An endpoint is something which is measured in a clinical trial or study. Measuring the selected endpoints is the goal of a trial. The response rate and survival are examples of the endpoints.
A clinical trial will usually define or specify a primary endpoint as a measure that will be considered success of the therapy being trialled (e.g., in justifying a marketing approval). The primary endpoint might be a statistically significant improvement in median overall survival (OS). A trial might also define one or more secondary endpoints such as median progression-free survival (PFS) that will be measured and are expected to be met. A trial might also define exploratory endpoints that are less likely to be met.
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These are expressed as a period of time (survival duration) e.g., in months. Frequently the median is used so that the trial endpoint can be calculated once 50% of subjects have reached the endpoint.
The disease free survival is usually used to analyze the results of the treatment for the localized disease which renders the patient apparently disease free, such as surgery or surgery plus adjuvant therapy. In the disease-free survival, the event is relapse rather than death. The people who relapse are still surviving but they are no longer disease-free. Just as in the survival curves not all patients die, in "disease-free survival curves" not all patients relapse and the curve may have a final plateau representing the patients who didn't relapse after the study's maximum follow-up. Because the patients survive for at least some time after the relapse, the curve for the actual survival would look better than disease free survival curve.
The Progression=Free Survival is usually used in analysing the results of the treatment for the advanced disease. The event for the progression free survival is that the disease gets worse or progresses.
The response duration is occasionally used to analyze the results of the treatment for the advanced disease. The event is progression of the disease (relapse). This endpoint involves selecting a subgroup of the patients. It measures the length of the response in those patients who responded. The patients who don't respond aren't included.
Overall survival is based on death from any cause, not just the condition being treated, thus it picks up death from side effects of the treatment, and effects on survival after relapse.
AR Waladkhani. (2008). Conducting clinical trials. A theoretical and practical guide. ISBN 978-3-940934-00-0