Is overall survival a realistic primary end point in advanced colorectal cancer studies? A critical assessment based on four clinical trials comparing fluorouracil plus leucovorin with the same treatment combined either with oxaliplatin or with CPT-11

Background: The adequacy of overall survival (OS) as study end point in phase III trials for advanced solid tumors is questionable. The present review highlights the limits of OS as study end point to evaluate the efficacy of new drugs. Methods: Four phase III clinical trials comparing a fluorouracil-based regimen with the same regimen plus either CPT-11 or oxaliplatin in advanced colorectal cancer patients were reviewed. The primary aim of the critical assessment was to explain the lack of OS advantage observed in two of the four trials, despite the presence of increased response rate (RR) and time to progression (TTP). Four possible reasons for the lack of OS benefit (i.e. statistical power, cross-over, magnitude of the effect on RR and TTP, non-tumor-related deaths) were systematically reviewed in the trials, and the detectable 1-year OS difference, assuming a statistical power of 80%, was calculated for each. Results: None of these reasons for the lack of OS advantage in presence of RR and TTP benefits convincingly explained the results of the evaluated trials. Three of the four trials had roughly the same statistical power to detect 1-year OS differences, while the fourth trial was underpowered to detect realistic OS differences. The lack of OS advantage observed in the two oxaliplatin trials is therefore likely fortuitous, and due to lack of statistical power. Conclusions: Although increase in OS remains the ultimate goal of many clinical trials, the choice of OS benefit as a mandatory requirement to register new compounds can lead to a serious underestimation of a drug's real efficacy.