Prescription Dispensing for Insulin Glargine After Interchangeable Biosimilar Designation.

Importance: The first US Food and Drug Administration-approved interchangeable biosimilar designation-that for insulin glargine-occurred in 2021, enabling pharmacy substitution for the branded originator. However, the impacts of this interchangeable designation on prescription dispensing are unknown.

Objective: To assess impacts of the transition of Semglee to interchangeable designation on prescription dispensing.

Design and setting: This economic evaluation analyzed changes in insulin glargine dispensing before and after the introduction of the interchangeable designation using data collected from IQVIA's National Prescription Audit, a nationally representative comprehensive database of pharmacy dispensing for the US, and PayerTrak. Data cover the time period from September 2019 through June 2024 and were analyzed from June 2023 to December 2024.

Exposure: Any medical diagnosis that would make insulin glargine a relevant treatment.

Main outcomes and measures: The primary outcomes were monthly US aggregate pharmacy dispensing of Semglee and insulin glargine-yfgn, measured both in prescription counts (in thousands of prescriptions) and as a proportion of the US aggregate insulin glargine market. Results were disaggregated into Semglee and insulin glargine-yfgn to show that changes in dispensing were associated with the interchangeable designation even after accounting for Semglee's formulary changes. This evaluation additionally examined dispensing channel and payer type.

Results: After the introduction of interchangeable Semglee and insulin glargine-yfgn in November 2021, there was a discontinuous increase in aggregate Semglee/insulin glargine-yfgn dispensing of 47.41 (95% CI, 19.45-75.38; P = .001), suggesting that the interchangeable designation was associated with substantially increased utilization. In addition, Semglee and insulin glargine-yfgn's share of the total insulin glargine market matched its dispensing trends, demonstrating that the jump in dispensing was not associated with changes in the market as a whole. When disaggregating by channel, there were also statistically significant increases in all 3 channels: retail (20.27; 95% CI, 2.58-37.95; P = .03), mail (6.63; 95% CI, 3.58-9.67; P < .001), and long-term care (20.52; 95% CI, 11.06-29.98; P < .001). This jump, however, coincided with advantageous formulary changes for Semglee but not insulin glargine-yfgn, the increased utilization of which was still associated with the interchangeable designation. In the Medicare Part D, Medicaid, and cash channels, insulin glargine-yfgn adoption grew faster than Semglee, reaching higher levels of dispensing in every single period measured after launch.

Conclusions and relevance: In this economic evaluation, the first US Food and Drug Administration approval of interchangeable status was associated with increased dispensing of the follow-on. This suggests that interchangeability designation may play an important role in decreasing costs and increasing access to biosimilar prescription drugs for patients.