Major US Formulary Change Triggers, but Does Not Sustain, Increases in Biosimilar Adalimumab Prescribing.

Abstract

CVS Caremark changed their national commercial formularies in 2024 to prefer adalimumab-adaz over originator. We assessed the formulary change's effect on biosimilar adalimumab prescribing in US community rheumatology practices. Data come from Rheumatology Informatics System for Effectiveness, a registry of electronic health record data from participating community rheumatology practices nationwide. We used interrupted time series to analyze the effect of the CVS formulary change on the proportion of patients prescribed adalimumab-adaz or other adalimumab biosimilars each month between January 2023 and September 2024 stratified by payor and among new users. We included 22,079 patients. Among those commercially insured, prescriptions for adalimumab-adaz went from nearly 0% to more than 25% following the CVS formulary change. In the next month, prescriptions for adalimumab-adaz dropped 12.3%. Prescriptions for other biosimilars increased slowly and linearly over the study period. Among patients with Medicare, we observed a similarly timed, but much smaller, increase in prescribing for both adalimumab-adaz and other biosimilar molecules without a large subsequent decrease. Among new users, prescriptions for adalimumab-adaz increased from 0% to 24% but was followed by a nearly 14% decrease in prescriptions in the first month after the formulary change. Other biosimilar prescriptions increased 0.6% per month throughout the period. Although biosimilar adalimumab prescriptions rose sharply after a major national formulary change, the increase was not sustained. These findings highlight that persistent barriers, including the nocebo effect and the administrative burden of switching, may continue to limit biosimilar adoption and contribute to the loss of substantial cost savings in the US market.