A surrogate endpoint is a laboratory measure or a physical sign that is intended to be used as a substitute for an endpoint measuring a direct, meaningful clinical outcome (clinical outcome assessment or COA). A surrogate endpoint does not measure the expected clinical benefit of the therapy being studied directly, but rather is expected to predict that clinical benefit. For example, intraocular pressure is an surrogate endpoint for loss of vision in studies involving therapies to treat glaucoma.
Surrogate endpoints may be used when a COA is unethical, for example waiting until a person goes blind, has a serioius stroke, develops other irreversible complications, or dies. Clinical outcomes may also take many years to observe which could delay a therapy being available to patients who would benefit.
Although all surrogate endpoints are a type of biomarker, not all biomarkers are surrogate endpoints. In order for a biomarker to be a useful surrogate endpoint, the relationship between the biomaker and the “direct” or clinically meaningful endpoint must be firmly established. Analyses of multiple studies, which assess both the direct and surrogate endpoints, are required to establish (and quantitate) the predictive nature of a surrogate endpoint.
U.S. Food and Drug Administration (FDA): Patient-Focused Drug Development Glossary
U.S. Food and Drug Administration (FDA): Table of Surrogate Endpoints That Were the Basis of Drug Approval or Licensure