Statistical considerations and endpoints for clinical lung cancer studies: can progression free survival (PFS) substitute overall survival (OS) as a valid endpoint in clinical trials for advanced non-small-cell lung cancer?
In the last decades significant progress has been achieved in the biological understanding of non-small-cell lung cancer (NSCLC) and its tumor heterogeneity has become more evident. The identification of novel tumor targets with different pathways has stimulated the search for anti-tumor agents with a specific target directed mode of action, stipulating the need of testing these agents in clinical trials with an appropriate choice of the study endpoint. Gold standard as an endpoint has been so far overall survival (OS). By definition there are 3 categories of classical endpoints applied generally in clinical lung cancer studies: survival time endpoints, symptom endpoints, and endpoints relying on patients’ reporting. Beside classical endpoints like OS which are tending to show the direct clinical effect of treatment, efforts have been taken to substitute these classical endpoints by surrogates. As a surrogate candidate for OS progression-free survival (PFS) should have the inherent considerable advantage, that it can detect subpopulations with longer PFS intervals early. Based on the (sub-) population treated and having in mind the risk-benefit profile of the drug under consideration, PFS can be considered for regulatory decision making. If accompanied by some independent measures like quality of life or treatment toxicity, PFS should be able to cover the clinical benefit achieved by treatment. Selecting PFS as primary endpoint in Phase III trials of advanced NSCLC may be based on a number of questions such as: Does the definition of PFS fit into the setting used by other trials? Are there accepted consensus standards? Are there consistent surveillance intervals? Is validation for each agent group planned? Is the incremental improvement of PFS big enough (≥30%)? And are there some additional measures to confine clinical benefit? OS is still accepted as the gold standard in trials investigating advanced NSCLC. OS is easy to measure and precise but it may be difficult to interpret if treatment action takes place only in a small subinterval of overall survival. PFS with some additional measures has become attractive when it seems advisable to make study results available earlier. Candidates for supporting PFS as "additional measures" may be treatment toxicity and quality of life measures. PFS allows a more precise detection and attribution to effects of the investigational treatment without being compromised by subsequent treatments. Therefore "enriched PFS" can be considered as an alternative primary endpoint replacing OS in studies investigating advanced NSCLC. The endpoint selection process should always be performed carefully considering all true and surrogate endpoint options in respect to the hypotheses to be proven.