State Substitution Laws and Uptake of an Interchangeable Insulin Biosimilar.

Importance: Although biosimilars have potential to reduce drug spending, their use remains low. Automatic substitutions of biologic drugs by their biosimilars at pharmacies can facilitate biosimilar uptake. Yet, state regulations limiting the types and circumstances under which biosimilars may be substituted could discourage efficient biosimilar adoption.

Objective: To examine associations of state substitution laws in the US with biosimilar adoption in the insulin glargine market, in which an interchangeable biosimilar was recently launched.

Design, setting, and participants: This retrospective cohort study using MarketScan commercial claims data examined fills for insulin glargine among users who were younger than 65 years and had fills for the insulin glargine Lantus (Sanofi) and its biosimilars, interchangeable insulin glargine-yfgn (Semglee [Mylan Pharmaceuticals]) and a noninterchangeable insulin glargine (Basaglar [Lilly]). Data were analyzed from August 2024 to January 2025.

Main outcomes and measures: Market share of the insulin glargine, interchangeable insulin glargine-yfgn, and the noninterchangeable insulin glargine. Unit of analysis was the per-person prescription fill. Changes in fills between those residing in states with less vs more restrictive substitution laws in a 1-year period before and after the launch of interchangeable glargine-yfgn (November 16, 2021) were compared.

Results: A total of 487 281 per-person prescription fills (mean [SD] age, 49.5 [13.3] years; 56.9% male) were included, with 158 141 and 329 140 per-person prescription fills from less vs more restrictive states, respectively. Following the launch of insulin glargine-yfgn, its market share differentially increased by 7.03 percentage points (pp; 95% CI, 1.89-12.18 pp; P = .008), coinciding with a 6.48 pp (95% CI, -11.70 to -1.26 pp; P = .02) differential reduction in the insulin glargine market share in states with less vs more restrictive laws. In the last quarter, the market share for insulin glargine-yfgn was 20.6% and 12.1% in states with less and more restrictive laws, respectively. There were not statistically significant differential changes in fills for the noninterchangeable insulin glargine (-0.24 pp; 95% CI, -1.40 to 0.92 pp; P = .68). Three restrictions had more pronounced associations with a lower uptake of insulin glargine-yfgn: enhanced physician notification (-8.15 pp; 95% CI, -12.49 to -3.81 pp; P < .001), refill notifications (-4.68 pp; 95% CI, -8.78 to -0.58 pp; P = .03), and patient notification (-3.52 pp; 95% CI, -8.44 to 1.40 pp; P = .16).

Conclusions and relevance: In this cohort study, insulin users in states with less restrictive substitution laws were more likely to fill the biosimilar alternative to insulin glargine, underscoring the role of state regulations of substitution as an important determinant of biosimilar adoption and the need for reforms to increase efficient biosimilar adoption.