Bias within economic evaluations – the impact of considering the future entry of lower-cost generics on currently estimated incremental cost-effectiveness ratios of a new drug

BACKGROUND Most economic evaluation models compare a new patented drug (NPRx) to a generic comparator. Drug costs within these models are usually limited to the retail cost of both drugs at the time of model conception. However, the retail cost of the NPRx is expected to drop once generic versions of this molecule are introduced following the expiration of the NPRx's patent. The objective of this study was to examine the impact on the incremental cost-effectiveness ratio (ICER) of the future introduction of lower-cost generic versions of the NPRx within the model's time horizon. METHODS We examined the impact of this parameter with the use of two approaches: 1) a mathematical proof identifying its impact on the NPRx's ICER; and 2) applying this parameter to a previously published economic model comparing a NPRx to a generic comparator and identifying what would have been the NPRx's ICER had this model considered this parameter. RESULTS As expected, both the mathematical proof and the application to the previously published economic model showed that considering the future introduction of lower-cost generic versions of the NPRx within the model's time horizon lowers the NPRx's ICER. The timing of the future entry of lower-cost generic molecules, their relative price compared to that of the patented version, and the discount rate applied to future costs all influenced the results. CONCLUSION An ICER estimated within economic evaluations comparing NPRx to generic comparators which ignore the future introduction of lower-cost generic versions of the NPRx within the model's time horizon will tend to be overestimated. Inclusion of this parameter should be considered within future economic evaluations.