US Fabry Disease Launch Readiness

Agalsidase beta (Fabrazyme, Sanofi Genzyme, approved 2003) remains the dominant US Fabry therapy at 60–65% market share and a 20-plus-year track record — a uniquely entrenched position because, unlike the EU and GCC markets, the US has never had a second FDA-approved ERT to compete with it directly. Migalastat (Galafold, Amicus, approved 2018), an oral chaperone for amenable GLA mutations, has reversed the historical order of preference: ERT-naive patients with a confirmed amenable mutation now start on the oral drug 50–60% of the time, up from 40–50% choosing ERT as recently as 2020. Pegunigalsidase alfa (Elfabrio, Chiesi/Protalix, approved 2023) is the first new US ERT competitor to agalsidase in two decades, but it is positioned narrowly, for patients with high-titre anti-drug antibodies (ADA) and documented suboptimal response to agalsidase beta, not the broad Fabry population.

That ADA-positive suboptimal-responder cohort is small and specific: an estimated 30–40% of male classic Fabry patients on agalsidase develop high-titre neutralising ADA within three to five years, but only 200–400 US patients currently have the combination of high-titre ADA and documented inadequate response (persistent GL-3 elevation, eGFR decline, or progressing cardiac hypertrophy despite ERT) that defines Elfabrio's label and the only genuinely open commercial niche in US Fabry disease today. Identifying them requires ADA ELISA testing that is not yet a routine part of Fabry monitoring — a diagnostic infrastructure gap any new entrant into this niche must close before launch. The much larger opportunity, the roughly 40–50% of treated Fabry patients with non-amenable GLA mutations who have no oral option, remains ERT-only territory contested only by agalsidase and Elfabrio.

A new Fabry agent's payer pathway depends entirely on its administration route: an infused ERT is billed under Medicare Part B (medical benefit, requiring its own HCPCS J-code, a roughly 12-month CMS application process) while an oral chaperone runs through Part D (pharmacy benefit, requiring 18 months of PBM formulary contracting). Prior-authorization criteria already in place require a confirmed GLA pathogenic variant, alpha-galactosidase A activity below 3% of normal, and symptomatic disease (proteinuria, cardiac LVH, neuropathy, or eGFR below 90) for any ERT; migalastat additionally requires a HEK-assay-confirmed amenable mutation, and Elfabrio specifically requires ADA-positive, inadequate-response documentation. ICER has never conducted a Fabry value assessment, a genuine first-mover opportunity to shape the class's cost-effectiveness narrative, given that untreated Fabry disease progression (renal decline of 3–4 mL/min/year, cerebrovascular events in 30–40% of classic males by age 50) carries its own downstream cost offset that has not yet been formally modelled.

This assessment synthesises three independently-verified research angles into one pre-launch view: competitive standard-of-care positioning, target-population epidemiology, and anticipated payer posture. Every factual claim traces to a primary source: FDA approval records, peer-reviewed trial publications, and named registry data.

Fabry sources: FDA Drugs@FDA, BALANCE (Elfabrio), ATTRACT and FACETS (Galafold), Fabry Registry US patient cohort, Amicus investor day 2024, and Chiesi's US Elfabrio commercial access plan.