Study design choices for evaluating the comparative safety of diabetes medications: An evaluation of pioglitazone use and risk of bladder cancer in older US adults with type-2 diabetes

Aim: The aim of the study was to empirically demonstrate the effect of varying study designs when evaluating the safety of pioglitazone in treating bladder cancer. Methods: We identified Medicare beneficiaries above 65 years of age with diabetes between 2008 and 2015 and with classified exposure (at least two claims within 180 days) to glucose-lowering drugs (GLD), pioglitazone or another drug. The effects of varying the following study design parameters on bladder cancer risk were assessed: use of a new vs existing drug, choice of referent (all non-users and users of GLDs, non-insulin GLDs and DPP-4s) and whether or not censoring accounted for treatment change. We used the Cox proportional hazards model to obtain adjusted HRs and 95% CIs. Results: We included 1,510,212 patients classified as pioglitazone users (N = 135,188) or non-users (N = 1,375,024). Users had more diabetic complications than non-users, but fewer than insulin users. The HR ranged from 1.10 (1.01-1.20) to 1.13 (0.99-1.29) when censoring ignored treatment change, suggesting a weak association or none between pioglitazone and bladder cancer, probably under-estimating risk. However, the HR was 1.20 (1.01-1.42) when cohorts were restricted to new users, censored upon treatment change, and when DPP-4 was used as the referent, suggesting an increased risk of bladder cancer associated with pioglitazone. Conclusions: The continued demand for new GLDs indicates the need for more robust observational methods to improve the value of generating real-world evidence in equipping clinicians to make informed prescribing decisions. Although there is no one-size-fits-all approach, we recommend active comparator new user study designs that compare therapeutically equivalent drugs and account for treatment changes during follow-up to present the least biased comparative safety estimates