Non-inferiority trials using a surrogate marker as the primary endpoint: An increasing phenotype in cardiovascular trials

Background/aims Non-inferiority trials are increasing in cardiovascular medicine, with approval of many drugs and devices on the basis of such studies. Surrogate markers as primary endpoints have been also more frequently used for efficient assessment of cardiovascular interventions. However, there is uncertainty about their concordance with clinical outcomes. Non-inferiority design using a surrogate marker as a primary endpoint may pose particular challenges in clinical interpretation. We sought to explore the publication trends, methodology, and reporting features of non-inferiority cardiovascular trials that used a primary surrogate marker as the primary endpoint. Methods We searched six high-impact journals ( The New England Journal of Medicine, The Journal of the American Medical Association, The Lancet, The Journal of the American College of Cardiology, Circulation, and European Heart Journal) from 1 January 1990 to 31 December 2018 and identified non-inferiority cardiovascular trials that used a surrogate marker as the primary endpoint. We assessed the non-inferiority margin reported in the manuscript and other publicly available platforms (e.g. protocol, clinicaltrials.gov). We also determined whether the included non-inferiority trials with surrogate markers as primary endpoints were followed by clinical outcome trials. Results We screened 15,553 publications and identified 247 cardiovascular trials that used a non-inferiority design. Of these, 37 had a surrogate marker as a primary endpoint (18 drug trials, 13 device trials, 6 others). All of these non-inferiority trials with surrogate outcomes were published after 2000, mostly in cardiology journals (13 in The Journal of the American College of Cardiology, 9 in European Heart Journal, 8 in Circulation, 6 in The Lancet, 1 in The New England Journal of Medicine), and their publication rate increased over time (p < 0.001 for linear trend). The median number of patients in the primary analysis was 300 (interquartile range: 202–465). The study protocol or a methods paper was publicly available for only 13 (35.1%) trials, of which the non-inferiority margin was not reported in 4 trials. In 16 studies (43.2%), the manuscript did not acknowledge the limitations of using a surrogate endpoint or the need for a definitive clinical outcome trial. Thirty-four trials (91.9%) concluded that the tested intervention met non-inferiority criteria. However, only five (13.5%) were followed by clinical outcomes trials the results of which did not always confirm non-inferiority. Conclusion Non-inferiority trials that use a surrogate marker as the primary endpoint are being increasingly performed. However, these trials pose particular challenges with design, reporting, and interpretation, which are not systematically and consistently addressed or reported.