Practical issues arising in an exploratory analysis evaluating progression-free survival as a surrogate endpoint for overall survival in advanced colorectal cancer.

This paper is based on a conference presentation in which several authors presented results from analyses of the same dataset concerning the evaluation of progression-free survival (PFS) as a surrogate endpoint for overall survival in advanced colorectal cancer clinical trials. In evaluating a potential surrogate endpoint, there is a hierarchy of information that might usually be considered desirable: 1) a biological rationale for surrogacy, 2) demonstration of the prognostic value of the surrogate endpoint in untreated patients and 3) in treated patients and 4) demonstration across randomized comparisons that differences in the effect of randomized treatments on the surrogate endpoint are associated with the corresponding differences in the effect on the clinical endpoint of interest. Results from analyses that might be used to address the third and four requirements are presented and some of the practical issues that arise in evaluating a surrogate endpoint, which would be relevant to many diseases, are illustrated. Although the results presented should not be seen as a definitive analysis of the value of PFS as a surrogate endpoint, concerns are identified about the potential lack of standardization of the definition of PFS or the frequency of evaluation of disease progression and the high leverage of one study in evaluating the association in addressing the fourth requirement.