Exploration of the regional effects of colchicine in the LoDoCo2 trial

Background: The Low Dose Colchicine 2 (LoDoCo2) trial randomized 5,522 patients with chronic coronary disease to colchicine 0.5mg daily or placebo in a 1:1 ratio and demonstrated the cardiovascular benefits of colchicine. In the trial, which was conducted in Australia and The Netherlands, a prespecified subgroup analysis suggested a difference in magnitude of treatment effect of colchicine by region (Australia: HR 0.51; 95% CI 0.39-0.67 vs The Netherlands: HR 0.92; 95% CI 0.71-1.20). The aim of this study was to explore possible explanations for the apparent difference in magnitude of treatment effect of colchicine by region in the LoDoCo2 trial. Methods: The analysis explored potential determinants of variations in the magnitude of effectiveness of colchicine treatment across the regions. This included investigating differences in investigational product, clinical characteristics, concurrent medical therapies and the duration of follow-up using a range of statistical techniques, including sub-group, landmark and effect modification analyses. Results: No differences were found in the colchicine product used in each region. Despite minor differences observed in baseline clinical characteristics and concomitant therapies, the effect modifier analyses demonstrated that these factors did not explain the difference in magnitude of treatment effect of colchicine by region. Randomization in Australia began more than 2 years before The Netherlands, with shorter duration of follow-up in The Netherlands compared to Australia. In a landmark analysis, over the period when more than 90% of patients in each region had been followed, the effects of colchicine were similar (Australia hazard ratio [HR] 0.58; 95% CI 0.34-0.97 vs The Netherlands HR 0.67; 95% CI 0.47-0.96). Conclusions: After examining several plausible explanations for the observed differences in the magnitude of treatment effect of colchicine between regions in the LoDoCo2 trial could be due to the differences in duration of follow-up but a substantial portion of the differences remain unexplained. Clinical Trial Registration: https://www.anzctr.org.au/ACTRN12614000093684.