Surrogate endpoints are parameters that can be easily measured, and that give an indication that the drug under study will provide a benefit to the patient. Surrogate markers are particularly useful in the early phases of drug development, when the objective is to understand the pharmacology of a drug based upon limited amounts of data. However, with some exceptions, one cannot rely on surrogate markers as endpoints for regulatory approval. In fact, unless the FDA believes that surrogate marker represents a significant benefit to the patient, it cannot be used as the primary endpoint in a clinical trial for approval purposes. Some apparent exceptions to this rule exist.
Blood pressure is actually a surrogate marker that can be measured in clinical trials .
However, lowering blood pressure does not necessarily make patients feel better, or even provide an immediate improvement in their quality or duration of life. Nevertheless, because well-controlled clinical trials have shown that controlling blood pressure ultimately leads to reduced morbidity and mortality, the FDA does approve drugs based upon the drug's ability to reduce blood pressure. Surrogate markers can provide a better understanding of the mechanism of action and dose response characteristics of a drug in phase I and II studies. When the development program moves into phase III however, it is best to choose a primary endpoint that specifically demonstrates an ultimate benefit for the patient, in either quality or quantity of life. In fact, it is best to explore a proposed primary endpoint during phase II , although statistical significance may not be achieved.