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Background and Importance Natalizumab is an effective therapy for relapsing-remitting multiple sclerosis (MS), usually reserved as a second-line treatment due to its high cost. The availability of an intravenous (IV) biosimilar may improve healthcare sustainability, while the subcutaneous (SC) originator offers shorter administration but its patent has not yet expired. Evaluating their economic impact is crucial for evidence-based and sustainable decisions.Aim and Objectives To conduct a five-year budget impact analysis (BIA) comparing IV natalizumab biosimilar with the SC originator in a hospital setting, and a cost-minimisation analysis (CMA) to identify the lowest cost option per dispensation, assuming therapeutic equivalence.Material and Methods The BIA model considered 1,168 annual dispensations (2024 baseline). Direct costs included drug acquisition (€1,090 SC; €873 IV biosimilar), administration (€12 SC; €33 IV), monitoring (€67 both), and adverse event management (€0.30 SC; €0.35 IV). Scenarios were: exclusive SC originator, exclusive IV biosimilar, and mixed (50/50 in year 1; 30/70 thereafter). Sensitivity analysis with 10,000 Monte Carlo simulations varied biosimilar uptake (50–90%) and annual dispensations (900–1,300). The CMA compared unit costs to estimate per dispensation savings.Results The IV biosimilar cost €975 per dispensation compared with €1,170 for the SC originator, yielding savings of €195 per dispensation. With 1,168 annual dispensations, this corresponded to approximately €228,000 per year and €1.14 million over five years. Over the same time horizon, total costs were €6.83 million for the SC originator, €5.68 million for the IV biosimilar, and €6.07 million for the mixed scenario. Sensitivity analysis of the mixed scenario, based on 10,000 Monte Carlo simulations varying biosimilar uptake (50–90%) and annual dispensations (900–1,300), showed an average cost of €5.63 million (SD €0.60 million; range €4.43–6.93 million).Conclusion and Relevance The CMA identified the IV biosimilar as the cost-minimising option, saving €195 per dispensation compared with the SC originator. The BIA confirmed that exclusive biosimilar use yields the greatest savings, while mixed adoption offers a pragmatic compromise, combining cost reduction with therapeutic continuity and an alternative for patients intolerant to the biosimilar. These evaluations may support sustainable decision making and improve patient access in MS care.Conflict of Interest No conflict of interest