Building External Control Arms From Patient-Level Electronic Health Record Data to Replicate the Randomized IMblaze370 Control Arm in Metastatic Colorectal Cancer

External control (EC) arms derived from electronic health records (EHRs) can provide appropriate comparison groups when randomized control arms are not feasible, but have not been explored for metastatic colorectal cancer (mCRC) trials. We constructed EC arms from two patient-level EHR-derived databases and evaluated them against the control arm from a phase III, randomized controlled mCRC trial. METHODS: IMblaze370 evaluated atezolizumab with or without cobimetinib versus regorafenib in patients with mCRC. EC arms were constructed from the Flatiron Health (FH) EHR-derived de-identified database and the combined FH/Foundation Medicine Clinico-Genomic Database (CGDB). IMblaze370 eligibility criteria were applied to the EC cohorts. Propensity scores and standardized mortality ratio weighting were used to balance baseline characteristics between the IMblaze370 and EC arms; balance was assessed using standardized mean differences. Kaplan-Meier method estimated median overall survival (OS). Cox proportional hazards models estimated hazard ratios with bootstrapped 95% CIs to compare differences in OS between study arms. RESULTS: The FH EC included 184 patients; the CGDB EC included 108 patients. Most characteristics were well-balanced (standardized mean difference < 0.1) between each EC arm and the IMblaze370 population. Median OS was similar between the IMblaze370 control arm (8.5 months [95% CI, 6.41 to 10.71]) and both EC arms: FH (8.5 months [6.93 to 9.92]) and CGDB (8.8 months [7.85 to 9.92]). OS comparisons between the IMblaze370 experimental arm and the FH EC (hazard ratio, 0.85 [0.64 to 1.14]) and CGDB EC (0.86 [0.65 to 1.18]) yielded similar results as the comparison with the IMblaze370 control arm (1.01 [0.75 to 1.37]). CONCLUSION: EC arms constructed from the FH database and the CGDB closely replicated the control arm from IMblaze370. EHR-derived EC arms can provide meaningful comparators in mCRC trials when recruiting a randomized control arm is not feasible.