Uptake of Bayesian robust information borrowing in Phase II clinical trials: a scoping review

Background and objective: Incorporation of external information in clinical study designs has received increasing attention in recent years. When external data can be considered consistent with the data collected in the current trial, efficiency gains and a reduction in the required sample size can be achieved. This makes it particularly attractive when, e.g., recruitment of patients is difficult. However, unexpected inconsistency cannot be ruled out, and various robust borrowing approaches have been developed to limit potential losses. These include, for example, the power prior, meta-analytic predictive prior, robust mixture prior, and Bayesian hierarchical model. However, their use in actual clinical trials remains underexplored. In this scoping review, we investigated the use of external information in Phase II clinical trials. Methods: Publications were identified by literature search in PubMed central, the Cochrane Central Register of Controlled Trials and complemented with a citation search in some of the main methodological papers on Bayesian borrowing in clinical trials. Results: Of the 1282 articles retrieved, 37 were eventually included. We investigated, among others, whether and how the suitability of external data was assessed, the borrowing method used, and the specification and justification of tuning parameters that influence the degree of borrowing. Fixed downweighting was found to be the most commonly used method for external information borrowing, with only slightly more than a third of the studies providing details on its implementation. Conclusion: Overall, we found poor reporting of various aspects relating to Bayesian borrowing, signaling a gap between the methodological developments and the practical uptake and reporting of such methods.