OBJECTIVES: Further validating a predictive model developed in 2023 to examine the relative impact of factors influencing originator US market share.

METHODS: We aimed to reassess the relationship between predictor variables and the impact influencing originator market share using the multivariate regression model developed in 2023. The analysis included 10 originators with 26 approved biosimilars. Variables included aggregate biosimilar market share relative to each originator, number of biosimilars, duration of biosimilar competition, originator orphan designation, price differentials between originator and biosimilar(s), and payer management. An analysis of 15 health plans’ formulary documents as of 12/1/23 was conducted to identify step therapy requirements for each originator. Market share and model inputs were captured from FDA, NORD, Drugs.com, and other publicly available sources. The significance level was α=0.05.

RESULTS: The 6-variable model including all 10 originators resulted in an R² of 0.589 (p=0.269). The only predictor variable to achieve statistical significance was payer management (𝛽_management=0.717 p=0.024) and was only significant when assessed independently from the other variables. When excluding orphan products, the R² increased to 0.931 (p=0.133), but no predictor variables achieved significance within the model.

CONCLUSIONS: Although R² values increased when excluding the orphan originators, the results continue to be limited by a relatively low sample size. Outcomes from these models were congruent to that of last year’s analysis with a relatively similar R² but increased p-value, suggesting that this model is a more accurate representations of the factors influencing originator market share. Payer management was the only significant variable to predict biosimilar market share but only when assessed in a simple linear regression, which differs from last year’s model which highlighted price differential as the only statistically significant predictor. Inclusion of list price likely limits the correlation given net price is more influential to payer access and thus market share.