OBJECTIVES: Explore the relative impact of factors influencing originator market share following biosimilar competition in the US.

METHODS: A multivariate linear regression analysis included 8 originators with commercially available biosimilars. Variables included aggregate biosimilar market share relative to each originator, number of biosimilar competitors, duration of biosimilar competition, originator orphan designation, price differentials between originator and biosimilar(s), and degree of payer management. An analysis of 18 health plans’ formulary documents as of 1/1/22 was conducted to identify step therapy requirements for each originator through their respective biosimilar(s). Market share and model inputs were captured from FDA, NORD, Drugs.com, and other publicly available sources.

RESULTS: The 6-variable model including all 8 originators resulted in an R-square of 0.581 (p=0.508). After an exhaustive analysis, no predictor variable achieved statistical significance independently or in any combination. However, when excluding the 2 orphan originators and their biosimilars, the R-square increased to 0.9618 (p=0.292). Further, this model resulted in a statistically significant relationship between price differentials and market share (p=0.005). This model also displayed significant results with similar R-square values in other scenarios, each with the inclusion of a single additional predictor variable alongside price differential.

CONCLUSIONS: This model better applies to non-orphan originators. Despite compelling R-square values, results are limited by the low sample size. Biosimilar pricing is more influential than originator step therapy requirements on market share. The difference between non-orphan originator prices and the average price of their biosimilar(s) was the only variable able to predict market share across all statistically significant model scenarios. Other variables such as the number of biosimilars, the duration of biosimilar competition, and step therapy requirements also partly influence market share. Perhaps, the number of biosimilars and duration of competition may influence price differentials. Similarly, price differentials may affect payer management decisions such as step therapy requirements.