Update on the clinical trial landscape: analysis of ClinicalTrials.gov registration data, 2000-2020

The clinical trial landscape has evolved over the last two decades, shaped by advances in therapeutics and drug development and innovation in trial design and methods. The tracking of such changes became possible with trial registration, providing the public with a window into the massive clinical research enterprise. The ClinicalTrials.gov website was launched in 2000 by the NIH National Library of Medicine and is the largest clinical trial registry worldwide. The purpose of this analysis is to describe the composition and methodologic features of clinical trials as registered on ClinicalTrials.gov and to identify trends over time. We analyzed data from the publicly available Clinical Trials Transformation Initiative Aggregate Analysis of ClinicalTrials.gov (AACT) database, focusing on trials (interventional studies) started between 1 January 2000 through 31 December 2020. Characteristics of design (e.g., phase, randomization, use of masking, number of treatment groups, sample size), eligibility criteria (age groups, gender), interventions, conditions, and funders (primary sponsor) were tabulated over time, by year trial started. There were 274,043 registered interventional studies (trials) included in the analysis. Most trials were reported as randomized (65%); single site (60%); parallel-group (56%); funded by other sources (e.g., individuals, universities, and community-based organizations) (65%); and involving drug interventions (55%). Notable trends include an increase in the proportion of registered trials without FDA-defined phases ("Phase N/A") over time, a decrease in proportion of trials that involve drugs or report treatment as a primary purpose, declining sample size and time to complete trials, and an increase in proportion of trials reporting results among completed trials. The proportion of missing registration fields has also decreased over time and more trials make protocols and other documents available. There is a current need to expand the registration fields in ClinicalTrials.gov to adapt to the evolving trial designs and reduce the number of trials categorized as "other." Observed trends may be explained by changes in trial regulations as well as expanding and evolving trial designs, interventions, and outcome types. Clinical trial registration has transformed how trial information is accessed, disseminated, and used. As clinical trials evolve and regulations change, trial registries, including ClinicalTrials.gov, will continue to pr