OBJECTIVES: Definitive sample sizes for clinical trials in rare diseases are usually infeasible. Bayesian methodology can be used to maximise what is learnt from clinical trials in these circumstances. We elicited expert prior opinion for a future Bayesian randomised controlled trial for a rare inflammatory paediatric disease, polyarteritis nodosa (MYPAN, Mycophenolate mofetil for polyarteritis nodosa). METHODS: A Bayesian prior elicitation meeting was convened. Opinion was sought on the probability that a patient in the MYPAN trial treated with cyclophosphamide would achieve disease remission within 6-months, and on the relative efficacies of mycophenolate mofetil and cyclophosphamide. Expert opinion was combined with previously unseen data from a recently completed randomised controlled trial in ANCA associated vasculitis. RESULTS: A pan-European group of fifteen experts participated in the elicitation meeting. Consensus expert prior opinion was that the most likely rates of disease remission within 6 months on cyclophosphamide or mycophenolate mofetil were 74% and 71%, respectively. This prior opinion will now be taken forward and will be modified to formulate a Bayesian posterior opinion once the MYPAN trial data from 40 patients randomised 1:1 to either CYC or MMF become available. CONCLUSIONS: We suggest that the methodological template we propose could be applied to trial design for other rare diseases.

AOU Meyer Viale Pieraccini 24 50139 Florence Italy

Department of Mathematics and Statistics Lancaster University Lancaster United Kingdom

Department of Medicine University of Cambridge Cambridge United Kingdom

Department of Paediatric Nephrology Great Ormond Street Hospital for Children NHS Foundation Trust Great Ormond Street London WC1N 3JH United Kingdom

Department of Paediatric Rheumatology University of Zagreb School of Medicine University Hospital Centre Zagreb Zagreb Croatia

Department of Paediatric Rheumatology Yorkhill Hospital Glasgow United Kingdom

Department of Paediatrics and Adolescent Medicine Charles University Prague 1st Faculty of Medicine and General University Hospital Prague Czech Republic

Department of Pediatric Nephrology and Rheumatology Gazi University Hospital Ankara Turkey

Department of Pediatric Rheumatology and Nephrology Universitary Hospital Ghent Ghent Belgium

Department of Renal Medicine Addenbrooke's Hospital Hills Road Cambridge United Kingdom

Department of Rheumatology UCL Institute of Child Health London United Kingdom

Department of Women's and Children's Health Institute of Translational Medicine University of Liverpool Liverpool United Kingdom

Division of Rheumatology Department of Paediatric Medicine IRCCS Bambino Gesù Children Hospital Piazza Sant'Onofrio 4 00165 Rome Italy

Great North Children's Hospital Newcastle upon Tyne United Kingdom

Hospital for Children and Adolescents University Hospital of Cologne Cologne Germany

Medicines for Children Research Network Clinical Trials Unit University of Liverpool Alder Hey Children's NHS Foundation Trust Liverpool United Kingdom

MRC North West Hub for Trials Methodology Research Department of Biostatistics University of Liverpool Liverpool United Kingdom

Oporto Medical School Integrated Hospital S João Porto Portugal

Sheffield Children's Hospital NHS Foundation Trust Sheffield United Kingdom

Wilhelmina Children's Hospital University Medical Centre Utrecht Netherlands

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