On the use of co-data in clinical trials
*Beat Neuenschwander, Novartis Pharma AG 

Keywords: Bayesian, exchangeability, hierarchical model, meta-analysis, nonexchangeability

Historical data are important for the design of a clinical trial. Yet, these data are rarely used in the analysis of the actual trial. While justifiable in certain situations, ignoring historical data can lead to less accurate inferences, and, therefore, suboptimal decisions. After a review of the main approaches to using historical data, the framework is extended to co-data, which comprise all relevant (historical and concurrent) trial-external data. These data can be used for the inference of the parameter in the actual trial via meta-analytic models. While the use of co-data in clinical trials is attractive, it is also ambitious. For example, avoiding undue weight of co-data (relative to actual trial data) is important, which can often be achieved by plausible assumptions about between-trial heterogeneity and by allowing for nonexchangeability among trial parameters. Two applications with co-data will be discussed: a phase III trial with interim decisions based on co-data; and, a phase I combination trial in Oncology, which takes advantage of co-data from completed and ongoing single-agent trials.