization etc. Second, crossover studies may sometimes suffer from a phenomenon known as the “carryover effect.” This means that, were a patient to be treated initially with the active drug, the biological effects of the drug might persist for an uncertain period of time after the patient was switched over to the placebo arm. Thus, it is important to know as much as possible about the duration of a drug’s biological effects so that an appropriate comparison can be made between the results obtained during both treatment arms.

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