Evolving options for the treatment of metastatic breast cancer: progression-free survival as an endpoint

Because of its direct clinical relevance, overall survival is the gold standard endpoint for measuring clinical efficacy. However, achieving improvements in overall survival can be confounded by factors such as crossover to active treatment arms and subsequent treatment with non-experimental active therapies. Powering studies to detect significant overall survival increases requires prohibitively large patient numbers and long follow-up and may not always be practical. Trials incorporating progression free survival (PFS) or time to progression (TTP) as primary outcome measures are likely to be shorter, require fewer patients and are usually more affordable, which may ultimately translate into a more rapid evaluation of potentially effective experimental therapies. In heavily pretreated metastatic breast cancer, significant improvements in progression-free survival may indicate a clinically meaningful benefit for patients with otherwise limited salvage therapy options available. Approval for several newer agents in the advanced resistant or refractory metastatic breast cancer setting has been based on prolonged progression-free survival or time to progression as primary trial endpoints. In this paper, clinical trial data relating to OS, PFS and TTP endpoints are reviewed and the use of surrogate markers of survival for the evaluation of new drugs is considered.