Immense research costs for a small user-group. There's no other explanation for the over 2 million euros (including VAT) in therapy costs for one person.
Zolgensma is a gene therapy drug and has been available in Germany since 2020. SMA (spinal muscular atrophy) is a disease caused by a genetic defect and associated with severe muscle wasting. The drug can delay the onset of the disease. The initial price charged by the manufacturer Novartis/AveXis is generally considered significantly too high.
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Sources
Magazine Welt der Wunder in article: "Was ist das teuerste Medikament der Welt", Edition 12/24, p. 30 (German)
Online portal der Betriebskrankenkassen (BKK e.V.) in article: "Das teuerste Arzneimittel der Welt ist nun in Deutschland auf dem Markt" (German)