Clinical Trial Design: Parallel and Crossover Studies
“Before a new drug or biologic can be marketed, its sponsor must show, through adequate and well-controlled clinical studies, that it is effective.” That directive comes from the U.S. FDA as a statement of purpose for why companies like Lilly invest in research for potential new medications. What constitutes an “adequate and well-controlled” clinical study is the subject of hundreds of pages of federal regulation, international guidelines and statistical proposals. In short, a study that is well-controlled is designed to compare two or more groups of people, with at least one of the groups taking the investigational drug (often called the study drug).
The way the groups get compared varies, depending on the study design. The most common design is called a parallel study. Participants are randomly assigned to treatment arms. Each treatment arm could include a particular dose of the study drug, a placebo or a standard of care treatment. Patients then remain in that same treatment arm throughout the course of the study. The challenge with a parallel study is that people dislike the possibility of receiving placebo, so it could be a deterrent for them to sign up to participate.
Another way of comparing groups in a research study is by using a crossover study design. This approach randomly assigns participants to one group, who then “crossover" to another treatment arm during the course of the trial. This means that even if they are initially put into a placebo group, they will also eventually receive the study drug or standard of care during the trial. Often, a washout period is used to ensure data integrity. The washout period is a predetermined amount of time during which patients receive no treatment. This period reduces carryover effects from the previous treatments and helps researchers determine whether the outcome of the study is due to the effects of the study drug.
Crossover studies typically require fewer patients than a parallel study since each patient acts as his or her own control, meaning that they receive both the study drug as well as the placebo or standard of care treatment. However, crossover studies can take longer to complete since patients will receive multiple treatments during the trial.
Check out pictorial depictions of these different study designs in the infographic below. (Long-time readers of our blog may recognize the infographic from an earlier post on LillyPad.)
Understanding clinical research can help you make an informed decision about whether to participate in a clinical trial. Beyond this short list of questions to consider, would having a better understanding of the study design help you make an informed decision? If so, what other information about study designs might be helpful? Drop us a line on Twitter to let us know!