An endpoint is the primary outcome that is being measured by a clinical trial. A cancer drug, for example, might use survival as an endpoint, comparing the five-year survival rate of patients using an experimental therapy against the five-year survival rate of patients using another treatment or a placebo.
A clinical trial might use a clinical endpoint or a surrogate endpoint. A clinical endpoint is an outcome that represents direct clinical benefit, such as survival, decreased pain, or the absense of disease. The most common and most meaninfgul clinical endpoint in cancer research is overall survival. A surrogate endpoint is a substitute for a clinical endpoint used in trials where the use of a clinical endpoint might not be possible or practical (if, for example, a drug's direct benefits would take many years to measure). Unlike clinical endpoints, surrogate endpoints do not represent direct clinical benefit, but instead predict clinical benefit. For example, tumor shrinkage can be used as a surrogate endpoint for longer survival in clinical trials for drugs intended to treat some cancers. Some surrogates are said to be "established" or "validated," meaning they have been proven to predict clinical benefit. Other surrogates have not been validated, but are "reasonably likely" to predict clinical benefit. This latter type of surrogate is the basis of the FDA's accelerated approval pathway.
The specific endpoint chosen for a given study has to do with clinical trial design, the nature of the condition being treated, and the expected effect of the experimental therapy being tested. Here are a few common endpoints for clinical trials of cancer treatments:
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